Blackbird
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F/A-18 Variable-Camber Flap System

Mon Jan 26, 2009 11:46 pm

I'm wondering here if the variable-camber flap system used by the F/A-18 which adjusts the leading and trailing edges automatically operates as a function of airspeed/mach-number or airspeed/mach-number/g-loading?


Blackbird
 
CTR
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RE: F/A-18 Variable-Camber Flap System

Tue Jan 27, 2009 11:51 am

Blackbird,

Add roll rate (leading edge can operate symetrically and differentially) and replace G with inertial sensors. The FCCs use a complex algorithm that utilizes all inertial sensor axis.

Have fun,

CTR
Aircraft design is just one big compromise,,,
 
Blackbird
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RE: F/A-18 Variable-Camber Flap System

Tue Jan 27, 2009 11:01 pm

CTR,

When you say "intertial sensors" you mean that the sensors have the LED's deflect down when the pilot pulls up first to produce a more unstable condition to improve pitch, then uses the trailing edges more to stabilize from that condition, and when simply sustaining high-g's the camber is optimized for a given airspeed and mach and of course the load-factor in question or something like that?

Out of curiousity, does the F-16 use differential trailing-edge flap movement with the ailerons only? Or does it use differential leading and trailing-edge flap movement with the ailerons?


Blackbird

[Edited 2009-01-27 15:03:42]
 
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RE: F/A-18 Variable-Camber Flap System

Wed Jan 28, 2009 1:28 pm

One of the early F/A performance complaints was lack of range. This was not a design flaw but a design compromise to accommodate a very thin wing for high speed performance.

What did turn out to be a design flaw however was that the torsional stiffness of the thin F/A-18 wing was inadequate and deflected excessively. During flight testing it was discovered that aircraft roll acceleration and rate were well below predictions. Investigations revealed that under high aerodynamic loads, large aileron and trailing edge flap commands would cause the wing to twist, resulting in reduced surface effectiveness.

The fix? I believe they squeezed a couple more plies onto the wing skins, but that was not nearly enough to fix the problem. What turned out to be the solution was to redesign the leading edge flap to allow them to operate independently and be controlled actively in all flight modes by the FCCs.

My background at MCAIR was mechanical systems not aerodynamics. So I cannot tell you all the flight envelope points that utilize the leading edge flaps. But I can tell you that a lot more that G and air speed are utilized by the FCCs.

Have fun,

CTR
Aircraft design is just one big compromise,,,
 
Mike89406
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RE: F/A-18 Variable-Camber Flap System

Thu Jan 29, 2009 2:58 am

Quoting CTR (Reply 3):
What did turn out to be a design flaw however was that the torsional stiffness of the thin F/A-18 wing was inadequate and deflected excessively. During flight testing it was discovered that aircraft roll acceleration and rate were well below predictions. Investigations revealed that under high aerodynamic loads, large aileron and trailing edge flap commands would cause the wing to twist, resulting in reduced surface effectiveness.

The fix? I believe they squeezed a couple more plies onto the wing skins, but that was not nearly enough to fix the problem. What turned out to be the solution was to redesign the leading edge flap to allow them to operate independently and be controlled actively in all flight modes by the FCCs.

IIRC yes they did beef up the wingskin, and also installed Leading Edge fences on both wings right around the Leading Edge areas to further stabilize the aircraft.
you can see them sticking up in the following pics
http://www.navy.mil/view_single.asp?id=14030
http://en.wikipedia.org/wiki/File:A-18D_VMFAT_101.jpg
Mike

[Edited 2009-01-28 19:03:38]
 
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RE: F/A-18 Variable-Camber Flap System

Thu Jan 29, 2009 4:07 am



Quoting Mike89406 (Reply 4):
installed Leading Edge fences on both wings right around the Leading Edge areas to further stabilize the aircraft.

Different problem and different fix.

During F/A-18 development, MCAIR recommended to NAVAIR extensive wind tunnel testing to evacuate the fatigue load effects of high angle maneuvers. NAVAIR told MCAIR to delete the testing because flight maneuvers at ridiculously high angle of attack served no useful purpose in combat.

Then the USSR flew the cobra maneuver with a SU 27 at an air show and Navy pilots decided to duplicate it. All of a sudden F/A-18s started to develop fatigue cracks at vertical fin roots. The fix? Perform wind tunnel testing, install cleats on the vertical fin roots, and install fences on the leading edge root extensions to breakup the damaging vortexes generated during high angle of attack maneuvers.

Have fun,

CTR
Aircraft design is just one big compromise,,,
 
Mike89406
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RE: F/A-18 Variable-Camber Flap System

Thu Jan 29, 2009 6:23 am

Quoting Ctr (Reply 5):
Quoting Mike89406 (Reply 4):
installed Leading Edge fences on both wings right around the Leading Edge areas to further stabilize the aircraft.

Different problem and different fix.

During F/A-18 development, MCAIR recommended to NAVAIR extensive wind tunnel testing to evacuate the fatigue load effects of high angle maneuvers. NAVAIR told MCAIR to delete the testing because flight maneuvers at ridiculously high angle of attack served no useful purpose in combat.

Then the USSR flew the cobra maneuver with a SU 27 at an air show and Navy pilots decided to duplicate it. All of a sudden F/A-18s started to develop fatigue cracks at vertical fin roots. The fix? Perform wind tunnel testing, install cleats on the vertical fin roots, and install fences on the leading edge root extensions to breakup the damaging vortexes generated during high angle of attack maneuvers.

Have fun,

CTR

Ok, Thanks for clearing that up. I do remember something about the fatigue cracks now. Weren't the fatigue cracks a result of over flexing or over extension on the wings because of the vortices?

[Edited 2009-01-28 22:25:08]
 
CTR
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RE: F/A-18 Variable-Camber Flap System

Thu Jan 29, 2009 11:53 am



Quoting Mike89406 (Reply 6):
Weren't the fatigue cracks a result of over flexing or over extension on the wings because of the vortices's?

The fatigue cracks were a result of the large vortices's generated from the leading edge root extensions at extremely high angle of attack striking the verticals.

Have fun,

CTR
Aircraft design is just one big compromise,,,
 
Blackbird
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RE: F/A-18 Variable-Camber Flap System

Fri Jan 30, 2009 8:23 pm

Mike89406,

What was McAir's initial plan to "evacuate the fatigue load effects of high-angle maneuvers"?


Blackbird
 
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RE: F/A-18 Variable-Camber Flap System

Fri Jan 30, 2009 11:29 pm



Quoting Blackbird (Reply 8):
What was McAir's initial plan to "evacuate the fatigue load effects of high-angle maneuvers"?

Spell check is not always a wonderful thing. The word should be "evaluate".

Have fun,

Carlos
Aircraft design is just one big compromise,,,
 
Blackbird
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RE: F/A-18 Variable-Camber Flap System

Sat Jan 31, 2009 12:46 am

Okay, I understand.

I remember an old poem that a social-studies teacher in 8th grade gave us regarding the effectiveness (or ineffectiveness) of spell-checks

"I have a spelling checker, it came with my PC -- It plainly marks four my revue mistakes I cannot sea. I've run my poem threw it, I'm sure you're please too no. Its letter perfect in it's weigh my checker tolled me sew."


Blackbird

[Edited 2009-01-30 17:03:29]

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