CloudHopper wrote:
Looks like your standard "up only" spoiler out there, with a bungee return. Thus, no downward deflection to worry about.
No, I meant that if you raise the spoiler (say to a 45 degree angle), the airflow pushes down on it, which pushes down on the sail. If the sail is free to flex down relative to the rest of the glider structure, then this will increase the angle-of-attack of the sail near the spoileron-- not what you want. This would generate a roll torque in the wrong direction. Perhaps on the "Tropi", a downward force at the location of the spoileron didn't flex the sail downward much. At any rate this would be an obvious concern that a designer would need to consider, and the further forward the spoileron, the less of a concern this will be, because the structure is more rigid further forward.
CloudHopper wrote:Hint: They are "spoilers" which not only spoil the lift but also add more drag. Any influence on pitching moment is minor compared to the yawing/rolling moment influence.
If they kill the lift at a location behind the CG, they will tend to pitch the nose up. John Coyne described to me how he had to fine-tune the location of his spoilerons so as not to pitch the nose up or down when they were opened.
Remember that a hang glider has negative "effective dihedral" at most angles-of-attack / airspeeds, especially at low angles-of-attack / high airspeeds. This means that yawing the nose to the left will create a roll torque to the right. If a spoileron had only a small lift-killing effect compared its drag effect, then deploying a spoileron from the left wing would tend to roll the glider to the right, especially at high airspeeds. Just like deploying a drogue chute from the left wingtip rolled the glider to the right, in my experiments in several different hang gliders. Especially at low angles-of-attack (high airspeeds).
John Coyne and others obtained good results from spoilerons with no wrong-way roll effects--even at high airspeeds-- which strongly suggests the lift-killing effect of their spoilerons was large compared to their drag effect.
I don't have first-hand experience that John's spoilerons created no wrong-way roll torque in the most extreme corner of the flight envelope where the glider's "effective dihedral" was most strongly negative (VG loose, bar fully stuffed), but when I asked John he said his spoilerons always were effective, at all airspeeds.
It does seem likely that a spoileron located further aft on the airfoil (like on the "Tropi" photo) would cause slightly less loss of lift, in relation to the drag it created, than a spoileron located as far forward as John's were.
In a "tip-dragger" that had no lift-spoiling effect, the pitching moment would indeed be minor compared to the yawing/ rolling moment. A tip-dragger would produce a "wrong-way" roll torque on a conventionally shaped hang glider, due to the glider's negative "effective dihedral" at most angles-of-attack, especially low angles-of-attack (high airspeeds).
aeroexperiments wrote:Also I'm wondering if at very high airspeeds (low angles-of-attack) when the tips are generating minimal lift, the spoilerons on the Tropi would have much roll effect at all?
CloudHopper wrote:Again, if you think of them as "spoilers" more than ailerons, and consider the yaw/roll moment arm distance verses the pitching moment arm distance, that question is easily answered. At high speeds, the roll effect is great, with very little deflection required.
If you mean think of them as generating drag rather than killing lift, then at high speeds they should be very ineffective. Or more precisely, they should generate a powerful roll torque in the "wrong" direction. Just as my wingtip drogue chutes did. After all, at high speeds (low angles-of-attack) is precisely where the "effective dihedral" of a flex-wing hang glider is most strongly negative. If the spoilerons continued to be effective at high airspeeds (as John Coyne's did), I would take this as a sign that 1) the spoilerons were mainly killing lift, not generating drag; and 2) the part of the wing where the spoilerons are located is still generating upward lift even with the bar well pulled-in. I would guess that the wing of the "Tropi" had relatively little twist?
PS I should add that without more photos of the wing, it's not obvious that the "Tropi" had negative "effective dihedral". But I suspect it did--especially at low angles-of-attack/ high airspeeds-- unless the designers decided to build a distinct amount of positive dihedral into the geometry of the leading edges relative to the keel. See my previous post on this thread on how even a glider like a Falcon or Spectrum with very little "airframe anhedral" still has an overall anhedral geometry, due to the way that sail billow creates an anhedral geometry. When you combine this anhedral geometry with a swept wing, you get negative "effective dihedral' at low angles-of-attack / high airspeeds, and the "effective dihedral" gets closer to zero as you approach min. sink. My experiments suggested that at min. sink the "effective dihedral" of many gliders is still slightly negative, but in some hang gliders it becomes slightly positive.
At any rate, from as little as we can see in the "Tropi" photos, it looks like a conventionally shaped hang glider.
Steve