Theory

Why are props better at low speeds and jets better at high speeds?

Why do prop engines have more power at low speeds and appear to lose power at higher speeds, while jet engines behave the opposite way? Why are prop engines measured in terms of horsepower while jets are measured in terms of thrust?

First off, let's clarify the difference between power and thrust. Power is the rate at which total energy is transferred. You have power input to an engine, in terms of the fuel delivery rate, you have total power transferred out of an engine, and you have power transferred to the plane, in terms of making it go faster or higher. Thrust is simply the amount of propulsive force applied to the plane by the engine. From the laws of physics, power delivered to the plane is thrust times velocity.

The power efficiency of different engine types depends on the operating situation. There are multiple effects operating against each other. The common wisdom is that props have higher power at low speeds than jets. But consider power delivered at standstill: Stand on the brakes and rev a prop engine. The engine is delivering lots of power, but none of it is going to the plane because the plane is stationary. The power is all dissipated in the propwash. So what's really going on here is that a prop engine delivers maximum thrust at standstill.

As your speed increases obviously the power delivered to the plane increases. However, eventually you start running into the limitations of a prop. As your speed goes up, the flight path of the prop blade becomes more and more straight ahead and less sideways; consequently, the "lift" generated by the blade more and more turns into rotational resistance and less into thrust. In other words, the thrust generated by the prop falls off as you get faster. So you reach the point of diminishing returns where the engine is no longer imparting any power to speak of into the air and its thrust drops to nearly zero.

A jet engine, on the other hand, imparts a much greater acceleration to a smaller amount of air. (OK - I'm talking about a pure turbojet here. I'll address bypass engines in a moment.) At standstill, this comes in part from just running the air through the compressor and turbine. However, most of the jet's thrust comes from heating the air. The air entering the engine is at atmospheric pressure; the air pressure leaving is only slightly higher, so by the old gas equation PV = NRT the air must be leaving at a velocity that's faster roughly in proportion to the amount by which it's been heated (in absolute degrees).

Just like a piston engine, at standstill a jet delivers zero power to the aircraft. However, as you accelerate, the thrust delivered by a jet does not fall off as fast as that of a piston engine because the exhaust velocity is much higher. As the jet gets faster, the compressor effectively does less work in accelerating the air into the engine - more and more the air just flows in at the freestream velocity. (But the compressor still has lots of work to do to compress the air inside the engine.) So you lose the element of thrust from accelerating the air into the compressor. But most of it is still there from heating the air. At very high speeds the air is actually being decelerated (relative to the engine) at the intake to keep the compressor blades subsonic. This is accomplished by expanding the diameter of the intake duct from the engine mouth to the compressor face. (Constant flow rate with constant pressure into an expanding volume means the flow velocity has to decrease.) This causes intake drag but is still compensated by the acceleration at the other end from heating the air.

With properly designed intakes, jet engines become even more effective at supersonic speeds. The air must be decelerated even more at the intake, but this is now done with supersonic flow rather than subsonic flow. The supersonic flow causes shock waves that compress the air as it decelerates; the compression of the air increases the total mass flow and therefore the engine's thrust. For a while, as your speed increases above Mach 1 the engine's thrust increases. There are a couple of limiting factors:
  1. Intake geometry. Above a certain Mach number the air can no longer be decelerated to subsonic speed before it hits the compressor face.
  2. Temperature limits. Compressing the air heats it up, so there is less "headroom" for heating the air in the engine and still staying within the engine's temperature limits.
  3. Ultimate limits on exhaust gas velocity. Intake drag finally exceeds the thrust that can be developed by heating the air to the maximum exhaust temperature.
One might ask about Mach limits to the jet exhaust velocity. "How can the jet exhaust flow faster than the speed of sound?" The answer is it doesn't. Remember that the speed of sound in a gas is proportional to the square root of the temperature. So the speed of sound is much faster in the hot engine exhaust, allowing it to flow much faster than the nominal 1000fps you get at normal atmospheric temperatures.

Another detail about flow and pressure... The air pressure at the engine exhaust is determined by the exhaust's flow resistance. EPR (engine pressure ratio) is the ratio of air pressure at the exhaust to the air pressure at the intake. It effectively represents the air flow rate through the engine as seen through the engine's flow resistance. Typical EPR for an airliner jet engine at full power is in the neighborhood of 1.4:1.

So what does this all come down to? Ultimately what matters to an aircraft is thrust, not power. You need to accelerate the plane at takeoff and overcome air resistance at cruise. Props and jets are rated differently because they are measured differently. A prop engine is rated in horsepower because that's what you can measure at the shaft: power = torque * rpm. But that's only loosely related to what it's really doing for the plane because the amount of thrust the prop can translate that power into greatly depends on the plane's speed.

Horsepower doesn't make any sense for a jet because most of the power delivered at the turbine shaft is used internally. And its thrust is more constant relative to speed, so static thrust is the more useful metric.

From an energy conservation point of view, props are more efficient at low speeds than jets because almost all the energy goes into accelerating the air. A lot of the jet's energy goes into heat; that part of the energy is effectively lost. The downside of props is that as you approach Mach 1 at the prop blades they can't accelerate the air anymore. Also from the energy conservation point of view: A jet engine typically generates a lot more total power than a prop engine. However, at low speeds a greater proportion of that power is wasted heating air and accelerating it to very high velocities. At higher speeds, more of that power is actually imparted to the aircraft.

Now about bypass engines... In a bypass engine, a portion of the air flowing through the first stage of the compressor simply passes around the rest of the engine (for high bypass engines, as much as 90% of it). Effectively, that part of the compressor is simply acting as a prop. Bypass engines (AKA turbofans) are in effect a hybrid of a jet and a prop engine. You get improved static thrust (vs total energy consumed) and higher energy efficiency at low speeds in return for reduced thrust at high speeds. The bypass ratio determines the extent of the tradeoff. From this point of view, a turboprop is simply the ultimate high-bypass jet engine. High bypass engines are a good fit for airliners because they are most efficient at the airliner's cruise speed of Mach .85. You get maximum thrust for takeoff and climbout, which are done at relatively low speeds, and you need relatively less thrust for cruise at Mach .85.

People have complained that the performance of jet engines in X-Plane doesn't match that given in published jet engine models and data. It's a little hard to compare jet engines in X-Plane to engines on real planes because you have to be sure you're comparing similar engines. The rate at which power drops off with airspeed depends directly on the bypass ratio. X-Plane doesn't have a lot of flexibility here - you get your choice between "high bypass jet" and "low bypass jet" with some built-in operating maps, and that's it. There's obviously a much greater variation in real life. Other factors are also important. The intake design makes a big difference in the engine's thrust vs speed characteristics, especially when you get supersonic. All that said, I haven't spent a lot of time studying X-Plane's jet engine model, so I make no representations about its accuracy.

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