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Author Topic: Moving wing centerline  (Read 5371 times)

Offline Will Moore

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Moving wing centerline
« on: March 20, 2018, 04:51:40 PM »
Igor’s Max Bee plans call for building wing with equal panels, assembling them together, building the fuselage over the wing but moving the wing I think 10 mm or so off center to make the inside wing larger. Is that the same, aerodynamically, as building the inboard wing bigger, or the outboard wing smaller, and centering the wing in the fuselage?
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Offline Tim Wescott

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Re: Moving wing centerline
« Reply #1 on: March 20, 2018, 04:58:39 PM »
Pretty much, yes.  But for some fractions of a mm difference here and there, you're effectively making the inside wing 20mm longer than the outside.
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Offline Geoff Goodworth

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Re: Moving wing centerline
« Reply #2 on: March 20, 2018, 07:45:10 PM »
By moving the finished wing centreline 10mm, you add 10mm to the inboard panel and remove the same 10mm from the outboard panel.

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Re: Moving wing centerline
« Reply #3 on: March 20, 2018, 10:55:52 PM »
Itll Be the SAME LENGTH , just 3/8 to the Left ! .  LL~ S?P :P .

Offline Will Moore

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Re: Moving wing centerline
« Reply #4 on: March 21, 2018, 12:03:46 PM »
Thanks for the comments. I understand the geometry ,  but what about the aerodynamics ?  Does the model feel the difference in flight, between the two techniques ?  Does one technique change the airfoil differently than the other ?  Are there advantages to Igor’s way of doing it ?   Does Igor know something we don’t? ( that’s a loaded question - can’t wait for the response to that question, LOL )
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Offline EddyR

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Re: Moving wing centerline
« Reply #5 on: March 21, 2018, 12:39:37 PM »
I think you asked this same question on Jan 22.
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Offline Igor Burger

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Re: Moving wing centerline
« Reply #6 on: March 21, 2018, 12:57:05 PM »
No, there is no magic  VD~

I had 2 main reasons:

1/ I use identical composite wing panels because they come from one mold. Milling aluminum cost lot of money so that is why it is made in one and therefore identical mold. However I did it also before as foam wing, it needed identical sheets (cut from one larger), identical templates for foam formatting etc.

2/ If I have centerline shifted, I can make holes for pushrod only in outer wing and hole will come in middle of fuselage.

disadvantage is that cut out for wing in inner side of fuselage is larger than that in outer side.

Offline Will Moore

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Re: Moving wing centerline
« Reply #7 on: March 21, 2018, 01:15:29 PM »
I think you asked this same question on Jan 22.

Hi Edd,

It does sound like the same question.  The previous post had more to do with whether to build equal rather than unequal panels.

The intent of this question is, if you build with equal panels, and then offset, as Igor does, is there an aerodynamic advantage, rather than just building inner wing slightly longer, or outer wing slightly shorter, and keeping the center of the wing joint in the middle of the fuselage.

Igor just commented and says he does it because both panels are cut from the same mold, so he moves the centerline over 10 mm to get the inner panel longer.






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Offline Ken Culbertson

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Re: Moving wing centerline
« Reply #8 on: March 21, 2018, 02:56:39 PM »
I am sure there is a difference but it will be so small that you most likely would not be able to measure it and it would be buried in the trim process anyway.  I never thought of using fuselage offset.  My building area used to be quite small and I didn't have room for a full jig so I built each wing separately then join them.  At least twice I used the same rib spacer on both halves so I had to cut off an inch on the outboard.  If only we had this forum in 1966!
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Offline Target

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Re: Moving wing centerline
« Reply #9 on: March 21, 2018, 03:42:28 PM »
No, there is no magic  VD~

I had 2 main reasons:

1/ I use identical composite wing panels because they come from one mold. Milling aluminum cost lot of money so that is why it is made in one and therefore identical mold.

That means you are very certain that the wing profile top and bottom are perfectly identical in your molds. Is there a way to check it?
We know for sure that the top of the right wing is identical to the bottom of the left wing then!

I always use one template when hot wiring foam wings with zero camber profiles for this reason. The hot wire has to be less accurate than the cnc cut aluminum mold, i would think!

Good info,  Igor, thank you.
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Offline Istvan Travnik

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Re: Moving wing centerline
« Reply #10 on: March 21, 2018, 05:09:22 PM »
Thanks for the comments. I understand the geometry ,  but what about the aerodynamics ?  Does the model feel the difference in flight, between the two techniques ?  Does one technique change the airfoil differently than the other ?  Are there advantages to Igor’s way of doing it ?   Does Igor know something we don’t? ( that’s a loaded question - can’t wait for the response to that question, LOL )

What about the aerodynamics? Good question!
Aerodynamics says a little bit differently to all all we think here, and all our precedessors made their models.
It is well known, the aerodynamical forces are proportional to airspeed up to the square. As the inside wingtip runs approx. 6% less speed, that means one elementary "step" of wing surface near inside wingtip makes .94^2 = 88.4% lifting force (and drag) of the outside.
Let us abandone the close area of wingtip, counting 5% speed difference, makes still .95^2 = 90.25%
It means, if we want to be really correct aerodynamically, the inside wingtip should be 10% longer chord & thicker airfoil than the outside! (This difference should be diminished not fully linearly, but hyperbolic way towards to the outside tip.)
I think our science is far from the theoretics, but sometimes it is good to know the truth... :)
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« Last Edit: March 21, 2018, 05:45:24 PM by Istvan Travnik »

Offline Igor Burger

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Re: Moving wing centerline
« Reply #11 on: March 22, 2018, 12:55:36 AM »
That means you are very certain that the wing profile top and bottom are perfectly identical in your molds. Is there a way to check it?
We know for sure that the top of the right wing is identical to the bottom of the left wing then!

I always use one template when hot wiring foam wings with zero camber profiles for this reason. The hot wire has to be less accurate than the cnc cut aluminum mold, i would think!

Good info,  Igor, thank you.

Good point, yes for sure I BUILD them "mirrorwise" so IF there is a shape mistake (but also template is CNCed) then it will be mirrored and it will fit nicely. And yes, alu mold which I use now is perfect, the precision is far better that what is necessary.

Offline EddyR

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Re: Moving wing centerline
« Reply #12 on: March 24, 2018, 06:50:31 AM »
 Many of the old construction articles from the 1950-70's say to build each half of the wing the same and slide it over 1/4"+. If you build a 1/2 rib shorter wing on the outside it looks odd as the tip is wider on that side. Some of the Palmer Thunder Bird plans say to move the wing over. Many will claim a great advantage to one method over the other .I think you can talk yourself into believing anything that is perceived to be better.
     NOTE  I am sure this will start a endless discussions as to the benefits of one way over the other.
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Offline phil c

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Re: Moving wing centerline
« Reply #13 on: March 25, 2018, 12:22:20 PM »
The air really doesn't care how you build the wing.  It only acts on the surface exposed.  So any method of using a wing with a particular airfoil and offsetting the panels will give the same result.
Istvan is correct.  The calculations as to what is going on are not a exactly a straight line since the drag and lift are affected by the velocity squared.  Wild Bill Netzeband's articles settled on calculating the effect based on the inboard and outboard tip speeds to calculate where the MAC(center of aerodynamic area) will be.  It works well enough for what we do.  Igor has refined the whole design process quite a bit to take other things like airfoil, airfoil, and drag effects into account.

But in practice, it boiled down to about a half an inch offset in the wing centerline one planes around 52-60in span.  Fine tuning depends on the plane, engine or motor, nose length that Igor wanted, etc.
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Offline Will Moore

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Re: Moving wing centerline
« Reply #14 on: March 26, 2018, 10:55:26 AM »
Appreciate all the comments.  In the Classic way of making the outer wing shorter, one would make the last rib bay spacing near the wingtip shorter, which is the narrower part of the wing chord section, since the leading edge rakes back a bit towards the trailing edge. Say we move it 1/2 inch, for the sake of discussion.  The other way, making equal panels , and sliding the assembled wing over, moving the centerline over a bit, say  1/4 inch, to be the equivalent 1/2 inch of movement, as in the Classic way, (disregarding the t-bird) exposes the larger area near the center of the wing, into the airstream. So, arguably (is that a word?)  I would suspect the airplane, aerodynamically would notice the difference. So how is it Igor was able to figure out moving the centerline over by 10 mm, making, in effect a 20mm adjustment, and that being just right. Is it that the adjustment isn’t as sensitive or criticial as we might think?
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Offline Will Moore

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Re: Moving wing centerline
« Reply #15 on: March 26, 2018, 12:20:29 PM »
Then again, wing tip is moving faster than center area, so maybe it’s a wash.....
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Offline Igor Burger

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Re: Moving wing centerline
« Reply #16 on: March 26, 2018, 12:37:53 PM »
Is it that the adjustment isn’t as sensitive or criticial as we might think?

No it is not sensitive ITSELF. Wing is the wing, it is only one area, no left or right and it need properly located idewise CG. That is the point. The fuselage and its mass makes some weight to move the CG to proper place. That is the point. Mine model for Perth had equivalent wings. However shifting fuselage left or right will need few moddifications:

1/ tip weight will be certainly different, less with longer inside wing (fuselage shifted right will add to the wing tip weight)

2/ trimming tab on outer flap will be of different size, probably smaller on longer inside wing (as the flaps centerline and thuse center of flap area will move )

3/ motor offset will be also larger on longer inside wing (to match prop thrust in relation to sidewise CG)

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Re: Moving wing centerline
« Reply #17 on: March 27, 2018, 06:56:05 PM »
Wondering WHY Tailplane dosnt seem to get the Same % offset. And if thats why extra outer flap chord on most U S late designs .  S?P

Offline Igor Burger

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Re: Moving wing centerline
« Reply #18 on: March 28, 2018, 12:29:46 AM »
Wondering WHY Tailplane dosnt seem to get the Same % offset. And if thats why extra outer flap chord on most U S late designs .  S?P

Because tail will need to be extended opposite side, so

1/ it will look ufly
2/ it is already done by shifting it with fuselage right,

 S?P

Offline Will Moore

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Re: Moving wing centerline
« Reply #19 on: March 29, 2018, 09:18:29 PM »

2/ trimming tab on outer flap will be of different size, probably smaller on longer inside wing

That opens up a whole bunch of other questions, like:

1.  Igor, how do you make the tab adjustable?

2.  How do you know how big it should be?

3.  Is placing it on the outer flap have the effect of making that flap bigger as Matt Spencer mentioned?

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Offline Igor Burger

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Re: Moving wing centerline
« Reply #20 on: March 30, 2018, 05:07:33 AM »
That tab is simply another trimming tool. Outer flap needs to be larger to make equal tip weight amount in soft round loops and hard corners. Equal flaps usually lead to too liitle tipweight in rounds and or much in corners. Larger outboard flap can solve that problem, so that tab size can adjust proper difference. That also tells how large it has to be. I have several sizes and I simply test which is the best. It is not very sensitive, so when I have it, I glue it on flap permanently and trimming finished, I do not touch it anymore. 

Offline Target

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Re: Moving wing centerline
« Reply #21 on: March 30, 2018, 07:44:29 AM »
A picture of the tab glued onto the flap would be great, if possible.
Regards,
Chris
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Offline Russell Bond

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Re: Moving wing centerline
« Reply #22 on: March 30, 2018, 03:38:35 PM »
Here you go...................
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Offline Will Moore

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Re: Moving wing centerline
« Reply #23 on: March 30, 2018, 05:10:31 PM »
That tab is simply another trimming tool. Outer flap needs to be larger to make equal tip weight amount in soft round loops and hard corners. Equal flaps usually lead to too liitle tipweight in rounds and or much in corners. Larger outboard flap can solve that problem, so that tab size can adjust proper difference. That also tells how large it has to be. I have several sizes and I simply test which is the best. It is not very sensitive, so when I have it, I glue it on flap permanently and trimming finished, I do not touch it anymore.

WOW. So, when I saw the photo of your Max Bee months ago with that tab , I thought it was a movable trim tab to get airplane to fly level, in case it didn’t. But it’s a trim tab to increase area of outside flap to compensate for the flight problem you describe above. Because you move the wing off center by 10 mm, the outside flap is already smaller, right? So, do you build the outside flap with same dimensions as inside flap, except for it being shorter, and then experiment with trim tab size ? Or do you build outside flap a little wider to try to bring its area equal to inside flap, then add the tab ?  Does that even make a difference which way you do it ?
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Offline Igor Burger

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Re: Moving wing centerline
« Reply #24 on: March 31, 2018, 01:20:00 AM »
Or do you build outside flap a little wider to try to bring its area equal to inside flap, then add the tab ?  Does that even make a difference which way you do it ?

Yes, that is the case, outer flap on plan is larger, but it could be even larger so with high probability, tab will need to be on the righ side (cold be also on left if the "problem" happened to be opposite - means outer flap is too large). And yes, too large tab can be even source of that problem if it is so large that it can cause stall on right side in really hard corner, optimal will be to have flap size optimaly extended without that flap.



I thought it was a movable trim tab to get airplane to fly level, in case it didn’t.

No, but it could be used in case that there is some warp on flaps. That warp can make model to be impossible to trim in positive/negative Gs. For example it is perfectly level normal and upright, also tension in positive and negative loops are equivalent, but it shows need of diffferent tip weght in positive and negative corners (unequal rolls in corners). Normally I would tweek flaps, but in that case rounds and level is again wrong. In that case little angled tab will help. However warped flaps are easy to replace and that is for sure better solution.

Because you move the wing off center by 10 mm, the outside flap is already smaller, right?

No, aerodynamically the left and right wing is defined by aerodynamical center of wing with respect to circular path - means its center of pressure and that is aproximately inch right of the geometrical cented of wing. So moving of fuselage right and left does not change length of wing, it only changes where is mass of the fuselage attached in relation to the CP. So if you move fuselage right (out of the circle), you make area of outer flap larger, not smaller - because longest chord of the flap is moved also right and thus outer flap is microscopically larger IN CHORD

Offline Will Moore

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Re: Moving wing centerline
« Reply #25 on: March 31, 2018, 11:32:58 AM »
Igor,

Thank you for those explanations , Igor.  And thank you for everyone’s input .

« Last Edit: April 02, 2018, 10:18:39 AM by Will Moore »
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Offline Ken Culbertson

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Re: Moving wing centerline
« Reply #26 on: May 25, 2018, 12:16:24 PM »
That tab is simply another trimming tool. Outer flap needs to be larger to make equal tip weight amount in soft round loops and hard corners. Equal flaps usually lead to too liitle tipweight in rounds and or much in corners. Larger outboard flap can solve that problem, so that tab size can adjust proper difference. That also tells how large it has to be. I have several sizes and I simply test which is the best. It is not very sensitive, so when I have it, I glue it on flap permanently and trimming finished, I do not touch it anymore.

I never thought that adding a trim tab to level the wings would also help the roll in corners.  Maybe because I wasn't looking for it until I got the wings level!  Thank you, Thank You, Thank You!  That tab sure beats dumping much needed tip weight!

Ken
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Offline Howard Rush

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Re: Moving wing centerline
« Reply #27 on: May 25, 2018, 07:06:06 PM »
I never thought that adding a trim tab to level the wings would also help the roll in corners.

It's not there to level the wings.  It's a straight extension of the flap.   A trim tab is not a good way to compensate for a warp.  From an earlier post:  A trim tab is a poor alternative to a straight wing.  Unlike some things that you can count on to add together predictably--weights on a spring, resistors in a circuit, for example--you can't count on a trim tab to balance a wing asymmetry.  That's because the amount that lift will vary with angle of attack will start to differ from one side to the other as angle of attack gets high enough to do loops.  You can counter a warp with a trim tab in level flight, but the airplane may roll in mild maneuvers and may stall one-wing-first in hard corners.

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Offline Ken Culbertson

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Re: Moving wing centerline
« Reply #28 on: May 26, 2018, 10:35:39 AM »
That tab is simply another trimming tool. Outer flap needs to be larger to make equal tip weight amount in soft round loops and hard corners. Equal flaps usually lead to too liitle tipweight in rounds and or much in corners. Larger outboard flap can solve that problem, so that tab size can adjust proper difference. That also tells how large it has to be. I have several sizes and I simply test which is the best. It is not very sensitive, so when I have it, I glue it on flap permanently and trimming finished, I do not touch it anymore.

Some feedback - Your suggestion cured the problem 100% y1  I never would have thought that a little thing like this tab could make such a difference.  It went from a roll so strong that I cold see the wingtips to Zero and I did not have to lower the tip weight!  #^  However, you took away my excuse for sloppy corners! n1
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Offline FLOYD CARTER

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Re: Moving wing centerline
« Reply #29 on: May 26, 2018, 09:26:51 PM »
Regarding unequal wing panels;  any time I see a plan calling for asymmetry in wing panels, I carefully fold up the plan and put it away.
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Offline L0U CRANE

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Re: Moving wing centerline
« Reply #30 on: May 28, 2018, 12:23:14 AM »
I'm glad to be back. I hope others will be, too.

Many years ago, when I had little opportunity to fly frequently - growing family and military reassignments - I tried to estimate how and why stunt models arrived at their level of performance.

I'd had some "Intro to..."  level courses involving basic aerodynamics the mid-1950's, at Brooklyn Tech, a great high school, which added to some "understanding" of the (usually wacko) theoretical 'explanations' of success in the modeling magazines. As other technical courses added awareness of structures, shapes and forms as well as loadings and design to meet them, I went a bit further.

I was humbled and proud that Igor mentioned my initial inadequate efforts, the main one of which was a BASIC program published in the 1980's on the now defunct CompuServe's Modelnet forum: STUNTIII, eventually. Some dynamics, some dimensional calculations, some loading estimates. Not rigorous, by any means. I found, nevertheless, that things I worked out helped me a lot on the infrequent occasions I could fly.

I'm not trying to claim any level of great insight. But a superficial awareness of the many factors involved intrigued me. Other things included?

Effects of our circular flight: the model is flown in a tethered circle. It meets what we usually mis-call Centrifugal Force. Fuel, a liquid, 'feels' a total force that results from the circling and from gravity. Three g out and 1 g down is a fair description for our level flight. Like the surface of water in a pail swung around, the result of those forces is a tilt of the free surface to be perpendicular to the resultant.

And on and on over several factors.

Lift across the wing's span? Inboard tip finishes one lap in the same time as does the outboard tip. Outboard flies a larger radius, so further in the same time. It goes faster, well, duh. Lift varies as velocity squared. The velocity of each increment from inboard tip to outboard tip increases with radius, so we could estimate what the difference at those two points would be.

But that ignores the incremental increases from one to the other. If we could find the point across the span where lift is equal for the inboard area and the outboard area, that would be where we should hang the weight of the fuselage with its engine, tail, etc. parts. If not, when we pull g's, those weights will be offset from the equal lift point, and will try to roll the fuselage. This is the basic argument for longer inboard panels!

So much else! Drag that comes with lift - Induced Drag - is small in level flight, and larger in rounds, much larger in corners. Like lift, that drag grows from inboard to outboard. If not centered, it is offset, and will try to yaw the model. So, 'centering' the lift effects is there again.

Power. Newton: F= MA. It takes more Force to Accelerate the same Mass from unmoving to a given speed. In level cruising fight, clean stunters have little drag. The Force to maintain speed is less that it was accelerating from standstill. When we pull g, drag increases: the model slows. Thrust load rises to try to maintain speed. (Modern engines and modes of operation make this less obvious, but the loads are still there.)

Torque? At cruise, not a whole lot. With sudden increase against effort to turn the prop? We get a greater torque reaction into the motor mounts, which could tend to roll the model.

On and on... So, I tried, with limited math and science, to estimate these and see which were significant and which were not. Guys like Igor, Brett and Howard can do the detail work, have done it and it shows in their results.
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Offline Brett Buck

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Re: Moving wing centerline
« Reply #31 on: May 28, 2018, 08:44:05 AM »
Regarding unequal wing panels;  any time I see a plan calling for asymmetry in wing panels, I carefully fold up the plan and put it away.

   Then you have missed the boat, quite comprehensively.

    Brett

Offline Ken Culbertson

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Re: Moving wing centerline
« Reply #32 on: May 28, 2018, 11:44:33 AM »
Regarding unequal wing panels;  any time I see a plan calling for asymmetry in wing panels, I carefully fold up the plan and put it away.

Then I guess you don't have many plans to choose from.  The purpose of unequal lengths is not to directly compensate for the speed differential, it is to place the mass of the fuselage over the center of lift which because of the speed difference will not be in the center of the wing.  There is no way you can do that with equal panels without adding unnecessary tip weight.  That is the layman's answer.  Maybe someone with a better engineering background can explain it better.

Ken
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