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Constant speed prop use simplified...

I know this thread has been beat to death but I am old and still confused! In Mike Bush's webinars and seminars I understood him to say that once at cruise to set the prop at the lowest RPM possible and firewall the throttle to the highest MP that you can get. My RV is fixed pitch and does not apply but I just had a second instructor In a Comanche 250 that I own a share of say to set it at 24 squared and never let the MP be higher than RPM. the operating handbook offers many different pitch and power settings at different altitudes, so who is right?

John Morgan
 
Check the power chart for the engine from the engine manufacturer. It is always correct.

Your instructor is teaching what he was taught. MOST engine power charts show that you CAN operate over square. Over Square is higher manifold pressure than engine RPM / 100. One example of a higher manifold pressure than RPM / 100 is on every fixed pitch aircraft when you advance the throttle on takeoff and only get partial RPM. Yes you are over squared with a higher manifold pressure than engine RPM / 100.

My O-320 Lycoming power chart shows that I can operate 6" MP over Engine RPM / 100 at most (not all) RPMS. I attempt to limit continuous MP greater than Engine RPM / 100 to 3 inches.
 
morganjp said:
I know this thread has been beat to death but I am old and still confused! In Mike Bush's webinars and seminars I understood him to say that once at cruise to set the prop at the lowest RPM possible and firewall the throttle to the highest MP that you can get. My RV is fixed pitch and does not apply but I just had a second instructor In a Comanche 250 that I own a share of say to set it at 24 squared and never let the MP be higher than RPM. the operating handbook offers many different pitch and power settings at different altitudes, so who is right?

John Morgan
Click to expand...

If you are paying him to sign something, the instructor is right until he gets the **** out of the airplane. :) As has been said multiple times in this thread, the old tribal knowledge of "never oversquare" is BS. Your operating handbook sounds like a reasonable person wrote it.

I used to follow those old rules when renting. Now, with my bird, I use the full range of RPM and MP combinations allowed on the Lycoming charts that came with my engine (No Hartzell limits). In effect, I use the prop control for power adjustment more than the throttle. Pretty much how Deakin describes in his Pelicans Perch articles. I REALLY like the technique of full throttle LOP decent with drastically reduced RPM. Its nice on the CHTs.
 
the answer to the balls to the wall question is, not exactly. In the T-6 we have a very accurate digital tach. On the course at Reno during a race, the RPM, with WOT was increased 25 RPM. We lost about 2 MPH. We have worked hard to find the exact RPM that gives us the best speed. Interestingly, that RPM changes with different props of the same model number. so full RPM at WOT may not get you all that you can get.

bob burns
RV-4
N82RB
 
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