Propeller Basics

Airplane Techtalk for the Non-Technical: 

By Jennifer Lesher: 

I have signed up to take the first of my FAA certification exams in just 10 shorts weeks (eeep!) so expect more of these techtalk posts as I work through the material I need to have down cold for my exams.

A word about those exams. There are three sections to the exams, two of which are attached to licenses. The sections are General, Airframe, and Powerplant. Within the three sections there is one written test, one oral and one practical. Generally one takes the General exams at the same time as the first license exam. For me, this means I’ll take General and Powerplant on the same day. Again, eeeep!

Now, on to propellers. I will confess that until about a week ago, I had inadequate respect for propellers. Sure, I thought they were OK, but I hadn’t really reckoned just how important they are and how hard they work, nor did I know how efficient they are. Now I know – propellers are awesome.

Propellers work by essentially screwing themselves through the air (snerk). They spin against the air then push it over their blades (which are essentially wings), and then behind them, producing thrust. I find it helps to think about a propeller moving itself through a solid, to get a good sense of what they do.

When a propeller’s blades are more or less parallel to the plane of rotation (think flat) they can move easily through the air, taking a smaller “bite” out of the air, but providing quick “pickup.” This angle is used for takeoff where you need to get up to speed in a hurry. It’s similar to the first or second gear of a car – good to get going, but not good for cruising.

Here's a good example of propellers at low pitch.

When the blades are pivoted so they’re closer to perpendicular to the plane of rotation, they take a bigger “bite” of air, which is good for cruising, but would require the plane to have a really long runway to get up to speed at takeoff.

And, high pitch. These are probably feathered, which is a whole other thing, but they make it easy to see high pitch.
Because of this most modern propellers have the ability to vary pitch according to the phase of flight. Believe it or not, in the old days planes were made with several different propellers which would be swapped in and out on the ground according to runway length and expected flight parameters. This was a pretty clunky solution, so the variable pitch propellers were a huge improvement.

Propellers are about 90% efficient and are much more efficient than pure jet engines, which just explode fuel to get where they’re going.

When I learned how efficient props are, I wondered why we even have jet aircraft, if propellers are so great. There are several reasons. One is that while propellers are extremely efficient, reciprocating engines are horribly inefficient. Shamefully inefficient. Many propeller aircraft are also recips, so they’re not very efficient overall. If we all flew around on reciprocating prop planes we wouldn’t be able to afford to fly more than a few times per lifetime.

The efficiency problem is solved by turboprop aircraft, which use a turbine to turn the propellers. Turboprops are really, really efficient.

So, when I go overseas, why don’t I fly there in a turboprop plane? It’s because despite the propeller's many good qualities, it can only go so fast. This is because when a propeller’s blades move through the air at near the speed of sound, shock waves build up on them and they lose all their amazingness and become clunky blades of failure and death. And because the tips of a propeller move much faster than the center (because they travel farther per revolution) they begin to approach the speed of sound much sooner than the rest of the aircraft. Hence the speed of the aircraft needs to remain at about 60% of the speed of sound (or, Mach .6 if you want to sound techy).

Another type of engine takes advantage of propellers’ efficiency while raising the allowable speed: the turbofan. Turbofans are the type of engine that would carry you overseas. They have a turbine engine with a giant, many bladed propeller (fan) up front. To eliminate the speed problem, the fan is put inside a duct which slows the air down and also heats it. Hotter air increases the speed of sound inside the duct, allowing the aircraft to go faster without overspeeding the fans.

Oh, and one more thing – because aviation is designed to be confusing – if you’re looking at an airplane from the front, the surface of the propeller facing you is the … propeller back. My exam is going to be so fun!