Originally Posted by
byrne1157
Wow! I guess I don't have as much experience as some of the rest of the guys on here, but I wanted to post a few observations and/or questions that could be food for thought.
1. I notice that most, if not all of the props I see are what I would call "Graduated Pitch" profiles. I am going to guess that is because of the need to reduce pitch the further you get from center due to what I will call "Leverage Loading" of the prop next to the hub.
2. Considering that fuel economy is probably the best way to measure prop efficiency, aircraft speed could be the best indicator of this. (Least time in the air at a given altitude to cover a given distance, while burning the least amount of fuel.
3. It seems a little hard to compare full-scale aircraft propping to model aircraft propping, since most full-scale aircraft utilize variable-pitch props. Kinda like apples and oranges.
4. This is quite the dilemma, it makes me think all sorts of things. Lots of fun to be had here...
The twist on the propeller is actually to ensure it maintains better consistency in angle of attack along the blade length at normal rotating speed and in forward flight.
There are many diagrams to represent this here
https://www.google.com.au/search?q=p...&bih=576&dpr=1
Basically.. in any fixed pitch propeller (full size included.. eg Cessna 152) the inner part of the propeller is stalled at full power with zero airspeed.. as your airspeed is increased the angle of attack of the air striking the propeller blade reduces and at flying speed most of the propeller should be working efficiently.
The speed of the blade is faster at the tip and therefore the blade angle is reduced by the graduated twist you mentioned. Aerodynamically, this twist maintains efficient angle of attack along the length of the propeller blades at forward flying speeds.
A constant speed propeller widens the speed range at which the propeller can operate at more efficient angles of attack.
As for the original argument.. it is quite simple.
One Blade is very efficient, but causes asymmetrical loads on the engine shaft.
Two Blade is the next most efficient and has balanced loads on the engine
The propeller has to absorb the power output of the engine. So when using 2 blades, in theory, the more powerful the engine, the larger the propeller must be.
The limiting factors become.
a, Propeller tip speed (Noise and efficiency losses if tip speeds approach the speed of sound)
b, Physical size
So if you have too much power for a practically sized 2 blade propeller, then use 3 or more blades, and accept a slight efficiency loss.
