ORIGINAL: sandal
ORIGINAL: Lnewqban
Volume of air pumped by Propeller 1 <compare?> Volume of air pumped by Propeller 2
(...)
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.
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.
ORIGINAL: sandal
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.
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