Wing Design Question

[Burner2k]

Hi,
I was discussing online about Wing design for fighter aircrafts and one of the posters made an interesting comment. One of the US fighter is designed to carry a missile on its wing tip always so that it reduces the peak wing deflection during flight and hence decreases the bending stresses and ultimately fatigue. The idea behind this philosophy is that aircraft will spend probably 80% of its life in training missions where a missile (usually dummy ones most of the time) are carried on wing tip. The above practice also reduces the wing weight.

Apparently, the same philosophy (not on the wing tips) are followed in commercial aircraft design as well.

https://en.wikipedia.org/wiki/Podded_engine

Placing engines on the wing provides beneficial wing bending relief in flight. The further the engines are away from the fuselage the greater the wing bending relief so engines buried in the wing root provide little relief. Almost all modern large jet airplanes use engines in pods located a significant distance from the wing root for substantial wing bending relief. The pods are in front of the wing to help avoid flutter of the wing which, in turn, allows a much lighter wing structure.

I had never thought about under wing engine placement from the above POV! What are the advantages and disadvantages of the above type of design compared to empennage mounted engine arrangement?

I have never worked on Military aircrafts but my question revolves around load case considerations. How are the DUL & DLL load cases defined for a wing structure wrt to pay load considerations? Are payload weights included in DUL/DLL calculations?

 

[rb1957]

generally weight on wings is "good" weight, in that it'll reduce wing bending moment (you've taken weight out of the fuselage). wing fuel similarly.

The Huge problem is aero-elastics (flutter, etc).

There could well be a benefit to the wing for carrying a dummy store on the tip, mine you, you've surrendered about 100 lbs of fuel, but that may not be a problem.

On a similar vein, I saw a pic of the F35 with wing (pylon, not tip) mounted AA missle and thought "I thought the big deal with the F35 was low observability, and hence internal carriage of stores ..."

another day in paradise, or is paradise one day closer ?

 

[KENAT]

Wing tip mounted stores aren't necessarily just 'anti flutter weights' or similar, they may also be used to tune the aerodynamics. Same would probably apply to the engines.

Placement of major components in aircraft are balancing multiple requirements, for instance wrt engines just some of the requirements are:

[ul]
[li]Global C of G/stability concerns.[/li]

[li]Local mass effects (i.e. countering the 'up' force from wing lift via 'down' load from the engine mass) which then impacts structural loads and hence structure mass.[/li]

[li]Serviceability/upgrade-ability (a problem with in fuselage engines be it on military A/C or the trijets of the 60's & 70's)[/li]

[li]Maneuverability i.e. moment of inertia effects of distributed mass V mass concentrated near the center.[/li]

[li]Line of thrust - move them too far from center line and in engine out conditions you get a lot of asymmetric yaw which is problematic.[/li]

[li]Safety - such as bird strikes or impact to passengers during emergency exit ...[/li]

[li]Routing of all the electrics, pneumatics, hydrualics, fuel lines ... to & from the engines[/li]

[li]Interference with other components such as the controls & high lift devices or even landing gear etc.[/li]

[li]Aerodynamics, both how they locally impact airflow and things like 'area rule' for compressible flow regime.[/li]
[/ul]

rb1957 - that's one of the issues with stealth for high performance Aircraft. Due to other constraints they are not going to have large internal weapons bays. So when operating 'stealthily' they will have small payload. This is off set somewhat by use of precision guided weapons which for most targets allows fewer much smaller weapons to give the same probability of effect as it would have taken multiple larger dumb stores historically. In fact at some point when using PGMs the limit can be having enough fuel/crew endurance to be airborne long enough to use up all the PGM. Instead of thinking about how many aircraft carrying how many bombs you need to give a high probability of taking out a target, you instead start thinking of how many targets can an aircraft hit in one sortie. With the specific case you mention the idea is probably that for high intensity or even 'first day of the war' missions you fly stealthily, in lower risk environments and/or after the opposing air defenses have been degraded then stealth is less useful so you strap on the external pylons and away you go with increased payload (which if history is anything to go by will probably end up being fuel tanks more often than not assuming they are wet pylons).

Taking this latter idea to the extreme you'd have a small fleet of super stealthy aircraft that you use to suppress enemy defenses on the 'first day of war' then a larger fleet of lower cost non stealth aircraft to finish up the conflict. The original operational concept of the F-117 more or less followed these lines as I understand it.

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[rb1957]

yeah, I forgot about the follow up questions about engines ... research into an airplane design book (Ramyer, Torenbeek, Niu) there are tons of trade-offs about engine placements. When Boeing first did wing mounted podded engines (as opposed to engine buried in the wings) there was a huge debate about the aerodynamic impacts (other than the obvious weight and aero-elastic issues) ... turns out that podded engines affect the lift curve slope (or so I believe).

and, yes, payload is an important consideration for wing loads. Funnily enough (until you think about it, a lot) minimum weight produces many wing design loads.

another day in paradise, or is paradise one day closer ?

 

[Burner2k]

Thanks folks for the illuminating response.

I like the term good weight...rare to hear weight is good in aerospace structures.

 

[WKTaylor]

External and internal stores carriage has rewards and risks.

Rewards. In theory, there is lots of space to 'hang' stores on fuselages and wing stores pylons. At these locations, 'static analysis' often show significant benefit at some losses due to drag... however... each external load combination represents a unique mass/aerodynamics [dynamic] distribution, which must be analyzed and tested to ensure structural and aerodynamic stability through the full flight and weapons delivery envelope. This can mean hundreds of combinations need to be analytically and flight-test evaluated... at great expense and some risk to a test aircraft. Nasty surprises have arisen when unpredicted aerodynamic and/or aero-elastic interactions occur... especially at the 'corners of the envelope'... A classic accident of an unsuitable combination of ordnance and aerodynamics is shown here...

https://www.youtube.com/watch?v=8WKDHsrh3mk

Internal load carriage, other than physical and loading restrictions, almost never pose significant aerodynamic/aero-elastic challenges.... except for the opening/closing of Bomb-bay doors... and the energetic jettison of weapons from their 'racks' thru/into/below the acft boundary layer/slip-stream.

Regards, Wil Taylor

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[KENAT]

WKYaylor, I'd love to know what configuration that store was. For an actual 'bomb' to do that is pretty impressive. Looked like maybe it was a retarded bomb and for some reason the 'petals' began to open early but hard to say. Would be less surprised if a drop tank or similar did that as they are a lot less dense (especially when empty) and so prone to that kind of thing, typically for a simple 'bomb' though mass trumps aerodynamics.

That said, was at an airshow at Boscombe Downe one time and at the stand for the British test agency (pre DERA/Qinetiq etc. so probably RAE or something) they had movie reel of similar accidents happening.

My own personal experience was on a certain program the 'quick disconnect' MIL-DTL-38999 C/MIL-STD-1760 connector "didn't". On the trials footage I was looking at it showed up as store starting to separate cleanly then 'hanging up' before breaking away. However at least one service A/C came back from dropping one with part of the harness that should have gone with the store still attached!

I can vouch for what you say about all the flight test gradually expanding the envelope, seems ridiculous when it takes years to integrate an existing store onto a new A/C but that's a big part of it.

(By the way & as you probably know, despite what the U-tube caption said it would have been very unlikely that if that had been a live store it would have gone 'bang' from impact with the A-4. The primary charge isn't that easy to set off and the fuze chain has multiple safety features to stop it going off too soon.)

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[WKTaylor]

Burner2k...

Also, a less well-published reason for placing engines on struts.

Older [early generation] engines of all kinds... piston [especially turbo/super-charged piston engines] and turbine [turboprop/turbojet/turbofan/prop-fan engines] were notorious for catastrophic failures.

Piston/turboprop engines mounted on the wing leading edges, and jet engines mounted within the wing or wing root, caused massive damage when high energy fragments and fire erupted in the cowling or engine bays... all-too-often leading to incredibly tense emergencies... that often went out of control causing loss of the aircraft.

Due to propeller dynamics, piston and turboprop [and hybrid prop-fan] engines, have to be mounted relatively close to primary structure [fuselage or wing] for rotational stiffness and strength. These installations present challenges over-and-above simple catastrophic engine failures, since propellers have their own unique failure modes [added to the failure modes for the engines... a double whammy].

By placing turbojet/turbofan engines on rugged struts, set away from wing or body primary structure, the effects of 'MOST' catastrophic failures... such as fragmentation, massive fires and even engine separation... can be 'managed/contained' away from most flight critical structures and systems, allowing [multi-engine] most aircraft to divert for a safe landing with remarkable damage. Obviously, if more than one engine of a multi-engine aircraft suffers simultaneous catastrophic failures [IE: multiple large bird strikes affecting the core engines], then very bad things can still happen... but at least primary structure and systems usually remain intact and functional.

NOTE. Classic examples/photos of these type failures are everywhere on the web.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]