negative dihedral

I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

In article ,
Tina wrote:

I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

Don't know why, but don't those transports with negative dihedral also have
wings above the hull? So those aircraft have the CG below the wings.

--
Bob Noel
(goodness, please trim replies!!!)

Tina wrote:
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!


"Negative dihedral" may have more to do with keeping the landing gear
legs shorter. I know that's how they solved the problem they
encountered with the longer gear legs required on the Avro Arrow. On
the original mock-ups, when they first tried to retract the main gear
legs, they "crunched together" with each other at the point where they
entered the fuselage.

Dihedral (whether "positive" or "negative") does tend to *add*
stability, not take it away. Fighter jets and large airliners use "fly
by wire" because the stick forces required to move the control surfaces
may be too much for the average pilot (or might cause a good deal of
fatigue which, on longer flights, could be detrimental to the safe
operation of the aircraft). "Fly by wire" does little toward enhancing
the overall stability of an aircraft. "Stability" has more to do with
the overall design of the aircraft (and it's intended use).

Bob Noel wrote:
In article ,
Tina wrote:

I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

Don't know why, but don't those transports with negative dihedral also have
wings above the hull? So those aircraft have the CG below the wings.



And large transport aircraft are amongst the most aerodynamically stable
aircraft to fly.

Tina wrote:
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

No idea of why or how it stays upright.

From the pictures I've seen of it in the air, most of that negative
dihedral seems to go away while flying.


--
Jim Pennino

Remove .spam.sux to reply.

On Jun 3, 7:56 pm, Frank Olson
wrote:
Tina wrote:
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

"Negative dihedral" may have more to do with keeping the landing gear
legs shorter. I know that's how they solved the problem they
encountered with the longer gear legs required on the Avro Arrow. On
the original mock-ups, when they first tried to retract the main gear
legs, they "crunched together" with each other at the point where they
entered the fuselage.

Dihedral (whether "positive" or "negative") does tend to *add*
stability, not take it away. Fighter jets and large airliners use "fly
by wire" because the stick forces required to move the control surfaces
may be too much for the average pilot (or might cause a good deal of
fatigue which, on longer flights, could be detrimental to the safe
operation of the aircraft). "Fly by wire" does little toward enhancing
the overall stability of an aircraft. "Stability" has more to do with
the overall design of the aircraft (and it's intended use).

well what occurred to me is, with negative dihedral, as one wing lifts
it generates MORE lift, while the wing going down generates less. I
thought that would encourage the roll, but maybe the wing design
itself somehow takes that into account..

"Tina" wrote in message
...

well what occurred to me is, with negative dihedral, as one wing lifts
it generates MORE lift, while the wing going down generates less.


Why did that occur to you?

On Jun 4, 12:30*pm, Tina wrote:
On Jun 3, 7:56 pm, Frank Olson



wrote:
Tina wrote:
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization *for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid *maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

"Negative dihedral" may have more to do with keeping the landing gear
legs shorter. *I know that's how they solved the problem they
encountered with the longer gear legs required on the Avro Arrow. *On
the original mock-ups, when they first tried to retract the main gear
legs, they "crunched together" with each other at the point where they
entered the fuselage.

Dihedral (whether "positive" or "negative") does tend to *add*
stability, not take it away. *Fighter jets and large airliners use "fly
by wire" because the stick forces required to move the control surfaces
may be too much for the average pilot (or might cause a good deal of
fatigue which, on longer flights, could be detrimental to the safe
operation of the aircraft). *"Fly by wire" does little toward enhancing
the overall stability of an aircraft. *"Stability" has more to do with
the overall design of the aircraft (and it's intended use).

well what occurred to me is, with negative dihedral, as one wing lifts
it generates MORE lift, while the wing going down generates less. I
thought that would encourage the roll, but maybe the wing design
itself somehow takes that into account..

To pionts: The swept wing also add roll stability and for heavy low CG
airgraft with high swept wings the anhedral may be large to reduce
excessive stability.

Cheers

"Tina" wrote in message
...
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

Dihedral (or anhedral - negative dihedral) are both used to reposition the
aircraft's Center of Lift (CL) vs the aircraft's CG. This effectively
creates a pendulum, which wants to stabilize with the heavy part at the
lowest possible position. Like a pendulum, relatively speaking, the farther
above the CG the CL is, the more stable an aircraft will be. The whole
"dihedral increases roll stability" issue is based around this, not
increased or decreased roll due to lifting forces stemming from the attitude
of the airplane. The roll forces which create stability are due to this
pendulum effect.

If you want a stable aircraft in roll, add as much dihedral as possible to
raise the CL. If you want an unstable aircraft, do the opposite. The
anhedral on some on high wing transports is probably designed to give a
desired amount of stability. Some level of stability is great for a
transport, but you don't want to create an aircraft so stable it requires
oversized ailerons to generate the desired roll rate.

KB

On 2008-06-04, Kyle Boatright wrote:
If you want a stable aircraft in roll, add as much dihedral as possible to
raise the CL. If you want an unstable aircraft, do the opposite. The
anhedral on some on high wing transports is probably designed to give a
desired amount of stability. Some level of stability is great for a
transport, but you don't want to create an aircraft so stable it requires
oversized ailerons to generate the desired roll rate.

Don't forget, also, that the wing in flight may well be flexed upward by a
considerable amount compared to its position on the ground. Check out
inflight pictures of a B-52, and compare them to pictures of the same
aircraft on the ground, for a dramatic example. That anhedral you see on the
ground may well be a significant amount of dihedral at max gross in flight.
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
Fairmont, MN (FRM) (Yes, that's me!)
AMD Zodiac CH601XLi N55ZC (ordered 17 March, delivery 10 June)