Giant scale Hellcat

[jpsaxnc]

I built this model to try to fly it with a compressed air motor, I chose this model because I could fit a one pint water bottle into the fuselage that I use as an air tank. I have a question, the wing area is about 184 square inches, and the model weights 5.9 OZ. the CG. is in the right place. My question is, do you think I could test glide this model or is it to heavy for the wing area? Thank You James

[kittyfritters]

I know that it's hard to find these days these days but find the proverbial tall, soft grass and test glide it. You need to be sure that it's not tail heavy before the first flight.

KF

[jpsaxnc]

Thanks KF. I found a spot I just have to wait for the wind to die down, I'll let you know how it goes.

[jpsaxnc]

The test glide was successful, but one side of the landing gear broke off. I tried to make everything as light as possible and only had the landing gear wire secured with a drop of super glue and a couple layers of tissue over that, with a drop of super glue. I will go back in and epoxy the wires to the support. but the glide was wonderful.

[Mitch]

Glad to hear you had a good first glide.
I built this model for rubber power and flew at an FAC event several years age.
When I build to fly... I build with "Gear Up", or, you could build with removable gear and not have gear in during flight.
I believe this would save on your weight and also the drag on the plane.

Good Luck and it is only a suggestion; build them the way you want.

Mitch

[jpsaxnc]

Thanks Mitch, I had thought about removing the landing gear after one side broke off. I will probably never fly in a compressed air competition but it requires the model to rise off the ground. Another reason to keep the landing gear is that I covered the model with Steven's Aero Lite covering, it's very light and easily punchers or torn. My dream is to find a place where I could see if it could rise off the ground, I'm confident it can.

[jpsaxnc]

I have the landing gear back on. The air motor I'm using is based on Edward Pachasa's 1928 design, I built my motor as a single cylinder and out of aluminum. I also used smaller id tubing for less wasted air. The original motor used two flats on the crank shaft to serve as the intake and exhaust valves, but that wasted a lot of air on the intake cycle because the intake stayed open much longer than necessary. The id of the tubing I'm using is 1/16" So instead of making a flat for the intake I used a 1/16" ball end mill to mill a slot 1/16" wide by 1/16" deep, so the slot had the same cross sectional area as the tubing, so I could get a full charge of air with a very short intake time. the result was that I can make the same power and run time from .6 litters/1 pint of compressed air that I was getting from 1 litter of compressed air using the flats. The motor makes 15-20 seconds of climbing power, with a one minute run time on a fill. The motor, prop, tank, tubing and fill port, weight 60 grams. I can safely charge the air tank to 80 psi. I tested the bottles to 100 psi. and they failed to return to there original shape having ballooned out. Another problem with higher pressure is the down force on the piston which puts a big side load on the crankshaft making the motor hard to get a full rotation to start.

[jpsaxnc]

If anyone is interested in compressed air engines for model planes, I bought a book on eBay, Expansion Engine Powered Model Aircraft by Bertram Pond, It's a gold mine of information. Out of print and expensive, I paid $70. but worth every penny. Diagrams of many different types of expansion engines, model plans, and all of the engineering explained and easy to understand.

[kittyfritters]

The air power guys in my club have been using tanks made from Red Bull cans. They take two cans, cut the top ends off of them, then they add the inlet valve (Schrader valve?) and outlet tubing to the ends. and epoxy them together with some overlap for the epoxy joint. These test reliably to over 180 psi and they usually only inflate to 140 for flying. Sorry, I don't know how much overlap, what brand of epoxy or any of the other details since the guy who developed these tanks moved away.

Have you ever considered using a tiny Tesla turbine as an air engine? Even with a high RPM prop you would probably need some reduction gearing but the power would be incredible. I don't know how the air usage would be but it would be relatively simple and interesting to try.

KF

[jpsaxnc]

Hi Kf, According to Pond's book my model needs 42 grams of thrust/25% of it's total weight, to rise off the ground and fly. The model was able to drag a 100 gram weight tied to the tail wheel across the floor, so I think I have enough thrust. I had used cans in my experiments, but settled on specialty water bottles, like Smart Water there made from a thicker material than ordinary water bottles.

[jpsaxnc]

I don't know if a turbine would work well in a compressed air model, because the pressure would be dropping from the moment the engine starts. The Pachasa engine design produces high torque even when the volume and pressure are low. I'm using a 11/32 bore x 3/8 stroke, turning an 8x6 slow flyer prop. I like the specialty water bottles, because you can find bottles with the same volume in deferent lengths and diameters for a better chance of finding one that will fit in a model.