Engine Placement

Configuration Design

part 3

Engine Placement

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


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


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


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


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


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.


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


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


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.


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


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 CL 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


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.


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


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


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.


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


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


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

Documents

Engine Placement