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May 11, 2010, 05:42 PM
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Change in thrust relationship to change in speed


For a given prop, what is the relationship between a change in pitch speed and the resultant change in thrust developed?

If a prop develops (for example purposes) 24 ounces of thrust at 32 mph pitch speed, then what thrust does it produce at 16 mph pitch speed? I'm pretty sure it's not a linear relationship.

Thanks,

Chuck
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May 11, 2010, 08:15 PM
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Lnagel's Avatar
Thrust varies with the square of the RPM. 1/2 the RPM (i.e. pitch speed) will give you 1/4 of the thrust.

Larry
May 11, 2010, 08:29 PM
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Originally Posted by Lnagel
Thrust varies with the square of the RPM. 1/2 the RPM (i.e. pitch speed) will give you 1/4 of the thrust.

Larry
Thank you, Larry!

Chuck
May 11, 2010, 08:32 PM
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That sounds familiar, for some reason. Isn't that the same relationship for current drawn?
May 11, 2010, 09:58 PM
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Quote:
Originally Posted by NoFlyZone
For a given prop, what is the relationship between a change in pitch speed and the resultant change in thrust developed?
The Advance Plots below may be used to determine the Thrust or Power as a function of airspeed.

1. Determine your Pitch/Diameter ratio

2. Determine your Pitch Speed (Pitch Speed = RPM X Pitch)

3. On the appropriate Advance curve, the X-Axis may be considered airspeed. Pitch Speed occurs at an Advance equal to the P/D ratio.

Ct is the Thrust Coefficient
Cp is the Power Coefficient
n is the Efficiency
May 11, 2010, 10:03 PM
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Thank you for that, Martyn. I'll print that out and study it. Thanks again!

Chuck
May 12, 2010, 04:13 AM
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Bruce Abbott's Avatar
To show how those graphs relate to a real prop, here's some wind tunnel test results for an APC 8x5 (data courtesy of Wilhelm Geck).

Note that these results are for constant rpm. In practice the rpm will increase as the prop 'unloads' (absorbs less power due to lower effective pitch) at high airspeed. The exact amount of rpm increase depends on how hard the motor and battery are being pushed (a lightly loaded setup will be 'stiffer' ie. less rpm variation occurs as loading changes).
May 12, 2010, 10:14 AM
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Lnagel's Avatar
Quote:
Originally Posted by NoFlyZone
That sounds familiar, for some reason. Isn't that the same relationship for current drawn?
Yes, it is the same.

The propeller produces thrust by accelerating air. The amount of thrust produced is proportional to the amount of air accellerated and the speed to which it is accellerated. For a given propeller if you reduce the RPM by 1/2 then the propeller blades will be intercepting 1/2 as many air molecules in a given period of time and accelerating the molecules by 1/2 the speed. Therefore, thrust is reduced to 1/2 * 1/2 = 1/4.

This, of course, is static thrust. You asked about thrust vs pitch speed which I interpreted as thrust vs RPM. I see that others have interpreted this question as thrust vs airspeed. That is a whole different ball game.

Larry
Last edited by Lnagel; May 12, 2010 at 10:24 AM.
May 12, 2010, 11:10 AM
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Originally Posted by Lnagel
Yes, it is the same.

The propeller produces thrust by accelerating air. The amount of thrust produced is proportional to the amount of air accellerated and the speed to which it is accellerated. For a given propeller if you reduce the RPM by 1/2 then the propeller blades will be intercepting 1/2 as many air molecules in a given period of time and accelerating the molecules by 1/2 the speed. Therefore, thrust is reduced to 1/2 * 1/2 = 1/4.

This, of course, is static thrust. You asked about thrust vs pitch speed which I interpreted as thrust vs RPM. I see that others have interpreted this question as thrust vs airspeed. That is a whole different ball game.

Larry
Hi Larry,

Actually, I was looking for a down and dirty way to ballpark an estimate of how much thrust was being produced at a certain motor rpm, when looking at motor and prop data.

An actual real world example is what I used in my original post.

If a motor/prop combo in a typical run of the mill plane produces 16 ounces of thrust at a pitch speed of 36 mph, then would it be safe to say that when I back off the throttle to about 18mph, the thrust would be about 4 ounces.

I'm just looking here for a general idea or rule of thumb, disregarding things like prop unloading, etc. Sort of a down and dirty approximation, if you will.

Thanks again!
May 12, 2010, 11:45 AM
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Dr Kiwi's Avatar
I think it depends a lot on the prop you are using (both diameter and pitch)... here is my "visual contribution"... I need to really look at numbers, but perhaps, with larger props the % reduction in thrust, from halving RPM, is greater than with smaller props.... but 1/4 the thrust, after halving RPM, seems about right.
Last edited by Dr Kiwi; May 12, 2010 at 11:52 AM.
May 12, 2010, 12:09 PM
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Originally Posted by Dr Kiwi
I think it depends a lot on the prop you are using (both diameter and pitch)... here is my "visual contribution"... I need to really look at numbers, but perhaps, with larger props the % reduction in thrust, from halving RPM, is greater than with smaller props.... but 1/4 the thrust, after halving RPM, seems about right.
Thanks, Phil

I'm a rather simple minded type, and first like getting into the ballpark before I wend my way through the aisles to get to my seat...

Chuck
May 12, 2010, 12:29 PM
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Lnagel's Avatar
Quote:
Originally Posted by NoFlyZone
If a motor/prop combo in a typical run of the mill plane produces 16 ounces of thrust at a pitch speed of 36 mph, then would it be safe to say that when I back off the throttle to about 18mph, the thrust would be about 4 ounces.
I think that should be safe to say. As Dr Kiwi says, it may depend somewhat on the prop, but the square of the RPM change rule of thumb should get you pretty close.

Larry
May 12, 2010, 01:01 PM
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whitecrest's Avatar
Per DriveCalc Simple Prop Calculator:

GWS HD 6x3 6000 RPM: Thrust 57g
3000 RPM: Thrust 13g

GWS HD 12x8 6000 RPM: Thrust 1200g
3000 RPM: Thrust 273g

In this case, the rule seems to be pretty good. The variation from the predicted change (@10 percent) is about the same for the large and small prop.
May 12, 2010, 01:16 PM
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Excellent information, and exactly what I was hoping to find. Coming to within 10% is more than adequate for my simple needs. I like the word 'about' a lot...

Thanks again,

Chuck
May 12, 2010, 02:47 PM
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Dr Kiwi's Avatar
I eye-balled my spreadsheet and for the dozen or so props I looked at (for which I had appropriate data) the "1/4 thrust from 1/2 RPM" rule was "about" right!


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