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I am studying for my PPL, and after going through aerofoils, lift, drag, optimal L/D ratio, propeller blades and the like, the need for a variable pitch propeller is absolutely clear to me.

It is also clear to me how a variable pitch propeller works, both from an operational and a mechanical point of view.

What I really don't understand is why. What is the basic reasoning behind its implementation?

Why does a propeller have to be decoupled from the crankshaft in the first place? How did we switch from talking about a blade's angle, to caring about its RPMs? If the problem was the blade's efficiency because of its AoA, why are we even talking about RPMs?

Why can't I just have my big simple knob by which I could directly control the blade pitch angle?

Controlling a blade's pitch by its RPMs and manifold pressure, to my very uneducated mind, sounds indeed like a very counterintuitive way of solving a problem.

update

After stumbling on the concept of two-pitch propellers, I managed to find this excellent page which clearly describes the evolution of propeller technology, from the "Ground Adjustable Pitch", to the "Two-Position", to the "Controllable Pitch" (which is basically what I described with my knob example), up to the "Constant Speed" which is an evolution of the latter, for reasons already exhaustively described in some of the answers.

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    $\begingroup$ for a slightly different perspective: direct blade pitch control is the function of a collective in a helicopter. So it's not completely unheard of, but you still end up monitoring RPM in your feedback loop for the same reasons Peter outlines in his answer. $\endgroup$
    – Erin Anne
    Sep 11 at 7:03
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    $\begingroup$ Sure, you could only fly aircraft with full-authority FADEC systems, only ever fly on autopilot, and never need to know anything about all this stuff. But then what happens if the FADEC or the autopilot malfunction? (Answer, either you crash or you learn how to fly manually...) $\endgroup$
    – alephzero
    Sep 11 at 10:57
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    $\begingroup$ Just as in an automatic transmission, constant speed props are simpler to use and more efficient than having to constantly adjust a variable pitch propeller. $\endgroup$ Sep 11 at 11:48
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    $\begingroup$ Note that the earliest variable-pitch propellers weren’t constant-speed. That was a later improvement in usability, efficiency and safety, for the reasons given in many answers below. $\endgroup$
    – StephenS
    Sep 11 at 17:20
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    $\begingroup$ The embedded question "why does a propeller have to be decoupled from the crankshaft in the first place?" suggests there is a misunderstanding about how constant-speed propellers work - they are, in fact, rigidly coupled to the engine driveshaft, either directly or through a fixed-ratio reduction gear chain. The propeller/driveshaft RPM ratio is therefore fixed, and the governor of the propeller, in maintaining a fixed propeller RPM, is forcing the engine to run at the corresponding RPM by varying the load placed on the engine. $\endgroup$
    – sdenham
    Sep 12 at 16:09
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How did we switch from talking about a blade's angle, to caring about its RPMs?

Because of the engine. A piston engine will only run well in a narrow range of RPMs. Going too fast means

  • lubrication may break down, resulting in excessive wear,
  • incomplete cylinder filling and fuel burn, resulting in power loss,
  • increased inertial loads on crankshaft, connecting rods and pistons, resulting in cracks and eventual destruction of the engine, and
  • Valve float from insufficient valve spring forces, resulting in the piston crashing into the valve heads.

So, clearly, the red line of the RPM gauge should be respected.

Running the engine too slowly is equally undesirable:

  • Since power is proportional to RPM, the engine produces less power when run too slowly. If you try to force more power from a slow engine, internal pressures must go up, risking to overstress the engine.
  • There are RPM ranges which must be avoided because of resonance which may eventually result in mechanical damage.

So it is best to run the engine at its specified RPM range (near the red line if full power is needed, a bit less for partial power) and adjust propeller pitch such that power consumed by the propeller equals power produced by the engine. Any difference between both means that the engine will either speed up or slow down!

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  • $\begingroup$ you beat me to it! -Niels $\endgroup$ Sep 11 at 1:07
  • $\begingroup$ ah ok.. so it's basically an engineering limitation. I mean, with my hypotetical pitch knob I could easily make the engine suddenly jump well out of its allowed RPM range? So instead of trying to manually adjust for an optimal blade angle, we focus on keeping the engine RPM and manifold pressure under control by changing the pitch angle. If I understood it well. $\endgroup$
    – user815129
    Sep 11 at 11:16
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    $\begingroup$ I wouldn't call it "keep under control", more like "auto-optimize engine power" (or "auto-optimize propellor pitch" if you like to think the other way round). If you'd have a separate pitch knob, you'd constantly have to adjust engine power as to get the best of it. Maybe compare it to a CVT in a car. Where the engine rpm should stay roughly the same because that's in its powerband. $\endgroup$
    – ontrack
    Sep 11 at 19:23
  • $\begingroup$ So to flip it around, a motor with a really wide RPM power range (like an electric motor) would not need any prop pitch control, it would just change the engine RPM for simplicity ? $\endgroup$
    – Criggie
    Sep 11 at 21:55
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    $\begingroup$ @Criggie: Potentially, yes. And on the flop side, constant speed is even more important for turboprops than for piston aircraft, since a gas turbine's powerband is even narrower than a piston engine's. $\endgroup$
    – Vikki
    Sep 11 at 22:57
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Because engine RPM is the variable in the feedback loop.

If you did have a nice big knob to directly set blade angle, how would you know when to adjust it, to what setting? You would need to have a feedback signal, like IAS, and a graph that indicates knob setting. No way to verify if you have adjusted correctly, and you’d be very absorbed by the process without being able to scan the horizon.

Engine RPM is a good feedback signal because:

  • the pilot can hear it while still looking outside of the cockpit;
  • but more importantly, a change in RPM indicates a change in engine torque, which is a direct indication of blade AoA.

So using engine RPM it is simple to build in an automatic feedback loop: if RPM goes up, increase blade AoA until RPM returns to the original set-point.

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  • $\begingroup$ Thanks, yes I thought about that as well, thanks for pointing out the feedback loop. $\endgroup$
    – user815129
    Sep 11 at 11:48
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Controlling a blade's pitch by its RPMs and Manifold Pressure, to my very uneducated mind, sounds indeed like a very counterintuitive way of solving a problem.

It depends on the problem. And here the problem is keeping the RPM and the manifold pressure within design range of the engine.

RPM is limited because the acceleration of the pistons is proportional to RPM and the engine is only designed to handle so much, and because the propeller is only designed to handle so much centrifugal force. On high-power engines a run-away prop (that went full flat at full power) will destroy the engine in seconds.

Manifold pressure is limited because it is proportional to the pressure inside the cylinders and the cylinders are only able to withstand so much. If you exceed it, the cylinder heads will break off. And large supercharged engines can go way over MP red-line on the ground at full throttle.

So controlling the engine by the two main limiting parameters seems the optimal way of solving the problem.

And since the manifold pressure is roughly proportional to the torque (well, if you ignore leaning out; leaning out reduces the pressure in the pistons, and thus power, but not manifold pressure), and power is torque times angular velocity, these two parameters also come as close as practical to telling you the engine power.

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  • $\begingroup$ Thanks for your answer, it helped. $\endgroup$
    – user815129
    Sep 11 at 11:45
  • $\begingroup$ How does having the pilot control throttle and an automatic mechanical system control blade pitch compare with having the pilot control blade pitch and a mechanical speed governor control throttle? $\endgroup$
    – supercat
    Sep 13 at 15:29
  • $\begingroup$ @supercat Directly controlling the pitch and using governor for the throttle is used for helicopter rotors. It provides immediate lift response unlike the throttle/power that for turbocharged and turbine engines has noticeable lag, but it only works that way because significant energy is stored in the rotation of the rotor so the rotor speed only changes a little before the engine catches up. $\endgroup$
    – Jan Hudec
    Sep 16 at 17:11
  • $\begingroup$ @JanHudec: I hadn't thought about the effect of a turbocharger, but I can see how one could create a disastrous scenario in manually-pitched-controlled plane. If quickly adding pitch causes rotation speed to fall to the point that the turbocharger loses effectiveness, the engine might not be able to get back up to speed, even with wide-open throttle, unless or until the pitch is dialed back. If one quickly added pitch because of a sudden need for power, this could be very bad. Rotational-speed-controlled propeller pitch would take care of that scenario automatically. $\endgroup$
    – supercat
    Sep 16 at 17:23
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There is a fairly narrow rpm range that is optimal for a given phase of flight. To maintain that optimal rpm as engine power is changed (for example for the airplane to go faster or slower) requires the blade's pitch to change. When adding power/torque the blade will increase its pitch (AoA) to maintain the set/desirable rpm. The opposite is true when reducing power/ torque.

Without a constant speed propeller performance and efficiency are reduced.

By the way, most constant speed propeller general aviation airplanes do indeed have a knob that allows the pilot to control the blade's rpm. (The prop governor mechanism changes the blade's pitch angle to maintain the selected rpm).

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Why can't I just have my big simple knob by which I could directly control the blade pitch angle?

For operating the aircraft, how would that help you? What pitch would you want at any particular flight phase?

Controlling a blade's pitch by its RPMs and Manifold Pressure, to my very uneducated mind, sounds indeed like a very counterintuitive way of solving a problem.

What problem are you trying to solve?

For many flights, you want to run the engine at a known power setting (especially during cruise) and at an efficient RPM. The prop control allows this to be dialed in and maintained over a range of flight conditions. The specific blade angle that supports those two may vary depending on the conditions.

So if you had a "blade angle" control, you'd either need additional information and more calculations to find the desired angle, or you'd need to optimize for some other parameters to use it.

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For the same reason that your car has gears. Why can't you just put your car in first and drive at 70mph with your engine running at 50,000rpm? Or put your car in top gear and pull away from stationary?

In a car, an automatic gearbox does that for you. But you're basically driving a stick-shift here, so you need to do it yourself. Just that instead of fixed 4 or 5 gears (and neutral) to choose from, you get to dial it in more accurately.

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    $\begingroup$ Actually directly controlling the pitch would be like stick-shift—accelerating increases the engine RPM and you have to adjust it back by shifting. Constant-speed prop is the aircraft equivalent of a perfect automatic variable-speed drive. And the prop lever is basically equivalent to the lever switching sport or economy mode—keep higher RPM for more power in sport mode, or lower RPM for better fuel economy. Some modern aircraft like the PC-12 even don't have smooth RPM, just the two settings for max power and economic cruise. $\endgroup$
    – Jan Hudec
    Sep 11 at 10:50
  • $\begingroup$ In my opinion the most accurate answer. And I would add race cars have a bit more gears than common cars because they are more optimized. $\endgroup$
    – mins
    Sep 11 at 16:27
  • $\begingroup$ This answer addresses the need of a variable pitch propeller, but does not explain why it should also be constant speed, i.e. why it must be done using RPMs and manifold pressure and not by directly changing the AoA like a "Controllable Pitch" propeller. $\endgroup$
    – user815129
    Sep 11 at 16:38
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    $\begingroup$ @user815129: You misunderstand the answer, it does say the rpm is constant for the same reasons a car engine works in a very narrow range of rpm. The very root cause is engines have torque and power curves with maxima, and therefore they are used at these specific rpm, else you would burn a lot more fuel and could not obtain the full power/torque. That's true for a car and true for a plane. You can read more here. $\endgroup$
    – mins
    Sep 11 at 17:30
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The AoA of a prop's blades depends on both their pitch and the prop RPM, relative to true airspeed. You're carving a screw through the air some amount faster than the plane is actually going.

When you say "why are we even talking about RPMs", I guess you mean why are we talking about keeping it near-constant? Or building a feedback loop to make that happen?

If it wasn't kept near constant, we'd definitely still have to talk about it, in connection with appropriate settings for prop pitch, to maintain a good AoA for the blades at various true-airspeed and power settings. (Where needed power varies as you climb or descend, and with drag which depends significantly on indicated airspeed.)

And that's of course assuming that we had an engine that could produce the desired power across a range of RPMs, or the extra weight of a multi-speed or continuously-variable transmission like you need in a car where constant wheel-speed isn't an option. We don't have that with internal combustion engines, as some other answers explain.

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For what it's worth:

I recall my father's Piper Cherokee Six D 300. It had a constant speed prop; the throttle quadrant had three controls: throttle, mixture, and RPM.

During start-up, taxi, take-off, and initial climb-out, the RPM lever was always set fully forward (max RPM). The prop would, for the most part, go to and stay at its fine pitch limit because ground operation never called for enough power that the prop would need to coarsen the pitch to load up the engine. The exceptions were during run-up checks when a checklist item was to cycle the RPM lever (cycling the prop into coarse pitch) and it seemed during take-off when the advance to full throttle was accompanied by engine rev-up to an RPM which, once achieved, didn't change with increasing airspeed or pitch attitude.

After climb-out, when engine power was reduced, the RPM lever was also backed off to achieve a cruise or cruise-climb RPM.

As power was reduced for descent, the RPM lever was usually put full forward again. This was mostly to simplify power management - in the same way the mixture control was usually set full-rich while not in cruise.

This should illustrate a few key points:

  1. At low power settings, with the prop at its "fine" stop, the engine is basically following a power/RPM curve just like it would if turning a fixed-pitch prop, and that's OK because there's no great demand on it, and the thrust it generates is all you need for taxiing.
  2. At full throttle, there is an optimal RPM the engine should be run at (as explained in another answer). Since this is the maximum RPM the engine should ever be run at, the prop should be calibrated for this RPM at its forward stop.
  3. At cruise, throttle is reduced, so RPM should be too (again, there is an optimum). Since the aircraft is moving through the air at speed, prop pitch needs to be increased so that the selected engine power is applied at that reduced RPM.
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