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Why a Swept-Wing?



 
 
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  #11  
Old January 11th 04, 06:37 PM
Tarver Engineering
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"Ed Rasimus" wrote in message
...
On Sun, 11 Jan 2004 09:55:02 -0500, Air Force Jayhawk
wrote:

An aircraft is only unstable if the aerodynamic center is forward of
the center of gravity. If the wing root is sufficiently aft and the
AC stays aft of the CG, stability remains.

Why? Well it was tried with the X-29 but I never have read why no one
has pursued it since. The advantage was supposed to be that the
boundary layer (the thick air right next to the surface caused by
friction and very annoying) builds up as the air moves aftward along
the wing. With a FSW, the thickest part of the BL is at the root
rather than near the control surfaces, enhancing control while at high
angles of attack. There are other advantages but it's been a while so
I can't recall them off the top of my head.

I knew the USAF pilot on the X-29 project...he said it flew fine and
had no issues with it.


As I recall the X-29 project, one of the objectives was evaluation of
the instability as a means of gaining agility for future highly
maneuverable aircraft. The "urban legend" was that the aircraft
required minimum of triple redundant FBW augmentation as loss of the
augmentation would result in immediate excursions from stable flight
and structural failure within seconds. The ultimate in "JC maneuvers".

Always thought it made for an extremely ugly airplane.

Wasn't the basic structure from an F-16A?


Grumman modified an F-5.


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  #12  
Old January 11th 04, 08:10 PM
Kevin Brooks
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"Ed Rasimus" wrote in message
...

snip

As I recall the X-29 project, one of the objectives was evaluation of
the instability as a means of gaining agility for future highly
maneuverable aircraft. The "urban legend" was that the aircraft
required minimum of triple redundant FBW augmentation as loss of the
augmentation would result in immediate excursions from stable flight
and structural failure within seconds. The ultimate in "JC maneuvers".

Always thought it made for an extremely ugly airplane.

Wasn't the basic structure from an F-16A?


Ed, I believe the basic structure was from an F-5.

Brooks




Ed Rasimus



  #13  
Old January 11th 04, 10:00 PM
WaltBJ
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All rearward swept wings suffer a loss of lift to some degree because
of span-wise flow. Hence wing fences on some. Forward swept wings do
not, for obvious reasons. Forward swept wings do suffer a weight
penalty because the bending moments are self-generating - any twist
results in a force tending to increase that twist, thus they must be
considerably stronger than the alternative. IMHO aircraft designed for
lower G limits would profit efficiency-wise from forward sweep.
Walt BJ
  #14  
Old January 12th 04, 03:35 AM
Tarver Engineering
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"Alan Dicey" wrote in message
...
As far as I can recall, forward sweep confers the advantage that
spanwise flow is now inwards, and the wingtips (with associated control
surfaces) stall last instead of first, so control authority is retained
at higher angles of attack or "deeper into the stall". In the X-29 they
were combined with canards, a supercritical wing and aerodynamic
instabilty in a search for enhanced maneuverability. See here

http://www.dfrc.nasa.gov/Newsroom/Fa...-008-DFRC.html

for the NASA Dryden infosheet. I seem to remember that the advantages
gained did not warrant the construction costs/difficulties (aeroelastic
tailoring with composites in the wing structure, as I recall) and so the
technique was not carried forward into new fighter design. Perhaps Mary
Shafer may know more of the projects findings?


LOL


  #15  
Old January 12th 04, 07:52 AM
machf
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On Sun, 11 Jan 2004 09:57:12 -0800, "Tarver Engineering"
wrote:

Actually, once the notch filter was adjusted such that the wing did not
delaminate, there was no benifit to forward swept wings.


Uh... excuse me, but what's a "notch filter" in this context?

--
__________ ____---____ Marco Antonio Checa Funcke
\_________D /-/---_----' Santiago de Surco, Lima, Peru
_H__/_/ http://machf.tripod.com
'-_____|(

remove the "no_me_j." and "sons.of." parts before replying
  #16  
Old January 12th 04, 03:28 PM
Kevin Brooks
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"Ed Rasimus" wrote in message
...
On Sun, 11 Jan 2004 09:55:02 -0500, Air Force Jayhawk
wrote:

An aircraft is only unstable if the aerodynamic center is forward of
the center of gravity. If the wing root is sufficiently aft and the
AC stays aft of the CG, stability remains.

Why? Well it was tried with the X-29 but I never have read why no one
has pursued it since. The advantage was supposed to be that the
boundary layer (the thick air right next to the surface caused by
friction and very annoying) builds up as the air moves aftward along
the wing. With a FSW, the thickest part of the BL is at the root
rather than near the control surfaces, enhancing control while at high
angles of attack. There are other advantages but it's been a while so
I can't recall them off the top of my head.

I knew the USAF pilot on the X-29 project...he said it flew fine and
had no issues with it.


As I recall the X-29 project, one of the objectives was evaluation of
the instability as a means of gaining agility for future highly
maneuverable aircraft. The "urban legend" was that the aircraft
required minimum of triple redundant FBW augmentation as loss of the
augmentation would result in immediate excursions from stable flight
and structural failure within seconds. The ultimate in "JC maneuvers".

Always thought it made for an extremely ugly airplane.

Wasn't the basic structure from an F-16A?


Here is a photo; the F-5 ancestry is evident in this view:

http://www.dfrc.nasa.gov/Gallery/Pho...EC90-357-7.jpg

Brooks




Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
Smithsonian Institution Press
ISBN #1-58834-103-8



  #17  
Old January 12th 04, 04:04 PM
Tarver Engineering
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"machf" wrote in message
news
On Sun, 11 Jan 2004 09:57:12 -0800, "Tarver Engineering"
wrote:

Actually, once the notch filter was adjusted such that the wing did not
delaminate, there was no benifit to forward swept wings.


Uh... excuse me, but what's a "notch filter" in this context?


In the feedback control system of the X-29 was a z-transform type filter at
3.2 Hz. This was a simple third order filter in the original airframe.
Once the filter was altered to eliminate the flutter problem on the second
airframe, there was no manuverability advantage. The first airframe having
a delaminated wing by that time.


  #18  
Old January 13th 04, 02:29 PM
machf
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On Mon, 12 Jan 2004 08:04:07 -0800, "Tarver Engineering"
wrote:


"machf" wrote in message
news
On Sun, 11 Jan 2004 09:57:12 -0800, "Tarver Engineering"
wrote:

Actually, once the notch filter was adjusted such that the wing did not
delaminate, there was no benifit to forward swept wings.


Uh... excuse me, but what's a "notch filter" in this context?


In the feedback control system of the X-29 was a z-transform type filter at
3.2 Hz. This was a simple third order filter in the original airframe.
Once the filter was altered to eliminate the flutter problem on the second
airframe, there was no manuverability advantage. The first airframe having
a delaminated wing by that time.

Oh, I see... I forgot that the X-29 had to rely heavily on electronics, and
thought maybe it was some mechanical equivalent or something. Thanks for
clearing that up.
Ah, z-transforms... It's been quite a while, but I used to be pretty good at
that stuff.

--
__________ ____---____ Marco Antonio Checa Funcke
\_________D /-/---_----' Santiago de Surco, Lima, Peru
_H__/_/ http://machf.tripod.com
'-_____|(

remove the "no_me_j." and "sons.of." parts before replying
  #19  
Old January 14th 04, 02:06 AM
Tarver Engineering
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"machf" wrote in message
...
On Mon, 12 Jan 2004 08:04:07 -0800, "Tarver Engineering"
wrote:


"machf" wrote in message
news
On Sun, 11 Jan 2004 09:57:12 -0800, "Tarver Engineering"


wrote:

Actually, once the notch filter was adjusted such that the wing did

not
delaminate, there was no benifit to forward swept wings.

Uh... excuse me, but what's a "notch filter" in this context?


In the feedback control system of the X-29 was a z-transform type filter

at
3.2 Hz. This was a simple third order filter in the original airframe.
Once the filter was altered to eliminate the flutter problem on the

second
airframe, there was no manuverability advantage. The first airframe

having
a delaminated wing by that time.

Oh, I see... I forgot that the X-29 had to rely heavily on electronics,

and
thought maybe it was some mechanical equivalent or something. Thanks for
clearing that up.


It could have been done mechanically, as the Russians have proven.

Ah, z-transforms... It's been quite a while, but I used to be pretty good

at
that stuff.


One of the people that worked for me at NASA got tasked with commenting the
original FORTRAN and came to me for help understanding what Mr. Main had
written. He was completely dissatisfied with what a third order filter does
and was certain the software did nothing.


  #20  
Old January 14th 04, 03:27 AM
Tony
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"James Dandy" wrote in message
m...
Pardon my ignorance on all matters concerning modern aviation but just
why the hell would you want to sweep a wing forward?

..........manuaverability

Doesn't that make any aircraft unstable?

..........Yes

If so, why would any pilot feel safe in it?

..........Computers

Has anyone ever made one work?

..........Yes, several


 




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