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As in reciprocating engines the gas generator of a turbo-prop engine is used to drive a propeller. It is the propeller that develops the thrust that drives the airframe.

To measure the power that is developed one needs to devise a system that can monitor the turning force on the propeller shaft.

If an engine produces torque (T) at N revs/min

Power

=

2 NT The Imperial Unit of Power is Horsepower.

Horsepower

=

2 NT 33000

Horsepower developed by an engine output shaft is known as shaft horsepower.

Brake Horsepower

To measure shaft horsepower it is usual to use a brake dynamometer. Hence, Shaft Horsepower is sometimes known as Brake Horsepower. Numerically it is the same.

Equivalent Shaft Horsepower

The turboprop engine uses the majority of gas power to drive the turbines, with the free or power turbine driving the propeller shaft. There is always a residue of gas power exiting the exhaust however. As long as the exhaust is directed parallel to the thrust line of the engine then this exhaust will add to the thrust the propeller is producing. The total thrust production of the engine is therefore the Shaft Horsepower plus exhaust or jet thrust. It is called equivalent shaft horsepower.

ESHP

=

SHP + Jet Thrust

I

If the aircraft is in flight then the efficiency of the propeller must be taken into account.

-

ESHP

=

SHP x prop-eff. + Jet Thrust

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Efficiency

Propulsive Efficiency

Propulsive efficiency is concerned with the efficiency of the engine to drive the aircraft in flight. If Pett

=

propulsive efficiency

Va

=

Aircraft speed Vi

=

Exhaust Velocity

Then Pett =

Consideration of the formula reveals that:

If Va

=

Vi then the efficiency will be 100%. But if Va

=

Vi there is no difference in velocity through the engine and hence there can be no thrust. Therefore 100% efficiency is impossible. Also note there would be no energy used to drive the compressors if 100% of energy was used for

propelling the aircraft.

If the aircraft is stationary on the ground then Va

=

0. In this case efficiency would be 0. This shows that propulsive efficiency is concerned with propelling the aircraft through the sky, not just producing thrust.

Propulsive Efficiency Graphs

The graph reveals how propeller-driven aircraft gain their efficiency first at low airspeeds

because the controllable pitch propeller is capable of moving large mass airflows. The curves all peak out as soon as more fuel energy is introduced to create an exhaust velocity increase. Work then comes out in the form of increase aircraft speed.

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governed by the statement

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De sign d i1 as. ociation "' tr t 10

club66pn ... ~orn question pracncs aid

-

c 80 41 c ,._

!

>- u 60

_z

w Sd u. u. 40 w w

>

(/') _J ::, ₂₀ 0.. 0 a: n,

{low by-pass ratio)

0 200 400 600 800 JOOO

AIRSPEED m p.h.

Figure 2.4: Propulsive Efficiency Graphs

The propeller aircraft (either piston or turbine driver peaks out slightly above 85%, after which

the propeller loses efficiency. That is, its exhaust wake velocity continues to increase from

added fuel energy, but aircraft speed does not increase proportionally. Note that after reaching approximately 375 mph, propulsive efficiency starts to decrease. Aerodynamic drag and tip

shock stall are involved here and by 500 mph efficiency decreases to 65%.

The ultra-high bypass turbofan curve peaks at approximately 560 mph (Mach 0.85), after which

the fan suffers the same losses in drag and tip speed as the propeller. In order to go to 700 mph (aircraft speed), the exhaust velocity will have to be increased to an uneconomical level.

The high bypass turbofan is the most widely use engine today in both large and small aircraft.

Its propulsive efficiency curve peaks out slightly lower than the UHB engine but at approximately

the same airspeed.

Subsonic aircraft with low and medium bypass turbofans all operate in the 500 to 600 mph range. Note that the curve shows a lower efficiency value than a high bypass engine in that

range. Because of this, high bypass engines are rapidly replacing low and medium bypass

engines in many aircraft.

The supersonic low bypass turbofan and turbojet have a theoretical propulsive efficiency peak

limit in the 2,000 to 3,000 mph range. Their narrow, low-drag profile allows this range. Any additional energy added (in the form of fuel) to increase speed further would raise the internal

engine temperatures to unacceptable levels.

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Thermal Efficiency

Thermal efficiency is the ratio of net work produced by the engine to the fuel energy input. As with propulsive efficiency it cannot be measured in the cockpit but can be calculated by utilizing a fuel flow indication

Thermal Efficiency

=

Net Power Output of the engine

Energy value of Fuel consumed

Overal I Efficiency

It is necessary to combine both of the above efficiencies when looking for a powerplant to suit a particular application.

Overall Efficiency= Propulsive Eff. x Thermal Eff.

For example if Pett = 70% and Thermal Eff. = 40% then

Overall efficiency = 70% x 40%

=

28%.

Thermal Efficiency Curves

OVERALL

AIRBPEEO

Figure 2.5: Propulsive, thermal and overall efficiencies, variation with speed Propulsive efficiency increases as airspeed approaches exhaust velocity values.

-

Thermal efficiency decreases due to added fuel needs at higher airspeeds.

Overall efficiency increases as airspeed increases because propulsive efficiency increases

more than thermal efficiency decreases.

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In document FACULTAD DE ARQUITECTURA (página 47-74)