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

On Thu, 02 Oct 2003 23:02:41 +0000, Dave Hyde wrote:

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.


Well, as you know, you can look at the wing tip during a dutch roll and
watch the path it makes against the horizon to estimate the phi to beta
ratio (ratio of lateral motion to directional motion for the non-flight
test folks). In a normal dutch roll you'll see the wing tip rising as it
moves forward, i.e. if the nose is moving left, the left wing will be
rising. So if you watch the left wing tip, you'll see it making a circle
or ellipse in a clockwise direction. If the lateral stablity if
negative, the left wing will make a circle or ellipse in the
counter-clockwise direction. If you are used to normal dutch rolls, it
feels very strange.

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

Thanks to everyone. I am working on a study that includes building
cheaply and safely.

The forward sweep came about because of two reasons, excellent
visibility and its not real common. I like to look down and fly low
and slow. The world doesn't need another strutted hi-wing amateur
design from me, that is for sure. I also considered a low wing, I own
a Fly Baby, but I do like looking at scenery. However the low wing
visibility in the pattern is very reassuring.

After doing some research on the web regarding the Cygnet, French Bleu
Citron, and the Andreasson Ba-7 the forward sweep really attracted me.
I want a single seater Canadian Ultralight with a heavy A65 in the
nose. The Cygnet would be nose heavy according to the designer from a
Sport Aviation article.

I will use the Cygnet plans as a reference and also study the Bleu
Citron, Tandem Airbike and such to understand a 500# empty 900# gross
airframe. The Cygnet uses geodesic construction for a rigid wing but
I want my fuel in the wings and not my lap or behind my head(these
forward sweep planes put the tank behind the cockpit head area).
Because of my size and the weight of the Continental with a metal prop
keeping the pilot as far rearward as possible will be important. A
35%C spar and some forward sweep 5 degreeswill work for me. By using
a tail with good authority and reasonable arm a forward range C of G
would be acceptable.