Forward Swept Wings

BD5ER wrote:

And you might need a bit more than normal dihedral..............

Oooohhh, good point. Anybody ever flown
an airplane with 'unstable' dihedral effect?
i.e. Step on the left pedal and the airplane
rolls right (but still yaws left)? It's
a very strange feeling.

In both examples (structural and dihedral)
a little forward sweep probably isn't going to make
much difference, but the smart designer should
understand both of these factors (and the ones we've
forgotten) and make an informed decision to neglect
them or not.

Dave 'Cl,beta' Hyde

Dave Hyde wrote:

BD5ER wrote:

And you might need a bit more than normal dihedral..............

Oooohhh, good point. Anybody ever flown
an airplane with 'unstable' dihedral effect?
i.e. Step on the left pedal and the airplane
rolls right (but still yaws left)? It's
a very strange feeling.

In both examples (structural and dihedral)
a little forward sweep probably isn't going to make
much difference, but the smart designer should
understand both of these factors (and the ones we've
forgotten) and make an informed decision to neglect
them or not.

Dave 'Cl,beta' Hyde


I've read that 15 degrees of (typical rearward) sweep
equals about one degree of dihedral. Four degrees of
forward sweep would require just a smidgen more dihedral.

I agree with the above posters that the biggest bugaboo
with FSW is increasing washin with load factor. Composite
planes (like the X-29) orient the fibers to counteract
that effect. I had the privelege of walking around and
climbing into an X-29 while it was at Grumman, the wing
had a spectacular amount of washin built into it, but the
wing skins were designed so that the amount of washin
didn't change as the wings deflected upward. As I recall
rather than the axis of the carbon fiber threads pointing
parallel to the spar, they pointed more forward than that.

Still, for a few degrees of sweep and a reasonably
torsionally strong wing construction, I can't imagine you'll
have too much trouble.

One idea I had was kind of odd, but it might work.
Rig the ailerons so that as the wingtips deflect upward
the ailerons move up, decreasing the lift at the wingtips.
You could rig the ailerons to do this automatically if the
control rod traversed the wing from bottom to top as it
moved out the wing.

thad

Thaddeus Beier wrote:
Dave Hyde wrote:


I've read that 15 degrees of (typical rearward) sweep
equals about one degree of dihedral. Four degrees of
forward sweep would require just a smidgen more dihedral.

Give the Blanik gliders a good look. They have as much forward sweep as
I've seen on a plane. Aluminum construction. Aerobatic. Here's a page
that shows the wing forward sweep.

http://www.nwinternet.com/~blanikam/ba/prod06.htm

On Thu, 02 Oct 2003 02:03:42 +0000, Dave Hyde wrote:

BD5ER wrote:

And you might need a bit more than normal dihedral..............

Oooohhh, good point. Anybody ever flown an airplane with 'unstable'
dihedral effect? i.e. Step on the left pedal and the airplane rolls
right (but still yaws left)? It's a very strange feeling.


The Alpha Jet's dihedral effect goes unstable at high Mach - above about
M0.86 IIRC. I agree that the "wrong" roll due to yaw would get your
attention if you ever had cause to use the rudder. The dutch roll was
absolutely bizarre too, as the roll rate at each point in the cycle was
very different than a normal dutch roll.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

Kevin Horton wrote:

The dutch roll was absolutely bizarre too, as the roll rate
at each point in the cycle was very different than a normal
dutch roll.

I think I just sprained my wrist trying to figure that one
out :-)
I've seen it in the Calspan Lear and a couple of simulators,
most of them accurate but one with a sign error :-)
It's not something that jumps right out at you, but once
you notice it it annoys the snot out of you.

....stability augmentation is your friend.

Dave 'wris****ch kill' Hyde