Why are there extra restrictions on constant-speed propellers?

Question

Constant-speed propellers are far more efficient than non-constant-speed propellers, allow the engine to be made considerably simpler (due to removing the need to continuously adjust the ignition timing based on how fast the shaft is rotating at any given moment), and, unlike traditional variable-pitch propellers, don't require the pilot to manually adjust the propeller pitch.

However, according to Wikipedia, propellers with constant-speed mechanisms have considerably tighter restrictions on them than other propellers:

A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU [constant speed unit; the part of the propeller mechanism that makes it a constant-speed propeller]. CSUs are not allowed to be fitted to aircraft certified under light-sport aircraft regulations in the United States.

Given that a constant-speed propeller performs its pitch adjustments automatically, without the need for pilot intervention, and that constant-speed propellers perform better, not worse, than other propellers, why all these extra requirements and restrictions?

Answer 1

The engine is the same, not simpler. There is one additional level of complication to the system -- the constant speed propeller has a number of extra parts.

The pilot needs to handle one more lever, selecting the speed. And needs more training in order to remember to put it at the right setting in various flight conditions -- not to mention how to handle an emergency when the constant speed unit falls (you need to prepare for every possible situation).

All in all, more training and more complicated systems sets up the requirements for restrictions.

Answer 2

I recently moved from a fixed pitch propeller (FPP) (cessna 152) to a constant speed propeller (CSP) (Piper Dakota). In the FPP scenario the engine RPM is controlled by the throttle only. High throttle means high RPM and high power/thrust. In the CSP scenario the engine RPM is controlled by the throttle as well as the propeller pitch. One important thing to remember while flying is that you don't want to redline the engine RPM. Thus various power settings are defined in the power settings table in the POH. Also, when you come in to land you need to remember to push the propeller back so that you will have maximum power available if you need it ( in case of an abort ). All this means that you need extra training to use the propeller control effectively and not to damage the engine in the process. In reality, if you want to go fast cruise at full power at a high enough altitude ( say 7500 ft), then you probably won't even need to touch the propeller control ( this is true for Piper Dakota, I can't say for other planes)

Answer 3

A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU

What jurisdiction is that? Who is the formal signoff satisfying? In the US, I went from flying a Cessna 150 with fixed pitch prop and 100 HP to flying a Cessna 177 with constant speed propellor and 180 HP, also fixed gear. No formal signoff was needed to satisfy the FAA, nor the insurance company that I used to insure the C177.

If I had gone to 200 HP, retractable gear, and constant speed prop, then a complex and perhaps high performance rating would have needed to be signed off. Also the insurance company would likely have required some number of hours to be flown with an instructor before being allowed to fly the plane solo.

So, just the constant speed prop alone should not be a driver for a formal signoff.

Answer 4

There are engine operating rules which govern the manipulation of throttle vs prop. Most commonly. When increasing power: 1) Increase Prop RPM, first. 2) Increase throttle, second. Then the opposite for decreasing power. This order of procedure is to protect the engine from preignition/detonation due to a sudden increase in Manifold Pressure at a low rpm.

When these restrictions were originally implemented, many engine manufacturers had models utilizing gear reduction of the prop, where the engine turned faster than prop rpm. Continental had an engine with a full 2:1 reduction. If the pilot used the more common rule for power increase/decrease, gearbox damage would occur as a result of sudden unloading and even negative torque (Backlash). In aircraft with these engines the procedure was the opposite. When increasing power: 1) Increase throttle, first. 2) Increase prop rpm, second. Then the opposite for decreasing power.

Going back a little further. Aircraft fitted with Hamilton Standard "counterweight" props had to be shut down in max pitch, so that oil would not cool and thicken inside the prop, pitch-change cylinder as it cooled.

A lot to remember for someone just transitioning from a fixed pitch prop, back in the day. Maybe there wouldn't be as much of a need for the restrictions in today's equipment.