The Fixed-Pitch Prop Trap

Scenario for more work

The pilot is climbing with full throttle at Vy (91 MPH in FIQX), nearing cruise altitude. It is a beautiful day with intermittent light turbulence from scattered cumulus clouds.

At his altitude the pilot levels off by reducing the RPM TO 2300. The airspeed settles at 100 MPH. The pilot is holding altitude with elevator, trading airspeed for altitude, kinetic energy for potential energy.

Emerging out from under a cumulus cloud, the aircraft enters some sinking air. By the time the plane reaches the shelter of the next cloud, the airspeed is 90 MPH. In the smooth air and slight updraft under the cumulus, the airspeed slowly returns to 100 MPH.

So far so good. But the layer of cloud is scattering out ahead. The next stretch of clear blue above is longer. In a matter of minutes the airspeed is 85 MPH and the aircraft is losing altitude. In frustration the pilot adds some power, not noticing that as he reaches for the throttle the RPM is 2150. He hears the RPM increase and concentrates on climbing back to his altitude. It takes a few minutes for the airspeed to get back to 100 MPH and the altitude to stabilize. Either he has been out of trim for all this time and holding back pressure, or he has trimmed for 85 MPH and now has to re-trim. He looks down and checks the RPM: it is 2400. He re-sets 2300 RPM and expects that all will be well.

Alas, the cycle repeats. And if he is a conscientious pilot and checks Carb Heat, it repeats that much sooner. Perhaps he also tightens the throttle friction, just in case.

What’s Going on Here?

We are driving a car with one gear. If we were driving a five-speed manual, we would be stuck in fourth gear.

Well, not exactly, of course. According to the POH, if we do a full-power static runup, the RPM should be 2270-2370.

But at flying speeds there is a lot less slippage, and our fixed-pitch prop behaves very much like a car in stuck fourth gear.

The red line in the illustration below is how the engine behaves with a fixed throttle position.

Let’s say your old beater is not only stuck in fourth gear, but it’s a long way to Saskatoon so you have made a crude cruise control out of a sawed-off broomstick. You wedge it between the seat and the gas pedal, and on level ground the car holds 90 kph without over-revving. Works like a charm, until you get to the rolling hills west of Yorkton.

You’re bored and there is no traffic, so you sit and watch.

Going uphill, the car slows. And slows. Are you going to make it to the top of the rise?

Well, you do but the speed is back to 55 kph. Now, going downhill, the car speeds up. By the time you cross the river at the bottom the car is doing 110 kph and you are seriously considering removing the broomstick. You want to get the transmission fixed in Saskatoon but you don’t want to have to replace the engine as well.

Well, this is what is happening in the C172.

You can see from the red line below that when the pilot set 2300 RPM at 100 MPH IAS, the percent power was 57%. By the time the airplane slowed to 90 MPH in the downdraft, The percent power had dropped to 51%!

Then look at the blue drag curve. At 90 MPH it is not at its minimum (best endurance), but it is close to crossing the Power Available line (red trace on the graph). With the speed below that the airplane will be unstable in speed and lose altitude (or speed, or both) because the Power Available is not sufficient to produce the Thrust Required for level flight.

 Fixed-Pitch Prop Problem

What’s the Solution?

  • Accelerate to design cruise IAS

  • Set POH cruise power (65% to 075%)

As long as the Power Available can produce the Thrust Required for level flight within the speed range expected at cruise, the pilot can enjoy a low-workload cruise technique – holding altitude by trading airspeed.

Indeed, that is the definition of Cruise Flight.