Airflow at a Hover in Helicopters
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- Опубликовано: 8 сен 2024
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In this lesson I'll be covering Airflow at a Hover. For the purposes of explanation, these diagrams refer to helicopters operating at a stationary hover and in a no wind condition. This is because motion or wind from any direction or velocity will change the lift characteristics due to dissymmetry of lift, transverse flow, and translational lift. In another video I will explain how all of these factors come together to affect the helicopter. But for now I'll keep it simple and cover one subject at a time.
Airflow at a hover can be broken down into two types: In Ground Effect (IGE) and Out of Ground Effect (OGE). This leads to the question, "what is ground effect?" Ground effect is the increase in lift that occurs when a hovering helicopter is within one rotor diameter above a surface. When a helicopter hovers, airflow through the rotor system comes down vertically and vortices develop around the rotor tips. While in ground effect, this down flow of air is restricted due to ground friction as the airflow is redirected out words and away from the rotor disk. Because of this, the high velocity of a downwards flow of air (induce flow) is reduced as well as the rotor tip vortices are pushed away from the helicopter. While hovering out of ground effect, there is nothing to restrict or redirect the downward flow of air. Therefore there is a high velocity of induced flow through the rotor system and the wing tip vortices are allowed to develop without any restriction.
Because there is more induced flow while operating out of ground effect, the pilot must increase the Angle of Incidence in the blades in order to maintain the same Angle of Attack. Keep in mind that Angle of Incidence is a mechanical angle that a pilot can directly control by manipulating the collective. However, the Angle of Attack is in aerodynamic angle that is the product of pitch and induced flow. Due to the fact that the pilot has to increase pitch in order to maintain Angle of Attack while out of ground effect, there is more drag on the rotor system and therefore requires an increase in power to maintain lift. This is why it requires more power to hover OGE than IGE.
One last factor to consider while operating at a hover ground effect is the surface condition. Tall grasses, rough terrain, ground obstructions, water, etc. can all affect airflow as it passes through the rotor disc. This can cause uneven air flow and therefore uneven lift.
If you enjoyed the video or have any questions or comments, hit the like button and comment below.
If you're just getting started and want more information, pictures, and more explanations, I'd recommend reading the Rotorcraft Flying Handbook - amzn.to/2ifPlnZ
If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle - amzn.to/2ifQGLx
After seeing your explanations: Who needs lessons at all! I'm going to buy me a helicopter and fly!!
🤣
I am a fixed wing pilot learning so much from you. I am a ~400 hr ppl SE Land, also A&P IA. 8 years fixing Navy planes and over 30 years fixing everything from the L1011 aircraft that belonging to the late King Hussein of Jordan, down to commercial, corporate, and GA aircraft such as the cessna 150 and virtually everything in between. Your videos are proof a man is always learning. After 40 years in aviation, I have decided to go after my rotor wing ticket. Your videos are great information and a big help. Some is new to me information and some refresher. Thank you and be safe.
Great clarity on this video, as usual. I know this one was all about airflow, however I really would have liked to have seen you use the remaining 90 seconds to bring it home as to why understanding these airflow differences is important to a pilot - understanding changes in performance. Maybe just quickly add some force vectors to show that as the angle of the aerodynamic force gets tilted backwards and away from the vertical in the OGE depiction, it has to increase in length so that the vertical component still matches the weight of the helicopter and maintains the hover. Therefore the rearward drag component also becomes longer (greater), requiring more power to maintain rpm. This would segue nicely into a follow up video about power required vs. power available and the max performance approach and landing. Please don't take this as a criticism, your stuff is great. Just a suggestion to increase understanding. And thank you for defending my freedom, sir!!
I appreciate the feedback. I’ve been putting together ideas for a reduced power maneuvers video and I plan on incorporating some of those very points.
when I become CFI I'm going to teach like you thank you !!!!!!! you are the best man
Thanks for the great lessons. Really appreciated the "he or she" catch.
Just thought I’d say that I’m very appreciative of the videos you post. The visual plus explanations really stuck with me and helped tremendously during my SIFT exam. I sometimes miss things by just reading alone and this did a lot more for me. I was able to walk away with a 64 on the exam.
Awesome! Congrats!
You mention that ground friction and ground resonance is the reason for a reduction in induced flow but that doesn't really explain why it happens. Perhaps you could mention the pressure increase below the disk as the air cannot escape fast enough. This increase in pressure restricts the downward component and therefore reduces the IF. Just a little further as to why it happens.
Also, in the U.K. We teach 2/3 of rotor diameter to be truly IGE. Interesting that we use different figures!
Martin Seward. Good point and that's true about the pressure differential under vs over the disk. Part of this is because the friction the airflow has with the ground and the wing tip vortices restricting the air's ability to escape from under the disk. But for this video I tried to keep it at the most basic level so aviators have an understanding about how airflow Relative to distance from the ground. I've never heard of the 2/3rds rule for true IGE. But it's easy to see on hover charts how the power differs with different heights AGL. Thanks for your input!
Thank you for your videos. I’m hoping to start training for my CPL(H) at the end of the year and your explanations for the theory side of things has been a great help in getting me to understand the aerodynamics of helicopters.
man, why didn't I spot this gem earlier.. subbed without any hesitation
Your stuff is so good, it's seriously helping with my study.
One thing though, in Australia we teach that ground effect is effective within the distance of 1/2 the diameter of the disc of the ground (or just the radius). I read the opposing information in Principles of Helicopter Flight (Second Edition) by W. J. Wagtendonk (an American book) saying it's the diameter of the disc. Then in U.K. I think they say its 2/3. Very confusing!
Great content. Keep it up!
Thanks! I'm glad you enjoyed it.
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Great videos. Simple and concise.
Good one. Keep doing it. Well explained. Thank you so much
Thank you brother for these videos :)
I'm so happy I found this channel! It's nice to have lessons so clearly and beautifully explained in bite-sized pieces. Well done! #Subscribed
Another great video. Use colors for air flow, etc.
So much to learn. Thank you.
Its nice explanation.
Great, video. I will share with my students!
Thank you great Videos
Great work sir ...I am in initial stage of helicopter flying .. hoping to see more videos on rotor Dynamics on this channel #subscribed
Good explanation, Thanks
Got my SIFT in 2 days, watching these videos have helped med grasp the material ive been studying thanks.
How’d you do? I test tomorrow
@@lh1229 I passed with a 52
@@manwhamenice9862 congrats. I just finished and passed with a 70
Cyclic collective and prouty doesn't explain vortices part great 👍
Grate job a clear principle. Thanks for sharing.
Thanks for the explanation
Ground effect being the friction of the surface of the earth and the air (vortices produced by the helicopter.) Of course a common mistake is to think a plane can't hover because it doesn't have ground effect. Well wait a minute when a plane lands it flares first just like a helicopter flares at the end of an auto rotation before landing. Primarily I suppose so that the back wheels Touch first and so that the plane doesn't want to flip over if the front touches and then the back wants to lift up if the plane is out of CG and the plane topples over. But it's also to ride out any additional vortices as the plane is coming down that would create ground effect and cause an imbalance in direction as the plane tries to land on the runway. Primarily a plane can't change the direction of its induced flow the induced flow is totally horizontal and not vertical as the propeller is pushing the air straight backwards. When the helicopter hovers and when the helicopter is not in ETL and the induced flow has not changed to a more horizontal state the induced flow is more vertical blowing downwards. Creating higher pressure underneath the helicopter.
You also need more power out of ground effect since you're increasing angle of attack
or pitch angle to maintain lift...
Nerd question. Is the increased power required for OGE driven primarily by overcoming the larger vortices or because of the increased drag on the rotor system due to the larger angle of incidence required to maintain the same angle of attack? My brain tells me it's the latter because of the positive correlation between torque required and drag.
Great explanation
Good looking out
Not sure you’re still making videos but I’m at fort rucker in common core in the last th67 class and your videos have been helping me with my aerodynamics oral knowledge a ton. Thank you.
Thank you so much for explaining. Wow, you helped a lot 👍🏼🙏🏼😃
IGE: ground friction prevents air moving away, higher pressure under the rotor disc, pressure prevents the through-disc airspeed, so AoA is greater. Higher pressure also increases lift for the same collective and power setting. OGE, air flows freely, the converse applies. Got it :)
You're the best. Thank you for this amazing explanations
Hi Jacob, So... No one has been able to answer this question. The books say that IGE Hover is better over asphalt or a smooth surface... it is claimed that this is because the surface doesn't disrupt the airflow like tall grass or obstacles. But IGE Hover requires less power and blade pitch because the down flow of air is disturbed causing the rotor vortices to stay smaller and slower, thus reducing induced flow. But something is wonky, wouldn't tall grass disrupt the downwash MORE and prevent the rotor vortices from gaining speed MORE than a smooth surface?
Seems like over a smooth surface would be worse given your explanation.
In ground affect the air flow hits the ground then pushes back up causing the helicopter to raise then float back down a little. Has nothing to do with vortex. And to get up to 100 feet you need more collective anyway in order get up their you do need more collective then ground affect yo maintain but once you reach the 100 foot mark you’ll now need to down collective to achieve the hoover
great video but pointing out: in every literature i've seen the rotor disk wake (inflow) converges in a funnel-like shape, it does not diverge. this is called the free wake theory or something similar if i remember right.
It both converges and diverges. The simplest anology is like turning on water from a faucet. It will initially converge to a concentrated point then diverge again. For helicopters this happens at roughly 1.5x rotor diameter.(75’ below a 50’ rotor diameter). Using this to your advantage you can hover OGE at this height if you want to have maximum surface disruption underneath (ex: blowing things away).
I did not read all the post below, so maybe someone already addressed this, BUT...
You said, "What is Ground Effect?, Well Ground Effect is going to be the increase in lift that occurs in hovering flight when a helicopter is within 1 rotor diameter of the surface". This is incorrect.
That sentence should have been, "Well Ground Effect is going to be the increase in lift that occurs when an AIRCRAFT is within 1/2 ITS WINGSPAN to the surface. (Or approximately 1 wings length and it applies to any configuration, not just a hover)
Since the rotor is actually a wing, if your helicopter has a 50 foot 'wingspan', the correct altitude for you to be utilizing ground effect (in any configuration) would be 1/2 of that wingspan. In your case, 25 feet or closer to the surface.
But let's forget all the numbers and just remember this. You have to be AT, OR LESS, one wings length (or blade length) to the surface.
Otherwise, great video and thanks for taking the time to try to explain this to people.
Great video!!! thanks.
Thanks for your great explanation. The way I understand from this video, restricted wingtip vortices lead to relatively small downwash over the blades, which generates bigger lift in comparison to OGE. Am I getting it across?
As far as I know ground effect is only valid till 2/3rd height of rotor diameter. For a 50ft rotor the ground effect will not be available for more than 35ft of height.
Depends on the reference. Most military manuals use 1 rotor diameter. And the math checks out when we compare our IGE torque required to OFE torque required. It could vary based on your operators manual
good job!! very well explained
Good vídeo 👍🏾🚁
My uncle was a Chinook pilot and instructor in the Army. He always had great things to say about Apache pilots. Except that they always made him slow down.
Lol. Well unfortunately not many other aircraft can keep up with a chinook going full speed. But we make due by usually taking off 5-10 minutes before them to get where we needed to go at the same time.
@@helicopterlessonsin10minut10 Haha nice. He was a SOAR guy and I grew up with a love for military helos bc of him. He likes to do some friendly ribbing but he really has a ton of respect for guys. And I do too!
If the helicopter was tethered to a platform below it that is light enough for the helicopter to lift and the diameter of the blade spin fits within the area of the platform, will the helicopter still fly up?
J. Palafox. Hah! Interesting question but yes. The rotor blades create lift by a combination of negative pressure above the airfoil and the action reaction effect of pushing the airflow downwards, not just by pushing against the ground. Check out my video “How lift is created” for more.
that was my first theory too maybe using the burnoulli's principle but i wasn't sure on why it does! thank you for clarifying.
Can you please explain the situation of hovering directly over a solid block wall, how that would or would not produce settling with power.and the airflow? What happened during the Bin Laden raid why did the pilot get into that crash?
U r great bro
Do you have a video explaining just Angle of Attack?
Check out my “Airfoil Characteristics” and “Forces Acting on an Airfoil” videos
We're having a debate over here about the wingtip vortices... Wouldn't it be more correct to draw them starting at the tip, expanding in a spiral downward first? You have them drawn in a way that looks like the air starts wide and tightens up toward the tip. Is that backward? (I'm trying to make an animation for a client.) To me it seems that the air's highest velocity will start at the tip where it interacts, then as it swirls, it widens. No?
Can you speak more to ground effect, at a hover, over water and soft surfaces, say mown turf, or tall grass? I have heard that ground effect is significantly lessened over such surfaces. Do you find that to be the case and, if so, why?
The one thing I’m confused on is that is the IGE from the top of the rotor or bottom of the landing gear? I know it sounds like a dumb question.
It’s from rotor to ground.
got it, spinny wind bad!
Do you have any videos of you teaching systems?
Is IGE vs OGE defined by the height of the rotor disk off the ground (Instead of AGL) with the crossover at the measure of the disk diameter? You seem to suggest this, but never state it. Great videos, I agree that some colors could help clarify the images.
Rotor disk within one rotor diameter of ground/terrain is IGE. Anything greater than one rotor diameter above ground/train is OGE.
kit headley. That's correct. Everything is based on the rotor's diameter and height above the ground which is why it can vary by helicopter.
It can also vary depending on your LZ, and remember that your rotor disc has to cover the entire surface area. Tall grass, water, sand vs concrete, asphalt, etc (smooth vs rough surface)
and i did see you mentioned it in the description...on it!
Hello sir,
I am extremely satisfied with the way u explain the rotor dynamics.
I have a question..,
When we draw forces acting on an Aerofoil, Where exactly are we drawing the vectors from ?
Is it from the blade midsection or Aerodynamic centre or something else ??
muito bom
Does helicopter lift the weight with bouncy force...?
Or barnouli
great explanation. hey brother where did you get your training at?
William Colt. Just some good ol' Army training and personal researching.
thanks bro my quad almost fall out of the air I was above 500m and descent down when VTR happened any tips I should now about. thanks
Can you make a video to explain ground resonance, please?
Joao Gabriel Amado. I’ll add it to the list. Thanks for the feedback.
What will happen if propellers are covered from above ,will it fly
nice explanations. But I would like to clarify. Ground effect is 1 diameter of the blade?, more than 1/2 of diameter, less than 1 diameter of the blade?
It’s generally 1 rotor diameter in height above the ground.
How do you compensate with the controls when your rotor blades are half over the water and half over the deck as taking off or landing on a carrier
One side of the disk produces more lift than the other which can cause directional drift. The only way to compensate is to apply cyclic to counteract the effect. If you’re drifting right due to uneven surfaces, apply left cyclic.
Hi, thanks for the videos.... Was wondering if you can do a mast bumping video
Quincy Kangea. Sure! I'll add it to the list.
Wirth all due respect , your flow geometry for OGE is incorrect.
First the rotor tip vortices:
Each tip sheds a vortex that first forming a spiral path moves about 4% to the inside, depending on rotor tip shape, then the vortices move down and form helicals with a final diameter of about 72% of the rotor diameter.
The flow through the rotor averages a mean velocity of:
√(lift) / d × √( 2/(π×density))
This velocity doubles quickly as it descends.
Here d is the effective diameter, about 88% of the rotor diameter . The mass flow is: density × this flow velocity × π/4 × d^2
The shaft power to turn the rotor is the sum of the induced power
Pind = lift^1.5 / d × √(2/(π×density))
and the rotor blade drag power
Pd = 5/3 × lift × tip speed × Cd/Cl
here Cd is the average blade section drag coefficient ( about 0.009 )
and Cl is the average section lift coefficient (depends on collective angle , around 0.6 at design hover conditions OGE)
in forward flight the induced power is reduced considerably.
in summary the flow (OGE) narrows and accelerates to twice the average rotor through velocity.
A real problem for the C-22
That was helpful ):
Day 423 of quarantine... learning everything about helicopters
I'm having a real hard time understanding what induced flow exactly is. I've always heard that it was easier to hover in ground effect because there is a "cushion" of air between the rotors and the ground. Is induced flow that 'cushion" of air? I'm thinking its not, because it just doesn't fit with your explanation, i'm trying to figure out how the flow would generate more lift within ground effect, but less out of it.
TheEarl69 999. Induced flow is simply the downwards flow of air through the rotor disk. It’s the flow that has been induced by the rotor system. It’s highest while at a hover compared to normal flight. Using the cushion of air idea isn’t necessarily wrong. The reason it takes more power to hover OGE than IGE is because the wing tip vortices grow and begin making less and less of the blade useful. So the pitch angle in the blades has to increase to be able to create the same amount of lift. This creates even more downwards flow of air (induced flow). However, when in ground effect the vortices cannot build as easily because the ground interrupts those vortices from growing as big. Some refer to it as a cushion because you can descend from an OGE hover to an IGE with only IGE lower applied and the helicopter can cushion itself down to an IGE hover. I hope this helps.
@@helicopterlessonsin10minut10 I absolutely love it when an explanation just clicks, which this is the third time this has happened on your channel for me. You are awesome! I'm currently an air traffic controller for the army and plan on taking the SIFT test and dropping a warrant packed very soon, so i'm using this channel and other pubs to get a nice score on the sift test. Thank you for so much help.
Hi! I am confused on this explanation. You discuss that IGE is the diameter of the rotor blade or less and OGE is greater than the diameter of the rotor blade. What is confusing me is something called IGE and OGE hovering ceilings which for example of a Robinson R22 is somewhere in the thousands of feet for IGE which is way more than the rotor blade of a Robinson R22 which is 25.2 Ft. I am new to learning about helicopters, so is the IGE hover for a Robinson R22 25.2 Ft or is it in the thousands for a hovering ceiling? Very confused? Please help. Tnx!
Mark Kiziuk. Thanks for the question. Hovering in or out of ground effect deals with rotor efficiency based on distance away from the ground (aka AGL altitude). This "in ground effect" is usually determined by being within 1 rotor diameter's distance above the ground. The max hover ceiling deals with the maximum pressure altitudes that either an IGE or OGE hover can be performed. As altitude increases, the air is thinner and it is harder to generate lift. So while your helicopter may be able to hover both IGE and OGE at sea level, it may not being able to do the same in the mountains at 12,000 ft MSL. It's max IGE and OGE ceilings would fall somewhere below that based on the aircraft's performance charts. I hope this helps answer your question.
Hey Jacob!! I have a ques. My hel does not prescribe IGE and OGE hover celiling separately. The common hover ceiling is written. So, does that mean my hel will be able to perform OGE hover at that ceiling?
TIA
Interesting. Most charts have both IGE and OGE. If that were the case I would plan on having no more than IGE power until you could safely verify OGE power when you get there. Avoid any maneuvers requiring OGE until you know for sure that you have it.
@@helicopterlessonsin10minut10 thanks a lot..
insert Subtitle when explain thanks
pitch oge
One question....do you fly an Apache?
John Tyrone I do
Huh, if I didn't know that I did not make this video, then I would say that THAT is MY handwriting exactly! Odd, I have never seen someone with my kind of scribble.
Hello, one question is still in my mind i i can't find the answer:
Can we estimate (more or less) how many tons per second of air flows through main rotor during OGE hover when mass ot helicopter about 3tons ( standard atmosphere)?
Great question. I’m not quite sure which physics formula would give it to you. But based on Archimedes principle, “The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid. The weight of the displaced portion of the fluid is equivalent to the magnitude of the buoyant force.” In this case, the fluid being air, the helicopter would need to displace enough air to to equal its weight. The tricky part is this is a continuous process for a helicopter at a hover. Tough question. I’m not quite sure of the answer.
@@helicopterlessonsin10minut10 thank you for the answer.
Could you ask someone in "work" about this?
the average vertical velocity under the rotor (OGE) could be helpfull i think.
can someone make a hovercraft by flipping the blades sideways so it hovers on its wings like a bird in strong wind
Great video exactly what i was looking for i have a question so the aircraft holds a hover due to the amount of wind hitting the ground beneath it which means that the rotor wash is carrying the air craft is that correct ? If so can the aircraft holds a hover at lets say 1000 feet or above ?
WRONG. The air comes off the rotor add a 20° angle with only the last 25% of the rotor blade providing lyft.
It's a hollow cone not a solid column
What helicopter are you flying?
dragons
I don't want you to get a big head so i have 1 criticism... uh... your marker is too stubbie.
But great tutorial!
who here is from BCIT AME Course
ruclips.net/video/WVYqjNYgae8/видео.html
Review HIGE & HOGE
He or she huh 😂 glad you corrected that, female student pilot here.
bro needs a new sharpie