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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.


Elevator power hinging

By Joe Supercool

A decade or so back, my daughter Remy and I passed thru Bakersfield, where we took the chance to see the Republic F84H Thunderscreech. This exotic aircraft was a gate guardian at Bakersfield Municipal Airport. I gather it is not there anymore, having been moved to the Airforce Museum in Dayton, Ohio.

From there we headed to Van Nuys airport, in the North-Western approaches to Los Angeles, where some of the heavy metal Reno race planes were hangared. Imagine my surprise, then, to find a rubber-powered man-carrying ultra-light in one of the hangars!

Now this could all be irrelevant to our modelling needs if it weren't for just one thing. This aircraft, the Rubber Bandit, had an electronic flight control system. Well, not really. The stab and rudder were moved by ordinary R/C servos, with the pilot using a regular R/C transmitter to operate them! Not so remarkable, perhaps, if it were not for the fact that the fully-flying rudder was at least 8 feet high and 2 feet wide!

So I bought a T-shirt and asked George Heaven, the designer, how he expected the servo to move such a huge control surface. It seems there is a trick to this: but first, did it work? Well George had mounted this rudder on the back of a pick-up truck, and used it to steer the vehicle down the Van Nuys Runway! All by a crummy little R/C servo.

Here is the trick. The rudder (no fin) was hinged at the 25% chord position. The rudder was a rectangle, so figuring out the 25% location was a no-brainer. Also, the airfoil section was symmetrical. Now it turns out that if you do this, there is no effort required of the pilot (or servo) to turn this surface against air loads, yet the rudder is fully effective at steering the airplane! Truly amazing!

I had to try this for myself. I restored the Bat flying wing and fitted Voodoo type elevators, which you can see in the photo. But I also hinged them at 20% of mean chord. I don't know how much all this moved the C/G back, but I can tell you the elevator was extremely effective, to the point of the model being hard to fly. I halved the elevator throw and was then pleased with the handling. There is still a need to move the C/G forward a little.

There was another unexpected result. Previously, the model would glide like a brick: there was no way it would glide for the lap required by the F2B rules. In fact, it was down in1/3 of a lap from full speed! I thought this was the draggy muffler and engine hanging out in the breeze. But with the new elevator, 2/3 lap glide was no problem at all. It seems that the Dominator style slab elevator hanging onto the trailing edge was very draggy, both losing lift and adding drag.

So this is something to take on board and think about before you build your next yo-yo. Certainly it is a fun thing to try!

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