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Model Forum / Radio Controlled / Air Models / August 2007static thrust
Hi, I made a crude setup to measure prop thrust on my electric mini edge 540. I used a digital scale connected to a laptop. Good accruacy. I measured 2 pounds of thrust in still air . I decided to see what would happen to the thrust value if I directed a large fan on the prop during tests. Surprising to me the thrust dropped about 10 percent. Not sure what happened to the RPM's. Fan was aimed at the prop nut at a distance of about 2 feet. I thought it might "unload" the prop and the thrust would go up. Not being a prop expert, what would you expect me to think?...... Anyone got any comments? Jimbo >Hi, >I made a crude setup to measure prop thrust on my electric mini edge [quoted text clipped - 9 lines] > >Jimbo Aircraft props generally work best in turbulence-free air... /daytripper >Hi, >I made a crude setup to measure prop thrust on my electric mini edge [quoted text clipped - 7 lines] >me to think?...... >Anyone got any comments? That is to be expected. Draw out some vector diagrams of the air movement and you should find that the draft from the fan reduces the angle of attack of the prop blades, which will indeed reduce the thrust. But the POWER the prop delivers to the airplane is equal to the thrust times the airspeed. As an airplane gains airspeed, the thrust goes down but the delivered power goes up. rj Great test! The scale is measuring thrust plus the drag of the plane. So the results might be indicating the drag effect. Did the prop unload? That is, did the RPM increase? > Hi, > I made a crude setup to measure prop thrust on my electric mini edge [quoted text clipped - 9 lines] > > Jimbo jbreeyea@uvm.edu wrote in news:1186358825.442531.308910 @r34g2000hsd.googlegroups.com: > Hi, > I made a crude setup to measure prop thrust on my electric mini edge [quoted text clipped - 9 lines] > > Jimbo you'd expect the thrust to drop when the prop unloaded. If it was a high- pitched prop and much of it was stalled, then the thrust could go up with airspeed, but otherwise it will drop. Current should drop, too. Further complicating the picture, as the other posters have suggested - the fan puts out a very 'dirty' airflow - very turbulent; that will cut the prop's efficiency. And the fan puts drag on the airframe, so you'd need to take measurements with the motor off & prop removed, then take them with the prop on & motor running, and see what the difference is if you want to remove the drag effect. Won't compensate for the lost thrust due to prop blast on the airframe, of course, but . . .  Signature"Whatever will have been, will have been." - Douglas Adams, "Life, The Universe, and Everything" Well I have done some more measurements and it seems it is only useful for comparing props, batteries and motors. I suppose I should collect more data ie; watts, rpm. I had thought about the air current beyond the diameter of the prop blowing on the airframe. I guess the prop wont see the airframe as a load until the plane gets moving. If I find anything of interests I will comment. Thanks for the comments. jjb |
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