By Bjorn Fehrm
April 5, 2024, ©. Leeham News: We started an article series about engine development last week. The aim is to understand why engine development nowadays dominates the needed time and the risks involved in new aircraft development.
To understand why engine development has become perhaps the most challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals. We start this week with thrust generation.
Figure 1. The principle for thrust generation using air as medium. Source: NASA.
The generation of thrust in an aircraft engine
All aircraft engines use the same principle to generate thrust, whether they are piston-based, gas turbine-based, or electric engines. To get thrust, they all accelerate air backward from the engine and thus the airplane.
When you accelerate air backward, the engine produces a reaction force that drives the aircraft forward. Figure 1 shows an example of a balloon that you blow up and let the air seep out. The reaction force drives the balloon forward.
The action and reaction are described in Newton’s third law. Newton’s second law says that the generated force is the mass that is moved times the acceleration of this mass. Air weighs 1.23kg per m3 at sea level and about 1/3 at an airliner’s typical cruise altitude of 35,000ft.
An aircraft engine can accelerate the air in different ways. As long as the captured air gets an increased airspeed when passing the engine, it creates thrust.
Over the years, airplane designers have sought the most efficient way to accelerate the air to achieve the thrust needed to overcome aircraft drag. Leonardo da Vinci wanted to use a screwing action in his “Arial screw” (Figure 2) to accelerate the air downwards to generate lift.
Figure 2. The Leonardo da Vinci “Arial Screw” from 1487. Source: Wikipedia.
Aeronautical pioneers like Samuel Langley, who competed with the Wright brothers for the first powered flight, used a fabric-covered propeller design for its paddle-type propellers (Figure 3), probably inspired by Windmill rotors.
Figure 3. The Samuel Langley Aerodrome Number 5 with its fabric-covered paddle propellers. Source: National Air and Space Museum.
The Wright Brothers were the first to conduct aerodynamic research into an efficient propeller blade profile. Figure 3 shows the homemade wind tunnel the brothers used for their research on airfoils (model 15) and propeller blades (model 31).
Figure 3. The Wright brothers wind tunnel with measured wing and propeller specimens and the balances used to measure forces. Source: Nasa.
Thrust at higher speeds
The propeller was an efficient thrust-generating device until about 500mph or 450 kts. To fly faster, the propeller tips, which added the rotational tip speed to the aircraft speed, went supersonic, which meant large propeller efficiency shock wave losses.
Another effect made propellers unsuitable for high-speed flight. The propeller delivers thrust by accelerating the air in the propeller-swept area to a higher speed. This air now has a delta speed compared with surrounding air, which we call air overspeed.
So, we have thrust = air massflow through the engine times the air overspeed
You can either deliver the thrust by accelerating a large air massflow to a low overspeed (the propeller case) or a small air massflow to a large overspeed (the jet engine case). If the sum is the same we deliver the same thrust at a zero forward speed of the aircraft.
At no or low speed, it requires less power to accelerate a lot of air to a lower overspeed than vice versa. The higher the air massflow and the lower the overspeed, the higher the engine’s propulsive efficiency. The opposite is also true. It’s inefficient to generate thrust at low aircraft speeds with an engine that accelerates a low air massflow to a high overspeed.
Both these cases deliver the same thrust at takeoff if the product air massflow times air overspeed is the same. But as the aircraft’s speed increases, the first case (propeller) will have the overspeed reduce proportionally faster than the second case (jet engine). The effect is called speed lapse of thrust, and it hits engine designs with low overspeed harder.
The effect means engine designers design the engines with a defined overspeed (called Specific Thrust in engine speak) adapted to the operational profile of the aircraft they shall be used on. For low-speed aircraft, you design with low overspeed, and for fast aircraft, with high overspeed.
So, the propeller is an efficient solution for low speeds, whereas the straight jet is the best solution for speeds over Mach 2. In between, the bypass turbofan is a good solution, where the bypass ratio shall be tuned to the aircraft’s speed regime. More about this in the next Corner.
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