LE, TE sweep, taper and spar location

[dave siegler]

Low to you lay out a wing with LE sweep, TE sweep and keep the high point in a line for a spar?  Assume a simple airfoil like the NACA00x  Like 12% at the root and 16% at the tip.

there has got to be a simple way to do this.  This is for foam templates, and I want the high point to be in a line from tip to tip. If I just "french curve fit" I can make it work, but there must be a btter way. 

[Howard Rush]

Low to you, too.  It the LE and TE are straight, which they would be for hot-wire-cut foam, the "high point" will be a straight line.  For the NACA 00xx, it will be at 30% chord. 

[Tim Wescott]

It the LE and TE are straight, which they would be for hot-wire-cut foam, ...

That assumes more skill at hot-wire cutting than I possess...

[Brett Buck]

Low to you, too.  It the LE and TE are straight, which they would be for hot-wire-cut foam, the "high point" will be a straight line.  For the NACA 00xx, it will be at 30% chord. 

    I think he means it's supposed to be straight from tip to tip.

    Brett

[Howard Rush]

    I think he means it's supposed to be straight from tip to tip.

Yes, of course.  Sorry, Dave.  I made my combat wing spars separate and joined them in the middle. Are you picking the airfoils and solving for taper and sweep, or are you picking taper and sweep and solving for the root and tip "high point" location that gives a straight line, or something else? 

[Howard Rush]

Oh, I read your post yet again. You want the front view straight, too.  I'll post something when I get back from flying. Better yet, send me the particulars and I'll plot you some airfoils. 

[dave siegler]

yes I want to cut foam, and I have experience to do that, but setting up the templates so that the high point is on a line tip to tip is always hit and miss. 

Constant cord wings are easy.  Single template cutting gets me close.  I can go on a drafting table and and and draw a template, but I have this lofting program I want to use and with and a laser cutter guy to make templates.   


I want a spar at the high point, one piece.  I can taper the spar on the table saw span wise. 

I'd like get parametric relationship between span, sweep and airfoil thickness to get a straight spar. 

[Howard Rush]

Is this what you want?  Thickness in inches will be the same from root to tip.

[dave siegler]

first of all thanks. 

the math is stupid easy, and I should have been able to get there. 

mental block I guess.

I was expecting the thickness to taper by some ratio, but I can work that out.

I needed help with the spar location which you provided.

Will a constant thickness wing like that work well?

let me lay out a panel and see what that looks like.

Thanks you got me over the hump

[Serge_Krauss]

I guess I'm pretty dense, because I read the original post to mean simply, "How do you line up differing wing and tip templates so that the high points of a hot wired wing all line up in a straight line from tip to tip?" My answer would be...

1) First choose your taper ratio.
2) Choose chord lengths of the root and tip airfoils in that ratio.
3) Mark the chord lines for the root and tip ends of the foam slab identically.
4) Position the root section template, and measure the distance from the front of the foam block back to its high point.
5) Mark that position (distance back) on the tip edge of the foam.
6) Position the tip template with it's high point at that mark.
7) Cut the foam.

It doesn't matter what differing airfoil section shapes or thicknesses you use.

If I've misunderstood, I'm sorry.

SK

[dave siegler]

the problem is for the tip airfoil I could either get the sweep wanted or the high point (spar) in the right place. 

if you pick an NACA0012 for the root an the tip, and want a straight spar, your LE and TE sweep is fixed by where the high point is on the airfoil, and the LE and TE sweep are the same because the high point is 30% of the cord. 

Add more sweep to the TE and the high point has to move forward ( as a % of tip cord).  its not an NACA0012 anymore but now I know where the high point has to be.

[Serge_Krauss]

You've pretty well defined the situation, and I agree...

Lining up the high points of the airfoils does set the sweep. Sweep is 0^o, if the high points are at the same percent chord. Otherwise there's some sweep. As you say, if you want a more drastically swept wing, then your high points will be swept and you cannot align them without really changing airfoil shapes ('wing sections'). You can still have your spar go straight spanwise from tip to tip (or edge to edge), but as has been said, you'll have to taper the spar differently, and it might not serve its purpose as well. You can compute this, but if you're cutting foam, you could just measure thickness at the points where you want the spar to run. It won't be linear (straight taper). Most plans do show swept spars at high points for swept wings.

I see sweep as - usually - a compromise in order to proportion flaps or imitate full-sized aircraft in semi-scale. Otherwise, for a stunter, a nice straight, tapered wing seems best to me. I built one (flapless) once with just such a straight, tapered spar, and it flew very well.

So, it looks as though everything's pretty well decided. Best of luck in setting up your plane!

SK

[dave siegler]

I modified Howard's spreadsheet to do the calculation a little different.

now you can vary the max thickness and the high point of both root and tip airfoils and it calculates the leading edge and trailing edge setbacks to a straight spar at the high point of the airfoil.

it also calculate the taper of the spar root to tip.  Pick a minimum tip spar depth and it gets you the root spar depth.

it helped me, I hope it can help you.