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Power Determination

Old 02-22-2002, 12:58 AM
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budcop
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Default Power Determination

Is there a formula to use in determining the proper power for an airplane? I have a Waco staggerwing 1/4 scale , 100" top wing, 85" bottom wing, 1670 Sq" weight should be 24-28lbs, want to use a gas engine and fly scale.....Thanks for any help available, Bud....
Old 02-22-2002, 02:16 AM
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Mike James
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Default Three ways that I know of.

Short answer: Depends on what kind of performance you want.

The first method would be to use your experience, and start with what has worked well for you in the past. Of course, you can use other people's experience as well, by looking at similar weight and type aircraft, and seeing what those models use.

The second method would be to determine several engine's thrust output, and make your choice based on a thrust-to-weight ratio. (At least 70 percent thrust-to weight would be good, I think.)

The third method is something I remember from Andy Lennon's book, and that is to aim for approximately a 240 to 250 oz. per cubic inch power loading.
Your plane, at 26 pounds (416 ounces) then would call for an engine of about 1.66 cubic inches. (416/250=1.664) This is only a general guidline, used for "sport" flying. yo may want more power if you're looking for extreme performance.

Hope this helps.
Old 02-22-2002, 03:17 AM
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budcop
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Default Power to weight ratio

Thanks Mike, I am not a aerobatic type flyer, more scale, the plans this model is built from are early 80's, the designer wrote about using a kawasaki engine, unknown to me! , I was hoping something along the 2.5 ci engine would work out so I could get a US 41, (cheap) and stay within a cop's budget, Thanks for the info....Bud :drowning:
Old 02-22-2002, 06:58 PM
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Ed_Moorman
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Default Engine Size

There is an old rule of thumb which says to divide the weight by 12 to find the engine size. It works pretty well for smaller planes and glow engines. For example, a 5 lb airplane needs a .40 and a 7 lb airplane needs a .58-.61 engine. Your plane at 26-28 pounds would need a 2.1-2.3 cu. in. engine.

But, this is glow so add some for running gas, then add some for a biplane which has tons more drag than a monoplane. A US 41 might do it, but you would be happier with a 50cc/3.2 cu. in. class engine.
Old 02-22-2002, 08:43 PM
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Default Power Determination

Thanks Ed, I would rather have a little extra, than not enough, maybe I should just go ahead and get a G-62......Bud :idea:
Old 02-23-2002, 11:48 PM
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Default Power Determination

Before you guys get away,

what is the power ratio on multi-engine powered A/C? I read way back when the ratio of horsepower to weight is different for twins etc.

Got any figures available?

Wm.
Old 02-24-2002, 05:51 AM
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Mike James
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Default Two engines - Power ratio

It seems to me that the power of the two engines combined should be a little more than the power required if there was engine only, to allow for continued flight in the case of an engine failure.

I'm guessing, but I believe the two engines should total something like 120 percent of the power of a single engine. (Say, two .46 engines instead of one .75-size.)

I don't know that there is any "official" standard. Hope this helps some, anyway.
Old 02-25-2002, 03:26 PM
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Default Last thing I heard

Last think I read was to downgrade the power for multiple engines. Rate each engine at 75% their normal, but it was written for a twin. Have a fellow now working on a 110" B-17 based on old plans, and we are trying to figure an appropriate power set-up for the A/C. We want to move around things within each nacelle to fit wahtever power is decided upon.

I would think maybe it to be even lesser on four engine. Maybe rate each at 65%. I cannot find such infomation in my design books, as they mostly pertain to single engine jobs.


Wm.
Old 03-06-2002, 01:08 AM
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budcop
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Default Power Determination

Thanks for the very helpfull information , you guy's are great!!!

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