Wing accuracy

[James Humphreys 1]

Ok so I've dry fitted the wing together over the plan cnc cut kit ( but not pinned) how accurate does it need to be,

[Cuban8]

That's a very interesting question. The obvious answer is "as accurate as you can get it" but in reality much conspires to often have us wind up with a structure that's not quite to spec. The smaller the model then the more accurate one has to be - a couple of mm out on a tiny indoor model may well play havoc with its trim, but on a larger sports model it'll have virtually zero effect. Aerobatic models will need to be built very accurately if they're to give of their best and people go through agonies to get them right. Most sport modellers accept a model's faults and work around them after a bit of fettling, and some never even realise a model is wrong in the first place.

A lengthy discussion is quite possible on this subject.

Edited By Cuban8 on 14/11/2020 09:51:54

[Jon H]

Cuban is right. I always aim for as close as possible but i dont waste time measuring everything to make sure its mm perfect so dont loose sleep over it. As long as both wings are similar, you will be fine but pay attention to twists in the wing. This is far more important than an extra half inch of span.

The only thing that really matters beyond the wing is that the tail is square to the wing. you can do this by eye. I have measured up models in the past and got them spot on, only for the whole thing to look crooked when using MkI eyeball.

My method is to assemble the model, then stand about 8 feet away in front of it and sight down the fuselage by kneeling down. Normally you get to a point where one side of the tail 'touches' the wing but the other does not. I then adjust it so they match. standng up on a chair and looking down on the model is normally enough to get the tail straight.

[Geoff S]

Obviously as accurate as possible but nothing's perfect and a mm or so probably doesn't make much difference. I'm a bit rough and ready and the beautiful workmanship I see from some posters here is totally beyond my abilities but I keep glueling bits of wood together and trying.

I think of ,most importance is getting evrything square. I spend some time measuring from each wing tip to the back of the fuselage to get them equal before drilling and fitting the wing bolts. As Jon says it's amazing how small discrepancies can be judged by eye. Like him I eye the model from the front to get the tail plane parallel to the wing and tiny variations are easy to spot. I bought a cheap plastic 90/30/60 set square and cut the 90 degree angle away so that it will clear the fuselage. I can them place the short side on the tail plane and check that the fin's vertical by checking with the long side against the rudder post.

Wing twist (or lack of it) is the big killer but even that can be overcome (albeit crudely) by aileron correction to make the model flyable as a sport aircraft.

In all honesty I've probably never bilt a perfect wing (and I build twice as many because I like biplanes ) But most of my models fly OK and I don't think my relative failures have been due to innacuracy.

Geoff

[Peter Miller]

Warped wings are the thing to worry about.

These days I have a building sequence for the wings which guarentees no warps.

The fin must be in line with the centre line of the fuselage although rudder trim will correct this.

[Martin Harris - Moderator]

A friend built a DH71 with an obvious and visible twist in the wing. I added anti-twist aileron and test flew it for him. I still ran out of trim and had to add even more - ending up with something like 1/2" of deflection each side with the stick centred. Competition aerobatics might have been a problem but as a sport scale model it flew just fine.

What will always grate is a tailplane out of line or an asymmetrical wing. Measure carefully and then double check with the most important measurement device yet invented - the Mk 1 eyeball!

Edited By Martin Harris on 14/11/2020 11:43:16

[Cuban8]

I know a lot of people don't bother, but I do use gauges to check incidences and thrust angles. One home made and the other a Robart that I've had for decades. Also gives a check on warps and washout etc.

Providing cg is correct, very rarely have to do much in the way with tranny trims. I do use string and a tape measure to check for wing to fuz squareness, for say a 70 inch span sport or scale model I aim for no more than 5mm error and check during the build.

[James Humphreys 1]

Quick update

When the left wing was dry fitted together there was no twist that I remember (but that was last weekend) anyway glued the ribs to the central spar along with the leading and the trailing edges and guests what a twist has occurred 🤦‍♂️

So I've dry fitted the right wing ribs to the central spar along with the leading/trailing edges and there's a twist there also but not as severe.

The wings will have a thin balsa covering can this be used to take out the twist or is there a better way. HELP

 

 

Edited By James Humphreys 1 on 14/11/2020 12:34:07

[Martin Harris - Moderator]

I don't know what you're building but if it uses a D Box around the (front) spar and leading edge it will get the vast majority of its torsional strength from that. Provided that the sheeting is done with the structure flat, it should remain so once the wing is removed from the building board.

The fact that you have noticed a twist suggests that you haven't left the structure pinned down - what model are you building and how do the instructions suggest you build it?

[Peter Miller]

If you have wing with a D box leading edge with a warp in it it can be removed.

In the long distant past I had this problem a couple of times.

What you do is slit the leading edge sheet down its length with several scalpel slits along the grain.

This will allow the D box to be twisted. Hold it in the right shape and apply thin CA down the slits.

This will lock the wing in the correct shape.

[ken anderson.]

I made a ripmax extra slim with a canny warp in the wing.....it flew well and was the best prop hanger to date.....I was given an award at our club AGM for the wing..

ken anderson...ne..1..keep them straight dept.

[perttime]

Soooo,

which is more accurate: the CNC cut parts or the plan?

What plan and kit is it?

Edited By perttime on 14/11/2020 17:26:35

[James Humphreys 1]

Ok here's a couple of pics

[EarlyBird]

Is this the Sky 40?

I can not see any problem as the wings have not been sheeted.

[James Humphreys 1]

Yes it's the sky 40

[Phil Green]

I've seen maybe a hundred Vic Smeed 'Tomboy's -- not one had a straight wing!

[James Humphreys 1]

I think I'll try pining the 1.5mm balsa sheet on and see if it corrects the twist

[Martin Harris - Moderator]

Pinning won't really give you any idea of the finished rigidity but if you sheet it pinned to the board and let the glue dry properly there should be no twist. Looking at the pictures you should probably sheet the lower part of the D box (spar forward), then the complete top sheeting before removing the assembly, removing the jigging tabs and completing the lower sheeting.

However, the build manual looks very much step by step so go by that if it differs.

[Martin Harris - Moderator]

P.S. Just a thought, the use of jigging tabs may be to deliberately build in a twist - known as washout - which helps to prevent a sudden wing drop at the stall. They will have been designed to sit flat on the building board during the earlier stages of the build.

[Cuban8]

 

Complete build here -  might be useful.

Edited By Cuban8 on 15/11/2020 09:13:36

[EarlyBird]

Cuban8,

Wow that is excellent. A step by step guide, which is ideal for the absolute beginner and full of useful tips for the more experienced.

Cheers

Steve

[Geoff S]

They look to be an excellent set of videos. I particularly like the choice of music - Wagner's 'The Flying Dutchman' which seems very appropriate. Incidentally it's the only professionally performed opera I've witnessed - though a lot of good amateur ones.

Geoff

[James Humphreys 1]

Ok so after contemplating what to do I decided to carefully cut out some ribs and some of the leading edge and re-glue, which has thankfully worked although looks a bit messy, but it'll be covered up with balsa and covering film.

THANKS to everyone who's tried to help.

[John Stainforth]

As Dave Platt points out in his videos, the crucial stage is the gluing of the last piece of sheeting, because this locks the structure into its final position. The wing must be held rigidly in place while the glue dries. Applying adequate weights is more reliable than pinning.

[Geoff S]

Weights are certainly a big help. I have several totally useless 12v leakproof lead acid batteries I keep solely because of their mass. I alaso find using a steel building surface and magnetic blocks much superior to pins for most building.

Geoff