constant speed prop question

Six pilotSix pilot
in General Discussion
My owners manual for my 1978 Cherokee 6-300 has 4 different rpm and manifold pressure settings for a given alt. an example is that to get 65% power at 3000 I can run 2100 rpm and 24.8 man. Or 2400 rpm and 22.5, or 23 squared. Why would I chose one over the other? Running at 2100 produces a quieter cabin so why is that not the way to go?

Nick

Comments

  • flyguydonflyguydon
    The lower the RPM, the slower it adds time to your tach... so TBO will last longer... It is best to run at the RPM that offers the smoothest performance with the least vibration.
  • kwoodworthkwoodworth
    Run at whatever rpm/mp configuration listed in the AFM that gives you and your pax a comfortable ride. The engine/prop/airframe combination has been tested and approved to be run there.
  • smalonesmalone
    I have a 1969 P-28-B with a CS prop and have the same issue. I tend to choose the lowest RPM to minimize tach time but I also never run a MP higher than RPM. I understand that running some higher MP than RPM is not neccesarily dangerous it is not recommended. SO I tend to run nothing less than square.
  • AKowatraAKowatra
    I recently started flying a PA28-235 and have very limited experience. I was told when I reach at cruise altitude I should keep the manifold pressure at top of green arc for any given rpm and then adjust the rpm / mp where engine is happiest (least vibration). MP should always stay in green arc. Any combination of rpm/mp in the range specified in POH is fine.
  • JoeBJoeB
    One more element in the equation is efficiency. The engine consumes significant horsepower just rotating - it takes about 25hp to spin a six-cylinder at full RPM (which you measure in a dynamometer with no ignition going on.) So the slower you rotate the engine the more efficient it can be. However, the slower the prop rotates the bigger bite it has to take out of the air which means more induced drag. Bottom line - you have to test it. My IO-540 with a 3-bladed prop is slightly more efficient at higher RPM's. So I knock the RPM's down a couple hundred from redline (2700) to make things quieter and leave it at that. Science. ;-)

    You can pick any of the combinations that the POH gives you for a given power level. Or you can interpolate and pick something in between. Whatever floats your boat is just fine with Piper and Lycoming as long as it's within the ranges published in the POH.
    smalone wrote:
    I understand that running some higher MP than RPM is not neccesarily dangerous it is not recommended. SO I tend to run nothing less than square.

    This is a memory aid which is a happy coincidence of our engines and our units of measurement. It's close, but not actually true, for most horizontally opposed normally aspirated GA engines. It's completely off base for turbos, radials, geared engines (e.g.: Rotax), and anything that uses the metric system. You want to learn what the POH limits are and stay within them. If this "don't go over square" rule of thumb keeps you inside the POH, fine. But realize it's only a rule of thumb for the kind of plane you're flying now and don't get married to it.
    AKowatra wrote:
    I was told when I reach at cruise altitude I should keep the manifold pressure at top of green arc for any given rpm and then adjust the rpm / mp where engine is happiest (least vibration). MP should always stay in green arc. Any combination of rpm/mp in the range specified in POH is fine.

    Your last two sentences are true for cruise. Your first sentence is not necessarily consistent with the last two, however.
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