AFP and Idle Manifold Pressure

[David-aviator]

Recently I got around to reading an article from a friend on rough idle issues. It was published in March 2009 in LPM (Light Plane Maintenance).

My engine idles OK, I read the article just to see what the author had to say about the subject. That led to an issue of relatively high manifold pressure at idle, an indication of a possible induction leak. The author says anything less than 16" is ok although 10-12" is better. I checked mine this morning and it is 14-15" during taxi, right on the edge of maybe a leak. Interesting. The intake pipe clamps need checking next time the cowl is off.

However, later in flight at idle power, manifold pressure was 8-9". Also very interesting some more. Does anyone know what's going on here? Does the FM200 controller have a feature to increases MP on the ground with low air flow through it?

With 8-9" MP in flight at idle, I don't think I have an intake leak.

I'd call Don at AFP but its a week end.

[Kevin Horton]

The MP at idle will be lower in flight than on the ground for at least the following two reasons:

1. The atmospheric pressure decreases as the altitude increases, so the MP for any given throttle position will also decrease.
2. Idle rpm will increase as the airspeed increases, due to the airflow through the prop. Thus the idle rpm will be higher in flight than on the ground, even at the same altitude. The MP will decrease as the idle rpm increases, as the engine is sucking harder against the closed throttle.

As a comparison, on my IO-360-A series engine, with Bendix fuel injection, the idle MP is about 10" on the ground, and about 7" MP at 2200 rpm at about 2000 ft (based on a quick look at some recorded flight test data).

[David-aviator]

Originally Posted by Kevin Horton

The MP at idle will be lower in flight than on the ground for at least the following two reasons:

1. The atmospheric pressure decreases as the altitude decreases, so the MP for any given throttle position will also decrease.
2. Idle rpm will increase as the airspeed increases, due to the airflow through the prop. Thus the idle rpm will be higher in flight than on the ground, even at the same altitude. The MP will decrease as the idle rpm increases, as the engine is sucking harder against the closed throttle.

As a comparison, on my IO-360-A series engine, with Bendix fuel injection, the idle MP is about 10" on the ground, and about 7" MP at 2200 rpm at about 2000 ft (based on a quick look at some recorded flight test data).

Thanks, Kevin. I have thought about the physics of the situation and already reached the conclusion expressed in #2. The in flight MP was observed turning final and for sure the windmilling prop is sucking manifold pressure down. It isn't turning 2200 but more like 1200-1400, enough to cause less than 10" MP.

The on ground figure is a bit high, but according to the article still normal.

I don't think there is a problem.

[islandmonkey]

Originally Posted by Kevin Horton

The atmospheric pressure decreases as the altitude decreases, so the MP for any given throttle position will also decrease.

Here in Europe, atmospheric pressure decreases as altitude increases.

[Kevin Horton]

Originally Posted by islandmonkey

Here in Europe, atmospheric pressure decreases as altitude increases.

Thanks. Fixed.

[islandmonkey]

Originally Posted by Kevin Horton

Thanks. Fixed.

Your welcome.

Hope the leg is getting better.