

Sep 16, 2005, 01:15 PM  

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 
Sep 16, 2005, 04:16 PM  
Toronto Canada
Joined Dec 2002
5,374 Posts

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. 
Sep 16, 2005, 04:24 PM  

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. 
Sep 16, 2005, 05:43 PM  
Hannover, Germany
Joined Aug 2005
263 Posts

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 
Sep 16, 2005, 07:52 PM  
Toronto Canada
Joined Dec 2002
5,374 Posts

Quote:
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. 

Sep 17, 2005, 04:42 AM  
Hannover, Germany
Joined Aug 2005
263 Posts

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 
Sep 17, 2005, 02:20 PM  
SE
Joined Mar 2003
365 Posts

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. 
Sep 17, 2005, 02:50 PM  
Hannover, Germany
Joined Aug 2005
263 Posts

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 
Sep 17, 2005, 07:08 PM  
SE
Joined Mar 2003
365 Posts

Quote:
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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