How do you figure different props for the same engine?

I take the diameter X 2 then add the pitch.

For example: a 13/4 = 2X13=26+4 = 30 So a 12/6 would work, but with less torque. Does this make sense????

sorry, i dont get it, whats your reasoning?

PROP LOADS
Down under
Back in about 1994 a guy in Norway went to considerable trouble calibrating various props and finding out how much HP was needed to drive them at different revs. APC were the main ones used as they had little variation in any given size. So basically all I did was look at the graph given in the April 1994 issue of Aeromodeller magazine. So now I can say that your OS has 1.37HP at 16.8K revs

I'm glad you mentioned the error in the load formula because I came up with one that seemed to work but didn't make sense when compared to the Diam^3*pitch so now I'll use the revised one for load figures.
10x4 load = 40,000
11x4 load = 58,564
10x6 load = 60,000
You can see that now the load figure for the 11x4 is much closer to the 10x6 than with the original (^3) formula.

OK, now this is what I did (and it's only approximate because you have to assume the same HP driving the prop). For starters I'll do the easy one first which is comparing the 10x4 with the 10x6. And I'll use your 16.8K revs this time too.

Divide the 40,000 by 60,000 which gives 2/3 or .666..... then find (.666)^-4 which gives .903 then multiply by the original revs of 16.8K to get 15.18K (near enough) which just happens to be almost exactly what the graph for an APC 10x6 shows at 1.37HP

Doing the same maths for the 11x4 prop's load figure gives .909*16.8K = 15.27K revs.
The ^-4 term is just finding the square root twice and that's where I couldn't figure why it worked with the ^3 load formula so I just didn't worry about it because I was coming up with the right numbers

Interestingly an OS site claims 1.2 HP @ 18K so maybe the Mousse can is working. But these formulas have to be used with some care because of different brands of prop varying in the HP needed to drive them at the same revs and changing the revs of the engine changes the HP available anyway. I'd take a guess the actual revs you might get with the larger prop would be a couple of hundred less than the calculated figures because there's less HP to drive it.

Copied from a previous post by Down under.

good luck,

Jim

If you want to find out how different props load an engine then use the formula Diam^4 x Pitch. Which just means multiplying the Diameter of the prop by itself 4 times then multiplying by the Pitch.
In the case of your 13x4 prop you'd have 13x13x13x13=28561 then times 4 = 114,244. For the 12x6 prop the figure comes to 124,416 so that prop will load the engine just a little more than the 13x4.
Of course, this is only a reasonably accurate guide when comparing two props having the same blade shape etc...a 12x6 APC will load an engine differently than a 12x6 MA for instance.

LOL...seems like w8ye and I sent in an answer at exactly the same time

What would be wrong with using a couple of props and a tach, I've never met anybody in this hobby that doesn't have a box full of props.

None of these prop load rules of thumb work very well. Dia x pitch probably works as well as any of these. The problem is that efficiency is not just Dependant on the prop design. For the same prop and engine efficiency is highest when the plane is sitting on the ground with the engine at idle. That is it moves the air at almost the same speed as the prop pitch. The efficiency is lowest when the plane is in a dive, the air is turning the prop and has a negative efficiency. When the plane is at full speed straight and level it would have a much lower efficiency than when running full speed on the bench. For the same design efficiency goes up with more diameter and down with more pitch.

One thing I see people do that is wrong is assume that RPM * pitch = plane speed. That is wrong. For one the prop pitch is not constant, it is the average pitch along the prop. The inside of the prop is at a much higher pitch. If efficiency is always 100% then the pitch * RPM would give the average speed of the air flow through the prop. If the plane is at full level speed and efficiency is 100% then the prop would have an average airflow through the prop that is RPM*Pitch+plane speed. That is the prop accelerates the air at that velocity, it is not the limit of the speed of the plane, it can be higher but usually lower.


Here is a prop tech site you might enjoy!

http://beadec1.ea.bs.dlr.de/Airfoils/propuls1.htm

What you say about the air turning the prop in a dive is not true, I've had many a plane in a dive and when you pull the throttle back to idle the engine immediately drops to very near idle speed and the plane slows.

Hobbsy,
Actually I should have stated that this would be true usually at low power settings, often at high power settings it would not be true. But a dive at idle would indeed cause the prop to turn the engine instead of the other way around. Even if the idle is 1 RPM over that on the ground then the prop is driving the engine and efficiency is negative.

This isn't strictly correct. The ANGLE of the blade relative to the hub axis varies continuously from close to the hub out to the tip but the actual PITCH remains constant. It's a matter of geometry for differing radius.

This is true, but many props do not follow a true helical model. I may have had it backwards. Don't recall but I believe that the pitch is modified from the helical model to compensate for centrifigal force causing the airflow to move from the hub to the tip.

well, I don't know about where you people live but where I'm at, the prop pitch situation is a little different.... I have a Prather prop pitch gauge. That I measured a lot of props with at one time trying to find out more about the variences between sizes and brands of props.

The whole outer half of the prop is like it says (for example, 10"diameter-6" pitch). This takes into account each and every incrimental diameter between 5" and 10". But to measure the inner half everything gets all screwed up. The amount of pitch falls dramatically to around 4" of pitch at the 2" diameter position and anything smaller diameter than that is not worth measuring.

Yet to look at this same 10-6 prop by eye, it appears to have substantually greater pitch on the smaller diameter rather than less pitch, Just looking at thesame prop, I would be willing to say that it appears to have 12" of pitch at the 3" diameter position.

Get out your trig tables and calculator, measure the angle at a particular position on a prop and compare mathmatically?

Enjoy,

Jim