Implications of a wing pod creating lift...
 

New to the forum, but read a few threads on fuel distribution, fuselage mounted engines versus wing mounted engines, etc. and their effect on wing bending and overall lift.

My question is, what are some of the general implications of a wing pod (my focus on military aircraft more than civilian) that created lift? I could imagine less drag, greater flight time, heavier payload capacity within the pod. But I could also see stability issues at the edges of the flight envelope, etc.

Any references you can point me to are also greatly appreciated.

Compared to the same wing (or a similar wing) with no pod, the wing with the pod will generate more drag for the same amount of lift, unless it's a really horrible wing design. The most efficient way to generate lift, in terms of low drag, is with a wing. A pod shape makes a bad aerofoil, so will be less efficient in terms of L/D. So the range will be lower.

What you're doing, when you design the wing/pod(nacelle) interface is trying to minimize the penalty of having some ugly lump stuck on the wing, screwing up the wing aerodynamics. I'll stick my neck out a bit and say that no external shape added to a wing other than for purely aerodynamic design reasons will improve the wing aerodynamics, compared to that you'd get if the aerodynamic design were not constrained by the need to have a pod or nacelle with something in it.

If pods really improved the wing, we'd put them on even if the engines were mounted at the rear!

Mad (Flt) Scientist,

CV990 ? But I would agree 'Kucheman bodies' are not quite the same as the 'pods' Ron.g is talking about.

CJ

Sure. Lots of similar cases where aerodynamicists have added something to a wing to improve it. That's what my caveat in italics was alluding to. And sometimes you can do something useful with the extra volume we may ask for.

But if you constrain the pod for some non-aerodynamic reason then I doubt it will be the best shape that it could be, almost by definition.

I would go on to say that as long as it is sucking and blowing the nacelle of an engine does not have significant drag and an attempt to construct some sort of airfoil under it would probably fail as a result of engine in operation. Do not forget that you do not create lift without creating drag. That motor only becomes a significant source of drag when it fails in flight.

It's not uncommon to design an engine inlet duct with a cross section in the vertical direction to add lift. The lift is a fuction of the flow through the inlet which in turn is a fuction of how much air is passing through the engine. Thus if you happen to break the pylon mounts on a running engine, most will lift up over the wing.

Tis true that there is significant drag associated with all this but its mostly associated with the inlet lip area. The fan engine area actually has less drag when its not running as the air can freely pass through the large fan blades without created a large amount of spillage around the inlet lip.

In the end the drag dynamics vary but the worst condition should be a highpower, low altitude engine surge.

What's missing from all this subjectivity is actual numbers, but it at least gets the thoughts out.

There exists somewhere a video of a fighter aircraft releasing a bomb that rose up and struck another fast jet above it, taking off its wingtip. The bomb must have been producing lift at the time.

After an excellent landing you can use the airplane again!

There's a few stores release videos around that show 'interesting' release dynamics. be careful, though - most of the ones that do the bizarre stuff are (empty) fuel tanks or (empty) rocket pods. If an actual bomb generated so much lift relative to its own weight that it could "fly" it'd be one Hell of an inaccurate dumb bomb (and yes, I know there are specifics of the upwash field, but still - you want your bomb to be a projectile, not a glider).

Any links to the videos? And were all those stores releases at 1g?

CJ

Here's one for you Christiaan

Thanks Brian !
'Interesting' store release dynamics indeed....
Looks as if something went seriously wrong, with the bomb tumbling.
I'l look again, but also it would seem the pylon departed from the a/c together with the bomb - not normal practice, it seems to me.... and that wouldn't have improved the aerodynamics of the bomb, either.

The video Brian linked to is obviously the one you were thinking of, too.I wouldn't really call the complex aerodynamic forces on a tumbling bomb in the wake of an aircraft "lift"....
Also, the A4 was flying 'chase', and not above the F18.

Ron.G,
I'm not sure we're really answering your question, are we ?
Were you thinking of reccy pods, ECM pods, and suchlike?
I would say they're most designed for low-drag as much as possible.... not to add a bit of lift - that would change the handling characteristics of the aircraft, something the pilot wouldn't appreciate.....

CJ

Good video but not the one I saw in which both aircraft returned to base.

After an excellent landing etc...

Try to find the link.....

CJ

it wasn't the pylon, it was the multiple bomb carrier (TRU? MRU?) that was ejected along with the bomb. Since it's attached to the pylon, the ejectors in the pylon can 9and in this case evidently did) act to jettison the whole assembly.

A bit of googling says its a Multiple Ejector rack - MER - knew it was a TLA at least!

PS I'll see if i can find any of the ones i recall. Many were 20 or 30 years old by now, and I'm sure some were 'internal' or 'restricted', though probably not now - I doubt anyone is going to learn any state secrets looking at a Buccaneer test cine film now!

Going a bit off-topic... but I still would say dropping the MER still joined to the bomb (rather than separately) sounds like a failure, or a dumb idea, since it would affect the bombs trajectory.

Would be interesting, but I suppose it's unlikely any of it was converted to video, and ended up on YouTube.....

CJ

Forgive my ignorance to the aeronautical engineers, I just can not see how the nacell and engine can produce lift Dynamic, esp. Static. In fact all advancements I have seen are to reduce disruption to the wing's lift producing capabilities. I have allways been taught that they are designed with the intent of producing minimal drag in operation and that a windmilling or locked engine esp creates allot of drag. An engine vibe failue is designed to shear the aft mount first creating an AOA to send the motor over the wing.

grounded27

You have been taught an old wifs tale, or what some want to believe because it sounded about right.

Discard such thoughts because they are scientifically not correct.

With a windmilling engine the core will still compress air creating a pressure front, which is drag. With a locked engine most of the air finds it easier to pass between the fan blades. and since the fan is not creating a pressure front the drag goes down (of course so does the thrust)

Unfortunately the simulators that train pilots don't recognize this and perhaps fortunately only one or two flights in fan jets have ever locked a fan rotor above 150 kts.

The bottom line is that the inlet lip anular area is the major drag producer and not the engine fan face.

And relative to designed mount separations, the regulation do not permit a failure condition by design, they expect cascading failure conditions to be minimized, not allowed.

Getting a bit back on topic...
 

ChristiaanJ (CJ)...

Quote:

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Ron.G,

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Yes, I was more referring to a pod used to carry electronics (optics, electronic warfare, etc.) versus an engine. I understand about anything added to the wing today (external fuel tank, weapons, etc.) adds a significant amount of drag and today's designs try to minimize the drag as much as possible. Has there been any studies that examine a pod that has more of an airfoil type of design? Would it help.

BTW...thanks to all responding to the question. The more I'm reading the more questions I have...

Just design the pod to be added to the fuselage, and save everyone a lot of headache....

you would not likely want an airfoil to destabilize the complex airflow over the wing...

in military apps, there are sensor arrays that are designed to be fitted to the standard armament connections, and these shapes will tend to follow the area rule.

I wish I could just add to the platform...BUT...that transforms a "pod development" program into a "platform modification" program. A transformation that brings a whole lot of baggage.

I know, I know...the fuse is off limits...

In reality, there should be plenty of guidance for connections to the bottom of the wing. (or add them to the tops of the winglets! hahaha)
Are you looking at sub or super sonic?

(vastly overlooked is the top of the tail, but you have little downward coverage)

I just can not see how the nacell and engine can produce lift

Three immediate considerations -

(a) curved surfaces with fluid flow can generate significant forces

(b) flow through the engine at high alpha or sideslip angles involves turning the airflow at the nacelle inlet - equals forces. Indeed, this sort of problem can create LSS implications in the missed approach as the lip lift force is destabilising. Also a problem with many piston to turboprop conversions at the prop disc.

(c) likewise, at high angles, the engine efflux thrust can be resolved into vertical and horizontal components.

Another one ChristiaanJ

Brian,
Many thanks !
I'll have to go through those step-by-step to see what I can learn.
A lot seem to be "things with fins" (tanks, bombs, rockets ...) where a slight misalignment of one fin is enough to start a violent tumble.

CJ