Friday, May 26, 2006


"2 blades or 3?"

Lots of friends ask what prop are you going to put on the front, and then quickly ask "2 blades or 3?" This lead me to think about the relative advantages of one over the other. I should say first of all I fly out of a short strip and a constant speed prop has a very significant advantage for landing, so I had already made that decision in favour of constant speed.

I am not an aerodynamicist, but after some thought it appears to me that the 3-blade prop has all the significant disadvantages. They are:

- it costs more.
- it is harder (more expensive) to ship.
- there is more of it to get damaged and repair.
- it is less efficient and therefore very slightly slower, though I suspect this is academic.
- it is heavier, therefore increases the empty weight.
- it makes the cowls significantly harder to get on and off since the lower cowl has to come forward to clear the airbox, (and the training gear leg if you still use one.)
- you can never put it out of the way in a crowded hanger.

The one genuine advantage is it is very slightly smoother, though the 2-blade MT is very smooth.
For an (I)O-320 the radius of the appropriate 3-blade MT prop is 1/2 inch less. If you have the tail that high it seems unlikely that this will save you from a prop strike. This difference in radius is hardly going to make any difference in tip speed so the noise difference is minimal. The most frequently quoted reason for 3-blade is that its "sexier". I guess if you are that is the basis of your decision then there is no point in reading this. Get down to the hairdresser :-)

There is one frequently stated reason which I think is bullshit. If it were true I would be very interested, but I can see no reason why it should be. The stated reason is that it acts as a better air brake. I can see no aerodynamic justification for this statement. If someone knows a good reason why it is true I would love to know.

Postscript dated 12 September 06.
I really wanted to run the air brake or disking issue to ground, so after an abortive attempt to find someone who understood these things on RVSQN, asked the same question on Vansairforce. It is an unusually complex subject. What I concluded from the conversation is that for a given diameter the 2-blade should probably provide better disking, however it is more complicated than that. eg How much of the swept area is solid? In practice, it is pretty clear from users of the 3-blade MT on 200hp engines, that for stopping, the 3-blade IS more effective. They also complain that at the top end it appears to limit the top speed. This conversation was useful to me in that it confirmed to me the intelligent choice for those not in doubt about their manhood was 2-blade! :-)

You can see the discussion thread here and draw your own conclusion.

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