By Alan Negrin, CFI, MEI
I like to use a rough analogy when comparing a fixed pitch prop and constant speed prop with a transmission in a car. The fixed pitch prop is like having only one gear whereas the constant speed prop is like having a normal transmission with multiple gears.
When to start out in a car, you start in low gear and then shift into higher gears as you reach your cruising speed and you need less power to maintain that speed. Similarly, in an airplane with a constant speed prop. You are starting with the prop cable all the way forward, which is known as “fine” pitch.
A fine pitch setting on a propeller takes a smaller bite of air, allowing more engine rpm (power) at lower speeds. A coarser pitch (higher number pitch angle) takes a larger bite of the medium, and when properly matched to the engine potential, produces a higher speed. The difference between the fine and coarse setting may only be a few degrees.
Course Pitch makes a larger angle between the blade chord and the plane of the propeller disc, thus producing a high forward speed for a given rotational speed. Unlike at takeoff and landing, when forward air speeds are low and the propeller pitch is fine, the blade angle needs to be increased as the speed increases, or coarsened, for the angle of attack to remain optimum. The change of pitch may be done automatically by a constant speed unit or manually in a variable pitch propeller. (Our props are both constant speed and variable pitch props)
So you can think of it this way. When you are taking off, you are in fine pitch (low gear). You start out with full power and after you lift off, it’s like climbing a hill in a car or even just a level road. Even in a car, you shift out of first gear after a bit into second. When you are in the airplane and we are established on our climb after a few hundred feet, we do essentially the same thing by reducing power with the throttle first and then reducing the rpm (coarser pitch) by rotating the prop knob counterclockwise which brings the cable out toward you, or if you are flying with lever controls, pulling the lever aft toward you.
Remember that setting was roughly 25 inches of manifold pressure and 2500 rpm (second gear). We are still climbing and need more power and rpm to get to our cruising altitude but usually not full power and highest rpm. Now this scenario would be a little different if we are taking off from a high altitude airport or short strip with obstacles on the departure end, and we need to have a steeper climb angle, so in that situation we would probably leave it “first gear” (high power and rpm) until we are well clear of any obstacles and can continue to climb in “second” and still be well clear of terrain.
When we get to our cruising altitude, now we shift into “high gear”. The engine and prop don’t have to work as hard to maintain level flight, just like the car’s power and transmission system doesn’t have to work as hard to maintain a steady state cruising speed. We reduce the throttle (manifold pressure) and rpm to a setting we want to produce a given airspeed. If we are just putting around the neighborhood why burn more fuel that you need. If we are not going very far, our arrival time is not really going to vary by much regardless if we are cruising at 110 knots or 140.
If while we are cruising along in a level attitude and want to climb higher or stay at the same altitude but increase airspeed, in the airplane, we would first slowly push in or rotate inward on the propeller knob or lever, thereby increasing rpm (finer pitch), then push forward on the throttle cable to increase manifold pressure. Just like in a car, if you want to increase your speed, you step down on the gas shift into a lower gear, either manually or automatically until you get to where you want to be in a steady state cruise again.
Now there is another factor we have to discuss here because when you have a constant speed prop, you have both the manifold pressure gauge and a tach compared to the fixed pitch prop where you only have a tach. If you leave the throttle cable set to one position, the manifold pressure decreases with altitude, in a non-turbo charged aircraft. Being a pressure instrument, it is much like a barometer or altimeter in the airplane.
As you well know a non-turbocharged engine produces less power as the altitude increases. That is why the number gets smaller on the MP gauge as we go higher. The MP is also expressed in inches of mercury just like the altimeter. Therefore the purpose of a turbocharger is to maintain the same sea level pressure in the engine at high altitudes as it produces at sea level. That is why you can get some much more speed at high altitudes. I am going to presume you understand the basics of turbo charging.
So if you are climbing in your aircraft to say, 10,000 feet or so and want to maintain up to 25 inches of MP, you will have to then push that black throttle knob or lever back in toward the panel. By about 4500 feet you’re going to be at full throttle and by 10,000 feet you might only be at 18 or 19 inches of MP.
I hope this helps you understand better the use of a constant speed prop vs. the fixed pitch prop in an airplane.
Alan Negrin is CFI,MEI and former factory demo, flight test and transition training pilot. He owns a Sportsman tail dragger and provides transition training for Glasair, GlaStar and Sportsman owners, including those who are about to commit first flight and those who have purchased flying aircraft.
He can be reached at firstname.lastname@example.org or 425-466-8472