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It's all about balance. Babe_b10
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Post  Oldenginerod on Mon Dec 31, 2018 6:50 pm

Just running some theories through my head. A while back I built a "The Cadet" 1/2A racer. I never got around to rigging up the hardware or mounting an engine. Not sure which engine I'll use. I do know now that it will need to be a Cox because I was thinking of using this plane for the Speed challenge. If I want real fast, I could use my Tach Race winning Silver Bee, but it might end up having too much overhang. I'm not all that keen on fitting and removing a multitude of engines to get the balance and power figured out. It just uses wood-screws to attach the engine, so the less swapping the better.

So here's my theory. The landing gear is right at the firewall and just forward of the leading edge. If I were to weigh the tail of the aircraft with the gear sitting on the deck, would that give me an approximate engine weight to aim for. I know the CG should be probably an inch back, so is there a formula I can use to calculate an optimal engine weight, or should I just give in and start bolting engines on and checking the balance for each? Or, just thinking, maybe just tape the engine in place for the weigh-in??
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Post  ticomareado on Mon Dec 31, 2018 8:01 pm

Tape.
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Post  Oldenginerod on Mon Dec 31, 2018 8:31 pm

Yeah, thinking back, I was just over-thinking it wasn't I? Guess I just wanted an excuse to use my scales and pretend I'm clever.
I just thought of the tape thing when I had nearly finished my post. Probably should have just deleted it before making a fool of myself. Embarassed
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Post  ticomareado on Mon Dec 31, 2018 8:43 pm

Think nothing of it. Go listen to a little tune.

https://www.youtube.com/watch?v=HQHnO6NoHFQ
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Post  944_Jim on Mon Dec 31, 2018 10:16 pm

So you want to use a scale, eh?
How about weigh all of the prospects and then trying the lightest, then the heaviest? Compared CG between the two.
Nose heavy isn't awful for a speed event. Although I do recognize the easier it lands, the more often you get to fly it!

Good luck, and tell us how you work this out.
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Post  KariFS on Tue Jan 01, 2019 2:38 am

Tape is the most straightforward, but you could do it with a scale and some geometry too.

The latter would be an interesting excercise. I’m not sure if the weight of the tail alone is enough though Huh...  Maybe if you support the plane from the wings at the desired place of CG, make sure the fuselage is horizontal and weigh the tail then. After that you need the horizontal distance from CG to the tail where it rests on the scale, and the horizontal distance from the CG of the plane to the CG of the engine, which I imagine to be a little aft of the cylinder on a long-tank bee. Then you can work out the desired weight of the engine.

Of course, calculations should be verified, so I’d weigh and calculate just for fun first, then do the tape thing.

If you just want to ”get it done”, then just skip the calculations, grab your favourite engines and start experimenting Smile
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Post  getback on Tue Jan 01, 2019 7:49 am

I find it best to pic the desired engine you want to use then weight it , that will give you an idea of what tail length and thickness , width to work with . If the plane is in the building stage you have the option . Tanked or not will make the difference to Very Happy
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