# Wing dihedral in a biplane

Would a biplane be able to fly if the upper and lower wings were in opposite dihedral? Imagine a wing configuration similar to the fictional "X-wing" design from StarWars. What would be the theoretical pros and cons of using such an approach? Ignore the engines, fuselage, tail, etc

What happens if you take it to its logical extreme and make all wings at 45 degree angles?

• Does this answer your question? What will happen if an X shape wings mounted on biplane? Commented Jan 19, 2021 at 23:53
• Sort of but not exactly. Here are the two area of difference. 1) In my question we are ignoring the engines - which are a major issue in that answer 2) In my hypothetical the wings are apart at the fuselage (just like a biplane) not touching as in that answer (which they say causes a ton of drag) Commented Jan 20, 2021 at 0:18
• Added an admittedly crude image to explain better. Commented Jan 20, 2021 at 1:59

There's a line of airplanes that have already done it, at least a bit.

The Rutan tandem wing Quickie designs are exactly that configuration, except that the wings are radically staggered because the forward one is the trimming/pitch control surface, and the dihedral/anhedral angles are shallower. But looked at from straight ahead, there it is.

The anhedral of the front wing is mostly there because that's where the wheels go. Versions of the 2 seat Q2 that have a conventional tricycle gear, have a nearly straight lower wing because the most of the anhedral is "in the way" (tips near the ground become a problem now without wheels there).

The problem with the configuration is other than being able to put the gear at the wing tips in Rutan's application, there's no point to it. That anhedral wings cancel out the dihedral wings, form a roll stability perspective, making the result more or less the same as making all the wings straight. If you do want to put the wheels there, you have to make the spars do double duty as really long leaf springs (bending loads from landing are much harder on the spars than flight loads), and there are ground handling issues with the wheels so far apart.

So might as well avoid the hassles of fabricating bent spar beams and make them straight across in the first place, or at least with more normal dihedral angles. It's more or less just something that looked cool from a science fiction perspective.

• Would you consider this a biplane? I guess maybe? My thoughts were strictly one wing above the other! Commented Jan 20, 2021 at 2:00
• @skanga: The term I've usually seen used to describe the Quickie is "tandem wing". Commented Jan 20, 2021 at 18:30
• @Fred Larson See paragraph 2 "Rutan tandem wing Quickie designs". Anyway, no it's not a biplane, it's a tandem wing. The front surface is the stabilizing surface, so some describe it as a canard, but tandem wing is more correct I think.. Commented Jan 20, 2021 at 19:21
• @JohnK It is wrong to describe a tandem fore wing as a "stabilising surface". Fore and aft wings are both primary lifting surfaces, with stability achieved through their relative arrangement (typically the fore wing has a higher AoA, but it gets complicated). All else is misguided sophistry (cue same, no doubt...) Commented Jan 21, 2021 at 20:08
• @Guy Inchbald I get your point. Better to say "trimming surface" since that is where the pitch control resides? Commented Jan 21, 2021 at 20:27

There's no particular need, from a physics perspective, to have dihedral in an airplane. It's there for stability. If you roll to one side, you naturally start slipping to that side. In an aircraft that has a positive dihedral, this slip means the lower wing will have greater lift than the upper wing. This creates a rolling moment back towards level.

An anhedral (negative dihedral) wing would have the opposite effect, increasing roll rate away from level. Note that this isn't necessarily a bad thing: it's possible for planes (such as the An-225) to be too stable, and thus, too difficult to roll when you actually want to. Having anhedral wings fixes that issue.

But there is another effect. Both dihedral and anhedral slightly reduce lift. Since the lift vectors of each wing aren't parallel, a small portion of the lift the wings might have generated is lost. This translates into slightly higher stall speeds, and slightly more drag.

In short, any "good" that the two opposite sets of wings might do would cancel each other*, while the "bad" would add together. You could get such an airplane to fly, it would just be less efficient than it otherwise would have been.

*Of course, if they're not at equal angles, or one is bigger than the other, or they have different airfoil shapes, etc., then they won't perfectly cancel, and you'll still get some effect based on which one is "stronger".

• So, if I understand you correctly you are saying that the aircraft would fly similarly to a biplane with the same wings being totally flat EXCEPT that it would have a higher stall speed, and slightly more drag. Commented Jan 20, 2021 at 9:22
• Yeah, pretty much. Commented Jan 20, 2021 at 16:26

Back in the 1980s (when Star Wars was still fairly fresh) there were a couple different radio control model designs that had the "X-wing" layout -- a strutless biplane with approximately equal upper dihedral and lower anhedral. One was a canard, the other was tailless (in order to look even more like an X-wing from the movie).

They flew fine, with the very neutral "goes where you point it" stability of a (pre-3D, pre-Turnaround) Pattern aerobatic airplane. They couldn't fly knife edge, because they had little rudder authority and couldn't keep the nose up at 90 degree bank (and were sloppy in point rolls for the same reason), but in non-aerobatic flight, they handled about as well as a conventional mid-wing, zero-dihedral Pattern design of the day.

• That's interesting. Thanks for sharing ... Commented Jan 21, 2021 at 17:23

The main issue with biplanes is the separation between the planes, in relation to the wing chord. They need to be well separated to avoid interference and hence inefficiency. Anhedral lower plus dihedral upper increases the separation towards the tips, making them more efficient. Another way of looking at it is that the centre sections are bunched close together, making them inefficient. But the tips already suffer greater inefficiencies due to air spilling round the ends, with the centre sections providing the main lift. So what you are actually doing is making the whole wing inefficient. This is why many biplanes raise the upper centre section well above the fuselage on cabane struts.

As far as dihedral goes, the two angles balance each other out. If you want a stable plane you will need to increase the dihedral over the anhedral.

At extreme angles, say 45 deg, the wing offers significant side area. This configuration is in fact common on high-speed air-to-air missiles, in conjunction with a cruciform tail, as the control surfaces can now generate side forces and this can greatly improve manoeuvrability.

• Rockets use cruciform wings because they have no clear up and down - no need to roll makes course changes more immediate. Banked wings create side forces, too, but take a while to reach the proper bank angle. Commented Jan 20, 2021 at 16:37
• Yes, with rockets it amounts to the same thing. Whether the cross is vertical/horizontal or 90 deg or 45 deg makes little aerodynamic difference, but angling the wings rather than the tail makes it easier to mount the rocket to a hardpoint. Commented Jan 20, 2021 at 17:03
• Brilliant answer, Guy. Thanks for sharing. Commented Jan 21, 2021 at 17:30