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new to wash out

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Andrew Cousins31/03/2020 12:49:09
168 forum posts
20 photos

Hello all i need a little help. I have not knowingly built a wing with washout. I am about to embark on a build that requires some wash out in the wing. I have done a little research and just wanted to see if the experts here can confirm if i am correct or not.

OK the plan calls for 1.5deg of washout. So i have measured from the route wing rib to the tip. Now using trig i should be able to work out how much i have to lift the T/E of the wing tip rib. So if my wing length is say 95 cm (adjacent) and my angle is 1.5 deg using tan 1.5 X 95cm should give me 2.48 cm. Is this the correct amount i have the lift the T/E of the wing tip rib? It seems a lot!!

I have another question but wanted to see if i have this correct before i ask the next question.

Many thanks in advance.

Martin Gay31/03/2020 13:06:22
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381 forum posts
254 photos

Hi Andrew,

That does not seem correct. The washout is a twist at the wing tip where the trailing edge is lifted.

The 1.5 degrees will be in relation to the wing tip chord NOT the span of the wing.

I will leave you to your trigonometry, but would expect the washout to only be a few millimetres (3-5mm at most).

Martin.

jrman31/03/2020 13:09:35
374 forum posts
3 photos

No. What you have calculated is 1.5deg dihedral. You should use the tip CHORD to calc the washout.

Andrew Cousins31/03/2020 13:23:36
168 forum posts
20 photos

Ok looks like i have made bo bo. That does make more sens Told you all i haven't done this before.

Ok so with a tapered wing i am going to have to measure the root from L/E to T/E then do my trig calculation. I will then have to raise the T/E of each wing rib by the result. Would i be correct in saying that as i get close to the WING root the rib will get bigger so the amount i have to raise the T/E will increase????

Or is this just done for the tip rib??

Martin Gay31/03/2020 13:34:43
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381 forum posts
254 photos

Andrew,

Measure the tip rib chord.

Do your trig calculations on that measurement. This will tell you how much you need to lift the tip trailing edge.

Then fit a tapered strip under the trailing edge. The tip end will be the calculated figure and the root end will be zero height.

This will give a gradual change in washout along the span of the wing.

Check the plan carefully as sometimes washout starts part way along the span. This point would then be the zero height of your tapered strip.

Martin.

Edited By Martin Gay on 31/03/2020 13:35:46

flight131/03/2020 13:36:16
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721 forum posts
36 photos

because many have explaiined this before me and better here is a link to read to explain it and how to with digrams from vaily aviation this one

hope it hepls

Peter Miller31/03/2020 13:59:31
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10946 forum posts
1272 photos
10 articles

The simple way that I always use.

Take a piece of wood and measure the 1 1/2 degrees on it and cut it out.

Wedge that under the tip rib with the LE touching the board.

If you do not have a protractor 11/2 degrees is about 1/4" at 10 inches

Andrew Cousins31/03/2020 15:49:23
168 forum posts
20 photos

Ok the fog is clearing. I know wear i went wrong. Its the root of the rib tip not the wing length i should be measuring.

Flight1 Many thanks for the link most helpful.

Ok Martin you are correct the wash out is only on the outer half of the wing (rib 7 to 15).

Now as the T/E of the ribs do not touch the building board i was thinking of making a tapered strip as you say Martin from the calculated height (at rib 15) to zero. To go from rib 15 to rib 7( rib 7 being zero). Then wear the rib meets the building board i will place the tapered strip. Lifting the ribs to place the strip under.

would the following be the best way to build the wing and lock in the washout.

1. fit bottom spar to plan.

2. fit tapered (wash out ) strip to plan wear ribs meet the building board. Then fit ribs and glue to bottom spar.

3. fit top and rear spar then false leading edge.

4. Fit L/E and T/E sheeting to top of wing. Will this then lock in the wash out?

sorry for all the questions bit nervous as i have not done wash out before and its a 1/4 scale. My first adventure into large scale.

many thanks

Andrew Cousins31/03/2020 15:58:34
168 forum posts
20 photos

Martin like your idea to. I take it its just the tip rib you do this to? How do you lock in the wash out? I was thinking of using the method in my earlier post. Whats your thoughts.

Many Thanks

flight131/03/2020 16:36:05
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721 forum posts
36 photos

What 1/4 scale plane is it and what plan is being used could lead to more useful info.

The tapered strip placed under the trailing edge of the ribs from the wing tip rib to where you want the twist to start, and with the bottom spare flat on the board is the most common method.

Don't forget not to glue the tapered bit of wood to the wing in the build process

Peter Jenkins01/04/2020 00:25:07
1501 forum posts
242 photos

Hmm - the link provided by Flight1 has the following text which is plain wrong I'm afraid

"Aerobatic models are aerobatic by the very fact that they are designed to be inherently unstable or right on the “hairy edge”. Hence, any means of establishing inherent stability is generally not used so it is not uncommon to see aerobatic models not use washout. Scale models on the other hand have a great desire to want to be stable so that the pilot in command stays calmly in command."

First, washout has nothing to do with stability. Stability is governed by the position of the CG. The effect of washout is to ensure that the wing root stalls before the tip thus avoiding a sudden wing drop and potential spin when flying close to the stall. A perfectly stable scale aircraft can have a vicious stall if it has highly tapered wings without washout but that has nothing to do with stability. The reason aerobatic aircraft do not have washout is that you spend 50% of your time inverted and then washout becomes wash in! You want the aircraft to perform symmetrically so you don't use washout. Today's aerobatic models do not have dihedral as again it becomes anhedral when inverted and affects the aircraft's yaw stability either way.

Second, aerobatic models are not inherently unstable. As far as traditional or precision aerobatics are concerned, the aircraft are always stable. Many pilots though tend to have too much movement on the controls as they believe that for an aircraft to be aerobatic you need large throws. That makes the aircraft twitchy but not unstable. Low control throws transform a twitchy, but stable, aircraft into a very pleasant flying machine that flies on rails.

I have buddied beginners to model flying using a 2 mtr competition aerobatic airframe and they manage just as well, if not better, than when flying a trainer.

Sorry, but we must correct gross errors when they are made and this is a gross error!

SIMON CRAGG01/04/2020 06:58:21
556 forum posts
5 photos

Have you tried an incidence meter?

I have used one for many years, and it is one of my most valuable aids.

Even if you build a model very carefully (even a foamie), wing twist can very easily creep in.

I have straightened, and also built in wash out on a variety of models from a fast EDF Hawk, to various pattern ships and several scale models.

I would advise getting one and giving it a try...........good bit of kit.

Good luck!.

Andrew Cousins01/04/2020 07:55:26
168 forum posts
20 photos

Hi All and thanks for the input.

Flight1. The plane is a 1/4 scale L19 Bird dog.

Peter Jenkins. Point taken but do you agree with the information on how to calculate and build in the wash out that is present in this document.

Simon Cragg. No i don't have an incident meter but it is on my shopping list as i intend on building a lot more.

Thanks for your input much appreciated.

flight101/04/2020 09:48:27
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721 forum posts
36 photos

The P J post is only picking out one paragraph of the the text the rest is right enough for our doings.

One thing to point out about that paragraph some planes are built unstable to make the more aerobatic/movable take for instance the euro fighter is so unstable a computer flight control is required to fly it.

hay ho lets not get distracted, this is about wash out and how to achieve it in the build. The main point of washout is stop the whole stalling at the same time. if the inner part of the wing stalls first you give some notice of it without the dramatic effect if it all stalled in one go. the outer wing has far more 'leverage'.

Peter Jenkins01/04/2020 18:55:15
1501 forum posts
242 photos
Posted by flight1 on 01/04/2020 09:48:27:

The P J post is only picking out one paragraph of the the text the rest is right enough for our doings.

One thing to point out about that paragraph some planes are built unstable to make the more aerobatic/movable take for instance the euro fighter is so unstable a computer flight control is required to fly it.

Flight1 is correct. It was just that para.

Only aircraft with a quadruplex flight control system are built to be unstable like the Eurofighter and all other 5th Gen fighter aircraft. We need not detain ourselves with discussion of such esoteric machinery here though.

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