Wing wash-out

rv9aviator · #1 01-11-2006, 07:02 PM

I know a lot production aircraft have wash-out built in to the wings to help the stall charactoristics. Why does Van's planes have no wash-out and don't seem to need it? After reading an article in Kitplanes it got me to thinking about my RV-9A wings and how we take great care to make sure they do not have any twist in them. I know the 9 is about as forgiving in a stall as you will find on any plane.

Thanks

1911pilot · #2 01-11-2006, 08:53 PM

I was wondering that myself after reading the cafe report. I'd never really thought about it before but I'm guessing its eaiser to build it straight than with a twist.

pierre smith · #3 01-12-2006, 04:11 AM

Tapered wings such as most Cessnas have a lousy, sharp stall without washout.
Stand in front of any Cessna single and look at the leading edges from a distance of twenty feet or so and the washout toward the tip is very visible. It's obviously there to induce the wing root to stall ahead of the tips/ailerons.
Van's "square" wing design is not only easier and cheaper to build, it's also much more forgiving thru the stall, so no washout is needed. A 'twisted' wing also creates additional drag.
Pierre

rv9aviator · #4 01-12-2006, 05:06 AM

So you are saying the constant chord wing design keeps the tip from stalling quicker because of not cutting down the surface area at the tip? Sounds reasonable.

Mel · #5 01-12-2006, 08:27 AM

That's correct. A tapered wing has less lift at the tip so the tip will stall at a slightly less angle of attack. Washout decreases the AOA of the tip thereby letting the wing root stall first.
Mel...DAR

Kevin Horton · #6 01-12-2006, 10:51 AM

Quote:

Originally Posted by rv9aviator

I know a lot production aircraft have wash-out built in to the wings to help the stall charactoristics. Why does Van's planes have no wash-out and don't seem to need it? After reading an article in Kitplanes it got me to thinking about my RV-9A wings and how we take great care to make sure they do not have any twist in them. I know the 9 is about as forgiving in a stall as you will find on any plane.

For aircraft that fly slower than the speed of sound, the air that is ahead of the wing is affected by the fact that the wing is coming. The air that hits the leading edge is already starting to move upwards by the time the leading edge gets to it. This has the effect of increasing the angle of attack. This upwards movement of the air ahead of the wing is called upwash.

Upwash ahead of the wing

There is less lift near the wing tips, as the high pressure air on the lower surface tends to spill over the wing tip. This creates the wing tip vortex, destroys some of the lift, and decreases the amount of upwash. The lower upwash near the wing tip decreases the local angle of attack.

If we have a constant chord wing, like our RVs, with no washout, there is more upwash near the wing root than there is near the wing tip. This causes the wing root to stall first, while the wing tips are still flying. This helps give good stall characteristics.

So, why don't all aircraft have wings like our RVs? Well, the higher local angle of attack on the inner part of the wing means that it has higher induced drag. It is possible to reduce the drag by having a tapered wing (an eliptical profile would be ideal). However, this type of wing tends of have an amount of upwash that varies less with spanwise location, so there is less of a tendency for the root to stall first. This leads to a need for washout, to ensure the root stalls first.

Stall progression vs wing planform