The way we roll

A discussion restricted to the topic of hang gliding.
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The Oz Report
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The way we roll

Post by The Oz Report »

<div id="1615424636"><i>With a flexible wing</i><br><p>Robert Osfield writes:</p><p class="BN"><b>Basics of how flex-wing hang gliders roll</b></p><p class="BN">I'm writing this post in response to Érico Miranda Schmitt's recent question on this topic. Back in the nineties I worked a bit for Airwave on the design side and wrote articles on theory for Skywings and the old hang gliding mailing list. (For those with long memories my wife and I combined surnames on marriage, you'll find all my ancient posts under Robert Osborn.)</p><p class="BN">The most simple explanation to start with would be roll of hang glider involves a combination of forces generated directly by weight shift and active/passive wing warping & stability. In this post I'll focus on the direct weight shift contribution.</p><p class="BN">It's important to understand that it's possible to roll a completely rigid hang glider purely from weight shift, but the roll rate you can achieve because of roll damping will be too small to be flyable, for all but very small wing span rigid wings where the roll damping can be reduced. My guess is a 8m span rigid wing would be controllable with weight shift. Sink rate and best glide would be poorer than modern flex-wings due to the stubby wings though.</p><p class="BN">Another important element of dynamic of roll of a weight shift hang glider is that when we shift our weight laterally we offset our mass from both the centre of lift, which creates the roll moment, and the center of drag, which creates a adverse yaw moment. If you have low lift and high drag at your current angle of attack then you'll get a larger adverse yaw moment for given roll input than you would when you have higher lift and lower drag. This can have a really profound affect on controllability of our wings so worth re-reading this paragraph and thinking about the forces in your head.</p><p class="BN">Consequences of the above relationship between roll and adverse yaw can help explain when controllability is diminished and how when it's optimized. When the L/D ratio is high we'll get the most favorable balance of roll for adverse yaw, and when we push out we also increase the roll forces, but when we pull in we temporarily lower the lift and our potential roll moment. So to optimize roll rate you want to pull faster than best glide, then ease the bar our as you shift left/right to give you some extra lift and roll moment whilst minimizing the adverse yaw.</p><p class="BN">Now controllability is compromised when the L/D ratio starts to drop, and it's most critical as we approach stall - here drag is increasing rapidly whilst lift can be dropping, perhaps rapidly. What happens here is our achievable roll moment for a lateral shift diminishes while the adverse yaw increases really rapidly. This adverse yaw in turn leads to both yaw that rolls you in the opposite direction and yawing motion that speeds up the wing you are loading, and slows the opposite - which shifts the lift towards the loaded wing preventing the intended roll and perhaps rolling you in the opposite direction.</p><p class="BN">This effect is why you often see folks drop a wing on landing even when they strongly weight shift to oppose a wing lifting. If you are flying around best L/D you'll have a chance to oppose the lifted wing, but if you are getting near stall the adverse yaw moment due to the increase in drag will overwhelm your roll inputs. In these situations it can be best to just stay centered and flare hard to stall the whole wing as quickly as possible rather than make things worse by attempting to counter the lifting wing.</p><p class="BN">This adverse yaw induced control reversal at low speed is something that some pilots and glider combinations take advantage of - at really low speed shift left and pushing out can be used to turn you right, and visa-vesa. As long as this doesn't cause a tip stall you can mush around "somewhat" in control. Something for advanced pilots to play with when high on a ridge soaring day when well away from other folks.</p><p class="BN">OK. So I've written lots and haven't even mentioned wing warping / billow shift / floating cross tubes, A-frame geometry / raised hang points, roll and yaw stability. These are all important but this post is already packed with enough crucial ideas that adding more will just confuse things. So first get your head around the above ideas. I will write about these other influences in a follow up post.</p></div>
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Tormod
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Post by Tormod »

Another important element of dynamic of roll of a weight shift hang glider is that when we shift our weight laterally we offset our mass from both the centre of lift, which creates the roll moment, and the center of drag, which creates a adverse yaw moment.
Doesn't drag also increase on the side the pilot shifts his weight? Both body and billow will increase drag. The common explanation for adverse yaw is that the loaded wing will increase airspeed and the increased lift will momentarily oppose the rolling force.
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