Wind tunnel instrumentation

[insideman]

I am attempting to design a "pocket size" wind tunnel to test small electric motor driven ducted fans. I need to measure thrust and torque as well as other things. Thrust will be less
than one pound, torque will be less than 0.6 inch pounds. Airspeed will be up to 73 feet per second.

Would strain gages on a cantilever strut do the job? I have in mind about 0.010 inch thick, with width and length tailored to suit. But the gage also has a thickness- will this affect the readings/

 

[btrueblood]

Yes, a cantilever strut with strain gages will/should work. You will need to calibrate your system by applying known loads/torques and measuring strain gage outputs. Make sure you measure all of the strain gages during calibration, you will be surprised that your "moment" gage reacts to forces, and force gage to moments. Also calibrate using different simulated test article weights. You'll end up with a matrix of correction terms to be applied to the strain gage outputs to convert to eng. units.

 

[solidworkswell]

I agree with force guages reacting to moments and viceversa. Instead of using one force guage. I would use a full bridge setup instead of quarter bridge setup u have. This way u can eliminate moment if u r measuring force or force when measuring moment.

Omega do sell half bridge (Two guages interlaid on each other at 90° to eachother). U can get two of these and put em 180° (Opposite sides) and you will end up with a full bridge. This way all u need to develop are slope and intercept for you stuff and forget the correction factors.

 

[btrueblood]

Sure, forget the correction factors, I was just blowing smoke. Just make sure you align those fans very precisely.

See Rae & Pope's book on wind tunnel instrumentation. I learnt how to do it from professor Rae. But he was often just blowing smoke too(*).









(*)Truly, he did a lot of smoke trace testing, as did his grad/undergrad students at the Kirsten Low Speed Wind Tunnel at U of Washington. Blew a lot of smoke, that man.

 

[Philrock]

Many "store-bought" transducers are carefully designed and manufactured so they measure one kind of load (torque, for example) but give no response to another kind of load (force, for example). If you want to distinguish between the two kinds of loads, you'll need to design your transducers with this in mind, or be prepared to do a lot of calibrating, interpolating, and extrapolating.