Ok! completely awesome experiments! Im retired Aeronautical Engineer from NASA. Yes I have done experiments at all Re# for stepped airfoils. I found a KFm1mod to have the best performance. The mods were NLF65 series leading edge with a design lift CL of 0.2 12% thick and sanded smooth with 400 grit 2lb blue foam also polyester resin to high polish. All I found was that we could use a stepped airfoil to create more desirable stall characteristics for airfoils designed for pressure recovery. Polished airfoils had better pressure recovery and a nice drag bucket bringing the CD down to 0.01 from 0.047 vs sanded blue foam even at Re# as low as 4000. An unmodified NLF 65-212 could not produce a drag bucket at low Re#. The massive pressure recovery that was produce on the underside step was spread out from 50%cord and back changing the center of lift to 60% and 75% in a stall causing desirable stall recovery characteristics. Tests were done at Turbulent scale length of 0.1meter and 2meters to show the difference in Micro and Macro aerodynamics. The interesting thing was we were able to produce pressure recovery as low as Re# 4000 with the stepped version in micro aerodynamic range. With the NLF 65-212 we needed at least 1.2 MRe# to achieve a significant pressure recovery and drag bucket. This is not a problem with full size aircraft as we always operate in the MRe# range but it may be of use in your smaller gliders. We made some gliders and it was fun to throw them around we saw glide slope of up to 40/1 with aspect ratios over 12/1 of corse with wing loadings as close to nothing as could be done, Man! they just keeped going. But what we were trying to learn was if it would produce better stall recovery without compromising top speed with a fixed geometry on pressure recovery airfoils designed for speeds from M0.25 to M 0.7 and they did indeed work, so I'm glad you are having fun studying this we sure did. This stuff has never really been implemented, I did use what I learned to design some high pressure recovery airfoils for wind mills that reduced the kick in speed down to 10kts from 20+kts. Oh and the Clark Y sucks! don't ever use it.
@@MauricioHernandez-de8is Oh thank you. I did actually teach at Space Camp. I like encouraging youth to be Awesome. I also wrote CFD based Flight Simulator Software that is the industry standard and used at NASA for all flight training in every flight test programs since 1996. And well everyone else. This kid is really smart and explains things well Im very impressed with his videos, He has learned this all correctly I have zero criticism of anything he has explained. I don't think he needs my help he is doing it right!
@@anthonyb5279 At what approximate physical scale do these airfoils start to lose benefit? I'm a layman and don't really understand Re# conceptually all that well. However, picture a generic Cessna 172-ish airplane. Are the wings on those too large to benefit? How about the horizontal stabilizer which is a much smaller surface?
Ive been a flying professionally and teaching for almost 30 years. In my opinion this is a great experiment. It makes me very glad to see that we still have young people in this nation with enough curiousity and motivation to try things like this. Unfortunately the experiment while creative is flawed - mostly on the design of th first airfoil. Lift is created by a combination of two principals - Bernoulli's and Newton's. Newtons law says that as air molecules strike the lower side of the airfoil they create a higher pressure area which "pushes" the air foil upwards. This works well on an airfoil where the bottom is perfectly flat. Unfortunately the bottom of the fold over airfoil is NOT flat, while the other two ARE flat. This leads to the first flaw. In the experiment the first airfoil not only has a different topside but also a different bottomside. When doing an experiment it is important to only change one thing at a time. The second flaw has to do with the effect of the step on the bottom. As it moves through the air, it very likely creates a forward pitching moment due to the application of Newton's law as the air flows over the step. This pitching moment should be causing the airplane to try to nose down which the elevator must in turn counteract. The additional drag will reduce the aircrafts performance substantially. The third flaw has to do with the application of the elevator itself. In real life, a pilot operating an aircraft will trim the aircraft so that it is at its optimal speed where the combination of induced drag and parasite drag are at its lowest point. This optimal speed will allow the aircraft to remain airborne for the longest period of time. Unfortunately, the experiment here fails to take this into account. The elevator is simply 'set' prior to flight without taking into account which . The experiment needs to be repeated in a wind tunnel where total drag and total lift can be measured at various attitudes, or it needs to be repeated multiple times using the same airfoil, with different elevator settings. The last flaw is that the aircraft needs to be chronographed/released from a mechanical device so that we can verify that flow of air over the wings is exactly the same for each flight. Still, its a great idea, and I hope this young fellow take my observations as advice and NOT as criticism.
A Typhoon fighter, carrying full ordinance and drop tanks has very little lower wing surface exposed. Could Newton explain how it flies in this configuration? I understood that lift was mainly supplied by the intense low pressure created over the upper surface of the wing, which rises into it.
Remember that not all the parasitic drag is in the wing ... it's everywhere on the plane, fuselage, tail, etc. At low Re it becomes important to use a boom tail rather than a tapered cabin, and to use a rudder placed farther back for better leverage, where it can also be smaller with less drag and have the same, or better, control authority. Smaller slower aircraft have different design trade offs, than large fast aircraft.
Hey man, wanted to say thank you for your video series. I worked on a project trying to build large model gliders out of lego! We used your videos to learn all we could about how to get them to fly. Thanks so much!
the fold over has a marked step at the lower surface trailing edge, which by my assessment is actually creating a hybrid inverted kfm2 - which would suggest that the lift is being compromised by the "inverted lift" of that part of the airfoil run a fill-in layer of tape from the step to the actual trailing edge to "disappear" that step and then try again... i am keen to see the difference
Interesting experiment and useful for someone who was building simple wings with very thick airfoils out of foamboard, where the KF airfoils may indeed be 'less bad'. However a comparison against a conventional airfoil that is specifically designed for operation at low Re numbers (NOT a Clark-Y) I'm certain would yield very different results. One of the Mark Drela series airfoils would be a good option, find them at 'Drela Airfoil Shop'. I did my own testing of a 1m glider with a Drela wing vs a KF of same thickness weight and planform mounted on the same fuselage. The Drela with flew about 50% longer duration.
I built an A2 FAI class towline glider decades using an Eppler E59 airfoil. Floated of a 50 mtr. towline the model would consistently do 3 mins. 15 secs. in still evening air after sunset. As Dr Eppler explained, the centre of pressure movment of the E59 was "very marked" However in turbulent air the model would not go away in a thermal and despite geodetic wing construction in certain circumstances the wing would torsionally oscillate bringing the model down fast. Experimenting with airfoils you will find the nose radius has a BIG affect as does wing tip planforms. For those of us fascinated by flight, yours was an interesting informative experiment.
Yes! please do all of these variables. Extremely interesting, massive real world implications. In all the hobbies and interests I am involved in I am shocked I have never heard of this before
I used a 3 or 4 step foil on 8" indoor gliders back in the 70's. The interesting thing was I could increase the weight of the glider to many times the original weight and the glide time was the same. The section was laminated and a proper section was shaped back to the steps.
Highly instructive. Good testing technique, especially for an "at home" and "non-industrial" effort. You earned my subscription. I'll be lurking around your channel. Thanks!
If you do include wind tunnels and simulation I would prefer to still see the actual real world testing afterwards. To me, seeing the theory followed by your controlled experiment was very rewarding. Looking at the three wing designs at the beginning then measuring what I thought would happen as compared to what did happen was great. I assumed the Flite Test wing would reign supreme and was amazed the 2 & 3 step wings (actually flew and) proved more efficient. I fly RC planes but assumed the kind of content here would require some intimidating math to understand. Your intuitive explanation made perfect sense to me. Thank you for making this video. I subscribed and one day hope to design my own RC model.
RC models, although flying slower than life-sized airplanes, still fly much faster than gliders. So that's probably why you were so surprised: you're not used to speeds THAT slow
Glad to see your channel speaking on kfm airfoils. I had my graduation project based on lift and drag performance of kfm2 type modified clary-y airfoil over multiple reynolds number and across multiple depth and location of step over the airfoil and compared it to a typical clark-y airfoil. Results showed a substantial increase in aerodynamic performance in low reynolds number range.
I see many comments that tell me the audience does not understand Reynold's number. Reynold's number indicates the ratio of fluid momentum to dynamic viscosity. A very viscous fluid with low density will create lower reynolds numbers (Alternately stated, increasing kinematic viscosity reduces Reynold's number.). High speed or high density increases momentum, if viscosity remains fixed, will increases the Reynold's number. This is what drives the boundary layer formation, the boundary layer changes the effective airfoil thickness and shape with the appearent effect of being "skin drag". A longer cord also increases reynolds number(Sorry I don't have a clean explanation that would fit here, but it is rooted in the scaling of momentum and viscosity.). The boundary layer does not grow twice as thick with twice the cord, so relative to the change in scale there is a lower proportional change in the boundary layer and effective wing thickness and shape. Low reynolds number = proportionally thick boundary layer (manifesting as "skin drag"), high reynolds number = proportionally thin boundary layer. Reynolds numbers below 100k tend to have mostly laminar flow in the boundary layer, above 200k tends to cause mostly turbulent flow in the boundary layer. Laminar flow at high reynolds number is possible and produces extremely low drag but it is very sensitive to disturbances in flow and surface condition. Turbulant boundary layers have more drag but they are more consistent and reliable in adverse conditions. This is all for fully subsonic flows with neglagible compression effects, so below about mach 0.5; Still somewhat useful for estimations up to mach 0.8 although allowances need to be made for more significant compression effects depending on the particular fluid properties.
I used a thiner KFM-2 airfoil, just fording foambord, without that spacer. Used it on a several flying wings with nice results. Very strong and easy to build.
Absolutely amazing video. Every time a "how?" or "why?" popped into my head you immediately gave more details. At those speeds, you could get lots of cheap data with a bicycle and bits of string. Mount the wing to a boom in front, glue bits of string to the wing, and Bob's your uncle. It's not at elegant as a wind tunnel or a simulation, but it could easily work in that huge gym you have access to.
Yes, this is is intriguing. I would be very interested if improving the "vortex holder" would further reduce drag: Keep the sharp top edge of the step, but fill in the bottom corner with a radius equal to the height of the step. My thinking is that this may eliminate micro turbulence at the foot of the step inside that 90 degree corner.
@@johnbgibbs my gut feeling is that a filet this large would reduce the speed range at which the wing would maintain its advantages. So it might drop like a rock until the forward speed drops enough to allow the vortex to become established, worse yet fly well until the speed drops too low, then suddenly stall. think that with enough resolution you would find that there is a tiny counter-vortex hiding in that corner that might be really important. Another field to borrow from is stepped hulls in planning boat hulls. These usually actually are undercut, sort of the opposite of a filet.
You’ve conducted the experiment in the best way possible and still maintaining interest along all the video, very good! Thanks now I can’t wait for the next
Nice Comparison, but Sorry- you're making a crucial error: The maximum airfoil thickness which really works is about 7-8 % at this Reynolds numbers. Therefore every other airfoil is better: Let's explain: In my youth I was owner of "der kleine UHU". Clark Y. I lost my ballast chamber and I turned my wing by 180 deg's leading edge to the font. And it flew good.- not worse than standard. Airfoils which work best are usually thin airfoils with 3D-turbulator (zigzag). 40 Yrs ago it was "Junior" of Graupner.
A whole other area to investigate what about stepped foils for propellers? That might have huge applications in drones. Especially drones intended for longer duration where they're going to use big propellers spun at relatively low speeds. I am thinking that FPV racing drones probably move too fast, but drones for surveys or search and rescue might benefit from more efficient props that would allow more time up on the same battery.
It would make them quieter. Also just about all KFm airfoils have been used on helicopters. The other big advantage is they can manage twisting moment and flapping problems due to being low moment airfoils that are inherently strong and not need a spar, they can be mede of just a skin. Also good for over simplified rotors used on most quad copters that have no collective control for blade pitch as the have good high AOA performance and arrest a decent without going into blade stall.
Your technical and methodical approach immediately earned a new subscriber here! Would love to see more on this topic and what else you uncover. Excellent analysis.
This channel is gold. Really appreciate the succinctness, the inclusion of visualizations like graphs and diagrams to get the point across, and the thoroughness of the testing.
It's incredible that I found your video! This is a really interesting topic - keep up the great work, you're awesome. The stepped airfoil design is new to me, and I'd love to see more tests with different shapes. CAD simulations would be super eye-opening as well. The background theories on why these designs might or might not work are very helpful, especially since I'm an amateur in this field.
I appreciate your effort, and care about this as I love small balsa gliders. We used to make them 4 inches long, and had great results usually. The thing I wonder about with your experiments is if the air is doing what you think it is. There is enough going on that you are generalizing a lot in your conclusion. You need a mini wind tunnel with smoke, and then possibly some flights through smoke to see if the patterns seem to match. Then you can connect cause and effect better as you are really just retesting past experiments by others. I'd love to see that mini tunnel in action.
I seem to recall that Cessna did extensive testing of the KF airfoil, as well as a main wing that pivoted at its CG. Of course, a 1600# airplane flying at 100MPH is quite different from a lightweight glider.
Thanks very much ! I have not known stepped designs before. Also your explanation of the reasons for better flying with stepped wings is understandable. This is an extremely compressed presentation of days & weeks of work in 10 minutes.
Amazing video. Ref: at 4:06 is the KFm foils page. While watching it I was having 2 thoughts: 1) use the flat extension as aileron/flap positioning the hinge at the edge of the step. 2) in your flight videos some planes have a nose-up attitude and glide while others simply glide downwards. A clever way to adjust the center of mass is to make it as independent from speed as possible. So having the plane have the same nose attitude regardless of launch speed. When you reach this condition your CG is theoretically on the center of lifting force of the wings. This could potentially provide some other kind of results, since CG is then trimmed to be at the center of wing lifting and not optimized for distance.
Great test matey, I’ve been an Avionics engineer on large passenger aircraft for 30 years. Never heard of stepped wings before. Dunno why this vid popped into my feed but glad it done.
I have built a bunch of flying wing rc planes using the KFm4 and versions of the same wing using a an actual symmetrical airfoil. These were relatively heavy fast planes. The "real" airfoil wings tended to be more efficient and faster. The KFm4 planes were very stable in the air they had much less pitch instability and over all flew better. I mostly flew FPV and I liked the smooth flying of the stepped airfoil. I have a bunch of flight videos on my channel from a few years back. Great work on your experiment you got a new subscriber.
I was playing with this exact thing a couple of years ago but this guy gas done an exceptional job at explaining and producing valuable data and experiments. Good job sir
You might want to build the slat wing by Clough from plans on Outerzone under sport control line plans. That design wuld be a great eye opener for your follow up questions.
Thank you, my question too. You know the wings with more material were heavier. That being said, the stepped airfoils still did better with these slow speed park-flyer scale planes. I would assume stepped wings that are optimized for weight would do fantastic for foamies.
The difference in the weights of the lightest and heaviest planes was only 6%. I didn’t try to match them exactly since the weight of a glider does not affect the flight distance.
Suggestion for efficiency: close the hole at the wingtips. There is air circulating in the open space on the KFM wing, which is what you want, but that gets destroyed at the wingtips, thus you want a surface there to prevent the inrush of air.
Don't forget why golf balls have dimples! Another interesting direction in this type of experimentation with surface drag reduction is distributing micro-vented air across the surfaces, basically creating a low-pressure air bubble around those surfaces. Or a flexible wing covered in individually-adjustable scales, each creating it's own little low-pressure void, controlled independently over the entire surface. In other words, feathers!
This was a very educational video for me, thank you so much! I have built FlightTest planes before and also made my own planes, the flight test planes never flew great or needed a lot more power and speed, this video explains why :-)
Harlold Penrose was a test pilot (and aircraft designer) in the UK that testflew a full-size glider in the '40s using an airfoil similar to the one you tested here, so it isn't a new airfoil! He test flew many different aircraft during his years at Westland. In his eighties he designed and flew a canard microlight, with a Kasper Wing-like wing if I remember correctly!
I'm pretty sure that the stepped KF airfoild has never been used in a full size aircraft. They have undergone wind tunnel testing and performed very poorly, so no one ever took them into full size trials. You are right that they are not new though, paper planes used them since people first folded a paper plane.
Thanks @ErikssonTord_2, I was hoping a viewer would pull Witold Kasper into the ring. Kasper was an academic and his efforts to capture vortex lift at 0 KIAS is still a thesis looking for a PhD candidate.
Thank you for putting in the groundwork and sharing the results with us. I will be watching this channel for any updates on this subject. Subscription well deserved.
Here’s an experiment for you: Let’s say that not all parts of the wing are created equal. The portion of the wing close to the fuselage performs differently (to some degree) than the wing portion near the tip of the wing. Thus, would it be possible to create a wing structure such that the portion of the wing near the fuselage contain a stepped airfoil and can that step be slowly eliminated the further out it gets to the tip of the wing where (possibly) the air flowing over the tip reacts differently than near the fuselage. I’m suggesting that possibly a “hybrid” wing design may be possible where near the fuselage it is of a stepped design and near the wing tip it is more of a solid surface design. Just a thought.
to my knowledge this is done at the blades inside the rotary turbines where a thin layer of air does just this (and is good for cooling and isolating the blade material from combustion gases)
Hi. Excellent information, thanks. I now only build small lightweight rubber powered planes, and I can't wait to try a stepped airfoil. I'll chop the wings of my triplane (which climbs like a rocket, but glides like a brick), and see if I can improve the flight envelope. As there's little difference between the k2 and k3 I'll do the single step one. Thank you for speaking slowly and clearly. Cheers, P.R.
I would be curious how the stepped airfoils vs non-stepped affect powered flight. You mentioned "not for high speed" (generalization). I am curious about slow speed flight. Downside being *much* more time required for testing.
Just one airplane video reignite my algorithm to reccomend more airplane video, and the video of yours is really interesting and the concept of your channel is really amaze me, low minimum entry to do all the things you're doing for everyone to see and experiment with!
Very informative video. I 3D printed some airfoils before and never considered stepped airfoils as they have the stigma of being low tech foamboard things. 3D printing makes it easy to just copy any "high performance" airfoil of large scale aircraft. Maybe it would be interesting to make a Airfoil with tiny stairs, so essentially a rough surface to create a smooth-ish buffer layer of air that sticks to the wing.
YO Your experiment is so well done! It’s so much better than any of my attempts. Also you explained KF airfoils wonderfully, I learned more about it. Great job. Also I’ve found a relatively cheap and easy way to build a Hotwire cutter. I used these materials: -Hobbywing Eagle 20A Brushed Esc -26g nicrom wire -20 gauge wire -one of those cheap blue servo testers You can make a jig for the cutter out of like some thin basswood sheets, pvc pipes, or some other sturdy material, never use any conductive materials. I soldered the positive and negative ends of the esc to the 20 gauge wire, and directed both ends of the wire to the ends of the jig. I used two small screws to act as the mounting points for the nicrom wire to wrap around. Its should look like this |+|-----------|-| | | | | | |__________________| | |_____________ _________| | [] | | [] | -- The esc is mounted where the [] on the handle is, with wires directed on both arms. The + and - are where the screws should be and the -- is the nicrom wire, you should try to wrap it tightly to create tension. Solder whatever battery connector you often use (for example JST, XT30). To control the temperature, plug in the servo wire type cable into the output port of the servo tester. Dont connect the battery to both the servo tester and the esc as it will short circuit and create a boom. Only connect the battery (preferably 2s-3s) to the battery port of the esc and make sure to be careful of where the wire is. For safety you can wear some thick gloves, and gradually turn the dial of the servo tester to slowly increase heat. It should work after that.
I noticed you didn't mention anything about the weight difference between the fold-over, KFM-2 and KFM-3 airfoils. If you could add that to the Flight distance table you showed at 9:34 that could give some more insight into the performance of the airfoils.
It's known as the Kline-Fogeleman airfoil that was developed in the 1960s, but never found in any application other than paper airplanes and r/c airplanes due to a poor lift-to-drag ratio performance in wind tunnel testing by NASA. "Time" magazine published an April 2, 1973 article, "The Paper-Plane Caper," about the paper airplane and its Kline-Fogleman airfoil. Also in 1973, CBS 60 Minutes did a 15-minute segment on the KF airfoil. CBS reran the show in 1976. In 1985, Kline wrote a book entitled, "The Ultimate Paper Airplane." To publicize the book Kline traveled to Kill Devil Hills, NC, the site where the Wright Brothers first had flown where their first manned powered flight, of 122 feet (37 m). A crew from Good Morning America filmed the event. The longest flight by Kline with his paper airplane traveled 401 feet 4 inches (122.33 m).
A conjecture of mine is that the KFm3 airfoil, which is my heavy lifter, generates lift over most of the airfoil because the two steps produce vortexes that lower the air pressure over most of the upper surface. Normal airfoils only produce lift over the front half of the airfoil. This has been a wonderful adventure for me that started in 1964. ~ Dick Kline
The difference in the weights of the lightest and heaviest planes was only 6%. I didn’t try to match them exactly since the weight of a glider does not affect the flight distance.
Great work! I will certainly recommend my students watch some of your content for inspiration. I love that you use rubber bands instead of epoxy/super glue because it "makes for a better thumbnail." One thing I would note if this topics came up and I was wearing my professor hat during any of my Experimental Aerodynamics sections is that airplanes and gliders are not interchangeable. They are wildly different systems with different requirements that operate under vastly different regimes in a common medium. For example, the Space Shuttle is a glider. No one needed a FAA pilot license to fly it. Many astronauts were licensed test pilots but it was never required because it is not a plane. It is an unpowered vehicle. About the airplane-glider dilemma, most if not all advantage demonstrated is lost if you were to include the 4th component of flight for an airplane, thrust. SLF is true when lift, gravity, drag and thrust are balanced. In free-fall, stepped airfoils create high drag regions on the back half of the top of the wing which is like pulling on the reigns of a horse. While in free-fall they oscillate into and out of stall. You did a great job of capturing this effect during your tests. You can see the stepped wings nose down, gain speed, nose up, almost stall, back to nose down...etc until they find the perfect balance to allow them to fall almost linearly. That near stall condition which is when a wing is going to generate the most lift. If you move the mass on the rounded over airfoil so that it presented a near stall AoA (+3 to 8 deg speed dependent) where the vehicles longitudinal stability has roots spaced out far enough to allow it to oscillate into and out of stall (See Dynamics of Flight if what I am describing sounds foreign), it will outperform both of the stepped configurations because the Clark Y airfoil is a stepped airfoil with an infinite number of steps. It will have the beneficial characteristics of a smooth airfoil while traveling at a lower speed due to the larger XSA which will drive down your reynolds number and, thusly, most of the drag parameters you reviewed. This is good stuff. You are on the right track! Feel free to reach out if you need more clarification.
I remember some stepped airfoil profiles being tested in the wind tunnel at Daytona Beach campus of ERAU. It was amazing to see how well they performed. The shape was the rounded nose to spar then essentially a skin from the top of the spar to the TE. The idea was to use differential elevators for roll control.
Cool and well done! I would not have predicted this result. It would be neat to see if these stepped airfoils would outperform a well formed and faired Clark Y. I'm liking the whole concept and execution of this channel.
Clark Y has 12% thickness and is not suitable for Reynolds numbers below 100,000. If you slim it down to 10%, you might get away with 80,000. Or use the similar Eppler 205, or the more complicated 211 and 212. But even those do not work below 80,000.
Just one doubt , was the cg adjusted for each model cause you need to trum it beforehand as not all airfoils have their aerodynamic center at the same place. As i noticed in the video the ppane was literally goibg down ( it's clearly nose heavy ) adjust it by using a ballast weight or by changing the main wing position a little forward. And do the test again.
I don't think I've ever seen better R&D on such a frugal budget. In addition to your other suggestions, I'd love to see the foldover airfoil modified to round/smooth the trailing-edge transition. You could probably add Bondo or some kind of sandable putty. I suspect that the foldover airfoil is susceptible to flow separation, even at small angles-of-attack.
Have you tried to put either feathers or fish scales or similar items to disrupt the laminar flow of air over the top of the wing and hence make the vortices more complete
Great video. Although a little obnoxious at first, your narrating is actually very good. I too love to use the KFM airfoils on my scratch builds. Easier, faster, and more precise to build than that folding stiff.
It's interesting to see how repositioning a similar amount of material between one step and no step increased the glide ratio by nearly 50%. Thank you for this presentation, i wonder if the height of the step and the length of the step can be tiled to gain this benefit on full-size wings, or as you said, where the threshold is for significant gains decreasing with scale.
6:15 Was the weight/mass the same of each wing? also the videos of your testing was very nice. Maybe they could be used to collect data over time from the video?
I wonder if there is an optimal step size. Would the steps created by 3D printing still work? Does it have to be a step or rugosity of the right scale work too?
Are you able to simulate it? I use XFLR5, but have not tried stepped. I found that the shape at the leading edge has a very signficant effect on L/D. Even very small changes make a big difference. Just thinking of similarities to stepped hulls on boats. These cut down drag. Any thoughts on that?
FASCINATING! I wonder whether stepped airfoils might be useful for large, very low speed powered fliers like human powered airplanes or those really slow solar powered flying wings. Perhaps if they were pushers so no turbulence from the propellers strikes the airfoils. Nice scientific method! The consistent launch method, very clever.
i once did an experiment almost like what you did, The experiment was I modified one of my F1N indoor glider then did test flights. It did flew very well and did the best flight of almost 40 sec (
The difference in the masses of the lightest and heaviest planes was only 6%. I didn’t try to make it match up exactly because the weight of a glider doesn’t affect the flight distance.
Great video! I like your clear explanation. So many variables to change. I am wondering what swept back wings would add to the equation. (In combination with stepped aerofoils). Love to see more this. Thank you.
Congratulations on your experiments. The RC hobby field does not have much scientific basis. I have searched for scientific data for KFm airfoils for RC planes and have not found much. Like many others, I am particularly interested in airfoils for slow flying FPV drone RC planes used for agricultural and conservation monitoring. You have identified the correct questions (a) at what reynolds number do the KFm airfoils become less efficient than curved airfoils? (b) how effective are thin KFm airfoils? I look forward to further videos.
Check out "turbulator tape", aka "W tape" used by full size competition glider pilots. Supposedly, the turbulence induced by the tape promotes a turbulent boundary layer which improves the performance of the aerofoil.
Maybe I misunderstand, but isn't it possible the reason why the 'flat-bottom' fold over did so poorly was because it was actually a modified KFm1 design. It has a step on the bottom, not really a flat-bottom.
Fascinating stuff! I’ve built a bunch of Flite Test models, and always wondered about the efficiency of the folded wing. It’s very interesting to see how much better the KF wing works. Thanks!
Around 1970 when I was a kid 60 Min had an episode on the Klein Vogelman wing (literally "little birdman" in German.) It was one of the reasons I became a commercial pilot and Air Force Officer. They used the KFm- 1 wing for the show.
Nice experimental research explanation video. Regarding low Re numbers, I would love to see what - golf ball dimples - would provide in place of steps? Upper step experiments on low Re, sort of mimic what broad winged soaring birds do if you look closely at there overlapping top feathers displaying tufting and flickering. Experiments in Morphing wing panels is also fascinating.
A very interesting experiment that raises as many questions as it possibly answers. For instance, the material used lacked a polished / ultra smooth surface. What difference might it have made if all three wing types had very smooth/polished surfaces (which can of course introduce their own issues, perhaps in less stable boundary layers). Also, the conventional wing in this experiment was disadvantaged by having a crude airfoil section (angular over the top wing surface, and a gap on the underside near the trailing edge) which will have handicapped it. And no mention was made as to whether all three wings had the same weight, as this is also a consideration. Love this kind of experimentation though so keep it up 👍
Old designs form 60-70's? had "turbulators". String above the front of the wing or v corrugated add ons made with tailors Pinking shears. KFM have a straight edge, what if the edge had v slots? Spiral turbulence?
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Ok! completely awesome experiments! Im retired Aeronautical Engineer from NASA. Yes I have done experiments at all Re# for stepped airfoils. I found a KFm1mod to have the best performance. The mods were NLF65 series leading edge with a design lift CL of 0.2 12% thick and sanded smooth with 400 grit 2lb blue foam also polyester resin to high polish. All I found was that we could use a stepped airfoil to create more desirable stall characteristics for airfoils designed for pressure recovery. Polished airfoils had better pressure recovery and a nice drag bucket bringing the CD down to 0.01 from 0.047 vs sanded blue foam even at Re# as low as 4000. An unmodified NLF 65-212 could not produce a drag bucket at low Re#. The massive pressure recovery that was produce on the underside step was spread out from 50%cord and back changing the center of lift to 60% and 75% in a stall causing desirable stall recovery characteristics. Tests were done at Turbulent scale length of 0.1meter and 2meters to show the difference in Micro and Macro aerodynamics. The interesting thing was we were able to produce pressure recovery as low as Re# 4000 with the stepped version in micro aerodynamic range. With the NLF 65-212 we needed at least 1.2 MRe# to achieve a significant pressure recovery and drag bucket. This is not a problem with full size aircraft as we always operate in the MRe# range but it may be of use in your smaller gliders. We made some gliders and it was fun to throw them around we saw glide slope of up to 40/1 with aspect ratios over 12/1 of corse with wing loadings as close to nothing as could be done, Man! they just keeped going. But what we were trying to learn was if it would produce better stall recovery without compromising top speed with a fixed geometry on pressure recovery airfoils designed for speeds from M0.25 to M 0.7 and they did indeed work, so I'm glad you are having fun studying this we sure did. This stuff has never really been implemented, I did use what I learned to design some high pressure recovery airfoils for wind mills that reduced the kick in speed down to 10kts from 20+kts. Oh and the Clark Y sucks! don't ever use it.
Wish I could understand all your words😅 good to read
@@CraigLandsberg-lk1ep Sorry, I have a Ph.D in this stuff. I would be happy explain what vocabulary you don't understand.
You should colaborate together, you have so much knowledge to share
@@MauricioHernandez-de8is Oh thank you. I did actually teach at Space Camp. I like encouraging youth to be Awesome. I also wrote CFD based Flight Simulator Software that is the industry standard and used at NASA for all flight training in every flight test programs since 1996. And well everyone else. This kid is really smart and explains things well Im very impressed with his videos, He has learned this all correctly I have zero criticism of anything he has explained. I don't think he needs my help he is doing it right!
@@anthonyb5279 At what approximate physical scale do these airfoils start to lose benefit? I'm a layman and don't really understand Re# conceptually all that well. However, picture a generic Cessna 172-ish airplane. Are the wings on those too large to benefit? How about the horizontal stabilizer which is a much smaller surface?
Ive been a flying professionally and teaching for almost 30 years. In my opinion this is a great experiment. It makes me very glad to see that we still have young people in this nation with enough curiousity and motivation to try things like this.
Unfortunately the experiment while creative is flawed - mostly on the design of th first airfoil. Lift is created by a combination of two principals - Bernoulli's and Newton's. Newtons law says that as air molecules strike the lower side of the airfoil they create a higher pressure area which "pushes" the air foil upwards. This works well on an airfoil where the bottom is perfectly flat. Unfortunately the bottom of the fold over airfoil is NOT flat, while the other two ARE flat. This leads to the first flaw. In the experiment the first airfoil not only has a different topside but also a different bottomside. When doing an experiment it is important to only change one thing at a time.
The second flaw has to do with the effect of the step on the bottom. As it moves through the air, it very likely creates a forward pitching moment due to the application of Newton's law as the air flows over the step. This pitching moment should be causing the airplane to try to nose down which the elevator must in turn counteract. The additional drag will reduce the aircrafts performance substantially.
The third flaw has to do with the application of the elevator itself. In real life, a pilot operating an aircraft will trim the aircraft so that it is at its optimal speed where the combination of induced drag and parasite drag are at its lowest point. This optimal speed will allow the aircraft to remain airborne for the longest period of time. Unfortunately, the experiment here fails to take this into account. The elevator is simply 'set' prior to flight without taking into account which . The experiment needs to be repeated in a wind tunnel where total drag and total lift can be measured at various attitudes, or it needs to be repeated multiple times using the same airfoil, with different elevator settings.
The last flaw is that the aircraft needs to be chronographed/released from a mechanical device so that we can verify that flow of air over the wings is exactly the same for each flight.
Still, its a great idea, and I hope this young fellow take my observations as advice and NOT as criticism.
Very nicely put! Interesting but as with most experiments you end up with more questions.
A Typhoon fighter, carrying full ordinance and drop tanks has very little lower wing surface exposed. Could Newton explain how it flies in this configuration? I understood that lift was mainly supplied by the intense low pressure created over the upper surface of the wing, which rises into it.
seeing the planes smoothly glide through the air relaxes me, more long range flying video please
Remember that not all the parasitic drag is in the wing ... it's everywhere on the plane, fuselage, tail, etc. At low Re it becomes important to use a boom tail rather than a tapered cabin, and to use a rudder placed farther back for better leverage, where it can also be smaller with less drag and have the same, or better, control authority. Smaller slower aircraft have different design trade offs, than large fast aircraft.
Wow. I’d have loved this when I taught psychology in college. Marvelous material for starting a discussion about experimental design. Thanks ❤
I agree! Excellent execution of the scientific method.
Hey man, wanted to say thank you for your video series. I worked on a project trying to build large model gliders out of lego! We used your videos to learn all we could about how to get them to fly. Thanks so much!
the fold over has a marked step at the lower surface trailing edge, which by my assessment is actually creating a hybrid inverted kfm2 - which would suggest that the lift is being compromised by the "inverted lift" of that part of the airfoil
run a fill-in layer of tape from the step to the actual trailing edge to "disappear" that step and then try again... i am keen to see the difference
Interesting experiment and useful for someone who was building simple wings with very thick airfoils out of foamboard, where the KF airfoils may indeed be 'less bad'. However a comparison against a conventional airfoil that is specifically designed for operation at low Re numbers (NOT a Clark-Y) I'm certain would yield very different results. One of the Mark Drela series airfoils would be a good option, find them at 'Drela Airfoil Shop'. I did my own testing of a 1m glider with a Drela wing vs a KF of same thickness weight and planform mounted on the same fuselage. The Drela with flew about 50% longer duration.
I built an A2 FAI class towline glider decades using an Eppler E59 airfoil. Floated of a 50 mtr. towline the model would consistently do 3 mins. 15 secs. in still evening air after sunset. As Dr Eppler explained, the centre of pressure movment of the E59 was "very marked" However in turbulent air the model would not go away in a thermal and despite geodetic wing construction in certain circumstances the wing would torsionally oscillate bringing the model down fast.
Experimenting with airfoils you will find the nose radius has a BIG affect as does wing tip planforms.
For those of us fascinated by flight, yours was an interesting informative experiment.
This was really well put together. It feels like I'm watching a school thesis. Everything is clearly laid out. Great job!!
Was the difference in mass taken into consideration?
Yes! please do all of these variables. Extremely interesting, massive real world implications. In all the hobbies and interests I am involved in I am shocked I have never heard of this before
I used a 3 or 4 step foil on 8" indoor gliders back in the 70's. The interesting thing was I could increase the weight of the glider to many times the original weight and the glide time was the same. The section was laminated and a proper section was shaped back to the steps.
Highly instructive. Good testing technique, especially for an "at home" and "non-industrial" effort. You earned my subscription. I'll be lurking around your channel. Thanks!
If you do include wind tunnels and simulation I would prefer to still see the actual real world testing afterwards. To me, seeing the theory followed by your controlled experiment was very rewarding. Looking at the three wing designs at the beginning then measuring what I thought would happen as compared to what did happen was great. I assumed the Flite Test wing would reign supreme and was amazed the 2 & 3 step wings (actually flew and) proved more efficient. I fly RC planes but assumed the kind of content here would require some intimidating math to understand. Your intuitive explanation made perfect sense to me. Thank you for making this video. I subscribed and one day hope to design my own RC model.
RC models, although flying slower than life-sized airplanes, still fly much faster than gliders. So that's probably why you were so surprised: you're not used to speeds THAT slow
I really have to compliment you on your hand-gesture game.
Top notch. Idea for explaining missions to military pilots or speaking to italians.
Glad to see your channel speaking on kfm airfoils. I had my graduation project based on lift and drag performance of kfm2 type modified clary-y airfoil over multiple reynolds number and across multiple depth and location of step over the airfoil and compared it to a typical clark-y airfoil. Results showed a substantial increase in aerodynamic performance in low reynolds number range.
Hi, I'm a freshman from Viet Nam and we're doing a project on making a model glider. Can you suggest some resources, blueprints, etc.?
11:17 Yes! Please make all!
This is the kind of content i love. The scientific method applied on a shoestring budget.
I see many comments that tell me the audience does not understand Reynold's number.
Reynold's number indicates the ratio of fluid momentum to dynamic viscosity. A very viscous fluid with low density will create lower reynolds numbers (Alternately stated, increasing kinematic viscosity reduces Reynold's number.). High speed or high density increases momentum, if viscosity remains fixed, will increases the Reynold's number. This is what drives the boundary layer formation, the boundary layer changes the effective airfoil thickness and shape with the appearent effect of being "skin drag".
A longer cord also increases reynolds number(Sorry I don't have a clean explanation that would fit here, but it is rooted in the scaling of momentum and viscosity.). The boundary layer does not grow twice as thick with twice the cord, so relative to the change in scale there is a lower proportional change in the boundary layer and effective wing thickness and shape.
Low reynolds number = proportionally thick boundary layer (manifesting as "skin drag"), high reynolds number = proportionally thin boundary layer.
Reynolds numbers below 100k tend to have mostly laminar flow in the boundary layer, above 200k tends to cause mostly turbulent flow in the boundary layer. Laminar flow at high reynolds number is possible and produces extremely low drag but it is very sensitive to disturbances in flow and surface condition. Turbulant boundary layers have more drag but they are more consistent and reliable in adverse conditions.
This is all for fully subsonic flows with neglagible compression effects, so below about mach 0.5; Still somewhat useful for estimations up to mach 0.8 although allowances need to be made for more significant compression effects depending on the particular fluid properties.
I used a thiner KFM-2 airfoil, just fording foambord, without that spacer. Used it on a several flying wings with nice results. Very strong and easy to build.
Absolutely amazing video. Every time a "how?" or "why?" popped into my head you immediately gave more details.
At those speeds, you could get lots of cheap data with a bicycle and bits of string. Mount the wing to a boom in front, glue bits of string to the wing, and Bob's your uncle. It's not at elegant as a wind tunnel or a simulation, but it could easily work in that huge gym you have access to.
Yes, this is is intriguing. I would be very interested if improving the "vortex holder" would further reduce drag: Keep the sharp top edge of the step, but fill in the bottom corner with a radius equal to the height of the step. My thinking is that this may eliminate micro turbulence at the foot of the step inside that 90 degree corner.
is called a fillet, and might make things better, or worse.
@@johnbgibbs my gut feeling is that a filet this large would reduce the speed range at which the wing would maintain its advantages. So it might drop like a rock until the forward speed drops enough to allow the vortex to become established, worse yet fly well until the speed drops too low, then suddenly stall. think that with enough resolution you would find that there is a tiny counter-vortex hiding in that corner that might be really important. Another field to borrow from is stepped hulls in planning boat hulls. These usually actually are undercut, sort of the opposite of a filet.
You’ve conducted the experiment in the best way possible and still maintaining interest along all the video, very good! Thanks now I can’t wait for the next
Nice Comparison, but Sorry- you're making a crucial error: The maximum airfoil thickness which really works is about 7-8 % at this Reynolds numbers. Therefore every other airfoil is better:
Let's explain: In my youth I was owner of "der kleine UHU". Clark Y.
I lost my ballast chamber and I turned my wing by 180 deg's leading edge to the font. And it flew good.- not worse than standard.
Airfoils which work best are usually thin airfoils with 3D-turbulator (zigzag). 40 Yrs ago it was "Junior" of Graupner.
A whole other area to investigate what about stepped foils for propellers? That might have huge applications in drones. Especially drones intended for longer duration where they're going to use big propellers spun at relatively low speeds. I am thinking that FPV racing drones probably move too fast, but drones for surveys or search and rescue might benefit from more efficient props that would allow more time up on the same battery.
It would make them quieter. Also just about all KFm airfoils have been used on helicopters. The other big advantage is they can manage twisting moment and flapping problems due to being low moment airfoils that are inherently strong and not need a spar, they can be mede of just a skin. Also good for over simplified rotors used on most quad copters that have no collective control for blade pitch as the have good high AOA performance and arrest a decent without going into blade stall.
Your technical and methodical approach immediately earned a new subscriber here! Would love to see more on this topic and what else you uncover. Excellent analysis.
This channel is gold. Really appreciate the succinctness, the inclusion of visualizations like graphs and diagrams to get the point across, and the thoroughness of the testing.
Great video! Missing a regular air foil as a base line to compare with though.
It's incredible that I found your video! This is a really interesting topic - keep up the great work, you're awesome. The stepped airfoil design is new to me, and I'd love to see more tests with different shapes. CAD simulations would be super eye-opening as well. The background theories on why these designs might or might not work are very helpful, especially since I'm an amateur in this field.
Stepped airfoil theory for low speed flight has been around since the 1970's. Nice work overall.
I appreciate your effort, and care about this as I love small balsa gliders. We used to make them 4 inches long, and had great results usually. The thing I wonder about with your experiments is if the air is doing what you think it is. There is enough going on that you are generalizing a lot in your conclusion. You need a mini wind tunnel with smoke, and then possibly some flights through smoke to see if the patterns seem to match. Then you can connect cause and effect better as you are really just retesting past experiments by others. I'd love to see that mini tunnel in action.
Your methods are comprehensive, interesting and useful! Thanks for making these videos.
I seem to recall that Cessna did extensive testing of the KF airfoil, as well as a main wing that pivoted at its CG. Of course, a 1600# airplane flying at 100MPH is quite different from a lightweight glider.
Thanks very much ! I have not known stepped designs before. Also your explanation of the reasons for better flying with stepped wings is understandable. This is an extremely compressed presentation of days & weeks of work in 10 minutes.
Amazing video. Ref: at 4:06 is the KFm foils page. While watching it I was having 2 thoughts:
1) use the flat extension as aileron/flap positioning the hinge at the edge of the step.
2) in your flight videos some planes have a nose-up attitude and glide while others simply glide downwards. A clever way to adjust the center of mass is to make it as independent from speed as possible. So having the plane have the same nose attitude regardless of launch speed. When you reach this condition your CG is theoretically on the center of lifting force of the wings. This could potentially provide some other kind of results, since CG is then trimmed to be at the center of wing lifting and not optimized for distance.
Great test matey, I’ve been an Avionics engineer on large passenger aircraft for 30 years. Never heard of stepped wings before.
Dunno why this vid popped into my feed but glad it done.
Rigorous experiment is quite a rare thing in popular scientific content nowadays)
This guy is a blast at parties 🎉
But seriously, I could just hang with this dude and have the time of my life.
I have built a bunch of flying wing rc planes using the KFm4 and versions of the same wing using a an actual symmetrical airfoil. These were relatively heavy fast planes. The "real" airfoil wings tended to be more efficient and faster. The KFm4 planes were very stable in the air they had much less pitch instability and over all flew better. I mostly flew FPV and I liked the smooth flying of the stepped airfoil. I have a bunch of flight videos on my channel from a few years back. Great work on your experiment you got a new subscriber.
I love this series, mainly because of your incredible sincere presentation style.
I was playing with this exact thing a couple of years ago but this guy gas done an exceptional job at explaining and producing valuable data and experiments. Good job sir
You might want to build the slat wing by Clough from plans on Outerzone under sport control line plans. That design wuld be a great eye opener for your follow up questions.
grate worck, the only suggetion is to consider also the different weights of the wings
was the total weight of each wing equalized to adjust for different amounts of material used in construction
Thank you, my question too. You know the wings with more material were heavier. That being said, the stepped airfoils still did better with these slow speed park-flyer scale planes. I would assume stepped wings that are optimized for weight would do fantastic for foamies.
The difference in the weights of the lightest and heaviest planes was only 6%. I didn’t try to match them exactly since the weight of a glider does not affect the flight distance.
Suggestion for efficiency: close the hole at the wingtips. There is air circulating in the open space on the KFM wing, which is what you want, but that gets destroyed at the wingtips, thus you want a surface there to prevent the inrush of air.
Don't forget why golf balls have dimples! Another interesting direction in this type of experimentation with surface drag reduction is distributing micro-vented air across the surfaces, basically creating a low-pressure air bubble around those surfaces. Or a flexible wing covered in individually-adjustable scales, each creating it's own little low-pressure void, controlled independently over the entire surface.
In other words, feathers!
This was a very educational video for me, thank you so much! I have built FlightTest planes before and also made my own planes, the flight test planes never flew great or needed a lot more power and speed, this video explains why :-)
I love this video ! But did you compare the weight of each wing ?
Harlold Penrose was a test pilot (and aircraft designer) in the UK that testflew a full-size glider in the '40s using an airfoil similar to the one you tested here, so it isn't a new airfoil! He test flew many different aircraft during his years at Westland. In his eighties he designed and flew a canard microlight, with a Kasper Wing-like wing if I remember correctly!
I'm pretty sure that the stepped KF airfoild has never been used in a full size aircraft. They have undergone wind tunnel testing and performed very poorly, so no one ever took them into full size trials. You are right that they are not new though, paper planes used them since people first folded a paper plane.
Thanks @ErikssonTord_2, I was hoping a viewer would pull Witold Kasper into the ring. Kasper was an academic and his efforts to capture vortex lift at 0 KIAS is still a thesis looking for a PhD candidate.
Thank you for putting in the groundwork and sharing the results with us. I will be watching this channel for any updates on this subject. Subscription well deserved.
I definitely learned something new today, thank you. So very cool.
Very few times I subscribe the first time I see a video from someone. Thanks!
I really like the fact that these are all experiments I can repeat myself for only a few dollars invested. Cool channel! Subbed!
I really enjoy your scientific approach to this. looking forwards to the next episode
Here’s an experiment for you: Let’s say that not all parts of the wing are created equal. The portion of the wing close to the fuselage performs differently (to some degree) than the wing portion near the tip of the wing. Thus, would it be possible to create a wing structure such that the portion of the wing near the fuselage contain a stepped airfoil and can that step be slowly eliminated the further out it gets to the tip of the wing where (possibly) the air flowing over the tip reacts differently than near the fuselage. I’m suggesting that possibly a “hybrid” wing design may be possible where near the fuselage it is of a stepped design and near the wing tip it is more of a solid surface design. Just a thought.
to my knowledge this is done at the blades inside the rotary turbines where a thin layer of air does just this (and is good for cooling and isolating the blade material from combustion gases)
Hi. Excellent information, thanks. I now only build small lightweight rubber powered planes, and I can't wait to try a stepped airfoil. I'll chop the wings of my triplane (which climbs like a rocket, but glides like a brick), and see if I can improve the flight envelope. As there's little difference between the k2 and k3 I'll do the single step one. Thank you for speaking slowly and clearly. Cheers, P.R.
I would be curious how the stepped airfoils vs non-stepped affect powered flight. You mentioned "not for high speed" (generalization). I am curious about slow speed flight. Downside being *much* more time required for testing.
Just one airplane video reignite my algorithm to reccomend more airplane video, and the video of yours is really interesting and the concept of your channel is really amaze me, low minimum entry to do all the things you're doing for everyone to see and experiment with!
Excellent! Well done science. I feel compelled to share this with my educator friends. And my engineer brothers-in-law.
Great video to refresh topics I have long forgotten. You are good at presenting aerodynamics without stupidiying it for the masses.
Very informative video.
I 3D printed some airfoils before and never considered stepped airfoils as they have the stigma of being low tech foamboard things. 3D printing makes it easy to just copy any "high performance" airfoil of large scale aircraft. Maybe it would be interesting to make a Airfoil with tiny stairs, so essentially a rough surface to create a smooth-ish buffer layer of air that sticks to the wing.
Stepped airfoils have been used since the 1930s. Way before there was foam core. 🙂
Were all planes the same weight?
Yes, they were very close.
@@DesignYourOwnAirplanes-xd6lzAIRPLANES ARE MADE OF **AIRPLANES** BECAUSE THEY ARE MADE OF **AIR**
I'LL LET MYSELF OUT
YO
Your experiment is so well done! It’s so much better than any of my attempts. Also you explained KF airfoils wonderfully, I learned more about it. Great job.
Also I’ve found a relatively cheap and easy way to build a Hotwire cutter.
I used these materials:
-Hobbywing Eagle 20A Brushed Esc
-26g nicrom wire
-20 gauge wire
-one of those cheap blue servo testers
You can make a jig for the cutter out of like some thin basswood sheets, pvc pipes, or some other sturdy material, never use any conductive materials.
I soldered the positive and negative ends of the esc to the 20 gauge wire, and directed both ends of the wire to the ends of the jig. I used two small screws to act as the mounting points for the nicrom wire to wrap around. Its should look like this
|+|-----------|-|
| | | |
| |__________________| |
|_____________ _________|
| [] |
| [] |
--
The esc is mounted where the [] on the handle is, with wires directed on both arms. The + and - are where the screws should be and the -- is the nicrom wire, you should try to wrap it tightly to create tension.
Solder whatever battery connector you often use (for example JST, XT30). To control the temperature, plug in the servo wire type cable into the output port of the servo tester. Dont connect the battery to both the servo tester and the esc as it will short circuit and create a boom. Only connect the battery (preferably 2s-3s) to the battery port of the esc and make sure to be careful of where the wire is. For safety you can wear some thick gloves, and gradually turn the dial of the servo tester to slowly increase heat.
It should work after that.
I noticed you didn't mention anything about the weight difference between the fold-over, KFM-2 and KFM-3 airfoils. If you could add that to the Flight distance table you showed at 9:34 that could give some more insight into the performance of the airfoils.
It's known as the Kline-Fogeleman airfoil that was developed in the 1960s, but never found in any application other than paper airplanes and r/c airplanes due to a poor lift-to-drag ratio performance in wind tunnel testing by NASA.
"Time" magazine published an April 2, 1973 article, "The Paper-Plane Caper," about the paper airplane and its Kline-Fogleman airfoil.
Also in 1973, CBS 60 Minutes did a 15-minute segment on the KF airfoil. CBS reran the show in 1976.
In 1985, Kline wrote a book entitled, "The Ultimate Paper Airplane." To publicize the book Kline traveled to Kill Devil Hills, NC, the site where the Wright Brothers first had flown where their first manned powered flight, of 122 feet (37 m). A crew from Good Morning America filmed the event. The longest flight by Kline with his paper airplane traveled 401 feet 4 inches (122.33 m).
Wow! You nailed it. Thanks! Dick Kline
A conjecture of mine is that the KFm3 airfoil, which is my heavy lifter, generates lift over most of the airfoil because the two steps produce vortexes that lower the air pressure over most of the upper surface. Normal airfoils only produce lift over the front half of the airfoil. This has been a wonderful adventure for me that started in 1964. ~ Dick Kline
5:57 The spar in 1 or 2 is half the spar in 3. That changes the weight, doesn't it?
The difference in the weights of the lightest and heaviest planes was only 6%. I didn’t try to match them exactly since the weight of a glider does not affect the flight distance.
I've been flying a 16" span delta with a KF airfoil for some years. It's just about impossible to stall and can transistor into a hover easily.
Great work! I will certainly recommend my students watch some of your content for inspiration. I love that you use rubber bands instead of epoxy/super glue because it "makes for a better thumbnail."
One thing I would note if this topics came up and I was wearing my professor hat during any of my Experimental Aerodynamics sections is that airplanes and gliders are not interchangeable. They are wildly different systems with different requirements that operate under vastly different regimes in a common medium. For example, the Space Shuttle is a glider. No one needed a FAA pilot license to fly it. Many astronauts were licensed test pilots but it was never required because it is not a plane. It is an unpowered vehicle.
About the airplane-glider dilemma, most if not all advantage demonstrated is lost if you were to include the 4th component of flight for an airplane, thrust. SLF is true when lift, gravity, drag and thrust are balanced. In free-fall, stepped airfoils create high drag regions on the back half of the top of the wing which is like pulling on the reigns of a horse. While in free-fall they oscillate into and out of stall. You did a great job of capturing this effect during your tests. You can see the stepped wings nose down, gain speed, nose up, almost stall, back to nose down...etc until they find the perfect balance to allow them to fall almost linearly. That near stall condition which is when a wing is going to generate the most lift.
If you move the mass on the rounded over airfoil so that it presented a near stall AoA (+3 to 8 deg speed dependent) where the vehicles longitudinal stability has roots spaced out far enough to allow it to oscillate into and out of stall (See Dynamics of Flight if what I am describing sounds foreign), it will outperform both of the stepped configurations because the Clark Y airfoil is a stepped airfoil with an infinite number of steps. It will have the beneficial characteristics of a smooth airfoil while traveling at a lower speed due to the larger XSA which will drive down your reynolds number and, thusly, most of the drag parameters you reviewed.
This is good stuff. You are on the right track! Feel free to reach out if you need more clarification.
Very easy to understand comment.
I remember some stepped airfoil profiles being tested in the wind tunnel at Daytona Beach campus of ERAU. It was amazing to see how well they performed. The shape was the rounded nose to spar then essentially a skin from the top of the spar to the TE. The idea was to use differential elevators for roll control.
This is really interesting stuff, thank you for the lesson!
这真是一个令人激动的发现，我之前都没有见过KF翼型，感谢博主的分享
I'm the same.
Cool and well done! I would not have predicted this result. It would be neat to see if these stepped airfoils would outperform a well formed and faired Clark Y. I'm liking the whole concept and execution of this channel.
Clark Y has 12% thickness and is not suitable for Reynolds numbers below 100,000. If you slim it down to 10%, you might get away with 80,000. Or use the similar Eppler 205, or the more complicated 211 and 212. But even those do not work below 80,000.
@@memyshelfandeye318 I'd rather see experimental results.
Just one doubt , was the cg adjusted for each model cause you need to trum it beforehand as not all airfoils have their aerodynamic center at the same place.
As i noticed in the video the ppane was literally goibg down ( it's clearly nose heavy )
adjust it by using a ballast weight or by changing the main wing position a little forward.
And do the test again.
I don't think I've ever seen better R&D on such a frugal budget. In addition to your other suggestions, I'd love to see the foldover airfoil modified to round/smooth the trailing-edge transition. You could probably add Bondo or some kind of sandable putty. I suspect that the foldover airfoil is susceptible to flow separation, even at small angles-of-attack.
Have you tried to put either feathers or fish scales or similar items to disrupt the laminar flow of air over the top of the wing and hence make the vortices more complete
Could the airplane body have steps to reduce drag?
Great video. Although a little obnoxious at first, your narrating is actually very good. I too love to use the KFM airfoils on my scratch builds. Easier, faster, and more precise to build than that folding stiff.
It's interesting to see how repositioning a similar amount of material between one step and no step increased the glide ratio by nearly 50%.
Thank you for this presentation, i wonder if the height of the step and the length of the step can be tiled to gain this benefit on full-size wings, or as you said, where the threshold is for significant gains decreasing with scale.
6:15
Was the weight/mass the same of each wing?
also the videos of your testing was very nice. Maybe they could be used to collect data over time from the video?
Yes, the weights were very close.
Maybe its worth to try making surfaces with pits similarly to golf balls?
I wonder if there is an optimal step size. Would the steps created by 3D printing still work? Does it have to be a step or rugosity of the right scale work too?
Are you able to simulate it? I use XFLR5, but have not tried stepped.
I found that the shape at the leading edge has a very signficant effect on L/D. Even very small changes make a big difference.
Just thinking of similarities to stepped hulls on boats. These cut down drag. Any thoughts on that?
Dude I just subscribed and you released a new vid that's amazing!
FASCINATING! I wonder whether stepped airfoils might be useful for large, very low speed powered fliers like human powered airplanes or those really slow solar powered flying wings. Perhaps if they were pushers so no turbulence from the propellers strikes the airfoils.
Nice scientific method! The consistent launch method, very clever.
i once did an experiment almost like what you did, The experiment was I modified one of my F1N indoor glider then did test flights. It did flew very well and did the best flight of almost 40 sec (
Was the wing weight the same or is that negligible?
The difference in the masses of the lightest and heaviest planes was only 6%. I didn’t try to make it match up exactly because the weight of a glider doesn’t affect the flight distance.
It looks like the third wing had more material than the other two. Was it heavier? That seems like another variable in the mix.
The weights of the gliders were very close.
Great test! Very informative - I might adjust my design based on this!
Great video! I like your clear explanation. So many variables to change. I am wondering what swept back wings would add to the equation. (In combination with stepped aerofoils). Love to see more this. Thank you.
Congratulations on your experiments. The RC hobby field does not have much scientific basis.
I have searched for scientific data for KFm airfoils for RC planes and have not found much. Like many others, I am particularly interested in airfoils for slow flying FPV drone RC planes used for agricultural and conservation monitoring.
You have identified the correct questions (a) at what reynolds number do the KFm airfoils become less efficient than curved airfoils? (b) how effective are thin KFm airfoils?
I look forward to further videos.
Check out "turbulator tape", aka "W tape" used by full size competition glider pilots. Supposedly, the turbulence induced by the tape promotes a turbulent boundary layer which improves the performance of the aerofoil.
It would be interesting to see the effects of different wing to vertical stabilizer ratios
Maybe I misunderstand, but isn't it possible the reason why the 'flat-bottom' fold over did so poorly was because it was actually a modified KFm1 design. It has a step on the bottom, not really a flat-bottom.
Fascinating stuff! I’ve built a bunch of Flite Test models, and always wondered about the efficiency of the folded wing. It’s very interesting to see how much better the KF wing works. Thanks!
Around 1970 when I was a kid 60 Min had an episode on the Klein Vogelman wing (literally "little birdman" in German.) It was one of the reasons I became a commercial pilot and Air Force Officer. They used the KFm- 1 wing for the show.
Nice experimental research explanation video. Regarding low Re numbers, I would love to see what - golf ball dimples - would provide in place of steps? Upper step experiments on low Re, sort of mimic what broad winged soaring birds do if you look closely at there overlapping top feathers displaying tufting and flickering. Experiments in Morphing wing panels is also fascinating.
reminds me of dragon fly wings. They are corrugated and each folds contains a turbulence. I guess they fly at low Reynolds number
A very interesting experiment that raises as many questions as it possibly answers. For instance, the material used lacked a polished / ultra smooth surface. What difference might it have made if all three wing types had very smooth/polished surfaces (which can of course introduce their own issues, perhaps in less stable boundary layers). Also, the conventional wing in this experiment was disadvantaged by having a crude airfoil section (angular over the top wing surface, and a gap on the underside near the trailing edge) which will have handicapped it. And no mention was made as to whether all three wings had the same weight, as this is also a consideration. Love this kind of experimentation though so keep it up 👍
The weights were all very close.
Old designs form 60-70's? had "turbulators". String above the front of the wing or v corrugated add ons made with tailors Pinking shears. KFM have a straight edge, what if the edge had v slots? Spiral turbulence?