AE 172 Aircraft PerformancePropulsion Characteristics

Prof. Dr. Serkan ÖzgenDept. Aerospace Engineering

April 2019

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Outline

• Propulsion system alternatives• Thrust and efficiency• Specific fuel consumption• Specific thrust• Jet engine thrust• Piston engine performance• Propeller performance

Propulsion system alternatives

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Propulsion system alternatives

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Propulsion system alternatives

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The choice of specific device is a compromise between thrust and efficiency.

Thrust and efficiency

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• Thrust is created through the pressure and shear stress distributionsacting on the exposed surface areas (internal or external) of thedevice. • Due to Newton’s third law (for any action, there is an equal and

opposite reaction), the device exerts an equal and opposite force T on the air.• ! = #$ = #̇∆' = (')* ' − ') ; thrust,• -. = !'/ = (')* ' − ') '); thrust power

• -.,1234. =565.= 7

8#̇'8 − 7

8#̇')8 =

78#̇ '8 − ')8 .

• :2; =<=

<=,>?@A== 8

⁄C CDE7; propulsive efficiency.

• CCD≈ 3.0 for turbojets, ≈ 1.5 for propellers.

Thrust and efficiency

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• Propellers: large diameter, processes large mass of air but gives air small acceleration ⇒ high efficiency, low thrust. • Jet engine: small diameter, processes small amount of air but

gives air high acceleration ⇒ low efficiency, high thrust.

Specific fuel consumption• Specific fuel consumption is the rate of fuel consumption per

resulting thrust:

Jet engines:"#$% &'(( "%)*/ℎ)#-

.ℎ-#(.=%0/ℎ%0

or3/ℎ3

=1ℎ

(6)

Propeller engines:"#$% &'(( "%)*/ℎ)#-

8)*$-=%0/ℎ0ℎ8

or9:/ℎ;

(6<=>)

• Propeller efficiency, ?> =@ABC= @AB

DDE=>

⇒ 6 =;G/.H&$

I= 6>

JK?>

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Specific fuel consumption

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Specific fuel consumption – typical values

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Typical jet SFC: 1/h (mg/Ns) Cruise LoiterPure turbojet 0.9 (25.5) 0.8 (22.7)Low bypass turbofan 0.8 (22.7) 0.7 (19.8)High bypass turbofan 0.5 (14.1) 0.4 (11.3)

Typical propeller Cbhp: lb/h/bhp (mg/Ws) Cruise LoiterPiston-prop 0.4 (0.068) 0.5 (0.085)Turboprop 0.5 (0.085) 0.6 (0.101)

Specific thrust• Specific thrust is the thrust generated per unit mass flow of air:

Jet engines: "ℎ$%&"'(&& )*+, $("- ((/$) =

234/& =

'&

6 = '̇ 8 − 8: ⟹ &<-=/)/= "ℎ$%&" = 8 − 8:• If 8: = 0 then &<-=/)/= "ℎ$%&" = 8 ?-" -@/" A-*+=/"B• Specific thrust is a measure of how well the engine makes use of its

working medium (mass flow of air).

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Jet-engine thrust

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• Turbojet cycle parameters: overall pressure ratio, turbine inlet temperature, bypass ratio, flight condition.

!!"#

= %%"#

up to 40 000 ft.

• As ↗ '(, * ↗ due to ,̇ ↗ but ' − '( ↘ as '/ ↗.• For subsonic flight, the gases at the exhaust are at a choked

condition (0 = 1). ⇒ ' = 3(, almost independent of flight speed.⇒ thrust is almost constant with velocity upto transonicspeeds.

Piston engine performance

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• Power produced ~ mass flow of air into the intake manifold.• Power ≈ constant with "#.• ℎ% = 620+̇ (lb/s) or ,- = 1019+̇ (kg/s).

0012

= 3314

− 67 ⁄3 3129.;;

• <@ intake manifold ≈ <>?@.•Manifold pressure can be increased by a supercharger or a turbosupercharger.• Supercharger is a centrifugal air compressor driven by a shaft

from the engine.• Turbocharger is driven by a turbine placed in the exhaust pipe.

Piston engine performance

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• Supercharging or turbocharging is usually used to maintain sea-level pressure in the intake manifold as the airplane climbs.

Propeller performance

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• A propeller is a rotating wing that generates thrust just like a wing produces lift.• Like a wing, a propeller is designed for a particular flight

condition, i.e. lift coefficient, which is usually around 0.5.• The twist of the propeller is selected to yield the optimal airfoil

angle of attack at the design condition.• Overall pitch of a propeller refers to the blade angle at 75% of

radius.

Propeller performance

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• Advance ratio; ! = #$%& : the distance the airplane moves with

one turn of the propeller.

• Power coefficient; () = *+%,&- =

../01)+%,&-

• Thrust coefficient; (2 = 2+%3&4

• Propeller efficiency; 5) = 2#$* = 2#$

../01)• Thrust; 6 = *78

#$= ../01)78

#$; forward flight

6 = 9:9;

*%& =

9:9;

../01)%& ; static

Propeller performance

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Propeller performance

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Propeller performance

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• Fixed pitch propeller: maximum !" is achieved at a specificvalue of #$ (or %). #$ is usually chosen to be the cruise velocityor velocity for maximum rate of climb.• Variable pitch propeller: maximum !" is achieved at different

values of % for different pitch angles. Maximum !" = 0.85.Allows a high propeller efficiency to be achieved over a widerrange of #$.• Constant speed propeller: pitch is varied automatically by a

governor mechanism so that the rpm of the engine is constantand remains at its optimum value.

Propeller performance

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