# Selection of airfoil and sizing of propeller

1. May 8, 2015

### shaft

Two problem to solve.
1) I have 4 propellers built with different airfoils. I have the graphs of coefficient of thrust and efficiency in function of advance ratio. There is a mathematical method to select which one is the best or the only way is just look at graphs
2)I know the maximum thrust necessary at my aircraft so I need to found the right dimension (diameter) and number of blade for a propellers .
I can calculate the thrust produced by the propeller but this change with the advance ratio and the pitch angle so really i don't know how to select the best diameter and number of blade.

2. May 8, 2015

### SteamKing

Staff Emeritus
A propeller is a device which converts torque from an engine into thrust. Selecting a propeller is more than just picking one which produces the right thrust. You also must consider the engine driving the propeller and what its torque characteristics are.

3. May 8, 2015

### shaft

but in the ideal case that I can have always the best engine appropriate for the propeller ?

4. May 8, 2015

### SteamKing

Staff Emeritus
That's not the point. You are selecting a real propeller driven by a real engine to add thrust for a real plane.

If the propeller can't absorb the torque output by the engine at its speed of revolution, then the propeller also won't produce the amount of thrust you think that it will.

5. May 8, 2015

### shaft

yes you have right, but in the case that I can select the engine after the selection of propeller, in this way it's the propeller that give the characteristics to the engine.

6. May 8, 2015

### SteamKing

Staff Emeritus
Good luck with that approach then.

7. May 8, 2015

### shaft

so if i don't know the characteristic of the engine I can't built a propeller ?

8. May 8, 2015

### SteamKing

Staff Emeritus
You can build any number of propellers, but you want to build (or select) the propeller which is the most efficient at turning the available torque produced by your engine into thrust for your plane.