Bertie the Bunyip wrote:
Dudley Henriques wrote in
:

Bertie the Bunyip wrote:
Dudley Henriques wrote in
:

Paul kgyy wrote:
On a 3 hour cross country today I was amusing myself by flying with
rudder pedals only (all right, OK, a little yoke usage to maintain
altitude). But then I got to wondering why applying rudder
pressure causes the plane to bank. All I could think of was that
rudder usage produces asymmetric lift because one wing is somewhat
blanked by the sideways motion induced by the rudder? Also, the
rudder surface is above the plane's center of lift but I don't know
how much of a
factor
that is.

It's called a yaw/roll couple. As you create yaw you acellerate
the
outside wing which then has more lift. It raises coupling with roll
and
you have turn.

Gotta disagree there Dudley. While it is true, and that's what
happening to some degree initially, the majority of the yaw roll
couple in lightplanes comes from the dihedral. the wing opposite the
direction of yaw has a higher angle of attack and generates more lift
then the opposite, which now has a lower alpha. Airplanes with no
dihedral will still roll slightly in the direction of yaw but it's
nearly zilch. can prove the first statement for yourself by
introducing the yaw so slowly as to make the diffrence in speeds
insignificant. The airplane will still roll in the direction of the
yaw. The V1 cruise missile had no dihedral and no ailerons and was
easily upset for this reason. Once it was off a wings level flight
path it's gyros had no chance of getting it back into straight and
level. Swept wing airplanes can have a huge yaw roll couple because
as you yaw, the forward moving wing's aspect ratio becomes massive
just as the aft moving's wing shrinks.(transonic ones have a reverse
effect couple at altitude, but that's another story)


Bertie

Not so much disagreement really. What you are saying is correct. All
these things happen.


Yep, but I think the airspeed portion is a minor one.

It is minor, especially for a light GA type airplane. High performance
airplanes get into a much more complicated dynamic concerning coupling.


Technically however, the exact moment the yaw
induced higher angle of attack of the outside wing causes the excess
lift produced by the higher speed and alpha to introduce roll, a
couple has occurred and the aircraft is in an axis change from yaw
only to yaw/roll. It's a couple. Don't forget; there are complementary
couplings as well as adverse, and not all couplings result in
divergence or departure.


Yep, agreed. I stil think the speed element is insignificant in
practice. As a means of demonstration, the student can take the 172 or
whatever,and introduce some yaw smoothely and slowly whilst stopping any
roll with the ailerons.Then, leave the rudder in and nuetralise the
ailerons. With the yaw stabilised, i.e., no differnece in the speed
between the two wings, the roll will be almost as quick as if it was
introduced from co-ordinated S&L flight.

As soon as you neutralize the aileron in this condition you are in
effect removing the opposing force preventing the coupling from
occurring . The airplane should instantly react to this by entering
into the couple which is consistent with your comment.


It's really a matter of semantics and amplified explanation.
The dihedral actually stabilizes the airplane in roll and acts as you
have said.


Oh I know it's picking a nit and from a practical point of view is
nearly immaterial, but I'm a chronic nit-picker.

Don't think so. In fact, you're right on with this stuff.


A Cessna 195 would be one example of an airplane that will couple in
yaw without dihedral with no ill effect. A T38 however is an example
of an airplane that will couple in roll to departure if rolled at .9
mach with a full lateral stick throw.





OK, the 195 is a bad example because of it's parasol element and because
it's high wing and there are issues with blanketing and what not.

Well, I would agree with this in that there are blanketing effects,
actually in high wings generally.
One of the midwing giant model airplanes they're flying aerobatics in
nowadays would be a better example.

Could very well be.


Actually, are you sure the 195 is zero dihedral? Most high wing
airplanes that have zero dihedral look like they have anhedral. (Swick
T-cart, f'rinstance)
It's tapered as well. so even zero dihedral on top would still give some
below!

I've flown them, BTW Nice.

If I remember right, the 195 tapers all the way out and the wing's
thickness varies as well. I believe it is a 0 dihedral wing.


The T 38 example you're going to have to break down for me because:

a. I've not flown that class of airplane and
b. I'm full of whiskey.

Never drink and fly the T38. Pull off to the side of the taxiway and
drink, then go fly.

The 38 has a high fuselage loaded mass IYMP (inertia yaw moment
parameter) which translated into normal language :-) means that at high
rates of roll, the airplane can actually suffer a divergence on the roll
axis as a coupling takes place with yaw and a new rolling axis is formed.
This can easily be visualized when you realize that at a speed around .9
mach, the roll rate of the airplane actually doubles with the last third
of a lateral stick throw. It's possible to get very close to 720 degrees
/sec roll rate out of a clean Talon.
The airplane is actually restricted to less than a full stick throw at
..9 for this reason.
When a coupling occurrs under these conditions, the 38 will depart, as
the offset roll axis produced by the coupling is unstable due to the
high mass loading of the fuselage vs the wing area.
It is however, a source of much amusement to bounce helmets off the real
canopy in this airplane even with a restricted roll rate.
:-)




Bertie


--
Dudley Henriques