Giant scale Hellcat

[kittyfritters]

I really don't know. I will have to dig out my 1927 edition of the Audel's Engineers and Mechanics Guide and see how it applies to steam engines and whether it carries over to air motors.

KF

[jpsaxnc]

The two scale compressed air models I could find online were Tom Nallens Secan and John Morrills Beaver. I bought a Dumas 30" Mr. mulligan kit, a one pint bottle will fit inside, but the wing area might be to small. Tom Nallen used a motor and tank from an Italian made model, the (Johnathan Z.) The motor, prop, tank and accessories could be bought in a sperate bubble pack. It might be possible to find the bubble pack on an Italian version of eBay.

[jpsaxnc]

In an advertisement for the Johnathan Z motor, it said the motor came with a pressure relief valve set at 102.9 psi. But Tom was only charging his motor to 70 psi. But he still got good flight times at that.

[jpsaxnc]

I took the HellCat out to see if it could rise off the ground and fly. It would not ROG with the 8x6 SF prop, so I put on a 9x6 SF prop. I had also given it a couple degrees of up elevator. It lifted off and went into a left hand stall, but landed on it's wheels, a success for now.

[jpsaxnc]

I might have found the issue. Laying a straight edge across the full size plan, alined with the down thrust shown on the plan. The angle runs from the centerline of the down thrust to the top of the back end of the fuselage. My line of thrust is about 3/8" above the back end. I will tweak the motor mount.

[jpsaxnc]

I fixed the down thrust issue, so the model lifts off the ground. The model tests glides just fine, it seems perfectly stable. But under power it wants to roll over to the left when it lifts off. Measuring the height of the wing at the wing tips, the left wing is about 1/2" higher. It's frustrating because it tests glides so well. I'm thinking of putting a trim tab on the right wing. What do you all think the issue is?

[kittyfritters]

I'm thinking torque which an air engine should have plenty of. You might try some right thrust and also see if it will ROG with the smaller prop now.

KF

[jpsaxnc]

Hi Kf, Yes, it rose off the ground with the smaller prop. I was thinking the same about the right thrust, it only had about 2-3 degrees so I went to about 6-7 degrees. I can't see any washout in the wing. Waiting on the weather to try again. When it ROG last time the power was very low because there was two kinks in the plastic feed line from the tank, I fixed that.

[jpsaxnc]

I'm seeing that my system is minimal at best. In the house at 74 degrees the motor makes good power, but outside at 53 degrees the motor only seems to make half as much power at the same pressure. I don't understand why?

[jpsaxnc]

Another issue is the right thrust, the model did manage to ROG once and fly level while turning left, a very short flight. But the right thrust issue is the centerline of the motor cuts across the centerline of the model, pulling sideways while the model is trying to go forward and that slows take off speed.

[jpsaxnc]

I started work on a lighter weight motor the original motor weighs 23 grams, the new motor will weigh about 15.5 grams. I machined a new crankcase from black Delrin, with A Peek polymer bushing. The new crankshaft is 7075 aluminum, and a 6061 cylinder and head, and Peek piston. I shortened the stroke on the new motor to .340, the bore to stroke ratio is .970-1. It's a lot of work and I don't know how the Peek bushing will hold up. But the model is a little nose heavy with the original motor and the one will fix that.

[jpsaxnc]

The new motor is 8 grams lighter, it might have been a mistake to shorten the stroke power wise. I have a tachometer coming so I can compare the the prop speed. I will run in the new motor for a while to see how the aluminum crankshaft holds up. I plan to order a piece of PEI polymer to machine a new crankcase, it is supposed to be stiffer than acetal and slippery. So I'm thinking of trying the PEI without a bushing, just having the PEI serve as the bushing. PS. The new motor with the shorter stroke produces noticeably less torque. I can go back to the original stroke without adding any significant weight.

[jpsaxnc]

I'm still waiting on a tachometer. But I've been thinking about the exhaust port on the motor, I noticed that if I cover the exhaust port even slightly that the motor slows down. I made the diameter of the exhaust port the same as the ID. of the transfer tube, I'm thinking that there might be back pressure in the motor if the exhaust flow is restricted. Because the volume of air that entered the motor at a given pressure, is still there on the exhaust stroke of the piston. And since the air expanded, I would think the upstroke of the piston might be recompressing the charge of air at least to some degree to force the air back out the transfer tube, and that the volume and pressure of the exhaust might be restricted by the exhaust port opening. When I get the tachometer I can test that too.

[jpsaxnc]

I'm still working on the PEI crankcase, I have trashed two so far. The molecules in exotic plastics don't like to seperate, so when I drill and ream the hole, the hole is always under size. This causes the tool to heat up the deeper the hole goes. I don't have a coolant feed, so I go in 1/8" withdraw the drill or reamer cool it down and go in another 1/8". This working, But then the problem is the front part of the crankcase that I'm holding in a collet is sticking further into the collet, past the gripping surface, so the hole in the front part of the crankcase is smaller than the hole in the back of the crankcase. So I have to withdraw the crankcase so the front part is clamped in the collet and run the reamer through again, but then cut the entrance to the hole over size for some reason, something was misaligned. I'll stay with it because it's worth the weight saving. I have a piece of Peek on the way, it's more dimensionally stable but with the same heating problems.

[jpsaxnc]

The tachometer came, the rpm of the 8x6 SF prop peaks at about 2500 rpms. at 60 psi. I'm still waiting on my PEEK, but in the meantime I was thinking about trying to mold a hub on a rubber power prop, that I could bore out to fit my 3/16" shaft, another can of worms. I wonder if it would be possible to 3d print a rubber power prop, as cleanly as the props Peck Polymers sells with a 3/16" hole?