Adverse Yaw

In the July issue of Soaring a letter writer contends that, during a
steep stationary turn, the outside wing creates less drag than the
inside and adverse yaw is caused by: 1)The inside wing flying at a
greater angle of attack than the outside wing, therefore creating more
drag on the inside wing (refers to lift vector diagrams) and 2)Down
aileron on the inside wing needed to create equal lift with the outside
wing while flying at a lower speed. He concludes, "Of course, the
incresed drag of the lower wing, caused by both 1) and 2) above, is the
source of adverse yaw." With all this drag on the inside wing why
wouldn't the glider yaw to the inside of the turn instead of the
outside? This is counter to everything I've learned. What am I (or is
he) missing here?

Mike Fadden

Forget it and just keep that string in the middle.

Stefan

It does, and this is not 'adverse'yaw. My ASW17, whch
has flaperons for the complete span, is a perfect example
of this. In a moderately banked turn, where I am using
a small amount of aileron to 'hold off' increasing
bank I need no rudder to maintain a balanced turn.
Gerhard Waibel was a very clever designer I think.
I do of course need rudder to conteract the prodigious
amount of adverse yaw when I apply aileron to enter
the turn, hence the fin and rudder the size of a small
flat (condominium).

At 14:00 11 July 2005, Mike wrote:
In the July issue of Soaring a letter writer contends
that, during a
steep stationary turn, the outside wing creates less
drag than the
inside and adverse yaw is caused by: 1)The inside wing
flying at a
greater angle of attack than the outside wing, therefore
creating more
drag on the inside wing (refers to lift vector diagrams)
and 2)Down
aileron on the inside wing needed to create equal lift
with the outside
wing while flying at a lower speed. He concludes,
'Of course, the
incresed drag of the lower wing, caused by both 1)
and 2) above, is the
source of adverse yaw.' With all this drag on the inside
wing why
wouldn't the glider yaw to the inside of the turn instead
of the
outside? This is counter to everything I've learned.
What am I (or is
he) missing here?

Mike Fadden

"Mike" wrote in message
ups.com...
In the July issue of Soaring a letter writer contends that, during a
steep stationary turn, the outside wing creates less drag than the
inside and adverse yaw is caused by: 1)The inside wing flying at a
greater angle of attack than the outside wing, therefore creating more
drag on the inside wing (refers to lift vector diagrams) and 2)Down
aileron on the inside wing needed to create equal lift with the outside
wing while flying at a lower speed. He concludes, "Of course, the
incresed drag of the lower wing, caused by both 1) and 2) above, is the
source of adverse yaw." With all this drag on the inside wing why
wouldn't the glider yaw to the inside of the turn instead of the
outside? This is counter to everything I've learned. What am I (or is
he) missing here?

Mike Fadden


The way I was shown adverse yaw was to turn the glider using just the
ailerons, this made the nose first turn in the oposite direction to the
stick motion, but as the glider banks the nose falls into the turn. the
advers yaw is the first bit when you bank the glider and the outer wing is
producing lift and the inner wing isn't. There is more drag at this moment
on the outer wing which yaws the nose out of the turn.

Peter.

Confusion of terms, maybe?

"Adverse yaw" is typically used to desribe the motion of the nose (if
left unchecked by rudder) when the stick is used to roll the aircraft.
Roll to the left, the nose swings to the right, the reuslt of
diffential lift/drag across the span resulting from deflected ailerons.

As for the lift/drag across the wing resulting from a turn, while it
will have some effect on yaw, I agree with your sense of this being the
opposite of what's taught and what's observed: stick against the turn
to prevent overbanking (for moderate to steep turns) and rudder into
the turn to trim the tail (which is far enough outside the circle
transcibed by the cg that it tends to put the aircraft in a slip).
Maybe the author is ascribing the effect of the vertical stab in a turn
to the lift/drag distribution? You see, if you slip in a shallow turn,
dihedral effect will roll you level, so you need stick into the turn.

I think someone suggested just keeping the string straight. Can't argue
with that.


Mike wrote:
In the July issue of Soaring a letter writer contends that, during a
steep stationary turn, the outside wing creates less drag than the
inside and adverse yaw is caused by: 1)The inside wing flying at a
greater angle of attack than the outside wing, therefore creating more
drag on the inside wing (refers to lift vector diagrams) and 2)Down
aileron on the inside wing needed to create equal lift with the outside
wing while flying at a lower speed. He concludes, "Of course, the
incresed drag of the lower wing, caused by both 1) and 2) above, is the
source of adverse yaw." With all this drag on the inside wing why
wouldn't the glider yaw to the inside of the turn instead of the
outside? This is counter to everything I've learned. What am I (or is
he) missing here?

Mike Fadden

BTW, what is a steep, stationary turn, I wonder?

At 14:00 11 July 2005, Mike wrote:
In the July issue of Soaring a letter writer contends
that, during a
steep stationary turn, the outside wing creates less
drag than the
inside and adverse yaw is caused by: 1)The inside wing
flying at a
greater angle of attack than the outside wing, therefore
creating more
drag on the inside wing (refers to lift vector diagrams)
and 2)Down
aileron on the inside wing needed to create equal lift
with the outside
wing while flying at a lower speed. He concludes,
'Of course, the
incresed drag of the lower wing, caused by both 1)
and 2) above, is the
source of adverse yaw.' With all this drag on the inside
wing why
wouldn't the glider yaw to the inside of the turn instead
of the
outside? This is counter to everything I've learned.
What am I (or is
he) missing here?


It's not a very clear explanation to me since 1) and
2) are interrelated.

I think of it as follows: For the saliplane to be in
a steady turn (constant bank angle), the total rolling
moments need to sum to zero. Most of these forces will
come from the two wings and therefore in a steady turn
both wings are producing roughly the same amount of
lift.

For the wings to produce equal lift in circling flight
the inner wing needs to produce a higher lift coefficient
than the outer wing because the outer wing is experiencing
a higher average velocity. This is why you need outside
aileron to counter overbanking. At high lift coefficients
and for tight circles (i.e. big difference in average
wing velocity) the induced drag on the inner wing goes
up more than the parasite drag on the outer wing does
and the nose will tend to yaw to the inside of the
turn.

For instance - on my sailplane flying at 48 knots in
a 45 degree bank the inner wing has roughly 8 percent
lower paraasite drag but 18 percent higher induced
drag -- and induced drag is 60% of total drag. (I think
I did the math right).

As I recall, adverse yaw is defined as the nose going
the other way and is the yaw associated with establishing
a roll rate, not a steady turn.

9B

Actually, if the string is in the middle, you are in a skid (unless the
string is attached at the CG of the glider, which usually makes it a
bit hard to see). Ball in the middle would be correct - but few US
gliders have slip balls (may be more common elsewhere?).

See Dick Johnson's explanation in a recent Soaring magazine article -
or Moffat in his first book.

A little slip helps reduce the aileron needed to counteract the
overbanking tendency, and often inproves climb rate. I know it does in
my LS6.

I doubt a little skid hurts much, and the difference is probably pretty
small - better just to bank steeper and slower!

Kirk