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Making a Prony Brake to Measure the Power of My Hot Air Engines

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Country Bubba:
I agree and was thinking that when  you first proposed the diagonal idea.  It made me think back to my ME lab many years ago and it was the horizontal distance that was measured.  Sometimes we think to deeply.  KISS! (Keep It Simple S) :hammer:

vtsteam:
Thanks Art, Bill!  :beer:  :beer: I'm chucking the diagonal notion out the window!  :hammer:

One other flaw in my messing around with what I already had was:

True, a hole at 9.74 cm would indeed give me an easy way of calculating watts in my setup, however, with a hole already at 10 cm, drilling results would be bad.

What I can salvage from all the unnecessary calculation of that post is the correction factor of 1.027.

I can just multiply the product of my readings by .0001027 to get true watts. I don't have to go through all the operations with radians, etc.

vtsteam:
I made a 9.0 gram compensating weight to solve the auto-shutoff problem with my scale.

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When the engine is stationary and the arm is loose on the drum, the brake arm puts 9.0 grams of weight on the scale. If at any point while the engine is running I need to tare the scale, I can now just lift the arm, and substitute the brass weight, and hit tare. Then remove the weight and lower the arm again.

On Professor Chaddock's Prony brake, he uses a permanently affixed lead counterweight behind the arm to balance it. I could have done that also, but to me this puts additional dead weight on the drum while running, applying additional unwanted braking pressure at all times. His system works on the principle of a balance scale, so this is necessary in his case.

vtsteam:
Extremely preliminary test on No. 83 with the Prony brake not broken in yet and a nearly empty Sterno can and a handheld tach (somewhat flakey: to be replaced) = 4.0 grams at 982 RPM = 0.403 watts. That's about what I expected for a starting figure.

vtsteam:
Scale 4.5 @ 1060 RPM = .490 watts. With a fuller can of Sterno, again really too preliminary to take as anything more than ballpark. Lots of consistency problems controlling brake pressure and reading RPM and scale simultaneously.

Issues:

1.) The brass thumbnuts loosen from vibration. They are very free running on the brass screws. No springs yet. Need to wear in the brake pads enough to allow them.
2.) The laser tachometer is difficult to get focused on the flywheel, the reading button often doesn't work (it's old), the flywheel black paint wears (not intended to be permanent) so creates false readings from scratches.
3.) The heat source is variable.
 

To do:

1.) Varnish arm (not pad wear surfaces) and then oil pads, and just wear-in so they seat better -- they are already greatly improved from running this small amount.  Get lighter springs onto the brake, add some sort of sticky compound to thumbnuts to inhibit loosening while running.

2.) Add a hall effect digital tach for continuous reading of RPM.

3.) For proper testing, use the electric heater at a consistent wattage.

4.) Disassemble and clean the engine.

I am presently using a 3/4" thick padding block of white pine on top of the scale (per Bill Todd's suggestion above) rather than the long brass stylus. The main advantage of that is lessening of the weight of the arm. The block is self-taring when turning on the scale, unlike the stylus attached to the arm.

I would normally need a new compensating tare weight as well for the now lessened weight of the arm. Also if I varnish the arm, that compensating amount will change. I think the easier course from here on out would be to eliminate a brass weight altogether, and just compensate by subtracting the most current compensating figure from the scale reading when tabulating the data. That would allow for quick adjustments whenever the arm weight changes, rather than making new weights each time.

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