Question about variable-pitch propeller aircraft & slow flight

[Messerschmitt Man]

This question is specific for a normally aspirated Cessna 172RG with variable-pitch prop.

First & foremost I know the aircraft flight manual will shed some light on this but I wont get a chance to have a good read of it until next week, so I'm just after a generalized answer until then.

Say I have pax on board & we are sight seeing over a small city. I want to fly at a slow speed suitable for this situation, I would even put on a stage of flap.

What is better? Reducing the manifold pressure then bringing the pitch control to full fine & hence treat the aircraft as fixed pitch (throttle controls RPM etc) OR find the MP/RPM configuration suitable for the purpose of sightseeing at slow speed given the aircraft weight etc, for example 20"/2050RPM or 2100"/2150RPM etc etc.

The first option I think would be more suitable, especially if the turns are steep enough to require more power in the bank. Imagine doing a 45 degree turn with the second option, I'd need to increase the RPM then increase the MP & vice versa once I've leveled off, so much work compared to the first option!

[Gabriel]

This question is specific for a normally aspirated Cessna 172RG with variable-pitch prop.

Not saying that it doesn't exist, but I've never seen a 172 with either variable pitch or RG, let alone both together. Maybe a 182?

About your question, my experience with variable pitch is very limited so take what follows with a grain of salt.

First, it's not true that with the prop in the full fine / high RPM setting the engine behaves like a fixed pitch plane. As long as the prop has available range of pitch to change even in full fine the pitch change with constant RPM will have priority over an RPM change. If the pitch movement hits a stop and cannot change any further, then the RPM will have to change.

Second, I'd use a climb RPM, or even cruise RPM. Then with throttle movement alone that should cover a good range of power to cover from slow straight and level to a level or climbing turn at any sound bank angle. So you are operating one singe lever. If you set full fine and then during some maneuver you feel you need full power and apply full throttle you might exceed the engine limitations.

But what if you set the prop for 2150RPM, as you've said, and then when you need power you advance the throttle and the manifold goes to 23"? That would give you a manifold higher than the RPM (23 higher than 21.5).

There is a myth that the manifold should never exceed the RPM. In the few POH I've come across and from talking with other pilots I've never heard of that requirement. In fact, since several combinations of manifold and RPM can give you the same power output, I've heard advice from very knowledgeable people that in cruise it's better to use the slowest RPM and highest manifold within the approved engine envelope that will give you the desired power. Slower RPM means less power is lost in internal friction, and higher manifold means more open throttle which means that the air flows more freely (with less resistance) into the engine. Both things mean that, from the power the engine is developing, more power is used to turn the prop and less power is wasted in loses. Which means less fuel burn for a given speed or more speed for a given fuel burn (in any case, better MPG).

[Curtis Malone]

Not saying that it doesn't exist, but I've never seen a 172 with either variable pitch or RG, let alone both together. Maybe a 182?

They're not real common, but they do exist. The model was marketed as 172RG "Cutlass", but was lovingly referred to as "Gutless".

Here's one:

http://www.controller.com/listings/aircraft-for-sale/CESSNA-172RG-CUTLASS/1981-CESSNA-172RG-CUTLASS/1159563.htm

[Gabriel]

They're not real common, but they do exist. The model was marketed as 172RG "Cutlass"

Thanks. I don't know why but I thought that the Cutlass was a 182RG. My bad.