KingAirPIC

Why do they put the 1 and 4 engine on the 747 and even 707 so far out from the fuselage? Why not closer to 2 and 3?

During a low speed V1 cut when the engine that fails is one of the outboard engines the loss of thrust being so far away from the fuselage creates a huge and fast yaw movement. Coupled with low airspeed where rudder effectiveness is minimal can cause a dangerous situation if not for very quick action in the abort. Why not put the two outboards closer in? Sounds to me like this would reduce the effect of an outboard failure.

A recent chat with a Boeing stress test engineer didn't provide any answers in his department. Curious if anyone has talked about this?

ryguy

It's really not that bad of a yaw. I'm sure some engineer out there knows why they are where they are but I don't.

OrionFE

CG would be one main reason. Like any other four engine airplane, you have to be on the right rudder input for the V1 cut or you are going for a ride. One of my SIM partners had a rough time with it. Once it gets going in the wrong direction, it aint coming back!

tomgoodman

Better weight distribution -- lower stress on the wing.

Cubdriver

This is a complicated topic that defies a quick and simple summary. Here are some of the issues:

1. Aeroelastic effects- the location of the engine pylon may interact in such a way as to introduce oscillations to the aircraft and must be placed to avoid divergent conditions.
2. CG placement- as mentioned the cg moves dramatically when engines are moved out from the fuselage, particularly on swept wings.
3. VMC- vmc is very sensitive to engine location on the wing.
4. Low speed effects on the wing- engine placement can affect flow turning around the wing in low speed, high-lift configurations especially.
5. High speed effects on engine- if an engine can be placed farther out the mass flow rate and efficiency can be optimized by avoiding disturbed air close to the fuselage.
6. Structures- if an engine is farther out there are higher bending moments to support structurally which increases weight of the aircraft.
7. Practical considerations- maintenance must have access to the engine and to the wing and fuelers must have access to the fueling points.
8. Noise footprint- nowadays the noise footprint even figures into it, as cities grow larger and more people live near major airports than ever before.

Some of these considerations favor moving an engine in the inboard direction while others favor movement outwards. Engineering is the business of deciding where to make tradeoffs and there are optimization objectives to every design. To determine an optimal placement for an engine involves mathematics, cost, practicality and surprisingly even appearance in some situations. The 747 was designed in the mid-1960s and there may be model specific information to be had on the subject, but I am sure the explanation would have to touch on these areas as a minimum.

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ryguy

Like I said....someone knows the reason!

Cubdriver

A few more...

9. Ground clearance for engine, given the amount of wing flex expected.
10. Damage control in the case of uncontained engine failure for the aircraft and for the engine in terms of FOD ingestion.
11. Effects on high lift devices located in proximity to the engine. This usually means no flaps behind the engine will be used due to drag and heat problems, it is better to push an engine farther out to avoid the loss of flap area.
12. Pylon effects on wing upwash- affects stall speed, total lift, etc.
13. Noise in airplane cabin- farther from cabin is usually but not always, desirable to reduce noise.

All of the above items are important, and any can affect the choice for engine placement. However aeroelasticity, VMC, weight, and aerodynamic effects are the chief concerns. An aircraft engineering team puts together studies of these issues and how they interact with one another then arrives at an optimum. Such studies take tons of time to perform. Large companies like Boeing develop standardized practices over time, but every new design requires model-specific studies. In the last 25 years computer codes have been developed to reduce the work involved to determine design optimums, and a lot of money is spent on such programs. Much of the scale-model wind tunnel testing done in prior years has been eliminated by codes.

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SmoothOnTop

Still not buying the Bernoulli effect?

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SmoothOnTop

before you get your pants all bunched up, I like all your answers and I'm just busting you about "magnus."

But for aviators excluding my good friend cubbie:

a wing with a finite aspect ratio does have a circulation flow that causes a strong vortex trailing downstream - watch the movie "pushing tin."

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Cubdriver

So does a wing of infinite span have circulation and a vortex. Fluid circulation is the unequal travel between top and bottom side of a wing developing lift, and the unequal travel is modeled as a vortex filament, a hypothetical line inside the wing. Since by the 3 laws of vortices (you know those, right), this line has to continue past the end of the wing and rolls into trailing lines commonly felt by pilots as wake turbulence if the wing is finite. But circulation and vortices are two dimensional, as well as being found inside wings of theoretically infinite span.

There are more satisfying sources on this topic than Hollywood. I like Richard Shevell's Fundamentals of Flight (used $80). I can read his books for years, whereas a movie like Pushing Tin is over after maybe an hour and a half if I can stay awake that long.

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