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I have been trained in a single engine fixed pitch prop ( C152). I'm trying to move to Piper Arrow II which has an adjustable pitch propeller. I understand how constant speed propellers work. I'm trying to understand how to use this new feature in cruise flight.

My understanding is that in the highest RPM configuration, the propeller will provide the highest thrust. This implies that the lower RPM will result in lower thrust ( true? ). If the objective is to get to the destination in the shortest possible time ( the most common objective ) why would anyone want to mess with the pitch of the propeller?

I understand that using a lower RPM may have other advantages like lower engine temperature, lower noise and possibly longer engine life. I'm hoping there is an answer like: ( you can reduce the pitch but maintain cruise speed ..) :) OR for a significant reduction in noise level you will sacrifice 5kts in speed.

This answer doesn't answer my question.

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  • $\begingroup$ Until now, more throttle means more RPM and more thrust for you. With a constant speed propeller, you need to see the effect of the controls differently: Now the throttle controls only power and the prop pitch controls RPM. Since power and RPM are still related, make sure you have enough RPM so the propeller can absorb the desired power. If you reduce power, reduce RPM a bit as well. That's all. For the figures (how many kts per 100 RPM reduction) consult the manual. $\endgroup$ – Peter Kämpf Nov 30 '17 at 23:58
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why would anyone want to mess with the pitch of the propeller ?

A propeller runs at its highest efficiency when used at a particular combination of forward speed and RPM. By changing pitch, you change this combination, so the best efficiency can be shifted over a range of airspeeds.

For the physical background please consult this answer.

This implies that the lower RPM will result in lower thrust ( true? )

Yes, lower than maximum thrust. If you need less than maximum thrust, a lower RPM is sensible to reduce engine wear. Reduce RPM only if the remaining prop speed can still absorb the power; in other words, make sure the RPM is high enough to allow the engine to run at the speed needed for the desired power. But do not control thrust with RPM - this is what the throttle is for.

A piston engine outputs a constant amount of power, regardless of airspeed. This power output grows about linearly with engine RPM. You control this power with the throttle (and measure the power indirectly with the manifold pressure) and use the prop pitch to allow the engine to run fast enough to create the desired power at all. If you run the prop faster by reducing pitch, it will operate at a blade lift coefficient which is lower than that of optimum blade L/D, so you lose efficiency. Also, the engine will run faster and be in the partial-load operational range where it also runs less efficiently. Run the prop slower by increasing pitch and you will slow down the engine so it cannot produce the desired power. Also, now the propeller will run at a higher lift coefficient, efficiency will drop and part of the prop blade might even stall if you increase pitch too much.

As to what is a significant reduction in noise: This is subjective and I advise you to find out for yourself.

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  • $\begingroup$ I'm confused by your answer. If I understand your other reference answer correctly, an increase in efficiency of the propeller should result in increase in thrust. But you also confirm that the increase in blade angle which results in an increase in efficiency will lower the RPM and thus reduce thrust. So is it just a matter of playing with the throttle control to find the sweet spot where the thrust as measured on the airspeed indicator will be maximized? $\endgroup$ – Prashant Saraswat Dec 2 '17 at 19:27
  • $\begingroup$ @PrashantSaraswat: It's not that simple. The engine outputs a certain amount of power at a specific RPM, and you adjust blade angle to convert that power to thrust in the most efficient way. If you increase blade angle more, some of the blade will stall and thrust will drop again, even at the same RPM. The sweet spot you want to find is between RPM and power (measured as manifold pressure), where RPM must be enough to allow the engine to produce this power but not more to keep the propeller at its best efficiency. $\endgroup$ – Peter Kämpf Dec 2 '17 at 20:27
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Thrust of the propeller is a function of propeller RPM, and of the blade angle of attack:

  • With a constant blade angle of attack, thrust increases with RPM.
  • When the airplane picks up speed, the inflow angle into the blade changes: local AoA reduces, and thrust reduces. This can be compensated for by either increasing RPM or by increasing blade pitch, which is the better option.

At airspeed zero, angle of attack is the same as the blade pitch. You can adjust the pitch of the propeller to keep the local angle of attack the same when the plane picks up speed.

You get the lowest specific fuel consumption at the highest L/D of the blade, usually at a blade AoA of about 6°.

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  • $\begingroup$ Understood, so you are saying that for a higher prop pitch angle, you will reduce fuel consumption without sacrificing airspeed and might actually gain airspeed ( as thrust will increase ). Unfortunately, in the FSX with CLS piper arrow II I didn't see any change in airspeed. Maybe they just didn't design the plane properly :). I will try out in an actual airplane. $\endgroup$ – Prashant Saraswat Nov 30 '17 at 23:20
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What a constant speed propeller really offers is the ability to operate at the maximum theoretical effiency of the propeller for a desired engine speed throughout the airplane’s entire flight envelope. Propeller efficiency is defined as ratio of thrust horsepower available to propel the aircraft forward over the brake horsepower delivered to the propeller by the engine crankshaft.

Gasoline engines produce specific power outputs at specific engine speeds. Maximum power output orrurs at the some particular engine speed, generally redline (maximum engine speed). Since a fixed pitch propeller cannot alter the propeller pitch, it has to be designed for certain flight regimes where it provides the best propeller efficiency. This generally doesn’t not occur on the ground at zero airspeed in order to prevent engine overspeed as the aircraft gains airspeed and allow for a larger operating envelope at full throttle. Airplanes fitted with cruise propellers having higher blade angles of attack and consequently lower max engine RPM during the takeoff roll, making for longer ground rolls and sluggish climbs to trade off for faster cruising speeds. Planes fitted with climb propeller accelerate and climb better at lower airspeeds but can’t cruise as fast without engine overspeed risks.

Since a constant speed propeller can alter the pitch of the blades to adjust engine speed in flight, it can then increase or decrease the workload applied to the engine crankshaft, maintaining a desired engine speed and power output for said speed which is then converted into thrust with minimum drag penalty.

Engine power output settings are then a specific combination of manifold pressure and engine speed. Manufacturers publish these for specific engines and propeller combinations in the form of power charts. These guidelines should be observed to obtain the required power outputs and fuel burn during all regeimes of flight. An example power chart is below from a Piper PA-28RT-201T Turbo Arrow IV.

enter image description here

Example: Suppose you wanted to cruise at 65% power at 12,000ft pressure altitude, assuming standard conditions. Using the power chart 28.2” and 2575 RPM will be required for the desired power output at the lowest manifold pressure.

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You can think of a constant speed propeller similar to the transmission on a car. Once at cruise altitude, you start dialing in more propeller pitch (seen on RPMs), and lowering the throttle (seen as Manifold pressure) which has similar effect to shifting up gears in a car. If you want to climb more, you have to undo that, similar to downshifting in a car. Manifold pressure drops with altitude, so at some point you can't reduce the throttle anymore as you'll have it full in. Don't forget to lean the mixture, generally above 3,000', so you're not running too rich and just wasting gas. Then enrichen on the way back down.

The advantage is more speed at a lower power setting, and lower fuel consumption.

Arrow II has a 360 cubin inch, 180 HP engine, similar to my Cardinal, except yours will be fuel injected (IO-360, vs O-360).

You can run flat out if you want - the propeller governor should limit the RPMs to 2700, you are running over redline if you go higher, bad for the engine. There is a safety-wired screw adjustment on mine to set the max RPM allowed.

And it's also noiser. Settle in at 2400 RPM, 24" manifold pressure, enjoy the ride, you'll be burning 9-10 gallons per hour (see page 9-4, 9-5, 9-10 of the POH). You can bump both up, but you won't have the horsepower to go all that much faster due to drag, and you'll use a lot more gas doing it.

Example Pilot's Operating Manual (also known as POH)

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Here is the answer I got from my piper dakota instructor:

When you are taking off and climbing you do this at max power and lowest blade angle. When you reach cruising altitude and start leveling off, the RPM starts increasing and threatens to go beyond the red line (even though the engine is rated at 2400RPM). In this situation to continue to get maximum thrust within the power envelope of the airplane, you can adjust the pitch to lower the RPM.

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