Piston Engines - Propellers — Page 164, Lesson 218

Let’s pick up with the propeller blade itself, because everything else in this chapter hangs on one physical fact: the blade is twisted, and that twist is called blade twist or wash-out. Here’s why it exists. As any point on a propeller blade spins, its rotational speed increases with its radius from the centre of the hub. So the tip is moving much faster than the root. Because of that, the magnitude of the total reaction generated along the blade also increases with radius. That means the outer part of the blade produces markedly more thrust than the root area. And that uneven thrust exaggerates the bending forces along the blade — the blade gets bent hard near the root because the tip is pulling so much harder. To even out the thrust developed along the blade, we maintain the angle of attack by reducing the blade angle from root to tip. So the blade is twisted — steeper at the root, flatter at the tip — so that every station along the blade sees a similar angle of attack and produces a similar share of thrust. That’s the whole point of wash-out. Now, the fixed pitch propeller. A fixed pitch propeller receives its relative airflow from a direction governed by two things: the aircraft’s true airspeed, TAS, in the direction of flight, and its own rpm in the plane of rotation. The operating angle of attack is the angle between that relative airflow and the chord line of the propeller blade. And that chord line is set at an angle to the plane of rotation — that angle is the blade angle, or the propeller pitch angle. So think of it this way: the blade is a wing, the relative airflow is the wind it feels, and the angle of attack is the angle between that wind and the chord line. The blade angle is the fixed geometric setting of the chord line relative to the plane of rotation. Now the key relationship: an increase in TAS reduces the angle of attack, whereas an increase in rpm increases it. If you fly faster, the relative airflow comes from more forward, so the angle of attack drops. If you spin faster, the airflow comes from more sideways, so the angle of attack rises. That leads straight to propeller efficiency. At high forward speed and low rpm — think of a power-off dive — you can reduce the angle of attack all the way to zero. At low TAS and high rpm — think of a climb — you can stall the propeller blade. Both extremes are inefficient and undesirable. So the conclusion is that for a given fixed pitch, a propeller will only work efficiently at one combination of TAS and rpm. One sweet spot, and nowhere else. The efficiency achieved is usually in the range 80 to 90 percent, and it’s properly rendered as: propeller efficiency percent equals thrust power times 100, divided by engine power. So efficiency is the ratio of the useful thrust power you get out to the engine power you put in, expressed as a percentage. Now, with a fixed pitch propeller driven by a piston engine, the rpm depends on the power setting — the throttle position — selected by the pilot, and on the TAS of the aircraft. And that creates two practical problems. In a dive, if you don’t back the throttle off — close it — you could overspeed the engine. Conversely, with the aircraft stationary on the ground, you may not be able to achieve rated rpm even with the throttle fully open, because there’s no forward speed to unload the blade. That brings us to the variable pitch, or constant speed, propeller, and its advantages over fixed pitch. The power setting of a piston engine is defined by a combination of manifold pressure — that’s the boost — and rpm. Where separate power lever and rpm lever controls are provided, you can vary one while leaving the other constant. That lets you optimise the engine and propeller combination for best efficiency, best fuel economy, and least engine wear and tear. To achieve that, you need a variable pitch propeller. It lets the pilot select a propeller pitch, and thus vary rpm independently of manifold pressure — provided the propeller is operating between its internal fine and coarse pitch stops. Once an rpm has been selected, a control unit — the CSU, the constant speed unit — takes over. And that’s exactly where we’ll go next: how the CSU actually holds that rpm constant.