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3 Answers

Why Do Constant Speed Prop Airplanes Have Manifold Pressure Gauges? (91.205)

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Aircraft Systems, FAA Regulations, General Aviation

As far as I know, 91.205 only requires manifold pressure gauges for aircraft with high altitude engines, which I understand to be engines with turbo/superchargers. Yet, every controllable pitch prop plane I've seen has a manifold pressure gauge. Is it because the higher pitch that can be used at higher altitudes yields seal level performance and therefore is categorized as a high altitude engine?

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3 Answers

  1. Best Answer


    John D Collins on Nov 26, 2014

    A minor correction, 91.205 specifies the requirement for a manifold gage for each altitude engine, not high altitude engine. The term altitude engine is defined in 1.1 as “Altitude engine means a reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude”. So your basic understanding is correct for the term, even though you added the word “high”.

    The reason that an aircraft with a constant speed propeller uses a manifold pressure gage is for setting the power, particularly with rich mixtures. This is because power under these circumstances is determined by the amount of air (oxygen) being delivered to the engine as the supply of fuel is greater than needed. If one had a form of an air density flow meter, this could be used for this purpose. But alas these are not available in engines. With a fixed pitch prop, the parameters that affect the power are RPM, OAT, and Density Altitude, so RPM is the primary indicator of power, but one must use look up tables to factor in the other parameters to determine the power setting, With a constant speed prop, the RPM is set by a governor and the prop pitch is varied as more or less power is demanded as long as the prop is inside of the governing range (ie not up against a high or low pitch stop). As you advance the throttle in with a fixed pitch prop, the RPM increases. With a constant speed prop, the RPM remains unchanged and the MP increases. Therefore, you can’t determine power (air flow) from the three parameters listed for a fixed pitch prop and need the fourth parameter, manifold pressure. From these four parameters, you can establish power settings by looking it up in a power table for that aircraft and engine, There is more than one combination of MP and RPM that may be used for a desired power setting, although many POH will simplify the choices be selecting a single RPM.

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  2. Lucas on Nov 26, 2014

    “if a picture is worth a thousand words, an animated one most be worth trillions”
    Here is a video we created a while back on propellers. Hopefully this gives you a clear picture that words just can’t.
    http://youtu.be/TR7KZdHLkE4

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  3. Drew on Nov 28, 2014

    Thanks for both your helpful answers.

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