Question on MCP, Power and Prop Settings

[tbaac]

The manual mentions that cruise should be at 75% power with a new engine.

This I guess is 75% Max Continuous Power (MCP).

I'd thought that power was indicated by manifold pressure, regardless of the prop setting.

However, the Cruise Performance tables on pages 52 and 53 of the manual seem to suggest that for the same % of MCP, MP will need to be different depending on the prop setting.

I'm missing something I think. Is there a page in the manual that I would help?

Thank you.

[ft]

For a given RPM, power is (roughly, there are other factors such as exhaust backpressure) governed by MAP. However, the same MAP will give different amounts of power at different RPMs.

The amount of mixture burned is a measure of the power produced - each gram of gas contains a given amount of energy. The amount of mixture sucked into the cylinders on the intake stroke is controlled by the pressure in the intake manifold (MAP) and the speed of the stroke (which is RPM dependent). Higher RPM means less mixture per cycle for a given RPM (as the pressure drop over the intake valve will be greater with the higher flow) and less power per cycle. However, higher RPM also means more cycles per unit of time and thus more power. The latter effect should generally dominate.

Cheers,
/Fred

[Great Ozzie]

This I guess is 75% Max Continuous Power (MCP).

Yes, correct.

I'd thought that power was indicated by manifold pressure, regardless of the prop setting.

However, the Cruise Performance tables on pages 52 and 53 of the manual seem to suggest that for the same % of MCP, MP will need to be different depending on the prop setting.

Power setting will be based on both the MAP and RPM... so you are reading the chart correctly.

For example, at a D.A. of 2000', you could get 74% power with 23" / 2400rpm or (about the same power) 73% using 26" / 2000rpm.

[tbaac]

Thanks for the response Fred and Rob.

There's more numbers to learn than with the C172

[Great Ozzie]

Well... the relationship how MAP & RPM effects Horsepower... that good to be clear on.

A little pic below to help. Btw, if you print checklists, I'd consider printing out the performance charts. Makes it easy then to pick a % Power based on the current chosen altitude.



(from Aircraft Propellers & Controls - An IAP Training Manual)

[tbaac]

Thanks for that Rob.