Piston Engines - Propellers — Page 164, Lesson 216

I want to walk you through the propeller chapter now. We're starting fresh with the introduction and blade geometry, so let's build this up from the very beginning. First, the purpose of a propeller. The propeller takes the power delivered by the engine and converts it into propulsive thrust, which is what actually propels the aircraft forward. How does it do that? By accelerating a comparatively large mass of air rearwards. And because of Newton's third law — for every action there's an equal and opposite reaction — pushing that air backwards produces forward thrust. One important point: the acceleration applied to the air is not large when compared with other reaction systems. So it's a gentle but massive push of air, not a violent one. The aerodynamic details of all this are covered in the Principles of Flight book, and I'd recommend reading those chapters alongside this one, but for now let's focus on the mechanical side. Now, blade geometry. A propeller consists of two or more aerodynamically shaped blades attached to a central hub. That hub is mounted onto a propeller shaft, which is driven by the engine. The whole assembly rotates around that shaft, and you can think of the blades as rotating wings — they're essentially wings spinning through the air. Since a blade is like a wing, it shares wing anatomy. It has a root and a tip, a leading edge and a trailing edge, and a cambered cross-section. The chord line of that cross-section passes from the centre of the leading edge radius to the trailing edge. Now, the blade has two distinct faces. The forward, cambered side — the curved side — is called the 'back' of the blade. The flat, rearward-facing side is termed the pressure or thrust 'face'. So the back is curved and faces forward, and the face is flat and faces rearward. Then we have two more specific terms at the base of the blade. At the root area, where the blade section becomes round, that's called the blade 'shank'. And the very base of the blade, where any pitch-change mechanism would have to be attached, is called the blade 'butt'. So remember: shank is the rounded root section, butt is the base where a pitch-change mechanism attaches. Let me show you the blade nomenclature so you can see all these parts laid out. Now, blade terminology. Most of the terms in the next diagram are explained fully in the Principles of Flight book, and they're repeated here as a reminder. The ones that matter from a mechanical point of view, we'll discuss further. Let's get into the two key mechanical concepts. First, pitch, or blade angle. The propeller blade is set into its hub so that its chord line forms an angle with the plane of rotation of the whole propeller. That angle is called pitch, or blade angle. So the blade isn't flat in the plane of rotation — it's twisted relative to that plane, and that twist is the blade angle. Second, angle of attack. The path of the propeller blade through the air is a helix — a spiral path. That helix determines the direction from which the blade receives its relative airflow. And that path is the resultant of two velocities: the blade's rotational velocity and the aircraft's forward velocity. So the blade is moving both around the hub and forward with the aircraft, and the combination of those two motions gives you that helical path. The blade angle is chosen so that the leading edge is pointing into the relative airflow at a small angle of attack — ideally 2 to 4 degrees. That's the sweet spot for efficient operation. So to tie it together: the blade angle is the fixed geometric setting of the blade in the hub, and the angle of attack is the actual angle between the chord line and the relative airflow, which depends on that helical path. The blade angle is set so that, in normal flight, the angle of attack stays in that ideal 2 to 4 degree range.