Giant scale Hellcat

[jpsaxnc]

A 7075 crankshaft is only compatible with two materials, eglide E and 954 aluminum bronze. I will put a 954 bronze bushing in the PIE crankcase that trashed. Hopefully I can still build a lighter motor.

[jpsaxnc]

This is the best solution, because the most critical factor is the clearance between the crankshaft and the bushing, to prevent air leakage. I can do it in metal with the simple tooling that I have.

[jpsaxnc]

Well, the 954 bronze bushing didn't workout either. I trashed two, I just couldn't get them to ream straight. 954 is resistant to being machined, and my lathe is a joke. The morris taper in the tail stock spindle, was bored at an angle, so nothing lines up. I have shimmed and fiddled with the tailstock, to get a reasonable hole. Finally I settled on making a Peek bushing, it is installed in the PEI crankcase, all looks good, so I can continue with the new motor. The crankcase and bushing weight 5.1 grams.

[jpsaxnc]

The new motor is coming together slowly, I'm making a new cylinder from 7075 aluminum. I tried a cylinder from 6061 aluminum but it was to soft and showed signs of wear pretty fast. I still have to make a new crankshaft, I have a cylinder head I can use from another motor, so things are coming along.

[jpsaxnc]

The new motor is running, it still needs some fine tuning. I had to use a bronze bushing along with the 7075 aluminum crankshaft, so it's only six grams lighter than the pervious motor, but I was able to make a prop hub from Peek polymer and attach the blades of a 7" Peck polymer prop to the hub and that came out 2 grams lighter that the 8x6 sf. prop. So I was able to shave 8 grams total off the motor weight. The rubber power style prop is a much better match the the rpm output of the motor.

[jpsaxnc]

The 7075 aluminum crankshaft was a problem to build, cutting the exhaust flat caused the shaft to warp. Trying to straighten it caused the flat to form a bulge, then it occured to me to leave the shaft oversize, cut the flat to the finished depth, and then turn the shaft to the finished size. That worked but there were still stresses in the shaft, so when I milled the 1/16" deep and wide intake channel, the shaft had a slight warp, about .006 I straightened it again and then drilled and tapped the hole for the prop screw, checked it for warpage and had to straighten it again, not from drilling the hole but the stresses had a memory and creeped out of alignment again. Gave it a final straightening and all was good. It had never occured to me to check the steel crankshafts that I made for straightness because they seemed to go into there bushings just right. But after checking them out of curiosity, they were all off which affects how good of a seal they would make in there bushings. Anyway the new motor looks to be the most leak resistant yet, because it has the straightest shaft and best fit in it's bushing. I'm going to make an 8" rubber style prop for the motor. Now I'm just waiting for warmer weather and no wind.

[jpsaxnc]

I had made a 7" rubber style prop by cutting the blades off a 7" Peck polymer prop, and machining a new hub for the prop that would fit the prop shaft on my motor. I super glued the blades into slots in the hub. I charged the air tank to 60 psi. The peak rpm. was 2700 rpms. That's a good match to the output of the motor. I have some 8" props on the way, I'll make one up and test the rpm. so far it looks promising.

[Ray Klein]

need video and pictures please!

[jpsaxnc]

I don't have the skills to post pictures, I get flustered if Gmail changes the way there header looks. My wife has taken pics. for me and posted them in emails. Well I made up an 8" Peck polymers prop. and the motor could not turn it fast enough to reach the effective rpm. range of the prop. It make the hub for the prop. by machining slots in the hub to glue the blades into after I cut them off from the stock hub. So the slots were cut on the centerline of the hub. Then I made a new hub, offset by 19 degrees and cut the slots, that reduced the pitch of the prop, and the motor could turn the prop. into the effective rpm. range of the prop. I will test it to see if I'm getting more thrust than with the 8x6 slow flyer prop.

[kittyfritters]

You can re-pitch a Peck prop by dipping a blade in hot water, twisting it to the pitch you want and holding it there until it cools. Do the same with the other blade and you've got it. Also look at it from the side when it is turning slowly to make sure that you didn't pull one blade back or forward so they are not turning in the same plain. Actually, they sometimes come that way. You can correct that with hot water also.

The other thing is balance. Let the prop swing freely on a prop shaft and see which blade is heavier. You can put tape, or glue, or lead tape from a golf shop on the light blade tip until it hangs level. Give the prop a swing and see if it comes back to level by itself. if it does it's balanced and should run without vibration. Have fun!

KF

[jpsaxnc]

The 8" Peck Polymer prop I made up with the stock pitch, had to much resistance for the motor to turn it fast enough. The prop. I made up with 19 degrees less pitch seemed to produce about the same thrust as the 8x6 slow flyer prop. The latest prop. has 10 degrees less pitch than stock, and produces noticeably more thrust. I tied my 100 gram scale weight to the tail wheel and it dragged it further and could continue to pull it at declining pressure. I'm happy with that.

[jpsaxnc]

Well, I tried to fly the Hellcat again with the lighter weight motor. No luck, I tried 3 different props. I don't know anything about engineering, but it finally occurred to me that the output of the motor is the same regardless of the prop I use. I had kept the swept volume of the cylinder small to conserve air, then realized that the intake cycle was the same, regardless of the swept volume. So I bored out the cylinder and made a new piston. I still have to make a new cylinder head, but now the bore to stroke ratio is closer to 1-1 @ .954-1 so the motor will produce more torque.

[kittyfritters]

I have to admire your persistence!

KF

[jpsaxnc]

I'm done experimenting with lighter materials, the aluminum crankshafts don't hold up, they galled even in the Peek bushing. I'm glad I tried though.