Good news and Bad News re Vertical Thrust Stand

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Good news and Bad News re Vertical Thrust Stand

Post by hinote » Mon Aug 25, 2008 1:55 am

I've tried my version of a vertical thrust stand to measure the thrust of my entry in the 1.5L Valveless PJ contest.

The good news is, the thrust measurement seems to be quite successful; in fact--in spite of a very high refresh rate on my digital postal scale, the value indicated was quite steady. I think this method has some merit based on my result.

The bad news (for me) is, the postal scale doesn't have any lateral stability built into it; the platform to set the measured item on just floats freely on 4 locator pins, to keep it from walking off, sideways. The actual point of contact with the load cell that measures the value is very small--and the dynamics of the operating pulsejet create an unstable effect, with the potential for the motor to be pitched off the platform during a wilder excursion.

I'm tempted to build a smaller version of Larry's proposal, which will be portable but reliable.

All comments invited!
Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

larry cottrill
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Re: Good news and Bad News re Vertical Thrust Stand

Post by larry cottrill » Mon Aug 25, 2008 12:38 pm

Bill -

Since the vertical movement is extremely small, it should be possible to tether the engine laterally without influencing the thrust force transmitted to the cell. This could be done with short lengths of music wire or small welding rod or some such. While lateral forces can exist, their magnitude should be quite small, so stabilizing it shouldn't take much.

Where a small contact point is necessary, I'd like to see a rubber pad inserted -- not soft (like foam) but fairly stiff, like maybe a short piece of small rubber hose or V-belt or something on that order. It's interesting that you've had good success with the load cell, considering earlier reports that they were hard to use.

You're welcome to try a version of my "force multiplier", of course. An important part of it is that the guts of the scale are fairly heavy, and seem to be well damped against vibration / oscillation. Fully de-bounced keying - ha.

L Cottrill

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