Fixed vs. constant speed propellers


Photo by Metin Ozer on Unsplash

Despite the terrible weather this last month I have been doing various lessons for students in the Decathlon, and those students while experienced enough in the basics of flight and aircraft control have no prior experience behind a constant speed propellor, which which the Decathlon is equipped.

As I’ve written previously, there are a range of differences between a typical training airplane (like the Grob) and the Decathlon: tailwheel undercarriage, no flaps, tandem instead of side-by-side seating, and a glass panel display. But in this essay I’m going to focus on the differences in engine management brought on by the more sophisticated propeller.

The Grob and the Decathlon have simiar power engines (160HP vs 180HP) and not too dissimilar empty weights (1522lbs vs 1301lbs) but there’s a quite significant difference in the ‘feel’ or avaiability of that power. This is particularly evident during low speed movement.

It turns out that in cruise the Grob is actually a little faster than the Decathlon. That’s because the Grob is a much smoother and less draggy airframe and is equipped with what you might call a “cruise” prop. The propeller’s fixed pitch is set at manufacture to be most efficient when the airspeed is around 100 knots or more. It’s as though the Grob is permanently stuck in a high gear. Mechanically simple, but not conducive to efficient takeoff and climb.1

On the other hand, the Decathlon’s propeller is able to adjust its pitch efficiently to convert engine power to thrust at a much wider range of airspeeds, including when the plane is stationary on the runway or flying slowly, when the fixed pitch of the Grob propeller means it’s at too high an angle of attack and so inefficiently thrashing through the air as it turns.

That means that during the takeoff roll the Decathlon acceleration is much more rapid – and you get the “boot in the small of your back” feeling as you open the throttle which you never get with a similarly powered fixed-pitch propeller airplane. The Decathlon takeoff roll is short in distance and time so you’d better be prepared with your rudder and aileron inputs to keep the airplane under control: stick forward for tail up very soon after opening the throttle, then shortly after that, nose up for departure. You definitely don’t have time to read a book or chat about the weather, like the more experienced pilot feels they do in the Grob.

Similarly, the Decathlon rate of climb is much higher. Partly that’s a factor of the reduced weight of the Decathlon compared with the Grob, but more especially that at the (fairly slow) best rate of climb airspeed (Vy) for both aircraft (73 knots in the Grob vs 70 knots in the Decathlon) the Decathlon constant speed prop converts a lot more engine power into thrust. If you’re flying a circuit you’ll be at circuit altitude much sooner than in the Grob.

The flip side of the extra efficiency of the constant speed prop includes the extra weight and complexity of the mechanism to adjust blade pitch and also extra operating complexity for the pilot. And this is something I’ve been noticing, that new pilots feel a bit lost at sea when faced with the trinity of engine controls of throttle, mixture and RPM vs the more familiar duo of just throttle and mixture. That single extra lever does throw people off their game.

Principles of constant speed propeller operation

So how can we help students over this hurdle?

First principle: Don’t lose track of the basics: the primary power control is still the throttle. Push it forward for more power, pull it back for less.

Second principle: if you never touch the propeller RPM control but simply leave it at maximum RPM (which is where it should be at takeoff) you effectively have a fixed-pitch propeller. One with a very fine pitch (behaving like a very low gear in a car) so you can takeoff and climb rapidly, but when you level off and try to accelerate to cruise speeds you’ll find that even at high RPM you’re flying quite slowly. Safe – but slow.

Third principle: coarsening the pitch by slowing the propeller down gives you access to higher airspeed at lower RPM, just like changing to higher gears in a car.

Fourth principle: at cruising power settings (70% say or less) use the mixture control to reduce the fuel flow without loss of power for better economy and less pollution. Just as you do with a fixed-pitch propeller.

Fifth principle: when the engine is running in constant speed (therefore at cruising power and after you reduce RPM) you can no longer measure power with RPM – moving the throttle changes power but not RPM. To calibrate and describe power settings we use manifold pressure (MP) instead. More manifold pressure – more power. Less manifold pressure – less power. So rehearse the right lexicon in your head by saying to yourself things like “reduce power by three inches of MP” instead of “reduce power by 300 RPM“. The first means to a constant speed prop pilot what the second means to a fixed-pitch propeller pilot.

Sixth principle: In contrast, when you need a very low power setting and you close the throttle accordingly, the RPM drops below the governor range regardless of where the RPM control is set. You’re back to having that fixed pitch propeller again, and back also to describing power with RPM.

Putting principles five and six together, a sequence of six consecutive power reductions from cruise power might see you acknowledge to yourself power as follows, switching from MP to RPM as the power decreases: “23 inches …. 20 inches … 17 inches … 15 inches …. 2000 RPM … 1700 RPM … 1500 RPM.

Seventh principle2: to a first approximation the actual RPM number doesn’t matter very much. After you’ve slowed the prop from maximum (for takeoff) to somewhere in the 2200-2400 RPM range for cruise, you can achieve decent efficiency at a wide range of power settings without further adjusting RPM at all.

Operating advice and rules of thumb

So much for principles. Let’s simplify life with some rules-of-thumb.

Firstly: always takeoff with prop control at maximum speed (high RPM, fine pitch) and mixture rich. The same for a go-around. On landing you should prepare for the go-around ahead of time by moving the RPM control forward on short final, if not earlier. You can set the mixture rich at the same time.

Secondly: with the engine turning, move the RPM control smoothly and not quickly. You’ll see why the first time you (mistakenly) punch the prop instantly to maximum RPM while under power and the engine surges and shakes and complains (almost as much as your instructor does.) Also move the throttle smoothly and not quickly, slower than you would in a fixed-pitch plane like the Grob, for very much the same reasons.

Thirdly: when you want to level off from a climb, reduce throttle first (to decrease power), then slow the prop down by dialing back the RPM, and then lean the mixture. Move the controls in that order – throttle back, prop back, then mixture back. Whereas in a fixed pitch plane you might have in mind a cruise power of 2300 RPM, your go-to setting here is both a manifold pressure and RPM combination: 21″ of manifold pressure at 2300 RPM, for example. If you can’t think of anything else, go for 21″ and 2300 RPM.

A big change for a student to get used to is to give up listening to the engine pitch (as in musical note) to judge power adjustments. Because small power adjustments around cruise power settings don’t cause a change in RPM, you can’t hear the power decrease. You have to check your manifold pressure gauge.

Fourthly: you can effectively manage an entire flight with the RPM control in only two different positions: maximum RPM position for takeoff, climb and landing, and (say) 2300 RPM for cruise and descent. Don’t allow the apparent flexibility of being able finely and continuously to adjust prop speed sucker you into thinking it makes much difference to you or the aircraft. For a novice pilot (or any pilot who’s busy) anything around 2300 RPM is good enough, and will do until you identify a clear need for something different.

Fifthly: to re-enter to a climb from level flight at cruise power: enrichen the mixture first, then smoothly dial the propeller faster, then, increase pitch and last of all, open the throttle. Move the engine controls in the order mixture forward, prop forward, then throttle forward.

Sixthly: training flights have a weird requirement for lots of power-on/power-off/power-on sequences. Other than aerobatics practice, no real pilot is repeatedly putting their airplane to the edge of or through the stall, flying very slowly, then very quickly, then slowly again and so forth. If you must do so then (as a lazy pilot, like me) you’d like to avoid making a lot of rapid RPM adjustments along with all this throttle-dancing.

So I suggest before engaging in what is loosely described as “air work”, you set the propeller to 2500 or 2600 RPM and leave it there for the duration. 2500 or 2600 RPM is a compromise between an efficient cruising RPM and an efficient RPM for flying very slowly. It’s not particularly efficient for either, but it is effective enough, being the aerial equivalent of second gear. You might as well apply full rich mixture too at the same time. You can then comfortably reduce the power to idle, enter a stall, and add more than enough power to recover from the stall all by manipulating only the throttle control. Just like you can do in an airplane with a fixed pitch prop. Later, when you want to flee the area and go somewhere else, then it’s time to dial the RPM back to 2300 and lean the mixture for cruise again.

As an aside: do you need full throttle for a stall recovery? Not really. If you’re flying the Decathlon with the prop control set to 2500 RPM and you enter a stall you’re in “second gear” and already have way way more thrust available than the “fifth gear” prop in the Grob provides you even at full throttle, in the same circumstance. If you move the throttle smoothly in accordance with the second principle above then you’ll have recovered from the stall and be well into the climb before you have time to reach full throttle.

Finally, seventh: When flying continuous circuits, when you reach circuit altitude leave the propeller control at its takeoff setting of maximum RPM and reduce throttle to a very slow cruising power of 15″ manifold pressure, just enough power to maintain level flight without catching up on the Cessna 150 ahead. There’s rarely a need to fly quickly in the downwind.

So there you have it – the beginner’s guide to moving up to a constant speed prop. To recap my suggestions for the Decathlon:

  • Propeller control at max RPM position for takeoff, and before landing to be ready for a go-around.
  • Cruise: propeller at 2300 RPM, and throttle at 18-22″ manifold pressure with mixture leaned.
  • Manoeuvring: set 2500 RPM and a full rich mixture for the short duration.
  • Circuit downwinds: leave the prop control at max speed and reduce throttle to 14-15″ MP.
  • Always move the RPM and throttle controls smoothly and slowly.

Anticipate rapid acceleration on the runway, and be ready to get the tail up very soon after adding power.

Happy flying!

  1. In the 1994 Spanish Grand Prix Michael Schumacher’s Formula One car got stuck in fifth gear (out of six) a third of the way through the race, and he still finished in second place. I believe he said afterwards that if you’re going to run a race using only a single gear, one below top is the one you want.
  2. Perhaps I shouldn’t have numbered them after all

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