Forward swept wings

[Leighton]

okay guys, (orion, im looking your direction on this one) enlighten me.
Besides the obvious, (it just plain looks cool) why did they look into building aircraft with forward swept wings? what are the pro's and cons of it vs a normal planform??

Leighton.

[orion]

As with any configuration, there are pluses as well as minuses and as such, the application of forward sweep has to be carefully analyzed for any particualr use and/or airframe in order to determine whether it's really worth it.

The pluses are:

1) Most relatively highly loaded wings generate a certain amout of span-wise flow, especially at high angles of attack (actually, even Hershey-bar wings do this). This component of the flow, since it moves perpendicular to the chord, produces no lift and thus decreases the effectiveness of the wing. A slight forward sweep reduces or under limited circumstances, even eliminates this flow, thus making the wings somewhat more effective at high aoa (this is however very subjective - not a hard and set rule). A more moderate foward sweep can actually induce a root-bound flow, which can slightly reduce the induced component of the drag number.

2) The above discussion especially applies when discussing a wing with deployed flaps. By sweeping the wing forward, the inboard directed flow actually makes the flaps more effective, allowing the wing to achieve a higher lift coefficient before a stall occurs. This however is only true for the case when the flaps are down, not for an unflapped wing.

3) The primary reason for sweep is to increase the wing's critical Mach speed. The direction of the sweep is irrelevant as far as the Mach effects are concerned. However, when you sweep the wing aft, the resultant configuration has a substantially greater nose down pitching moment. While this results in a more stable airplane, it can also result in an airplane that suffers from more trim drag (and/or forces the designer to move the allowable CG envelope further aft). Sweeping the wing forward creates a more positive pitching moment, destabilizing the airplane. For normal uses this is considered a negative effect (although it does allow you to place the allowable CG envelope further forward) however for military aircraft, where maneuverability is often paramount, the forward sweep results in an airplane with superior maneuvering characteristics.

4) From a configurational standpoint, sweeping the wing forward allows the structural carry-through of the wing to be located further aft on the fusleage. So, if you're designing a business jet for instance, and you do not want to mount the wing below the fusealge tube structure or you do not want the spar penetrating the pressurized volume of the airframe, then you can sweep it forward, and place the structural members aft of the cabin, thus arriving at a cleaner structural layout.

On the negative side, the primary penalty is weight. In most conventional designs where the wing sweeps aft, when the airplane maneuvers at high Gs the wing loading and geometry twists the wing in such a way as to create washout (tip twists leading edge down). This reduces the angle of attack at the tip, thus reducing the bending moment on the wing, as well as somewhat reducing the chance of tip stall.

In a swept forward design just the opposite is true. If the twisting effects are not taken into careful account, the wing can suffer what we call structural divergence, and simply peel off from the airframe - not good. But even if it does not peel off, the increased wash-in could result in an upredictable tip stall, making high G maneuvers somewhat risky.

Assuming though that the structure is stiff enough to prevent the tip effects, the forward sweep has two other issues that might not be as good. First, the forward swept wing does tend to stall at a somewhat lower angle of attack, and second, it also does so at a lower lift coefficient.

As far as our airplanes are concerned (general aviation), there is really no reason to sweep the wings in either direction since the speeds at which we operate are nowhere near the critical Mach region. If, on the other hand, a designeer is trying to achieve a certain look, or needs to position the wing in such a way that the structure needs to be displaced in order to clear some other space or volume, then a slight amount of sweep should be OK as long as the structural design accounts for the effects.

[wally]

I have seen several biplane types, (my Pitts for one) that have the top wing swept back a little. After what you said just now, the only reason I can see for it is to align where the flying wires attach from the top spar to the fuselage.

Is that perhaps why Mr Pitts did the top wing sweep-back?
Wally

[orion]

Biplanes usually have the top wing swept back so that the center part of the wing can be a bit forward, thus allowing the pilot easier access in and out of the airplane.

I have also heard some time back that this sweep geometry has also had something to do with tailoring the stall and recovery characteristics of the airplane however, I've never come accross anything that would verify that.

[Canuck Bob]

I hope renewing old threads is acceptable.

Orion, may I ask what "root bound flow" is and how it might help reduce drag?

In a LSA type plane can you estimate a range of how the stall happens at a lower AOA? Is it a dramatic change? This is the first reference I've heard about this effect and it is interesting.

I am designing a plane using the Sisler Cygnet as the main design study.

[Rom]

I was wondering about how the stall characteristics would affect stibility due to the shift in the center of lift during a stall. If the forward swept wing stalls at root as would an unswept wing, the center of lift on the aircraft would move forward as the lift would move out towards the wing tips, thus the COG and the center of lift would diverge make the aircraft want to pull nose up during a stall. Would there be any validity to this?

[orion]

I hope renewing old threads is acceptable.

Orion, may I ask what "root bound flow" is and how it might help reduce drag?

In a LSA type plane can you estimate a range of how the stall happens at a lower AOA? Is it a dramatic change? This is the first reference I've heard about this effect and it is interesting.

I am designing a plane using the Sisler Cygnet as the main design study.

Darn - I just answered this and it disappeared somewhere. Try again:

All wings have a span-wise flow component associated with their performance, even ones with a rectangular planform. The span-wise component of the flow however does not contribute to the wing's lift characteristics and thus is penalizing. Couple that with the increased tendency for tip losses and you can quickly see that any mechanism that reduces the outboard span-wise flow would increase the wing's efficiency and performance. And although the effect is most prevalent with higher planform loads, even lower loadings that are after some level of optimization can benefit - example, the Czech Blanik glider.

When utilizing forward sweep you are in essence directing said flow inward rather than outward. This reduces tip losses and increases the chord-wise flow component, thus increasing the L/D.

As the forward sweep increases so does the effect, to the point where now you have inward flow. Here however the symmetry of the airplane and the presence of the fuselage prevent that flow from achieving too large an inbound flow vector and so you get sort of a compressing effect near the root. This will delay the stall and does aid the wing in achieving more lift during agile maneuvering.

Regarding the stall and an LSA, I'm not sure what you're asking.

[orion]

I was wondering about how the stall characteristics would affect stibility due to the shift in the center of lift during a stall. If the forward swept wing stalls at root as would an unswept wing, the center of lift on the aircraft would move forward as the lift would move out towards the wing tips, thus the COG and the center of lift would diverge make the aircraft want to pull nose up during a stall. Would there be any validity to this?

There might be some validity to this but I've never seen any data that singles out the phenomenon. In the case of aircraft like the X-29, the characteristic would most likely be accounted for by the flight computer so it's not anything you might clearly see, unless of course you're on the programming side.

And with the low levels of sweep we see in GA, the effect might not be dramatically significant - however off the top of my head I don't have a specific answer for you. But if doing a design like this, yes, it's something that might be worth investigating, or at least being a bit on the conservative side when assigning you margins.

[Rom]

And with the low levels of sweep we see in GA, the effect might not be dramatically significant...or at least being a bit on the conservative side when assigning you margins.

I sort of figured that the stall characteristics may not be that significant, but the calculated COG range might have to be reduced to allow for a shift in the lift geometry during a stall.

[DaveK]

There might be some validity to this but I've never seen any data that singles out the phenomenon. In the case of aircraft like the X-29, the characteristic would most likely be accounted for by the flight computer so it's not anything you might clearly see, unless of course you're on the programming side.

Not an exact example but the didn't the AD-1 oblique wing test aircraft have some funny handling characteristics because of the differences in lift between the forward swept side and the backward swept side? I remember that when stall was being approached it would start at the trailing tip and move forward toward the forward swept wing causing a roll to be introduced. There was also some pitch trim issues due to the shifting center of lift (for lack of a better way of saying that.)

[BBerson]

A forward swept wing will delay tip stall as does a rectangular planform. So a rectangular wing with a bit of forward sweep should have good spin resistance.

ref. NACA Report 927 excerpt in Light Airplane Design by L. Pazmany.
BB

[orion]

The oblique wing program was quite fascinating, especially in the aspect that it proved a theory that stipulated that this configuration would be more efficient in cruise since the flow will see the oblique wing as one swept, high aspect ratio planform rather than two, lower aspect ratio swept planforms as you might get with a more conventional layout. I attended one of the summary presentations at Moffet Field back then and if i recall right, there was a drag difference by about a factor of four. I remember they showed the math behind this but I don't recall the details.

But you're right, the wing that was swept forward did exhibit a higher level of effectiveness and thus did introduce a non-linear rolling moment into the trim profile. And if I recall right, I think they did mention that the effect became more pronounced at lower speed but it was not considered critical since at lower speed the wings would be unswept.

I wish I remembered more of the meeting though - keep in mind I'm trying to recall the details of the presentation from the mid 70's - since I was in high school at the time, some of the nuances were most likely lost on me back then.

[bmcj]

I wonder if helicopters have to deal with the same issues while they are in a high speed cruise? The retreating rotor blade's lower overall speed combined with a sweep back angle during a portion of its rotational arc?

OK, I know the second sentence was not really a question or syntactically even a sentence, but you get the idea.

Bruce

[Canuck Bob]

"Assuming though that the structure is stiff enough to prevent the tip effects, the forward sweep has two other issues that might not be as good. First, the forward swept wing does tend to stall at a somewhat lower angle of attack, and second, it also does so at a lower lift coefficient." from above


I am wondering about how noticeable or large is this lower stall AOA and lift coefficient reduction for a LSA type aircraft?

Thank you for this help.

Would you consider 6-7 degrees of forward sweep manageable in a small single seat 125 sq. ft., 28' span, 1000 lb. gross aircraft? It seems that 3-5 degrees is common but I can find little info regarding the range of sweep tat would be acceptable for a design study.

[orion]

The problem with this discussion is that above we have the X-29 and here we're talking about an LSA class airplane. While the forward sweep has similar effects for both classes of airplanes, not everything applies as equally. For instance, while there is evidence that the forward swept wing may stall at a lower aoa and at a lower cl, for lightly loaded wings the opposite may be true due to the stabilizing effect the inboard flow has on the boundary layer at the root. Simply said, in order to apply this type of technology, it's best to do a bunch of research prior to adopting it for your design. And then most importantly, make sure that the references you use are applicable to your project. After all, you don't want to use the same rules on your LSA that were used for the X-29. It took five flight computers to keep that thing in the air.