Using Profili for Airfoils??

[mb571]

I am trying my hand at cutting eps foam wings. I am trying to scratch build a high wing aileron trainer. Can anyone recommend an airfoil? What is the dotted lines on the airfoils printed out on Profili??

Mark

[raptor22]

Virtually any flat bottom or semi-symmetrical airfoil used in modeling.

I'll start with sugggesting the trusty ole Clark Y. Its simple, proven, and not hypersensitive (a tiny cutting error will not screw everything up). It also providas alot of lifting capability at moderate drag.

--Alex

[mb571]

Thanks. I'll look that one up. Now what is the dotted line on the airfoil printout? It may be called the chord line. Is it the angle to keep parallel with the rear stabilizer?

Mark

[Mike Taylor]

Mark,

The 'dotted line' is the chord or reference line. It runs from the LE to the TE, and yes, that is used to determine the tail's angle. The tail is usually set 1 or more degree less than the reference line to provide stability in the airplane. The reference line is also often set a degree or more up from the Fuselage reference line.

(added)
The Profili database has a lot of great airfoils, all designed for different criteria. Some handy ones are the Horten for flying wings and deltas, the MA-409 is great for small to micro aerobatic models, the S8035 for slightly heavier, faster fully aerobatic models, the Clark 'Y' (as mentioned) for a general purpose forgiving parkflyer.

If you find an airfoil you like, you can click the 'show airfoils similar' button, and you can select from similar airfoils. If you want to see airfoils of some % camber at some % chord, use the 'filter by parameters' button and enter the numbers. A list of airfoils is selected.

[mb571]

Excellent info Mike. Help me clarify your answer to a newbie designer. Wing up 1 degree at the leading edge. Is that correct? Tail (horiz stab) 1 degree down on it's LE. Is that correct?
I am also still stuggling with foam types. I was going to cut a wing for this foamy out of what I believe to be EPS. Is that okay ? It is mint green in colour from Home Depot here in Canada. They also had some denser stuff - "Extruded Polystyrene Insulation" that is pink in colour. What do you think?

Mark

[Mike Taylor]

Well, then, Profili is just what you need for making the foam cutting templates. One tip to make you planes less likely to tip stall - add a degree or two of washout in the tip templates; you can cut the twist right in.

EPS is OK - as with all foam cutting - don't breathe the fumes. The pink foam is great stuff with a very fine grain. It cuts clean and makes a very smooth surface. On either, run tape across the top and bottom of the wing, spanwise, to act as a spar. There is blue foam (like the pink stuff) as well that works great.

Yes, the wing is positive, and the tail is neutral or negative. The relationship depends on the airfoil's characteristics (CP and CP travel) and what the planes designed for.

For a fully aerobatic model, the motor, wing and tail are all set at 0%. The plane will be as happy upside down as it is right side up, but you have to fly it constantly or it turns into a lawn dart with no tendency to right itself.

A sport plane with a lifting airfoil is set up with positive wing, negative tail, and a forward CG. This gives a stabilizing platform that can be trimmed to fly hands off. Typical setting are +2, 3 or 4 degrees for the wing, tail -1 to 0 degree, CG near 30%.

There is no one answer. It ultimately depends on the individual design. That's part of the fun in juggling the data around...

[mb571]

Great info again Mike. The cutting part I got down pat. Made the cutting bow already. That was easy being an electrician and having easy access to nichrome wire.
The new part is the designing.
What is washout? How do I accomplish this?

Mark

[Mike Taylor]

A wing that 'washs out' has a lower angle on the tip than at the root. The theory is that you want a stall to start at the center and work its way out. If you start with the tips a few degrees lower, that is pretty easy to do. CLick the 'building tabs management' from the interpolated ribs design screen. Add the building tabs and where it says 'tip angle of incidence', enter a negative number (like 2). The flat feet lay the templates out with the 2 degree twist drawn in. You woulldn't leave the tabs in place to cut the bottom surface of course, but you can use the bottom between the tabs as a cradle to get the ribs angled correctly.

[durone]

CLick the 'building tabs management' from the interpolated ribs design screen. Add the building tabs and where it says 'tip angle of incidence', enter a negative number (like 2). The flat feet lay the templates out with the 2 degree twist drawn in. You woulldn't leave the tabs in place to cut the bottom surface of course, but you can use the bottom between the tabs as a cradle to get the ribs angled correctly.

For Foam templates it's better to use the washout angle in template setting....

ciao
Stefano

[mb571]

Another question guys. It seems that the foam block is never perfectly straight. The first block I am working with bends a bit along the length. How can you compensate for this? I don't want secure the block for coring, then after it is cut, have the wing curve back to the natural bend of the block.

Mark

[Sparky Paul]

A common practice in carpentry is to "joint" one side of a crooked workpiece; figure out where and which way the twist is, and cut one side to be flat. and use that side to measure from.
You only need one side flat for a starting location. For a wing of course, this should be be the bottom side.
The shuck left after the cut becomes the master for holding the core and its sheeting, as it will be flat on the bottom.

[raptor22]

About the washout. If your wing is rectangular you don't need it. You only need it if you have a swept or highly tapered wing. It doesn't really hurt anything on a trainer, but will at unnecessary drag at higher speeds because the tips can end up at a negative angle. You can also prevent it by using a thicker/more cambered airfoil at the tips. THis is called "aerodynamic" washout.

The tail angle: for future refernece its called "incidence", and if you need or how much you need it depends on your airfoil and tail length. start with 1" but know you may have change it later or use up trim for level flight.

--Alex

[Ollie]

Washout design:
http://aero.stanford.edu/WingCalc.html

[Tony D.]

mb571, what you want to do is 'skin' the foam buy taking your hotwire and removing about .25" from the top and bottom of the foam sheet before you cut your cores. This will relax the foam and help reduce undesirable bending of the finished cores. Just attach straight edges at .25" lines marked on the sheet and pull the wire over them. You will get a smooth surfaced foam sheet that will lay flat on the board.

raptor22, "The tail angle: for future refernece its called "incidence", and if you need or how much you need it depends on your airfoil and tail length. start with 1" but know you may have change it later or use up trim for level flight."

Did you mean 1 degree?


T.D.

[raptor22]

Yep.

--Alex