Since this is the engineering board, I thought I might show you some work I have been doing.
SOme years ago I had an application that analyzed the sound from a recorded flight and then plotted the rpm you get from the plane in flight.
I managed to lose that application in a software upgrade, but just recently I rewrote the application.
My motivation is to compare how a IC engine compares with an electric motor in a CL plane.
Most of us flying electric use a speed control that has a governor on the motor--so the rpm stays constant during the entire flight. I have been curious to see how the common IC setups handle the power needs during the flight. This first post is my first installment. Just to whet your appetites (this is the engineering forum after all), I will continue to update this post with more info as soon as I can get things measured.
Today I will show you some flight info from Fred Krueger's Magnum 36 powered Tucker Special. Fred has modified the engine (basically blocking the boost port and adding some head gaskets and playing with the venturi size. More or less the engine is running is a rich 2 stroke/lean 4 stroke mode.
This data was taken by placing my Mac laptop near the pilot and recording the sound. This was done (thanks to a suggestion by Brett Buck) to minimize the Doppler effect---which I have directly verified completely dominates the apparent rpm variations when the laptop is located on the circumference of the CL circle. There is still a small Doppler effect from any residual wind, but its magnitude is small as long as its velocity is ~10 mph vs the 50+mph of the airplane speed. This needs to be compared to the speed of sound, or about 770mph. So a 10mph wind will give an apparent rpm variation of (770+10)770 or about 1.3% (or 130 rpm at 10 krpm).
Briefly (I know, I know), I record the sound. After the fact, I run the sound file back into a fourier analysis--I analyze the sound in 0.1 s slices looking at the frequency components. Fourier analysis tells us that the engine sound can be decomposed into its fundamental harmonics. So for a 2 stroke at 10k rpm, the 1st harmonic is 10k, the second is 20k, the 3rd is 30k......you get the idea.
Now this would be true if the engine was running at a perfect 2 cycle combustion, firing exactly the same every rotation. But if it is partially 4 stroking, or firing non-evenly, you will also generate other harmonics. If it was a perfect 4 stroke, firing every other cycle, and perfectly off in between, then you would see the first harmonic at 5000 rpm, the second at 10k, the third at 15k, ....
The actual engine is somewhere between these two extremes---at least Fred's is running this way.
So one thing I do is to claim the first harmonic is 5000 rpm, the second 10k,... In this extreme, if the engine is running in a pure 4 stroke, I would see the roughly equal numbers of "odd" harmonics (5000, 15000, 25000 or 1, 3, 5....) as "even harmonics (10k, 20k, 30k....). As you will see the actual run is a complicated mix, but I define a measure which is =1 if a pure 2 stroke --only even harmonics) and =0.5 if pure 4 stroke (even and odd).
So after all that here is a graphic of part of Fred's flight. I show the calculated rpm, the "harmonic measure", and the loudness. Since the laptop speaker was fixed, the sound intensity did vary during the flight, and was particularly high during the overheads--when the engine was directly over the microphone.
I'll post the plot of the engine start up to the inside loops in the next post.