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Old Sep 16, 2005, 01:40 PM
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Bookham, Surrey, UK
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Prop size Pitch v Diameter

Can anyone answer what the differences are between a prop with large diameter/small pitch and one with small dia/large pitch assuming the theoretical volume of air moved in one revolution is the same?
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Old Sep 16, 2005, 01:45 PM
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That's the point. The pitch differentiates the amount of forward movement the prop will create with one revolution. Hence lower pitch props are setup for thrust and higher pitch props are setup for speed.

I believe diameter contributes to an overall larger mass to turn creating even greater thrust/speed for a given pitch at the expense of higher amp draw. I believe someone said here that alot of the power generated from the prop was from the tips.

ymmv,
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Quote:
Originally Posted by gdavey
Can anyone answer what the differences are between a prop with large diameter/small pitch and one with small dia/large pitch assuming the theoretical volume of air moved in one revolution is the same?
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Old Sep 16, 2005, 02:15 PM
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large diameter & small pitch = more torque like a low gear in a car.
small diameter & large pitch = more speed like a high gear in a car.

It is difficult to go 80 in first gear, but riding over hills and through dirt at slow speeds is no problem, so large diameter & small pitch is better suited to 3D or aerotowing, or hauling more weight at slow speeds, usually in these cases diameter is nearly or more then double the pitch.

It's difficult to go up a hill in 5th gear, but you can cruise at 80 comfortably, so small diameter and large pitch is good for faster airplanes such as pylon racers where the pitch and diameter are nearly the same.


-dave
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Old Sep 16, 2005, 02:52 PM
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Old Sep 16, 2005, 05:16 PM
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For props with P/D ratios greater than 0.6, statically, most of the prop is stalled and as a result the Thrust/Watt ratio is less than that of props with P/D ratios less than 0.6.

Props with P/D ratios less than 0.6 are good for slow flying aircraft and hovering.

Props with P/D ratios greater than 0.6 are more suited for higher speeds.
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Old Sep 16, 2005, 05:24 PM
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The major difference is:
Thrust is related to the mass of air moving through the prop and the amount of extra speed it is given. (mass x speed = momentum)
Power is related to the mass of air and the *square* of the speed.

This means that if you halve the swept area of a prop and double the pitch (so half as much air is moved twice as fast) the thrust stays the same BUT THE POWER REQUIRED DOUBLES!

However, when the plane is moving, it will have no thrust when it reaches the same speed as the air leaving the prop. So, if you have enough thrust, the smaller prop can go twice as fast.

Also, high pitched props (where the pitch:diameter ratio is more than 1:1.6) will be stalled when stationary, so they won't generate as much thrust and can draw more current than you expect when they are in the air.
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Old Sep 16, 2005, 05:42 PM
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For instance, a 12x6 prop at 6000 RPM takes about 140W to produce 37 oz of thrust.
To produce the same thrust a 4x2 prop needs 48500 RPM and takes about 300W.
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Old Sep 16, 2005, 06:43 PM
cp1
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There seems to be an optimal relation between pitch (P) and diameter (D) at a given airspeed and rpm. It can be found in the NACA report 141, which is some dozen years old but is nevertheless still valid.

Helmut Schenk has evaluated this very old report and published some of the results here in Germany. The formula is:

1) (P/D)opt = 0,07 + 1,1*J

with
J = 60*v/n/D = progression of the prop
v = airspeed [m/s]
n = speed of the prop [rpm]
D = diameter of the prop [m]

(Values are metric!)

There is also a simple way to approximate an airplane's minimum airspeed, if you know the weight and the area of the wing. It applies to most standard wing profiles of motorized aircraft:

2) vmin = 1.26 * square root(L)

with
vmin = minimum airspeed [m/s]
L = wing load [gr/dm^2]

With formula 1) you can calculate the optimum airspeed for a given drive. With the motor and the prop you choose, you have D, P, and n. Then the optimum airspeed (at which the prop achieves it's best effiency) is

vopt = -0,0151515 * D *n * (0,07 - P / D)

with
vopt = optimum airspeed [m/s]
D = prop diameter [m]
P = pitch [m]

The optimum airspeed of your drive should be at least one and a half of the minimum airspeed in formula 2).

Regards
Christian
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Old Sep 16, 2005, 08:52 PM
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Quote:
Originally Posted by cp1
Then the optimum airspeed (at which the prop achieves it's best effiency) is

vopt = -0,0151515 * D *n * (0,07 - P / D)

with
vopt = optimum airspeed [m/s]
D = prop diameter [m]
P = pitch [m]
After thinking about this equation for awhile, I now see that it may be approximated by using Pitch Speed (RPM X Pitch) or (D * n * P/D) and that Pitch Speed is only an estimate of the speed at which a propeller is most efficient.

For example a 10X8 prop (P/D ratio = 0.8) at Pitch Speed would have an Advance Ratio of 0.8.

Using the formula above would give a maximum efficiency at an AR of 0.73 which the graph below reveals is closer to the point of maximum efficiency.
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Old Sep 17, 2005, 05:42 AM
cp1
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Hi Martyn,

As I understand it, efficiency is defined as the ratio between the power delivered by the propeller for propulsion of the airplane and the effective power delivered by the engine to the propeller.

At pitch speed the propeller cannot contribute to the propulsion of the plane. So efficiency is zero.

I have seen the diagram you included in one of the articles written by Helmut Schenk. But it did not contain the lines and remarks in red color and on the lower side (like P/D=0.8, Pitch Speed). This seems to be an interpretation which I do not understand immediately.

Anyway, the diagram itself seems to show the torque coefficient CT, the input power coefficient CPi, and the efficiency of a certain propeller in relation to the progression degree J (= v/(D * n)), which is used in formula 1) too.

As you can see, best efficiency for this propeller (with given pitch) occurres around J = 0.7. So, the pitch/diameter ratio should be about 0.85.

There is no theory behind formula 1) (as far as I know). It was derived from experimental results only. I'm including a diagram from an article Helmut Schenk published in 1991, in which you see the experimental results. Some stem from the NACA report 141; the others are from model prop measurements done by Helmut. Pitch is named H in the diagram. (I should have sticked to that convention in order to avoid confusion with power).

Christian
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Old Sep 17, 2005, 06:11 AM
cp1
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Well, now we've done some cross editing. Shouldn't you be in bed at this time, Martyn?
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Old Sep 17, 2005, 09:32 AM
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You're probably correct, but I thought the information that you provided was significant and worthwhile looking at more closely.
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Old Sep 17, 2005, 03:20 PM
flying low & slow
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Assuming that:
Prop's absorbed power = thrust x pitch speed
Prop's output power = thrust x airspeed
Then:
Prop's efficiency = (thrust x airspeed) / (thrust x pitch speed)
or = airspeed / pitch speed

But since thrust becomes zero when the airspeed reaches the pitch speed, the prop's output power and efficiency become also zero at that point.
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Old Sep 17, 2005, 03:50 PM
cp1
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Well, but according to your calculation efficiency would be 1 in that case. So that can't be right.

In fact, you don't get very far with this pitch speed stuff. It may be useful to calculate the airspeed at which you definitivly don't get any more thrust at all, but that's it.

Remember, the prop blades do not cover the whole diameter area. And prop blades do not simply screw their way through the air. They have a profile and behave like wings. So, unfortunately, things are somewhat more complicated. You can get a good impression of the relationship of power consumed by the prop and and the power produced by the prop if you look at the diagram that Martyn posted. The curves are quite typical.

Regards
Christian
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Last edited by cp1; Sep 17, 2005 at 04:04 PM.
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Old Sep 17, 2005, 08:08 PM
flying low & slow
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Quote:
Originally Posted by cp1
Well, but according to your calculation efficiency would be 1 in that case. So that can't be right.

In fact, you don't get very far with this pitch speed stuff. It may be useful to calculate the airspeed at which you definitivly don't get any more thrust at all, but that's it.

Remember, the prop blades do not cover the whole diameter area. And prop blades do not simply screw their way through the air. They have a profile and behave like wings. So, unfortunately, things are somewhat more complicated. You can get a good impression of the relationship of power consumed by the prop and and the power produced by the prop if you look at the diagram that Martyn posted. The curves are quite typical.

Regards
Christian
The formula:
Prop's efficiency = (thrust x airspeed) / (thrust x pitch speed)
is valid as long as the thrust produced is greater than zero.
When the thrust is zero, the efficiency becomes also zero as the formula suggests.
Since the airspeed only reaches the pitch speed in a dive, the airspeed is always lower than the pitch speed during level flight and climb (as long as the prop produces thrust), which means thrust is > 0.

Also agree with the diagram posted by Martyn, except for the reference to pitch speed, which I think should be coincident with zero thrust speed.
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