RPM vs. MP

[Michael]

Hello everyone, hope you all had a nice Thanksgiving.

Yesteday I took a friend flying who has owned many planes and logged 4000 hours. He currently flys a bonanza. I was checking him out to fly my plane solo. He asked me some questions that I couldn't answer other than "because that's the way everyone does it."

The first was why do I fly with RPM at 2450 (and MP 22-23) when I could achieve the same power with lower RPM and higher MP. His point was the plane would be quieter.

Another was can I fly to the ground (or very near) while keeping the MP in the green if I use more flaps? He then demonstrated this by bringing down 40 degrees of flaps and pushing MP to 15 giving us a normal glideslope that was in the green. On short final he went to 20 degrees and pulled power back (mostly because I told him it really wasn't all that safe to land with flaps 40 because you might forget your trim on a go around and have a fight to keep the nose down).

So, those of you more experienced guys now have an opportunity to explain why we do the things we do!

Michael
97AV

[Todd Peterson]

I normally tell everyone to use 2,450 rpm as this is the smoothest spot to run at. For some reason the IO-470 really seems to like 2,400 to 2,450 rpm at most any altitude and I am a firm believer in running an engine where it's happiest. Extra vibration just doesn't do the engine any good, likewise with the baffeling, exhaust system, etc. Normally when you start bringing the rpm back into the 2,250 to 2,350 rpm range you will feel an noticeable increase in vibration. I have run many of these engine at 23 square and it just doesn't feel all that happy there. With all that said there shouldn't be anything wrong with the lower rpm if you don't mind the extra vib's.

Normally I don't recommend dropping full flaps then reducing the flap setting on final to help out with the engine temps. Anytime you change the flap setting you must then readjust trim, speed and power. For a lot of pilots this is just adds to their workload and I would say reduces the safety factor somewhat. Personally I'm much more into a good stabilized approach where you can concentrate on airspeed and glide path control with a minimum of effort. I think it's just safer that way. Also once in the pattern slowed for the approach shock cooling is just not much of a problem. Keep the cowl flaps closed and in the course of the approach you will have some power on anyway so there really isn't a problem. Many pilots will also leave the mixture leaned as the higher EGT's will really stabilize the CHT's. Just remember to push the mixture back in if you have to go around.

Jo did a dead stick aerobatic routine for years with one of our airshow aircraft. After a full power climb to altitude Jo then shut off the engine and stopped the prop. Once this was done she then pushed the nose down for airspeed to start the routine. You can't shock cool an engine worse than that and we never had to pull a cylinder.

[Mike Sullivan]

Wouldn't balancing the prop help with vibration at all rpm's?

Mike