prop pitch V diameter

In a word, YES.

I will give this a whack.

Let us say for a given engine, it can develop its peak power of 2.2 HP at

13,000 RPM. Let's say that it will turn both a 12.25 X 3.75 and a 10 X 6 at the 13,000 RPM.

If I did my math correctly, in the perfect world, the larger prop will fly the model at about 46.2 MPH while the smaller prop would fly the model at about 73.9 MPH.

In general, for more thrust ( climb power ) you want to move a larger amount of air slowly as opposed to speed where you want to move a smaller amount of air quickly.

I hope this helps,

Douglas Kaip

Sorry about the top post ... I meant to delete the original text.

GIVEN THE SAME RPM: Larger diameter/less pitch = more pull, less top speed (like low gear on a car). Smaller diameter/more pitch = less pull, more speed (like high gear on a car). Dr.1 Driver "There's a Hun in the sun!"

close

big diameter AND lower pitch ~ more climb smaller diameter AND higher pitch ~ more speed

Doug is right. A F-1 plane uses like a 9x8 or thereabouts and turns like

23,000 rpm, where a sport 40 uses a 10x6 at about 14,000. Formula for prop loading is Diax2+pitch. A 10x6 would equal 26 and a 9x8 would equal 26.

| Doug is right.

Doug's are often right! :)

| A F-1 plane uses like a 9x8 or thereabouts and turns like 23,000 | rpm, where a sport 40 uses a 10x6 at about 14,000. Formula for prop | loading is Diax2+pitch. A 10x6 would equal 26 and a 9x8 would equal | 26.

Your formula is ... wonky.

At some level, prop loading might be poportional to --

(prop diameter)^2 * pitch

but that formula would be a very gross approximation, only valid at a fixed RPM with no forward speed (i.e. on the ground, calculating static thrust), and is probably wrong to begin with.

But certainly, `doubling the diameter and adding the pitch' is flat out *wrong*.

For electrics, Motocalc is a very good tool for determining the effects of prop changes on the performance of a plane. (For a glow plane, things aren't so easy to nail down.)

well double the dia. and add pitch will get you different props of comparable loading. Say you want to use a bigger prop on your .40. MGf call for a 10x6. equals 26, so if you want a 11 inch prop a 4 pitch would be a good substitute. The example was given because the guy wanted to know if more pitch gets more speed or if it gets more power. At a given rpm the smaller dia and higher pitch will give more speed and less verticle and the opposite is true. As to the formula, take it up with some of the more respected people at RCM and other places because that is what they put in their articles as a general rule for prop substitution. The reference to fixed RPM is "wonky".. how else would you compare prop performance.. 2 different props,a 10 x6 and a 9 x 6, at 13,500 RPM.. which one makes more thrust... You can use whatever math you want.. involve NASA if you like but I like to at least start with some simple rules that that keep things undestandable... Like where do I balance my wing.. a good starting point is 28% -32% Of the MAC or if you like Mean Aerodynamic Chord or just the Mean Average Chord.. Both points might be different but either one would serve the purpose.

Sorry but this is not the formula for prop loading. The formula for prop loading is the diameter to the fourth power times the pitch times a constant to take care of units. Or loading = Dia x dia x dia x dia x pitch x a constant. This formula is very accurate if the plane is traveling at an air speed close to the speed governed by the pitch and the rpms the prop is being turned at. It is a rough approximation at zero air speed. At zero air speed a given prop absorbs a bit more energy then indicated by the formula, which is why the rpms are a bit lower then in flight. This formula clearly shows there is a lot more to the functioning of a prop then simply how much air it blows.

so.. just what would a constant be????

A constant is exactly what it sounds like it should be. It is a constant. An example would be pi. The area of a circle = pi X radius**2. pi = 3.14 no matter what circle you are talking about. Now, if you happen to measure the radius in inches and want the area to come out in acres you also need a second constant to account for the unit changes. Then the formula would be: Area (in acres) = pi X radius**2 (in inches) X second constant.

I believe he knows the definition of a constant. Although poorly phrased, I believe he is asking for the value of "a constant" in the equation.

Dr.1 Driver "There's a Hun in the sun!"

Of course, in the "real world" we all know that constants aren't, and variables won't

for a discussion on prop loading etc, I recommend a visit to Brian Morris web site. He did a good bit of work on this some years back, and posted some results, conclusions, and even formulae.

bob

I first read the Subj line as meaning a "variable diameter propeller". Sort of difficult to do with the engine running, but not impossible. Check out the 5th photo. Also gives a rough but indelible measurement of pitch.

The factor (Diameter^4*Pitch) appears in the power, torque, and thrust of a prop. Just my 2 cents

Ray S.

Depends upon the units your working with and do you want the power constant, the torque constant, or the thrust constand. All three are different and there is also some variation depending upon the prop manufacturer.

Ray S.