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#11
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Ed Sullivan wrote:
On Sat, 11 Jun 2005 00:04:10 -0400, vincent p. norris wrote: Friend of mine said at lunch today that the upper wings of biplanes have no dihedral. Hementioned specificlaly the Stearman PT-17. I believe he was mistaken. I know the Sopwith Camel upper wing was flat but I think the PT-17 and most other biplanes have dihedral in both wings. Any expert comments? Thanks. vince norris Some biplanes have sweptback wings which provide the same effect as dihedral. My parasol Jungster II has sweepback and no dihedral. Ed Sullivan Why is that Ed?...dihedral provides lateral stability by making the 'downgoing wing' increase it's lift while the 'upgoing' wing's lift decreases, so how does the 'sweep-back' provide lateral stability? -- -Gord. "I'm trying to get as old as I can, and it must be working 'cause I'm the oldest now that I've ever been" |
#12
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On Thu, 16 Jun 2005 02:09:30 GMT, Gord Beaman
wrote: Ed Sullivan wrote: On Sat, 11 Jun 2005 00:04:10 -0400, vincent p. norris wrote: Friend of mine said at lunch today that the upper wings of biplanes have no dihedral. Hementioned specificlaly the Stearman PT-17. I believe he was mistaken. I know the Sopwith Camel upper wing was flat but I think the PT-17 and most other biplanes have dihedral in both wings. Any expert comments? Thanks. vince norris Some biplanes have sweptback wings which provide the same effect as dihedral. My parasol Jungster II has sweepback and no dihedral. Ed Sullivan Why is that Ed?...dihedral provides lateral stability by making the 'downgoing wing' increase it's lift while the 'upgoing' wing's lift decreases, so how does the 'sweep-back' provide lateral stability? The reference escapes me, but I have read that so many degrees of sweep back is the equivalent of an estabilished amount of dihedral. All I know for sure is that this little sucker only has a 21' span and it is as steady as a rock. It will go for miles without touching the stick. |
#13
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Ed Sullivan wrote:
On Thu, 16 Jun 2005 02:09:30 GMT, Gord Beaman wrote: Ed Sullivan wrote: On Sat, 11 Jun 2005 00:04:10 -0400, vincent p. norris wrote: Friend of mine said at lunch today that the upper wings of biplanes have no dihedral. Hementioned specificlaly the Stearman PT-17. I believe he was mistaken. I know the Sopwith Camel upper wing was flat but I think the PT-17 and most other biplanes have dihedral in both wings. Any expert comments? Thanks. vince norris Some biplanes have sweptback wings which provide the same effect as dihedral. My parasol Jungster II has sweepback and no dihedral. Ed Sullivan Why is that Ed?...dihedral provides lateral stability by making the 'downgoing wing' increase it's lift while the 'upgoing' wing's lift decreases, so how does the 'sweep-back' provide lateral stability? The reference escapes me, but I have read that so many degrees of sweep back is the equivalent of an estabilished amount of dihedral. All I know for sure is that this little sucker only has a 21' span and it is as steady as a rock. It will go for miles without touching the stick. Ok, thanks...I know that sweepback will increase fore and aft stability by making the critical fore and aft CG limits 'farther apart' but I sure can't see how it affects lateral stability...anyhoo...if you find out could you tip me off? Thanks Ed. -- -Gord. "I'm trying to get as old as I can, and it must be working 'cause I'm the oldest now that I've ever been" |
#14
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On Fri, 17 Jun 2005 01:06:53 GMT, Gord Beaman
wrote: stick. Ok, thanks...I know that sweepback will increase fore and aft stability by making the critical fore and aft CG limits 'farther apart' but I sure can't see how it affects lateral stability...anyhoo...if you find out could you tip me off? Thanks Ed. This is not a complete explanation, but it might help. http://avstop.com/AC/FlightTraingHan...Stability.html |
#15
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On Fri, 17 Jun 2005 00:20:28 -0700, Ed Sullivan
wrote: On Fri, 17 Jun 2005 01:06:53 GMT, Gord Beaman wrote: stick. Ok, thanks...I know that sweepback will increase fore and aft stability by making the critical fore and aft CG limits 'farther apart' but I sure can't see how it affects lateral stability...anyhoo...if you find out could you tip me off? Thanks Ed. This is not a complete explanation, but it might help. http://avstop.com/AC/FlightTraingHan...Stability.html It does help - but in a disconfirming sense - let me quote: "The contribution of sweepback to dihedral effect is important because of the nature of the contribution. In a sideslip the wing into the wind is operating with an effective decrease in sweepback while the wing out of the wind is operating with an effective increase in sweepback. The reader will recall that the swept wing is responsive only to the wind component that is perpendicular to the wing's leading edge. Consequently, if the wing is operating at a positive lift coefficient, the wing into the wind has an increase in lift, and the wing out of the wind has a decrease in lift. In this manner the swept back wing would contribute a positive dihedral effect and the swept forward wing would contribute a negative dihedral effect. " This suggests that sweepback is directionally destabilizing..... (?) Brian Whatcott Altus, OK |
#16
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Brian Whatcott wrote:
On Fri, 17 Jun 2005 00:20:28 -0700, Ed Sullivan wrote: On Fri, 17 Jun 2005 01:06:53 GMT, Gord Beaman wrote: stick. Ok, thanks...I know that sweepback will increase fore and aft stability by making the critical fore and aft CG limits 'farther apart' but I sure can't see how it affects lateral stability...anyhoo...if you find out could you tip me off? Thanks Ed. This is not a complete explanation, but it might help. http://avstop.com/AC/FlightTraingHan...Stability.html It does help - but in a disconfirming sense - let me quote: "The contribution of sweepback to dihedral effect is important because of the nature of the contribution. In a sideslip the wing into the wind is operating with an effective decrease in sweepback while the wing out of the wind is operating with an effective increase in sweepback. The reader will recall that the swept wing is responsive only to the wind component that is perpendicular to the wing's leading edge. Consequently, if the wing is operating at a positive lift coefficient, the wing into the wind has an increase in lift, and the wing out of the wind has a decrease in lift. In this manner the swept back wing would contribute a positive dihedral effect and the swept forward wing would contribute a negative dihedral effect. " This suggests that sweepback is directionally destabilizing..... (?) Brian Whatcott Altus, OK Yes, I see your point Brian and on a slightly different tack, I always saw dihedral like this. A wing has max lift when it's 90 degrees to gravity, (or 'down') it has zero lift when pointed straight 'up'...now, when a gust knocks a wing (with dihedral) 'down' (towards level) then it's lift increases while the other wing's lift will decrease (as it goes upwards 'toward' the 'zero lift angle'. Hell, if you had 'enough' dihedral then you couldn't roll the a/c because of this powerful effect, the ailerons wouldn't have enough authority to overcome it. Taking things to a ridiculous extreme you could have say, 45 degrees of dihedral where it'd be possible to have a gust of wind take one wing 'down' to 90 degrees so that it has the max lift that it's capable of while the other wing is pointed straight up and has 'no' lift at all. I imagine that the correction then would be somewhat sudden. -Gord. "You are completely focused on RPM as the single factor producing rotational velocity" -Dude Henrickles |
#17
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On Fri, 17 Jun 2005 20:07:30 GMT, Gord Beaman
wrote: This is not a complete explanation, but it might help. http://avstop.com/AC/FlightTraingHan...Stability.html It does help - but in a disconfirming sense - let me quote: "The contribution of sweepback to dihedral effect is important because of the nature of the contribution. In a sideslip the wing into the wind is operating with an effective decrease in sweepback while the wing out of the wind is operating with an effective increase in sweepback. The reader will recall that the swept wing is responsive only to the wind component that is perpendicular to the wing's leading edge. Consequently, if the wing is operating at a positive lift coefficient, the wing into the wind has an increase in lift, and the wing out of the wind has a decrease in lift. In this manner the swept back wing would contribute a positive dihedral effect and the swept forward wing would contribute a negative dihedral effect. " This suggests that sweepback is directionally destabilizing..... (?) Brian Whatcott Altus, OK Yes, I see your point Brian and on a slightly different tack, I always saw dihedral like this. A wing has max lift when it's 90 degrees to gravity, (or 'down') it has zero lift when pointed straight 'up'...now, when a gust knocks a wing (with dihedral) 'down' (towards level) then it's lift increases while the other wing's lift will decrease (as it goes upwards 'toward' the 'zero lift angle'./// I imagine that the correction then would be somewhat sudden. -Gord. I think that I must have read this thread once too many times...in that when I read what I posted, I see that the "Traing" text implies just the opposite of what I thought it did. Oh my: he says sweep back is like dihedral, but the argument seems to point the other way. Think I will shut up, now! Brian W |
#18
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I think that I must have read this thread once too many times...in that when I read what I posted, I see that the "Traing" text implies just the opposite of what I thought it did. Oh my: he says sweep back is like dihedral, but the argument seems to point the other way. Think I will shut up, now! Brian W I'm really sorry I started this. Perhaps my parasol also benefits from the pendulum effect. It does have a slight dutch roll tendency, but the controls are so light that this is overcome easily. According to what I have read my aircraft has far more sweepback ( 15 degrees ) than is healthy subsonic, but it doesn't seem to care. It does very reliable moderate aerobatics and yet is not a monster on cross country trips. Ed Sullivan, me too |
#19
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On Fri, 17 Jun 2005 15:45:41 -0700, Ed Sullivan
wrote: I think that I must have read this thread once too many times...in that when I read what I posted, I see that the "Traing" text implies just the opposite of what I thought it did. Oh my: he says sweep back is like dihedral, but the argument seems to point the other way. Think I will shut up, now! Brian W I'm really sorry I started this. Perhaps my parasol also benefits from the pendulum effect. It does have a slight dutch roll tendency, but the controls are so light that this is overcome easily. According to what I have read my aircraft has far more sweepback ( 15 degrees ) than is healthy subsonic, but it doesn't seem to care. It does very reliable moderate aerobatics and yet is not a monster on cross country trips. Ed Sullivan, me too I take it back here is some more confusing info:http://www.av8n.com/how/htm/roll.html |
#20
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This suggests that sweepback is directionally destabilizing..... (?)
Aren't a/c with sweptback wings subject to dutch roll? vince norris |
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