
Mar 11, 2002, 08:00 PM  

For thrust measurement it's enough to hang the plane from the ceiling of your garage with two wires. The deflection from the vertical will give you the thrust, you don't need a scale (In any case upside down placement on a kitchen scale will only work if the there is no front intake).
For efflux speed measurements we use a $2 electric motor hooked up to an ordinary voltmeter. It needs to be calibrated, otherwise it will give you only relative speed measurements: http://aeneas.ps.uci.edu/edf/fan/p8300058.jpg It's accurate enough to measure small deviations from uniform speed throughout the fan diameter. 
Mar 12, 2002, 12:20 PM  

Dave Ms rig looks the best. You dont want to measure the force of the air as there will be losses from divergence?? What you want is how much thrust can the fan put into the airframe.
With Dave's rig like he said the moment arms must be = so the distance from the axle of the fan to the piviot and then from the pivot to the scale contact point must be the same, calibrating it with two scales like he said it the way to go, then you can change the moment arms to account for any loss from the mechanics. Its also as cheap and simple as you could want, his is made from foam. I too use the in jet, nose on scale but there are fluxuations as you try to hold the jet up plus you are to far from the fan moment arm at the nose, the ideal way to do that would be to make a foam cradle that grabs the wings and sits on the scale. Measurent of the fan thrust inside and outside of the plane is usefull to see your ducting losses but ohter than that, its the thrust in the plane that is the most relivant I think, again this is only staic thrust, but thats the best we have,,,,, . 
Mar 12, 2002, 02:37 PM  

Quote:
also (voltage out) proportional to (rpm of sensor probe). Thus measured voltage is lclose to linear in efflux speed. Static thrust can also be measured locally, it is proportional to the rpm^2 or measured voltage^2 . 

Mar 12, 2002, 03:59 PM  
Seattle, WA
Joined Jan 2002
186 Posts

Herb,
I think your math is a little off. Thrust would equal weight at 90 degree (the motor would be hovering). At 45 degree, the thrust would be 71% of the weight. I think you forgot to account for the tension in the string that is supporting some of the weight. It has been a while since I did static analysis, but I think it goes something like this (see attached image): horizontal components  t * cos a = s * sin a s = t * cos a / sin a vertical components  w = s * cos a + t * sin a combining to replace s  w = t * cos a * cos a / sin a + t * sin a w = t * ( cos a * cos a/sin a + sin a ) w = t * ( ( cos a * cos a + sin a * sin a ) / sin a) t/w = sin a / ( cos a * cos a + sin a * sin a) Using this equation, at 0 degress t/w = 0, at 90 degrees t/w = 1, at 45 degrees t/w = .7071. Tony 
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