Originally Posted by
TURIN
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