Aerodynamics of Rogallo wings

[hugh flung_dung]

I'm just converting onto a weightshift microlight so have been trying to understand the basic aerodynamics.

I understand that Pitch stability comes from washout and reflex, and that reflex causes a rearward CP movement with AoA - but please can someone explain why reflex causes the CP to move rearwards?

Some of the articles talk about pendulum stability but I can't see how this is possible when the trike is able to freely pivot beneath the wing (presumably at the Aerodynamic Centre?). I could accept something based on centripetal force causing sideslip which leads to roll-in because of the sweep, but haven't seen this discussed anywhere. What's actually going-on?

I can see that billow-shift is a good thing, and that it can be improved with a floating cross-tube, but how do I explain the sequence of events when entering a turn with a wing having both of these "features"?

HFD

[pilotmike]

Very briefly: Reflex is the general weightshift 'implementation' of a lower angle of incidence in a tailplane of a conventional 3 axis aircraft, with respect to the main wing. It is just that it is achieved in the one weight-shift wing, rather than the conventional two. Given the lower angle of incidence for any given pitch angle, and incremental change in pitch will give a greater percentage change to the lower angle of incidence 'tail', giving proiportionaltely more lift, hence the nose-down pitch caused by the effective rearward moving C of P. Reflex is held in weightshift wings by trailing edge support wires, which are in turn supported by a centrally mounted post. Care must be taken to maintain sufficient reflex, as this is a critical safety feature in such wings: reduction of reflex can lead to a fatal manoeuver known as a tumble.

Washout acts much as in conventional wings, ensuring that the tips remain flying when other sections of wing are stalled. Combined with the swept-back wing design, this has the added advantage of moving the centre of lift aft in such circumstances, giving a strong restoring pitch down moment.

Billow shift is the mechanism by which roll is achieved. Without a 'floating crosstube', many wings would be so 'stiff' in roll as to be un-flyable. The use of floating crosstube effectively adds a 'servo' mechanism. This facilitates the billow shift, which in turn makes the roll control suitably light for practical use, in much the same way as servo tabs are used in conventional control design.

I hope this brief treatment helps.

PM

[hugh flung_dung]

Thanks PM. I've read several papers over the last few days, including GGs PhD thesis and others:8, (BTW, where is GG when you want him?) and understand the basics; it's the "why"s that I was after.

I understand what reflex is and does, and can see the "conventional tailplane; lower angle of incidence" explanation but can't see how this works when it's all in the same aerofoil.
BTW, I thought I'd read that lack of reflex could lead to an irrecoverable luffing dive but that tumble was caused by momentum and the inherent control limitations of the bar passing through the triangular structure.

Also, I'm totally happy with washout giving pitch stability for swept-wing tailless deltas; it's the concept of pendulum stability when the mass is suspended on a pivot that I'm finding difficult to accept.

Finally, it's the sequence of events for turning that I don't yet understand. Here's my best attempt: pilot displaces weight to one side, because of the relative masses of trike/wing this causes the wing to slightly bank to the same side, gravity now allows the mass of the trike to move the keel to the same side (compared to the leading edges, permitted by the floating cross-tube), this results in less tension in the sail fabric on the "down" side, so this billows and decreases the effective angle of attack of that half of the wing (the opposite on the "up" wing) and this gives the "servo" system. The luff lines running over the king post amplify this effect.
Is this a correct sequence?

HFD

[chornedsnorkack]

I understand what reflex is and does, and can see the "conventional tailplane; lower angle of incidence" explanation but can't see how this works when it's all in the same aerofoil.

Many planes do without tailplanes. Concorde for one. I think that the basic stability-generating effect of smaller angle of incidence at the back are the came whether there is a gap between stabilizer and main wing or not.

[TURIN]

You also need to remember that a flex-wing will alter in shape dependent on wing loading. As has been explained, the billow shift during roll makes turns a lot easier, but in pitch the luff lines will ensure the reflex creates a stronger pitch up 'feel'. Early hang-gliders had a tendency to 'tuck' in a dive before the reflex function was fully understood. The combination of reflex and wingtip washout ensures that pitch stability is maintained through the speed range. Obviously the more stability you have built in with reflex etc can also cause extra drag which is why modern microlights have adjustable reflex/luff lines. (I think, my hang-gliding days are, I fear behind me now:{)

[Genghis the Engineer]

You also need to remember that a flex-wing will alter in shape dependent on wing loading.

True, but this hasn't much bearing on the rest of your post. The main effect of this is to move the spanwise lift distribution inwards, raising the aerodynamic wing loading inboard, and thus increasining the stalling speed beyond what would normally be predicted.

As has been explained, the billow shift during roll makes turns a lot easier, but in pitch the luff lines will ensure the reflex creates a stronger pitch up 'feel'.

This is primarily a high speed effect, lufflines didn't do much at low speeds on most wings until Bill Brooks invented the luffline pitch trimmer.

Early hang-gliders had a tendency to 'tuck' in a dive before the reflex function was fully understood.

This was referred to as a "luffing dive" - essentially the wing flattened out, removing the reflex and washout that maintain pitch stability, thus allowing the wing to lock into a high speed dive - usually until it hit the ground.

The combination of reflex and wingtip washout ensures that pitch stability is maintained through the speed range.

Indeed, although it is important to remember the benefits of tipsticks here. When they were introduced in hang-gliders, this something like halved in one year the fatal accident rate, since even if lufflines fail or the tension is wrong, these ensure *enough* pitch stability to allow a pilot to pull out of a high speed dive. There's quite a good explanation of this in the PhD thesis of a lady called Elizabeth Kilkenny from Cranfield in the mid 1980s.

Obviously the more stability you have built in with reflex etc can also cause extra drag which is why modern microlights have adjustable reflex/luff lines.

Ah now, here's a contentious point. The extra drag is largely theoretical - the effects are so small they can't easily be measured in flight. A few 3rd generation microlight wings in particular, particularly the Mainair Flash 2 alpha wing, had such adjustments. You could make a reasonable case that a number of fatal accidents over the 1980s and 1990s were down to f***wits who knew less than they thought they did, adjusting wings to try and obtain this, largely illusory, reduction in drag, and in practice reduced the pitch stability to a point where either a luffing dive could happen (the Alpha didn't, for example, have tipsticks - although they were re-introduced on the Blade through the sheer common-sense of Mainair's then Chief Designer) or the low pitch stability made it too easy to mishandle the wing into a tumble.

Mainair, very sensibly, brought in a mandatory check after a while to ensure that all wings were checked for sufficient preset reflex and pitch feel - this seems to have eliminated the batch of dead F2a pilots and severely scared F2 pilots.


(I think, my hang-gliding days are, I fear behind me now:{)

Must admit, I don't go near flexwing microlights much these days either. No lack of desire to fly them, just far too damned busy. But, they remain fascinating bits of technology.

G

[Genghis the Engineer]

.

Also, I'm totally happy with washout giving pitch stability for swept-wing tailless deltas; it's the concept of pendulum stability when the mass is suspended on a pivot that I'm finding difficult to accept.

That's because it's largely rubbish - there is no wing pendular stability, beyond the very small amount due to the hangpoint being below the chordline of the wing. As a pilot you can change this of-course by hanging on hard to the bar - so in effect you can act as a roll damper through your arms introducing a pendular stability term. But, hands-off, it's not there for all reasonable purposes, for the wing - it does help the trike stay the right way down however.

Finally, it's the sequence of events for turning that I don't yet understand. Here's my best attempt: pilot displaces weight to one side, because of the relative masses of trike/wing this causes the wing to slightly bank to the same side, gravity now allows the mass of the trike to move the keel to the same side (compared to the leading edges, permitted by the floating cross-tube), this results in less tension in the sail fabric on the "down" side, so this billows and decreases the effective angle of attack of that half of the wing (the opposite on the "up" wing) and this gives the "servo" system. The luff lines running over the king post amplify this effect.
Is this a correct sequence?

HFD

Roughly, but there's an additional important factor (shown in an otherwise very poor paper by some chaps from Cranfield a couple of years ago, attempting to explain hang-glider stability in Aeronautical Journal which is that once the wing is tilted it moves the lift vector to one side of the whole-aircraft CG, creating a rolling moment about the CG.

What the relative contributions are of billow shift and movement of the vertical lift vector, I'm not sure anybody really knows. The largely useless Cranfield paper didn't add much to the science.

G