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3 Answers

How to use constant-speed prop

Asked by: 18304 views Aircraft Systems

One plane I occasionally rent is a 172K that has been retrofitted with a 180-hp engine and a constant-speed prop.  I guess because it's a retrofit the plane's POH doesn't cover operation of the prop control.  I know enough to use full RPM for takeoff and landing, and to use a coarser pitch for cruise, but I'm not really sure how to know the optimum manifold pressure and RPM settings for any given mode of flight.  The FAA's Airplane Flying Handbook doesn't have a lot to say about this, other than to increase RPM before increasing MP, and decrease MP before decreasing RPM. Another pilot who flies it told me "Use 24 inches and 2400 RPM for en route climb."  Beyond that, and keeping the needles in the green arcs, what else should I know to use this system properly?

3 Answers



  1. Nathan Parker on Aug 16, 2011

    You surely have performance chart supplements for the new prop arrangement.  The settings there should be your guide, not the 24/2400 superstition.
    You basically have a takeoff setting of Max RPM/Full Throttle, and then a cruise setting that comes from your AFM based on your desired performance.  Then leave the prop control alone until you advance it on final.
     

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  2. John D. Collins on Aug 16, 2011

    A constant speed propeller/governor is not a controllable pitch prop in the sense that the pilot sets the prop control to determine the RPM, not the prop pitch.  The governor will adjust the propeller to whatever pitch it needs to maintain the RPM, as long as the prop pitch is not at the high or low stop.  Normally the propeller control is set to full forward or full in for maximum RPM.  Full throttle and maximum RPM are used for takeoff and climb.  In the old days, you would be taught to reduce the manifold pressure and RPM after takeoff and add back manifold pressure as you climbed, for example 25 square, which means set the manifold pressure to 25 inches and the RPM to 2500.  This was found to be harder on the engine than leaving the throttle at the wide open position. The reason for this is that the throttle linkage provides for additional fuel at the full forward position and provides extra cooling in the climb.  When you retard the throttle, the mixture is leaned, the engine runs hotter, and performance is reduced. Most engines are rated for continuous operation at full power which means full throttle and maximum RPM.  Typical maximum RPM values are between 2500 and 2700 RPM. 
     
    I would recommend a full power climb to altitude and then reduce the RPM to the cruise setting.  If you level off at an altitude below 7,000 feet, you may want to reduce the manifold pressure to a cruise setting.  Above the 7000 foot point, most engines will be run at wide open throttle.  There should be a performance chart that goes with the installation and included in your POH.
     
    When you are operating the engine on the rich side of peak EGT, the amount of air you deliver to the cylinder will determine the power setting. The amount of air delivered to the engine is based on several factors, the manifold pressure, the  altitude, and the temperature, so you will see these factors in the POH performance table.  If you are at full throttle and leveled off, the only control you have to set power is the RPM.  A higher RPM will result in more power.  One can always reduce power by either reducing the manifold pressure or the RPM or both. If you set a power setting and can adjust both manifold pressure and RPM, the lower RPM with the highest manifold pressure allowed will normally be the most efficient.
     
    Most engines have RPM limits for a given manifold pressure, for example on my engine, I can use any RPM down to 2200 with a manifold pressure of 24 inches.  Below 2200 RPM, I have to reduce the manifold pressure according to a chart.  I never operate at the lower RPM’s because the engine runs smoother above 2300 RPM, in fact my engine has a sweet spot between 2400 and 2500 RPM. 
     
    So where to run your engine.  Follow the POH and use it to determine and set your power.  There are more choices than needed, but it is up to the pilot.  In my case, I climb full power and after leaving the pattern will just make a reduction of my RPM from 2700 RPM to 2500 RPM for the cruise climb setting.  I lean as I climb. When I level off, I let the airplane accelerate and then lean to cruise setting. For me, I don’t touch the prop control for the rest of the flight, with a possible exception to advance it on short final, although that isn’t really necessary as I would have more than enough power for a go around if I needed it at 2500 RPM.
     
    If you want to get the airplane anywhere near the service ceiling, you will have to increase the RPM or you won’t have the power to get there. So for me above 12,000 feet I will increase the RPM to 2600 and above 14,000 I will go to 2700 RPM and keep it there for the cruise at high altitude.
     

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  3. James MacGregor CFI on Aug 16, 2011

     On most engine conversion STCs there will be an addition to the POH or a separate handbook just for that mod.
     
    Sounds like yours went AWOL, many of the STC addition handbooks tend to get filed away or lost, especially in rental aircraft.
     
     Find out who the STC is from (i.e. the name of the conversion, like “Delair 180hp engine & prop mod”), this will be in the aircraft log books, then call up the shop that sold the STC.
     
     I’m sure the holder of the STC can fax/email you all the numbers you need for rpm/man settings, fuel burn numbers, misc other info you might want to know as well.

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