Why a Swept-Wing?

"Air Force Jayhawk" wrote in message
...
On 11 Jan 2004 05:39:04 -0800, (James Dandy)
wrote:

Pardon my ignorance on all matters concerning modern aviation but just
why the hell would you want to sweep a wing forward?

Doesn't that make any aircraft unstable? If so, why would any pilot
feel safe in it?

Has anyone ever made one work?

James Dandy

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.

USAF got shafted on the X-29 and would have never built the second airframe,
had they known about the flutter problem. NASA falsified the flight test
reports such that they indicated the wing flutter sensor was within limits,
when in fact, the data went full scale and drew a straight line.

What would a pilot know about a vehicle? You've been there Rosco, how could
they possibly know?

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?



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

"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

"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

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?



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

"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.

On Sun, 11 Jan 2004 18:20:34 GMT, Ed Rasimus
wrote:

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".

Well, it didn't have to have all three computers working, just one,
which could have been the fourth, back-up one. But that wasn't a
long-term sort of thing.

However, it didn't hang around for seconds before it pitched up.
stalled, and departed controlled flight. Time to double amplitude was
a small fraction of a second, although I can't remember the number.
It was smaller than that of the F-16, but the F-16 isn't very unstable
(it's neutrally stable clean and full of fuel and could be flown,
albeit rather oddly, without augmentation until enough fuel burned
off, not that anyone except VISTA would try this).

The X-29 was statically unstable because the project was a technology
demonstrator for agile aircraft with forward-swept wings, aircraft
that were stall-resistant. It wasn't statically unstable because it
had a forward-swept wing.

Always thought it made for an extremely ugly airplane.

I thought it wasn't all that bad looking, myself. The X-31 was rather
plain, but the X-29 was OK.

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

No, that was the X-31, I think, at least for the gear and cockpit.
The X-29 used a couple of F-5s for the fuselages. I don't remember
how far aft the F-5 airframe went, but it definitely included the
cockpit and surrounding structure, as well as the gear, as I recall.

Mary

--
Mary Shafer Retired aerospace research engineer

"Mary Shafer" wrote in message
...

snip
However, it didn't hang around for seconds before it pitched up.
stalled, and departed controlled flight. Time to double amplitude was
a small fraction of a second, although I can't remember the number.
It was smaller than that of the F-16, but the F-16 isn't very unstable
(it's neutrally stable clean and full of fuel and could be flown,

This is closer than Mary's claim that the F-16 is statically unstable, but
the F-16 continues to remain 5% pitch stable.

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

"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