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Posted: 10/10/2009 3:09:42 PM EST
I am an aerospace engineer myself but deal primarily with space stuff. Even so, I know a bit about aerodynamics and know other engineers that know more about aerodynamics. Yet there is one question that I have yet to find an answer to, so I thought I'd ask Arfcom.
Why do so many military planes have obliquely chopped wingtips / aerodynamic surfaces? I see it a lot on different military craft, including missiles, but I do not know why designers do it. I also have not seen any commercial aircraft with such surfaces so I am assuming that it has something to do with high maneuverability and control of the wingtip/aero surface vortex. Anyone know? As an example, the twin vertical stabilizers of the MiG-25: 
MiG-29: 
MiG:31 
Su-35: 
The horizontal stabilizers of the Su-35: 
Both the vertical and horizontal stabilizers of the Su-34: 
Both the horizontal and vertical stabilizers (but not the wingtips, for some reason) on the MiG-31: 
Both the main wings and horizontal stabilizers (but not the vertical stabilizers) of the F-15: 
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Posted: 10/10/2009 3:12:34 PM EST
[#1]
Maneuverability. "bite" into the air if you will. Smooth in a straight line, but as soon as angled, offering more 'dig' than 'git'
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Posted: 10/10/2009 3:13:28 PM EST
[#2]
Just talking out my rear end, but I'd imagine it has a lot to do with transsonic and supersonic flight. Particularly the shockwaves that build up as the aircraft approaches the speed of sound.
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Posted: 10/10/2009 3:22:11 PM EST
[#3]
Increased compatibility with treadmills.
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Posted: 10/10/2009 3:23:31 PM EST
[#4]
Quoted:
Maneuverability. "bite" into the air if you will. Smooth in a straight line, but as soon as angled, offering more 'dig' than 'git' I do not understand why, if this is the case. Aerodynamicists used to put "dog tooth" leading edges for such purposes (and I understand why - it sheds its own little vortex at high angles of attack to energize the boundry layer over the main wing, thereby preventing wing stall) but an obliquely chopped wingtip would not do that - it is on the tip. 
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Posted: 10/10/2009 3:33:16 PM EST
[#5]
Quoted:
Quoted:
Maneuverability. "bite" into the air if you will. Smooth in a straight line, but as soon as angled, offering more 'dig' than 'git' I do not understand why, if this is the case. Aerodynamicists used to put "dog tooth" leading edges for such purposes (and I understand why - it sheds its own little vortex at high angles of attack to energize the boundry layer over the main wing, thereby preventing wing stall) but an obliquely chopped wingtip would not do that - it is on the tip. Can you locate any wind-tunnel tests of both (straight & oblique) surfaces? I don't have an answer but I'd be interested to see how the vortices look on both and, as mentioned above, the flow when at any "angle of attack" (actually a yaw for the horizontal surface). ETA - and I think Merrell's point has merit. Without the oblique cut the side could become the leading edge with airflow at angle. Unsure how significant that is but... be interesting to see. |
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Posted: 10/10/2009 3:36:27 PM EST
[#6]
The largest reason is that wings like that are more efficient in terms of supersonic drag. The delta wing shape, however, contributes to the nose downward portion of a phenomenon called "mach tuck". Essentially, mach tuck is when the airflow over an airfoil (which is faster than that going under the airfoil) gets up to mach, causing a shockwave rooted on the upper wing surface, which causes boundary layer separation for the upper surface, in turn causing a loss of lift at the portion of the wing where this occurs. Trans/supersonic airflow is pretty complex and interesting stuff.
ETA: Sorry, misunderstood the question, didn't realize you were asking about the shape of the back portion of the tips. About that, I have no idea. I'll have to ponder it. |
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Posted: 10/10/2009 3:38:19 PM EST
[#7]
Funny how Mikoyan Guerovich(sp) has done this on every aircraft since the Mig 23; now we see it on Sukhoi.
Sorry, I have no experience on why, I've just notice that they've done that forever now. |
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Posted: 10/10/2009 3:44:15 PM EST
[#8]
Oh, one other thing that really doesn't have bearing on the question, but is kind of "gee wizz" info. The tips of the verts on the MiG-29... the dark grey/almost black part... are antennas. Lotta years ago, my unit was TDY to Malaysia to fly against their "new" (used, new to the Malays) 29s, and at the end of the trip we got to take a good close look at one of their new toys. I can't recall if both were labeled so, but one clearly was labeled "IFF antenna", and the other was also an antenna of some kind.
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Posted: 10/10/2009 3:45:44 PM EST
[#9]
Why do so many military planes have obliquely chopped wingtips
I'm no engineer but my guess would be a reduction in aerodynamic drag by reducing wingtip vortices. They probably work like tip sails or winglets on civilian aircraft. Just a guess. |
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Posted: 10/10/2009 3:58:08 PM EST
[#10]
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic
surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.  CJ |
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Posted: 10/10/2009 4:07:59 PM EST
[#11]
Quoted:
Why do so many military planes have obliquely chopped wingtips
I'm no engineer but my guess would be a reduction in aerodynamic drag by reducing wingtip vortices. They probably work like tip sails or winglets on civilian aircraft. Just a guess. This for the win. Reduced spanwise flow and corresponding vortex at tip. Posted Via AR15.Com Mobile |
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Posted: 10/10/2009 4:11:16 PM EST
[#12]
This for the win. Reduced spanwise flow and corresponding vortex at tip. Awesome, a learned answer, and more stuff for me to educate myself on. Noted for further study of mentioned characteristics. |
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Posted: 10/10/2009 4:16:36 PM EST
[#13]
Quoted:
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.
CJ The trailing surface length (I do not know how one defines "trailing area") is still the same as the sine component of the oblique edge still counts. As far as subsonic drag goes, the lift generated by the wing is directly proportional to the vortex strength. There is no way around it. The most efficient distribution of this drag results in an elliptical wing. This is why the Supermarine Spitfire had such a wing. 
Winglet tips are an attempt to move the axis of the vortex outward, allowing more of the wing to be used for lift but it does not reduce drag. In essence, it is like a long wing with the tips folded up. This allows airliners to have effectively longer wings without taking up more space between gates at airports. I understand that the actual performance improvement is still debatable when one considers the extra weight and surface drag of the winglet. |
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Posted: 10/10/2009 4:58:45 PM EST
[#14]
Quoted:
Quoted:
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.
CJ The trailing surface length (I do not know how one defines "trailing area") is still the same as the sine component of the oblique edge still counts. As far as subsonic drag goes, the lift generated by the wing is directly proportional to the vortex strength. There is no way around it. The most efficient distribution of this drag results in an elliptical wing. This is why the Supermarine Spitfire had such a wing. http://homepage.ntlworld.com/alemarinel/Spitfire/Spitfire_profile.jpg Winglet tips are an attempt to move the axis of the vortex outward, allowing more of the wing to be used for lift but it does not reduce drag. In essence, it is like a long wing with the tips folded up. This allows airliners to have effectively longer wings without taking up more space between gates at airports. I understand that the actual performance improvement is still debatable when one considers the extra weight and surface drag of the winglet. i always thought that winglets were intended to control spanwise flow and redirect the airflow chordwise, resulting in more efficient lift for a given airfoil. |
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Posted: 10/10/2009 5:01:10 PM EST
[#15]
Quoted:
This for the win. Reduced spanwise flow and corresponding vortex at tip. Awesome, a learned answer, and more stuff for me to educate myself on. Noted for further study of mentioned characteristics. The F-15 prototype had squared wing tips, and no dogtooth on the stabilizer, so these detail at least at the time weren't easy to predict. |
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Posted: 10/10/2009 5:23:52 PM EST
[#16]
Quoted:
Quoted:
Quoted:
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.
CJ The trailing surface length (I do not know how one defines "trailing area") is still the same as the sine component of the oblique edge still counts. As far as subsonic drag goes, the lift generated by the wing is directly proportional to the vortex strength. There is no way around it. The most efficient distribution of this drag results in an elliptical wing. This is why the Supermarine Spitfire had such a wing. http://homepage.ntlworld.com/alemarinel/Spitfire/Spitfire_profile.jpg Winglet tips are an attempt to move the axis of the vortex outward, allowing more of the wing to be used for lift but it does not reduce drag. In essence, it is like a long wing with the tips folded up. This allows airliners to have effectively longer wings without taking up more space between gates at airports. I understand that the actual performance improvement is still debatable when one considers the extra weight and surface drag of the winglet. i always thought that winglets were intended to control spanwise flow and redirect the airflow chordwise, resulting in more efficient lift for a given airfoil. Basically correct. They are an attempt to counter the span-wise flow of the vortex from off of the bottom of the wing up over to the top. Here is a picture of the issue. Note that the distance between the centers of the vortecies is less than that of the wing span. The winglets attempt to spread the vortex distance to be the wingspan. They do not reduce the strength of the vortex. 
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Posted: 10/10/2009 6:16:40 PM EST
[#17]
Quoted:
Quoted:
This for the win. Reduced spanwise flow and corresponding vortex at tip. Awesome, a learned answer, and more stuff for me to educate myself on. Noted for further study of mentioned characteristics. The F-15 prototype had squared wing tips, and no dogtooth on the stabilizer, so these detail at least at the time weren't easy to predict. I did not know this. I guess that means they needed to fix something and that this configuration was the solution. Thanks. |
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Posted: 10/11/2009 5:50:56 AM EST
[#18]
No definitive answers yet? I would have thought that the Hive Mind would have this one licked by now.
Thanks to those that did give answers, even if they are not definitive. |
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Posted: 10/11/2009 5:57:55 AM EST
[#19]
Quoted:
No definitive answers yet? I would have thought that the Hive Mind would have this one licked by now. Thanks to those that did give answers, even if they are not definitive. The answer I think is correct has been given, but I have a reservation about it. It makes sense on the wing and horizontal stabilizer as both are lift producing surfaces (therefore vortex producing) on a vert stab the airfoils are typically symmetrical as they do not generate lift or any resultant force vector until the rudder is used. That being said clipping the vert. stabs to reduce a vortex's effect on the airfoil in normal flight is pointless un It's still early.. |
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Posted: 10/11/2009 6:07:23 AM EST
[#20]
Cool. Threads like this make all the "cool story bro" posts and poop threads worth it.
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Posted: 10/11/2009 6:18:09 AM EST
[#21]
I can't answer about anything but the F-15. However, according to the book Eagles Engaged the wingtips were raked to:
"The production F-15 would feature raked wingtips to improve load distribution on the wing, reduce high-AoA buffet and improve transsonic performance. This raked wingtip had the added benefit of increasing specific excess power during supersonic, maximum power accelerations and maneuvers, and was therefore especially welcomed." Eagles Engaged by Steve Davies and Doug Dildy is a must have for the aviation buff. |
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Posted: 10/11/2009 6:20:02 AM EST
[#22]
Quoted:
I can't answer about anything but the F-15. However, according to the book Eagles Engaged the wingtips were raked to: "The production F-15 would feature raked wingtips to improve load distribution on the wing, reduce high-AoA buffet and improve transsonic performance. This raked wingtip had the added benefit of increasing specific excess power during supersonic, maximum power accelerations and maneuvers, and was therefore especially welcomed." Eagles Engaged by Steve Davies and Doug Dildy is a must have for the aviation buff. Yeah that part makes sense, same as the stabilator. But the clipped vert stabs on the sukhois I cant figure out |
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Posted: 10/11/2009 6:34:36 AM EST
[#23]
Supersonic air flow is very different than sub-sonic air flow.
Mach waves attach, and you need to be very careful to minimize them, instersecting other parts of the airframe. The problem is that you have to get to supersonic. Reducing span length flow is effective on any surface. The forces on the tips also have more leverage requiring a stronger structure, often at a weight penalty for a small increase in control area. |
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Posted: 10/11/2009 6:49:10 AM EST
[#24]
Quoted:
Quoted:
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.
CJ The trailing surface length (I do not know how one defines "trailing area") is still the same as the sine component of the oblique edge still counts. As far as subsonic drag goes, the lift generated by the wing is directly proportional to the vortex strength. There is no way around it. The most efficient distribution of this drag results in an elliptical wing. This is why the Supermarine Spitfire had such a wing. http://homepage.ntlworld.com/alemarinel/Spitfire/Spitfire_profile.jpg Winglet tips are an attempt to move the axis of the vortex outward, allowing more of the wing to be used for lift but it does not reduce drag. In essence, it is like a long wing with the tips folded up. This allows airliners to have effectively longer wings without taking up more space between gates at airports. I understand that the actual performance improvement is still debatable when one considers the extra weight and surface drag of the winglet. I am installing winglets on Falcon 2000's The improvement is 6% plus. The winglets add 200 lbs total weight to the aircraft and increase wingspan by 6'9", but we are really adding 18 feet of lifting surface. The vertical portion of the winglet is using what would be the vortex to help create more lift as the air rises upwords along the outboard surface instead of curling around to the upper surface. Watching the wind tunnel smoke trail is wild. The French engineers involved have told me that a 2000 wing with or without winglets have almost the same drag due to the way the vortex is moved to a tip that is 1/4 the size. I hope he wasn't just giving me a line of shit to have me stop asking questions. 
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Posted: 10/11/2009 6:52:10 AM EST
[#25]
Interesting that all the Soviet aircraft have this.
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Posted: 10/11/2009 6:54:49 AM EST
[#26]
It's a partial fix to reduce the intensity of buffet at high angles of attack and high speeds that generate localized transonic flow.
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Posted: 10/11/2009 7:56:55 AM EST
[#27]
Quoted:
I can't answer about anything but the F-15. However, according to the book Eagles Engaged the wingtips were raked to: "The production F-15 would feature raked wingtips to improve load distribution on the wing, reduce high-AoA buffet and improve transsonic performance. This raked wingtip had the added benefit of increasing specific excess power during supersonic, maximum power accelerations and maneuvers, and was therefore especially welcomed." Eagles Engaged by Steve Davies and Doug Dildy is a must have for the aviation buff. Quoted:
Supersonic air flow is very different than sub-sonic air flow. Mach waves attach, and you need to be very careful to minimize them, instersecting other parts of the airframe. The problem is that you have to get to supersonic. Reducing span length flow is effective on any surface. The forces on the tips also have more leverage requiring a stronger structure, often at a weight penalty for a small increase in control area. Quoted:
It's a partial fix to reduce the intensity of buffet at high angles of attack and high speeds that generate localized transonic flow. Ok, there seems to be a consensus here. I do not know why this shape helps reduce high-speed transonic buffeting, or why is increases specific excess power, but I will accept that it does. 
I still do not understand why the Russians use it on their vertical stabilizers, though. I assume these surfaces rarely see high angels of attack at transonic speeds, and I also do not understand why many of their planes have it on the vertical stabilizers and not their main wings. Go figure.
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Posted: 10/11/2009 7:59:54 AM EST
[#28]
Quoted:
Quoted:
Quoted:
Note, it's on the F-15 everywhere but on the vertical stabs. The F-16C has clipped tailplane corners, too. It's an integral design feature of the F-22 and F-35 aerodynamic surfaces as well. I could easily be wrong because I'm NOt an aerodynamic engineer, but the first thing that comes to mind is that the clipped section results in lower drag. Imagine a wind tunnel test on the surface in question, and you'll figure out that the less trailing surface area you have, the lower the induced drag will be. The tip of the structure induces a vortex, and if you can clip back what's behind the tip, that surface won't be in that vortex. If the surface is in the tip vortex, then this is a high drag scenario and it'll ALWAYS be subject to that high drag factor. Cut it away. Let the vortex slide away without hitting any structures. At least, that's how I'm analyzing it. But I'm not an aeronautical engineer, don't play one on TV, and haven't even had the pleasure of beating one up at a Holiday Inn Express.
CJ The trailing surface length (I do not know how one defines "trailing area") is still the same as the sine component of the oblique edge still counts. As far as subsonic drag goes, the lift generated by the wing is directly proportional to the vortex strength. There is no way around it. The most efficient distribution of this drag results in an elliptical wing. This is why the Supermarine Spitfire had such a wing. http://homepage.ntlworld.com/alemarinel/Spitfire/Spitfire_profile.jpg Winglet tips are an attempt to move the axis of the vortex outward, allowing more of the wing to be used for lift but it does not reduce drag. In essence, it is like a long wing with the tips folded up. This allows airliners to have effectively longer wings without taking up more space between gates at airports. I understand that the actual performance improvement is still debatable when one considers the extra weight and surface drag of the winglet. I am installing winglets on Falcon 2000's The improvement is 6% plus. The winglets add 200 lbs total weight to the aircraft and increase wingspan by 6'9", but we are really adding 18 feet of lifting surface. The vertical portion of the winglet is using what would be the vortex to help create more lift as the air rises upwords along the outboard surface instead of curling around to the upper surface. Watching the wind tunnel smoke trail is wild. The French engineers involved have told me that a 2000 wing with or without winglets have almost the same drag due to the way the vortex is moved to a tip that is 1/4 the size. I hope he wasn't just giving me a line of shit to have me stop asking questions. Thanks for the insight. The nearly equal drag for each type of wing matches the articles I have read. Do you know if the French engineers meant that the 6% improvement was lift or fuel efficiency? Thanks. |
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Posted: 10/11/2009 8:23:18 AM EST
[#29]
Wouldn't horizontal stabilizers function as air brakes? If you can turn tighter and slower than your opponent he will over-shoot you or will have to peel off.
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Posted: 10/11/2009 8:35:20 AM EST
[#30]
Quoted:
No definitive answers yet? I would have thought that the Hive Mind would have this one licked by now. Thanks to those that did give answers, even if they are not definitive. There chopped for super sonic flight and drag, with the regime of AoA we maneuver in (all), drag kills energy sustainability-resulting in larger turn circles and slower angle rate Chopped wing usually give better rate overall, while non chopped wing usually give better instantaneous performance, but suck over time. delta fighting wings,=good initial turns, high gs, suck at sustained maneuverability chopped wings, good initial performance, less drag over time=better sustainable It's a design consideration ETA If you chop a triangle off a wing you get rid of the drag of that entire surface area of the triangle, you do have to deal with the vorticies but the performance gained overall will be better |
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