Does anyone know of any formulas for calculating engine thrust?

Here is what I am planning on doing.... Any suggestions

I have a nice engine test stand I will mount on a a moving platform.. Actually a skate board mounted on a rail.

I will mount the engine, prop and associated equipment on the skate board.

So I will have the weight of the associated engine components and the weight of the moving platform.

When I run the engine I will measure the engine thrust with a fish scale connected between the moving platform and a fixed stop.

How would I get the accurate thrust of the engine? I realize I need to calculate the weight of the engine and it's components and deduct the weight of the test stand and the moving platform.

I think interesting data could be developed with actual measured thrust of various engines, props and fuel.

I am a short distance from Joe Noll so I am thinking about doing it there as well.

Anyone interested?

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If you added the weight of the mounted and rolling items to the weight pulled on the fish scale, would that not give you a close enough thrust output figure?

Sometimes I think we might be trying to get too technical for our own good.

Hi Four-stroker
C'mon, ... you have got to be kidding us about the skateboard setup?
If it's static thrust you want, why not calculate it instead?
Go to http://www.bmaps.net/ and click on 'Goodies' - where there's a software static thrust calculator for download

Calculated thrust is really not worth much.
There isn't a calc chart that is even remotely accurate .
Thrust can be measured -easily -on the plane or on a simple rig.
I am adding a pic of a rig I use for electric motors .
This could be done for gassers but it would be a much sturdier setup .
Why aren't calc figures any good?
Simply - you can't add in the variables of the prop shape
you can add in rpm and the "calculated prop shape -but props change in efficiency as they pass thru different speeds .
I know some people swear by calculated data - but I have never seen prop charts which are any good.
Try and compare - you will see what I mean

I have always heard of the inaccuracies of the thrust charts. It just seems this would be more realistic. You could change fuel or prop and see what would be better. I realize a prop unloads in flight and there would be no way to duplicate this except possibly in a wind tunnel.

All we have ever done is to tie the plane to a fence post through a spring scale and measure the thrust with any combination of prop, fuel, Etc. Old mother earth is supporting the weight of the plane and engine and vibration takes care of any of the friction involved in the set-up. anyway, what we are really interested is the relative thrust of the various combinations and this will show it up quite easily. [8D][8D]

RJ

you can also make your own wind tunnel - but -that would hardly be a convenient thing for most of us
I use static thrust tests --on gassers --to compare the improvements in torque -using various exhaust setups -etc.
for example --on a Pitts muffler - -say you get 6000 rpm - on a header and muffler 6200 rpm - on a header and tuned can 6400 rpm - etc.,
These all represent big increases in torque- if you had all other things equal - same prop same day etc..
comparing someone elses figures from 1000 miles away is of little value - as they may be at different altitudes and their prop -even same brand may be a couple of hundred different in load -or unblanced , etc..
A GOOD 50 lb Fish scale (new Peizo types) can give accurate readings by having an assistant hold the model - adding power -- then pulling the model back away with the scale and a harness - this way the model is always under control.
For engines ofer 100 , you need a bigger scale -

You need to do a friction test on your set up first. That co-effecient will need to be factored in. There is some information on the net about friction tests but you can do a simple one by pulling your rig with an accurate scale and taking a reading. Do it three times and average them. Try to be consistant with the time/distance. You can then start the engine and get some readings. Once you have them you can add the friction reading and have an over all thrust measurement. Hope this helps.. BobH.

Hi BOBH!

That's the info I am looking for.

Do I need to weigh the platform, the engine test stand and the engine and it's components and figure it into the final calculations?

There is no need to allow for the weight of the engine and test mount in your set-up when you measure thrust, the engine is not trying to lift it... assuming you are testing with engine crankshaft horizontal. Just be sure your set up is as friction free and level as possible. I wouldn't bother trying to allow for the little friction that you have in the system as the vibration will reduce it's affect to negligible anyhow.

I think your basic idea of using a skate board is a good one, and you'll have a lot of fun comparing props, mufflers, etc., and getting familiar with your engine without having to bother mounting it in an airplane.

Be sure to add safety travel limits in all directions so that there is no way your test stand could go on an unscheduled flight!