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Fourier Analysis of in-flight RPM---Magnum 36 running in rich 2 stroke

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Alan Hahn:
I wasn't too sure how to organize these posts. As I mentioned, I want to see how various engine setups perform in the pattern. This Forum has a "flat view" of threads (unlike SSW which allows particular posts to start a sub-thread), and if I post all this stuff together, it will be difficult to keep track of the different types of engine runs.

So the topic of this post is Fred Krueger's Magnum 36 powered Tucker Special. Fred has made some modifications of this engine (blocking the boost port and adding some head shims) and has been pretty happy with its performance, first on an Arf Nobler, and now on the Tucker Special. It has no habit of running away. I think he is currently using an APC 10.5x4.5 prop.

I will put in some plots of the various maneuvers starting with the takeoff, and eventually to the clover. The analysis is just that I showed in my Fourier analysis post ( http://stunthanger.com/smf/index.php?topic=14196.0 )with one added feature---I put the actual sound recording device, like before, near the pilot to minimize doppler effects form the airplane velocity. The new feature is that we placed a video camera in the upwind location so that it has a good view of the maneuvering area. This lets me figure out what the plane was doing at the time I am looking at the engine details.

In the plots there are three traces--the engine rpm represented by a white trace, a loudness trace in red (how loud the sound was at the recorder), and in green a trace (labeled "even%") that represents whether the engine is in a 2 stroke (value =1) or in a 4 stroke (value =0.5). Usually the green trace is somewhere in between 0.5 and 1.0 which simply means the power strokes were not exactly equal for each firing. Each time point represents a tenth of a second.

I will mention that during the 1 hour period of the test runs, the wind was an average of ~4mph, with a maximum gust up to 8mph. Pretty much stunt heaven winds.

Alan Hahn:
Take off and level laps. I'll add text if it isn't clear from the picture. I include vertical light-blue markers which give some info about what the plane was doing.

Just before release, the engine is running at ~10875 rpm. After it steadies in the air, the average rpm is 11250 rpm. The slight oscillation is just the upwind and downwind unloading and loading. The blue markers I set to be directly downind of the camera, where Fred is going to do the maneuvers. I note the lap speed is 4.96s/lap upright.

Alan Hahn:
Reverse Wingover

This one really was a puzzle at first look.
When you look at the first corner, the rpm drops ~1500 rpm, and persists even during the climb (the climb time mark here is roughly with the plane half way up climbing vertically). The rpm recovers just as the plane goes over the top.

What is interesting, you don't see such a dramatic effect for the inverted pullup into the next wingover, and finally note the engine "coughs" again as it pulls out at the end of the complete maneuver. This engine is mounted inverted, so we figure that positive "g's" must be throwing some fuel into the cylinder. Note how on the first pullup, the green trace indicates the engine is going into a 4 stroke. This isn't what you normally think should happen. So this was a surprise to both Fred and me.

Alan Hahn:
Here are the inside loops, inverted flight, and outside loops. I note the inverted lap times are 5.0 s. I include the 3 maneuvers so you can compare the inside and outside---the inside loop rpms are also seeing sensitivity to the positive g's

Alan Hahn:
Here are in inside squares, the outside squares, and the triangles. I'll note the inside maneuvers rpm plots tend to be dominated by the "positive g effect" (for want of a better name).

However let me note that Fred is pretty happy with the way this is running, so rpm plots alone are just a factor of what the plane is doing. The plane speed, although affected by the engine rpm changes, tends to average some of these fluctuations out.

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