Manifold pressure for FP
 

It's interesting to be tooling along and everything in the plane is fat and happy and starting thinking, "I wonder....."

Today's flight "I wonder" question:

"I wonder what the value is of the manifold pressure indication is for a fixed pitch propeller? What is the significance of, say 22" at 2400 vs., say, 24" at 2400?

Significance is less power (i.e. less torque to spin the crankshaft/prop) which equates to less pull through the air, which equates to less speed in cruise or less climb capability or, well, less anything that requires the ability to pull or push (if the case may be) the aircraft through the air.

I installed a MP gauge in my O-360 FP RV-4. I used it for the 48 rule of thumb. That is, if the MP in inches and the RPM in thousands totals 48 or less than the engine is running at no more than 75% power, therefore leaning can be safely performed.

Or so I am told....

I remember when I was working on a certification project with the FAA. They wanted a backup manifold pressure indicator along with the electronic one.
At some point, one of the FAA engineers, exasperated by his fellow regulators... said: " if you can hear engine noise.... it is still running!!"
Having said that, most of us have MP on a readout somewhere. But, we all flew FP for many years in trainers without one. You really won't miss it at all I suspect.

So I assuming 22 at 2400 isn't as efficient as 24 at 2400, what is the process to increase to 24?

If the prop is swinging through the air at 2400 in either case, does it matter? It must be swinging through at the same speed, right?

I've always used MP to adjust power. MP is somewhat instantaneous while it takes several seconds for rpm to settle.

I've played with the idea the, hey its just a barometer, so I should be able to use it to set my altimeter, before starting the engine. However, mine doesn't have the resolution to be very accurate.

I have also heard it can be an indicator of carb icing, I believe it will start falling slightly if ice builds up.

This question depends on what your definition of efficient is. If you are referring to efficiency of burning fuel, 22 inches of MP will burn less fuel than 24 inches, due to the decrease in power produced by the engine which will take less fuel to produce. However, in terms of efficient movement of the airplane through the air, 24 inches of MP is going to move the airplane more efficiently than 22 inches of MP will. If your definition of efficiency is related to the speed of the aircraft through the air then more MP will be more efficient than less MP.

As you can see, the idea of efficient is a very relative term.

As for your second question concerning swinging the prop at 2400 RPM and getting 24 in MP at the same time. A fixed pitched prop airplane is going to be limited on its ability to run at "24 squared" (24 in. MP/2400 RPM). You will be able to do so perhaps at a specific range of altitudes while in cruise but will not be able to do so during high power climb or at high altitudes. The engine [Upower[/u], whatever it may be, is not going to be able to overcome the physical resistance of the weight of the airplane, air pulling against the prop, decreased amount of air flowing through the engine, etc. that is restricting the ability of the engine pistons to push the crankshaft.

Bob, if you have more MP then your engine is producing more power. You can have a steeper pitch on the prop and go faster for the same RPM. It generally takes more throttle and more fuel to do that.

Efficiency is a different thing. I'd think of that as fuel consumption for a given speed, and while that's dependent upon the overall power output, it also depends on many engine design characteristics. The compression ratio is a major factor, for example.

Dave

MP is one of parameters (along with RPM, TEMP, and DA) that allow you to go to the Lycoming engine performance charts to find the engine output HP.

The value for fixed pitched installations is to allow the pilot to determine engine performance should an alternate prop pitch is being considered. Obviously aircraft performance can indirectly give you that info too...

BTW, the "square rule" and others are simplifications or approximations of these charts for pilot/operational convenience.