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Fourier Analysis of in-flight RPM---Fox 35

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Alan Hahn:
I got a chance to run my L&J FOx 35 on my Skyray 35 last Saturday. Here are some plots from a flight.

The setup isn't really performing that well right now. I can't tell if it is fuel foaming (I am a little suspicious), or just bad luck that the profile resonant frequency is just between 4 stroking and 2 stroking. Anyway as a result of these issues, I can only post the results through the outside loops, since the Fox went into a strong 2 stroke mode and stayed that way for the rest of the flight. Oh well, the charm of a Fox!

Just to reprise, I have an audio recorder located near the center of the circle--near the flyer, and a video camera on the outside of the circle, basically where the judges would stand. I record the flight from both perspectives, using the audio recorder to analyze the rpm and 2/4 mode of the engine, and the video camera to place where the plane is in the maneuvers. I synchronize the two sources together by locating the time mark where I start the engine. I guess I am good at the 0.1-0.2 second level of associating the sound on the audio recorder with the video recorder.

I show 3 plots on each graph, rpm (white trace), the %even (green trace) is ~1.0 when the engine is in a pure 2 stoke operation, and ~0.5 when it is in a pure 4 stroke mode, and in red I show how loud the sound is at the audio recorder. Its microphone is facing the downwind direction, so the sound gets louder at that time and also when the plane exhaust is higher off the ground.

The details of how I get the rpm are described in my post  http://stunthanger.com/smf/index.php?topic=14196.0  .

I note that the analysis is working pretty good right now, most of my time is spent correlating the maneuvers with the sound, and annotating the plots!

A couple of details---the fuel is Sig 25% castor, 10% nitro, and I am using a plastic MasterAirscrew prop (after an inverted flameout claimed on of my precious Top Flite 10-6 props. :'(  )

Alan Hahn:
Engine Start, takeoff and level laps.

I think the title says it all here. You can see the engine blipping occasionally into a 2 stoke on the green trace (~0.5->1.0) and also an increase in rpm.

The lap speed is almost exactly 5.0s with an in-flight rpm of 9250 rpm.

Alan Hahn:
Wingover


The big rpm dip is the dreaded Fox burp, occurring at the inverted pullout.

Alan Hahn:
Inside Loops and inverted flight.

The burp here occurs as I exit from the last loop and pull negative g's and enter the inverted laps.

I also note the inverted lap time was just under 5.0s (4.96 actually) very close to the level speed, but strangely enough, the flying rpm is closer to 9600rpm. I see similar things in my electric plane, where the lap speeds are slightly different but the rpm is exactly the same. I interpret this to mean that the inverted and upright flight configurations have slightly different drag (maybe the AoA is different due to small misalignments between the wind and tail.)

Alan Hahn:
Outside Loops.

They start out ok, but the engine finally breaks into a 2 stroke and stays that way for the rest of the flight.

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