(and Climb Power)
19 August 2019
The primary reference is the Pilot Operating Handbook (POH). All the numbers and recommendations below are taken from the POH.
Full throttle. At climb airspeeds the RPM will be in the green band. The POH says mixture can be leaned above 3000 feet. From the POH: Normal climbs are performed at 80 to 90 MPH and full throttle for best engine cooling.
The POH says 2200-2700 RPM (no more than 75%). Normal cruise in most piston aircraft is 65-75% power, with 65% as the conservative setting.
The low end of the RPM range 2200-2300 RPM can result in an airspeed of 100 MPH or less, especially in summer. There is nothing wrong with this, but it does make the pilot’s job more difficult.
Why? Because the pilot has very little airspeed to trade for altitude. Then she has her hand on the throttle if she gets low in a downdraft.
The ideal of Cruise Power is set it and forget it. Or, at cruise altitude, get cruise speed, set cruise power, lean, and forget it. Then maintain cruise altitude by trading speed in small corrective climbs or descents. Is this always true? No. In mountain waves airliners sometimes cannot maintain altitude even at Max Continuous Thrust. Nevertheless, that’s the goal. Set it, lean, and forget it.
For the same percent power, RPM increases with altitude. The table below is taken from the C-172 Cruise Performance Chart in the POH.
For instruction or below 3000 feet, 2400 or 2500.
Above 3000 feet, 2500 RPM.
It does no harm to cruise a C-172 at 100 mph. But it’s a lot of work.
Why? Because there is a negative feedback loop that can get going, making work for the pilot. A bump nudges the airplane out of equilibrium. It sags below altitude. The pilot pulls to bring it back up. It get slower and slower. Frustrating for the pilot.
It’s the fixed pitch prop.
Imagine you are driving on an autoroute. There are hills. Up and down every few kilometres. Instead of cruise control, you have a device to hold the throttle steady. It is a standard shift, and you are in high gear. You can’t change gears. It is like a fixed pitch prop.
You gain speed going downhill, and hit a maximum in the valley. Then, as you start uphill, the car slows. You (or the cruise control) are not pushing on the gas pedal, adding power. The throttle is fixed.
But as the car slows, so does the engine – it is connected to the wheels in a fixed ratio. As the engine slows, it produces less power. Power (at the fixed throttle angle) is dependent on RPM.
Just so the C-172 with a fixed pitch prop. Maybe you have set 2300 RPM. But there is a downdraft, and you have to apply back pressure to climb the hill, to get back up to your cruise altitude. The airspeed is back to 90. If you glance at the RPM, it is now 2200.
The downdraft is over. You are at your altitude. But the airspeed is 90 and the RPM is still 2200. Will it accelerate back to 100 mph and 2300 RPM?
Maybe. But at 90 mph you are close to the bottom of the drag curve, where it flattens out. So the speed stability is going from positive to neutral.
And the fixed pitch prop seals the deal. Instead of 57% power, you now have 51%1. So the airplane is already unstable in speed. That means if you hit another downdraft and pull, the airspeed will be 80 mph and the power down well below 50%. You will have to add power – the instability, the negative-feedback loop, has taken you where you don’t want to be.
Fly the airplane at 65% power, where it was designed to cruise. At low altitude, that’s 2425 RPM and just over 120 mph. Then the speed stability is still positive. You can hold altitude with subtle forward and back pressure on the yoke. You can leave the throttle alone.
1All these numbers come from the POH