Bjorn’ s Corner: New engine development. Part 2. Thrust generation

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.

15 Comments on “Bjorn’ s Corner: New engine development. Part 2. Thrust generation

  1. it is an optimax problem.

    Creating overspeed takes energy in a square relation.
    E = 1/2m * v²

    Thrust is impulse is mass times overspeed.
    i = m*v

    covering a large area to accelerate more mass at a certain speed …

    • The results of an internet search for ‘optimax’ are ‘interesting’ 🙂

      • Didn’t realize that.

        optimax ~= optimize a system for best overall performance based on some metric or other.

        ( interesting here for example the shift from smaller widebodies to the A380 where the best drag target optimizes down to a lesser aspect ratio.

        • The A380 development dragged on too long and it became too heavy, expensive and a bit short on range. Today you have suitable engines like RR Ultrafan and GE9X and you have big carbon structure fabrication with robots, hence designing a 600-700 pax plane for SYD-LHR och DBX-LHR is easier but you need firm orders for 500 ea to get started. Today only EK, Qantas, Cathay, JAL, ANA, Turkish, KLM, BA, SQ, Qatar and Air India have the volume and central hubs to be interested.

          • Your screed never links to anything i wrote.
            Please stop.

          • You will see the exact opposite happen. Airlines fly the smallest plane that will connect city pairs. Single aisle aircraft can fly upwards of 10+ hours. The A380 went under, the 747 is done and 777 is even struggling to sell.

  2. My head is once again spinning (pun accepted not intended). Its such an amazing area of flight and Bjorns very good presentation to go with it.

    That is far more in depth than I had ever seen described though its dropped down tech wise by most as you want to get the point across without getting into the physics (I believe that is correct word usage) involved.

  3. The diagrams used by Bjorn really describe the ‘pulse jet’ system. No Moving parts.
    Once you get to a front fan sucking air in through compressor stages and
    then burning fuel to add even more energy to the airflow which is mostly absorbed by a turbine driving the compression stages leaving some left over for thrust ….then its really complicated.

    • it directly describes any “push” system that does not take mass from the environment.
      a rockets, a kids blown up balloon going off on the moment of releasing stored air under pressure. …
      A guy in space on a surfboard shooting arrows to the rear.

      a pulse jet takes in mass at the front “does something energetic to it” and expells it at the rear. about the same what a jet engine does.
      only less complicated.

  4. Propellers were of course first used on marine propulsion. What’s interesting there is how efficient they were straight out of the starting gate. Brunel’s SS Great Britain was one of the first to have a prop, and crude though it appears to be compared to today’s elegantly sculptured bronze behemoths, it was surprisingly efficient. There were other problems, like the prop shaft ate through the bronze bearings at an alarming rate. On the whole though, surprisingly good.

    Everything since has been of incremental improvement.

    It’s also interesting comparing aeronautical solutions to other maritime solutions. The most interesting is the propulsor, as used on some torpedoes and UK RN and then also USN submarines. This is akin to the turbofan that airliners use.

    What’s interesting is that I hear that, compared to a prop, they’re slightly less efficient. But, with endless nuclear power on tap, that doesn’t matter and the other beneficial properties are worth it (the low net torque characteristic, noise control, speed flexibility, damage resistance, etc). It took the submarine world longer to adopt ducted fans than it took the aviation industry, but now that’s all they use.

    I don’t think submarines are going to adopt open fan technology anytime soon. But it would make for an interesting tail end of a submarine.

  5. So, as propellers run into sonic speed problems, wouldn’t the solution be to reduce the diameter of the propeller and increase it’s RPM and have more propellor blades.

    I thought so until I saw that this is what they had on Shorts 360 aircraft, which look like a shoebox and are decidedly low speed. But fantastic fuel economy. Anyone have any idea what was going on there?

    • shorter blades -> less diameter -> less area -> less air mass to work on -> more overspeed -> depresses efficiency.

      • Quite so.

        So why does the Shorts 360 have a small, six blade prop instead of a larger diameter 5, 4, 3 or 2 or even 1 blade prop (there was a light single engine American plane that had a single blade prop initially, I forget which it was – they changed to a two-blade prop).

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