I know there's a way to do this, based on the sound pitch of the motor, but am not certain as to the procedure. I believe you record a sound sample at a known RPM on the ground, then record the in air sound, then use spectrum software to anaylize the two, but how do you campare them to determine the in air RPM?

You would measure the increase in pitch or freq. then translate back to RPM.
The only problem is that there is no way to measure the pitch of an air-born eng. from the ground. The 'Doppler Effect' shifts the pitch higher as it comes toward, lower as it recedes. It would only be accurate flying at a constant diameter circle around the microphone.

Ever notice on TV lately how the RPM of top fuel drag cars appears to hold constant or even drop as it goes down the line.
When the mic is at the line, the car accelerates away from the mic faster than the increase in RPM!!! AMAZING!!!!

Kirk

Yes you can do it with spectrum analysis software. In the early 90s back in Georgia we had a guy in our group with the software running on a lap top. He used a directional microphone and would record a fly-by. Using frequency and doppler shift he could calculate engine rpm and aircraft speed. He recorded the data in ram. I don't know how many data samples he took nor how much band width he used. It took his old lap top a few minutes to do the calculations but it seemed to be very accurate.

If you record frequency both coming and going then average, you would negate the doppler effect.

Jim

For the full low down on how to do this then go to
http://www.supercoolprops.eftel.com/...article_17.htm
and then the following page
http://www.supercoolprops.eftel.com/...article_18.htm

Sounds like the Prop doctor is just averaging the coming and going freq. to get the RPM at a theoretic 'stand still'.

It doesn't take in to account altitude changes, straight line variations, time delay, wind, a hot/lean motor or speed change from coming to going. It will tell you the average but not the peak RPM.
Seems like a lot of work for a ballpark figure.
Why not use an on-board tach that stores max/min RPM or, better yet, transmit in real time with a down-link so you can see RPM in a straight line, increase in a dive or decrease in a climb.

Kirk

I'm not looking to take this too far, just wanted to get a close extimate of what one of my planes is doing. A reasonably accurate average is easily enough done, and without expense. Just the ticket. Thanks for the help guys!

You read my mind W8YE "The average of coming and going", great minds think alike hee-hee.


JJ
KB9MXE

Why not have two people standing on the flight line and fly over their heads. The first person, signals to the second to start his stop watch and the second person stops the watch when the plane is over head. DO the math to figure out the airspeed and propeller speed.