How to improve the efficiency of a propeller?

[PhyIsOhSoHard]

Homework Statement

One of your friends is building his own airplane. He has received a quotation for a two-bladed propeller with an efficiency of $\eta = 80\%$ . He asks for your advice on whether it is possible to get a higher efficiency - what is your answer?

• no, the efficiency based on physics not be improved
• use a large diameter propeller
• use a propeller with more blades
• use a higher rotational speed
• use propeller blades with a larger chord

The correct answer according to my book is "use a large diameter propeller" however his propeller already has an efficiency of 80%, is it physically possible to make it higher than this?

Here is a graph that shows the efficiency of an aircraft but it doesn't go higher than 80% efficiency:

So how is it possible to increase this efficiency by making the diameter larger?

For the record, the speed of advance coefficient (J) is:
$J=\frac{V}{nD}$
Where V is the velocity of the aircraft, D is the diameter and n is the revolutions.

 

[Simon Bridge]

The graph shows how the efficiency depends on speed, (for a fixed diameter).
A larger diameter may move more air with a smaller increase in drag compared with the other approaches.
But it may not translate into gains against the advance coefficient.

If you increase D, you decrease J ... what happens to $\eta (J)$?

iirc - 80% is pretty close to the top efficiency of a prop.

 

[AlephZero]

The ideas in your OP might give you more power or thrust, but not higher efficiency. You will need a more powerful motor to drive the bigger/faster prop.

You can't make a prop that is 100% efficient, because it can only generate thrust by moving air about, and some energy must be "wasted" in the propellor wake left behind the plane.

 

[rcgldr]

If you increase D, you decrease J ... what happens to $\eta (J)$?

Assuming V is to be the same, then n would need to be decreased if D is increased, and J could end up being the same, and the curve for $\eta (J)$ might shift upwards by a small amount. If the propeller is already at 80% efficiency, then it's probably at a point of diminishing returns for increasing D.

 

[Simon Bridge]

@rcldr: thanks but - that question was pedagogical - I know what I'd answer ... I want to see what OP answers.

Comes down to how you interpret the question though doesn't it?
Take each choice in turn - all of them have something objectionable about them.

 

[CWatters]

One simple explanation for the book answer is that the tips of any wing are less efficient than the mid span. So the higher the aspect ratio the less effect the inefficient tip part has on overall performance. It's one reason why gliders have high aspect ratio wings.