Why are the back of the wings of some aerobatic planes swept forward?

Question

I've seen that in some aerobatic planes like the Edge 540, the back of the wings are swept forward. Why is that? Does it help in maneuverability?

Photo from https://en.wikipedia.org/wiki/File:Edge_540_In_Flight_1.jpg

With the engine in the front those planes tend to be a bit front-heavy. By making the leading edge straight and trailing edge swept-forward, the center of lift could be moved forward a bit. — user3528438, Apr 13 '23 at 15:00
It's fascinating that there are a number of completely different thoughts on this matter, unusual for this site! Great question man !!!!!!! — Fattie, Apr 14 '23 at 16:46
@Fattie: as I wrote at the beginning of my answer, the aerospace world is made of compromises, so it is actually normal that different answers/points of view suit the same question — sophit, Apr 15 '23 at 12:09

score 10 · Answer 1 · answered Apr 12 '23 at 22:40

It is not so much that the trailing edge is swept forward as that the leading edge is held to zero sweep.

One way to approximate an elliptical lift distribution (without twisting a wing) is to taper the wing with a taper ratio between 0.4 and 0.6. The 540's taper ratio is about 0.46. Enough to improve the load distribution, but without being so aggressive as to make tip stall a problem.

Often general aviation wings (solidly subsonic) will have a straight quarter-chord. This allows them to have a straight spar and load carrythrough structure.

The 540 instead has a straight leading edge. This is most likely to give it a striking appearance from the ground during aerobatic displays. Maneuvers like vertical flight, 16-point rolls, and 16-point rolls during vertical flight will look better with a wing that meets the direction of flight at 90-degrees. This will also help aerobatic judges appreciate the precision of the maneuvers being executed.

Interesting point about "striking appearance for the judges" wonder if this is also the reason behind the straight trailing edge of Hirth's Acrostar — user721108, Apr 13 '23 at 09:25

sophit · Answer 2 · 2023-04-15T12:13:38.447

This is actually a very nice example of the compromises normally encountered in the aerospace world.

This being an aerobatic airplane, the wing should ideally deliver the same handling qualities both when flying upright and also when flying upside-down. This implies that all the standard techniques normally used to tailor the flight characteristics of the airplane cannot be used because they are normally not symmetrical when the wing is upside-down. For example, the wing should be vertically positioned in the middle, i.e. not on the bottom of the fuselage like in a jetliner (so called low-wing) nor on the top of the fuselage like in a cargo plane (high-wing). And already this single choice has a lot of consequences like the fact that the wing's structure just cut the fuselage in the middle and therefore nothing can be put there: see for example how the pilot seats behind the wing and the engine in front.

For the same reason, dihedral cannot be implemented: if you look at a jetliner's wing from the front, you'll notice that the wing goes up a bit, while in a cargo plane it goes down; dihedral is normally used to adjust the stability of the wing but, as before, in an acrobatic airplane if it would positively contribute to the stability when the airplane flies upright then it would be destabilising when flying upside-down.

Another way to optimise the aerodynamic characteristics is by using washout i.e. adjusting the local angle of incidence of each section of the wing. But, again, this method would have the same problem as before i.e. a washout optimised to fly upright would be detrimental when flying upside-down.

So in an aerobatic airplane if some in-/stability is needed, all that's left to do is sweeping the wing. In particular, sweeping the wing forward moves the lift at the tip a bit forward reducing stability, something which might be beneficial for this kind of airplane.

Note that this wing is not only swept forward but also tapered i.e. the chord at the tip is smaller than the chord at the root. This is done to optimise the spanwise lift distribution and generate less induced drag.

score 5 · Answer 3 · answered Apr 14 '23 at 14:48

The primary reason for forward swept wings in aerobatic aircraft is maneuverability and controllability. Wing sweep is measured at quarter chord. Trailing edge sweep is the result of wing sweep and taper. It is not normally thought of as a design driver. Other answers deal sufficiently with taper.

Forward-swept wings also are able to maneuver at high angles of attack. The Grumman X-29b was found to be able to maneuver and not stall up to 67 degrees angle of attack. Good maneuverability was maintained at 45 degrees angle of attack.

Robert DiGiovanni · Answer 4 · 2023-04-14T12:11:46.760

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why are the back of the wings of some aerobatic planes swept forward?

To optimize performance in high G loading and high angle of attack turning maneuvers.

This is an answer a pilot from 1918 might understand better than many of the higher Mach cruise-performance bred engineers of today.

Vortex formation is much more pronounced at high angles of attack, and tapering of the trailing edge is a well known technique of reducing drag, making the aircraft more responsive when power is added to accelerate$^1$.

Aerobatic aircraft also engage in many maneuvers with a high degree of side-slip, where yaw-roll coupling is undesirable. So one takes their weight saving$^2$, elliptical span loading tapered wing and yes, indeed, sweeps it forward a bit, eliminating the yaw-roll couple$^3$.

Aerobatic pilots want pitch, yaw, and roll effects to be separate and predictable, making advanced maneuvers, such as a knife-edge, 8 point roll, or inverted flight, much easier to fly.

$^1$ and providing more (T-D) excess thrust to climb

$^2$ also very helpful for aerobatics

$^3$ "mostly upright" planes, like the Cessna 172, can also do this by increasing area underneath the CG, making maneuvers, such as the forward slip, easier to fly. Some call this making it more "spirally unstable".

edited Apr 14 '23 at 12:11

answered Apr 14 '23 at 11:44

Robert DiGiovanni

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We can't see how the wing is mounted in the photo, but I assume it's probably mid-wing-- wouldn't the coupling between slip and roll be closer to zero with zero sweep? – quiet flyer Apr 15 '23 at 14:46
Consider also that any coupling due to sweep reverses sign whenever the wing is negatively loaded -- unlike coupling due to actual dihedral. I'm finding this especially aggravating when practicing slow rolling maneuvers, sustained knife edge, etc with a r.c. F-16 model with it's delta wing--I can really feel how the couple varies from significant positive when wing is + loaded, to near zero at knife edge, to significantly - when the wing is - loaded--I'd rather just have a constant (though preferably mild) coupling in one direction! (such as the case w/ my high-wing a/c with slight dihed) – quiet flyer Apr 15 '23 at 14:49
@quietflyer good points. The leading edge is straight. (It seems to be technically "forward swept" by connecting root and tip centres with a line). A swept leading edge may have more dihedral effect, as per F-16. And yes!, high wings are very good at reducing "dihedral effect" (in side slip), even though they have dihedralled wings, by virtue of their "keel" and wheel side areas. – Robert DiGiovanni Apr 15 '23 at 15:34
One can only imagine what a biplane/turboprop Edge 540 could do! (They seem to be catching on with Vstab cable supports). – Robert DiGiovanni Apr 15 '23 at 15:38

DKNguyen · Answer 5 · 2023-04-13T17:34:20.653

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It's the easiest planform to build after a constant chord planform. Both have the spar at the 1/4 chord position along the entire wing but the plank planform has more area and mass than is optimal at the tips.

Any additional modifications to approach the theoretically ideal eliptical planform would start to require things like curved edges or spars that do not always sit at the 1/4 chord.

edited Apr 13 '23 at 17:34

answered Apr 13 '23 at 13:41

DKNguyen

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Swept angle refers, by definition, to the line passing through the quarter chord, so that wing is actually swept forward. – sophit Apr 13 '23 at 14:16
@sophit Even with that definition it still looks like zero sweep to me. – DKNguyen Apr 13 '23 at 14:38
Try to connect the quarter chord point at the tip with that at the root and let me know if you get a line perpendicular to the fuselage – sophit Apr 13 '23 at 14:44
@sophit Sweep is most commonly defined in terms of either sweep of c/4 or LE -- but I've also seen it defined in terms of c/2. I wouldn't be so strict that the definition of sweep is only about c/4. – Rob McDonald Apr 13 '23 at 16:59
These other definitions are sweep are so strange to me since the objective of sweep is to present the leading edge differently to the air. Maybe it's just more convenient in drawings to use a chordline or something. – DKNguyen Apr 13 '23 at 17:33
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@RobMcDonald: science is not democratic, definitions are not chosen by show of hands. At subsonic speed, swept angle is given in respect to the quarter-chord point because that's the point where lift and pitching moment act, full stop. We can discuss if at supersonic speed it's better to choose the 50% of the chord (aerodynamic center) or the leading edge (to know if the wing lies inside the Mach cone). And at transonic speed we can discuss if it's better to use the point at the maximum thickness (after which the boundary layer might detach). But at subsonic speed definition is straightforward – sophit Apr 13 '23 at 18:04
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@sophit Good thing this is an Engineering question, not a Science one. Definitions and conventions are arbitrary and are tools to suit a need. You can believe that sweep is always in terms of c/4 -- but in the real world, I have observed countless times when engineers instead worked in terms of LE sweep. The Wright Brothers were set back a year because they did not catch the difference in the 0.5 term in 0.5rhoV^2 in data they received -- that mistake led them to build their own wind tunnel and take their own data (which probably advanced their knowledge more than what was lost). – Rob McDonald Apr 13 '23 at 19:50
@DKNguyen There are (at least) two purposes for conventions in things like sweep angle, taper ratio, aspect ratio (and other parameters). One is simplicity of drawing -- a design-first (whether drawing board or CAD) perspective. The other is related to collapsing data onto a trend -- a physics-first (often observed before it is understood) perspective. As sophit points out, there are times when different dominating physics (and your interest) will cause the data to correlate in different variables. – Rob McDonald Apr 13 '23 at 19:54
@RobMcDonald: the 0.5 comes out after several integrals and it is not a convention since it is mathematics. Leaving it out is a convention but a dangerous one as your example of the Wright brothers shows. That being said, at subsonic speeds the Edge 540 posses a wing which is swept forward, period – sophit Apr 13 '23 at 19:55
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Nobody ever worked with leading edge sweep. They worked with leading edge rake. For anything other than 1/4 cord, the terminology is rake. Talking about leading edge sweep will cause aero guys to walk out of the room. – Phil Sweet Apr 13 '23 at 20:17
@sophit I understand where the 0.5 comes from. My point is that using it or not was once convention. There are multiple conventions for Cp/Cq/Ct for propellers/rotors -- depending on which side of the Atlantic you are on and whether the rotating axis points up or forward most of the time. Choosing Sref as the area of a trapezoidal equivalent wing is just a convention. The sweep of a B737 (or any transport with a planform break) will likely be described by the LE. I can look at the 540 and tell you it has zero degrees of LE sweep. Can you look at it and tell me what the sweep angle is? – Rob McDonald Apr 13 '23 at 20:20
The tapered forward-swept wing will optimize with a larger wingtip span (here listed as 0.46 of root) compared to an unswept wing (about 0.4) or swept wing (less than 0.4). The aeorbatic plane has large high-gain aileron controls for quick roll accelerations and precise control. The large tip is more roomy for controls and provides better roll accelerations. – Phil Sweet Apr 13 '23 at 20:23
@PhilSweet I have only seen rake when talking about propellers. Lots of folks work with leading edge sweep. And trailing edge sweep. Look at any edge-aligned low observable aircraft. They certainly work in terms of LE and TE sweep all the time. I just took a flip through a DATCOM manual -- $\Lambda_{LE}$ shows up frequently there (I also saw some $\Lambda_{c/2}$)-- I guess aero guys never use DATCOM. Has anyone ever described a delta wing in terms of $\Lambda_{c/4}$? By far the most common convention I have seen is to always specify what x/c you're talking about when working with sweep. – Rob McDonald Apr 13 '23 at 20:31
@sophit I would not say that the circumference is $\pi R$ - that is wrong. However, there are people in the world who define $\tau=2*\pi$ and would prefer that we say $D=\tau R$ and that we celebrate $\tau$ day on 6/28 https://tauday.com/. Conventions are arbitrary. They are meant to be useful. Therefore, people choose whatever is useful at the time and you should always communicate your convention. – Rob McDonald Apr 13 '23 at 20:43
@sophit I agree that the c/4 of the Edge 540 is swept forward. That was never in question. However, if you wanted to communicate the sweep to me in a quantified manner -- Can you easily and precisely do that? I can say that $\Lambda_{LE}$ is zero with pretty high precision and confidence with just a glance. In terms of a design decision, do you think the Zivko engineers decided the wing should have a $\Lambda_{c/4}=-2.23417^\circ$? Or do you think they decided the wing should have a $\Lambda_{LE}=0^\circ$? – Rob McDonald Apr 13 '23 at 20:57
@RobMcDonald I've actually already answered that in my answer, i.e. they chosen a negative c/4 sweep angle based on aerodynamic/flight dynamic aspects. And they have used the c/4 to do the maths because that is the important point at subsonic speeds, any other geometrical point is just a consequence. I seriously doubt they used the leading edge to do aerodynamic (or even structural) calculations.. p.s. I'm going to clean up a bit the comments – sophit Apr 13 '23 at 21:10

Why are the back of the wings of some aerobatic planes swept forward?

5 Answers5