HP-24 Project

Over a year ago I agreed to do a one-hour course on aeronautics or aerodynamics or some such thing for the GATE (Gifted And Talented Education) program at my daughter's middle school. A couple months back, we finally got around to agreeing on dates for the course, Thursdays 3 May and 10 May 2007. A few weeks back, I finally started getting serious about distilling down what I could possibly do with an hour (actually an hour-twenty) that makes any sort of sense. Over the last couple of weekends, I started assembling the information, tools, and ideas that I would present.

Here's what I decided:

• It all starts with Bernoulli's Principle. Very little in aeronautics makes sense unless you actually demonstrate that increasing the velocity of an airmass decreases its pressure.

• Aeronautics revolves around airfoils. Airfoils are relevant to virtually everything that flies (including many rockets).

• It's important to depict the scale of the pressures you're talking about. Yeah, your wing might only make a pressure differential equal to 1/500 of the air pressure in a car tire. But the square inches add up fast, and by the time you get to 100 square feet you can lift 1000 lbs.

• Once you've introduced airfoils, you can start to talk about the four basic forces of flight.

• Once you've introduced the four forces, you only have to subtract thrust to discuss the three forces of gliding flight.

• You can get their attention and hold it by promising that something will go "bang!"

The 7075 let go with a satisfying crack at 1960 lbs, showing a good 90 ksi. The pair of rivets let go at a 807 lbs, about 403 each, showing a shear strength of about 31 ksi. Both of those tests showed numbers comfortably above handbook values, and that gave me a good jumping-off point for a few words about design and safety margin.

At the end of the coupon tests the kids were good and wound up, so we just sent them out to play in the courtyard with flying toys and paper airplanes.

Just in case you're interested, you can get a copy of my aeronautics slide deck here (Adobe Acrobat .pdf format).

In an interesting confluence of events, the guys who loaned me the wind tunnel (Steve Smith and Kurt Long) had an aeronautics presentation for a fifth-grade class on the same day and also used my slides. Please let me know if you have any questions, comments, or concerns about them. You're welcome to use them for the purpose for which they're intended, so long as you credit me for their development.

Oh, we've also got some stuff done on the HP-24. Most of the latest progress is on the plugs for the flaperons, getting ready to cut the hinge radii on the Radius Maximus machine (name courtesy of one Dave Nadler).

Here's the latest photos, in chronological order.

I decided that since the Break-O-Tron is going to have its public debut, it might as well be portable and look nice. So I quick-like welded up an axle and a couple of wheels for it, and sprayed on a quick coat of paint.

The outboard end of the outboard flaperon (I still have to stop myself from calling it an aileron) plug, being prepped for gudgeon installation.

A batch of 7075-T6 test coupons for the aeronautics classes. These are no good for determining elongation or modulus, but are just fine for showing breaking strength and going "bang!"

The rig for demonstrating the relationship of lift and angle of attack. The digital scale is the unit I use for measuring epoxy.

The whole wind tunnel setup. I borrowed the wind tunnel from the CFD group at NASA Ames.

The same setup, this time reconfigured to show pressure drop at a Bernoulli constriction.

Overall view of the wind tunnel and also the manometer I used to measure pressures.

The Break-O-Tron loaded with the 7075-T6 coupon.

The start of the aeronautics class. I started by showing the Bernoulli box, and I think that all of the kids who looked at it said that the pressure would be greater at the constriction.

After the aerodynamics, I get the Break-O-Tron ready.

Ready! In the back is the piece of glider canopy we used as a blast shield, just in case one of the tests liberated any particles (none did).

Steve Lauterbach explains tensile versus compression strength way better than I would have thought to do.

Back at the shop, the elevator plug showing the hinge radius sanded, varnished, sanded again, and waxed. Now it's ready for us to make an elevator mold off of.

One of the flaperon hinge points installed with five-minute epoxy.

A view down the wing mold showing the inboard and outboard flaperon sections with all hinge points installed and nicely colinear. Colinearity is nice for keeping control friction down, but is often overrated in glider development. Look at any ASW-20 in a thermal and you begin to wonder whether gliders don't impose their own non-Euclidean environment.

Same thing, outboard in. Yeah, I need to dust off those molds and stop putting stuff on them.

A prototype of the ASK21 disassembly tool I'm making. One end is threaded to extract the drag spar pin, the other is hexed to remove the tailplane bolt, and the hood over the hex disengages the bolt's retaining wire.

Homebuilt aviation is not for folks who don't try things at home.

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