Calculating thrust of a rubber motor airplane

[thomasd538]

Hi All,

How do I calculate thrust on a rubber powered plane?
I have found plenty of information on electric motors, but what about rubber power?

Thanks
Tom

[JetPlaneFlyer]

It would be difficult to do accurately. Probably the easiest/most accurate way would be to rig up the prop rubber motor and physically measure the static thrust.

[thomasd538]

Hmmm....

I guess I could make thrust tables. Has anyone done this? I could add more information.

Thanks,
Tom

[eflightray]

The crude way.

Hang it from a spring scale to weigh it, then run the motor, note the new 'weight', deduct the models weight to get the static thrust.

But it is static thrust, once the model starts flying the thrust figure will change.

[Brandano]

consider the variables. rubber quality, motor length, number of strands, number of turns, whether it's lubricated or not, how long it has been wound, what temperature it is... you can at best hope for a ballpark figure, there's just too many things to consider. Not to mention what prop is being used and whether it's a geared setup or not.

[slipstick]

Probably the biggest variable in any thrust discussion will be the propeller. And rubber model props are notoriously variable themselves since many are hand made.

Add to that the power of the rubber motor changing from second to second as the turns unwind and the variable quality of the rubber available today and I reckon that while you could measure thrust for a specific setup it wouldn't really tell you very much even about the next rubber motor you used with that same propeller and it would tell you even less about e.g. the thrust that someone else putting together a motor and prop from the same plans would get.

Static thrust is simply not something that any rubber fliers I know ever even mention (I don't even take any real notice of it for electrics myself ;)).

Steve

[JetPlaneFlyer]

There are some breaking torque figures for Tan II rubber here: http://www.modelflight.com/rubber.html

From torque and rpm you can calculate power and from power you could take a stab at thrust but thrust greatly depends on prop efficiency and flying speed. I’m not sure what is a reasonable figure to use for efficiency?

You could perhaps drop the torque and rpm figures into whatever calculation you are using for the electric motor which presumably makes some assumptions on prop efficiency.

Thrust would be overestimated because of the reduction in run torque due to hysteresis, plus torque drops off quickly as you run the motor down from break torque.. You would obviously never intentionally wing to break torque anyway.