I recently had an experience that I thought the solution to might be helpful to arf fliers in particular. As you know with an arf/arc you do not have control of many of the things we do when we do a full build. A major one is the cg. Some planes will have heavy wood in the tail, some light. I have seen huge differences in this with the same model that I have had multiple planes of. At times I have rebuilt components, most often tails to change the weight and help the cg.  I have been recently working up a Brodak SV-11 arf that I had put an arrowind 2832/05 electric system in. The plane flew ok but it did not pop corners. And the line tension was really excessive. One of the issues you always have to deal with in electric conversions is the cg. Usually an electric motor of the same power is considerably lighter than the comparable glow motor. When you add the battery it generally winds up about the same weight for the power system as the glow motor with full fuel give or take a little. But the CG's on the plans are of course for the glow motor with no fuel. So you ballpark it and then fly it and see what you can do to change work it in.

I observed that moving my leadouts forward on the SV the lap times go down. The problem was with the way things were in the plane I could not move stuff around very much. I could change out spinners and gain a little, but not enough to significantly change the cg. Battery size was already as low as I wanted to go. I posed the problem to some of my flying partners. Rick Sawicki confirmed that the lap times dropping with leadouts going forward meant nose heavy... the plan was flying yawed outwards excessively and moving the leadouts forward decreased this with less drag the lap times decreased. Dennis Adamisin came up with a suggestion that I had just not thought of... basically because I think I was just too close to the problem. But it was a solution that can be used by glow and electric fliers alike: change the motor to a different weight motor, in my case a lighter one.

In electric this is pretty easy to do. The arrowind was an 8.5 ounce motor. I was using the motor because I wanted to see how it compared to the axi 2826/12 power system I normally use on a plane this size. The axi will turn the same prop the same rpm and weighs a full 2 ounces less. There had seemed to be no advantage to the heavier motor in how the plane flew as I did not seem to need the additional power. I have not that I can recall ever seen this suggested in glow motor power on the forums, though I am sure it must have been somewhere. Usually people are looking at putting in more and more power and that is it. But our planes can generally be powered with quite a large range of motors. They just need to have appropriate props and exhaust for the application and be trimmed to it. As an example, you can fly a Strega arf with a Rojet .76 and rear muffler. You can in fact fly this very heavy plane reasonably well, certainly to Advanced class standard with a PA .51 on a tuned pipe. I have had several powered this way. I'm not trying to say that the smaller motor has the same power, but it can be tuned to and used. I cannot think of a more extreme example, though I think I have read of LA .46's going into nobler arfs... that is a pretty good stretch from a fox .35.

Of course if you are tail heavy you can go in the different direction. At any rate when Dennis mentioned it to me it was one of those "dah" moments in life and I thought I'd share it. It can help when other options just don't provide what you need. Adding lead to a plane is not something I ever "need".

bob branch

As Bob said, electrics do cause a re-think on the "more power" question.  Electrics drive us to the realization that the power is delivered not as the displacement, but as thrust through the prop.  Pick a big enough motor to drive the prop you want.  Only use a larger motor if you need noseweight (as Bob did NOT) and/or to generallly de-stress the powertrain.

  Very similar with conventional IC engines. I still people droning on about "more power" and beginners talking about engine mods, 4-2 breaks, etc. What little need there was for that ended conclusively in 1988 or so when practical piped engines 40 on up, had sufficient power to fly just about any airplane. After that it was a matter of picking one that worked well, not getting more power.

     Brett

Bob, can you post some pictures?  Specifically of power train and battery placement, what timer, what speed control, and so on?  Any mods to aircraft for electric power?  Thanks.

Tom

Great, thanks Bob.

Tom