i was looking at the great planes pt-40s features in Real flight and noticed that it has 3 degree washout on its wings whaich are rectangular. I thought washout was necessary only on tapered planforms. Is it needed on rectangular planforms as well. Regards

It should not be needed.
A rectangular wing which tip stalls has abuilding fault, like a twist or badly formed Leading Edge.
Adding washout to a properly made rectangular (constant chord ) wing is pointless. It just makes it even less efficient, and difficult to fly inverted.
I would suggest leaving out the washout, or building a better design.
Alasdair

I read on the beginner thread, that the Great Planes PT-40 has washout as well.

I get the feeling that GP suggests adding washout as "warp insurance". Even if the builder puts a warp in the wing, there will be some washout to compensate and keep things tame.

Washout can be used to simply -unload the tips-, reducing the bending stress at the wing root.

Several trainers recommend washout on their rectangular wings. Goldberg Eagle 2 is one. I believe since the structure doesnt respond that well to torsional loads the addition to washout can help compensate for that and maintain a lower or same incidence at the tip than the root.

Washout is a pretty good way to adjust spanwise lift distribution without sacrificing Reynolds number at the tips.

I'm trying to imagine the engineering meeting where Great Planes senior staff signed off on the PT-40's bending loads, torsional rigidity, spanwise lift distribution, washout, and elastic band strength. [X(]

From where I sit, I reckon that Shoe has the best hit yet.

Add to that idea the fact that washout also (in my book) prevents the whole wing from stalling at once, making it go "mushy" before falling out of the sky. I reckon that is a good reason for having washout on any wing for starters.

I was at the meeting, they had chili hotdogs and orange gatorade afterwards. [sm=bananahead.gif]

Washout can be used on any wing to help control tip stalling. Many things enter into the manner in which a wing stalls. The leading edge shape can be wrong or different along the length of the wing due to novice builders, the covering sag in the wing doesn't help and then there's just the issue that some pilots figure they can do anything they want by just cranking the sticks and the model should follow like an obiedient computer mouse pointer.

Which all goes to say that there's not right or wrong about using washout but it can be a good thing on any trainer model.

Yes, on a trainer or any other plane that's inteed for slow stable flight, washout can be a good thing, but when we move to more acrobatic planes, then forget about it. Have you ever seen a Fun fly with washout??? Even if it has a rectangular or tapered wing??

Wouldn't the span loading and area loading at the tip of a rectangular planform be low enough that it should not need washout. I would think the inboard section would separate before the tip. I tend to agree with Alasdair (not that he needs it).

The thing on a tapered wing that makes washout worth the effort is because of the loadings at the tip compared to the area, etc. compared to the inboard sections.

For good airfoils and moderatley skilled pilots I would agree that a constant chord wing would not need any washout. But for novice builders and pilots that may not shape the leading edge quite correctly enough or try odd control mixes at the sticks at low altitudes I can't help but think it's a good idea.... or at least a SAFE idea. Add to that the lovely Florshiem 10% airfoils that often look like they came straight off the insole of the designer's shoe and there may be more reason for it. An otherwise nasty stalling charactaristic may be softened by some washout so the wing doesn't just let go all at once.

At least that's my story and I'm sticking to it..... Especially after flying a semi scale ARF Piper J3 that did about 3 stalls that flipped into true spins that needed careful and lucky control inputs to recover from. This model had exactly 2 flights on it before being retired. I had the misfortune to be the pilot on both occasions. The first one I barely got it back on the ground in one piece after one spin and the landing was done at Mach 0.8 to ensure the wing wouldn't stall and spin again. Even then it DID try while I was flaring at about 1 foot of height and still moving quite fast. The second flight a couple of weeks later with the CG moved slightly forward and some washin added proved that it was slightly better but still nasty enough that it spun once from high altitude which I corrected and the second one got me on a shallow turn at about 100 feet. No time or room to do the push down, opposite rudder and then up slowly that it needed for the previous recovery. It was buried as the piece of junk that it was.

So don't tell me that constant chord wings don't tip stall.

Did you mean, washout instead of washin?

I have a semi scale J-3 that is about 25 years old. It is a little heavy and it sounds just like what you describe. I figured it was my luck with J-3 designs. As long as the speed is in the big fractions of Mach number range it seems to fly OK. But it always is making me wonder when it is going to bite. Landings are OK but I have to admit I am a very good flier!! Humble too!

Perhaps the rounded wing tips are not truly representative of what a square wing tip is gong to give?

OOPS, yes I did. After the first debacle I checked the model over VERY carefully. They had warped in about 3/16 of washOUT over about an 8 or 9 inch chord. It helped but only enough to ensure it was up high enough to ensure it's total destruction over TWO spins instead of just one like the first flight....

Ben, if your's is a ARF or ARC type of about 48 inch span I wonder if they are the same product.

I seriously doubt it's the wing tips. Cubs with marvelously gentle flying qualities are much more the rule than the exception. The old Goldberg 6 foot one was one lovely flying model that I would not hesitate to train a new pilot on.

I seriously suspect that a host of configuration mistakes makes a bad model even with a constant chord wing and that demon Cub had "The Right Stuff" to spoil your day big time.

But the point here was about constant chord wings with constant shaped airfoils and the desireability of washout to avoid tip stalling. I think that Cub shows that there's more to the soup recipe than just the wing shape and perhaps even the airfoil choice and that while some washout won't cure a model that's intent of commiting suicide it can help turn the tide on those with a marginal problem brought on by other factors. So I come back to the idea that a little washout for a trainer or other "upright flying" model is probably a good idea. It certainly won't hurt.

A rectangular wing with a constant airfoil section, accurately built will stall from the root outward, and shouldn’t need any wing washout. (but like BMatthews said, It won’t do any particular harm in a trainer.) However, understand that if wing stalls symmetrically, even if it stalls at the tips first, It won’t drop a wing, it will merely pitch forward. The problem comes when a wing stalls concurrent with yaw. In that case one side stalls while the other is still lifting and it rolls toward the stalled side. If the wing tends to stall from the center outward, this rolling moment is rather mild, but if it stalls first at the tips, the roll can be sudden and violent. The fact remains that the yaw is the basic cause of the typical “tip stall” incident.

The ultimate solution is to eliminate the yawing moment. Use of the rudder whenever the ailerons are deflected will help. Differential aileron motion will help although it is a little more effort to install. Coupling the rudder with the ailerons either mechanically or with mixing will help. All of these have been used on full scale aircraft with some success. If the student wants to advance to more aerobatic aircraft, the use of rudder is the only real solution. An airplane designed to reduce effects of tip stall will be difficult to snap roll. And washout becomes washin in inverted flight which makes the problem worse.

Lou, while I agree with you in theory that damn Cub stalled and spun during one of those 2 times in the second flight while flying dead straight but climbing at a shallow angle. I don't know if it was just pure 'ornery or if a turbulent gust simulated a yaw.

But hey, if we didn't have an exception to every rule then where would these discussion boards be, eh?