Question about manifold pressure to propeller rpms in twin engine aircraft?

Question

My question is, if you were flying in a twin-engine aircraft such as a beechcraft baron, and when you increase/decrease manifold pressure with the throttle, do you always have to match it with the propeller rpm speed when changing altitude and in cruise altitude? And, do you set max propeller rpm when you are on final approach or sooner?

I'm just trying to figure out how important this ratio is with the propeller rpm when you are doing manifold pressure adjustments in climbs, decents and level flight. Thanks in advance!

Answer 1

a manifold pressure change does not always have to be followed by an rpm change. For the most part, rpm is set for the phase of flight and manifold pressure is changed as needed.

For example, in a climb, it is common procedure (every airplane is different) to pull the rpm back a little bit, a few hundred rpm from redline, but the manifold pressure must be changed throughout the climb.  As you increase altitude, manifold pressure decreases with a constant power setting, so every 1,000' you will loose 1" of manifold pressure.  And the change in pressure does not result in a change in rpm.  The reverse is true with a decent.

With level flight, pick a power/rpm setting that gives you the speed and fuel burn you want. the POH has all that information for you. Just avoid high manifold, low rpm settings. Some people say it is ok, Jimmy Doolittle proved its safe to do years ago, but its still said to be bad practice.

As for when to go to full rpm, different people like to do it at different places. Check your POH for the right answer. I have a 182RG checklist in front of me, full rpm is under the "before landing" check list, so really any time before landing is ok.  The point here is to have rpm to full so if you have to go around, you will be prepared ahead of time. Personally, I go full rpm with my before landing flow. Fuel, gear, flaps, mixture, prop, throttle, carb heat, lights, seat belts.  I know people that like to keep the rpm low through the pattern to be easier on the engine and go full rpm on final.

Answer 2

There are several things to consider; phase of flight, what power setting (% max) you want, etc.

Think of MP as power, like the accelorator on your car, and RPM as the gear you're in.  On takeoff, you want lots of power, so full throttle to max MP and the prop full forward (what the British call "fine pitch") to get the most RPM.   Once at altitude in cruise, you want less strain and more efficiency; so, MP back to whatever power you're using (see the POH) and RPM to a similar cruise setting.  This is like shifting to overdrive on your transmission in your car, higher ration/pitch gets you less RPM but the same forward motion.

Then, if the engine is turbo- or super-charged, you have to take that into consideration with the old adage:  "never exceed squared".  

For example, the GSIO-540s on a Cessna 421 have the waste gates closed at about FL220 -- you can't get any more MP if you go higher and since the engine is geared, the props are out there turning about 1800 RPM at over 200 true airspeed.  Yet on an R-1340, with a supercharger, you have 10" more MP any time, any where -- so, 27"/1750 RPM is perfectly acceptable as a cruise power setting [with the 10" boost, you're still squared].

Hope that helped.