Determining Change in Chord from Wing Sweep and Span?
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I would have thought it'd be the leading edge, but the idea was if I could determine the span, and the chord, and I sweep the wing, I was curious if I could compute the new span with that data. That's probably why I figured trig would be necessary,
Yeah, you didn't exaaaaactly state it clearly.
I meant that you needed trailing edge sweep in addition to trailing edge.
So, you actually want to take a wing of known span and chord with no sweep, then sweep it. Aside from everything else, span is affected by what you do with the wing tips. Are you cutting them so that they're a chord, or are you leaving them as-is so that they look like the tips on the B-2?
You'll lose wing area as you sweep if you sweep around the point where the leading edge of the wing contacts the fuselage. Part of the wing ends up "inside the fuselage."
Assuming that you do it that way and you don't crop the tips, chord is just cos(sweep angle). It's handy to know that a 30 degree sweep in nearly 0.9 x starting span, 45 is about 0.7, and 60 degrees is exactly half the original chord.
I meant that you needed trailing edge sweep in addition to trailing edge.
So, you actually want to take a wing of known span and chord with no sweep, then sweep it. Aside from everything else, span is affected by what you do with the wing tips. Are you cutting them so that they're a chord, or are you leaving them as-is so that they look like the tips on the B-2?
You'll lose wing area as you sweep if you sweep around the point where the leading edge of the wing contacts the fuselage. Part of the wing ends up "inside the fuselage."
Assuming that you do it that way and you don't crop the tips, chord is just cos(sweep angle). It's handy to know that a 30 degree sweep in nearly 0.9 x starting span, 45 is about 0.7, and 60 degrees is exactly half the original chord.
I think you have to consider a lot of things like the centre of lift to avoid producing a lawn dart.
Yes, but it was done successfully with several early jets. F9F Panther into the Cougar, for example.
It was done with the 262 to adjust CoG.
Not to move it, but to maintain it with heavier engines. From wiki "The Me 262 is often referred to as a "swept wing" design as the production aircraft had a small, but significant leading edge sweep of 18.5° which likely provided an advantage by increasing the critical Mach number.[22] Sweep, uncommon at the time, was added after the initial design of the aircraft. The engines proved heavier than originally expected, and the sweep was added primarily to position the center of lift properly relative to the center of mass. (The original 35° sweep, proposed by Adolf Busemann, was not adopted.)[23] On 1 March 1940, instead of moving the wing backward on its mount, the outer wing was re-positioned slightly aft; the trailing edge of the midsection of the wing remained unswept.[24] Based on data from the AVA Göttingen and wind tunnel results, the inboard section's leading edge (between the nacelle and wing root) was later swept to the same angle as the outer panels, from the "V6" sixth prototype onward throughout volume production. "
Eh? In the link it says "position the center of lift properly relative to the center of mass. " The adjustments were to get the centre of mass and the centre of lift in the same place.
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GregP
Major
To be stable, COL has to be behind the COG. That way, the tail must "lift" downward to maintain stable speed. If you slow down, the tail loses lift and the nose drops to get back to stable speed. If you speed up, the tail gains lift downward and noses up to regain stable speed.
You can move CoG and CoL independently along the length of the aircraft. They weren't trying to shift the COG along the length of the aircraft to get the CoG and CoL positioned correctly relative to each other, they were trying to change the lift distribution to move the CoL. Sweeping the wings actually moves the CoG in the wrong direction.
Sweeping the leading edge more than the trailing edge actually moves the CoG backwards a little, assuming that you're not also moving the wing forward, etc., etc., etc.
As GregP astutely points out, Messerschmidt specifically did not want the CoG and CoL in the same place. "Same place" and "position[ed] ... properly" are not the exact same thing. You'll notice that that text you quoted states: "On 1 March 1940, instead of moving the wing backward on its mount, the outer wing was re-positioned slightly aft ..." If they were trying to move the CoG aft, this would have been a far better solution.
Yes of course, and also by changing the engines construction and materials you can change its centre of mass. Now do you start changing all you worked out about the engine and its aerodynamics or keep the same mounting and jiggle with how it sits in the airframe, which could mean sweeping the wings back a little, but no more than something like a DC-3. I presume what GregP pointed out was a relationship that was true for the original design and for the revision, the position of CoL and CoG would be relatively the same.
GregP
Major
As long as the COL is behind the COG, the aircraft is stable. The farther aft it is, the better the static margin. When the COL and COG are in the same location, you have an aircraft with no pitch stability and no static margin. That is, if you disturb it from trim speed, there is no tendency to automatically correct back to trim speed. If the COL is ahead of the COG, the aircraft is unstable. That is, if you disturb it from trim speed, it tends t diverge faster and faster unless you manually catch it. If you don't, it will swap ends and tumble. Think of an F-16 without the three computers or a P-39 with no ammunition up front. In WWII, without autopilots, it would have been a real handful in IMC ... sort of like flying a P-51 with the aft fuselage tank full. You can fly it if you pay strict attention, but it ain't any fun, and it is easy to get into trouble with only slight inattention.
special ed
2nd Lieutenant
- 5,662
- May 13, 2018
A brief side track into practical (model) CoL vs CoG. My lifelong friend in all things aviation, photos, models and full size flying became very interested in the Curtiss XP-55. He wanted to build a control line model and asked me for plans. I provided 1inch to the foot plans and he produced a model. When he tried to fly it, the results were a brief take off run followed by a flip onto it's back on the runway. After several tries, he destroyed the model with out allowing me to fix the problem. As close friends do, I told him I could have flown it, he said it was impossible. I told him I would build and fly a proof of concept model. From this I learned the all flying canard elevator/stab is not practical on models as the airflow will keep the canard "up", over powering the control from the handle and lines. It was modified to a forward fixed stab and hinged elevator. The next problem was balance. Although static balance based on using 1/4 of the mean chord, aerodynamically it needed nose weight, as the engine mount had been moved forward three times bringing the prop tips close to the wing T/E. See the wheel weight the front L/G strut. A takeoff was achieved at max power (thought needed because of the lead weight) and it flew fast and squirrilly and difficult to control as if tail heavy. When the engine quit, the glide
was completely under control and made a good landing. Conclusion: at speed, the center of lift moves to or just behind center of gravity, but at reduced speed CoL moves forward allowing control.
was completely under control and made a good landing. Conclusion: at speed, the center of lift moves to or just behind center of gravity, but at reduced speed CoL moves forward allowing control.
