Hi Folks,

Just trying to get my head around the efficiencies of the Jet Engine and Propeller driven aircraft.

I'm actually confused as to the term propulsive efficiency.

The normal story goes that the turbo prop is more efficient at lower airspeeds than the turbo jet, and then the turbojet becomes more efficient as the speed increases above roughly 350nm/hr.





I can understand the turboprop efficiency dropping off after a certain speed due to the increased drag and shock waves that form when the prop tips go supersonic, but can't really understand why its more efficient at a lower airspeed (and also is better out of short airfields)

As I understand, the propeller adds thrust by giving a relatively large portion of air a small acceleration, and the jet engine, a small portion of air a large acceleration, but don't see how this alone can give one combination a better advantage over another at high or low speeds. What part of the story am I missing here?

Is part of the reason for the jet engine being more efficient with higher forward speeds due to the ram effect, which helps to maintain the 'effective' thrust the engine produces due to the mass airflow increasing?

This is really doing my head in!

Thanks in advance for any advice or input

Hi Basketball,

This is getting into ATPL theory but a gross simplification of the reasons are:

Turbofans are most efficient at high RPM so the compressors are rotating at high speeds improving the compression inside the jet. Higher compression means greater heating of the air which leads to better core thermal efficiency and better fuel efficiency.

With the denser air at low altitude there will be too much thrust produced which is great for take-off but will is way too much for normal level flight and the aircraft will rapidly accelerate beyond its structural limits. Therefore the RPM needs to be reduced which reduces efficiency. Since the air is less dense at altitude, the RPM of the engines can be increased and even though the air is thinner, the engine will still actually operate more efficiently. Also the reduced IAS due to the thinner air means lower stresses on the airframe and the aircraft can operate at a higher TAS than would be possible at low level.

Another reason is the effect of altitude on propulsion efficiency. The efficiency of propulsion is a function of the speed of the exhaust compared to the speed of the outside air. The closer the aircraft velocity is to the exhaust gas velocity, the greater the propulsive efficiency. For example, you would have maximum propulsive efficiency when the speed of the exhaust gases is the same as the velocity of the aircraft. This would mean all the thrust generated is being used to propel the aircraft and no energy is being wasted on just accelerating the exhaust gases alone. Another way to look at it is if you were standing behind a jet which applied full power and it had 100% propulsive efficiency, you would feel no jet blast but the aircraft would instantly accelerate away from you at the speed of the exhaust gases (e.g. 1200kt). The gases leave the jet at 1200kt but the jet is moving forward at 1200kt. You would feel no jet blast. This obviously doesn't happen in reality. Other factors are slowing the aircraft down and preventing it from reaching the same speed as the exhaust e.g. parasite drag.

If you can get higher TAS you will be improving the propulsive efficiency since the difference between TAS and exhaust velocity will be less. As you know, as you get higher in the atmosphere, the TAS increases for the same IAS. So, at high altitude you will be getting better propulsive efficiency for the same IAS when compared to the efficiency at low altitudes.

After a quick google I found these demo figures:

"Assuming an aircraft speed of 375 mph and a jet velocity of 1,230 mph the efficiency of a turbo-jet is approx. 47%. At 600 mph the efficiency is approx. 66%. Propeller efficiency at these speeds is approx. 82% and 55% respectively.
So gas turbine engines like to fly at high TAS."

The propeller loses its efficency at higher speeds for the reasons you mentioned and its propulsive efficiency is best at a lower range of airspeeds compared to a turbofan engine.

So, to summarise, you want the turbofan to operate at high RPM which it can do safely at higher altitudes and you want as high a TAS as possible to maximise propulsive efficiency and this also occurs at higher altitudes.

Cheers,

Rich

Thanks Richard. After reading your explanation I can see where my "thinking" went astray.

Glad to help. Have fun!

Cheers,

Rich