Airfoil for a delta wing?

[GregG]

I'd like to do an own design fast delta winged plane, perhaps with canards. Would one of the newer pylon racing airfoils be a good choice?

[nmasters]

With deltas the airfoil section doesn't matter very much. Anything with a low pitching moment will do, even a flat plate. A symmetrical section will be a little more stabile at top speed.

[JetPlaneFlyer]

As the reply above... With a tailless delta you would best go with a airfoil with a low or better still positive pitching moment... perhaps a symmetrical one or one intended for tailless use.

If you add a canard (or tail) then your choice of airfoils would be wider because you could tolerate higher pitching moment.

Steve

[lincoln]

You might tolerate a higher pitching moment with a tail, but you wouldn't need it. The low aspect ratio means lots of drag if you go to high lift coefficient, so might as well keep camber low and keep drag down for high speed. I suspect a low camber pylon racing airfoil might be good. (I'm not quite sure how much camber they start with.)

I think either TWITT or the Sailplane Homebuilder's Association (which has a new name) offers a tape of Barnaby Wainfan talking about the Facetmobile, which has some interesting insights about delta wings, but not high speed ones.

[GregG]

The airfoil I've been thinking of is the MH54. It looks like there is very little chamber and unmodified it's only 8% thick.

Right now I have an old F15 with a 33" WS that needs a new wing as the original airfoil is fairly thick and has what looks like negative chamber. :rolleyes: I was thinking that this would be a great place to try the MH54. Maybe even thin it down a bit? What do you think?

[nmasters]

I think either TWITT or the Sailplane Homebuilder's Association (which has a new name) offers a tape of Barnaby Wainfan talking about the Facetmobile, which has some interesting insights about delta wings, but not high speed ones.

I've been a member of TWITT for most of this century :D so I have a few copies of the newsletter. They tape the guest speakers and advertise on the back page for a few years. It was even easier to find than I expected. I found the add in the first issue I pulled out of the box. Mr. Wainfan spoke at the September 1994 meeting and the editor's notes were published in October '94. Some of his speech didn't get into the newsletter so, if you're really interested in the technical details, you should order the tapes (if they're still available some older tapes got recycled).

SHA is now the Experimental Soaring Association (http://esoaring.com/) .

[nmasters]

Here's that newsletter with Barnaby Wainfan's presentation to TWITT. I left out several pages of regular business but it's still a pretty big file. If you're on dial up it may take 15 of twenty minutes to download.

[BMatthews]

It doesn't matter how little camber there is. If the airfoil camber line isn't a peak forward shape with a reverse S curve to the rear it won't be stable until you put in enough up trim to turn it into a reverse camber airfoil. And the more rearward that max camber point is the more extreme the S at the rear needs to be.

Frankly I'd say go with the symetrical option and build in two degrees of twist to the tips. On a delta with the twist being located back there it is behind the CG so it's adding stability and it's also helping to reduce to a slight extent the tip vortices by not being at as high an angle of attack.

[Tom Harper]

Don't agonize over the section. Bruce is right about washout.

Deltas are not efficient. Make them big, light and overpowered. Flat plate works fine unless you want a thick section to house components. In that case go symmetrical.

Start with the CG forward of the half chord point!

This model was 14 sq ft, ST .91, flat plate with tip wash out. Handled like an LT-40.

Tom

[GregG]

From that photo I've added a few lines. Does this represent the washout accurately?

[BMatthews]

That would be a nice amount of washout for a slower flying delta. But if you want to have a speed model then it gets a little more complex. In that case you need to figure out what the angle of attack would be at a very high speed and set the twist so that the tips are flying at around a zero to one degree negative angle. Otherwise you could end up with the tips generating a lot of drag if the twist is too much for the speeds.

Truthfully if this is supposed to be an extreme speed sort of model then thin and symetrical and no twist would be my suggestion.

[Tom Harper]

I agree. The model shown was an experiment in construction. I'll show the washout detail when I get time.

This model has too much washout - probably why it needed to be nose heavy and was such a nice flyer.

Tom

[Tom Harper]

I was trying to use Deltas for AP. I built 9 models with engines from .15 to .91 and areas from 4 to 14 sq ft. My conclusions are:

Deltas are great fliers - rugged, easy to build and they look good in the air.

Deltas are not efficient. The low aspect ratio increases drag in every situation except high speed level flight.

For reasonable performance 100 watts per pound is a minimum. 140 W is great.

50 to 100 watts per square foot will give good performance.

On a 14 sq ft model a wing loading or 10 to 12 ounces per square foot allowed easy hand launching. When scaled down to 7 sq ft that has to be 7 0r 8 oz per sq ft.

Since these were flat plat wings, washout was difficult to get without an ugly joint. The solution was to angle the joint so that it gave both dihedral and washout without drooping the leading edge. On the above model the angle was too great giving too much washout.

The number should be 18 degrees. At that angle you get one degree of washout for each degree or dihedral. Three degrees of dihedral then solves both problems.