crosbyjcrsby

Can somebody give me some much needed information? WASHOUT. What is it? What does it do? How much is to much? How much is not enough? And exactly what does it do for the air plane? I am speeking about gliders and sailplanes. Ant information on this subject will be appricated. Grizz.

rscarawa

Washout is used to help control the model during the initial stage of a stall. Sometimes planes drop a wing when they stall. Sometimes they just fall forward (desireable).

Washout is where the wing twists down as you go from the root to the tip. How much? That is hard to say. Anywhere between 0 and 2 degrees at most depending on how the model acts. If you do not have bad stall tendancies, I would leave it between 0 and 1 degrees.

evan-RCU

No, washout is the wing twisting up as it goes out to the tips, the tips have negative incidence compared to the root to prevent stall... as much as 5 degrees...

evan-RCU

Hmm... guess it depends on how you look at it... Still negative incidence compared to the root......

mocgp

A good way to understand the way washout works is to visualize the airplane at the upward angle that represents the stalling point. If the wing was nice and straight all the way out to the tips, the entire wing would be stalling at the same time. If your tips have a bit of washout, their angle of attack is just slightly less than the rest of the wing so the tips are still flying (producing lift) while the inner portion of the wing is at or near a stalled condition. This bit of lift at the tips tends to keep the airplane going straight instead of falling off to one side or the other.

If memory serves, the typical standard (100") sailplane will call for about 1/4 inch of washout in each tip. I tend to remember viewing washout as the trailing edge at the tip sticking up higher than the rest of the wing. That helps avoid the confusion of trying to figure out what reference you are using when you say the wing "twists."

rscarawa

It is a perspective issue. I think he should have the point by now.


You are also looking at washout from a powered plane perspective. Since this is a sailplane forum, have you seen a sailplane with 5 degrees?

rscarawa

I think washout also tends to be recommended on highly tapered wings and usually only on the last 20% or so of the wing (the tip part). Form the explainations I have received this is due to the shorter cord (tip) stalling at an earlier angle than the root cord. It is desireable to have the stall start at the root and propogate to the tip as you go deeper into the stall. To even out the stall and force the stall to start at the root, washout has to be great enough to keep the tips flying.

Consider this. You have a plane with no table and a totally true wing. Now washout or wash in. As you slow down and attempt to keep flying you increase elevator to increase the angle of attack. This works until you hit the critical angle. For many airfoils, this should be between 14 and 17 degrees. So you are continiously increasing the angle of attack to keep your altitude then, BAM, you stall out and the whole wing stops flying at once. I would expect this type of plane to fall forward when stalling but if it did not, a degree of washout should help.

Case two. A highly tapered wing with no washout. As before, you pull back on the elevator (or you increase the wing loading for the given speed) you are now at 13 degrees of attack. The outside of the wing is ready to stall, the inside of the wing may be two degrees away from stalling. You increase the angle one more degree. The tips stop flying but the root is still flying. This is very unbalanced and I would expect a tip to drop. WHich one? There are many factors that effect this and sometimes it comes down to wind. Maybe it has to do with the planes current orientation (flying slightly left or right), wing weights being different, ... Remember that in this case, the tips are not flying so your ailerons will not work. If you have to correct and keep flying, only a rudder will help. Otherwise, you can decrease the angle of attack to make the whole wing fly again.

Case three. A highly tapered wing with 2 degrees of washout. Similiar to before but now the root is at 13 degrees attack. The tips are now at 11 degrees. You pull back more and now the root is stalled but the tips are flying. You still have aileron control. If you keep her straight and level, I would think all you would have to do is drop the nose some. I would not expect it to drop a tip unless you were banging on the rudder or ailerons. Tip stall could still happen, just should be less likely.

That is my understanding of how it works. On highly tapered wings, there are other performance reasons for washout too such as equalizing lift between the root and tip. With no washout, the tips would actually provide more lift than the root. More lift equals more drag and decreased performance. The washout should make each square inch of the wing pull its fair share of the weight.

Here is a link to a pretty high performance sailplane. Mark Drela (designer) recommends .5 to 1 degree of washout.

http://www.charlesriverrc.org/articl...PDFs/bd_V3.pdf

I hope this is helpful.

michpittsman

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Some full-size aircraft mfgs don't use washout at all; they attach a short piece of aluminum angle to the leading edge near the root. I have seen military trainers at a local airshow with an 18 inch piece about 18 inches from the fuse. Apparently this enough to assure the root stalling before the tip. JIM

Aquila223

The angle produces airflow seperation ahead of the wing which then reduces the tipstalling of the aircraft.

crosbyjcrsby

Thanks for the information. I have a better understanding of washout. I'll give it a try and keep you informed. Grizz

jooNorway

Might just mention this in addition to rscarawa`s good explanation: When the wing is close to stalling the use of ailerons might provocate a brutal tipstall. If, sa he said you are close to stalling and add ailerons, the aileron which move downwards increase the angle of attack and the plane actually stalls the opposite way of what you expect when you add ailerons... The aileron which move upwards in fact give you washout and that wing decrease the AoA.

This principle is used when we fly. With ailerons at the outer half of the wings we often rise these when landing. It is called spoilerons, and ensures you don`t get tipstalls when flaring and landing. If you in addition have flaps which are set you get a wing with a lot of washout. This is called butterfly or crow-mix.

rscarawa

As JooNorway said, spoilerons work well for increasing stability when drag is needed.

As Michpittman said, adding what some call a 'Stall Strip' on the leading edge causes that part of the wing to stall first. I had a semi-aerobatic plane in the 80's that used these. It made landing a breeze and the spins were the most beautiful I have ever seen. I am not sure now well it would work on a sailplane but if it did work, it would be worth trying if you found you needed tons of washout. Performance wise, I wonder if using stall strips increases drag on the wing that would be noticable on your glide ratio?

I wonder why we do not see stall strips used anymore?

evan-RCU

Yes, 5 degrees on a long wing is not much... 5 degrees on a powered airplanes short (compared with cord) wing is too much. I was talking sailplanes...

kwmtrubrit

If you are building the wing and are doing the tip sections, place a piece of 1/4" scrap under the far outer tip of the trailing edge (TE). This means that the leading edge (LE) will be flat on the board, but the TE will slope upwards from the di/polyhedral to the tip. Basically, three corners of the wing panel will be in contact with the board and one won't. Construct that section making sure the 1/4" space is always there at the tip. If the wing is already constructed, tape the wing down with the same 1/4" under the far outer tip of the trailing edge.** Assuming you have a monocote type covering, there will be wrinkles caused by the twisting of the wing. Get your heat gun and work on the wing to remove the wrinkles. This "should" put in the desired washout. You may need to increase the amount of space at the tip to allow for the torque of the wing wanting to go back to its original shape. Hope this helps.

Keith




** If this is a dihedral wing, you will need to support the other panel of the wing. If it's a polyhedral, you will have three panels to support.

rscarawa

Not saying it would not work, but I would be interested in seeing a side by side comparison of the glide ratio between a non washed out wing and a 5 degree wing.

If you were flying at best l/d or faster, a normal non-washed wing would fly between 2 and 4 degrees of positive angle of attack. With 5 degrees washout, you would actually have a section of the wing that is not providing lift, but pushing down against the lifiting section. Then the lifting section would have to compensate by providing more lift and that equals more drag. It would be a flyable solution but I would thing the glide ratio would be terrible. It would be like flying with partial 'crow' which is used to induce drag.

Which sailplane recommends 5 degrees? There might be an odd example out there somewhere, but I bet they are hard to find.

Cheers!

rscarawa

Take a look at the 'Drela' post.

http://www.rcuniverse.com/forum/m_1120310/printable.htm

The point here is that any change you make to the wing has a positive and negative impact at the same time. Sure, you are making the plane more fogiving stall wise with mega washout. But how does that impact the planes performance on a high speed run? With powered planes, who cares unless you are racing!? In a sailplane, it is real important.

evan-RCU

Scot, do you realize how much 5 degrees is? That link has nothing that i saw in my quick look that talks about degrees. If you prop the TE up on a Gentle Lady that has about what a 4" cord at the tip by 1/4 like the link you come up with just under 4 degrees? If it's a 5" tip then 1/4" would be 3 deg.

Max lift would be a straight wing but stability would be poorer.

Heck, you're right I'm wrong, I'll stop designing/ building/ flying gliders now.... want to buy any?

rscarawa

Evan
I am merely pointing out the performance implications of excessive washout in a sailplane. When one fix is put in, another problem develops. The question is "Was this fix worth the cost of the new problem?". Performance tuning had never been about fixing everything. Any expert will tell you it is about moving the bottleneck from one location to another.

In the case of a gentle lady, that is probably going to be the most extreme case of washout needed due to its very light wing loading and floating characteristics. Very old school! This make sailplanes like the Spirits and Aspires look like high performance sailplanes. This is not to offend GL owners. They are just different paradigms of flight. On one end, you have the old GLs, on the other end you have planes like AVA, Supra, Pike, etc (non of which I can afford to own). Most planes today fall very much in between. If you intend to fly at min sink all the time, then the 5 degree washout would have an insiginicant penalty. When you have to fly l/d or 1.25l/d, do you expect the plane to fly the same?

Granted, I have never designed any planes before. Therefore, I reference the notes of an expert in the field. Dr. Drela, referenced above has a Phd in some aerospace field and is employed at MIT where one of his roles is to design airfoils for various applications. He is well known in the thermal duration community for his AG series of airfoils. I bet if you asked Dr Selig the same question, you would receive a response that is very similiar to Drela's.

Whether one understands how little 5 degrees is or not is almost irrelevant. Almost all of flight takes place between 1 and 15-17 degrees AoA ruling out aerobatics. That is a pretty thin margin and makes 5 degrees look like a pretty big deal.

I am not trying to be offensive in my posts. I just ask on what authority is 5 degrees needed in a sailplane? I cannot find any manuals where that much is recommended. Many of my low dollar machines say to have between 0 and 1 degree. 5 degrees is five times as much as the link I posted. It is how you look at it I suppose.

Kind of like filters. One say it is 98% efficeint, one say it is 99% efficient. Seems like not such a big deal until you realize that one is letting 1% of particals thru and the other lets 2%. That is a 2x difference.

--Note: For those researching this forum in the future.

The above is my opinion which is built from personal experience and references others' opinions. After all is said and done, they are just opinion. I believe Evan's solution is very workable. Feel free to implement the full washout. There should be no danger to your ship but do not venture too far down wind on windy days ( that should go for anyone with minimal experiance no matter what the washout though). The sailplane should be very stable in high bank turns and during slow flight when stalls are near or imminant.

Should you feel that the plane does not range between thermals well when the elevator is trimmed down, then try taking out the washout or atleast most of it. If you do not see an improvement in glide ratio or wind penetration and you have the tip stalling problem in the turns, you can always put the washout back in.

By the way Evan, you do have my permission to use 5 degrees, even in a sailplane.

-- End note

Take care.