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Easy Formula for Engine Choice

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Old 06-20-2003, 01:25 AM
  #1  
doctorgo
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Default Easy Formula for Engine Choice

Some months ago, I read some posts asking which engine would be appropriate for any described model. Of course there were answers, but nothing specific.

I was recently reviewing the performance of the 37 ARFs I built and re-built so far and found something interesting.

If I took the engine size, say a .46, moved the decimal to the right one digit, (4.6), then added .5, we have the number 5.1.
This is the maximum weight of the model in pounds, ready to fly, to be able to hover! (Depending on fuel & prop size)

It worked for all my engines, All TTs from .36 to 1.20, including a Magnum .91. Strange, doncha think?

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DGO
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Old 06-20-2003, 08:14 AM
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Mike James
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Default Power loading

Another way to look at power loading is to compare the overall weight to the displacement, for the task you have in mind. I first started to think about this after reading Andy Lennon's articles and his book, "The Basics of RC Model Aircraft Design".

Just divide your model's weight in ounces by the engine displacement in cubic inches.

For example a 5 pound model is 80 ounces. If you use a .46 2-cycle, that is 80 divided by .46 = 173.91 ounces per cubic inch displacement. (CID)

Once you know the performance you're looking for, it's easy to know what power loading to use as a goal.
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Old 06-20-2003, 09:41 PM
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doctorgo
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Default Easy Formula for Engine Choice

That's much too technical for us poor ol' retirees.
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Old 06-20-2003, 10:03 PM
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Default Easy Formula for Engine Choice

DGO,
This may work with planes in the .40 to .90 (2 stroke) range, but that's about it. (Doesn't work for a .90 four stroke all that well.)

Take my Quadra 35. 35 cc equals about 2.2 ci. By your method, 22.5 pounds. A Quadra 35 won't take 15 pounds vertical, let alone 22.5.

I'm sure there are other examples of how it won't work, but nice try! I'm always looking for good rules of thumb.
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Old 06-20-2003, 11:09 PM
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Default Engine Formula

You could take the recommended size and double it.
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Old 06-21-2003, 12:16 AM
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doctorgo
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Default Easy Formula for Engine Choice

Geez, guys. It was just an observation.
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Old 06-21-2003, 12:26 PM
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RobStagis
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Default Easy Formula for Engine Choice

*grin* How's about this one? When the kit arrives, weight it as it came from the manufacturer - don't unpack it. That'll be the rough finished weight. Try it!

I just use the mfg's recommendation. When building my Goldberg Cub, which is .40 - .61, all I had was a well-used Enya .40 bushed engine. I was worried just a little about the power, and asked around. Replies ranged from not-enough-power to you-won't-be-able-to-balance-it - it's a light engine.

Not having any cash, I built it anyway. It balanced with no weight. It flew perfectly. It flew perfectly with a second, clipped wing. It flew perfectly on the Goldberg floats!

Tell that to the guys with 4-stroke 90's in their Cubs.......
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Old 06-22-2003, 01:44 AM
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Default Easy Formula for Engine Choice

Truth is, the majority of people overpower their planes. Virtually every recommendation for a 40 size trainer I read discusses which 46 to power it with.

While extra power CAN get you out of trouble at times, most people never learn to fly "on the wing". They rely on excess horsepower to yank the plane around.

Try flying that 46 powered trainer at no more than 1/2 to 3/4 throttle throughout a few flights. It's amazing how much you'll learn about "how" a plane actually flys.
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