jetblueguy22
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Winglets On Fighters

Thu Dec 01, 2011 3:07 pm

I was thinking the other day while looking through some photos of fighters, Why don't any have winglets? I understand the forces a fighter goes through is much more than a commercial aircraft but couldn't it help efficiency? Or would it just blow off in a dog fight?
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Spacepope
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RE: Winglets On Fighters

Thu Dec 01, 2011 4:10 pm

Aren't wingtip=mounted AAM's kinda doing a similar thing?
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connies4ever
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RE: Winglets On Fighters

Thu Dec 01, 2011 4:45 pm

Winglets are added to commerical airliners to improve cruise efficiency.

Basically, on swept-wing a/c, there is always energy moving towards the wingtip. These create the vortices we can see when it is very humid, but also a lot of the energy is lost basically spilling off the end of the wing. Winglets reduce this energy loss and therefore the air spillage along the span can be put to good use. It makes the wing effectively longer and increases the aspect ratio (span vs chord), which is important for cruise efficiency.

You could just make the wing longer and finer, but then aeroelasticity considerations and simple wing strength become an issue. Not to say the winglet doesn't weigh anything, and on 737s, for example, some local strengthening needs to happen to support the winglet.

There are other issues as well.

On a fighter, seems to me a winglet wouldn't be of much value since cruise mode isn't likely going to be very long, I do not know how a winglet would affect a/c handling under high-g / rapid roll flight modes. I am certainly not an expert, but I think the larger winglets you see on a 737 for example might act as additional keel surface and make a fighter less responsive to yaw inputs. ?? And, importantly, would it interfere with wingtip missiles ?
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vikkyvik
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RE: Winglets On Fighters

Thu Dec 01, 2011 8:47 pm

Quoting connies4ever (Reply 2):
Basically, on swept-wing a/c, there is always energy moving towards the wingtip. These create the vortices we can see when it is very humid, but also a lot of the energy is lost basically spilling off the end of the wing.

To clarify: trailing vortices exist on straight-wing aircraft as well, and indeed there are straight-wing aircraft that have winglets:


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Vortices will be generated basically anywhere you have a pressure difference around a finite barrier.

Quoting connies4ever (Reply 2):
I am certainly not an expert, but I think the larger winglets you see on a 737 for example might act as additional keel surface and make a fighter less responsive to yaw inputs.

I believe that some aircraft have different crosswind limits for winglet-installed aircraft, but I'm not positive about that.
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Slcpilot
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RE: Winglets On Fighters

Thu Dec 01, 2011 10:42 pm

Another reason.... Radar cross section.

Many notional future fighter images have no vertical surfaces at all. I suspect we will eventually see transport aircraft start to see more relaxed stability as well in the search for efficiency.

It is my understanding (in layman's terms), that the efficiency gained by a winglet is roughly equal to increasing the span by half the height of the winglet.

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connies4ever
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RE: Winglets On Fighters

Fri Dec 02, 2011 12:28 am

Quoting vikkyvik (Reply 3):

Vortices will be generated basically anywhere you have a pressure difference around a finite barrier.

Thanks for reminding me !    I deal with a fluid mechanics code so I should actually know this. But, 60 now, senior-type memory from time to time. What was your name ?
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prebennorholm
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RE: Winglets On Fighters

Fri Dec 02, 2011 2:05 am

Winglets are angled (toe-out) at an angle which fits cruise speed and a one G environment. A fighter shall be efficient at various G environments, including negative G's.

Winglets fly in the air at a combined speed of the TAS and the speed of the vortex, therefore always faster than TAS. They reach transonic drag before the rest of the wing. Fighters shall be efficient also at speed faster than traditional transport planes.

Most fighters are made for supersonic flight. Winglets will not work at all at supersonic speed, but will add disastrous drag only.

These days fighters are designed to be as stealthy as possible. Winglets will add considerably to the radar signature from the most critical angles.

The only modern military plane which might benefit aerodynamically from winglets is the B-2A bomber. For stealth reasons it is completely out of question.
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Legs
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RE: Winglets On Fighters

Fri Dec 02, 2011 4:45 am

Quoting prebennorholm (Reply 6):
modern military plane which might benefit aerodynamically from winglets is the B-2A bomber

The B-52, perhaps? Firmly subsonic, spends a lot of time at cruise, old-ish engines. (Yes, I fully understand that it may not fit into some definitions of modern, but hey, it's still in service!).
 
rwessel
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RE: Winglets On Fighters

Fri Dec 02, 2011 8:00 am

Quoting legs (Reply 7):
The B-52, perhaps? Firmly subsonic, spends a lot of time at cruise, old-ish engines. (Yes, I fully understand that it may not fit into some definitions of modern, but hey, it's still in service!).

Aerodynamically, it probably would, as would any aircraft with flying long cruise segments. As would the B-1, although integrating winglets with a variable geometry wing is likely to be far more costly than any possible gain. The B-2's wing loading is fairly low, so the value of winglets would be reduced (less of a pressure difference between the upper and lower surfaces). A problem specific to the B-52 is its very flexible wing, which would likely have issues with the considerable torque from the winglets.
 
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bikerthai
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RE: Winglets On Fighters

Fri Dec 02, 2011 9:43 pm

As I recall, the wing loading on a fighter is much different than a wing loading on a commercial aircraft.

Can't recall the math, but because of this difference (high maneuverability vs. high efficiency) you tend to get wings with short aspect ratio on fighter vs high aspect ratio for passenger planes.

Quoting prebennorholm (Reply 6):
A fighter shall be efficient at various G environments, including negative G's.

Never thought that a fighter need to be fuel efficient doing G maneuvers.
Thought they just have to be highly maneuverable to create those G conditions  

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connies4ever
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RE: Winglets On Fighters

Fri Dec 02, 2011 9:46 pm

Quoting bikerthai (Reply 9):
Never thought that a fighter need to be fuel efficient doing G maneuvers.
Thought they just have to be highly maneuverable to create those G conditions

Not fuel efficient, but efficient at turning, i.e., high roll rate.
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JakeOrion
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RE: Winglets On Fighters

Sat Dec 03, 2011 12:09 am

Quoting rwessel (Reply 8):
A problem specific to the B-52 is its very flexible wing, which would likely have issues with the considerable torque from the winglets.

What about raked? Winglets/sharklets I can see, but raked would have considerable less torque.

For the thread, fighters on average are up in the air 2 to 4 hours at a time, generally less, which negates the purpose anyway. The only logical choices for winglets would be cargo, refueling and the likes aircraft.
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flyingturtle
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RE: Winglets On Fighters

Sat Dec 03, 2011 3:06 am

...but then, dogfights with their many tight turns involved are rare today. I thought fighter jets on a typical mission (like in Afghanistan) would cruise for some hours until called in for close air support... but as seen in Vietnam, changes in combat doctrine may come quickly.

Quoting JakeOrion (Reply 11):
What about raked? Winglets/sharklets I can see, but raked would have considerable less torque.

...the F-4 Phantom comes to mind... did her angled "wingtips" have any advantages?
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prebennorholm
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RE: Winglets On Fighters

Sat Dec 03, 2011 3:09 am

Quoting bikerthai (Reply 9):
Never thought that a fighter need to be fuel efficient doing G maneuvers.

Then go and talk to a fighter pilot.  

A high speed and high G maneuver creates immense induced drag. So much drag that the fighter is unable to sustain speed. It will slow down even in full afterburner. Especially so at some altitude, where the thinner air reduces engine power significantly, while induced drag for a certain G load increases.

The less you slow down, the more advantage you have over an opponent in a dogfight. It's all about maximizing lift to drag ratio, on commercial transports for saving fuel-$$$, while on fighters it is more for maximizing maneuverability with the available power.

Everything, which possibly can be done, is done to make fighters as efficient (high lift to drag ratio) as possible in all ways of flight. Most notable is "Direct Lift Control" on the wing.

A transport plane has leading edge and trailing edge devices which at take off and landing are set at certain positions to increase lift capability. An F-16 for instance constantly optimizes leading and trailing edge flaps for the best lift to drag ratio throughout the flight. They move up and down instantaneously like the elevator on any control stick input.

Direct Lift Control is implemented differently on various configurations of fighter planes such as tailless deltas (Mirage), canards (Thyphoon, Gripen), or more conventional configurations like most Russian and American fighters.

Winglets work very well on commercial transport planes as fuel-$$$ savers. But they are not miracles. They are optimized in shape, profile, twist and toe-out angle for a specific flight regime, say save 3% fuel (reduces drag by 3%) at one G cruise at Mach 0.8. Divert a little from that regime, and the gain reduces significantly and soon becomes negative. If you cruise for instance in a comfortable tailwind and slow down your airspeed to take advantage of the tailwind, then it is no longer 3% gain.

For a fighter to take any advantage of winglets they would have to be variable both in sweep-back and toe-out angle, and they would still be pure disasters in supersonic flight as well as radar signature wise.
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bikerthai
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RE: Winglets On Fighters

Mon Dec 05, 2011 3:25 am

Quoting prebennorholm (Reply 13):
It's all about maximizing lift to drag ratio,

Yep and in the current generation of fighters, the lift to drag ratio is maximized during high turn rate and maneuver by use of thrust vectoring (and/or canards) which kills your fuel burn efficiency, considering during these maneuvers, the flow over the wing would surely be highly turbulent already.

Quoting prebennorholm (Reply 13):
Then go and talk to a fighter pilot.

Don't know any fighter pilots. But from what I read, when in a tight spot they mostly want more thrust and leave the wing efficiency debate in the class room.   

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prebennorholm
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RE: Winglets On Fighters

Mon Dec 05, 2011 4:21 am

Quoting flyingturtle (Reply 12):
...but then, dogfights with their many tight turns involved are rare today. I thought fighter jets on a typical mission (like in Afghanistan) would cruise for some hours until called in for close air support... but as seen in Vietnam, changes in combat doctrine may come quickly.

That's only because the later wars have all been so called "asymmetric wars".

Last time there was a little air to air battle, I think, was during the Iraq "No Fly Zones" 1991 to 2003.

Just this year the Royal Danish Air Force flew 923 sorties over Libya. Many were air to ground missions - tank busting with smart bombs and such, but many were also air patrol enforcing the declared no fly zone. The latter were trained for and ready to take on any Libyan fighter which left the ground. None did.

There may have been some air to air battles during the Russian - Georgian War in 2008. I don't know.
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