GreenAce92

I was told that it is important to either stall the wing tips first (in a turn) or not to stall I can't remember which one.

The idea though is having the washout get over extended, so that the wing tips are down facing (if that makes sense).

Here is an image of what I'm trying to describe.

I'm not sure if I drew it right, but basically it's like over "feathering"

rcers

You want the wing tips to stall last so the plane won't snap as easily. A small amount is all that is needed or desired.

I can't make heads or tails of your drawing sorry. You don't want to over do the washout as that starts to create other issues (especially in aerobatics and inverted flight).

Mike

NJSwede

Doesn't that depend on what flying characteristics you're after? If you want to do crazy snap rolls and stuff, you may want the tips to stall first. Also, if you're after insane speed, you may sacrifice some low speed stability in order to reduce the tip vortices. But if you want a stable plane that's easy to land and that flies well at slow speeds, you definitely don't want the tips to stall first.

YMMV, I'm no expert and all other disclaimers apply...

rcers

My point exactly on some planes you want aggressive snap and spin characteristics. Aerobatic airplanes for sure. But.....you would want to be mighty careful with having the tips stall first. That would mean slow down too much and the wing snaps and spins.

I had a plane like that once. A snappy Cap. It had a short but spectacular life.

GreenAce92

that would explain the flat constant chord wings on 3d planes

sorry not chord but taper

yeah i apologize about the drawing

quorneng

GreenAce92
It is worth remembering that washout becomes 'washin' when flying inverted which will ensure the tip stalls first!

Washout does cause a slight reduction in the wings efficiency however you can use other aerodynamic effects to ensure that the wing root stalls before the tip.

The most obvious is to use a different wing section on the outer portions (ie one that stalls at a greater angle of attack) or by aerodynamic 'additions' such as leading edge slots or stall strips at the root.

JetPlaneFlyer

Washout is a useful trick in many ways but as others have said needs to be used with extreme caution if the model is intended to fly inverted.

BTW.. washout does not necessarily always reduce a wing's efficiency. On un-tapered wings washout can actually increase efficiency because it can be used to optimise the spanwise lift distribution to be closer the elliptical... Elliptical being the ideal for reduction in induced drag.

You can demonstrate this with the little application here: http://aero.stanford.edu/WingCalc.html

Put in an unswept wing with no taper (taper ratio 1) and you can achieve about 4% increase in efficiency by adding 2.5 -3 deg of washout. Wing efficiency factor is the 'e' value.

quorneng

How the ailerons perform can have a big influence on a plane's stall behaviour.
I shot this video of my Wing Dragon with separate flaps to demonstrate its speed range.
It has a tapered wing with no washout so might be expected to stall aggressively but it actually retains adequate roll control at very slow speeds with only a gentle and controllable wing drop if taken too slowly.

Note the planes angle of attack remains more or less the same when the flaps are down so the wing tips do not stall and the aileron remain effective albeit with progressively less authority as the airspeed reduces.

norcalwelder

Basically you only need to worry about washout if you have a wing plan form other than a rectangular wing. (tapered, delta, elliptical) An aircraft's stall characteristics are determined by its wing plan form, with some stalling at the root, (rectangular) some stalling at the tip (tapered, delta) and some stalling all at once (elliptical). If you don't want to use washout, you can force a stall at the root with stall strips and a few other devices.

simonrich

hi just a quick message for me inlaw.. he's building a fast slope soaring plane of a 5m wingspan..he's using the programme compufoil, with this cord and settings.... 30%>2.40"-> HN218PRO.COR/8"c/.0625"s/0.3"k/ .875"h/9.84%thk/2.33%cmb....... 10inch rootcore for the first metre, and from 10-5inch for a 1.5mtres thinned 25% on the outter pannel.. wht he wants to no is does he have to put wash out in.... or does he need the tip cord broader or not to get away without putting wash out in? any help is much appreciated, thanks Simon

quorneng

As a fast glider the performance will be marginally improved without wash out, providing the wing has sufficient torsional stiffness to hold its incidence accurately all along the wing when at speed.
With such a high aspect ratio the way the structure of the wing performs at various loads and speeds is likely to be just as important as the static wing geometry.
There is no easy answer.

F22trainer

If the plan calls for it - do it, someone already figured out the why. Just depends what you want the airplane to do. Low wing full size trainers have stall strips and washout, in addition to stronger dihedral than high wing. Cessna high wings have a a nice surprise for the student that is not coordinated when stalling the plane - Pipers on the other hand don't break as hard. The nose drops and you accelerate. Wing root stalls first while the tips maintain some control beacause the are at a lower AoA. Put washout in a swept wing design and bad things happen at high speeds. Good discussion!

solentlife

Wash out ? I never add more than the kit / model has. I believe that unless its deformed by shipping - that designer may have had an idea what he was doing when he set the wing.

On this subject - lets take the HK Lancaster. Out of box with no damage, the washout is hard to see, because its set by the aerofoil used in the outer section. But some people go to the lengths of 'steaming in' or even shaving off foam to create visible washout.
Does it add any advantage to the model ? Being human - we will always 'perceive' improvement and then claim its worked. But in all honesty - small models in full size air ? This modification to the HK Lanc is work for very little reward. But another model that may not have washout / twisted in shipment will benefit.

My 2c's worth ...

Nigel