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Thank you!

Hi there, thanks for your comment. The point is that at the point of entering a tumble you don't have any weight, you are literally weightless, this isn't just semantics, this is important. You can't put weight in any direction if you don't have any. The weight isn't even applied downwards if the glider is pointing vertically down your weight isn't pulling down on the glider, both you and the glider are in freefall at that moment. You're right that the only force can act through the basebar, but this doesn't change the logic. It will only be truly weightless for a moment, at some point the wing will start generating some force as it goes through the air, that force will act on your body, you will feel it as g force (regardless of which way it is relative to the horizon at this point) If the force is in a positive direction (lift) then the glider should recover regardless of whether your weight is forwards or backwards. Being forwards is still a good idea in this situation though as you want to recover into a dive so you can build speed and regain full control. The big problem is if the wing isn't generating lift but downforce. This is actually pretty likely in a severe tipping, if you've been travelling forwards and suddenly tip into a vertical nose down attitude then you still have momentum in the same direction, which is now air pressing on the top of the sail. Now if the wing is generating downforce then having your weight forwards is really important. Having a centre of gravity below the wing ('Pendulum stability', which is a term that makes aerospace engineers wince) is only stabilising when the wing is generating lift. As soon as the wing starts generating downforce then it's destabilising (effectively it's now an upside down pendulum). However having weight forwards (transmitted through the basebar) is pushing the glider in right direction.

There's a video of a tumble here: ruclips.net/video/Y-1IDDDuZ4A/видео.htmlsi=J9Cm5W0Fx7TwSTaM, entirely created by the pilot's actions. He pitches up strongly. Just as the glider is about to stall he pulls his weight forwards violently. In this situation, pulling forwards like that wasn't a good idea, it gave the glider a lot of angular momentum pitching down. However, as it gets close to vertical nose down the pilot's body comes backwards. I don't know if this was intentional (maybe the pilot was thinking "Oh s**t, I'm pointing downwards, I need to push out") or maybe it was the momentum that threw him backwards and he didn't hang onto the bar, but for whatever reason his arms go straight, his weight comes all the way back, his feet hit the keel and it goes over. You can see that weight on the glider far back is acting in the opposite direction, pushing in the nose down direction. Imagine if somehow the pilot's weight wasn't pushing on the glider far back, but was pushing on the glider far forwards. It would have the opposite effect, right?

Hi again! Thanks a lot for the answer! I agree about the fact that you are weightless and thus pendulum stability is temporarily unavailable. In this brief moment do you agree that pushing out gives the glider the right momentum to not tumble? Once the wing starts gaining airflow again, you have described two cases: Normal lift or downward force. The whole point of pushing out in the weightless phase is to keep the glider in an incident angle that will lead to the first scenario, and I have the feeling that pulling the bar will add a momentum that increases the likelihood of falling into the second scenario. And the video you shared showing someone pulling hard during a stall seems to prove that. Once the glider recovers air speed with positive lift, it isn't a problem to temporarily have the weight back: The glider is already in an incidence angle that will give him air speed. Obviously you should not stay there and as soon as you feel you're not weightless anymore you have to pull the bar again to avoid a nose up. Now if the push out wasn't sufficient and you are in the second scenario with downward force, yes being forward is better, but with the pushing motion you have reduced the tumbling momentum so the glider has a greater chance to stop turning. Then yes you should put the weight forward to make the glider tip back in the correct direction.

@@ailesdeployees307 I don't agree that pushing out gives the glider momentum not to tumble, other than a tiny component of rotation. In normal flight, when you have a tight hang strap then the strap acts as a rigid link, so pushing forwards and backwards on the bar does rotate the glider. However if the weight is off, the hang strap effectively isn't there so there is no pivot to rotate the glider about. In the video clip in the link, the pull forwards as he stalled was certainly a bad idea, but what really tumbled it was when he got to a very steep dive his arms went straight. By your logic, pushing out at that moment exactly like he did should have saved him, but it clearly does the opposite. At 12 seconds (just before his hang loop goes slack and his feet hit the keel) he is fully pushed out, arms straight. Up until 11 seconds, he put himself into a daft situation by whip stalling and pulling in, getting himself into a situation that could also happen through extreme turbulence, but from 11 seconds onwards he does exactly what you advocate, pushing out all the way. It does not prevent the tumble, it does exactly the opposite.

@avianhanggliders1985 I watched the video again in slow motion, and what I see is the pilot pulling the bar up to 12s (nose down to 45-60 degrees) then neutral up to 13s (nose down to 80-90 degrees) and slightly pushing only at the very last moment when the glider is over 90 degrees and the downforce starts pulling him towards the glider. So I think for most part he followed more your way. About the effect of pushing out when weightless: I try to imagine how would the glider behave in space, the pilot+harness weight is 2 to 3 times that of the glider so the push has to have some effect on the angle, though I don't know how much...

Thank you Thomas. Regarding the sprogs, I don't know for all gliders in all situations, but I think generally they are starting to exert some pressure on the sail before reaching full VG. It also depends on the loading. My predecessor at Avian did experiments with push switches on the sprogs connected to LEDs on the basebar. Doing dolphining type manoeuvres as he pulled extra +ve g he could get the lights to go off and come on as he went to less than 1g. Also turning you would see them come on one side then the other. More VG then they were on more of the time, less VG and they were on less of the time. For the 2nd question, it's a good question! I'm not sure if pitch pressure is 100% constrained to static stability but the correlation is very close. Unfortunately a light bar pressure does indicate small static stability. However (as the message of the video) static stability is necessary but not sufficient for tumble resistance. A greater static margin helps, but it's not the only thing. Image you have one of the 'plank' unswept flying wings, all the stability created by reflex. Well if you had a REALLY reflexed section you could have a pretty good static stability. It would still be very susceptible to tumbling as it would have very little pitch damping as all the area is concentrated close to the CG. The other extreme would be a tandem wing design, with a 'tailplane' as big as the main wing and it could be A LONG WAY away from it. Even if that was set up to have almost no static margin then it would still be basically impossible to tumble, you've got so much area so far from the CG.

A term used to describe this is "Tail volume" this is tail area (and for 'tail' we can also count the outboard sections of a swept wing) multiplied by distance from the aircraft centre of mass. Since Area x Distance has the units of volume, then it's called tail volume. It doesn't seem a very helpful term in understanding though as there's no actual volumes involved.

@avianhanggliders1985 I guess the way I have my topless (T2C) set up then is ideal... a s**t ton of bar pressure VG off...moderate pressure at full VG, and if I go beyond full having removed the stop that limits rearward pull on the Xbar, (reserved for smooth air) I get very light pressure at high speeds. I wouldn't fly slowly at this extreme VG setting. Trim increases to mid 30s in mph when VG is very tight. Pitch pressure is easily adjusted by the angle of the tip wands. I have raised them to have light but positive pressure at extreme VG.

@@ThomasLowFlyer That sounds like a well set up glider. The official word from WW is that the sprogs don't touch even at full VG, so lowering them below the stated minimum doesn't have any effect on performance. Personally, I'm slightly sceptical, I suspect an element of psychology there (perfectly understandably trying to keep pilots safe). If you tell a bunch of comp pilots "Don't lower your sprogs, it's unsafe" you'll probably find half of them do it anyway. if you tell them "Don't lower your sprogs, it won't make you any faster" then they might actually pay attention!

A combination of OpenVSP, OpenFOAM (using the CfdOF workbench within FreeCAD) and custom written code.

No, not any that I'm aware of I'm afraid.

The inverse method isn't being done by me, it's someone else who wrote that tool, so I'm not expert on it.However, it is just working on the sail tension. For a hang glider the shape of the sail is the most aeroelastic thing, so if you take a constant spanwise tension line, work out the lifting load at each point along that line they you can get the billow shape and then the twist. He's not treating the flex of the tubes as part of the aeroelasticity. Although ideally they should be included, the sail is the dominant factor so just fixing the curve of the tubes isn't too bad as an approximation.

@@NevilleStyke that or nearly tripping over stepping backwards on the stage!

@@cekuhnen recently support for OpenFOAM was added to FreeCAD, which is a game changer. OpenFOAM is a very powerful and capable CFD solver, but it has no GUI, you have to program it all through text files, so it's only suitable for expert users, the learning curve for even getting a simulation to run at all was pretty steep! The FreeCAD Cfd-OF workbench means you can do all the preprocessing in FreeCAD, generate the OpenFOAM input files and run them. Expert users can still hand modify the input files for more advanced functions, but for simple cases it's now pretty easy to use. Search for FreeCAD OpenFOAM tutorial videos for more info.

I flew a seagull 3 in 78 as a trainer. No luff lines and a very poor glide ratio but easy to launch and land. I flew it of high launches numerous times but in very calm conditions before moving on to a new glider. I still have its bones in my metal pile to repurpose into other projects. Very good thing it was fall and winter flying with no lift else the story might be different.

@@mermaid10x My training was done in a Seagull Seahawk, which seemed somewhat similar to the Seagull 3. Good memories.

I flew standard rogallos in the early to mid 70s (Eipper Flexi-Flyer and a Skyports Lark 17). The only design mod to recover from dives was to tighten the top line from the kingpost to the keel to give the keel a bit of reflex. The Seagull 3 was considered one of the higher performance models back then!!! Probably a good thing I quit flying in '77. Now I'm starting up again at 69. Am I nuts?

Hi, so firstly Rogallo wings were the original (60s and early 70s) HG designs. Those could enter an uncontrollable (luffing) dive as a result of a gust (as I describe in the video) however modern gliders will recover themselves from that situation (also as described in the video). Secondly, I don't think a flat spin is even possible. Hang gliders are pretty spin resistant, some pilots do it for fun, but it's actually quite hard to get it to do it and you have you put quite a lot of effort into holding it in the spin. Centralising the controls stops it almost immediately. Also, yes kinetic energy is proportional to the square of speed, but you have to also include the original speed. So if you are flying at 25mph and hit a gust which increases your speed by 5mph then you are doing 30mph. If you work out the increase from 25 squared to 30 squared then it's 1.44x, not 5x. 6mph increase from 25mph is 1.53x, not 36x the energy. Also this isn't directly related to pitch input anyway, if the speed of the glider suddenly increases then it will pitch up, that's actually a function of stability, it's converting excess kinetic energy into potential, any aircraft will do that. The worst situation for a hang glider is actually a tumble, but that has nothing to do with aerodynamic controls or power, it's a function of being tailless and rotational inertia, which I'll explain in the next video.

@@avianhanggliders1985 Yes you are correct, but the total mass of the pilot and the aircraft is a constant (m/2) and the velocity components can have a greater value plus direction. I have experienced loss of pitch control in Rogallo wings and it is not an enjoyable experience. Wind shear is not fun.

@@crimestoppers1877 You fly old-school rogallos? I stopped flying mine in the 70s and sold the last one as a trainer about 1980. I do remember one sketchy tactic to recover from a full luff dive. You are diving so fast that you are nearly weightless, so weight shifting is useless. To push the nose up and re-establish a positive AOA, you had to use inertia rather than weight shift. The idea was to grab the down tubes hard and swing your feet up against the basetube in a crouch, then kick the bar forward really hard with both legs. A violent action to create a reaction. Sometimes the reaction would (supposedly) punch the nose up and the sail would fill, pulling you out of the dive. Thank god I never had to try that to see if it worked!

@@kimp8079 nice! Thank you.

Ah, thank you!

Yes, using carbon but keeping everything round tubes doesn't seem to me to make a lot of difference. Same for technora. The only wings that seem to be making the best use of carbon are the rigids (currently only ATOS). And maybe the Combats with oval leading edges.

Good question. Kind of no. Paragliders can get away with pitch negative airfoils because the wing is simply so light that the pilot swings ahead of the glider. However, imagine if a hang glider with pilot is dropped totally vertikal the nose facing the ground... the hang glider wing is so heavy that it would freefall as fast as the pilot, hence it has to pull away from the pilot by being pitch positive .

@@kimp8079 Thanks for that explanation. I find it counter-intuitive, but I'm not expert enough to really "get it". To me, it feels like lift-induced drag would prevent the wing from keeping up with the pilot, except in the extremely unlikely event of the vehicle being pointed straight down and having no pitch momentum at all. Otherwise, if there's the slightest difference in the downward velocity of the pilot and wing, the wing will lack lift-induced drag for only a moment.

@@IsaacKuo Yes. It is a little complicated, as Tim states. If the pilot has locked arms he/she changes the aircrafts center of gravity and you get partially pendlum stability (the pilot has also drag ;) ). However, trust me, you do not want to fly a hang glider that feels that it is is dropping its nose at speed so you need to push and hold on to the control bar. What you want, is for a the glider to be able to fly hands off (pilot acting at the gliders centre of gravity by only one hang strap) and from speed to pull up gently by itself, hence pitch positive behaviour.

@@kimp8079 I have test flown a glider that was badly adjusted (and was in a fairly bad shape), and with full VG at high speed was pitch negative. I quickly pushed out, released VG, landed, handed back to the seller and told him to scrap that kite. It was one of my scariest experiences, and only lasted a few seconds. Oh well, not quite, the wing also didn't handle predictably, and on approach I was thrown to the other side when leveling into final, and landed on the other side of the fence to avoid hitting our club's bar/shed/wing storage. Yeah, pitch negative and bad adjustments can cause bad problems.

"Hanging below the wing" is basically only true if your arms are loose and you're free to swing. In this state then it's all about the stability of the wing by itself, your weight under it is doing nothing for stability. As others have said, it's important that the wing is stable like this. If you lock your arms solid (this is just a thought experiment) then the CofG of the aircraft is now moved a long way below the wing. You're not really 'hanging' now though, as the centre of rotation of the overall aircraft is you. This does increase the stability when the wing is generating lift, but consider what happens if the wing goes negative AoA for a moment. Now everything is reversed and effectively you're 'above' the wing and destabilising it! Note a PG just can't generate negative lift, the lines would go slack and it will simply collapse. We expect a HG wing to recover from a momentary negative AoA. A PG is expected to collapse (possibly only partially and momentarily before reinflating itself) at negative AoA. So a stability system that only functions when the wing is generating lift 'works' for them (for a given definition of 'works' that includes just accepting that your wing will collapse from time to time!)

Yes, I guess in principle it might be possible for one of those to do half an outside loop and then start flying inverted! In fact I half remember someone telling a story of hang gliding 'back in the day' of something like that happening and the pilot being OK. I can't remember any more about it though.

@@mikunan I believe the deflexors were mostly aiming at improving performance by stiffening the leading edges. I think that was mostly driven by the limited availability of tubing at the time. We now use much thinner wall, but larger diameter 7075 (or even carbon) which is way stiffer in bending so the deflexors just aren't needed.

Yes, that would improve the glide. Most hang gliders have a horrible distribution of loading over the span in terms of lift induced dag. They are overloaded at the root, underloaded at the tips. What you did was reduced that non-ideal span loading with a resulting reduction in vortex drag. You also made the wing less pitch-positive (more prone to tucking) but fixed it with the addition of the horizontal stailiser. There is a good reason that the highest performance sailplanes all have a stabiliser behind the wing! That being said.... there are other ways to fix this. Watch this space.

As @HowesAero said ;-) To me, 'tucking' and 'tumbling' have subtly different meanings. I think what you're concerned about here is what I'd call a tumble, which is about the rotational inertia and rate of rotation. A tail also gives a lot of pitch damping (resistance to the rate of rotation around the pitch axis), which also helps prevent a tumble. Although also as Jon says, there are other ways to fix this ;-) My next video will go into tucks and tumbles in detail.

The only downside I could think of for your mod was if you wanted to do a spot landing, bar full out, and drop onto your feet. Did you ever have a tail strike?

@@travelbugse2829 The funfex has a long keel that helps rigging, and can be shortened before takeoff. If you leave it long and go flying, you might hit it on the ground before your feet and the wing drops forward. You still land a "no stepper" but it falls on the control bar half-gently. Not too bad. That's an intermediate so I'm not sure how a topless would compare. A hinge with a fuse could also be used, that allows the tail do bend up on landing, but not down, so you get the dive recovery and pitch dampening, but if you tail-strike the landing, it will bend up.

@@ericoschmitt Many thanks - can't help wondering whether you were joking! It reminded me of my schoolboy days long long ago, when we fitted de-thermalisers to our free flight models. Hinged tail, a rubber band, some string impregnated with potassium nitrate(?), light its end and wait for some variable moment before it stopped flying!

@@wrdturkey thank you ☺️