As a layman with a interest in aviation principles I really love the way you break down complex ideas so they are more easily understood. You are a great teacher! Thanks 🙏
Lovely, thank you. I tried hang gliding some years ago, the instructor briefly described the purpose of the luff lines and dive rods, but it's really interesting to see a structured and more detailed explanation. Very interesting.
Wow. You do a fabulous job of translating a lot of very technical ideas into clear, visual, and intuitively comprehensible explanations that non-engineers like myself can grasp. I look forward to your next video addressing dynamic stability in hang glider design.
Gratulation, great video. I was flying 45 years, since the beginning of this sport. Now I see this vid and recognize the sum of improvements from the beginning in the early days. I wished I could go back in time. 😢
What a great video. I used to hang glide between the late 70s and early 80s, and my glider was a Seagull Aircraft "10-meter". It did have anti-dive struts at the wingtips to enforce washout at all times. It was not a fixed type and wasn't limited by a cable, either. Rather, it was the geometry of its attachment do the leading edge that allowed it to "float" to more negative angles relative to the rest of the wing, while also establishing a minimum guaranteed negative, washout angle.
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.
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?
Thanks Tim for the great addition to the series on stability. I had a glider back in the late 1970s that had deflexers and extra cables forward of the leading edge and they claim that was for tuning the glider and you can adjust them. Apparently that was not the greatest thing because that was the end of that when they used smaller diameter. Leading edges too as a result, but it did have anti-dive struts and battens . They also used a connector that would lift the rear section above the keel and connect to the aft leading left line.
@@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.
At 10:30 the picture you showed of the parked Rogallo triggered a memory of me kite flying for my children years ago (long after I'd given up HG and trike flying). I bought a cheap Aldi or Lidl kite which had its frame on the top surface. After a couple of flights I couldn't help modifying it and flipped the whole thing - the frame was now underneath. The difference was amazing. Before, the string angle (as I called it, the angle between the ground and the paid-out line) was about 45 to 50 degrees. After, it was 65 to 75 degrees! I even got neck ache flying the kite, looking up at it with my head back, although my kids didn't care either way. So, at the very least the old Rogallos wouldn't have flown well once they started diving - never mind the luffing!!
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.
Lots of straight wing tailess aircraft out there... MW9 Microlight Markse XM1 Marske Pioneer Marske Monarach Al Backstrom EPB-1 the list goes on but these are ones from memory.
Thank you for explaining that so clearly. After several bad "close to tuck-Situations" under turbulent alpine conditions, I installed a horizontal stabilizer from AEROS on an elongated keel tube and lowered the pitch of the wingtips to compensate for the additional pitching moment. The problem was gone. I never experienced anything close to a tuck anymore (without changing my XC-flying habits). As far as I can see that modification also improved the performance of the glider a bit. It is not a pure flying wing anymore, but it seems to help. What do you think about that solution to deal with very turbulent conditions?
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!
Thanks for your project. According to what all my veteran colleagues tell me (30-40 years flying) the current wings are little evolved, increasingly expensive and the few things that are implemented are carbon, technora (delicate) and four other nonsense, to justify an exorbitant price.
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.
What happens if I am flying a Rogallo wing ( typical HG kite design) and encounter unstable air and lose pitch control (momentary crosswind, turbulence or gusts and sink)? This can place me into a flat spin or dive as a result. Without power and no elevator the only solution I can think of is to place my Center of Mass as far ahead of the Center of Pressure so that the aerodynamic twist or reflex can help me restore pitch stability. Move further downward while in a flat spin or dive. I have been in this unfortunate situation and it is frightening that only luck seems to help me recover. Think of this Kinetic energy (KE)= 1/2 mv squared. A gust of 5 mph (delta V) will require 25 times the energy to restore pitch. A gust of 6 mph requires 36 times the energy to restore pitch, a 10 mph gust requires 100 times the energy. Your mass is a constant ( glider plus pilot = m/2) and instantaneous velocity and density altitude are the only variables. This is not fun. Controllable aerodynamic control surfaces and lots of power can be employed to restore stability. Dihedral also requires a large vertical tail structure which adds mass moving the CG aft ( not good). I think this is why the Wright brothers received their patents for wing warping and gas engine power for aircraft in 1903? Aerodynamic stability and control!
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!
interesting topic. i will go look at the next videos. question :we know hang gliders tumble. are there examples of the ridgid hang gliders with the small tails tumbling?
If you're hanging sufficiently below the glider, couldn't that provide adequate stability by itself? I mean, that's how parafoils work, and they don't have either sweep or reflex.
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!)
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As a layman with a interest in aviation principles I really love the way you break down complex ideas so they are more easily understood. You are a great teacher! Thanks 🙏
Complex ideas simplified, but not over simplified. Brilliant explanations. Thanks for these videos.
Lovely, thank you. I tried hang gliding some years ago, the instructor briefly described the purpose of the luff lines and dive rods, but it's really interesting to see a structured and more detailed explanation. Very interesting.
Wow. You do a fabulous job of translating a lot of very technical ideas into clear, visual, and intuitively comprehensible explanations that non-engineers like myself can grasp. I look forward to your next video addressing dynamic stability in hang glider design.
Thank you Tim for continuing with your talks/special information on HG design!
Gratulation, great video.
I was flying 45 years, since the beginning of this sport. Now I see this vid and recognize the sum of improvements from the beginning in the early days. I wished I could go back in time.
😢
Excellent mate the Marske Monarch is great flying wing
SPROG explanation - SUPER!
Probably the last of a kind my dear amigo. Keep up the good work!
Great vid. I have no flying experience at all, but I understood all the principles you explained very well.
Very good video. Very informative. Nice illustrations. I learned the basics of the wing when I was learning to hang glide. This is a great refresher.
@@wrdturkey thank you ☺️
Marske Pioneer, Backstrom EPB1, Fauvel AV36, MW-9, several successful “flying plank” sailplanes and powered aircraft.
Ah, thank you!
Great vid, Tim! I found it fascinating and you break it down very well. Cheers
What a great video.
I used to hang glide between the late 70s and early 80s, and my glider was a Seagull Aircraft "10-meter". It did have anti-dive struts at the wingtips to enforce washout at all times. It was not a fixed type and wasn't limited by a cable, either. Rather, it was the geometry of its attachment do the leading edge that allowed it to "float" to more negative angles relative to the rest of the wing, while also establishing a minimum guaranteed negative, washout angle.
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?
Thanks Tim for the great addition to the series on stability. I had a glider back in the late 1970s that had deflexers and extra cables forward of the leading edge and they claim that was for tuning the glider and you can adjust them. Apparently that was not the greatest thing because that was the end of that when they used smaller diameter. Leading edges too as a result, but it did have anti-dive struts and battens . They also used a connector that would lift the rear section above the keel and connect to the aft leading left line.
@@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.
Thank you very much. I will wait for your next episode 🎉
At 10:30 the picture you showed of the parked Rogallo triggered a memory of me kite flying for my children years ago (long after I'd given up HG and trike flying). I bought a cheap Aldi or Lidl kite which had its frame on the top surface. After a couple of flights I couldn't help modifying it and flipped the whole thing - the frame was now underneath. The difference was amazing. Before, the string angle (as I called it, the angle between the ground and the paid-out line) was about 45 to 50 degrees. After, it was 65 to 75 degrees! I even got neck ache flying the kite, looking up at it with my head back, although my kids didn't care either way. So, at the very least the old Rogallos wouldn't have flown well once they started diving - never mind the luffing!!
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.
This was excellent and fascinating. Thank you!
Lots of straight wing tailess aircraft out there...
MW9 Microlight
Markse XM1
Marske Pioneer
Marske Monarach
Al Backstrom EPB-1
the list goes on but these are ones from memory.
Thank you for explaining that so clearly. After several bad "close to tuck-Situations" under turbulent alpine conditions, I installed a horizontal stabilizer from AEROS on an elongated keel tube and lowered the pitch of the wingtips to compensate for the additional pitching moment. The problem was gone. I never experienced anything close to a tuck anymore (without changing my XC-flying habits). As far as I can see that modification also improved the performance of the glider a bit. It is not a pure flying wing anymore, but it seems to help. What do you think about that solution to deal with very turbulent conditions?
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!
Great video! very educational :)
Thanks for your project. According to what all my veteran colleagues tell me (30-40 years flying) the current wings are little evolved, increasingly expensive and the few things that are implemented are carbon, technora (delicate) and four other nonsense, to justify an exorbitant price.
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.
1 war german fighter hansa branderburg has deflex wings profile ,very stable .
What happens if I am flying a Rogallo wing ( typical HG kite design) and encounter unstable air and lose pitch control (momentary crosswind, turbulence or gusts and sink)? This can place me into a flat spin or dive as a result. Without power and no elevator the only solution I can think of is to place my Center of Mass as far ahead of the Center of Pressure so that the aerodynamic twist or reflex can help me restore pitch stability. Move further downward while in a flat spin or dive. I have been in this unfortunate situation and it is frightening that only luck seems to help me recover. Think of this Kinetic energy (KE)= 1/2 mv squared. A gust of 5 mph (delta V) will require 25 times the energy to restore pitch. A gust of 6 mph requires 36 times the energy to restore pitch, a 10 mph gust requires 100 times the energy. Your mass is a constant ( glider plus pilot = m/2) and instantaneous velocity and density altitude are the only variables. This is not fun. Controllable aerodynamic control surfaces and lots of power can be employed to restore stability. Dihedral also requires a large vertical tail structure which adds mass moving the CG aft ( not good). I think this is why the Wright brothers received their patents for wing warping and gas engine power for aircraft in 1903? Aerodynamic stability and control!
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!
interesting topic. i will go look at the next videos. question :we know hang gliders tumble. are there examples of the ridgid hang gliders with the small tails tumbling?
If you're hanging sufficiently below the glider, couldn't that provide adequate stability by itself? I mean, that's how parafoils work, and they don't have either sweep or reflex.
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!)
Here is a full size "plank" - wing.
ruclips.net/video/mG1mxrCuV-s/видео.htmlsi=bdTF03oeSqdym0Id
@@kimp8079 nice! Thank you.
Wrong.
en.wikipedia.org/wiki/Fauvel_AV.36