Generally speaking what am I going to get the most thrust out of, I have a 12*8 turning 8500 approx, if I could get a 11*8 to do 9500 do you think I would get more thrust, eg smaller prop doing more rpm or larger prop turning slower

depends what is the blade shape?
If the shape is scaled up or scaled down is a simple matter of relating the area of one blade to the area of a similar blade on the other prop to determine how much air is displaced per rev (ignoring other losses and compressibility effects).

And is the engine capable of delivering the required rpm to deliver the thrust?</p>

What kind of flying do you want to do? If you are going for speed go with a higher pitch prop. If you want better climb performance go with a lower pitch. You will get more static thrust with a lower pitch prop also. Best way to find out what will work best is to test them out on your plane and see what you like the best.

Zach

You'll probably get more "static" thrust from the bigger 12x8 @ 8500 but that may or may not equal better performance in the air.

Can you upload this app. It would be very useful

Ameyam

I downloaded from http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm
I have heard that it gives "optimistic" numbers but it at least is a simple comparison tool.

Edited to add
The MVVS web site http://www.mvvs.nl has a Prop Power Calculator spreadsheet in the "Utilities" section. It was developed by Pe Reivers.

Let's make a little calculation for comparison:

Volume of air pumped by Propeller 1 <compare?> Volume of air pumped by Propeller 2

Air density x axial velocity of propeller 1 x area of propeller 1 <compare?> Air density x axial velocity of propeller 2 x area of propeller 2

Let's remove air density from both sides,

Axial velocity of propeller 1 x area of propeller 1 <compare?> Axial velocity of propeller 2 x area of propeller 2

Rpm1 x Pitch1 x (Diameter1)^2 x Pi/4 <compare?> Rpm2 x Pitch2 x (Diameter2)^2 x Pi/4

Let's remove Pi/4 from both sides,

Rpm1 x Pitch1 x (Diameter1)^2 <compare?> Rpm2 x Pitch2 x (Diameter2)^2

Substituting values:

8500 x 8 x (12)^2 <compare?> 9500 x 8 x (11)^2

Let's remove 8 from both sides,

8500 x (12)^2 <compare?> 9500 x (11)^2

1,224,000 <compare?> 1,149,500

Propeller 2 (11x8) @ 9500 rpm will generate 94% of the thrust produced by propeller 1 (12x8) @ 8500 rpm

That is only in theory

Regards!

Iuse the Prop Power Calculator but at the field I will grab a hand full of props in the general size range and start trying them out.

Yup.

I've used it extensively to compare actual wattage draw versus power claims.... and glow and gas engine power output predictions versus observed.

IMHO it is about 20% too high in it's estimates for most cases.

However if you adjust for this, it is a very good tool.

A propeller is more than just an "air pump" or a fan. A prop is shaped like a wing, and low pressure on top (front side of the prop) typically creates 2/3 to 3/4 of the pulling force. Only 1/4 to 1/3 is created by air being blown backwards. These numbers apply to full size airplanes like Cessna 172s and Piper Cherokees, but the principles are the same. The shape of the prop airfoil decides how the prop will perform. Thus calculating the volume of a cylinder won't be very helpful.

Back to the original poster's question: Comparing two props of different diameter, the larger-diameter prop will *generally* be more efficient; it will turn a higher percentage of the engine power into thrust. Some power is always "wasted" (noise/turbulence/...), and larger props tend to run "cleaner". Generally speaking. Pe Reivers' spreadsheet has been mentioned, and I recommend that you experiment with different prop sizes and see how the results change.

The 12x8 vs the 11x8: The 12x8 will most likely produce more thrust, and the 11x8 will most likely give you more speed. In theory. Type of airplane and engine will play a major part. The 12x8 will produce more thrust if the engine can pull it. My Irvine 53 will probably produce more thrust with the 11x8 because it lets it rev up to "where the power is".

If you go down in pitch to a 12x6, rpms will be closer to the 11x8. The 12x6 will definitely produce more thrust than the 11x8, but top speed will be lower.

Hope this helps more than it confuses.

Some engines are picky about the prop used. My Irvine 36 for example doesnt like to be loaded down with a heavy high pitch prop. It wants to build rpm fast or it tends to stumble. Iwent from a 10X5 to a 10X4 today and found the take off and vertical performance much improved despite the calculator telling me based on the rpms Iwas seeing the 10X5 was more powerful.

I disagree with sandal here:

That is the "Bernoulli versus Newton" old argument:

http://www.rcuniverse.com/forum/m_75...tm.htm#7510316

I would say it in the opposite way: that most of the fans are propellers wanna-be, or rustic propellers (no airfoil and no constant pitch), but the principle of moving a fluid based on acceleration and pressure differential still applies the same to both machines.

For bigger prop diameters (equivalent to bigger wing span), the lower pressure on the top, combined with the higher pressure on the bottom, act over a bigger area of propeller blade (or propeller disc).
As we all know, propeller thrust (equivalent to wing lift) depends directly on the area.

Air being blown backwards by a propeller is nothing less than the downwash or air moved downwards by that wing in the process of generating lift (considered as thrust in this case).
That downwash is produced by the difference in pressures between the top and the bottom of the surface (propeller's blade or wing).

As a comparison, my calculation is close enough, I believe.
Note that the calculator results shown in Post #4 above (which considered efficiency) estimated 88% of the static thrust of 12x8 for 11x8 at respective rpm's.

I agree with sandal here:

A longer wing is always more efficient than a shorter wing, even if both have the same area.
The main reason is that for the same loss of pressure differential and lift at the wing tips, there is more area "feeling" that pressure differential in the longer wing.

In the case of propellers is even more so, because the central area of the propeller disc produces less lift than the exterior area of the disk.
In the central area, the blades move slower, and the airflow is disturbed by the spinner and engine.

noginblue,

As others have explained, ideal propeller selection is more a practical art than theorical calculation.

As a rule, try to use the bigger prop diameter that your ground clearance and engine allow (based on desired pitch or plane speed versus recommended rpm's).
Airplanes with low drag can benefit from high pitch props; however, there is no much benefit in trying to speed up a draggy airframe (most of biplanes and trainers) by selecting high pitches and sacrificing always beneficial diameter.

Check this chart:

http://www.top-flite.com/accys/topq5000a.html