negative dihedral

"Kyle Boatright" wrote in
:


"Tina" wrote in message
news:d9fb071f-4d30-45c3-916d-3e4c00f89d52@
34g2000hsh.googlegroups.com..
.
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.


All correct! Also, anhedral helps a bit with the high alt dutch roll
case. I'd have to go into a lengthy explanation to cover it all, but
suffice it to say that swept wing airplanes are subject to a roll yaw
reversal at high altitues that can get rather unpleasnt ( complet loss
of control) Most must fly with a yaw damper, which is a gyro-controlled
rudder input that keeps the thing going straight. Anhedral offers a non
mechanical ad to ammelierating this effect. In fact, the russians are
notoriously clever at this sort of thing, which is why thye have madly
maneuverable fighters with more or less conventional control systems
while contemporary western aircraft relied on computers to achieve the
same results.


Bertie


Bertie