Hang glider Tucks and Tumbles

[@VTSifuSteve]

Excellent presentation, and very helpful to understanding the forces at play in these situations. I’m looking forward to your next video.

[@avianhanggliders1985]

Thank you!

[@ThomasLowFlyer]

Thank you Tim...a great series of videos. A couple of questions: Do sprogs have any aerodynamic effect in normal flight at less than full VG? It seems like the are adjusted to not engage at AOAs that we can achieve through the range of pitch control and roll billow shift. Second question: Is it possible to have very light pitch bar pressure, but still have a tumble resistant design, or is light pitch pressure a direct indication of low static margins?

[@avianhanggliders1985]

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.

[@avianhanggliders1985]

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.

[@ThomasLowFlyer]

@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.

[@avianhanggliders1985]

@@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!

[@ailesdeployees307]

Hi Tim ! Thanks a lot for your very instructive video about tumbling. However I disagree on the last point that you talk about: If you are entering a tumble and your nose is facing downwards, I would recommend to push out. This has brought some debate among fellow pilots, but on this point I feel it's important that we get it right so here are my arguments. To see why, your drawing should not picture a horizontal flight but a glider entering a tumble: Nose down. I this situation your weight, directed downwards aswell, no longer pulls on the strap and thus no longer affects the wing through the strap as shown on your picture. In this situation your weight only affects the wing through the speed bar: *If you remain static and just grab the bar, your weight will naturally add a benefical momentum to the wing to come back to horizontal: Pendular stability. Though this is not enough if the tumbling momentum is strong. *If you pull the bar as you advise, you will briefly give the glider the opposite momentum and help it tumble. *If you push out, you give a temporary extra momentum in the good direction to exit the tumble. This will at least keep the glider with nose downwards an extra second, allowing it to gain back the airspeed it needs. I have been in rough conditions and used this method quite a few times, never tumbled. I spoke about this to someone who used to be test pilot and he agreed, saying that to enter a tumble on the tested gliders he had to pull the bar when nose down.

[@avianhanggliders1985]

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.

[@avianhanggliders1985]

There's a video of a tumble here: kzbin.info/www/bejne/j16Uend6edqNaqMsi=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?