Wsrtjtyk

In jet engines, I read that thrust is related to the fuel flow rate, whereas in turboprop engines power produced is related to the fuel flow rate. What is the reason and brief math behind this?

Ultimately what you want from all three types of engines is quantification of thrust available to push an airplane through the sky. The turbofan/jet engines are self contained and produce thrust directly but a turboprop engine requires the addition of a propeller, which may have differing characteristics based on the installation.

Since thrust is not known for a turboprop until its installation has been determined, manufacturers instead quantify the power available to drive a prop. This allows engines to be compared so that an airframe manufacturer can make the proper selection.

Fuel flow is then related to either thrust or power as a measure of efficiency, depending on type of engine. There is generally no published mathematical relationship between power and thrust for a given engine. It is not needed for turbofans/jets and not possible to determine at time of manufacture for turboprops.

Jet engines directly produce thrust by exhausting gas (and in a modern turbo fan also moving a lot of air around them), so fuel flow rate is directly related to the thrust that is generated.

In a turbo prop the engine produces power which, via a gear box spins a propeller that generates the thrust. Since most turbo props have the ability to adjust the propeller pitch the engines power output is not always directly related to the thrust generated at a given time.

To expand on what Dave said, the term Power is used for turboprops because the thrust produced by the prop is a function of horsepower applied to the prop, that is, torque @ RPM.

The gas generator of a turboprop - the jet engine part - has its output indicated as a percentage of maximum torque it can apply to the propeller gearbox, whereas a pure jet engine, who's push results from the mass airflow accelerating through the engine, has its output indicated by Engine Pressure Ratio, the difference between the air pressure going in vs the air pressure going out.

Turbofans are kind of in the middle of the two, being sort of a turboprop with a fixed pitch many bladed propeller. Because the fan is fixed pitch and has no constant speed regulating ability, you don't need to know the torque being applied to it and it's sufficient to go by just fan RPM (N1, indicated as percent of max). Turbofans also show core engine RPM (N2), but the fan speed N1 is the primary power setting measurement.

For turbofans vs turboprops, its similar to how piston airplanes with fixed pitch props just measure RPM, like a turbofan, whereas piston planes with constant speed props need to show RPM and manifold pressure (MP being more or less equivalent to torque in a turboprop).

Not sure what you mean by the math part.

1) Why do we refer to “power” for turboprop engines and “thrust” for turbojet/fan engines?

2) why you wrote that turboprop engines generate power, whereas while writing for jet engines you wrote that they generate thrust. This was my actual question? I understand mechanics of both, but why is it intentionally written different? Am I missing something?

1. Take a Turboprop like an ATR-72.


2. Set Prop pitch to neutral.


3. Set the engines to high RPM (just don't floor it or you'll damage the engines).


4. The aircraft will not move.

Both of the engines are producing a lot of power but still the airplane is not moving, because with a neutral prop pith they are producing negligible thrust and that's why its called power at that point.

Do the same to an A350. You'll get your answer as to why they are called that way.

You wrote in a comment on Dave's answer:

why you wrote that turboprop engines generate power, whereas while writing for jet engines you wrote that they generate thrust. This was my actual question? I understand mechanics of both, but why is it intentionally written different? Am I missing something?

So it sounds like the main question you're trying to ask is: what is the difference between power and thrust?

Power and thrust are two different measures of the output of an engine (just like how height and weight are two different measures of the size of a person). All engines generate power, and all engines generate thrust, but they're two different numbers with two different meanings.

Thrust is a bit easier to describe. Thrust is often measured in pounds or newtons. If an engine is producing 500 pounds of thrust, then it's pushing on the aircraft with the same amount of force that a 500-pound weight would push on it. The difference is that the engine pushes forwards while the weight pushes downwards.

Output power is a bit harder to describe. Power is often measured in horsepower or kilowatts.

For rocket engines, the formula for output power is simple:

$$text{output power} = text{thrust} times text{exhaust speed}.$$

Unfortunately, most airplanes aren't powered by rocket engines, so this simple formula no longer applies, though. But the basic principles are the same:

• As an engine produces more thrust, it will also produce more power.


• Even if an engine is producing a constant amount of thrust, as the airplane flies faster, the engine will end up producing more power.

Power production is directly related to power consumption. If you make an engine produce more power, then it will consume more fuel.

Finding a conversion factor for thrust and horsepower
may at not be impossible when one looks at the original definition of horsepower and how its meaning, and math, became garbled over time.

First thrust: Is a force = mass x acceleration (gravity). Units of expression: kg meters/second squared or simply weight in pounds F = kg × G

Now horsepower: based on a horse pulling a rope attached to a weight across a pulley. Horse walks forward a 1 meter in 1 second of time lifting the 75 kg weight. Work W = weight x distance

Formula P = W/t = Fd/t = Ma x d/t = Ma x v!

Checking the units we have kg meters/second squared x meters/second = kg meters squared/second cubed.

What is going on here? Is this a steady state, or is it an accelerating system? Looking at the accelerated case provides the link between thrust and horsepower, and cleans up the math quite nicely.

To qualify as a horse, you have to lift the weight, but why include distance over time? Because a clever pony could put in a compound pulley attached to the weight and lift it half the height in the same distance walked forward. But this is where the true meaning of "horse power" was lost.

No mechanical advantage allowed! The draft horse, being stronger, simply walks forward and lifts the weight. Looking closer, the horse velocity is 0 and accelerates to walking speed. The act of lifting the weight 1 meter in one second not only matches gravitational force, but accelerates the weight upwards. Two draft horses will either accelerate it faster or lift double the weight in the same time: 2 horsepower! Since force vectors can be added, the math cleans up to:

Force = kg G + kg v/t = kg (G + a) kg meters/second squared

Naturally the horse does not continue to accelerate to a gallop, but how it is useful in the 18th century?
If the horses are taking longer to reach walk speed, they are getting tired and need to rest!

In modern times, horsepower is fuel consumed, so is thrust! The explosion pushes the piston, the rest are torque forces. It is thrust. How to compare it? Have any engine (rocket, piston engine/prop, husky team etc.) accelerate the weight without mechanical advantage. Call it what you will. It is Force.

But the usage in our language, 120 years past "horsedrawn days", remains in many forms.