Wing Shape Effects

[Stoney74]

For any of  the tech experts:

What are the advantages of the different wing shapes?

For example, why is the leading edge of a P-47/P-40 straight with a trailing edge that is tapered?  Contrast that with a more "hershey bar" wing like that on a Corsair.  What advantage does the leading and trailing edge taper give the Yak?

[Charge]

AFAIK, the different wing "planforms" have most effect on those aircraft's drag but e.g. Corsair and Yak have such different shapes that those affect the slow speed handling too. More of that later.

 E.g. the Spitfire's planform is often referred as being an acrobatic planform but I think this an erroneous claim. The Spitfires leading edge is shaped to provide as even pressure distribution as possible along the wing to provide less drag which is again caused by the big wing. The even pressure distribution does have its negative effects too. Because the pressure distribution is even the lift is often removed off the whole wing when stalled. In case of Spit this is countered with a "warp" or "wash-out" but this affects the wing angle against the flow and is out of scope of your question.

The Yak has quite a small wing with most of it surface at the root and it also has some sweepback at the leading edge. The sweep-back in leading edge provides less drag in high speed and enables the use of thicker wing profile than would be possible with straight edge for certain mach number. From its wing design I'd say that Yak likes to fly and maneuver and rather high speed and its departure in loaded slow speed maybe somewhat sudden.

Corsair needs a large wing even at the tip because it needs to have good control at slow speeds and it thus cannot afford to be a tip staller. I'd say that because of that thick tipped wing it needs to have much angle in the wing to make it again stable in higher speeds so the option is to put the wings more angled upwards, but after certain point it starts to hamper the vision so one option is the structure seen in Corsair and Stuka with the "bend". The bend structure also makes the wing stronger -but again heavier.

I think that in Ju87 the bend is to counter the effects of straight leading edge in high speed dives. Why use the straight leading edge then? To have better control at slow speeds i.e. take-off and landing.

The less there is surface in tip of the wing and trailing edge the less the wingtip vortice hits the wing and the less it creates drag and (probably) eats lift.

Why all those planforms and wing profiles? Why not use some universal "überwing" (sry Spittydrivers, no pun intended ;-))?

Because the wing design is always a compromise.

But I'm not an aeroengineer so I don't really know for sure...

-C+

[Angus]

Since you mentioned the Elliptical planform, it is optimal for minimum of (lift) induced drag. But a total ellipse has a somewhat wide chord.
The difference between the induced drag of an ellipse and a square is AFAIK 10%. Not sure how the tip vortices also behave. But the difference is markedly less with a tapered wing vs an elliptical one. And tapered is easier to build.
There is at least one aerobatics aircraft with an elliptical wing on the market, just can't remember the type. Will have a lookie.
BTW, I thought the wing shape of the F4U was because of UC and Prop issues, and turned out to have some nice extra effects. Anyone?

[Stoney74]

With regard to the Corsair, I wasn't asking why it was bent.  I was asking why the leading and trailing edge stay roughly parallel almost all the way to the tip, i.e. "hershey bar".  Regardless, thanks for the posts...I look forward to some more.

[Saxman]

Correct, Angus. Any extra affects of the inverted-gull wing design were an after-thought and in SOME cases unexpected benefits. The reason they were bent was to give sufficient clearance for the shallow-bodied Corsair's prop (contrast with the Hellcat and P-47, which have very deep bellies allowing them to use a similar-sized propeller with a normal wing configuration).

[Stoney74]

Constant chord = opposite of tapered?  I.e. chord being the thickness between leading and trailing edge, correct?  

I suppose constant chord would have been a better description that "hershey bar".  I got that from the slang for Piper wings...

[Charge]

"With regard to the Corsair, I wasn't asking why it was bent. I was asking why the leading and trailing edge stay roughly parallel almost all the way to the tip, i.e. "hershey bar"."

"Corsair needs a large wing even at the tip because it needs to have good control at slow speeds and it thus cannot afford to be a tip staller. "



-C+

[Stoney74]

Sorry Charge--I got lost in the part about the wing bend and missed that part.  

Cheers,

[Mace2004]

As you can see from the discussion, wing shape (planform) is a complex series of tradeoffs between lift, drag, stall, speed and construction costs.  

A Spit has an elliptical wing designed to provide an elliptical lift distribution from wing root to tip.  Elliptical lift distribution means the wing contributes equally to lift all the way from the wing root to the tip but, most importantly, ellipitical lift distribution is the most efficient (with lowest drag due to lift).  The tradeoff is that it's very complex and expensive to build this wing.  The Jug has a semi-elliptical wing.  The straight leading edge makes it easier to mount guns and is easier to build while the elliptical trailing edge helps to keep the lift distribution elliptical.  Aero designers found that you can come very close to elliptical lift distribution without actually making the wing elliptical in shape.  The P-51 with straight leading and trailing edges also has a near ellipitical lift distribution because of the taper.  An F-4U has much less of an elliptical lift distribution but needs more total lift and a larger wing to for slow-speed performance while operating off a CV.

So the taper provides the elliptical lift distribution, why are some leading edges swept and others are not?  It depends on a lot but some key factors are dihedral effect and stall.  The greater the sweep the greater is dihedral effect (which is roll due to sideslip).  In simple terms a swept wing has a greater tendancy to roll in the same direction as rudder input and, for fighters, it's a useful way to improve roll rate by combining dihedral and ailerons for roll. Also, I just wanted to clairify this, the slight amount of sweep we're talking about here has almost no effect on drag since it's all subsonic flow we're talking about.  Highly swept wings on jets are a completely different issue.

Decisions regarding whether you sweep the leading or trailing edges (or both) depends on the total design and is very complex.  But for just one example, let's say you sweep just the trailing edge forward.  You get no additional contribution to dihedral effect but if your wingtip stalls the wing will tend to pitch down reducing AOA and restoring unstalled flight.  If you sweep just the leading edge back you get more dihedral effect but if the tip stalls the wing will want to pitch up worsening the stall.  By adjusting the sweep of both the leading and trailing edge you can adjust both the stall characteristics and dihedral for the desired balance.

I don't mean to oversimplify this, there are literally thousands of considerations that are taken into account when building a wing but these are some of the most basic and important.

Mace

[Simaril]

Thanks for the discussions. You guys rock...I love reading this stuff!

[Angus]

Very nice Mace

Then there is chord and span, and the loading. I've always been looking into the wing shape effecting stall in banking. Do you have some words on that?

[Charge]

Further reading:

http://www.ae.uiuc.edu/m-selig/apaLinks.html

http://www.geocities.com/hlangebro/J22/Aerodynamics.htm

-C+

[hitech]

The greater the sweep the greater is dihedral effect

Mace can you briefly describe what causes this?

My guess is change in airfoil shape in relation to the air stream, I.E. cross section acrross the air flow?

[Mace2004]

Originally posted by hitech
Mace can you briefly describe what causes this?

My guess is change in airfoil shape in relation to the air stream, I.E. cross section acrross the air flow?

This is caused by simple geometry.  The effect is somewhat small for small sweeps but is easily demonstrated with highly swept wings.  Consider an airplane with wings swept back 45 degrees.  If you yaw the airplane to the left by 45 degrees the right wing "sees" all of the relative wind while the left wing "sees" none.  In effect, the right wing becomes both longer (with respect to the relative wind) and greater in cord while the left wing becomes almost non-existant.  The right wing develops lift, the left does not and so the airplane rolls left with the rudder input.   This is of course a gross exageration but the principal remains the same for small wing-sweep or side-slip angles.  Also, you also get the effect with a straight wing because of wing mounting (high, mid, or low), pressure on the side of the fuselage (related to mounting) and blanking of the wing due to the fuselage.

Then there is chord and span, and the loading. I've always been looking into the wing shape effecting stall in banking. Do you have some words on that?

I'm assuming your talking about a steady-state bank as opposed to rolling into or out of one so there are two major issues.  First of course is increased overall wing loading but the important aspect here is the nature of the stall, i.e., where does it originate and how does it propogate.  The other comments about taper ignore other aspects such as changes in the airfoil section, wing twist (washout) and structural weight.  

As Charge mentioned, a pure elliptical wing stalls all at one time which is not good.  It's preferable for the wing to stall at the root before the wingtip for a couple of reasons.  Because the tips provide a greater lever arm for roll a tip stall can result in a snap departure, also, tip stalls affect aileron effectiveness.

You see several different kinds of aerodynamic "fixes" to ensure the stall progresses from the root to the tip including mechanical devices (i.e., small tabs designed to "trip" the airflow on the inboard section of the wing), washout at the wingtips (to lower the angle of incidence, delay stall onset, and reduce structural loads), and changes in the airfoil section.  

Generally speaking designers want a lot of taper to provide lots of room inboard for landing gear, guns and fuel while still getting elliptical lift distribution and reduced cruise drag of a high aspect ratio...all this in a package that is not too heavy or complex to build.  A problem that you run into with a tapered wing is that the Cl is higher at the tip (because of the shorter cord) than inboard which means that for the same AOA the wingtip will reach Cl max and stall sooner, just the opposite of what you want so you have to counter this with washout.  The downside of washout is increased drag.  You can also change the airfoil from root to tip but that adds complexity.  Everything's a tradeoff so small changes in airfoil are combined with washout so that the combined effect gives you what you want with less penalties.

One of the more bizarre "fixes" for the stall problem was a 1950s USAF design (I think it was based on the F-84) that had a reverse taper where the wing had smaller cord inboard with long cord wingtips.

Mace

[gripen]

Originally posted by Mace2004

So the taper provides the elliptical lift distribution, why are some leading edges swept and others are not?  It depends on a lot but some key factors are dihedral effect and stall.  The greater the sweep the greater is dihedral effect (which is roll due to sideslip).  In simple terms a swept wing has a greater tendancy to roll in the same direction as rudder input and, for fighters, it's a useful way to improve roll rate by combining dihedral and ailerons for roll. Also, I just wanted to clairify this, the slight amount of sweep we're talking about here has almost no effect on drag since it's all subsonic flow we're talking about.  Highly swept wings on jets are a completely different issue.

Well, generally a bit of sweep (assuming no any sideslip) increases airflow in the tips due to vortices along the wing span, increasing the stall speed of the tip. So  basicly the sweep has the same effect as the washout decreasing the tendency of the tip stall. But in practice very few WWII planes used this trick due to losses in drag (at subsonic speeds), infact in most cases sweep seem to have been added due to structural reasons (balance point etc.).

Otherwise I have noticed the dihedral effect of the sweep in some RC-planes; often it's possible to do well coordinated turns with rudder alone despite wing has no any dihedral, just some amount of sweep.

gripen