Understanding the constant speed prop and 2-4 degrees optimum

Question

I've read through some answers on here and just wanted to write my own description of how this works and let someone check for me. Currently I've just started to fly an King air 100 with PT6A

We generally have 3 RPM settings to use. 2000RPM for takeoff, 1900RPM for climb, 1750 for cruise.

The CSU only knows the desired RPM to maintain on the propeller, (The RPM is measured from the tachometer on the 1st stage reduction gear box) as set by the pilot. The way is knows what desired RPM to maintain is via the position of the speeder spring.

At takeoff, we ensure the props are in fine and once we increase the power to the point the propeller comes out of the low pitch stop and is within the governing range, (should govern at 2000RPM and should not go above that, otherwise the CSU has failed but the prop should be protected by the OSG at 2100RPM) the CSU will now be able to change the blades angle.

At the start of the takeoff roll, our KTAS is very low, hence the horizontal compoent of velocity is low (Hv) and our blade angle will be basically at the low pitch stop in order to maintain the optimum angle of 2-4 degrees. Once we start to accelerate our KTAS increases, hence the Hv increases. Now as our Hv has increased the blade angle of attack has now decreased, The CSU has no clue the blade angle of attack has decreased but what it has sensed is the RPM slightly increasing above the set value of 2000RPM as the centrifugal force has increased causing the flyweights to move outwards. Now that the flyweights are moving outwards away from This pushes the speeder spring down and entering a overspeed condition. Now the checkvalve moves down and oil pressure decreases until the blade is back onto an onspeed condition, where the flyweights are no longer moving out and the angle of attack should be around 2-4 degrees again.

So the CSU will continue to constantly increase our blade angle as we acclerate and climb away. Now once in climb are TAS is considerbly higher, hence our Hv is considerbly higher. Now (heres my confusion) we reduce to climb RPM, down to 1900. We do this in order to maintain again the 2-4 degrees on the blade angle of attack. As we bring back the prop levers to 1900RPM what we are doing is changing the position of the speeder spring. We are moving it down, so the oil pressure decreases until the flyweights are no longer moving out?

We do this process again in cruise, but more significantly down to 1750 as the TAS increase from climb to cruise is greater than from inital takeoff speed to climb speed. Using the 1750RPM it should give 2-4 degrees angle of attack a the high TAS.

This is all to maintain propeller effiency. If we were to fly in cruise say at 1900 RPM our blade angle of attack would be too small and we would be getting drag (what type of drag?)

Note: Im also aware of wanting to favour max power over max efficency on takeoff and landing phases via Power= Thrust x RPM, hence we would get the most power with props fine. Do this mean the aircraft will accerlate the quickest with the highest power?

Thanks ,ollie

Answer 1

When we talk about "Power" there is engine output, or horsepower, as Torque × RPM and aircraft power, which is Thrust × Velocity.

If you want maximum thrust from a prop (at any airspeed), it's a combination of maximum (allowable) RPM and best angle of attack to the relative wind for the prop blade.

The best angle of attack of the prop blade, which is a spinning airfoil, will produce the highest Lift to Drag ratio, just like a wing.

If the angle of attack is not optimal, then the prop must absorb more power from the engine to generate the same amount of thrust.

If the engine can't give any more, then thrust will drop.

At any airspeed, with any throttle setting, the best prop AoA will produce the most thrust, therefor, the best acceleration.

The reason one changes from fine while climbing to coarse while cruising is the engine RPM slows down in cruise while airspeed is generally higher than in climb. We can see the changes in prop RPM and airspeed will reduce prop AoA (to the relative wind) unless the blade is opened up a bit.