I am unsure about exactly what determines what size prop best suits each plane.
I read somewhere that if you multiply diameter by the pitch you get a certain number and the higher the number the more speed?
Presuming that I am somewhat correct. A 9x6 prop would be very close in performance to a 11x5?
Could someone please enlighten me?

This topic has been covered alot so do a search and youll find tons of info.
Basically it works like so the higher the diameter the more thrust, the higher the pitch the more speed. Usually when choosing a prop for an engine you can plus 1 and minus the other. exp if you have a .40 engine you can either have 10-6 11-5 or a 9-7. However having to big or small a prop can be harmful to an engine. Theres tons of charts that show which prop best fits which engine. Hope this helps.

It's also important to remember that most engines will actually run fine with a large range of props. The "best" prop always depends on the engine, airframe, air density (ie flying in denver) and how you want to fly.

The forumlas (like twice the diameter plus pitch) are rough guides, but really don't tell you that much. They do keep you from putting on a prop that is way to big or way to small though. In both cases, you can damage your engine.

To complicate things even more, different brands of props act differently in the air. For example, an APC brand prop will fly noticeably different from the black MAS at the same size.

You should do the search mentioned, props are a common topic

A good rule of thumb is called prop loading. For example lets assume the manufacturer recommends a 10-6 (10" diameter by 6" pitch) the 6" is the theoretical distance the plane would move forward in one complete revolution of the prop. Multiply the diameter by the pitch (10 x 6 = 60) so your prop load is = 60.

If you want to go to a larger diameter prop (for example if you have a large cowl to get the prop beyond for more efficient air flow) lets say you use a 12" prop at 6" pitch and your prop loading would be 72 (12 x 6). This would load the engine too much so lower the pitch. A 12 x 5 = 60 which would be the same loading as the 10 X 6 and a better choice. A 12 x 4 would be a loading of 48 (which seems low but may actually work best).

Your example:
9x6 = 54 (faster)
11x5 = 55 (more torque)
is actually an almost equal loading match. The higher number has nothing to do with speed (prop loading number) as the higher pitch lower diameter prop combination will pull the plane faster. Think of this as gears in your car. Lower ones to get going and higher ones to cruise. Variable pitch props anyone?

EXCAP232

quote 11x5 = 55 (more torque) originally posted by excap

Excap great explanation but i believe the larger the diameter the more thrust not tourque (torque for you americans).

The easier way to remember it (for me anyhow) is that a steeper pitch equals more speed, but also means less acceleration (static thrust).

An apt analogy that I always use is to think of a larger diameter, smaller pitch is just like running in first gear. LOTS of thrust at slow speeds, but no top end power. Make the diameter smaller, and increase the pitch, and it's like running the plane in second gear. Still a lot of low speed thrust, but the greatest thrust comes at a moderately slow speed. As the plane gets above this speed, the thrust drops off, and eventually reaches zero, but this point will be higher up in the speed range than the "first gear" prop. Continue this analogy and you have third, fourth, and overdrive. Since you dont get to change pros "gears" in flight, you have to choose what you want to do. Either go fast, but take more time getting there, or go slow, but accelerate to that slow speed rapidly; or some mixture of the two in between.

. . or tourque for all you who spell it wrong!



All other things being equal, larger diameter = more thrust and larger pitch = more speed, but, generally speaking all other things aren't equal!

As with the selection of gear ratios in a car ( a very close analogy in many respects) it all depends at what revs the torque is produced, what shape the torque curve is, how much 'torque back up' there is, what the maximum usable revs are etc etc.

- my 2c (p) worth!

-David C.

IMHO, the forumla of pitch x diameter = "loading" isn't really accurate, while you might use it as a ballpark, you should also keep in mind that it is one rough approximation. (my personal experience tells me that a 9x6 and an 11x5 aren't even close to the same load, but ymmv)

Another forumla for a "loading" is the 2 x diamter + pitch, so a 10x6 and an 11x4 would have the same loading of 26.

I've heard of both of those forumlas, and a few others as well, and none of them are accurate enough to tell you what prop to use, you have to bolt one on and try it, nothing else really works.

Some of the factors that make the various "load formulas" not work well in the real world include:
- Engine torque curves are not linear
- prop efficiency isn't 100%
- prop efficiency actually changes with RPM, espeically at high RPM
- different brands of props in the same size will load the engine differently and fly differently (try a MAS and an APC in the same size some day)
- different prop materials will act differently, wood vs plastic for example.

The points everyone made about diameter vs pitch is right on in concept, lower pitch is good for climb and acceleration, high pitch is good for straight line speed Assuming the same RPM! Sometimes you can actually go faster by lowering the pitch and allowing the engine to run at a higher RPM for example.

Prop selection also depends a lot on the design of the engine. Some engines are "long stroke" designs, with a small bore and long piston stroke. These engines prefer to turn larger props at lower RPMs. Other engines are "overbore", with a large bore and short stoke. These engines want to rev much higher and turn a smaller prop for better performance. Most sport engines are roughly "square", with the bore and stroke about the same. And there are other factors as well, such as port timing and carb design. So, it is quite possible that two different engines will do different things with the same pair of props. I've seen this myself, the Mag .15xl will turn a MAS 7x4 better than an OS .15LA. But if you put an 8x3 on both, the OS seems to have an edge or at least is much more equal.

Generally, when I have a new plane, I'll pick up a prop or two in a few different sizes from different companies and try them to see what happens. I don't try everything out there, just a couple based on past experience or what other guys are using. The more I care about getting every last bit out of the plane, the more props I am going to try. After a little while, you wind up with a box of different size props, which makes experimenting later on easier. I've also been known to borrow or trade props at the field to see how something works.