Hi there,

Given a particular wing planform, and a particular airfoil, where do I locate the spar? The planform I'm trying to replicate is shown in the diagram, and the airfoil I would like to use is the Eppler 479. The red lines represent 25% of chord at the ribs.

I'm guessing the spar should be on the 25% chord line?

What if I want to use a straight spar (say, to facilitate the use of a carbon tube).

Thanks.

The wings I have seen have had the main spar located at the high point of the airfoil.

Merry Christmas. Ron

At the thickest point is much more common but it's not a rule. But if you try to put it back TOO far then the spar won't be at the point of maximum load on the wing and the wing will try to twist. But if you're using a tubular spar in the middle of the ribs it doesn't really matter where you put it within reason. Anywhere between about 25 and 35% would be acceptable. Bear in mind though that a carbon tube spar is not going to be as strong or rigid as a proper outer surface spar and webbing combination. Carbon is great stuff but it needs to be used in the correct manner to get the most out of it and a tubular spar that is much smaller than the wing thickness is not using the material to it's best.

Thanks for the input.

I'm planning for this is to be a built up Depron wing. 37" wingspan. I guess I'll go with 2 carbon tubes through 25%, and "foam sheer webs" around the tube to join the upper and lower surfaces of the wing. Should be okay?

I ran some number through this online cg calculator. and resulted in the CG position in the picture below.

The tubular spar would offer a degree of flexibility if that is important to you. A lighter and stiffer spar that would better control the structure would be an I beam made from a couple of 050 carbon rods with vertical grain balsa between them. I'd use 1/16 balsa and then wrap the whole mess with a double criss crossing wrap of kevlar or carbon thread to bind the whole lot together. The thread forming an XXXXXXX pattern on the webbing. The whole thing would then be painted with something to tie the kevlar thread to the spars.

The spar is preuly structural, and has no bearing on aerodynamics, so hgh4est point and deep4est spar is where it does teh most good.

Having said that, it ought to be near the center of pressure to minimise twisting on the wingtips.

If its to be straight you will have to just eyeball it. won't make a huge amount of difference.

That spar construction method sounds interesting, but I wouldn't want to go too overkill for a parkflier. ...or maybe I do.

Another question if I may. What's the generally accepted principle with regards to dihedral and differing rib heights. What I mean is, that the height of the ribs decrease from root to tip, so, do I centre all my ribs, or do I arrange them to form a flat wing top, but "dihedral" wing bottom? Please refer to my diagram.

Thanks.

Your choice. It's not that much dihedral to really matter. A pattern flier would notice if they are good enough so it may make a difference. But for a parkflyer I'd just do it whatever way is easiest. Technically your second sketch is going to be the most neutral flying option provided your wing is going to be mid mounted as shown. The upper one will tend to look less like the wing is drooping though.

Iso by the looks of that plane and wing platform I'd go back another inch fron the LE at the root for your CG.

Okay, I'll be going with my second drawing.

CG position noted.



I'm going to step off topic a bit and throw out another question. What are the effect of the wing and the stabilizer being positioned off the thrust line? I would rather not do things by the TLAR method, but I've been looking at a lot of glow widebody pattern type plane photos and it seems that most have the stab near or on the thrust line, with the wing below it. Are there some guidelines as to what the positions are?

There are but it mostly relates to how the model performs in verticals and knife edge. It's all very subtle and mysterious if you don't fly pattern. Remember those guys are looking for Precision with a capital P and if you just want to hack around and do 3D stuff and hover the reality is that you can get away with a lot more. Even 3D has rules but they are more related to ensureing that the profile area is properley distributed to simulate a wing in knife edge flight and that the controls are massive enough.

I probably wouldn't notice that little bit of extra precision, nor could I acheive it since I lack any kind of professional experience in airplane design. However, given the chance, I'd rather have something better (better in this case, a more precise airframe) and more optimized (optimized for better flying characteristics, and I happen to think pattern is "better" than 3D when it comes to flying, not hovering), rather than something that is merely adequate.