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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


F2B: “Heart of Gold” Gyroscopic Precession

By Supercool

[Before I start, you might like to look up “gyroscope” in Wikipedia]

Test flying and trimming of the “Heart of Gold” F2B flying wing stunter has advanced to the point where the model could be flown in a contest. It has been flown with ST35C, STG21/40, STG21/46, Enya 45 6001 and Enya 45 6002 engines. The lightest engine, the ST35C was flown with the engine as far forward as permitted by the engine mounts, but still tail heavy. Turns were extremely tight!

A problem appeared during outside squares. The model wanted to yaw in and float on the lines: not good! Peter White suggested this may be a problem related to the gyroscopic effect of the propeller, a problem previously corrected by Al Rabe, using a movable rudder. Hmm, sounded good, but my model has no fin or rudder!

I was puzzled that the gyroscopic yaw problem was so bad on the “Heart of Gold”. The lack of a fuselage and fin possibly meant that there was little in the way of yaw damping: that, plus the low moment of inertia, seemed to make the flying wing model more susceptible to this problem. But what to do about it?

First I reviewed the little I know about gyroscopes. Back in an early Zaic Yearbook (Model Aeronautic Year Book, by Frank Zaic,1951-52: also Model Airplane News, April, 1950, by Don Foote) there were diagrams of the effect of forces on gyroscopes, so I drew on that for my own movable rudder on my first “Rivets” F2B, back in 1968. The first thing to realise is that gyroscopes are weird. When you push on them, they don’t just tilt over in the direction you push them! Rather they tilt over somewhere else, which is rather unnerving! The reaction force takes place 90 degrees later in the direction of rotation, which takes some getting used to.

For example, on a model, down elevator produces a reaction force that makes the model want to yaw in: up elevator makes the model want to yaw out. High RPM and heavy props make the reaction force stronger. This seemed to be what I observed on the “Heart of Gold”, so it was apparent that the elevator was causing the problem. Well, I could hardly leave the elevator off: having no flaps or fins to start with, this could become an uninspiring model!

Being a proof-of-concept model, I have no hesitation in cutting pieces off it to correct a problem, so the logical thing to attack with the knife was the trouble-causing elevator. My first thought was to yaw the elevator hinge line. By moving the port side of the elevator forward, that would induce a yawing force, out on down and in on up. (you gotta fly stunt to follow that mish-mash) (hey, my spell-checker thinks “gotta” is a real word!).

I ran this idea past Peter. Hmm, could work. But a week later I was in Sydney for the wedding of Stumax, with a chance to drop in on mountain-man Andrew Heath. I ran the idea past him, and was amazed to discover the Russian F2D guys were already doing this: nothing new under the sun there. Then I considered tilting the whole elevator: this would do the job too. But both methods required rather more hacking than my lazy mind could handle. I needed something easy, you now, just one slash and the job done!

Then it came to me: bend the port elevator down. One slash, some epoxy left over from making a prop and the job done! I guessed the angle, slashed and glued, then next morning off to the field. Not so much a “Rabe Rudder”, more a “Supercool Slash”. Honour and Advancement, here I come!

After a week of high heat, wind and humidity, the day dawned cool, calm and overcast: no blinding sun. Perfect for testing. The mighty Enya 45 6002 purred into life first flick, and off we went, roo poo flying everywhere. Damn, wish there was some wind; I’m flying into my own turbulence. Now for the outside squares. Yes, yaw cured! I’m definitely a genius! Think I’ll tell Ricky Ponting how to bat! I even felt like hugging Shane Warne! How sick is that!

Now you will notice I have been circumspect in my language re assigning this problem to gyroscopic precession. There is another effect which behaves in the opposite way, the so-called “P” effect. Up-elevator pitches the nose up, with the result that the prop blade on the inner disc gets to a higher angle of attack, while the inner blade gets an increased angle of attack. The net result is yaw in on up, yaw out on down. If you are lucky, the “P” effect can cancel the gyroscopic precession.

I have noticed that during an outside square turn, the model first yaws out, then back in rather more strongly. It is as though the “P” effect operates first, then followed by the reverse gyroscopic effect. More work is needed here.

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