Does a biplane need wing twist?

I understand wing twist is used to:

  1. creat a soft/partial stall ( hopefully create a partial stall before a full stall)
  2. balance the wing so a smaller horizontal tail is needed with less drag.

So if the top wing on a biplane is canted upward, in a mild stall, it will stall first, with the bottom wing still flying, creating a partial stall.

If the bottom is at a lower AOA, it's like having wing twist on 50% of the plane. Wouldn't this act like wing twist on a monoplane and reduce the total moment, therefore needing less of a horizontal tail, like the self balancing wings on modern gliders. Is this correct, at least conceptually?

  • $\begingroup$ The reasons for twist mainly apply to swept wings, which would naturally stall tip-first (due to the outward flow velocity component) and that would create a very nasty pitch-up moment (as the root still producing lift is forward), so this needs to be compensated somehow, usually with twist. But straight wings don't produce this effect and many straight-wing monoplanes don't have any wing twist. $\endgroup$
    – Jan Hudec
    Aug 2 '19 at 22:00

Wing twist is not required for any wing, although it is often used (as you said) to make sure that not the entire wing stalls at the same time, but also to tune the spanwise lift distribution.

Both of these are much lesser issues for biplanes:

Stall on a biplane

Any wing (when creating lift) creates a flow field around it, where the flow below it is slowed down and the flow above sped up, the flow in front is pulled up, and behind it pushed down. In a biplane, the upper wing is usually a little ahead of the lower one. This means that the upper wing sits in the sped-up region from the lower wing, and gets a higher angle of incidence -- that makes it quicker to stall once angle of attack increases too much. The lower wing is the opposite, though. It gets a lower velocity flow, and the lower side of the upper wing is helping to turn the flow, so it doesn't separate from the lower wing as easily. This means that biplanes have a naturally very gradual stall behaviour, which makes them much easier to control in near-stall conditions than most monoplanes. Triplanes are even more stable.

Lift distribution

The reason modern passenger aircraft use wing twist to control lift distribution is that it's easier than using wing planform. In the 1930s and 1940s, elliptic planar wings were all the rage because they're the aerodynamic optimum for planar wings, but they are not the overall optimum (structural weight, manufacturing costs...), and that's why modern passenger aircraft have wing planforms with straight edges, and lift distributions half-way between an ellipse and a triangle (the higher inboard lift can be borne easier because the inboard wing can be made thicker, and it causes less bending moment at the wing root). For bi-planes, they usually have struts between the two wings, so they're incredibly stiff and can bear much higher loads with equal structural weight and despite using quite thin profiles -- This, the small moment of inertia for rolling and the benevolent stall behaviour is why biplanes are still a thing in aerobatics. That's also why tailoring wing twist isn't as critical for biplanes as it is for e.g. large passenger aircraft: There's not as much to gain from it.

This would probably be different if somebody wanted to build a large long-range highly efficient biplane. In that case, there would probably be some gains to be made from introducing wing twist. However, unless that plane had extremely highly stretched wings, it would likely still have worse efficiency than a conventional one because the struts between the wings cause quite a bit of drag, and you'd still have to make relatively thick wing profiles to fit the fuel in, negating the other advantage of biplanes. And that's why such biplanes don't exist.

  • $\begingroup$ The Zeppelin Staaken Riesenflugzeuge had aspect ratios in excess of 10 and would come close to a long-range biplane. But even they did not have twist. $\endgroup$ Dec 12 '19 at 20:08
  • $\begingroup$ @Peter: nice planes... Those concepts certainly made sense at the time. I would think, though, that the potential efficiency gains from introducing wing twist to them would have been tiny compared to all the other improvements (like using monoplanes...) found between then and the 1950s -- at which point wing twist, rather than planform, started to become the method of choice to control lift distribution. $\endgroup$
    – Zak
    Dec 14 '19 at 22:38
  • $\begingroup$ Yes, those giants produced horrible zero-lift drag – but were extremely light, which helps in low speed flight. The limited engines of that time did not allow for more. $\endgroup$ Dec 15 '19 at 8:04

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Evidently, they don't. The picture shows a fully aerobatic Pitts Special from 1966 with no visible wing twist.

  • $\begingroup$ Small, light, aerobatic plane, no. Huge aircraft that could not pull out of a dive in time following a full blown stall, might not be a bad idea. $\endgroup$ Aug 2 '19 at 5:36

Go back to the early 1900s and have a look at the "tailless" Dunne biplane. This designer very ingeniously realized wing twist (washout) is possible, making stalling characteristics much more benign, and, taking advantage of wing sweep to use lower AOA aft wing tips to pitch the nose down. This is the fore-runner of todays slats, which can be retracted in cruising flight to save drag.

Other early biplane designs did put the upper, forward wing at a higher AOA, as a stall safety measure, but now with two equally sized wings, one cannot be set to its most efficient AOA.

So, especially if you strive towards a higher aspect, more efficient wing, where do you put your pitch control? Traditionally, aft, taking advantage of the longer torque arm of the fuselage.

Birds deal with this issue by fanning their tails open at high AOA for pitch control, then folding them at cruise to save drag.

But where else could it go? Yes, forward! Leave the empennage on and put another control surface forward. Instead of having a bi-plane, a much smaller forward wing would serve for stall warning, passively dropping the nose (it would stall first).

So if you want tailless, you need something else to control pitch. Lower aspect, washout, placing trims and control surfaces closer to CG come at a price of lower stability and/or more drag.

Computers, which can trim several times a second, have made the reduction in size of passive pitch control "fins" possible, but their function remains critical for safety. Washout and/or slats do add drag, but make the aircraft much safer in low speed/high AOA flight.

So, how about bigger, retractable slats (for swept wing aircraft), or a "safety canard" for all.

  • $\begingroup$ Or perhaps a more robust AOA measuring device. $\endgroup$ Aug 1 '19 at 19:43
  • $\begingroup$ havinig a forward control surface go into a stall could be exciting, $\endgroup$
    – Jasen
    Aug 1 '19 at 23:59
  • $\begingroup$ @Jasen Good point, that's why it would only be a wing. No control surfaces. Just something to stall before the main wing. $\endgroup$ Aug 2 '19 at 5:22
  • $\begingroup$ still a poential roller-coaster though. $\endgroup$
    – Jasen
    Aug 2 '19 at 5:50
  • $\begingroup$ That's why we keep the empennage. A relatively small canard simply would drop the nose and unstall itself. Moving the whole stabiliator rapidly is the roller coaster. I'd rather close the barn door before the horses leave. $\endgroup$ Aug 2 '19 at 6:17

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