So far we have said that for the jet engine fuel flow is proportional to thrust produced. For the propeller engine fuel flow is proportional to power produced. This has lead to the following conclusions about range and endurance.
Our conclusions were based on our earlier assumption that:
· TSFC is constant for a jet
· SFC is constant for a piston or turbo-prop
If the above is true then:
Our assumptions were only approximations however. We must now consider the factors which change the TSFC and SFC.
Jet Engine Efficiency
Jet engines are designed to turn at much higher rpm than piston engines. They are also quite sensitive to changes in rpm. In other words, they loose efficiency if throttled back to low rpm. Jet engines also convert fuel more efficiently in low outside air temperatures. We can summarize by saying jet engines are:
2. More efficient at high rpm
It is also worth noting that jet engine efficiency is NOT affected by velocity (in subsonic flight) or air density.
Turbo-prop Engine Efficiency
Turbo-prop engines are actually just jet engines with gear boxes and propellers attached. Therefore, the same factors affecting the efficiency of the jet engine described above also apply: In other words turbo-prop engines are:
1. More efficient at colder air temperatures
2. More efficient at high rpm.
Piston Engine Efficiency
Piston engines are remarkably consistent in their efficiency, compared to turbine engines. In other words efficiency does not change much with air temperature or rpm.
The only significant factor affecting the efficiency of the piston engine is throttle setting. When the throttle is retarded (making the manifold pressure lower than surrounding air pressure) the engine looses efficiency.
Pilots should keep in mind that a given amount of power can be produced by an infinite number of manifold pressure (MP) and rpm combinations. For example the following MP x rpm combinations all produce equal amounts of power:
· 22 x 2400
· 23 x 2300
· 24 x 2200
The engine will be more efficient if the pilot choose the higher MP and lower rpm combination (i.e. The third choice above.)
It is critical to note that none of the above theory will do any good at all unless the pilot leans the mixture to the maximum economy setting, as specified in the Pilot Operating Handbook.
Turbo-chargers allow a piston aircraft to fly faster and higher. However, the turbo-charger also tends to make the engine run hot, because it heats the air as it compresses it. Therefore, above some critical altitude the engine will overheat unless the mixture is richened to help cool it. As soon as this is happens the overall efficiency of the engine begins to decline.
Based on all the above, any power setting will have an optimum altitude(s) for the piston engine. These will be the altitude(s) at which full throttle produces the desired amount of power, with the mixture set for maximum economy.
In addition to the engine efficiency factors described above the piston and turbo-prop aircraft must also contend with the efficiency of the propeller at converting the power into thrust. The following discussion about propeller efficiency applies equally to piston and turbo-prop aircraft.
Propeller efficiency refers to the percentage of Brake Horsepower (BHP) which gets converted into useful Thrust Horsepower (THP) by the propeller. The propeller is never 100% efficient. Therefore the propeller efficiency is always a number less than one. The definition is:
Neta is propeller efficiency.
In the last chapter we saw that the efficiency of a wing (as measured by the maximum L/D ratio) depends upon the aspect ratio of the wing and the angle of attack at which the wing operates. The efficiency of a propeller depends upon the same things. In other words propellers with high aspect ratios will be more efficient than short stubby propellers. Additionally, each propeller will have an optimum angle of attack. When operated at the optimum angle of attack the propeller will be most efficient.
So, all we have to do is figure out what affects the angle of attack of a propeller.
Propeller Angle of Attack
We therefore know that the thrust produced by the propeller, which is nothing more than lift by another name, will decrease as the TAS increases because the propeller will be operating at a smaller angle of attack. This will reduce the coefficient of lift for the propeller and thrust will drop off.
When the pilot increases rpm the angle of attack of the propeller will increase. Thus, thrust will increase and the aircraft will accelerate.
As we learned before, any given wing will have a certain angle of attack at which it if most efficient. This will be true for the propeller as well. It is after all just a wing flying around in a helix pattern.
Since the angle of attack of the propeller depends on both rpm, diameter and TAS, the propeller efficiency will vary according to the ratio of these factors. The ratio Velocity/rpm x diameter is called the Advance Ratio. The formal definition of advance ratio is:
where J is Advance ratio, n is rpm and D is propeller diameter, V is TAS.
Available Thrust Horsepower