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Washout Questions (Read 1213 times)
Reply #13 - Sep 22nd, 2018 at 8:45am

alfakilo   Offline
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Me too on the math and aero issues!! What I don't need is a lot of 'book learnin' when I'm supposed to be having fun.

Now the big "yeah, but...". Too often in posts or in YouTube videos we are advised to add or change washout in the trimming process, and this is done often enough to lead a newbie to think that this is not only a big deal but a necessary one as well.

As one of these newbies, I have found that to be an example of the kind of help I am looking for. Unfortunately, not much has been said, at least so far as I have been able to find, about how to do this. The info is probably out there, here or elsewhere, but digging it out of the surrounding noise is a chore.

This particular discussion brings the subject of washout into clearer focus. Knowledgeable folks have the chance to give us specifics to make the building process easier and more effective. Thanks to everyone for their input!
 
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Reply #12 - Sep 21st, 2018 at 9:35pm

terryman   Offline
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terryman wrote on Sep 21st, 2018 at 9:33pm:
alfakilo wrote on Sep 21st, 2018 at 9:42am:
I was reading about the subject and this came to mind. If we see washout as a twist in the wing, then that twist will have an axis to rotate about.

I have been seeing this as the leading edge. I would pin down the leading edge and then elevate the trailing edge by some amount. How to determine how much is still unclear to me. In any case, I ended up with a wingtip that looked definitely twisted "up".

One of the pictures on Google showed what seemed to be a wing where the twist axis was aft of the leading edge. The result is a trailing edge that is not as far "up" while retaining the same angle of attack.

Should we be considering this when we build in washout?

I've attached the pic. Only the washout section is relevant. I interpret the little red dot on the chord line as the axis of rotation.

I doubt many modelers have considered the axis of rotation along the wings span for washout, it being sufficient to just get the washout in there.  I mean we add on trim tabs and Gurney flaps to flight trim without considering the effective position of axis of rotation they impart and btw those devices are usually mounted on the trailing edge.  For this reason and simplicity of jigging the wing I think most of us add shims under the trailing edge to impart washout while the tissue is drying.  If any have a dissenting opinions please cry out.

As to shimming I have used stacks of balsa and plywood shims of 1/32 and higher thicknesses to graduate the amount trailing edge is lifted from the board while drying.  Starting at no shim and increasing the stack height of shims until reaching the maximum at the tip.  The spacing of the shims I use has been sometimes every rib, every other rib, or a fractional distance from 0 (zero) to full stack at the wigs tip.

Where to start the twist?  I would use no shim at the third rib and increase the stack thickness linearly from the third rib to tip so the trailing edge does not look obviously bent.

Terry

 
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Reply #11 - Sep 21st, 2018 at 2:28pm

Sky9pilot   Offline
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As your illustration shows, the washout effectively changes the incidence along the leading edge of the wing.  You're getting way too technical for this modeler.  I'm sure Reynolds numbers etc. would begin to come into play in all this calculating....for me I tend to use the "TLAR" engineering method.  As long as I can get a glide and flight I'm looking for.  As I've said before English and Literature was my college focus...my brother was the mathematician!  Enjoy fellas and go for it!  I can still learn!!!  Teach me!
Tom
 

If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Reply #10 - Sep 21st, 2018 at 9:42am

alfakilo   Offline
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St Louis, MO

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I was reading about the subject and this came to mind. If we see washout as a twist in the wing, then that twist will have an axis to rotate about.

I have been seeing this as the leading edge. I would pin down the leading edge and then elevate the trailing edge by some amount. How to determine how much is still unclear to me. In any case, I ended up with a wingtip that looked definitely twisted "up".

One of the pictures on Google showed what seemed to be a wing where the twist axis was aft of the leading edge. The result is a trailing edge that is not as far "up" while retaining the same angle of attack.

Should we be considering this when we build in washout?

I've attached the pic. Only the washout section is relevant. I interpret the little red dot on the chord line as the axis of rotation.
 

1_twist.jpg (38 KB | 32 )
1_twist.jpg
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Reply #9 - Sep 21st, 2018 at 1:48am

Sky9pilot   Offline
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Affirmative.  From rib 3.
 

If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Reply #8 - Sep 20th, 2018 at 10:41pm

alfakilo   Offline
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OK, that makes it easier to build! One other thing, because of the gull configuration, won't I have to start the washout/twist from rib #3 (where the bend in the gull shape is located)?
 
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Reply #7 - Sep 20th, 2018 at 8:36pm

Sky9pilot   Offline
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Marlin is right...don't pin the trailing edge except at the root of the wing!!!! You want that subtle twist in the wing!  Rib six is just where the scrap balsa of the proper thickness is to start under the trailing edge to the tip.
Tom
 

If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Reply #6 - Sep 20th, 2018 at 10:53am

strat-o   Offline
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AK, I think the idea is not to put washout localized to the tip, I think the general approach is to give the overall wing a slight twist from root to tip.  So I would eliminate your green circle pinning location on the trailing edge and let the full span of the wing take the washout.  It looks like Tom is suggesting otherwise.  Not to say this approach won't work (of course it should) but haven't seen it done this way.  One thing to consider with Tom's approach is your trailing edge will curve upwards towards the tip when sighted from root to tip.

Marlin
 
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Reply #5 - Sep 20th, 2018 at 7:55am

alfakilo   Offline
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I'm at the point in the wing construction where I need to add the washout. As you suggested, I'll use a 3/32 spacer. First, I'll pin the leading and trailing edges (green) down at rib #6. I'll also have to keep the leading edge pinned down. Where to place the spacer isn't clear to me since its placement will change the amount of twist. The picture shows a location about #8 to the tip (purple). I'll put in the twist prior to covering. How does this look?
 

l46_twist_2.jpg (27 KB | 32 )
l46_twist_2.jpg
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Reply #4 - Sep 17th, 2018 at 7:05am

alfakilo   Offline
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I have read the same regarding the elliptical wing. I'll build the wing as you suggest and add the washout prior to covering.

Thanks for the tip (no pun intended)!!
 
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Reply #3 - Sep 16th, 2018 at 11:58pm

Sky9pilot   Offline
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with the elliptical planform of this wing you might want to increase the washout to 3/32" from the tip to rib #6 placed under the trailing edge.

From what I've read re: Spitfires they require much more washout with their elliptical wings.
Tom
 

If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Reply #2 - Sep 16th, 2018 at 9:04am

alfakilo   Offline
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St Louis, MO

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Thanks, Tom...I'm getting there!!

Given the 6.5" wing, I'll go with 1/16". In this pic, where do you recommend adding the washout?
 

l46_wing_twist.jpg (338 KB | 28 )
l46_wing_twist.jpg
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Reply #1 - Sep 15th, 2018 at 12:54pm

Sky9pilot   Offline
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Washout is mostly a Free Flight (always capitalized) technique for setting up stable flight characteristics.  It helps in keeping the tips from stalling before the root of the wing. 

Some build the the washout into the wing at construction of the wing.  This is done by placing a scrap of balsa at the recommended thickness (usually 1/16" for models up to 24-27" span) can be larger is the designer has specified.  Larger spans can use 1/8" or so. 

Other's will fix this washout at covering time as the tissue is shrunk on the framework.  This is done as the tissue is wetted and then pinned in place to dry.  You'll want to add strips of balsa to allow air to pass under the wing and then on top of these at the trailing edge you add the washout thickness from the tip to the end of the second bay of ribs or so. This will put a gradual twist in the wing giving the tip the washout desired.  I hope this was clear enough.

I believe Spitfires and Planes with elliptical wings require more than the usual washout due to the wing shape. 
Here's a pic of my Mig-15 with approximately 3/8" washout for the 27" span swept wings.  I used a clothes pin as you can see for the washout as the wing panels were built.
Tom
 

wing_washout_001.jpg (105 KB | 33 )
wing_washout_001.jpg

If God is your Co-pilot...switch seats...
Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Sep 15th, 2018 at 10:57am

alfakilo   Offline
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Retired USAF and TWA.
St Louis, MO

Posts: 1132
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I suppose we all understand the benefits of washout, but, speaking for myself, I'm unclear on exactly what to do. Here are my questions:

One, Assuming that the washout technique is to keep the airfoil the same while twisting or bending the wingtip up, where along the trailing edge is the bend made?

Two, how much is the trailing edge bent up? Is there a rule of thumb for this?

Are there pros and cons regarding when (before covering or after) the washout is made?
 
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