TurboFan vs TurboProps Question - Need Help

[airbuscopilot]

Q1. Why does a TurboFan have better performance at high Altitude?

Q2. Why do the Turbo-Props fly lower than TurboFans?

Answer to the first According to me:

Fuel Flow = SFC X Total Drag (Jet Engines)

Now to keep the Fuel Flow Low, The SFC should be minimum & the Total Drag should be minimum.



Total Drag is min when flying at Vmd (Jet).
SFC is low when Temperatures are Low &,
When Engines are running at 90 - 95% Design RPM (Which occurs at high altitude as due to decreased density at high altitude, the engine has to run at 90-95% RPM to fly at Vmd at high Altitudes)


Thus Less Fuel Flow Means more fuel available, thus both Range and Endurance increase (Thus increasing performance)

Now what is confusing me is that, As Turbo-Props are also Gas Turbine Engines which Thrust Large Mass of Air at Small Acceleration and have similar characteristics to the TurboJet except for the Propeller Then why do they have to fly Lower than the Jets?

Please clear my confusion. Thanks

[grounded27]

Compression=thrust

[airbuscopilot]

The TurboProp uses its 95% of the Exhaust to move the Propeller. Only Residual Thrust is left.

Whereas the Jet has more than 50% of Thrust after moving the N1 and N2.

Is that the idea sir?

But still i don't understand why the Turboprop cant fly higher?!
could you please explain a bit more?

[Turbine D]

airbuscopilot,

I think you need to look at the two systems as being different, even if a "jet engine" is the source of power for both. In a turboprop engine, the engine provides power to the propeller through a gear reduction mechanism (gearbox), reducing the rpm's of the rotating turbine shaft to the rpm's of the propeller for safe operation. In this system, the measure of power is "shaft horsepower". Therefore, as the airplane gains altitude and the air becomes thinner, the shaft horsepower diminishes. Ideally, the system works to an altitude of ~20K feet or so. Beyond that, not enough shaft horsepower is present therefore limiting the operational altitude.

For the turbofan engine, power is define in terms of pounds of thrust. Pounds of thrust is determined by the amount of air entering the fan, part of which is compressed by the compressor, mixed with fuel in the combustor and expelled through the turbine which drives both the fan and the compressor. The largest portion of air that passes through the fan by-passes the core engine and provides more thrust than the core engine by itself. The larger the fan, the more air processed and the more pounds of thrust. Therefore, a typical commercial turbofan engine is efficient to ~42K feet before the air becomes so thin, loss of thrust becomes very apparent.

When thinking of a turboprop engine, think of it as your car engine at various altitudes. At sea level it may generate 250 horsepower, but at 10,000 feet, it may only generate 150 horsepower.

Hope this helps your understanding of the difference between the two power sources.

TD

[lomapaseo]

I believe that it is segment fuel burn vs time in flight.

The passengers care about shorter flight times and both long and short flight legs
If you try operating the pure jet at lower altitudes for the short segments it burns a lot more fuel changing the rotor speeds in the innards.

If you try operating the near constant rotor speede prop jet at high altitude the frontal drag will eat up all your performance.

So the crossover point is pretty much floating around making the innards of the engine run efficiently, while using a gearbox to drive a propeller and at the same time ducting the propeller to increase its effeciency while keeping the frontal drag as low as possible.

[barit1]

The primary advantage of a turboprop is its ability to create a lot of thrust per unit of fuel at low airspeed. This is because the prop pumps a large mass of air at a relatively low velocity. (The natural extreme of this is to turn the prop shaft 90 degrees, i.e. helicopter, where the "prop" generates enough thrust to directly lift the craft at zero IAS.)

But the blades "get in the way" at higher airspeeds. And the engine is sized for economy, not super-duper cruise speed.

The typical jet transport has its engine size determined by the desired cruise altitude, and is thus overpowered at low altitude. (Thus, derated/reduced takeoff thrust is in common use, unless loads or field density altitude are very high, or runway very short.)