Wing Washout, any hard fact anywhere??

[Alan Cantwell 1131]

I made a comment on another forum about WW, i stated that the trailing edge should be propped up the required amount (what do you do about this, guess??) but the leading edge should be left straight, giving the wing the required washout, but not making it like a propellor, but, am i right? should the entire wing be twisted? it makes sense to me to have just the TE up toward the tip, it doesnt make sense to twist the entire wing, but would it matter if it was??????

[Dave Lowe]

Hi Alan
If I'm cutting a foam wing I always align the tip rib template to give me 2.5 degrees of washout, over the years this has resulted in a very stable flight pattern with little tendency to tip stall, this must mean the whole wing is twisted, So I guess it dosent matter.
Regards..........

[John Rickett]

My understanding is that washout results in the incidence of the tip being less than the root, the purpose being to to ensure that when the wing stalls, its progressive from root to tip. Having the tips stall last gives a more gentle stall and a less dramatic wing drop. Whether the trailing edge is curved upward or the leading edge downward or a bit of both, doesn't matter, just make sure both wings are the same. How much to build in also doesn't really matter. From an efficiency point of view, any washout is going to decrease the efficiency of the wing due to drag, but in the modelling world we hardly care about the slight increase in power required to overcome the drag. For our purposes 2 to 5 degrees seems to be about right - wings with a high aspect ratio and/or taper will benefit from a greater twist.
........now I'm sure someone will disagree with that!

[Chris Lane]

I'd just like to add that the point of having the root stall before the tip is to keep the aileron effective for as long as possible in the stall process. For our sort of aeroplanes the normal cruise incidence would be around 3 degrees and the stall occurs at about 16 degrees. This implies that any wing twist of more than 3 degrees would mean the tip is at negative incidence! But then have you seen a modern sailplane diving for the line? the tips are curved down with the load. . . . two degrees of washout should be plenty. WW2 multi's had parallel centre sections and the washout confined to the outer wings.

Finally if you need washout you should also consider differential aileron - little down but plenty up.

[Keith Mitchell]

This is a highly interesting thread. Many years ago when I was doing my Large Model Column for RCM&E I discussed it at some length in relation to large model design.

To cover it adequately requires a bit more than a posting. Dave P. has just asked me to submit something for the Journal. I was going to do something on my love of 'Multi's' but as this topic seems to be generating some interest, I think I'll rework my old articles on washout & wing sections (both need to be addressed together).
K.

[Phil Clark]

I've cut washout in foam wings where I raised the TE on one set and dropped the LE on another. Both wings worked fine, but I did note the ones where the LE was dropped did have a slight tendency to 'tuck under' in a turn (not sure if this was washout related, but the 'TE up' wing didn't do it) The other issues was that of appearance.....a TE up washout looks fine whereas the LE down can sometimes give the illusion of less dihedral or even anhedral on a model with little dihedral.

I've always gone with TE up since then.................

Phil

[Alan Cantwell 1131]

Gentlemen, some great replies to a question that many are not sure of, i have, as i have said, posted this on another forum, where i have recieved quite a few replies, but one in particular i thought i would share with you, for me, it seems to put the matter to bed,
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Alan, you know me - never one to be shy where a technical debate is concerned! Sorry not to have picked up on this but in my defence i have been busy

Anyway - to the subject of washout and your interesting question. I well remember your post on the Stuka thread and, with the greatest of respect, thought at the time that you were confusing what washout actually is, which is what FB3 asked and I answered, and the practicalities of how you achieve it!

In an theoretical aerodynamic context washout is any arrangement that results in the angle of attack progessively decreasing in a spanwise direction from the root to the tip. To be strickly accurate this should be achieved by the gradual rotation of the aerofoil section about its aerodynamic centre - a mathematically defined point which typically lies at about one third of the chord or so from the leading edge. So its neither the depression of the leading edge, or elevation of the trailing edge. Strickly speaking its both achieved as a result of a rotation! So, in your terms - yes, it is "like a propeller"!

The problem with this definition - which is the theoretically definative one - is that its extremely difficult for any modeller (no matter how skilled or knowledgable) to build a wing that complies with this. The problems are:

1. Depending on the plan profile of the leading and trailing edges, the path of the aerodynamic centre along the wing is not necessarily a straight line.

2. How exactly would we achieve the required rotation? Given that the spars we use are not twisted they tend to pull the wing straight. Mechanically how would we depress the leading edge and hold it there accurately whilst simultaineously elevating the trailing edge so as to get perfect rotation of every rib exactly about the point which is its aerodynamic centre and by exact correct amount? Its almost impossible!

So, how do full manufacturers do it? Well as a fellow engineer I'm sure you can guess. They have jigs in which they build the wing. The jigs have fixtures clamping the assembly and everything is carefully designed to ensure just the right rotation about the correct point is applied to each section. But of course even if we had the skill we don't generally have the resourses, in machines or money, to make such jigs. So we have to find a simplier way that approximates to what we want.

You don't have to think about it for very long to realise that anything that involves depressing the leading edge downward is a practical no-no. It would create all kinds of problems.

But, if you let the rib just sit on the building board, with its mean chord line approximately horizontal, it will contact the board at a point which is not a million miles from where the aerodynamic centre is. The aerodynamic centre, as I said, is about one third of the way across the chord, this is also approximately the point at which a semi-symetrical aerofoil is thickest and that is more or less where it will contact the building board surface. So if you now lift the trailing edge of that rib - obviously we are only talking small amounts - the contact point will move forwards, as the rib sort of "rolls" on the building board. But, and this is the important bit, it will only move forwards very slightly. So, if the normal point of contact, with the mean chord line nominally horizontal, is very close to the aerodynamic centre, then this small rotation by "rolling" will only very slightly alter things and the new point of contact will also be close to the aerodynamic centre.

And so, by just lifting the trailing edge and leaving the rib in contact with the building board, we have approximated rotation about the aerodynamic centre simply because the aerodynamic centre and the thickest part of the wing section are always very close together.

Of course this approach has a major advantage - in that its realively easy to do. You pin the leading edge flat and straight and pack under the trailing edge. This is achievable accurately without recourse to a jig.

So, your question: "Is washout packing under trailing edge?" Strickly speaking the answer is "no". But, trailing edge packing is a practical and very close approximation of it

[Andy Boylett]

I think that there may be a small error in that last write up? It was an excellent detail of washout and very helpful, but....

If the leading edge is pinned in place and then spacers added under the trailing edge, then the aerofoil is rotated about the leading adge and the ribs lift slightly off the building board.

Alternatively if weight is applied to the spar area of the ribs, so the ribs are held down at their contact point, then spacers are placed under the trailing edge, the ribs rotate about their contact point, which obviously stays in contact with the board. Then after fitting the spacers, the leading edge can be pinned in place.

A lot of kits started to adopt this second approach by building in tabs on the rear end of the ribs so that they sit on the building board with the correct amount of washout. For my half scale Spacewalker I used this approach......if you look at the drawings you can see the rear tab increasing in size from rib 2 to rib 13 ........

Spacewalker 2-Drg 1.jpg (34.49 KiB) Viewed 8455 times

Regards, Andy

[Mike Booth]

Good thread to which I can add only one thing as the subject is so well covered. If you are talking model warbirds after 1936, you will be enjoying landings for years longer with washout than without.