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Old Mar 29, 2005, 10:38 AM
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Prop Diameter vs Model Size

Is there a formula or general guideline for choosing the proper propeller diameter & pitch in relation to a model's physical size and weight? An example would be like, both my 20 oz. parkflyer and 4.2 lb warbird both use a 10X7 prop. Yet they both fly very differently, especially at low speeds. The prop on both models satisfy the current draw allowed and power available from their batteries. Incidently, they both use 3s packs. The parkflyer can fly at half throttle while the big warbird cannot, obviously from a lack of thrust at that throttle setting.
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Old Mar 29, 2005, 11:31 AM
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I'm afraid there's no simple plane/prop rules you can apply.
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Old Mar 29, 2005, 11:50 AM
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Old Mar 29, 2005, 06:54 PM
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Thanks for the info Ralph. Looking at a variety of different planes and their power systems, it seems to average around 25% of the wingspan. Thus my 51" warbird should probably be using a 12"-13" prop for good overall performance across the speed range.
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Old Mar 29, 2005, 07:32 PM
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Faster models tend to use less than 25% slower ones may use more.

Multi ebngines all bets are of
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Old Mar 29, 2005, 07:43 PM
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Sorry about my excessively short post earlier!

It depends entirely on the type of flight the aircraft is intended for and the actual design of the plane. The prop is a function of the power you're trying to transmit and the speed at which you want to do it. It's not related to the geometry of the plane in any specific way. The airframe only affects the prop diameter if you need prop wash over the control surfaces for 3D type stalled flight or if you have landing gear that restricts the prop size that will not hit the ground or to ensure you don't get excessive roll due to torque effects or the helical wake fom the prop.
If you look at a pylon racer weighing 45oz and spanning 51 inches with power input of say 500W it might have a 5" prop. A 3D style aerobatic plane with the same span, weight and power would probably have something like a 16" prop. The pylon racer needs pitch speed to produce thrust up to high speeds. The 3D aerobatic plane needs lots of thrust at low speed but doesn't need the ability to fly very fast.
Generally a bigger prop is more efficient. A static pitch speed just above your target airspeed is more efficient than a very high pitch speed. Usually the best way to go is to trade pitch speed for thrust until the pitch speed drops close to the sort of max speed you think the plane should be flying in level flight.
For a warbird of 4.2lbs and 51" span you'll need a reasonable turn of speed but not so much that you can't use a decent sized prop to help out with low speed thrust for take offs and manoeuvres. Like Ralph said use the biggest prop that gives sufficient pitch speed and doesn't cause other problems as mentioned above.

Aidan
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Old Mar 29, 2005, 09:31 PM
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Thanks Aidan, so it sounds like that I need to first establish a "max and or target airspeed" that I expect the model to achieve. Then use the biggest prop with sufficient pitch to attain that desired "speed". This is my first warbird of this size. What speed do you think I should realisticly expect for good performance? The model is a Sureflite kit.
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Old Mar 29, 2005, 10:57 PM
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I'm not exactly sure but I would imagine around 50mph would be a good guess. If in doubt aim a little high on pitch speed. Say aim to produce 55-60mph static pitch speed would be my guess. If you want to get it just right you'll need to play around with a few different props in flight anyway and I think this would be a safe starting point.

Vintage & Ralph,
Does that sound about right to you guys?

Aidan
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Old Mar 30, 2005, 01:19 AM
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Old Mar 30, 2005, 04:01 AM
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Quote:
Originally Posted by Joe Papa
Thanks Aidan, so it sounds like that I need to first establish a "max and or target airspeed" that I expect the model to achieve. Then use the biggest prop with sufficient pitch to attain that desired "speed". This is my first warbird of this size. What speed do you think I should realisticly expect for good performance? The model is a Sureflite kit.

You MUST be able to do stall+50% at part throttle. Otherwise the plane will be a dog to fly.

Pitch speed up to 3 times stall - or more for super top speed - is a general rule.

If the plane is a heavyish slimer style, expect stall around 20-22mph, if built light this may be lower. So 60mph pitch speed is a good target.

If you them aim for 70-100W/lb for a warbird, and use whatever diameter absorbs that and gives you the pitch speed on a 1.4:1 ratio (diameter to pitch) you will be in the ballpark.
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Old Mar 30, 2005, 08:27 AM
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Excellent information here, thanks guys. I guess I need to accept a more realistic level of speed.
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Old Mar 30, 2005, 07:28 PM
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This may open up a can of worms, but I tend to look at the "progam" pitch speed with a bit of a jaundiced eye.

First of al it's a given that a DC motor would very much like to run at it's design Kv. Now, if we choose to load that motor so that it must run below that rpm, should we not assume that as the aircraft accellerates the motor will maintain it's draw(A) until it sees its design Kv?

I think that rather than unloading gradually during acceleration, the motor would maintain it's torque and increase rpm until Kv was attained.

Am I out of line in this thinking? Tex.
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Old Mar 30, 2005, 07:41 PM
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Not completely. The worse the motor the more it speeds up as it unloads. Better motors simply draw less current.

Its the lessening of the current, times the internal resistance, that gives the extra volts, that makes the motor spin faster. Sort of.

If you had a superconducting motor, it would barely blink its revcounter as load was applied.

Worst motor I have is the good old IPS. Really revs up in a dive that one! My speed 400's are a bit better
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Old Mar 31, 2005, 07:52 PM
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Aaaah, so. Tex.
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