With the possibility that someday we SR20ers will have a turbo option, either diesel or add-on, I can’t help but wonder which of the props available to us from CD is better at altitude. The notion that the 2 and 3 bladed props are equal to each other in all phases of flight, be it low altitude climb, cruise, and high altitude flight, strikes me as counter-intuitive. Any insight on which prop climbs better when the altimeter reads, say, 15,000 feet? The answer may be useful in future years when a new turbodiesel is available to swap for our IO-360s. aa
The following is from the Hartzell web site (lin included at the end):
(Hope this helps)
If two-blade propellers are the most efficient, then why donÂ’t all propellers have two blades?
The short answer is because efficiency doesnÂ’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will be highest. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.
http://www.hartzellprop.com/engineering/index_engineering.htm
With the possibility that someday we SR20ers will have a turbo option, either diesel or add-on, I can’t help but wonder which of the props available to us from CD is better at altitude. The notion that the 2 and 3 bladed props are equal to each other in all phases of flight, be it low altitude climb, cruise, and high altitude flight, strikes me as counter-intuitive. Any insight on which prop climbs better when the altimeter reads, say, 15,000 feet? The answer may be useful in future years when a new turbodiesel is available to swap for our IO-360s. aa
http://www.hartzellprop.com/engineering/index_engineering.htm
This is a fascinating site, which essentially makes a strong “it’s not just about sex appeal” argument in favor of 3-blade props. Yes, yes, I understand Hartzell’s self-interest in making this argument. Still their points about acoustic dynamics are interesting, and more complicated than I had thought. I had thought it was just a matter of blade-tip velocity: the greater the number of blades, the shorter each blade is, and the shorter the blade, the lower the tip velocity. But there apparently is more to it than that.