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Author Topic: Unequal semi span wings?  (Read 877 times)

Offline Charles Hofacker

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Unequal semi span wings?
« on: July 19, 2020, 12:26:43 PM »
It looks to me that many classic era planes had unequal semi span measurements with the outboard panel about 2 inches shorter than the inboard panel. Current design thinking makes both inboard and outboard panels the same. As I understand it the unequal lengths were because the outside wing was flying faster than the inside. Panel lengths were adjusted to equalize the lift across the entire span. When and why did this theory fall out of common practice?


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Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #1 on: July 19, 2020, 03:21:11 PM »
That's in interesting question.  My planes have a 5/8" longer inside wing.  Yaw and roll moments are the sums of a bunch of things, so an airplane can be trimmed with various wing span differences by changing other things, most notably wingtip weight.

I wonder how the effect of span difference changes with line length.  I'll try to cipher that. 
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Offline Dan McEntee

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Re: Unequal semi span wings?
« Reply #2 on: July 19, 2020, 04:02:57 PM »
  In the early days, many or most of the designs had longer inboard wings of varying lengths. The longest I know of is the All American Senior old time stunt legal design that has about two inches! This presents some problems with trimming and flying the airplane. As time progressed, the wing offset, as it became to be known, got shorter and shorter. You see a few designs that are equal span, and some have very subtle differences that you have to measure them to see them? I don't know that there is a "best" offset dimension, I just build what the designer put on the paper, or these days, on the CAD screen! There is a mathematical explanation, and Howard is the man for that. But the basic reason is like you suggest, the outboard wing does fly faster than the inboard wing and generates more lift, and you can see all sorts of little accouterments in addition to that such as different flap lengths, widths, and even Bob Palmer had differential flap movements in some of his designs! Most of us like to keep things simple and straight forward.
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #3 on: July 19, 2020, 07:22:11 PM »
Current design thinking makes both inboard and outboard panels the same. As I understand it the unequal lengths were because the outside wing was flying faster than the inside. Panel lengths were adjusted to equalize the lift across the entire span. When and why did this theory fall out of common practice?

   It didn't  and equal-span wings are the exception now. One thing that did change was that someone figured out that the required asymmetry (to get the lateral CP on the fuse centerline) is about 1/2"-1" rather than 2" or 3".

   You can get it to work about equally well with anything from around 1.5" to zero, as long as you are willing to adjust tabs, thrust vector offset,  and tipweight accordingly. It's not that critical.

    Brett

Offline Perry Rose

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Re: Unequal semi span wings?
« Reply #4 on: July 20, 2020, 07:06:53 AM »
Has the crab angle and the fuselage disrupting a bit of the airflow over the outboard wing been taken into consideration?
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Offline BillP

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Re: Unequal semi span wings?
« Reply #5 on: July 20, 2020, 07:22:12 AM »
I always thought the offset was for inboard line weight. The only plane I've noticed the offset on was the All American Sr. when taking off.
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Offline Tim Wescott

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Re: Unequal semi span wings?
« Reply #6 on: July 20, 2020, 09:47:40 AM »
... As time progressed, the wing offset, as it became to be known, got shorter and shorter. ...

At least in stunt.  (Photo: Flying Lines)

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Offline Ted Fancher

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Re: Unequal semi span wings?
« Reply #7 on: July 20, 2020, 12:59:53 PM »
I always thought the offset was for inboard line weight. The only plane I've noticed the offset on was the All American Sr. when taking off.

The AA Sr. was the first thing I thought about when I saw this thread.  It was a gross example of the genre and a good study of the subject you brought up; problems with the ship wanting to chase the pilot during the take off roll!

The tendency to come in at the pilot "is" peripherally involved in the longer wing issue but is more an issue of the CG location (here we go again) than the naked fact of the longer inboard wing...which does exacerbate the issue, however.

The AA Sr. can be made to eliminate the tendency to come in on take off and still maintain the stock span asymmetry although you're pretty much playing games with the trim to do so.

The airplane's tendency to attack the pilot on takeoff is a function of the lateral (wingtip to wingtip) location of the CG and the thrust of a conventionally mounted engine with zero to a couple of degrees of offset.  The "attack the pilot" event is the result of the thrust of the propeller at release is direct well to the "outside" of where the spanwise CG is located with an "appropriate" amount of tip weight for flight performrance (very little due to the wing offset).  The CG "IS" the airplane at release and with the thrust directed beyond (outside of) the CG the airplane rotates into the pilot and sometimes "chases" him/her...which can be embarassing and potentially hazardous!

The correct solution of the AA Sr. with "stock" offset wing panels is to offset the engine fairly dramatically so that the thrust at rest (and for the first bit of the takeoff roll) is at least directed "at" the CG or even inboard of it.  Another (albeit undesirable) solution is to add a lot of wingtip weight to bring the CG back toward the centerline of the ship thus eliminating the thrust vector on takeoff issue but, however, producing an inflight condition whereat the excess lift of the much longer inboard wing combined with the significant amount of  extra wingtip weight to allow safe takeoffs combine to make the ship roll away from the pilot during at positive or negative G maneuver states.

The whole "two inches off wing offset concept" of the late forties, fifties and much of the sixties fairly rapidly lost its cache in the seventies and beyond.

Ted Fancher

p.s.  This "issue" with CG location vice aerodynamic maneuvering for takeoff is a good example of why fore and aft CG locations have become much greater subjects for discussion,, experimentation and re-evaluation of stunt critter design in the last couple of decades.

Online Ken Culbertson

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Re: Unequal semi span wings?
« Reply #8 on: July 20, 2020, 02:25:48 PM »
The AA Sr. was the first thing I thought about when I saw this thread.  It was a gross example of the genre and a good study of the subject you brought up; problems with the ship wanting to chase the pilot during the take off roll!

The tendency to come in at the pilot "is" peripherally involved in the longer wing issue but is more an issue of the CG location (here we go again) than the naked fact of the longer inboard wing...which does exacerbate the issue, however.

The AA Sr. can be made to eliminate the tendency to come in on take off and still maintain the stock span asymmetry although you're pretty much playing games with the trim to do so.

The airplane's tendency to attack the pilot on takeoff is a function of the lateral (wingtip to wingtip) location of the CG and the thrust of a conventionally mounted engine with zero to a couple of degrees of offset.  The "attack the pilot" event is the result of the thrust of the propeller at release is direct well to the "outside" of where the spanwise CG is located with an "appropriate" amount of tip weight for flight performrance (very little due to the wing offset).  The CG "IS" the airplane at release and with the thrust directed beyond (outside of) the CG the airplane rotates into the pilot and sometimes "chases" him/her...which can be embarassing and potentially hazardous!

The correct solution of the AA Sr. with "stock" offset wing panels is to offset the engine fairly dramatically so that the thrust at rest (and for the first bit of the takeoff roll) is at least directed "at" the CG or even inboard of it.  Another (albeit undesirable) solution is to add a lot of wingtip weight to bring the CG back toward the centerline of the ship thus eliminating the thrust vector on takeoff issue but, however, producing an inflight condition whereat the excess lift of the much longer inboard wing combined with the significant amount of  extra wingtip weight to allow safe takeoffs combine to make the ship roll away from the pilot during at positive or negative G maneuver states.

The whole "two inches off wing offset concept" of the late forties, fifties and much of the sixties fairly rapidly lost its cache in the seventies and beyond.

Ted Fancher

p.s.  This "issue" with CG location vice aerodynamic maneuvering for takeoff is a good example of why fore and aft CG locations have become much greater subjects for discussion,, experimentation and re-evaluation of stunt critter design in the last couple of decades.
I flew the "cr**" out of one of these back in the late 60's.  My solution was to take off with a tail wind and to cock it out about 30 degrees.  I also bent my gear so that the outboard wing was nearly on the ground. Ground a few props doing that.  Mostly we hand launched though and we did it ala Combat style with the outboard wing down about 30 degrees.  I still got chased a few times.  If it made it 1/4 lap I was in for 5min of pure fun.

I feel sad for the younger generation who's first venturing into stunt is an LA46 on a Twister.  You could have been a National Champion with one of those in the 60's.  They missed all of the fun!

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Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #9 on: July 20, 2020, 03:18:57 PM »
Did most folks who built All American Sr's eliminate most of the wing offset?
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Offline Mike Alimov

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Re: Unequal semi span wings?
« Reply #10 on: July 20, 2020, 03:50:45 PM »
The Genesis article dedicated quite a bit of time to this issue. Bob wrote that he kept building different versions of it, until hinging was completely eliminated with equal span panels.
Gabe's Junior- winning design Sour Skittles uses a Crossfire wing with equal panels and flies very well with no ill side effects.
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #11 on: July 20, 2020, 07:11:21 PM »
I always thought the offset was for inboard line weight. The only plane I've noticed the offset on was the All American Sr. when taking off.

    No, it's for the lateral shift of the CP due to the velocity gradient. What the early pioneers like Pappy DeBolt thought about it was unclear, but *probably*, they were moving the fuselage over to the side to eliminate the need for tip weight, which it does, sort of. It would have been better if he had not moved the stab to the side, too, but the entire force arrangement is just screwy.

    Brett

Online Ken Culbertson

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Re: Unequal semi span wings?
« Reply #12 on: July 21, 2020, 11:52:16 AM »
Did most folks who built All American Sr's eliminate most of the wing offset?
Are you kidding me?  I was 18 when I first started trying to understand why things did what they did. At 16 we just flew.  Also had a Thunderbird back then with that crazy differential flap setup.  Man that extra roll to keep it out on the lines really was cool.

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Offline Serge_Krauss

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Re: Unequal semi span wings?
« Reply #13 on: July 22, 2020, 10:10:59 PM »
Martin Hepperle's site is a treasure trove of interesting concepts and information for models or full-scale aircraft. If you click on "Index" and go down to "C", you'll find a brief control-line section with this article in which he computes the span-wise "eccentricity" due to flying in a circle:

http://www.mh-aerotools.de/airfoils/control_line_aero_2.htm

(Edit: Here's the link to Martins site: http://www.mh-aerotools.de/airfoils/index.htm)

If you understand freshman calculus, you'll follow him easily as he computes opposing torques and arrives at the center of pressure. If not, and you want to get a "ball-park" figure for a plane of your preferred size, you can go to the end and find three equations where you can substitute measurements and come up with an offset for a rectangular wing of like size. If you understand the math, you can adjust for a tapered wing and re-compute. Anyway, you'll see that the computed amount for the "Hershey Bar" wind is comparable to what is often seen.

As most of my contributions, this is just a FWIW. Those offended by the language of science can kindly ignore this post.

SK

Online Brett Buck

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Re: Unequal semi span wings?
« Reply #14 on: July 23, 2020, 09:37:24 AM »
Martin Hepperle's site is a treasure trove of interesting concepts and information for models or full-scale aircraft. If you click on "Index" and go down to "C", you'll find a brief control-line section with this article in which he computes the span-wise "eccentricity" due to flying in a circle:

http://www.mh-aerotools.de/airfoils/control_line_aero_2.htm

(Edit: Here's the link to Martins site: http://www.mh-aerotools.de/airfoils/index.htm)

If you understand freshman calculus, you'll follow him easily as he computes opposing torques and arrives at the center of pressure. If not, and you want to get a "ball-park" figure for a plane of your preferred size, you can go to the end and find three equations where you can substitute measurements and come up with an offset for a rectangular wing of like size. If you understand the math, you can adjust for a tapered wing and re-compute. Anyway, you'll see that the computed amount for the "Hershey Bar" wind is comparable to what is often seen.

As most of my contributions, this is just a FWIW. Those offended by the language of science can kindly ignore this post.

SK

   At quick persusal, he calculated the correct lateral CP motion as 2-5%, then dismissed it as "negligible". 5% of 60" is *3"*, which is far from negligible.

   The primary and most obvious effect is that even small amounts of asymmetry greatly reduces the required tip weight. With 1.25" on a typical 40-60-sized airplane, it will be around 3/4 ounce, and with 0, it might be 3-3.5 ounces.

    I still contend that it works about equally well from 0 to about 1.5" on this sized airplane, *as long as you are willing to adjust the differential flap area to even out the roll at various load factors*. If you aren't going to adjust it, and just extend the inboard wing and flap proportionally, then, yes, it will tend to hinge, because the stab is still centered on the fuselage.

     Brett

Offline Mike Alimov

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Re: Unequal semi span wings?
« Reply #15 on: July 23, 2020, 10:27:23 AM »
   At quick persusal, he calculated the correct lateral CP motion as 2-5%, then dismissed it as "negligible". 5% of 60" is *3"*, which is far from negligible.

   The primary and most obvious effect is that even small amounts of asymmetry greatly reduces the required tip weight. With 1.25" on a typical 40-60-sized airplane, it will be around 3/4 ounce, and with 0, it might be 3-3.5 ounces.

    I still contend that it works about equally well from 0 to about 1.5" on this sized airplane, *as long as you are willing to adjust the differential flap area to even out the roll at various load factors*. If you aren't going to adjust it, and just extend the inboard wing and flap proportionally, then, yes, it will tend to hinge, because the stab is still centered on the fuselage.

     Brett

Empirical data (Sour Skittles/Crossfire , ~ 60" span, equal panels, outboard flap 1/2" longer) shows that symmetrical panels only require about 1 oz of tip weight.  Our Legacy 60 ARF, however, has 1" longer inboard wing AND flap, and hinges noticeably.  I want to try cutting the inboard flap to make it 1/2" shorter and see if it fixes it. 
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #16 on: July 23, 2020, 07:43:25 PM »
Empirical data (Sour Skittles/Crossfire , ~ 60" span, equal panels, outboard flap 1/2" longer) shows that symmetrical panels only require about 1 oz of tip weight.  Our Legacy 60 ARF, however, has 1" longer inboard wing AND flap, and hinges noticeably.  I want to try cutting the inboard flap to make it 1/2" shorter and see if it fixes it.

   I would suggest adding a tab to the outboard flap, then re-trimming, before cutting anything. And don't cut the span, cut the chord.

    Brett
« Last Edit: July 24, 2020, 08:19:08 AM by Brett Buck »

Offline Steve Fitton

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Re: Unequal semi span wings?
« Reply #17 on: July 24, 2020, 08:00:31 AM »
Infinity is equal span wings?
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #18 on: July 24, 2020, 08:15:34 AM »
Infinity is equal span wings?

  No - only a few modern (past 25 years) stunt planes have equal-span wings. If nothing else, you don't want to put 3 ish ounces of tip weight in it.

     Brett


   

Offline Mike Alimov

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Re: Unequal semi span wings?
« Reply #19 on: July 24, 2020, 09:07:55 AM »
   I would suggest adding a tab to the outboard flap, then re-trimming, before cutting anything. And don't cut the span, cut the chord.

    Brett

Just back from the field. Taped a 1/2" x 4" tab to the outboard flap, and voila! Hinging is gone.
Now, going forward, why is it better to have flap chord difference rather than flap span difference?
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Offline Ron Santia

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Re: Unequal semi span wings?
« Reply #20 on: July 24, 2020, 09:49:32 AM »
I have a question for all you guys that know way more than I do..

Would there be any advantage to build a flappless plane like a Skyray with a 1/4 inch or so longer inboard wing...

does an Primary force have equal length wings ?     Thanks,  Ron
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Offline Matt Colan

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Re: Unequal semi span wings?
« Reply #21 on: July 24, 2020, 10:26:27 AM »
  No - only a few modern (past 25 years) stunt planes have equal-span wings. If nothing else, you don't want to put 3 ish ounces of tip weight in it.

     Brett


 

I find this really interesting. I checked Dracula, and I have just  3/8” offset in the wing and I’m only running 3/8oz of tip weight plus maybe 3 grams of clay as well. I’m curious as to how this trimmed out as it did. When I fly it in the winter, I have to put some clay on the inboard wing to get rid of some minor hinging and wing throwing
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #22 on: July 24, 2020, 06:02:20 PM »
I find this really interesting. I checked Dracula, and I have just  3/8” offset in the wing and I’m only running 3/8oz of tip weight plus maybe 3 grams of clay as well. I’m curious as to how this trimmed out as it did. When I fly it in the winter, I have to put some clay on the inboard wing to get rid of some minor hinging and wing throwing

   Determined how? Counterbalance, or just how much you put in?    You might also have more weight in the outboard structure, which relieves the need for some tipweight.

    Brett

Offline Matt Colan

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Re: Unequal semi span wings?
« Reply #23 on: July 24, 2020, 07:36:13 PM »
   Determined how? Counterbalance, or just how much you put in?    You might also have more weight in the outboard structure, which relieves the need for some tipweight.

    Brett

That’s just what was in my tip weight box. I checked my notes and only made notes for the weight of my outboard wingtip. Both wing panels weighed the same and my outboard tip weighed 17 grams. I don’t have any notes on my inboard tip so I don’t have a truly accurate comparison.
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Offline Serge_Krauss

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Re: Unequal semi span wings?
« Reply #24 on: July 25, 2020, 06:37:13 AM »
   At quick persusal, he calculated the correct lateral CP motion as 2-5%, then dismissed it as "negligible". 5% of 60" is *3"*, which is far from negligible.

     Brett

I have to edit my response here, due to re-calculating the offset again, after several years. My P-Force is smaller than the planes you mention, as is my line length, but not extremely. My calculated value of offset is actually smaller than that 2% and smaller than the plane's actual offset of 13/16" (from my plans). I computed only .6" for my 51" span on 60' lines (l = 63.5'). I think that taper has to reduce the offset from what suits rectangular wings anyway, since the ac's are closer to the root.
« Last Edit: July 25, 2020, 07:18:56 AM by Serge_Krauss »

Online Brett Buck

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Re: Unequal semi span wings?
« Reply #25 on: July 25, 2020, 08:04:23 AM »
I have to edit my response here, due to re-calculating the offset again, after several years. My P-Force is smaller than the planes you mention, as is my line length, but not extremely. My calculated value of offset is actually smaller than that 2% and smaller than the plane's actual offset of 13/16" (from my plans). I computed only .6" for my 51" span on 60' lines (l = 63.5'). I think that taper has to reduce the offset from what suits rectangular wings anyway, since the ac's are closer to the root.


   Half an inch is about right for 35-sized airplanes, as Al himself somehow determined back in the early 70’s and generally consistent with Heperle (and everyone else who tried figuring it out). The real takeaway is that just getting it close is good enough, and can be trimmed out within normal tolerances - as long as you are willing to adjust the flap area differential.

    I note again that the same issue (velocity gradient) is also present for the tail, and generates non-trivial torque. You will end up doing something about that when trimming whether you know why or not. So I think scholarly calculations are very useful for explaining or giving insight to the problem, the actual numbers don’t really help that much. 3” is obviously too much, 0 is not the right answer, either, but obviously close enough that you can make it work.

 Brett

Offline Jim Svitko

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Re: Unequal semi span wings?
« Reply #26 on: July 25, 2020, 09:31:40 AM »
This thread brought back some memories about wing asymmetry.  I found my copy of the Dick Mathis Coyote article, published around '67 or so in Flying Models.

Mathis designed this plane with equal span inboard and outboard wing panels.  Also, no rudder offset, no engine offset.  After reading the article and liking the looks of the plane, I built one, according to plans.  It flew quite well, from what I can remember, even with my limited capabilities (both flying and building) at that time.  Today, if I tried to get a nice finish on it, it would probably be too heavy without flaps.  Mathis, being a free flight modeler as well as CL, was very good at building light.

Mathis' Excalibur designs (both the original and the Excalibur II) were typical of the day (i.e. longer inboard wing) so I wonder what made him go with equal span wings on the Coyote.  Did flapped vs. unflapped have something to do with it?

Offline Bob Hunt

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Re: Unequal semi span wings?
« Reply #27 on: July 25, 2020, 12:03:51 PM »
For years I bounced back and forth between equal panel wings and unequal panel types, where I used from 1/2 to 3/4 - inch difference depending on the span of the model. My verdict: Not too much difference, except that the equal panel models were easier to trim, but did indeed need a bit more tip weight. Not anywhere near the three ounces that was mentioned here in a post, but perhaps 1/4 to 1/2 ounce more. Now, I also weigh the sheeting that goes on my wings and I use the heavier sheets on the outboard wing panel, so that may be a factor.

Later - Bob Hunt
« Last Edit: July 26, 2020, 02:59:06 PM by Bob Hunt »

Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #28 on: July 26, 2020, 01:07:33 AM »
So I think scholarly calculations are very useful for explaining or giving insight to the problem, the actual numbers don’t really help that much.

Wise words. 

I've been doing some ciphering.  It will take awhile to write up and make pictures.  I may have miscalculated, but the actual numbers look weird.  To wit, for an airplane of approximately Impact dimensions flying a 70-ft. radius, asymmetry would be 1.86" to compensate for speed variation along the wing by asymmetry alone.  The effect of asymmetry also varies a lot with line length.   It takes about twice the asymmetry at 50 ft. radius to have the same effect as that at 70 ft. 
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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #29 on: July 26, 2020, 06:51:53 AM »
Wise words. 

I've been doing some ciphering.  It will take awhile to write up and make pictures.  I may have miscalculated, but the actual numbers look weird.  To wit, for an airplane of approximately Impact dimensions flying a 70-ft. radius, asymmetry would be 1.86" to compensate for speed variation along the wing by asymmetry alone.  The effect of asymmetry also varies a lot with line length.   It takes about twice the asymmetry at 50 ft. radius to have the same effect as that at 70 ft.

   That all makes sense, and is consistent with Heperle's calculation.  I am currently using 1.25" for my 58" span airplane, which is probably excessive, but again, close enough that you can trim it out.

     Brett

Offline BillP

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Re: Unequal semi span wings?
« Reply #30 on: July 27, 2020, 09:26:36 AM »
Didn't math calculations get us massive offsets in the first place...and trial and error found less better?   
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Offline Kees Roos

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Re: Unequal semi span wings?
« Reply #31 on: July 27, 2020, 11:18:49 AM »
Math analytical solution is a rather complicated intergral because one of the parameters is the planform shape. Did it once for fun for a barndoor wing, once was enough. Numerical solution is straightforward.
Recenty calculated the excentricity fot a wingspan 1.48m (58.25") on 20m(65.7') lines, tipchord/rootchord 0.83, elliptical tip with span 0.1 part of wingspan. Excentircity came out at 14.5 mm, which means the inboard panel would have to  be 29 mm (1.14") longer than outboard panel.

I think Howard did the constant chord calculation, which would result in a bigger effect. Smaller tipchord results in a smaller effect because more of the wing area is concentrated at the center section, where speed differences between inboard and ouboard locations are smaller.

Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #32 on: July 27, 2020, 12:17:33 PM »

I think Howard did the constant chord calculation, which would result in a bigger effect.

I did the calculation for a straight taper with tip chord / root chord = .7 using Schrenk approximation for the lift distribution, but I did the calculation wrong.  I’ll report further if I can find the error(s).
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Online Ken Culbertson

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Re: Unequal semi span wings?
« Reply #33 on: July 27, 2020, 12:40:51 PM »
I am curious how all of this affects the flap area differential.  I have always used 1/2" offset and I normally add 1/8" to the outboard flap chord.  This gives me a net of about 2sq" after deducting the 1" longer inboard flap area.  It seems to workout fine with no hinging and moderate tip weight.

So here is what has my brain twisted.  By adding area to the outboard flap we are also adding area to the outboard wing.  Wouldn't having zero offset and equal size flaps do the same thing?
I have had two ships like that over the years and I was not happy with them.  The 1/2" seems to be right.  I have also had the extreme inboard wing and liked that somewhat less than Zero. So If I understand all of this, and I probably don't, it appears that within a narrow range of about 1" offset it doesn't matter much where the fuselage is as long as you balance the flaps.  That means the only real benefit is the amount of tip weight, which = weight.

I don't need to know the formula if I understand the physics.  Help me with the physics.

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Online Brett Buck

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Re: Unequal semi span wings?
« Reply #34 on: July 27, 2020, 04:16:21 PM »
Didn't math calculations get us massive offsets in the first place...and trial and error found less better?

  No, not at all. If so, they surely missed it by a mile.

   As far as I can tell, they did it in order to use the fuselage and engine as tip weight - which is not entirely illogical. Note that this was about the time they *discovered* tip weight, before that, they used rudder and engine offset.

     There is a remarkable article on Dave Day's old site where the miracle of tip weight was introduced to the British modeling community.

    Brett


Offline BillP

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Re: Unequal semi span wings?
« Reply #35 on: July 28, 2020, 07:42:28 AM »
  No, not at all. If so, they surely missed it by a mile.

   As far as I can tell, they did it in order to use the fuselage and engine as tip weight - which is not entirely illogical. Note that this was about the time they *discovered* tip weight, before that, they used rudder and engine offset.

     There is a remarkable article on Dave Day's old site where the miracle of tip weight was introduced to the British modeling community.

    Brett

That's actually what I was talking about to counteract line weight in my first post. For the math part I assume some math wizard did calculations in the early days for the offset distance...but there are so many changing variables when a plane is flying I don't see math being the best answer. In the end it's still trial and error.
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Offline Kees Roos

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Re: Unequal semi span wings?
« Reply #36 on: July 28, 2020, 11:24:01 AM »
"That's actually what I was talking about to counteract line weight in my first post."

Aerodynamically counteracting line weight is fundamentally wrong. It will work in horizontal flight at shoulder height. In overhead flight, the direction of the line weight relative to the wing changes, while the extra lift of the inner wing panel does not. Outer tip weight does not have this disadvantage.

Kees

Online Brett Buck

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Re: Unequal semi span wings?
« Reply #37 on: July 28, 2020, 12:30:12 PM »
That's actually what I was talking about to counteract line weight in my first post. For the math part I assume some math wizard did calculations in the early days for the offset distance..

   I think you assume wrong. I doubt that Pappy DeBolt was a math wizard, or did any calculations of anything at all. Cut and try in the early wild days probably got the right answer in a matter of months. There's nothing wrong with that, they experimentally moved the fuselage over to induce outboard roll/use it as tipweight, the first attempt was clearly too much but acceptable for a beginner model, and they had more-or-less gotten it right in a year or two.

    Cut-and-try is a perfectly acceptable engineering approach if the cost and/or impact of getting it wrong is negligible. Good thing, because we are still doing it...

      Brett

Offline BillP

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Re: Unequal semi span wings?
« Reply #38 on: July 29, 2020, 02:09:05 PM »
   I think you assume wrong. I doubt that Pappy DeBolt was a math wizard, or did any calculations of anything at all. Cut and try in the early wild days probably got the right answer in a matter of months. There's nothing wrong with that, they experimentally moved the fuselage over to induce outboard roll/use it as tipweight, the first attempt was clearly too much but acceptable for a beginner model, and they had more-or-less gotten it right in a year or two.

    Cut-and-try is a perfectly acceptable engineering approach if the cost and/or impact of getting it wrong is negligible. Good thing, because we are still doing it...

      Brett

I have to think Pappy probably used some sort of math as a benchmark. Could have been in his head or on paper. Curtis Pitts designed/built the original Pitts bipe on the hanger floor without plans so it happens.
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Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #39 on: July 29, 2020, 05:13:56 PM »
I may have miscalculated, but the actual numbers look weird.  To wit, for an airplane of approximately Impact dimensions flying a 70-ft. radius, asymmetry would be 1.86" to compensate for speed variation along the wing by asymmetry alone.  The effect of asymmetry also varies a lot with line length.   It takes about twice the asymmetry at 50 ft. radius to have the same effect as that at 70 ft. 

I did miscalculate.  The airplane above would have an asymmetry of 1.3" to compensate for speed variation along the wing by asymmetry alone.  Asymmetry at 50 ft. is about 1.85 that at 70 ft.
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Offline Howard Rush

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Re: Unequal semi span wings?
« Reply #40 on: July 29, 2020, 05:25:04 PM »
Control line lateral-directional flight mechanics is (are?) pretty complicated.  Lots of effects are in play.  We use trial and error to pick a combination of values for wing asymmetry and other parameters that works.  Various combinations work, some better than others.  My Impact’s left wing is .624” longer than its right wing.  My ciphering here is to show how the effect of wing asymmetry by itself varies with line length and wing shape for simple trapezoidal wings.  All calculations assume a 60”-span wing of 700 square-inch area with no fuselage.

There are various ways of making asymmetric wings.  Which one you pick depends on whether you’re starting with a stack of ribs, a top and bottom wing mold, a pair of templates for cutting foam wings, or a stack of foam wings that are already cut.  For this analysis, I took a ratio of tip chord to root chord, a wing area of 700 square inches and span of 60 inches, then solved for root and tip chords.  I used the same root and tip chords for all wing length calculations for a given taper ratio.  To calculate asymmetry, I started with equal-length left and right wings, then iterated by lengthening the left wing and shortening the right wing the same amount until dCm/dα of the whole wing reached a target.  For most cases, the target was the dCm/dα of a wing with taper ratio .7 and span difference of .624”.  For fun, I also calculated asymmetry for a target of zero.  Zero dCm/dα is where the wing rolling moment is balanced by offset alone.  This is probably what folks calculated before the discovery of the miracle of tip weight. 

I’m assuming that the left wing is the one toward the inside of the circle.  Sign changes are left as an exercise to people who fly the other direction. 

I used the Schrenk approximation to calculate wing loading.  This looks kinda suspicious, but it seems to be pretty accurate for subsonic wings with little sweep and a reasonably high aspect ratio.  The Schrenk approximation assumes that the air load on one side of a wing is proportional to the average of its actual planform and a quarter ellipse with the same area (figure 1).  A further complication for control line airplanes—the reason for this exercise—is that airspeed varies along the wing, and the aerodynamic force at any point on the span is proportional to dynamic pressure (q), which is proportional to the square of the airspeed.  I weighted the Shrenk “chord” for right and left wings by r2/R2, where r is the distance of a point on the span from the center of the flight circle, and R is the distance of the wing root from the center of the flight circle.  Figure 2 shows the dynamic-pressure-weighted loading for a 60” total span, .7 taper-ratio wing of equal right and left lengths, with wing root 70 feet from the circle center. 

Figure 3 shows how asymmetry of the 60” total span, .7 taper-ratio wing varies with distance of the wing root from the circle center.  The red curve is for dCm/dα equivalent to my wing with .624” span difference at 70’ circle radius.  The blue curve is for rolling moment balanced by wing asymmetry alone.  The difference between the two is pretty constant at .704”. 

Figure 4 looks peculiar.  It says a constant-chord wing would need more offset than a tapered wing for a given dCm/dα.  That’s what I think Serge said, but opposite of what I think Kees said.  It’s not intuitively obvious to me, but I can’t find anything else wrong with my calculation. 

Figure 5 shows the effects of both taper ratio and wing-root circle radius on the amount of asymmetry equivalent to that of my .7-taper-ratio wing with .624” span difference at 70’ circle radius. 

Ken mentions flap effects, and thereby hangs a tale.  We typically separate wing loading distribution into two parts: “basic distribution”, which is the part due to twist and camber, and “additional distribution”, the part that varies with angle of attack (p. 10 of Abbott and Costello, if you’d like to sing along).  The stuff I did is for “additional distribution”.  Basic distribution is zero for unwarped stunt or combat planes without flap deflection.  Flap loading is generally part of basic distribution, but stunt flap deflection is related to angle of attack.  Tip weight required, for example, is a function of both.  To figure the effects of flaps, you’d need airfoil data vs. flap deflection all along the wing.  We can get this now from programs like Javafoil, and it would be interesting, but it’s beyond the scope of this short monograph. 
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Offline Kees Roos

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Re: Unequal semi span wings?
« Reply #41 on: July 30, 2020, 11:02:12 AM »
Figure 4 looks peculiar.  It says a constant-chord wing would need more offset than a tapered wing for a given dCm/dα.  That’s what I think Serge said, but opposite of what I think Kees said.  It’s not intuitively obvious to me, but I can’t find anything else wrong with my calculation. 

On the contrary, I fully agree with you. I said that for a wing with constant chord, the excentricity is bigger, because in that case relatively more of the surface area of the inner and outer panels is situated in the tip regions, which have the biggest relative speed diference. Actually I formulated it the other way around, in a tapered wing relatively more of the surface area is situated close to the center of the wing, where relative difference between inner and outer panels is smaller. Same conclusion of course.

The method you describe is actually identical to what I use. Only, because lift distrubution along wingspan is virtually indentical with the small difference between inner and outer span I ignore it. When I developed this method, I found a difference of some tenths of millimeters between the two algorthms, while the complexity of the calculation is a just abit less.

Offline Serge_Krauss

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Re: Unequal semi span wings?
« Reply #42 on: July 30, 2020, 11:05:20 AM »
Edit: I was writing while Kees' post appeared. Pretty much the same.

Wow, Howard, that's a wonderful study. I really like the last graph. Thanks for sharing.

"Figure 4 looks peculiar.  It says a constant-chord wing would need more offset than a tapered wing for a given dCm/dα.  That’s what I think Serge said, but opposite of what I think Kees said.  It’s not intuitively obvious to me, but I can’t find anything else wrong with my calculation."

I based my judgement just on the motion of the wings a.c. inboard with increased taper. For instance, for a taper ratio of zero for a pointed wing (triangular half-span part) you get the inboard extreme position (of the MGC) of only 1/3 of the half-span out. Of course that's for a simplified geometrical approach that assumes equally distributed lift - just finding the c.g. of a flat surface with the given wing shape - but tips have losses that bring tapered wings closer to elliptical in actual lift distribution. Otherwise, a severe taper  ratio (.37, I think) would be required to move the a.c. inboard to that of an elliptical wing.

It just seemed to me that with lift concentrated closer to the root, the lift vector would not have to move as far to generate the same moment difference as with a less tapered wing. I think that explains why the computation comes out large; our equations used here don't account for 3-D motion at the tips.


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