Propeller Management

This is an opposed piston engine (commonly called a "flat engine"), commonly used in the vast majority of piston-engined airplanes flying within the US.


This is a radial engine. They were widely used for high-power applications before the turbine engine was developed. This one is a Russian engine that is equivalent to a high-power flat engine.

This is a Pratt & Whitney R-2800, arguably the most successful radial engine produced in the US:



As you might imagine, the crankshaft look different. On a radial engine, there is one "master connecting rod" riding on a very large crankshaft throw . All of the other connecting rods in that bank of cylinders are bolted to the master rod.



This is a graphic of how the major parts of a R-2800 fit together:


The drawing was done by Ralph Jones, this is from a series of animations by him of the operation of a R-2800 (which is no longer available [edited to add 8/2020]).

This is a drawing of a crankshaft of a flat engine, which doesn't look a heck of a lot different from an automotive crankshaft.

Radial engines produce a lot of power, which require large propellers. That has two implications. First, the props have several blades to absorb the horsepower. Second, propellers do not do well if the tips are moving so fast to go supersonic. Also, a supersonic prop makes a lot of noise (look up the XF-84H "Thunderscreech"). So radial engines are geared down so they can drive large props.

An AN-2:


The AN-2's engine is essentially an evolution of the Wright Cyclone engine, which the Russians built under a license from Curtiss-Wright. The Wright Cyclone 1820 was used in a lot of DC-3s.

(As an aside, the Wrights and the Russian clones use a mixture control that is bass-ackwards from almost all others, in theat you push the control forward for lean and pull back for rich. A wise pilot makes damn sure whether the engine on a DC-3 is a Wright or a Pratt.)

F8F Bearcat and F6F Hellcat:


Constellation:


A-1 Skyraider, which the Navy called a "Spad":


FG-1 Corsair, which had much larger engines than the F4Us. FG-1s were made by Goodyear:


DC-6:

DC-6s had R-2800s, which were very reliable.

DC-7s had turbo-compound R-3350s, which also included "power recovery turbines", which were spun by the exhaust gasses and, through a fluid coupling, fed power to the crankshaft. Power recovery turbines were also known as "parts recovery turbines."


Dc-7s were known for requiring an engine shutdown, which probably is why some old hands referred to DC-6s as "four-engined airplanes with three-bladed props" and DC-7s as "three-engined airplanes with four-bladed props. Note, though, that the higher output of the R-3350 required an additional propeller blade to make use of the additional horsepower.

Flat engines do not require as large props, so the props can be bolted directly to the crankshaft. Most are two-bladed. For most applications, changing to a 3-bladed prop is more a question of vanity than practicality.


All this leads to a discussion of operating the propeller. (This only applies to airplanes with adjustable-pitch propellers. ) Generally, there are three controls: the Mixture control (a red knob) that controls the fuel-air ratio; the propeller control, which controls the prop pitch (blue knob); and the throttle, which controls the power output.

Constant-speed propellers are set so that rather than directly controlling the pitch, you set the prop RPM and a governor adjusts the pitch to get to the desired RPM. The throttle controls the power output as measured by manifold pressure.

The gearing of a large-power engine is set to push power one way; from the engine to the propeller. A major sin in operating a geared engine is to "let the prop drive the engine". This happens when the throttle is brought back and the propeller control is not. The airflow "windmills" the propeller. The thrust loads go the wrong way and the load bears on surfaces, especially the crankshaft, that are not designed for lubrication under loads.

What you do is that you do not put the prop control all the way forward until the airplane is on final approach to the runway. At that point, the airplane is moving slowly and there is not the same windmill effect that would happen earlier. You'll know that is working because the engine RPM will not change as there is not the airflow to change the angle of the blades of the propeller, as there is not enough power to change the setting of the governor. If you need to go around (abort the landing), you just move the throttle to cruise power, level off, clean up the airplane (flaps, gear), gain a little speed and then go to full power.

(You should do it this way in the event that you ever realize your dream of flying a WW2 fighter. If you are on short final and you need to go around, reflexively slamming the throttle all the way forward will cause the aircraft to "torque-roll" and that will kill you in a heartbeat.)

There is another reason to not push the prop control in on most engines and that is called being a good neighbor. On the largest flat engines, and especially on long propellers such as floatplanes, the propellor's tips do go supersonic at high RPMs. When someone approaches the airport and pushes the prop control in then, the prop goes to flat pitch and speeds up to maximum RPM. The blithe/uncaring/rude/ignorant pilot then flies over the countryside with his propeller howling away, which irritates the people living underneath (and shows to conscious pilots that he is a clod).

So, to be a considerate pilot and to keep peace among the airport's neighbors, do not move the prop control from the cruise setting until you are on final.

And honestly, it would also be a good idea if, upon departure, you brought the prop control back 50 to 100RPM soon after takeoff. You won't hurt the engine and you'll bring the propeller out of the supersonic range. Everyone within earshot will appreciate it.