Podded engines

[dimmolto]

Does anyone know what the advantages/disadvantages is with podded engines?

[A-FLOOR]

It's done primarily for noise reduction, but easier maintenance access, more room for cargo, pax and/or kerosene as well as aerodynamic considerations are also part of the equation. Engines podded below and in front of the leading edge make excellent places for vortex generators to help energize the boundary layer on the wing to help handling at high AOA and improve stall characteristics.

The primary drawback is that an engine fitted outside of the fuse or wing produces parasite drag.

[dimmolto]

Excellent answer, thank you..... but how would it contribute to noise reduction??

[A-FLOOR]

Originally Posted by dimmolto

Excellent answer, thank you..... but how would it contribute to noise reduction??

More distance (both in absolute and construction terms) between the pax and the noise source, and there is less vibration being induced into the airframe as the engine is basically hung from the wing on a few bits of metal instead of being contained within the airframe itself.

That is also why aircraft with the engines mounted to the tail are generally noisier than those with the engines in pods under the wings. The airflow that gets "touched" by the engine (both on its way in and on its way out) runs closer to the fuselage, and the engine is of course connected directly to the passenger compartment by means of a stub wing.

[Dani]

I'd say the main reason is aerodynamics, i.e. efficiency. There has been generations of engineers calculating it through. I read an article once (sorry, couldn't find the source) that you need less structural strength compared to the size of engine/aircraft with underwing engines, since the propulsion source is right on the center of gravity, the center of lift and (most of) all axis.

Before noise reduction I would also mention the safety factor of having engines as far away from all vital systems and passengers as possible. We all know of some uncontained engine failures that led to complete system failures with fuselage mounted engines while so far its rare to do the same on an underwing design (that's why the engines are mounted in front of the leading edge).

Anyone agree?
Dani

[ORAC]

And, of course, it allows the manufacturer to hang different engines on the aircraft to match the customers preference at minimal pylon/interface redesign cost, and to change them later in life at a similar low(ish) cost.

For a comparison, look at the cost and redesign needed to replace the wing on the Nimrod to replace the Spey.

[27mm]

A disadvantage of podded donks that springs to mind is their susceptibility to FOD - especially some of the new generation large fan engines - just look at the ground clearance under the pods on the re-engined KC-135, for example.

[earnest]

They also provide wing bending relief, ie as the airflow bends the wings up, the weight of the podded engines resists this. (This is also why many aircraft keep fuel in the wing tips for as long as possible).

[Mad (Flt) Scientist]

Originally Posted by Dani

I'd say the main reason is aerodynamics, i.e. efficiency. There has been generations of engineers calculating it through. I read an article once (sorry, couldn't find the source) that you need less structural strength compared to the size of engine/aircraft with underwing engines, since the propulsion source is right on the center of gravity, the center of lift and (most of) all axis.
Before noise reduction I would also mention the safety factor of having engines as far away from all vital systems and passengers as possible. We all know of some uncontained engine failures that led to complete system failures with fuselage mounted engines while so far its rare to do the same on an underwing design (that's why the engines are mounted in front of the leading edge).
Anyone agree?
Dani

Ooo - disagree.

Taking the second part, about failure effects:

Firstly, what about that 767 (?) at LAX (?) recently which had an uncontained engine failure that took out the 'other' engine; seems to me that would count as an "uncontained engine failures that led to complete system failures ....on an underwing design"

Secondly, the rules on pressurization loss following rotor burst failure for an aft mounted engine constrain the position of the aft bulkhead because you're not allowed to compromise the pressure vessel. There's no equivalent requirement for underwing engines, basically because it would be completely impossible for them to meet it. So the consequences of an uncontained failure of an underwing/podded engine may be far worse if you are unlucky and the disk element heads for the cabin.

And as for engine position effect on design:
"the propulsion source is right on the center of gravity, the center of lift and (most of) all axis."
The underwing engine is much further off-axis relative to the CG compared to, say, the aft-mounted type - there are significant pitching and yawing moments associated with thrust changes, and much more significant VMC concerns. Also, the wing/engine interface isn't without penalties - you;'re disrupting a clean wing design to mount the pod and pylon, and likely giving up some aerodynamic efficiency on the wing. There4 are, of course, benefits too, but it's not a one-way bet in terms of design choice.

[barit1]

Not sure I can go along with MFS's second point - an MD-80 uncontained failure has penetrated the cabin, resulting in death & injury to nearby pax.

And there is certainly a structural penalty (ZFW) for tail-mounted engines, compared to hanging them directly on the wing.

[Dan Winterland]

Historically, British aircraft had inboard engines. Think Comet, Victor, Vulcan and Valiant. The big disadvantage is that an engine failure is likely to take out the adjeascant engine and or a fuel tank. Not too good for survivability and this happened on more than one occaision.

The first podded production aircraft was the B707. It too had it's engine teething problems and engine fires were less of a hazard in these. The only experience I have of these is the early model KC135 (the military tanker version). The drill for the early models was just to shut off the fuel. There was no fire extinguisher. If the fire got critical and stated up the pylon, the fuse pin holding the engine on melted, the engine fell off - and end of problem!

Also, the weight penalty of rear mounted engines is significant. The BV10 was some 7 tonnes heavier that the B707, largely due to the supporting structure for the rear mounted engines.

[Mad (Flt) Scientist]

Originally Posted by barit1

Not sure I can go along with MFS's second point - an MD-80 uncontained failure has penetrated the cabin, resulting in death & injury to nearby pax.

Didn't say it couldn't happen, just that there are rules about it for rear mounted. Either that case was before the rule (and so the rotor disk zone did intersect the cabin) or the disk element(s) went outside the "predicted zone".