Equal or unequal wing panels

[Matt Piatkowski]

Hello,
Equal or unequal wing panels? That is the question...

Your comments will be appreciated,
Thank you,
Matt

P.S: propeller rotates CCW when looking at the model from the front. 

[Dane Martin]

Outboard wings just slow you down.

[Crist Rigotti]

Do a search on your subject.

[Serge_Krauss]

Martin Hepperle has a nice derivation of wing "eccentricity" hidden on his site here:

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

Eccentricity is the distance out from the center of the wingspan to where there is no rolling moment from the increasingly higher airspeed with distance from the handle. That means that he finds the point at which the outboard lift torque (moment) about fuselage center equals the inboard lift torque, and the plane doesn't tend to roll to the left, if pivoted there. That presumably (in a simpler world) is where you would place the fuselage. In his derivation, he assumes a rectangular wing, which is a wing that would have more tendency to roll than the usual tapered wing with smaller tip than root chord. In his final equations, "a" is the eccentricity to be computed, "b" is to wing span, and "l" is the distance from center circle to the fuselage center line. These are all that matter, since the other terms divide out. I have added a scan below of my old notebook page, where I have done the last (omitted) division to get eccentricity (a) in terms of span (b) and the fuselage flight radius (l =line length plus 'half'-span).

If you plug usual values into this equation, you get between .5" and 1.0". Since our wings (except for planes like 'Bellfry Bound' with bigger tips than roots) fall closer to .5" - .75" eccentricities, not much offset is indicated, and with the need for tip weight anyway, it really doesn't seem to matter much whether you make the inner wing larger than the outer (i.e. move the fuselage out on the wing). That's probably why both schemes seem to work OK for various designers. Just remember that an eccentricity of only a = .5" makes a difference of a full inch in wing panel lengths.

SK

[Tim Wescott]

Outboard wings just slow you down.

Yea, but how does it do in the hourglass?

[Dane Martin]

Yea, but how does it do in the hourglass?

Within acceptable parameters

[Tim Wescott]

Within acceptable parameters

 

[Dave_Trible]

I'll throw my two cents at it from a more practical standpoint as I see it.  I have used equal span wings for at least  30 years in all of my own designs and in fact the copies of my Music designs I built actually had the outboard panel 5/8" longer than inboard.  Serge is right on his post where he says both methods have been used and can work well.  My basic reasoning from way back is that I want a little more drag on the outboard wing panel for line tension throughout the sphere.  You'll get some of that simply with equal span wings because the outboard panel flies slightly faster and drag increases much faster than speed-like four times.  I've heard stories about this requiring 2-3 ounces of tip weight-no.  You aren't trying to counterbalance a longer, heavier inboard panel and the difference isn't all that.  I use around 1 1/4 ounce on a 60" wing flown on 66' eye to eye lines of .018 stranded.  That may be 1/4 ounce more- 3/8 in some cases.  There isn't a more powerful drag on the inboard side trying to yaw the nose inward at you.  This is most important and noticeable up high in vertical and overhead maneuvers. 

This is just my preference but a Magician with a 2 1/2" longer inboard panel can fly pretty well too.

Dave

[Brett Buck]

If you plug usual values into this equation, you get between .5" and 1.0". Since our wings (except for planes like 'Bellfry Bound' with bigger tips than roots) fall closer to .5" - .75" eccentricities, not much offset is indicated, and with the need for tip weight anyway, it really doesn't seem to matter much whether you make the inner wing larger than the outer (i.e. move the fuselage out on the wing). That's probably why both schemes seem to work OK for various designers. Just remember that an eccentricity of only a = .5" makes a difference of a full inch in wing panel lengths.

    You can make it work either way with the right tip weight and the right flap tabs, but you will need *much more* tip weight in general with equal-span wings, maybe 2.5 ounces more on a typical 60-sized airplane.

   The "moving the wing over" to act at tip weight analogy is the same mistake made on the All-American. The mistake is that you move the engine and the stab, too. You probably want an asymmetrical stab and some engine offset, too.

     As in Dave's case, you can make it work over a fairly large range, more-or-less well enough to make the effect negligible with respect to the other trim problems. The right answer is to make the asymmetry from about 3/4" to 1 1/4".

     Brett

[phil c]

I'll throw my two cents at it from a more practical standpoint as I see it.  I have used equal span wings for at least  30 years in all of my own designs and in fact the copies of my Music designs I built actually had the outboard panel 5/8" longer than inboard.  Serge is right on his post where he says both methods have been used and can work well.  My basic reasoning from way back is that I want a little more drag on the outboard wing panel for line tension throughout the sphere.  You'll get some of that simply with equal span wings because the outboard panel flies slightly faster and drag increases much faster than speed-like four times.  I've heard stories about this requiring 2-3 ounces of tip weight-no.  You aren't trying to counterbalance a longer, heavier inboard panel and the difference isn't all that.  I use around 1 1/4 ounce on a 60" wing flown on 66' eye to eye lines of .018 stranded.  That may be 1/4 ounce more- 3/8 in some cases.  There isn't a more powerful drag on the inboard side trying to yaw the nose inward at you.  This is most important and noticeable up high in vertical and overhead maneuvers. 

This is just my preference but a Magician with a 2 1/2" longer inboard panel can fly pretty well too.

Dave

I agree with you Dave.  I've tried this on lighter, faster planes.  Works even better on profiles since the motor usually provides all the tip weight needed.  Making sure the thrustline in on the centerline of the wing automatically offsets the MAC, just like a full bodied PA plane with equal panels.  The couple(offset) between the MAC and the thrustline make the plane turn outwards a bit, particularly when line tension goes down.  The extra outward turning force lets the leadouts to be set a bit further forward reducing squirrely behavior from the lines swinging around in maneuvers.

[Pat Chewning]

Even the designs that have equal left/right wings will end up with one longer than the other if you measure them with the correct amount of precision.....

[Serge_Krauss]

I agree, of course, that thrust line should always be set to avoid inward yawing. We don't let it pass outside the c.g.

[john e. holliday]

I flew Dave's Shameless design for a couple of years.  It had more out board wing than inboard.  Also the fuselage area was set up for windy conditions.  Did not pull so hard down wind or get to light up wind.  But, I had several judges tell me the reason for low scores was the plane didn't look right.  Dave has provided me with updated plans and I may build another one as it was a good flying plane.  I think he had a full fuselage version that I never got plans for.

[Brett Buck]

I agree, of course, that thrust line should always be set to avoid inward yawing. We don't let it pass outside the c.g.

   You would be quite surprised at how many people don't understand it, or actively disagree.

    Brett

[Brett Buck]

Even the designs that have equal left/right wings will end up with one longer than the other if you measure them with the correct amount of precision.....

   True, but it's a continuum, it's not like if you get .001 longer inboard wing, you suddenly need 3 ounces less tip weight. That's why you can more-or-less make it work well enough with anything from 2" to 0, with appropriate other adjustments.

     When talking about these sorts of design issues, always remember that errors in the trim, engine run,  and structural rigidity dominate the end result to a large degree. Most or nearly all airplanes are so far out of the ideal trim that there's absolutely no way to tell whether your killer design feature is having the effect you think or not. Even Dave's reverse asymmetry can be overcome, or at least swamped, by other issues.

     Brett

[Paul Walker]

     When talking about these sorts of design issues, always remember that errors in the trim, engine run,  and structural rigidity dominate the end result to a large degree.

     Brett
[/quote]



   You would be quite surprised at how many people don't understand it, or actively disagree.

[Randy Cuberly]

     When talking about these sorts of design issues, always remember that errors in the trim, engine run,  and structural rigidity dominate the end result to a large degree.

     Brett




   You would be quite surprised at how many people don't understand it, or actively disagree.

Well, after watching you make a relatively competitive airplane, by trimming, from what I considered to be a completely non-competitive design at VSC in March, I'm a believer.

I'm usually relatively good at trimming an airplane and have significant experience at trimming mine and other's but I admit that watching that one and you (I didn't have the opportunity to fly it of course), I admit that I was amazed at your determination and insights into the problems.

I just wish I could develop your insights into what is needed!

Randy Cuberly
PS:  For those that don't know, the airplane in question was a "loaner" to Paul and had not been flown prior to flying in practice at the VSC Competition.  I and most other folks I know would have given up on the thing after a couple of flights.  I really learned what it means to be a "Champion" watching Paul beat that thing into submission!

[Paul Smith]

The aeronautical engineers on this site should be able to calculate the true center of lift of a wing, considering the difference in airspeed from the inboard to the outboard tip.   But, since the airspeed changes across the wing, let's say from 58 MPH on the inboard to 62 MPH on the outboard and the airfoil also changes due to taper, the problem is not just simple arithmetic.

Another factor is the difference in airspeed between level flight (60?) and maximum turns (40?).  Maybe the offset needs to be based on maximum turning while ignoring the effect in level flight.

[Tim Wescott]

The aeronautical engineers on this site should be able to calculate the true center of lift of a wing, considering the difference in airspeed from the inboard to the outboard tip.   But, since the airspeed changes across the wing, let's say from 58 MPH on the inboard to 62 MPH on the outboard and the airfoil also changes due to taper, the problem is not just simple arithmetic.

Another factor is the difference in airspeed between level flight (60?) and maximum turns (40?).  Maybe the offset needs to be based on maximum turning while ignoring the effect in level flight.

More like 54 on the inside and 56 on the outside, for typical-to-fast lap times.

I suspect that what Paul and Brett are trying to tell us about trim vs. design is true -- within my own limited experience it certainly is.

[Brett Buck]

The aeronautical engineers on this site should be able to calculate the true center of lift of a wing, considering the difference in airspeed from the inboard to the outboard tip.   But, since the airspeed changes across the wing, let's say from 58 MPH on the inboard to 62 MPH on the outboard and the airfoil also changes due to taper, the problem is not just simple arithmetic.

   Right. It's the math from Hepperle from the post above.

     Brett

[Serge_Krauss]

More like 54 on the inside and 56 on the outside, for typical-to-fast lap times.

I get about a 4 mph difference for a 56"-span plane on 65' lines.(53 and 56.9) and 4.0 for 54" on 60'. So I think the difference is a bit greater.

From Paul: "also changes due to taper, the problem is not just simple arithmetic...Another factor is the difference in airspeed between level flight (60?) and maximum turns (40?).  Maybe the offset needs to be based on maximum turning while ignoring the effect in level flight."

I agree about taper and expect taper to diminish the effect  Maybe I'll try the math with a taper built in - 'wonder if the 54-year old remnants of my freshman calculus will hold up to the integration. Regarding the speed,...mine cancelled out, but clearly, since a stationary airplane has no speed difference across the span and thus no need for "eccentricity", speed matters, and "v" should not have been allowed to cancel in my final division. I'll have to think about that; it needs to be involved in the derivation. In those turns though, the higher g's require more aoa for more lift. That really muddies it up for me. I wonder whether the effects on "eccentricity" cancel out.

I have to bail out here, at least for the time being - two home projects have me hopping around here. Anyone else curious enough to see where this goes?

[Howard Rush]

...since a stationary airplane has no speed difference across the span and thus no need for "eccentricity", speed matters, and "v" should not have been allowed to cancel in my final division.

I think your intuition is being tricked by multiplication by zero.  Lift across the span is proportional to v², but if v = 0, v² = 0

[Tim Wescott]

I get about a 4 mph difference for a 56"-span plane on 65' lines.(53 and 56.9) and 4.0 for 54" on 60'. So I think the difference is a bit greater.

How odd.  I used a spreadsheet, though, so it must be right.

60 inch span (5 feet even)
65' from circle center to canopy.
62.5' from circle center to inner tip.
67.5' from circle center to outer tip.
5.25 second lap times

Distance = 2 * pi * r.
1 foot/second = 3600/5280 miles/hr (15/22, or 0.682.  Gotta love English units!)

2 * pi * 65' / (5.25 sec) * 15/22 (mph/fps) = 53 mph
2 * pi * 62.5' / (5.25 sec) * 15/22 (mph/fps) = 51 mph
2 * pi * 67.5' / (5.25 sec) * 15/22 (mph/fps) = 55 mph

Huh.  I wonder where I went wrong before.  Musta been the spreadsheet program -- it couldn't have been me.

At any rate, I suspect that you could come close enough in your calculations by taking the speed of the center of each half-span and multiplying by the area to come up with lift.  Then you'd have to complicate things by remembering that the moment arm is different for each, with the longer span having more effect.

Or just use somewhere between nothing and an inch, because that's been what's been winning contests for a good long time.

[Igor Burger]

And do not forget that air flow is not paralel to wing chord 

[Fredvon4]

I am not a math savant... in fact a math idiot.... despite being able to get a 208 lb projectile to land accurate every time into a 55 gal barrel 11 miles away only using a slide rule and established TFT* and NOT the FADAC**

Mean areo chord for each panel has to be calculated...AOA*** ...actual air speed in each attitude... wind direction and speed... fuselage side area...rudder effective side area...

Coriolis effect, gravity of the lat long, sun spots and cosmic wind

* Tabular Firing Tables computed for each and every range, lat lon, weight of pro jo, elevation, height of target vs height of weapon and about 10 other variables

** Field Art computer in the 50~60s...tape fed with 19 variables to be computed...I dare any one to define all the variables...hint powder temp and a particular tube chrono FPS is considered

*** AOA is NOT just relative to the flat plane circle and wind direction and speed... up against  grav.. down with grav

0-0-0 ship gyro precesion is one constant.... engine angle of off set changes the gyro precesion influence to something different than 90 degrees to any incidence line

Please remember or re-read my first 6 words...grin

[Tim Wescott]

I am not a math savant... in fact a math idiot.... despite being able to get a 208 lb projectile to land accurate every time into a 55 gal barrel 11 miles away...

Just one 55 gallon barrel?  It seems that even if you're just hurling slugs it'd wear out pretty quick.

[Fredvon4]

yes each pro jo has a 180 meter burst radius...thus a little hard on the 55 gal barrels...we usually did this fire power demo for Congress or Pres Kennedy..or Nixon .....in my day

I no longer remember ALL the variables we computed and admit a LOT came from the TFTs  ---the constants..
And a lot depended on a good Multi level WX report locally,
And we needed to know accurately the Chrono data from each tube and how many rounds since last chrono... bore erosion is or was predictable

BUT the fact that we could very accurately place and 8" Howitzer round at max distance was significant  as long as we had almost all of the variables accounted for.... today they are even MORE precise simply by having much more accurate LOCATION data to 10 digits vs the surveyed 8 digit data we had to work with...gotta love piggy backing off DOD GPS system...

In my day Field Arty could be accurate to 1 meter at max range...today they have it down to .1 meter

I assume a lot of folk may have seen the Movie Battle Ship and the use of 16" navel guns from the Missouri   with a LOT movie inaccuracy's.. not gonna hump a 2000LB pro jo by hand 500 feet...with 4 or 5 guys... ain't ever gonna happen

What a lot would not know was the targeting was purely analog and movements of the ship-- roll/aft/fore heave- was a variable that had to be accounted for..... and at that... 16" Navel guns were about as accurate as land based fixed location shots at very great distances

[Ted Fancher]

So...

All of the above has been extolled upon endlessly for decades and the inboard vice outboard spans' effect on the center of lift conundrum...despite its modest relevance, as suggested by Paul and Brett...remains of little consequence in the real world of flying the tricks once airborne.  Within any realistic realm of span differential the airspeed effects in flight can be accounted for and a ship trimmed to competitive levels.

If one is interested in a factor of actual consequence with respect to span differential and flying the pattern one is the effect of the location of the center of gravity which results due to trimming out any minor roll axis tendencies caused by the asymmetry.

Yes, center of gravity location...albeit, not in the usual to the stunt airman concern re its location on the longitudinal axis.

The amount of asymmetry and the amount/location of the dead weight necessary to trim out any roll/rolling tendency of the ship in maneuvers has a direct and predictable affect on the spanwise location of the CG and, by extension, of it's location with respect to the thrust which generates the energy to get the whole shebang rocking and rolling on the end of the lines.

The greater the asymmetry of the inboard/outboard spans the less tip weight will be required to to achieve the desired lack of roll in flight in both level and maneuvering flight.

The lesser the asymmetry (such as equal span wings) the greater will be the weight required in the outboard wing to achieve the desired roll condition above.

Remember, there is really only "ONE" wing.  What we are really talking about is where the fuselage and, thus, the powertrain that pulls the wing around is located on that "ONE" wing.

Since the power train and the fuselage which holds it is always the densest part of the entire airframe moving it inboard or outboard on that single wing moves the spanwise location of the center of gravity along with it (and also the thrust line as viewed from above or below the ship.)

If one's sole concern about a stunt ship is weight, a significant amount of otherwise dead weight can be eliminated by moving the fuselage outboard and allowing all that "necessary" weight to multi-task as tip weight.  Pretty brilliant, huh!

What might not be immediately obvious, however, is that the spanwise CG is now inboard of the fuse/thrust line as well...which might not always be a good thing.  (If that isn't clear try visualizing moving the fuse all the way to the right wingtip.  You've added zero weight to the airframe but now, despite not having added any weight, the left wing tip will likely fall to the ground as all the comoparatively modest weight of the wing is on the inboard side of the fuse/thrustline.  This is the extreme example but every inch you move the fuse toward the outboard wingtip the greater will the spanwise CG be inboard of the thrustline).

Remember the powertrain is mounted on that double duty fuselage/tip weight structure.  Thus, depending on the amount of engine offset employed (usually, nowadays, not a lot given the dearth of such attention necessary to generate line tension with modern powertrains)  the thrust of the powertrain will pass outboard of the spanwise location of the center of gravity, and will attempt to rotate the ship about that inboard CG.  This is no big deal in flight as line tension generated in tethered, un-accelerated flight more than offsets the natural tendency of the mass to want to turn "about" the objects center of gravity (like a good curve ball).

Where it can be a problem, on the other hand, is on launch when there is not yet any tethered motion to do the centrifugal/centripetal (who knows which) force thing.  At launch the initial tendency of the aircraft with the thrust line outboard of the spanwise CG will be to yaw the airplane inward toward the pilot and, if great enough or sustained long enough, can result in a temporary to fatal loss of control to the pilot hanging onto the handle in the center of the "planned" flight path.  The oft discussed poster boy for this sort of activity is Hal de Bolt's All American Senior with its ~three inch inboard asymmetry on a modest ~50" or so span.  The AA Sr was/is infamous for its desire to attack pilots if necessary remedial action has not been undertaken.

That remedial action is to provide adequate engine offset to, at a minimum, direct the longitudinal thrustline at or inboard of the spanwise location of the CG so as to eliminate the tendency to do so.

For those of you who experienced the conundrum of control-ability issues initially experienced during the near instant acceleration response of early electrics this effect may have been a factor.  The comparatively explosive acceleration produced by the electrics if combined with a light airframe and even a modest degree of "thrust outboard of the CG" could result in an inboard yaw scaring the dickens out of the pilot quickly overcome as a result of the rapid acceleration establishing rotationally produced line tension and, thus, pilot control .  If, however, this effect is combined with a less than ideal launch location with respect to high or gusty winds the results could be scary indeed.

After way too many words (now that I hardly build anything) my plan is to move more toward equal span wings inasmuch as they can clearly be trimmed to be competitive (Mr Rabe appears to have proven that repeatedly) and provide a degree of safety during launches in bad wind conditions inasmuch as equal span wings on a tethered stunt ship will require sufficient tip weight to result in a spanwise location of the CG that insures any yaw generated under such conditions will be away from rather than toward the pilot.

Ted

p.s. if this highly condensed version of the original document finds any of you still unclear on the subject matter please mail a very large self-addressed envelope to the author along with a check or pay pal payment in the amount of $10.00 to cover postage.  (this is intended to be witty...don't do it).

[Dane Martin]

Ted

p.s. if this highly condensed version of the original document finds any of you still unclear on the subject matter please mail a very large self-addressed envelope to the author along with a check or pay pal payment in the amount of $10.00 to cover postage.  (this is intended to be witty...don't do it).

Ugh, I had already started filling out the check as I happened upon the statement in parentheses.....

[Air Ministry .]

How odd.  I used a spreadsheet, though, so it must be right.

60 inch span (5 feet even)
65' from circle center to canopy.
62.5' from circle center to inner tip.
67.5' from circle center to outer tip.
5.25 second lap times

Distance = 2 * pi * r.
1 foot/second = 3600/5280 miles/hr (15/22, or 0.682.  Gotta love English units!)

2 * pi * 65' / (5.25 sec) * 15/22 (mph/fps) = 53 mph
2 * pi * 62.5' / (5.25 sec) * 15/22 (mph/fps) = 51 mph
2 * pi * 67.5' / (5.25 sec) * 15/22 (mph/fps) = 55 mph

Huh.  I wonder where I went wrong before.  Musta been the spreadsheet program -- it couldn't have been me.

At any rate, I suspect that you could come close enough in your calculations by taking the speed of the center of each half-span and multiplying by the area to come up with lift.  Then you'd have to complicate things by remembering that the moment arm is different for each, with the longer span having more effect.

Or just use somewhere between nothing and an inch, because that's been what's been winning contests for a good long time.

Or as 5 Ft goes into 65 Ft 13 times , & call it 52 mph as its four of them ( 13s ) thats FOUR mph , tip to tip .

or if we want to get really complicated 1/2 of five ft. is 2 1/2 , goes in 26 times , but theres two of them ( wings ) .

ANYWAY ,

For a intermediate type , a longer inner will fly the thing out / pick up the inner - lines'n'all , with 2 in assymetry .

Tho maybe No assymetry and generous tip Wt , It wont kick loose or come in . ? But a heavy plane in the ghourglass
slammed into the first top corner , or trying to kick it into the first cloverleaf Loop ,in wind
without a wobbly rudder
may attempt to engage the pilot, directly .

Id presume the symetric or 1/2 assm. , if set up to deal with a good stiff breeze , would have a fair pull on the lines , constantly .
other presumption is the big flat ( plank like ) fuselage , if near even in side area fore & aft , are as big a factor .

Using near 2 in assymetry on 63 span , extra tip Wt steadied it up discernably , mainly the pilots nerves .
tension increased dramatically in a gale ,downwind .
Going to the finer pitch'd get it ' locked in ' .

as theyre all interrelated variables ! .

Thats with This Fuse , which you can see ISNT a Plank Type .

In fact the rear surfaces ' lock in ' after it gets past the ' waffling along ' at 40 mph stage.



assume its 1/8  per bay . Spruce spars silk covered so stiff sinued . rip off of the Lampo & Free Bird , both national title winners .

[Serge_Krauss]

 
Ted: “to do the centrifugal/centripetal (who knows which) force thing. “

They're a "reaction pair" of forces required by Newton's Third Law of Motion, acting equally, but oppositely on two separate interacting objects. The pilot wants to feel sufficient centrifugal force in the lines, when he exerts the equal and opposite centripetal force on the plane. In amount, the forces equal line tension felt at either end (Diagram below). So, contrary to some college teaching, both forces exist, and neither is an "imaginary" force. 

(confusion arises sometimes when switching reference frames to become a "passenger." One would then experience himself pushing the plane as it pushes him to take him along its circular path. In this case the plane is exerting the centripetal force on its 'passenger')

[Ted Fancher]

Ted: “to do the centrifugal/centripetal (who knows which) force thing. “

They're a "reaction pair" of forces required by Newton's Third Law of Motion, acting equally, but oppositely on two separate interacting objects. The pilot wants to feel sufficient centrifugal force in the lines, when he exerts the equal and opposite centripetal force on the plane. In amount, the forces equal line tension felt at either end (Diagram below). So, contrary to some college teaching, both forces exist, and neither is an "imaginary" force. 

(confusion arises sometimes when switching reference frames to become a "passenger." One would then experience himself pushing the plane as it pushes him to take him along its circular path. In this case the plane is exerting the centripetal force on its 'passenger')

Hi Serge, 

Happy Halloween to the Krausses! 

Nah, I don't question the existence of both.  I only mentioned the "confusion" factor vis a vis the two which rears its head several times a year on this and other fora dedicated to masses (as in weight, not large groups of folks) revolving about oneself on a tether!

It's a frequently "maddening to others" foible of mine to try to insert, quote: cleverness; unquote, in my often overly opinionated offerings (the alliterative formation of which you are now experiencing).

The sole intent of my previous post was to suggest that a division of inside to outside wingspans in a fashion that requires an extra ounce or so of dreaded higher wing loading via greater tip weight might be a worthwhile trade off when taking off in inclement stunt conditions while avoiding wasting a fewer number of thrust ergs by pointing the tractor away from the desired flight path of its trailer.

My apologies.  Can't help myself.

Ted

p.s. I know, I know, weight and mass aren't the same thing.  What probably is accurate is if an un-restrained "weightless" astronaut tried to swing a three pound (on earth) weight about himself at 60MPH the inside of the capsule could be at risk of getting redecorated!

[Mike Scholtes]

As a practical illustration of the need for additional tipweight on symmetrical models, I test flew my new Oriental Plus yesterday for first time. At the suggestion of none other than the designer Dee Rice, and mindful of Brett's comments about symmetrical models needing as much as 2 ounces rather than the more typical 3/4 to 1 ounce needed on asymmetrical models, I started with 1.5 oz to be safe. It will need another .25 oz at least to fly wings level upright and inverted (the out-of-level appears identical, outboard tip slightly high upright and inverted). This is not a large model, about 610 inches. I need .75 oz in my 680 inch Legacy, with about an inch of asymmetry.  I used the Brett-Paul-Ted bench trim protocol since I cannot run at 60 mph and wanted go home with the same number of parts I started with. Was able to remove temporary rudder trim tab after two flights, good tension at all attitudes.

[TDM]

Tim your numbers are a little off despite the use of the spread sheet.
The lift generated by a wing panel is in reference to the center of pressure for that wing. So instead of

62.5' from circle center to inner tip.
67.5' from circle center to outer tip.

Should be

x' from circle center to inner panel center of pressure 
y from circle center to outer panel center of pressure

I am wiling to bet it will cut all your number by a factor of more than one half as the center of pressure is at 50% if your wing has no taper and the flaps have no taper as well which i doubt that very much. Center of pressure for a panel is closer to the root of the wing rather then the tip. Consequently your number will end up (assuming some common sense taper on the wing and flaps)

2 * pi * 65' / (5.25 sec) * 15/22 (mph/fps) = 53 mph
2 * pi * 63.75' / (5.25 sec) * 15/22 (mph/fps) = 52.02 mph
2 * pi * 66.15' / (5.25 sec) * 15/22 (mph/fps) = 53.97 mph

and there you go less than 2mph


BACK TO THE THREAD TOPIC
That being said, a little more drag on outboard wing (due to the extra 2mph roughly) will counteract some of the massive drag the lies, also a little more lift on the outboard wing is also mot a bad thing to the overall balance (you want the outboard to lift a little faster than the inside right?). Did I mention easier not to build an equal span wing too?

There are two schools of thought
1 equal span equal everything get advantages listed above with a little extra tip weight
2 unequal span with a little less tip weight.

I say pick one and run with it.

[Serge_Krauss]

Hi, Ted!

I know you know all that stuff! I always enjoy reading your posts. Have a good one!

Serge

[Howard Rush]

This thread got me thinking about: A) calculation of the spanwise location of the mean aerodynamic chord, 2) the way folks think of balancing asymmetries, and iii) how to use this theoretical stuff to trim a stunt plane.

I’d seen the traditional formula for spanwise mac and even weighted it for q due to flying in a circle in combat plane designs, but hadn’t thought much about its physical significance. It’s the effective moment arm for aerodynamic forces acting on one wing (one side of the airplane) for an airplane not flying in a circle. It also assumes lift proportional to chord along the wing. Hepperle, in the link Serge gave, adapts that formula to our problem, considering the whole wing, as Ted says. It’s really cool and a useful way to think of “eccentricity”, I think. I have a couple of quibbles. It integrates from -b/2 to +b/2, which puts zero halfway between the tips. On a CL airplane, that point may neither be the fuselage center nor the place the wings join. I’d call the tip to the inside of the circle zero.  It would give the same result. I think I’d also toss in a more realistic lift distribution. Igor’s comment adds another complication. I don’t know how to adjust for that.

Folks tend to look at these assymetries one at a time and see what to do to cancel them: tip weight to balance lift eccentricity (or half the level-flight line weight), rudder to balance moment caused by precession of rotating machinery, etc. Brett said awhile back that you want the airplane to rotate about a line between it and the flier. I think you want to zero the sum of the moments about the other two perpendicular axes while the airplane is rotating about that line. There are a plethora of components to each of these moments. I don’t think I’ve ever seen them listed, let alone calculated.

Any theorizing we apply to trimming an airplane is probably incomplete. Just add a chunk of tip weight and follow the chart.

[Igor Burger]

Igor’s comment adds another complication. I don’t know how to adjust for that.

I think it is not so difficult. If simplify that the air stream is straight (it is also not, but I do not think it makes visible difference), simply separate front and side wind and make separate calculation for both. Then real center will be on line between them on place equal ratio of side and front wind. At least I hope :-P. In any case we know that effect exist, side wind moves center little left and it is known experience that model yawed outside needs less tip weight and flying in wind makes too much tip weight symptoms on downwind corners and too little tip weight upwind (begin of wingover). 

Any theorizing we apply to trimming an airplane is probably incomplete. Just add a chunk of tip weight and follow the chart.

True. I did my calculation when I saw program of Lou Crane (25 years ago?) which I reworked to excel for metric units and then expanded to 60 input values and aproximately 70 output values. Today I know it is hardly on half way and several ideas inside are plain wrong ( ot least very very average - LO position concept against CG force, 1/2 weight rail tip weight etc). The real value of such programs is understanding (designing), not PC trimming 

[PJ Rowland]

I think there are far more things you can do to an airframe to make it fly better than worrying about the lift differential.

For what its worth Ive calculated it and yes if you compare the outboard tip to the inboard tip there is a distance difference. Thus a minor speed difference. If you want to get completely technical ; to do it properly the entire wing needs to have the thickness percentage altered from inboard to outboard in a linear manner.  This is somewhat impractical but would yeild ( for example..  Inboard tip at 1" outboard at a reduced percentage.

I dont think it makes as much difference as you think it does vs talking about it. 

Brett is correct when you move the wing your moving the thrust line and stab.  But what if you build on the assemtry into the stab also..  Trust me it doesnt make it "the next big thing"

I think* (rarely)  how the airflows around the system is more critical than many other ideas.. as Igor eluded to the wing doesn't always see all the air....

[Ted Fancher]

Wow!  After reading all of our combined pontifications one is forced to look back in awe at Palmer and Aldrich who appeared to have figured it all out with a wet finger held aloft on a windy day.

Oft times methinks we think too much while WAGs have worked so well so long.  As long as Prather produces stick on weights we'll all be swapping lies at the bar with the Wright Bros.

Ted

[Howard Rush]

Wow!  After reading all of our combined pontifications one is forced to look back in awe at Palmer and Aldrich who appeared to have figured it all out with a wet finger held aloft on a windy day.

Oft times methinks we think too much while WAGs have worked so well so long.  As long as Prather produces stick on weights we'll all be swapping lies at the bar with the Wright Bros.

Ted

That’s my point, although I don’t think I expressed it very well. Even when somebody correctly identifies the cause of a roll or yaw moment, he often tries to balance it in isolation from other stuff. The only thing that counts is to zero the sums of all the moments in directions other than the one you want the airplane to rotate around. If you set out to make a + b + c + d = 0, you won’t get there if you only work on a and b. There may also be an e and f you don’t know about.

Guys like you and Paul (and Palmer and Aldrich) figured out how to design and trim by doing zillions of flights while continually experimenting and observing. We are fortunate that you share what you learned.