will a 12*6 give me more speed than a 11*7? my engine has the torque but i think my prop is too small to give me the wanted speed and pull.

Higher pitch gives faster speeds in general. To go higher in pitch, you may have to go down in diameter to keep your engine working efficiently. Lower pitches, often with larger diameters generally provide more low end pull. This is assuming that the high pitch and low pitch prop are both in the proper range for the engine of course.

Truth is though, experiment with a number of props in various diameters, pitches, manufacturers, designs, materials, etc. until you find what works the way you want.

No. That will give you more torque. You need to increase the pitch for top-end speed (second number). An 11x8 or 10x8 would give you more speed vs. an 11x7.

And, because life is all about compromises, the higher torque will give you better acceleration but lower final speed. (i.e. 0 to 30 in three seconds but 45 top end vs. 0-20 in three seconds but 55 mph top end).

As mentioned earlier - try a few and see which works best in your particular model.

We need to know the engine in question, could be a 40. We also need to know the plane, could be a GeeBee.

However assuming it is an average 60 the anwser is no. An 11x7 will usually give more speed than a 12x6, why you are more likely to be near the RPM for peak BHP.

This comes up often and the answer is always the same. It all depends on several factors. One being the engine/plane combination, weight, wing loading, and all that. The plane will not go faster than it is designed to do.. but you can put a bigger engine on it and get there faster, but it won't help beyond a certain point, and all you do is risk airframe damage, wings getting torn off, and so on.

Then there is how a prop works. Of course it rotates, and pushes a chunk of air through the prop arc. Over-exaggeration, but you get the idea (Gordon, don't say it.. ).

But, consider a prop is like the gears in the transmission in your car. If you want to climb a steep hill, you shift down to a lower gear (same engine, same car, but now a lower gear). So, the car "pulls" you up the steep hill, but at a slower speed AND the engine RPM increases dramatically.

Now, you are on a long straight road and you want to go faster. So, you shift up to al higher gear and the car goes faster, but takes longer to get there. Again, same engine, same car, but now a higher gear. So, the engine is now pulling you along at a higher speed, lower RPM, but is working hard to keep you there.

Relate that to a prop. Take a given prop 12-6, as your original post suggested. The 12 is the diameter, the 6 is the pitch. At 10,000 RPM, the prop is rotating 10,000 times per minute, and with a 6 pitch, that's 6 inches of air that passes through that prop each time it rotates. So, that's 60,000 inches of air in one minute. (actual numbers will vary because of other factors, but this is straight theory and numbers without the added factors which will lower that somewhat). So, again 60,000 inches per minute or 5,000 feet per minute, which is slightly under one mile per minute, or slightly under 60 miles per hour (88 feet per second or 5280 feet per second).

Lower the pitch and those numbers go down, so the aircraft will go slower, but the acceleration and "power" (not really, but you get the idea) will increase. It will pull that plane to it's maximum RPM for that prop at a faster rate. Rate of change = acceleration.

Now, for my last "comment". Do not equate noise to power. When you decrease pitch, the engine will tend to go to a higher RPM. That higher RPM will now give the appearance to more power and more speed. You won't go any faster, but you will get there faster and at the cost of more engine wear AND more noise.

As mentioned, get some props of various sizes and play with them. See what works out better. And, remember my last comment.. noise does not equate to speed.

CGr.

thnx alot guys! i asked cause my engine is broken in and i have been doing a vertical takeoff(i know thats dumb so dont say much plz, and plus i wait untill im high enough to come out of a stall) and my engine is still purring but the plane will stall and i was wanting to be able to hover or atleast climb somewhat vertically. i have a super trainer 60 by nitro planes with a magnem .61 fsx (i think thats the right letters).

Try an APC 13 x 4 W for wide. This will give more thrust in the hover.

Yeah, you want a good stiff prop for hovering. You definitely want as much thrust as possible out of the prop and some of the "lesser" props will not give you that. Some tend to flex quite a bit under load, and hanging on a prop is a pretty good load.

Of course, you could opt for carbon fiber prop.. (bring your checkbook, though).

CGr.

I have a carbon fiber prop on my Mag. 180 4 stroke. Going to the lake this evening and fly the Sea Dancer. Getting ready for the Midwest float fly in Sept.

If an engine is loaded with two different sizes of props - say the 12x6 and 11x7 - and both run at the same RPM, then the torque is the same. The larger diameter lower pitch prop does not "give more torque".

But you do have more thrust, with the lower pitch prop at lower speeds. The reason is that more prop blade length and area, and less of the prop is operating stalled.

The speed of the airplane is proportional to the one third cube root of the ratio of power applied. So if you double the power, use the correct prop, the airplane will go 26% faster.

Selecting the correct prop (pitch and diameter) is a matter of finding the load that allows the engine to produce it's maximum power and efficently tranfer the power to thrust. Often, the final selection is a question of deciding if maximum speed is the goal or maximum climb. No fixed pitch prop does it all.

Dropping down in pitch with the same diameter does offer in many cases the best overall performance. As long as the ground RPM is a couple of thousand below the rpm where the engine is rated for peak power, you are close to extracting the maximum power. Even with the lower pitch, the increased rpm will compensate for the lower pitch.

As far as engine life is concerned, the lighter loaded engine will have a much broader needle at the top end and be easier to set. Running lean is what kills engines.