the turboprop powerplant is rated by an 
“equivalent shaft horsepower.” 
T,y ESHP= BHP+325vp 
where 
ESHP=equivalent shaft horsepower 
EHP= brake horsepower, or shaft horse- 
power applied to the propeller 
T,= jet thrust, lbs. 
V=flight velocity, knots, TAS 
‘1s = propeller efficiency 
The gas turbine engine is capable of processing 
large quantities of air and can produce high 
output power for a given engine size. Thus, 
the principal advantage of the turboprop 
powerplant is the high specific power output, 
high power per engine weight and high power 
per engine size. 
The gas turbine engine must operate at quite 
high rotative speed to process large airflows 
and produce high power. However, high 
rotative speeds are not conducive to high 
propeller efficiency because of compressibility 
effects. A large reduction of shaft speed must 
be provided in order to match the powerplant 
and the propeller. The reduction gearing must 
provide a propeller shaft speed which can be 
utilized effectively by the propeller and, be- 
cause of the high rotative speeds of the turbine, 
gearing ratios of 6 to 15 may be typical. The 
transmission of large shaft horsepower with 
such high gearing involves considerable desi,gn 
problems to provide good service life. The 
problems of such gearing were one of the 
greatest difficulties in the development of 
turboprop powerplants. 
The governing apparatus for the turboprop 
powerplant must account for one additional 
variable, the propeller blade angle. If the 
propeller is governed separately from the tur- 
bine, an interaction can exist between the 
engine and propeller governers and various 
“hunting,” overspeed, and overtemperature 
conditions are possible. For this reason, the 
NAVWEPS Oo-ROT-30 
AIRPLANE PERFORMANCE 
engine-propeller combination is operated at a 
constant RPM throughout the major range of 
output power and the principal variables ofcon- 
trol are fuel flow and propeller blade angle. 
In the major range of power output, the 
throttle commands a certain fuel flow and the 
propeller blade angle adjusts to increase the 
propeller load and remain at the governed 
speed. 
The operating limitations of the turboprop 
powerplant are quite similar in nature to the 
operating limitations of the turbojet engine. 
Generally, the turbine temperature limnations 
are the most critical items. In addition, over- 
speed conditions can produce overstress of the 
gearing and propeller as well as overstress of 
the turbine section. 
The performance of the turboprop illustrates 
the typical advantages of the propeller-engine 
combination. Higher propulsive efficiency 
and high thrust and low speeds provide the 
characteristic of range, endurance, and takeoff 
performance superior to the turbojet. As is 
typical of all propeller equipped powerplants, 
the power available is nearly constant with 
speed. Because the power from the jet thrust 
depends on velocity, the power available in- 
creases slightly with speed. However, the 
thrust available decreases with speed. The 
equivalent shaft horsepower, ESHP, of the 
turboprop is affected by mass ,flow and inlet 
temperature in fashion similar to that of the 
turbojet. Thus, the ESHP will vary with 
altitude much like the thrust output of the 
turbojet because the higher altitude produces 
much lower density and engine mass flow. 
The gas turbine-propeller combination utilizes 
a number of turbine stages to extract shaft 
power from the exhaust gases and, as high 
compressor inlet temperatures reduce the fuel 
flow allowable within turbine temperature 
limits, hot days will cause a noticeable loss of 
output power. Generally, the turboprop is 
just as sensitive, if not more sensitive, to com- 
pressor inlet air temperature as the turbojet 
engine. 
133