How to operate a constant-speed propeller

fc-cover-photo-smI’m often asked by pilots moving up to complex airplanes, what the real-world operational procedures are for a constant-speed propeller.

I like to compare using a constant-speed prop to riding a multi-speed bike. In each case you control performance via two variables:
1. rpm / how fast you’re pedaling, and
2. “oomph” / how hard you’re pedaling.

First, the rpm:

  • Flat pitch/high rpm/prop-control-forward in plane corresponds to low gear on the bike: hence better acceleration and (hill) climb performance, but limited cruise speed.
  • Coarse pitch / low rpm / prop-control-pulled-back corresponds to high gear on a bike: reduced acceleration and climb but faster on the flat and downhill/descending. Pitch is correlated to rpm through a governor, and managed by prop control.

Now for the “oomph” part. (Ie, how hard you’re pedaling.) In piston airplanes that’s controlled by throttle and measured by manifold pressure (MP).

As with a bike, there are various combinations of oomph (manifold pressure) and rpm that can all result in the same speed. In aircraft the ratio is designed so changing MP up 1″ corresponds to changing rpm down 100rpm, and vice versa. So 22″ MP/ 2200 rpm = 21″ MP / 2300 rpm = 23″/2100 rpm. Consult your cruise performance charts for options.

From an operations standpoint, think bike. You’ll use high rpm/flat pitch/control full forward (think “low gear”) for takeoff, climb, and pre-landing in case you need to go around.

So on a normal flight:

  1. max MP and max rpm (prop full forward) for takeoff;
  2. adjust prop and MP to climb power after takeoff (if different than takeoff power)
  3. reduce rpm and adjust power when leveling in cruise, and leave it there throughout the flight and descent. (Unless you need to climb en route; then you’ll increase rpm for that purpose.)
  4. Increase rpm to full, pre-landing, so you’ll have maximum power available in case of go-around.

One nice thing about this arrangement compared to the fixed pitch prop in, say a 172, is that the RPM won’t change by itself, so when flying in up- and downdrafts you needn’t constantly adjust power to keep RPM within range.

©2014, 2020 Gregory N. Brown

Leave a Reply