Adjusting MAP on an FP Setup

briand · #1 04-23-2007, 10:24 AM

I thought I read in a thread where someone claimed to be using MAP to adjust the rpm of their FP prop. I have never heard of this. Can someone explain this in detail? Or is this just chain pulling.

n5lp · #2 04-23-2007, 10:30 AM

Quote:

Originally Posted by briand

I thought I read in a thread where someone claimed to be using MAP to adjust the rpm of their FP prop. I have never heard of this. Can someone explain this in detail? Or is this just chain pulling.

MAP does control the RPM of a FP prop assuming all else is equal. In level flight increase MP by increasing the throttle setting and RPM goes up. You can also increase RPM by leaving MP the same and lowering the nose.

Mel · #3 04-23-2007, 10:35 AM

I always use MP to adjust my engine. Once you are familiar with you setup, it is much faster to adjust using MP as it responds immediately. It take several seconds for the rpm to respond. Also during decent, you can use MP to reduce power an inch or two at a time to avoid shock cooling.

briand · #4 04-23-2007, 12:17 PM

Quote:

Originally Posted by Mel

I always use MP to adjust my engine. Once you are familiar with you setup, it is much faster to adjust using MP as it responds immediately. It take several seconds for the rpm to respond. Also during decent, you can use MP to reduce power an inch or two at a time to avoid shock cooling.

Would I need a three lever quadrant and 3 cables to do this?
Is it basically set up the same as if you had a cs prop only with no prop. gov. or hose going to the prop?

Mel · #5 04-23-2007, 12:19 PM

Manifold pressure is controlled solely with the throttle. Power is controlled with the throttle. Manifold pressure is simply a measurement.

RVbySDI · #6 04-23-2007, 01:14 PM

Quote:

Originally Posted by Mel

Manifold pressure is controlled solely with the throttle. Power is controlled with the throttle. Manifold pressure is simply a measurement.

This post is intended to clarify the idea being discussed in the above posts about Manifold Pressure posted by Mel and Larry. Either or both please correct my thinking if I am incorrect.

So to fly by the manifold pressure I will use my throttle lever to adjust the speed at which the engine is receiving fuel/air. This I will monitor not by observing the tachometer, which will read out the revolutions per minute (RPM's) that the engine is turning, but rather, I will monitor the MP gauge, which will read out the pressure of the air/fuel in the combustion chamber of the engine as it is processed? By monitoring the MP gauge I can more closely control the throttle setting for the engine and better manage the fuel burn, temperatures, etc.? In so doing the MP gauge will allow me to make more minute adjustments to the throttle setting?

Is this an accurate explanation of what you guys are saying?

Mel · #7 04-23-2007, 01:23 PM

Exactly! You may still monitor the tach for rpm, it's just that the MP responds immediately so you don't have you head "in the cockpit" waiting for the rpm to stabilize. If the rpm stabilizes lower than you want, crank in another inch of MP, etc.

Radomir · #8 04-23-2007, 01:42 PM

Can someone explain what the difference is whether you fly by RPM or MP.. why would you do MP over RPM on a FP setup (when you can't change 'em separate from each other like CS guys can)? Sounds like you're choosing a different arbitrary number to fly by...

airguy · #9 04-23-2007, 01:57 PM

Quote:

Originally Posted by Radomir

Sounds like you're choosing a different arbitrary number to fly by...

Exactly correct. The only big difference is that upon throttle change, MP difference is immediate, RPM difference may take several seconds.

hevansrv7a · #10 04-23-2007, 03:16 PM

Just my 2 cents, but MAP has a longer scale which lends precision and it is more linearly related to actual power. You can see the same RPM going, for example, up hill or down hill, but with different power. MP combined with RPM is power (yes, I know there are altitude corrections). If your prop is a Prince, then MAP is better because the prop flexes under different load conditions. Elsewhere on this site Walter Atkinson and others have discussed at length the relationship between MAP, RPM and power. It's just as true for FP as for CS except with FP you have less control of the variables independently. Walter says:
HP =100-((((2700/100)-(RPM/100))*2.5)+((29.92-MAP)*3.5)).
I think I have this right and the parens are for making it work in Excel.