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#11
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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 |
#12
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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 |
#13
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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 |
#14
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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 |
#15
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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 |
#16
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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 |
#17
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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. |
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