Principles of Thrust

 Article Principles of Thrust Date Updated March 28, 2008 Category Aerodynamics
Thrust is the force that opposes drag, and whenever there is more thrust than drag, the airplane accelerates along the flight path until the increasing drag restores the equilibrium. In conjunction with drag, thrust is the factor that limits the top speed of an airplane. Most airplanes use power generated by their engines to turn propellers, converting rotational energy into thrust. Jet engines produce thrust directly, through the expansion of burning gases. Interestingly, jets convert some of that thrust into rotational energy to turn their compressors and accessories.

The difference between thrust and power is fundamental to an understanding of aircraft performance. Thrust must equal drag in level flight in order for the aircraft not to accelerate or slow down. In other words, a pound of thrust must be present for each pound of drag at any airspeed. Parasite drag increases as the square of airspeed, so the thrust necessary for level flight at 150 knots will be four times the thrust needed at 75 knots, less whatever reduction there is in induced drag. On the other hand, power is the rate at which work is done, so it is tied directly to speed. It takes less power to do the same amount of work at a slower rate. If an airplane had the same amount of drag at low speed as at high speed, it would still take more power to fly at high speed, because the work of overcoming drag would be done at a faster rate. Of course, drag increases at any speed higher than L/D(max), so more thrust is required in addition to more power. When using our familiar units of pounds, knots, and horsepower, the power required for level flight is defined as thrust required times velocity over 325. (325 is simply a constant to convert horsepower in feet per second to knots.) If your airplane has 300 pounds of drag force at 100 knnots, it requires 92.3 thrust horsepower to maintain level flight. At 125 knots, the drag force will be nearly 469 pounds, and about 180 thrust horsepower would be required. In this example, increasing speed by 25% requires a 95% increase in power to overcome a 53% increase in drag.

 The thrust required curve (A) looks suspiciously like the total drag curve since thrust must equal drag in level flight. The power-required curve (B) is somewhat different, indicating that less power is required at very low speeds. While the low point of the thrust-required curve defines L/Dmax, the lowest point on the power-required curve is at a significantly lower airspeed, and marks the minimum power required for level flight. Typically, the airspeed for minimum power required is 76% of the speed for L/Dmax.

Propeller Efficiency
The propeller converts the engine power into thrust. In order to obtain maximum performance, the propeller must make this conversion as efficiently as possible. Because it is an airfoil, the propeller is subject to all the factors that affect airfoil efficiency, such as angle of attack and speed. There are additional factors that are unique to propellers because of their rotation. The shape of a propeller blade reflects many of the principles discussed in our lift section. It has a low speed airfoil at a high angle of attack near the root where the local speed is low, and a high-speed airfoil at a low angle of attack at the tip where speeds are much higher. It has a high aspect ratio to minimize induced drag, and most have an elliptical planform to distribute the load. Controllable-pitch propellers allow the pilot to vary the angle of attack of the blades. As with wings, high angles of attack creates high induced drag, and that is why the engine slows down when you cycle the propeller during your runup. For take-off, the blades must be in the low-pitch, high rpm setting to make maximum use of the engine's power. In cruise flight, the forward speed of the airplane changes the relative wind that the propeller blades encounter, reducing angle of attack. When you adjust the propeller pitch in cruise, you restore the blades to an angle of attack that provides a higher coefficient of lift, increasing the thrust provided. At peak efficiency, some propellers are able to convert 85% or more of the power produced by a reciprocating engine into thrust.

When the propeller's axis of rotation is different from the airplane's relative wind, the angle of attack of each blade changes continuously through each revolution. The angle of attack is at its minimum as the blade is ascending, and reaches its maximum as it descends on the other side. This varies the amount of lift, or thrust, produced by one side of the propeller disc compared to the other, and is most noticeable during climbs.

Maximum Level Flight Speed
There is an upper limit to how much thrust your engine and propeller can produce. When maximum thrust is produced, the airplane accelerates until the drag force is equal to the thrust. Power and thrust available vary with speed.