Engine Placement

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Configuration Design
part 3
ENGINE PLACEMENT
Factors to be considered:
Effect of power changes or power failures
on Stability & Control.
Drag of the proposed configuration.
Weight & Balance considerations.
Inlet requirements and resulting effect on
installed thrust and efficiency.
Accessibility and maintainability.
ENGINE PLACEMENT: Propeller-Driven AC
 Option 1: Engines placed along fuselage
centerline.
Pros:
1. No thrust asymmetry in the event of
engine failure. Cessna Skymaster
2. High thrust line. Dornier CD-2 Seastar
 Important for amphibian ac.
Cessna Skymaster
Dornier CD-2 Seastar
ENGINE PLACEMENT: Propeller-Driven AC
Option 2: Engines placed symmetrically
on the wing King Air
Pros:
1.Most attractive aerodynamically &
structurally.
Prop slipstream has favorable effect on stall (built-in
safety against stall)
Prop slipstream increases L, especially when TE flaps
are employed.


Beech King Air

ENGINE PLACEMENT: Propeller-Driven AC
Option 2: Engines placed symmetrically on the
wing
Cons:
1.Engine failure may cause high windmilling D
before prop is feathered. Induced YM and PM
present control problems especially during TO.
2.Variation of engine power changes the
downwash on the tail
NB: If props are placed behind the wing, prop
plane must be at least 0.5 c behind the TE to
avoid vortex excitation from the flaps or the TE
onto prop blades! (GP-166, B-36 : prop fatigue,
broken blades)

Piaggio GP-166 Avanti
B-36
ENGINE PLACEMENT: Propeller-Driven AC
 Pusher vs. Tractor
Pusher Pros:
1. Stabilizing tendency in both pitch and yaw
when compared to tractors. This feature
can result in reduced tail surface
requirements.
2. Lower cabin interior noise.


ENGINE PLACEMENT: Jet AC
Intake requirements:
Must
Provide const. airflow at different engine settings and flight
conditions.
Limit flow distortion and turbulence at the compressor face.
Have short length, otherwise increased W and p-loss.
Must not
Change excessively the direction of the oncoming air at
different aoa.
Allow the wake of a partially stalled wing to enter the inlet duct
(i.e., wing LE is unsuitable for intake location)
Generate unstable flow in sideslipping (air oscillating instead of
entering the duct; problem with split intakes).
Have pronounced curvature.

ENGINE PLACEMENT: Single Engine Jet AC
engine mounted inside the fuselage

Problem: Intake & exhaust duct location
Solutions
1.1 Pitot Type Intake
Fokker S-14, MIG-17
Pro: Provides the engine w. undisturbed flow
for all flight conditions
Con: Requires long inlet duct, which
generally has to be divided at the level of
the cockpit – low intake efficiency
Fokker S-14 MIG-17
ENGINE PLACEMENT: Single Engine Jet AC
1.2 Wing Root Inlet Hawker Hunter

Con: Difficult to meet the intake
requirements (i.e., supply the engine with
the required airflow at different intake
velocities) and cope w. changes in aoa &
aos. An additional constraint is that the
local airfoil shape must not be modified
excessively.

Hawker Hunter
ENGINE PLACEMENT: Single Engine Jet AC
1.3 Side Inlets (Scoop Type) X-35 JSF
Problems:
1. Additional D. To minimize this D, the airscoops
must not be too short and must be well faired.
2. A divertor is needed to prevent the fuselage BL
from entering the duct but this also increases D.
3. The inlet opening must be located far ahead of the
wing to avoid interference and excessive variations
in the intake conditions.

ENGINE PLACEMENT: Single Engine Jet AC
1.4 Top Inlets
Miles Student, North American YF-107A
Problem:
The opening must be raised far above the
fuselage to avoid BL and wake ingestion
at large aoa.
Miles Student Experimental Jet
North American YF-107A
ENGINE PLACEMENT: Single Engine Jet AC
1.5 Split Bottom Inlet
North American Rockwell Buckeye
Pro: Attractive for mid-wing and high-wing ac
Con: Measures must be taken to avoid
ingestion of debris during taxiing and TO.
North American Rockwell Buckeye
ENGINE PLACEMENT: Single Engine Jet AC
Exhaust requirements:
Must
Be as short as possible; exhausts cause T-loss = 0.3% per ft-
length or 1% per m-length. 2 tail-booms can be used for this
purpose, offering the additional advantage of excellent
engine accessibility.
Must not
Not allow the hot jet efflux to impinge on the ac structure; for
M<1 in parallel flow, the expanding gases form a cone with
semi-apex angle = 6 deg.
ENGINE PLACEMENT: Single Engine Jet AC

1.6 Rear End Exhaust F-16
Pro: Keeps efflux away from the ac w/o any
special precautions
Cons:
1. Structural problems
2. Complicated fairings must be used
around the exhaust
F-16
ENGINE PLACEMENT: Single Engine Jet AC
1.7 Split Exhaust Hawker Sea Hawk

Cons:
1. Structural problems
2. Complicated fairings must be used
around the exhaust

Hawker Sea Hawk
ENGINE PLACEMENT: Multi Engine Jet AC

2.1 Engines buried entirely within the wing root

De Havilland Comet, Avro Vulcan, Vickers Valiant, Handley Page Victor, Tupolev 104

Pros:
1. Low D.
2. Better maneuvering capabilities as a result of the low W/S (larger S) and
low C
L
in cruise. Also, no compressibility problems such as buffeting.
3. Smaller nose-up pitching moment due to sweep angle because of the
low AR.
4. Better low speed performance due to the low W/S.
5. Better from the aeroelastic point of view because the low R wing box
structure offers greater stiffness.
6. If LFC is used (ex. BL suction), low T engines integrated inside the
fuselage of the wing in combination with a low W/S may be used.

De Havilland Comet
Avro Vulcan
Vickers Valiant
Handley Page Victor
Tupolev 104
ENGINE PLACEMENT: Multi Engine Jet AC
2.1 Engines buried entirely within the wing root

Cons:
1. Accessibility to the engines: Detachable skin
panels are necessary at a location where
the wing is highly stressed.
2. Safety: In the event of an engine fire the
likelihood that the fire will spread to the fuel
stored in the wing is great.

ENGINE PLACEMENT: Multi Engine Jet AC
2.2 Pod-Mounted Engines
Pros:
1. Safety: in the event of an engine fire the likelihood
that the fire will spread to the fuel is limited (main
argument for the choice of the B-47 configuration)
2. Optimum engine operation due to short intake &
exhaust ducts
3. Engine accessibility.
4. Current high bypass ratio engines together with the
development of efficient HLS favor the use of high
W/S (smaller S) and pod-mounted engines
Con: Higher D penalty

ENGINE PLACEMENT: Multi Engine Jet AC
2.2.1 Pod-mounted engines suspended below the wing
B-47, An-225, AB-380, B-777
Pros:
1. Structural advantages: the mass of the engines &
pylons lead to a reduction in the root BM, thus
lightening the wing structure. If engines are placed
ahead of the wing flexural axis, they also constitute a
mass balance against flutter.
2. Easy engine accessibility for maintenance.
3. Favorable aerodynamic effects of the pylons at
large:
 Tend to counteract the nose-up PM of swept back wings.
 Act as fences, which are often used on “clean” wings.
Antonov 225 Mriya
Airbus 380
Boeing 777
ENGINE PLACEMENT: Multi Engine Jet AC
2.2.1 Pod-mounted engines suspended below the wing
Cons:
1. Engines placed too far outboard increase LND
impact.
2. Engines placed too far outboard require a large fin.
3. Higher D.
4. Large YM & PM in case of engine failure.

ENGINE PLACEMENT: Multi Engine Jet AC
2.2.2 Pod-mounted engines fitted to the rear of the fuselage
Sud-Aviation Caravelle, DC-9
Pros:
1. “Clean” wing.
2. Low door level.
3. Little asymmetric T in case of engine failure.
Cons:
1. Large c.g. travel w. variation in loading conditions.
2. Deep stall because of the T-tail.
3. W increase due to required local “beefup” of the structure.
4. Loss of useful space in fuselage tail; result = longer fuselage for same PL.
5. In general, OEW will be about 2% greater.
6. Engines are not easily accessible for maintenance.
7. At full PL large download on the tail; result = lower L/D.

Sud-Aviation Caravelle
DC-9
ENGINE PLACEMENT: Multi Engine Jet AC
2.3 Mounting of a central engine
2.3.1 Engine buried in the fuselage B-727, L-1011
Cons:
1. Long and curved inlet; loss in intake efficiency.
2. Heavier.


L-1011
ENGINE PLACEMENT: Multi Engine Jet AC

2.3.1 Engine pod-mounted on top of the fuselage

Problem: Fin forms an obstruction.
Solutions:
 Cigar engine (DC-10)
 Butterfly tail

DC-10

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