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August 8th, 2011, 11:24 AM   #1
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A question of Torque Measurement


I have several questions regarding a project for the construction of a torque measuring rig for a small (RC aircraft) engine. The idea is to mount the engine on a stand. The mount would be able to rotate freely around an axis parallel and concentric with the engine's crankshaft. Please see Fig 1 of the attachment png. The pale blue segment between the brown block and the engine is this axis (pivot). The block itself is attached to something solid, like a bench. When the engine is started, it would rotate around this pivot in the opposite direction of the rotation of the crankshaft. I hope you get the picture.

Now, onto measuring the force. The engine would be prevented from rotating around the pivot by fixing a rod to the mount, orientated out laterally at 90 to the crankshaft axis. Please see Fig 2 of the attached png. At 12 inches from the center of the crankshaft, out along this rod, we would place a small extension scale (like those for weighing fish), calibrated in some small measure of pounds. Now, as the engine ran it would - instead of spinning around the axis - extend the scale and measure the "force" on the lever at 12 inches, presumably giving us a measure of ft/lbs.

Now, my questions.

First, the effect of the masses of the engine/mount and what it is turning (eg; a propeller or a small flywheel). In the case of a propeller, I would assume the aerodynamic drag of the blades would produce a counter-torque and different propellers would certainly influence the measurement at the scale. In order to measure the torque this way, as such, I imagine it would be better to use a flywheel. Am I right?

Second, the engine mount, technically, is now restrained from rotating by the scale on the lever. If the engine were allowed to run free (ie, no restraint, floating around in a zero gravity situation, and the unit not mounted to anything), would not the engine and flywheel rotate counter to each other in proportion to their respective masses? By this I mean, if the flywheel weighed 0.5 lb and the engine 1 lb, would the flywheel not turn twice for every one turn of the engine in the opposite direction? For an example of what I am thinking of, picture a helicopter with it's tail rotor out. The main rotors are turning a good deal faster than the fuselage of the helicopter is spinning under it. Do I have to consider this effect when measuring the torque this way?

Or am I missing something, like the effect of the drag on the fuselage of the helicopter impeding the counter rotations to equal a half/half distribution?

Is it even possible that we would be measuring torque if we made this rig? The idea is to build up a table of the torque forces of the engine at different rpm's.

Please, any comments are most welcome! Thanks in advance.
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FeebleBrain is offline  
August 9th, 2011, 04:58 AM   #2
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Re: A question of Torque Measurement

These are questions I must refer to the Mechanical Department of a college or Jet propulsion laboratory.

The questions you have are related to a lot of things, but to make things simple you can do trial and error and have some money to throw away in case it fails.

Helicopters have rotating blades on the tail for counter torque--that's my common sense.

My other common sense tells me that the front propeller of a small airplane has high torque because it has to spin fast enough to create trust to move forward. The propeller cops the air so fast like the little household blowing fans. Like fan blades spin they do not spin the main body.

The only thing that could prevent a stationary engine from spinning must have a direct relationship with moment of inertia about the axes in two directions. In flying condition, the wings must play an essential role.

Read wikipedia for computing moment of inertia. It has to do with cross-sectional areas of your engine; it has to do with the mass of the cross-section; it has to do with the center of mass of the engine; it has to do with the rpm; it has to do with the length of you moment arms; it has to do with fluid dynamics and air compressibility; it has to do......etc. It's time consuming.

The question is too big, but it's interesting.

Perhaps, our aerospace engineers at this forum will design the whole thing for you--if -----if-----there's one here.

Good luck.
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