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Old Feb 26, 2007, 02:24 PM
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James Frolik's Avatar
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Washout on a Wing, especially a Polyhedral

I am building a Graupner Me-323 and had heard somewhat vague but negative slowflight and banking characteristics of this model due to having no washout in the design. So I am adding roughly 5mm washout in the outer polyhedral wing panels. (The panels are removeable.)

My major question: is it possible to have too much washout for safe flight?

Having finished both outer panels but only the right wing's aileron, with the aileron affixed it now appears the washout is possibly too much. That is, if the wing's upper surface remains safely profiled and continously sloping downward, the underside--primarily the aileron's chord--does not maintain washout and near the tip it returns to a flat angle of attack, or it even possesses a slightly extended flap-like profile. Similarly, if the washout continues unabated with respect to the aileron, the aileron's washout results in a slight upward deflection at the wing tip, something like a spoiler.

If both outer wing panels--left and right side--are exactly the same slope, slant, profile, etc., then what effect will this overdone washout have on this polyhedral wing?

It would be a real, real hassle to build completely new panels without washout. But, since I did make the outer wing panels removeable (for transport reasons), this would be possible, nonetheless.
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Old Feb 26, 2007, 07:35 PM
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Well..the most docile model I have has about 6mm washout at each tip of a 60" span.

All that will happen with excessive washout is that you will have a lot of extra incidence on the center part of the wing I guess...at slow speeds the stall will be a most progressive and slow affair..this particular model can be landed on a sixpence..you pull the stick back till it simply loses lift enough to land on its three wheels (tail dragger)..it never stalls and puts a wing or its nose down..it just sort of slows up and flops a bit!

At speed you will simply be operating at almost zero incidence on the wing tips..which probably helps keep induced drag and tip vortices down..so all in all I think that 5 degrees or more is no big deal.

The effect on low speed behaviour is dramatic tho. I have another model - very slender 60" span glider..and that was vicious at low speed till I twisted in about 3 degrees washout. Instant karma! suddenly it was predictable in the stall. No wing and nose drops..just a nodding and a slow nose down motion.
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Old Feb 26, 2007, 08:28 PM
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Quote:
Originally Posted by vintage1
.

All that will happen with excessive washout is that you will have a lot of extra incidence on the center part of the wing I guess...at slow speeds the stall will be a most progressive and slow affair..this particular model can be landed on a sixpence.. .
vintage1,

Is this what you are talking about...the arrowed line is the washout at the wing tip?

I have a small model with a lot of dihedral but still death spirals...it has no washout though.

Thanks

Bill
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Old Feb 26, 2007, 08:55 PM
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yes, but if you look at the picture there is more than your line shows.

Death spirals are more about insuffiicient fin area though..usually with a rather too rear CG.
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Old Feb 27, 2007, 07:25 AM
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Vintage1, nice of you to post here. I was hoping this science might draw your attention.

That model you pictured has a really nice wing—and I love it's washout! Just looking at yours I can imagine docile flying characteristics. And I love your flying field.

But, since my model's already got a polyhedral, and not a standard dihedral as your model has, would such extensive washout create any unexpected negative effect? I tend to think it would not, but I am no aerodynamic scientist and have been rudely surprised by modifications I made to other models in the past.

Additionally, here's another detail I forgot in my inital post. The washout begins after the spar and not at the wing's leading edge. Had I started the washout's incidence at the leading edge, I couldn't have made the outer wing panels removeable. In this case the washout would have intersected the spar's sliding joiner; there wouldn't have been enough wing thickness for the joiner.

My old Commander 680E had no washout and a rather large large dihedral (as all Robbe Commanders do). For added appearance, I created small 4cm wing tip extensions with a little winglet which also made the wing tip a little narrower. (The Me-323's wing tips are also very narrow compared to the wing's root.) But this really screwed things up: more than normal turns (banking) and the plane would snap into a stall; rolls were awkward and dangerous; it would always snap (or begin to snap) into a stall at the apex of a loop, regardless of the speed or loop's circumference—and once when being over-confident and daring, that's how I lost it. Death spiral. I'm sure if the model had had washout, it wouldn't have had such a bad tendency to stall.

Once I get the first Me-323 aileron sanded down and better to form, I will post a photo.
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Old Feb 28, 2007, 12:38 PM
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Polyhedral as opposed to dihedral doesn't make much difference to the effects of washout. If you look at profile viewa of the full sized Jodel you'll note that it has a polyhedral wing featuringa large amount of washout towards the tip. The reasoning behind this follows what Vintage1 states above; at cruise speed the tips fly at a relatively low AoA creating less lift than the centre section of the wing but also less induced drag, this aids the top speed of the aircraft. When the nose is raised the tip sections create lift and drag (although still less than the deep centre section) allowing a slower stall speed.

Your concerns about the wing section aft of the mainspar being the portion in which the washout is incorporated rather relates to another question;- that of a change in aerofoil section beyond the dihedral breaks. If the change in section is gradual and does not include and sharp points or ridges this should have the same effect as washout. I'm presuming you have merely raised the trailing edge a flat bottomed section towards the tips? If so then you're in effect creating a more symmetrical section at the tips which will create less lift during the cruise and thus less drag. In short, I don't think you need to worry about incorporationg too much washout!
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Old Feb 28, 2007, 05:20 PM
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Quote:
Originally Posted by Pete1978
I'm presuming you have merely raised the trailing edge a flat bottomed section towards the tips?
Yes, that is correct. The upper surface has no profile change, whereas the bottom surface—just aft of the spar—that was originally designed as a flat surface now encorporates washout in the outer polyhedral wing panels.

Your post is reassuring concerning the simplicity of what I'm trying to achieve. Although I am still a bit uncomfortable with the outer panels' chord (including the aileron). Because of the washout, as I stated earlier, in order to keep the upper surface's aft profile in a continuous downward slope—or in a normal profile—the aileron may deflect a little like a flap; in order to keep the bottom surface's profile with continuous upward washout, then the aileron may deflect a bit like a spoiler.

I will post some photos once I get everything sanded down to a smoother profile.
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Old Feb 28, 2007, 06:25 PM
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I did some rough work in Photoshop and created a depiction similar to what I am talking about in this thread. The larger profile is the inner rib where the wing panels join and the lower surface (with aileron) is completely flat. The two smaller profiles depict the wing tip, and each shows roughly what position the aileron has when the upper or bottom profile remains consistent.
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Old Feb 28, 2007, 07:07 PM
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Are they strip ailerons, then?

That's a pain..you want to rig them up a bit

In WWI they used to rig ailerons - outboard ones - permanently a little bit upwards..to create washout.

But then, a spin was an unrecoverable terminal life situation in those days.
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Old Feb 28, 2007, 10:01 PM
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Washout is still a valuable addition in polyhedral wings as well. Poly sailplanes typically use a few degrees in the tip panels to aid slow circling flight and prevent tip stalls and the resulting spiral dives. I've flown a model without it and then added it and it was a far nicer model to fly with the washout.

Your scale Gigant may technically have polyhedral but it's not very much. But any model of that style will look best flying slowly and that's where washout helps avoid troubles a lot. By all means include some. How much is the question. It's best to state the amount in degrees since that way it's not reliant on the size of the wing chord. Around 2 to 3 degrees is a good number in my experience.
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Old Mar 01, 2007, 06:41 AM
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The Gigant's center section dihedral is 4 degrees (2 degree slant on each side's center rib) and the outer wing panel's polyhedral is an additional 4 degrees (again, 2 degree slant on each abutting rib). This design is according to the Graupner plan.

My washout on each outer panel behind the spar is exactly 5mm at front of the aileron; at the aileron's trailing edge, the washout is either appr. 2.5mm more or less, depending on whether the upper or lower surface contour tapers continuously (as depicted in my diagram). I can't say exactly the "degree" of washout.

The aileron's are not strip ailerons (as such on the JU-52). They are a rather normal design and only extend the length of each outer wing panel. In the diagram I drew, I should have noted that with the neutral aileron in the large rib's depicted profile--which is where the outer panel butts against the wing's center section--the aileron at the wing tip would also be in the "neutral" centered position, neither deflecting like a flap or spoiler.
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Old Mar 01, 2007, 06:47 AM
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Well I think you will be fine..better than fine actually. If the ailerons are well outboard and it still wants to drop a wing, just rig the ailerons up a bit.
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Old Mar 01, 2007, 07:35 AM
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It also helps a bit with fighting adverse yaw, something that with large ailerons is usually an issue
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Last edited by Brandano; Mar 01, 2007 at 09:45 AM. Reason: spelling
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Old Mar 01, 2007, 07:38 AM
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Quote:
Originally Posted by Brandano
It also helps a bit with fighting adverse jaw, something that with large ailerons is usually an issue
Adverse Jaw? I though that as like a 'personal attack'
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Old Mar 01, 2007, 09:46 AM
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Oops, sorry, I meant yaw. I am not English mother tongue, and still make the odd mistake here and there. Pretty much all over the place actually.
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