Why a Swept-Wing?

[James Dandy]

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

[Simon Robbins]

"James Dandy" > wrote in message
m...
> Doesn't that make any aircraft unstable? If so, why would any pilot
> feel safe in it?

I imagine it would move the centre of lift forward of the centre of gravity,
which might help to increase lift at slow speed or high angle of attack. (I
think..)

Si

[Mike Marron]

> (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?

Depending on the type of aircraft, there are lots of good reasons
to sweep the wing forward as opposed to aft. I'm only familiar with
the reasons the Blanik L23 sailplane has forward swept wings and
that is to allow for more weight placed forward of the main wing spar.

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

The Blanik is stable in all 3 axes (e.g: pitch, roll, and yaw) and
aerobatics in it are a delight. It's an all-metal sailplane that's
been around for decades and countless pilots, including me,
feel perfectly safe in it.

>Has anyone ever made one work?

The Blanik is the USAF Academy's basic sailplane trainer of choice.

[C Knowles]

This is taken from Aerospace web
http://www.aerospaceweb.org/aircraft/fighter/s37/

"The advantages of forward sweep have long been known as such wings offer
lower wave drag, reduced bending moments, and delayed stall when compared to
more traditional wing shapes. Unfortunately, forward sweep also induces
twisting strong enough to rip the wings off an aircraft built of
conventional materials. To solve this problem, the Su-47 makes use of
composite materials carefully tailored to resist twisting while still
allowing the wing to bend for improved aerodynamic behavior. "

"However, Sukhoi has apparently decided to abandon the forward-swept wings
of the S-37, and the future production model will return to a more
conventional wing layout. If true, Sukhoi may have reached the same
conclusion as NASA did following testing of the X-29--the benefits of
forward-swept wings are just not worth the extra cost and complexity
associated with their design and manufacture."

Curt

"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?
>
> 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

[Air Force Jayhawk]

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.

Ross "Roscoe" Dillon
USAF Flight Tester
(B-2, F-16, F-15, F-5, T-37, T-38, C-5, QF-106)

[patrick mitchel]

The Hansa jet had forward sweep- the one pilot that had flown said type
stated no unkind words on the plane. Pat

[Tarver Engineering]

"C Knowles" > wrote in message
m...
> This is taken from Aerospace web
> http://www.aerospaceweb.org/aircraft/fighter/s37/
>
> "The advantages of forward sweep have long been known as such wings offer
> lower wave drag, reduced bending moments, and delayed stall when compared
to
> more traditional wing shapes. Unfortunately, forward sweep also induces
> twisting strong enough to rip the wings off an aircraft built of
> conventional materials. To solve this problem, the Su-47 makes use of
> composite materials carefully tailored to resist twisting while still
> allowing the wing to bend for improved aerodynamic behavior. "
>
> "However, Sukhoi has apparently decided to abandon the forward-swept wings
> of the S-37, and the future production model will return to a more
> conventional wing layout. If true, Sukhoi may have reached the same
> conclusion as NASA did following testing of the X-29--the benefits of
> forward-swept wings are just not worth the extra cost and complexity
> associated with their design and manufacture."

Actually, once the notch filter was adjusted such that the wing did not
delaminate, there was no benifit to forward swept wings. The program
falsified X-29 flight test data and USAF was quite punative in blackballing
the whole group. Perhaps Mary would like to speak to that issue, as she was
very close.

> "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?
> >
> > 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
>
>

[Tarver Engineering]

"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?

[Ed Rasimus]

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]

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

[Tarver Engineering]

"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.

[Kevin Brooks]

"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

[WaltBJ]

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

[Tarver Engineering]

"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/FactSheets/FS-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

[machf]

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

[Kevin Brooks]

"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/Photo/X-29/Small/EC90-357-7.jpg

Brooks

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

[Tarver Engineering]

"machf" > wrote in message
...
> 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.

[machf]

On Mon, 12 Jan 2004 08:04:07 -0800, "Tarver Engineering" >
wrote:

>
>"machf" > wrote in message
...
>> 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

[Tarver Engineering]

"machf" > wrote in message
...
> On Mon, 12 Jan 2004 08:04:07 -0800, "Tarver Engineering" >
> wrote:
>
> >
> >"machf" > wrote in message
> ...
> >> 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. :)

[Tony]

"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

[Mary Shafer]

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

[Tarver Engineering]

"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.

[JasiekS]

Uzytkownik "Alan Dicey" >
napisal w wiadomosci ...
> 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/FactSheets/FS-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?
>

I dug through my old notices on X-29A and X-31X and found these references:
[1] Bandyopadhyay G. - "Low-Speed Aerodynamic Characteristics of
Close-Coupled Canard Configuration at Incidence and Sideslip", Journal of
Aircraft, Vol. 28, No. 10, October 1991
[2] Er-El J. - "Effect of Wing/Canard Interference on the Loading of a Delta
Wing", Journal of Aircraft, Vol. 25, No. 1, January 1988
[3] Manoeuvring Aerodynamics, AGARD CP 497, Toulouse, France, May 1991
(especially papers)
[3.a] Ross Hannes - "X-31 Enhancement of Aerodynamics for Maneuvering beyond
Stall", Paper 2
[3.b] Kraus W. - "X-31, Discussion of Steady State and Rotary Derivatives",
Paper 13
[3.c] Ferretti A., Bartoli A., Salvatore A. - "Prediction of Aerodynamic
Phenomena Limiting Aircraft Manoeuvrability", Paper 5
[3.d] Visintini L., Pertile R., Mentasti A. - "Parametric Effects of some
Aircraft Components on High-Alpha Aerodynamic Characteristics", Paper 6

Close-Coupled Canard was my main area of interest these days so sweep
(-forward or -back) can be treated mariginally in these papers, but I hope
they can help you.

Regards
JasiekS
Warsaw, Poland

[Tarver Engineering]

"JasiekS" > wrote in message
...

> Close-Coupled Canard was my main area of interest these days so sweep
> (-forward or -back) can be treated mariginally in these papers, but I hope
> they can help you.

The major interesting fearure of the X-29 was the two poles in the right
half of the s-plane.

[W. D. Allen Sr.]

Two poles in the right half s plane made it inherently unstable, right?

Does that mean LOT, the Polish airlines, can have seats only on the lefthand
side of their airplanes.

WDA

end

"Tarver Engineering" > wrote in message
...
>
> "JasiekS" > wrote in message
> ...
>
> > Close-Coupled Canard was my main area of interest these days so sweep
> > (-forward or -back) can be treated mariginally in these papers, but I
hope
> > they can help you.
>
> The major interesting fearure of the X-29 was the two poles in the right
> half of the s-plane.
>
>

[Tarver Engineering]

"W. D. Allen Sr." > wrote in message
...
> Two poles in the right half s plane made it inherently unstable, right?

Pitch unstable, but the poles being complex in nature eliminates most folks
(including aero engineers) from the discussion.

> Does that mean LOT, the Polish airlines, can have seats only on the
lefthand
> side of their airplanes.

No.

[machf]

On Fri, 23 Jan 2004 10:34:23 -0800, "W. D. Allen Sr." >
wrote:

>Two poles in the right half s plane made it inherently unstable, right?
>
>Does that mean LOT, the Polish airlines, can have seats only on the lefthand
>side of their airplanes.
>
Engineering humor... LOL.

--
__________ ____---____ 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

[tonini]

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

Swept-wing spread - low speed and manouverability.
Unspread - higher speed and intercepting