Too Much Wash-Out?

[wannabe]

I built a little kit with a simple wing. I used the wrinkle-set method of adding wash out to the wings. I am not an expert a covering and it looks like I have shrunk the covering as much as it will go. It looks like I have too much wash out at the trailing edge wing tips and I cant really twist it back any without recovering the wings.

How much is too much and what will the flight performance be like with too much?

Any help is great.
Thanks,
-Todd

[seanreit]

Everything in Aviation is a compromise. In your case, speed for stall characteristics. Balanced properly, your model will fly with no issues. Go have fun!

[Brandano]

A straight rectangular wing with relatively low aspect ratio shouldn't need much washout, if it needs any at all. The lift distribution across the span is such that the center portion will always tend to stall before the tips, and therefore the stall should be straight and gentle. On tapered or elliptical wings, or on high aspect ratio wings, the washout is more important, since they tend to tip stall, while on highly swept wings washout is also used to add pitch stability and unload the stabilizer (if present!). it's really a question of "suck it and see".

[John O'Sullivan]

About 1/8" washout would be appropriate for your model and would as Brandano says not strictly be necessary. It would be mainly to ensure that there is no washin.
Excessive washout as you have will not be fatal, but will reduce the efficiency of the wing.

Even though you feel that you have shrunk the covering to its maximum does not mean that you cannot remove the washout. The covering is flexible enough when re-heated to allow twisting to reduce the washout.
John

[macboffin]

Provided you have equal washout each tip, no problem.You may have to fly with a small amount of UP trim dialled in however, since you have efectivly reduced the main wing incidence by about a quarter inch.

[Texas Buzzard]

When you used the term "washout" did you mean that the leading edge of the tip was higher than the trailing edge? Of did you mean the opposite w/ trailing edge was higher.

As a general rule I never twist my wings so that the tip is at a higher angle of attack, I'd rather have them at a slightly negative angle of attack. This prevent tip stalls which can result in bad things.

In that little bird I would try for a perfectly flat wing.

[wannabe]

Washout is trailing edge up.

Thanks to all for responses. I have been able to flatten out the wings with heat. Very challenging with a wing this small.

-Todd

[Texas Buzzard]

Way back when, I flew tow-line gliders and free flights. No radios used here.

Go look at a Cessna ( full scal - the real one) . Walk past the tip of the wing about 50 feet so you see the plane in sideways profile. Look at the wing. Notice how the tip has it's L.E. lower than the T.E. That is wash out. Cessna uses about 3 degrees of washout to make sure that the center part of the wing stalls before the tips. A tip stall is bad! It can kill you at low altitudes from ....hitting the grouns.

Sailplanes and gliders are made more stable like the Cessna is by using washout.

A non-radio plane is set to turn in one direction. If we make it turn to the left we would want the left wing tip to have a tiny bit of + incedence and the right tip would have a bit of washout.

Yes washout has its place in our models. But when we have total control via radio we can get along with a truely flat wing. But still I like a tad of washout in both wings for safety when taking off and landing. I dont like tip stalls.

[vintage1]

Washout can turn a rather snappy model into a docile one. I like it a lot.

I have two models....that are at opposite ends of the spectrum. On the 'bags of washout' is my vintage 'Black Magic'. This is the most docile plane I have. You can do tight turns at appalingly low speed and it NEVER drops a wing. The only damage on it is where I spapped a longeron putting te runbber bands on. :D

At similar span is my 'skimmer 400 ' glider..that was a complete beast with its long slender wing..it has very little rudder authority near the stall, like most sailplanes, and when turning it on landing, or indeed at any height, if it lost airspeed one wing or the other would drop into a snap tipstall, and lose you 20 feet in a blink. Not good if less than 20' altitude..I put a couple of degrees washout into it, and it fixed it pretty much. Now any modest stall just mushes out, it doesn't drop a tip.

Its not that you can't fly wthout washout - you can - but it demands constant attention at slow speeds. Stalls are not progressive, and usually result in wing drop, with longer thinner wings being much much worse...its far to easy to slow the inner wing under stall speed on a long wing.

On scale models I crank the ailerons up a tad, or arrange a tapering wing section to add 'effective' washout on a flat bottom wing.

On a short blocky wing like the one that started the thread, its probably not that critical. As long as its the same on boith wings, anything from none to rather a lot will not be a big issue.