Question about Flight Charateristics??????

LAST EDITED ON May-21-02 AT 10:51 AM (CST)

I have 10 flights on my new Bear. I know 10 doesn't seem like many but this thing pretty much flew right off the board. It has a very high aspect ratio wing. The span is 63" the root with flaps is 13 1/8". The plane flies good! You can really corner this plane with reckless abandon and it will just hang in there and exit flat. It is so fun to fly it. It flies very tail heavy. The CG is 4" after the leading edge at the wing joint. With nose weight added it is just like all other planes it gets more stable. But the wing can fly very stable with the tail heavy setup so I keep it there because that corner is so much fun.

Here are some measurements and numbers.

Span 63"
Root 13 1/8"(with the flaps that are 3 1/4")
Stab/ele 26%
Stab span 29"
Nose moment 9"
Tail moment 17 3/4"
Fuse length 44"
Motor PA 65 pipe at 18"
Prop Eather 4 blade
Tip weight 1 1/2 oz.(have tried more this seems to be the right amount so far.
LO are just a smidge behind the CG and the tension is excellent every where. There is no hinging or yaw that I can see or feel or have been told about so far.
Weight 59oz.

I have one question about the flight characteristics. When in level flight the tip seems to want to bump up and down all the time. The flight path is still level (no hunting). If you are looking at it from head on the ob tip seems to want move up and down slightly all the time. It will not just sit stable. My low aspect ratio wings are so stable it is easy to fly level and flat all the time. They sure don't turn like this one though. This thing has a monster turn and I love flying it. I have tried lots of tip weight and that didn't do anything to this situation it just effected the maneuvers. It tracks on straight legs and flys rounds by itself.

I am just curious about this wing tip movement.

Anyone got any ideas?

>
>I am just curious about this
>wing tip movement.
>
>Anyone got any ideas?
>
>DMoon

1) high aspect wing hitting turbulence? (You asked for ideas. For this, I don't have a solution.)

2) Are the flap hinge lines sealed? If not, I would suggest trying it to see if it helps. (Maybe, maybe not. Just another idea.)

Doug,

I know of no solutions. It's just a byproduct of the high aspect ration wing when it encounters any turbulent air. Since we fly so low to the ground, turbulence abounds even when it doesn't seem like it should. Good luck getting rid of it if you can, if you do, please let everyone know!

Doug,
What is the A/R of this plane? What is the tip dim and what is the wing area? I'm just curious as to what you consider a low A/R and a high A/R. Sounds like you got yourself a great flying plane. Seeing that it is "unbeatable", I expect to see nothing but 1st place trophies on your mantle!

Doug,

If the wing tip is rockin' and rollin', you have an instability in roll - "hunting", only in the roll axis instead of pitch. From what you describe, however, it seems strange that tip weight has no effect on it (not even changing its rate?). And moving the horizontal CG forward does not affect it at all?

The actual causes of oscillations such as this are often hard to track down. What happens is that a force (lift) imbalance in one position causes the wing to move and change its position. In the new position, the imbalance disappears, so the wing returns to its previous unstable position. And the cycle repeats.

The frequency of the oscillation is usually affected by the masses (or moments of inertia) involved, which is why I wondered about the tip weight effects. More mass has a slower frequency.

Are your wing panels the same length? That might be a factor somehow. Or a tiny little warp somewhere.

I believe that higher aspect wings are more sensitive to such problems, because the smaller airfoil chord length has less stabilizing force associated with it. One might surmize that the narrower wings offer some performance benefit by being a little less stable.

Since this is a level flight issue, we might assume that your controls are not affecting it, but that might not be the case either. Level flight may involve some small control bias, whether or not you are aware of it at the handle.

You didn't mention if the problem was worse when you flew inverted. If it changes between inverted and normal flight, that tells you something is not quite symmetrical.

And although the problem is in pitch, it might well be affected by horizontal CG, which might reflect in the wing attitude (and lift characteristics) very slightly.

How's your vertical CG? Ships with vertical CG problems often will carry themselves with a slight roll bias. You should be able to see that from the handle in upright vs inverted flight. If roll gets biased enough, it might cause some aberration in the lift at the wing tip, which could support oscillation.

I know I've offered no real ideas for fixing your problem, just some thinking about them.

Wouldn't it be great if this were a take apart model, so you could slap in another wing? That might reveal a lot.

BTW, how do you like those 4-bladed props?

Hope you find it soon. When you do, tell us what it REALLY was..

L.

Rocket,

That's strange. I flew a very high aspect plane (like 8.5 to 1)) last year and didn't have this problem. I had other ones, but in level flight it was rock steady. No bump at all.

Doug, I have a relatively high aspect ratio wing airplane called the "Backfire" that I designed for my Saito 72 motor. It has the exact same characteristics you described. It turns like nobodies business, but the wing tip rocks in moderate wind. I immediately asssumed that it was the high aspect ratio wing. I have just finished designing a lower aspect ratio wing version of the plane which is why I immediately answered your thread. Bob Whitely, who we occasionally fly with, described these problems and stated them as some of the reasons why they abandoned high aspect ratio wings. It is really a matter of finding the right balance for the conditions your intend to encounter.

I love the 4 blades. Just ordered one today. They are excellent. The combo here in Dalls is working well on the 4 blader.

Larry I am going to fly tonite, hopefully. I will be paying attention to what you wrote and see what I can see.

I am thinking maybe with a really long propeller I can calm it down some. The larger the diameter the more stabilizing effect.

As to other questions about aspect ratios. I will measure it out and get back to you.

DMoon

PS Maybe with the advent of the stump puller motors to stunt out there(I.E. Saito 72 or 91) the high aspect ratio wing might return. If my little experiment works with props. You see I am building another one of these jewels that is almost identicle except it will be fitted with the YS 63 and it should be able to throw 13.5" prop that is an 1.5" longer than I got on there now. That should have some stabliing effect.

Doug,
Sounds like you need a Gyro!!!
Tom N

Doug-

'hope you'll forgive me for taking a preliminary run at this. What you are experiencing should be a more subtly complex motion than you have sensed so far. Both rolling and yawing couples should be involved. If the outboard wing raises relative to the plane's path and the inboard wing, you have roll. Since roll is initiated by a lift differential, there is an induced drag differential and thus a tendancy to yaw. The lines and inertial forces tend to restrain these motions and return the plane toward an equilibrium attitude. But they have to occur.

For example, how about this sequence? For the plane not to be hunting, the inboard wing must momentarily be experiencing as much less lift as the amount of excess lift raising the outboard wing. This might happen if the exaggerated speed differential between inboard and outboard wings, brought about through your high aspect ratio, caused excess lift on the outboard wing nearly equal to the lift deficit inboard. Since lift and drag are proportional to the square of the speed, without sufficient assymetry in half-spans, the outboard wing will be affected more and there could be as much as, perhaps, a 7-8% difference. Even with normal small assymetry, the lift and drag of the outboard wing might be momentarily up to a few percent greater than the inboard values. Drag (profile and induced) differential would tend to yaw the wing outward (clockwise, slowing it) against the restoring force of the lines (rake vector, line drag, and centripetal force), until the restoring torque increased to snap the plane back toward neutral. Simultaneously, as the outboard wing slowed marginally relative to the inboard wing, it would also have lost relative lift and perhaps returned through "normal" pitch/roll angles. Then the cycle could repeat as the lines rotated the wing back counter-clockwise. If the frequency of this cycle is high enough, you might not see a change in altitude. But the plane should "corkscrew" as much as tight lines allow. Perhaps the only easily visible motion is the raising of the outboard wing.

An interesting complication to all of this might be Reynolds Number effects. The critical Reynolds Number for airfoils of the NACA 00XX series lies somewhere around half a million, which is in the area of those found on C/L stunt planes. For instance, a 12-inch (1.0 ft.) chord wing traveling 55 mph at sea level would have a Reynolds number near 1.0 x 9306 x 55 = 5.1 x 10^5, or about half a million. In this area, there is a more or less "sudden" increase in profile drag with reduced Reynolds Number for a given wing section. There are also losses in lift coefficient with decreased RN, but I don't recall whether there is any "sudden" effect. Wouldn't it be bizarre, if the high-aspect-ratio stunter has stumbled into this unpredictable realm?

Ah, well, it's probably something simpler, but I doubt that it's gusts for such a constant periodic effect. For gusts, I'd expect a frequent damping of the tip effect - between gusts.

Serge,

How does thickness affect Reynolds numbers? Would thicker tip airfoils exacerbate such problems or help?

My curiosity is piqued, because I'm working on a little project with an Excel spreadsheet for elliptical wings. I'm trying to deal with both the elliptical wing perimeter shape and elliptical thickness tapers.

I've always assumed that there was something inherently good about a thick tip airfoil (supposedly it stalls AFTER the root?). But I'd love to hear an informed opinion on this aspect of CL stunter wings.

One more thing I was wondering about, if Doug's ship is experiencing some effects of wing tip vortex.. Although Bob Geiseke's Bear has those little pointy tips, which I was assuming might be a design feature to deal with such. I didn't notice if Doug's ship had these or not.

It would be nice to find out what fixes this. Doug will probably have some more specific comments tomorrow.

At the risk of oversimplification....does Bob's high aspect ratio plane exhibit this roll dynamic?? If it does not, then there must be some tiny misrigging as mentioned above. If it does, then there may be a design flaw as Serge mentioned. I am assuming that the external dimensions of your Bear are identical to Bobs Bear of course.

>At the risk of oversimplification....does Bob's
>high aspect ratio plane exhibit this roll
>dynamic?? If it does not, then there must be
>some tiny misrigging as mentioned above. If
>it does, then there may be a design flaw as
>Serge mentioned... >
>Steve

It doesn't have to be either. There is a lot happening in the air in which we fly that we never think too much about. In calm air we can fly through our own turbulence. In windy weather the wind, itself, can cause the turbulence as it moves across the uneven landscape. Heat from the sun, evaporation of moisture, many things cause cause little updrafts and downdrafts that we are constantly flying through. The longer the wing span, the more chance we have of one tip or the other catching one of these that will then affect the whole plane (tipping the wing up or down). Higher aspect wings will have more problems, in general, with "squirrelly" weather.

> Higher aspect wings will
>have more problems, in general,
>with "squirrelly" weather.

An interesting approach to this "problem" is evidenced by Wild Bill's Doodlebug which has a very low aspect ratio wing of about 3 to 1. His stated intention is to use the extra drag of the fat low AR wing for speed control with enough power up front to offset it. With it's stubby wing I wouldn't be surprised if it handled turbulence quite well. Not only will the wind have less leverage on the wingtips but additionally any given bubble of turbulence will affect a greater area of the wing compared to conventional designs so even if wonky air causes the plane to bounce it may not jump around on it's roll axis near as much. It's a slick example of "behind-the-curve" aerodynamics.---Mark

"The longer the wing span, the more chance we have of one tip or the other catching one of these that will then affect the whole plane (tipping the wing up or down). Higher aspect wings will have more problems, in general, with "squirrelly" weather. "

Quite right, Leonard. The Oriental springs to mind as a plane that looks nervous on a bumpy day. But Doug will have a perfect chance to compare apples to apples when Bob is out there with his plane sometime. Under equal conditions the planes should groove equally well-or bounce equally as much. This will be a chance most of us never get-match our plane up with the designer's example and see how they work.

I think SK is onto something there about the speed each wing sees. As the wing gets longer there is a greater and greater effect of the speed changes from tip to tip. However I found a small fix that seemed to help a little.

Larger prop. 11 7/8" diameter to 12 1/2" diameter.

This really seemed to dampen some of the effects that were being seen earlier. Yes Serge when one wing was up the other was down. It is not hinging at the LOs. The roll happens around the fuse.

Tell me this stunt guys. Howcome it has less effect at slower speeds. I had a flight at 5.9 lap times at full 70ft lines also last night and it was stable as can be. I have had it as fast a 5.4 and it was moving all around. I would think that the faster it went the more the lines would help dampen the effect by creating tension across the airframe. However this does not seem to be the case. At 5.9 or 6.1 it is smooth and it flys really good there too. I attribute that to its total weight of 59oz. The wing has a total area of 714 sq in and has a ratio of 7.6.

Doug,
Please check your numbers again. A wing span of 63 inches and an area of 714 squares has an A/R of 5.56 to 1. About the same as an Impact. How did you arrive at 7.6 to 1?

>Tell me this stunt guys.
>Howcome it has less effect
>at slower speeds. I
>had a flight at 5.9
>lap times at full 70ft
>lines also last night and
>it was stable as can
>be. I have had
>it as fast a 5.4
>and it was moving all
>around. I would think
>that the faster it went
>the more the lines would
>help dampen the effect by
>creating tension across the airframe.
> However this does not
>seem to be the case.
> At 5.9 or 6.1
>it is smooth and it
>flys really good there too.

Lessee if this makes some sense. Since lift varies with the square of the airspeed the lower lap time is going to affect the wingtips in a more abrupt fashion. Obviously at the faster speed the light tip is more affected by a turbulence bubble than at the lower speed but due to the squared lift increase it jumps enough to make the behavior much more visible at a 5.4 than at 6.0. It may not calculate out to be a lot but it might well be producing the visual behavior you're seeing. The bobbles may be happening so fast that the stabilizing influence of the lines doesn't have time to take effect.---Mark

LAST EDITED ON May-21-02 AT 10:56 AM (CST)

LAST EDITED ON May-21-02 AT 10:46 AM (CST)

Crist, I did the aspect ratio without the flaps. I took the root and the tip added them together and devided by 2. Then I devided the span by this number. I did not use the flaps when coming to this number. Maybe I did it wrong I am not sure. Randy flew one with an 8.5:1. That must have a very long span.

By the way how did you come to your ratio number?

I also received this note from a fellow CLer.

"Hi Doug,

You mentioned the plane has a 63 inch wing span, that comes to 5 and a
quarter feet.

There is a phenomenon, called ground effect. A natural lifting tendency
of
turbulence generated by a planes wings when it gets close to the
ground.
This effect begins when the airplane reaches an altitude equal to it's
wing
span. Since your wingspan is almost smack dab in the middle of the
recommended level flight altitude.

The ground effect is certainly acting on your airplane. Now whether
this
is the main cause of the bobble is uncertain. I suggest an experiment
and a
day with little wind try level flight at about 15 feet or greater
altitude
and see if the wings rock constantly. If not you have found the answer
to
your question."

What do you all think of this?

Doug,
As Ted would say the flaps are part of the wing. I'm assuming they are included in the wing area you listed. Here is how I arrive at an A/R:
Wing area/wing span= Ave chord 714/63=11.333 (Average chord)
wing span/average chord= A/R 63/11.33=5.56

There is another way:
wing span * wing span / wing area
63 * 63 = 3969 / 714 = 5.58

Please describe the plane that has 8.5:1 ratio. What is the span of this plane. How did you come to 8.5:1 on your numbers?

>Tell me this stunt guys.
>Howcome it has less effect
>at slower speeds. I
>had a flight at 5.9
>lap times at full 70ft
>lines also last night and
>it was stable as can
>be. I have had
>it as fast a 5.4
>and it was moving all
>around...

>DMoon

Question, did you have any wind? In calm air you could be flying through your own prop wash. The faster you fly the less time it has to dissipate. Of course, if you were flying in wind, that should not be the case.

LAST EDITED ON May-22-02 AT 07:01 AM (CST)

Steve-

I wouldn't say a design flaw, but rather another challenge to be met in developing a plane with significant potential. All aircraft design involves compromise, some with heavy tradeoffs. But often enough to encourage us, problems can be solved without giving up too much. Doug's new plane's one quirk might turn out to have a convenient, clever fix!

Larry-

I am almost embarassed to submit this, since its relevant value is not proportional to its length! But having found some interesting stuff (some more relevant to earlier airfoil discussions) and re-written much of it due to being knocked off the internet while compiling the URL list below, I'll just go ahead, for whatever it's worth. Here goes...

I looked in my files for data to support some sort of "educated" response. The problem of addressing low-Reynolds-Number issues is lack of data from the major institutions involved in aeronautical research through most of the last century. Until the development of CFD and its availability to individuals, the only data I know of was from modelers themselves and some early wind-tunnel work done by agencies like our NACA (became NASA), which was still coping with tunnel interference and developing guestimates they called "effective Reynolds Numbers". By the time they solved these problems, they were more interested in higher speeds and more advanced airfoils. Besides those concerned with MPA's, ultralights, sailplanes, and models, few expressed any further interest in this realm until recently. There are probably some AIAA papers now (some data for AeroVironment's Pathfinder series??), but I don't have them.

Recently individual light plane developers and modelers, particularly in the R/C soaring ranks, have made progress in acquiring data with the more sophisticated tools. David Lednicer, Michael Selig, Martin Hepperle, Richard Eppler, Franz Wortmann, and others have furnished quite a volume of data, only some of which I have seen.

I don't know how reliable the current CFD codes are in predicting low-RN behavior, but I would expect them to give good comparative results. Moreso with the current inexpensive wing-section software. Using them, one could put in the NACA 00XX series coordinates (a couple already come with these in their own self-contained database) and use the mouse to thicken them in increments. The automatically generated graphs should show the stalling and max-lift trends.

Here are some sites to research:

NACA Technical Report Server (for free downloadable reports): http://naca.larc.nasa.gov/

NASA Technical Report Server (probably not much free text available): http://techreports.larc.nasa.gov/cgi-bin/NTRS

NASA CASI Report Server: sorry, couldn't get it to come up this AM.

UIUC Airfoil Data Site (Michael Selig, U. of Illinois; links to free shareware like X-FOIL, SNACK,...): http://amber.aae.uiuc.edu/~m-selig/ads.html

UIUC Airfoil Coordinate Database (David Lednicer; data to plug into shareware): http://amber.aae.uiuc.edu/~m-selig/ads/coord_database.html

Profili 1.2 (shareware that steals from Lednicer/Selig):
http://www.baronerosso.net/software/profili/profili1_2.htm

Lednicer's "Incomplete Guide to Airfoil Useage" (links to several sites): http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html

B-Squared Kuhlmans' article on effective dihedral (relevant to recent thread): http://www.b2streamlines.com/EffectiveDihedral.pdf

Now for what little I know...

"How does thickness affect Reynolds numbers...Would thicker tip airfoils exacerbate such problems or help?"

I'm not sure how to answer this, as my recollection is that RN is defined in terms of a linear dimension along the direction of flow. This provides a measure of scale effect, describing indirectly how many air molecules encounter the surface. While accelerations along curved surfaces are valid in determining Mach -Number effects, I don't think this would relate to scaling. Let's ask Brett!

I think that tip thickness does affect stunter handling because of stall behavior and 3-D flow phenomena, but here it gets complicated. Whether larger vortices might cause more problems than the tips otherwise solved (flap effectiveness near the tips? turbulence?) I don't know. It has been shown that extended tips giving narrower than elliptical chord distribution are the most efficient for a given root bending moment and that several small tips waste less energy to vorticity than single full-chord tips. These would be inherently thin.

"I've always assumed that there was something inherently good about a thick tip airfoil (supposedly it stalls AFTER the root?). But I'd love to hear an informed opinion on this aspect of CL stunter wings."

"Informed Opinion"...H-m-m-m-m... The blunter, rounded thickness taper should allow more circulation around the tip to delay stall as well as providing a "friendlier" airfoil - up to some limiting thickness. I know that Jim Bede (BD-5, at least) and others (possibly Jim Marske) employed thicker (in %) tips to avoid tip stall without washing out the incidence. Aside from early NACA work, I have seen little data on this.

Here is what I have found in the old literature - relevance varies, since this is derived from measurements of two-dimensional flow only:

1) At "normal" RN's (in the millions), the stall angle increases with thickness up to around 12-15% thickness and then decreases (but see #6 below). The greatest value of maximum lift occurs at about 13% thickness for symmetrical sections. This trend is biased by how far back the point of maximum thickness lies and by flap-induced camber. Flapped airfoils develop their significantly higher maximum lift at slightly lower stall angles of attack. Maximum lift coefficients seem to drop off as the point of maximum thickness is moved rearward.

2) At "normal" RN's, the minimum coefficient of drag (profile drag) increases with thickness.

3) At "Normal" RN's, The aerodynamic center moves forward from the 23%-chord point as thickness increases.

4) At "Normal" RN's, Lift curve (CL vs. angle of attack) slope decreases slightly with increasing thickness.

5) At "Normal RN's", and presumably for non-laminar series sections, maximum coeff. of lift decreases and profile drag increases as the point of maximum thickness moves rearward from the quarter-chord point.

6) Of the NACA symmetrical airfoils 0009, 0012, 0015, and 0018, the 0018 is the only one with only a gradually steepening minimum profile drag curve as RN decreases below 10^6. For high coefficients of lift (CL = .8 shown), the 0018 and 0015 may have the highest drag, but they are also the only ones not showing evidence of stalling, which has already occurred for the 9% section and is imminent at RN = 500,000 for the 12% section.

7) For NACA 230-series airfoils with split flaps, there seemed to be a trend of best efficiency for thicker airfoils as RN decreases, but the lowest RN shown was one million, where a 13-14% thick section was best.

8) In a report uncorrected for interference effects discovered later, the NACA 0018 airfoil has the highest maximum lift coefficient for all "Effective" Reynolds Numbers between 150,000 and 1,100,000. Above that, the 0012 and 0015 win out. That says that thick sections are best for C/L stunt. The 0018's lift-curve slope is however the lowest shown in the C/L stunt range. No thicker sections are shown.

Most of these trends/data come from NACA TR 610 (12/5/36), NACA TR 586 (6/24/36), NACA TR 669 (2/13/39), and Abbott and Von Doenhoff's *Theory of Wing Sections* (1949), the famous compilation of previous NACA work. Remember that these are basically two-dimensional effects whose application is compromised for 3-D flow at tips.

"Although Bob Geiseke's Bear has those little pointy tips, which I was assuming might be a design feature to deal with such..."

I really don't think shape has nearly as much effect on model tips as it does on the tips of "full-sized" aircraft wings. The success of rectangular-winged stunters seems to support this. This is an area where I feel that Reynolds Number is relevant. Since wing loadings of C/L stunters are much lower at 10 oz/ft^2 (=.6 lb/ft^2) than those of large aircraft (over 10 lb/ft^2), and tip area of C/L stunt wings of any shape are small compared to the pointiest of the large wings, it seems that models should be less affected. Shape does make a difference in vortex formation, but (I think) a proportionally much smaller one. However, this is a GOOD and convenient area in which to experiment - interchangeable tips aren't that difficult to make.

There must be some good CFD results out there for models by now. These would be able to predict low-RN tip effects much better than old infinite-aspect-ratio NACA results. While there is quite a lot of tip literature, I haven't seen data on tips at low Reynolds Numbers. Perhaps on a more practical level though, Doug is helping answer some of these questions.

Doug-

No solution, but a couple observations.

First, Mark has a point: the forces on your model are considerably greater at the 5.4 sec lap time than at the 6 sec lap. The ratio of speeds is 6/5.4 = 1.111... So lift and drag forces at any angle of attack would be in the ratio of (6/5.4)^2 = 1.23, or 23% greater for the 5.4 sec laps. With this much more lift, you would need less control input to maintain level flight or initiate pitch changes. Line tension is also 23% greater.

Second. If your root chord is 13 1/4" with flaps, and the c.g. is 4" back from the root leading edge, then your c.g. is definitely behind the wing's center of lift, until the flaps are deflected some. This creates a dynamic instability that the tail moment (and line tension) must overcome. Perhaps some sort of coupled oscillation - amplified with speed - has crept in here...??

Just some thoughts.

LAST EDITED ON May-24-02 AT 09:16 AM (CST)

Doug,
How did you come up with the aspect ratio? I fly a twin engine plane Called gemini. It has a 77 inch wing with average cord of 12 and it is only 6.4.It has 960sq. To have that kind of ratio you would have to have a average cord of about 8.28 and your 63 in. span. Which means you would only have 521.64 sq.inches.Tell us what your root cord and your tip cord is,In clued your flaps also.I haven`t had the problem your talking about with my twin. Hope you get it figured out. Like to know what you find.

Hi Doug,

I gotta say, you're ability to put out a new ship every two or three days does keep Leonard's forum hopping!

First, I must agree that you probably overestimated the actual aspect ratio pretty severely. Chris' comments were right on. The preferred method of determining aspect ratio is: span squared divided by the area...and, once again, as simply a part of the wing that is hinged, the flaps are very much a part of the wing.

I think Serge suggested the CG might be aft of the Aerodynamic Center (or center of lift, if you prefer) because of your description of the CG location at the fuse. Once again, I caution that common practice (and pragmatic reality) requires that we make our observations regarding wings based on the Mean Aerodynamic Chord (MAC) which, for our purposes, can usually be considered as halfway out the panel. Thus, if your ship has a significant taper to the leading edge it is entirely possible that the CG in relation to the Aerodynamic Center/center of lift might be right in the ballpark.

I bet if you measure the length of the chord at the MAC and check the location of the CG at that point it'll work out that the CG is right around 24-26% aft of the leading edge at that point. Check for us and see if that isn't about right. I'll go out on a limb here and guess you'll find the length of the chord at the MAC to be about 11-3/8" long and the CG will be located about 2-3/4" to 2-7/8" aft of the leading edge.

Such a location is about exactly where I would have set up a ship of your configuration for first flights. The fact that it flies so well off the board is likely due in no small measure to this relationship.

I think it can be generally stated (nothwithstanding Randy's exception which might the one which proves the rule!) that higher aspect ratio wings are more susceptible to roll induced by turbulence or (in tethered flight like ours) cross winds. Even if the degree of roll isn't much larger than a lower AR the amplitude at the tips is greater simply because they are farther away from the CG (about which movement in all axes happens). Ergo, you're going to see it more noticeably.

I'm actually more interested in what you find about flying in winds with the higher aspect ratio Bear. Although the 5.56 ratio isn't particularly high it is clearly greater than the five or slightly less of most stunters.

My experience with high A.R.s has been that they have a much greater tendency to wind up in maneuvers. They can be spectacular performers in calm to ideal conditions but when the wind blows they start chasing their own tails in consecutive maneuvers. Careful attention to wind placement and careful seleciton of props and power delivery become much more critical.

Let us know what you think after some of your well known Texas wind days.

>Hi Doug,
>
>I gotta say, you're ability to
>put out a new ship
>every two or three days
>does keep Leonard's forum hopping!

I just finished putting in the stab last night night on my next High AR Bear that will be fitted with the YS 63. As Brad walker sayd the Psycho building continues!!
>
>
>First, I must agree that you
>probably overestimated the actual aspect
>ratio pretty severely. Chris'
>comments were right on.
>The preferred method of determining
>aspect ratio is: span squared
>divided by the area...and, once
>again, as simply a part
>of the wing that is
>hinged, the flaps are very
>much a part of the
>wing.
>
>I think Serge suggested the CG
>might be aft of the
>Aerodynamic Center (or center of
>lift, if you prefer) because
>of your description of the
>CG location at the fuse.
> Once again, I caution
>that common practice (and pragmatic
>reality) requires that we make
>our observations regarding wings based
>on the Mean Aerodynamic Chord
>(MAC) which, for our purposes,
>can usually be considered as
>halfway out the panel.
>Thus, if your ship has
>a significant taper to the
>leading edge it is entirely
>possible that the CG in
>relation to the Aerodynamic Center/center
>of lift might be right
>in the ballpark.
>
>I bet if you measure the
>length of the chord at
>the MAC and check the
>location of the CG at
>that point it'll work out
>that the CG is right
>around 24-26% aft of the
>leading edge at that point.
> Check for us and
>see if that isn't about
>right. I'll go out
>on a limb here and
>guess you'll find the length
>of the chord at the
>MAC to be about 11-3/8"
>long and the CG will
>be located about 2-3/4" to
>2-7/8" aft of the leading
>edge.
>
>Such a location is about exactly
>where I would have set
>up a ship of your
>configuration for first flights.
>The fact that it flies
>so well off the board
>is likely due in no
>small measure to this relationship.

I will get back to you on this.

>
>
>I think it can be generally
>stated (nothwithstanding Randy's exception which
>might the one which proves
>the rule!) that higher aspect
>ratio wings are more susceptible
>to roll induced by turbulence
>or (in tethered flight like
>ours) cross winds. Even
>if the degree of roll
>isn't much larger than a
>lower AR the amplitude at
>the tips is greater simply
>because they are farther away
>from the CG (about which
>movement in all axes happens).
> Ergo, you're going to
>see it more noticeably.
>
>I'm actually more interested in what
>you find about flying in
>winds with the higher aspect
>ratio Bear. Although the
>5.56 ratio isn't particularly high
>it is clearly greater than
>the five or slightly less
>of most stunters.

All we been getting around here is wind from the east. that means it rolls over a very large set of Railroad tracks that arebuilt up on a mound about 30 feet in the air about 100 feet east of the circles. They are also lined with trees that are just as tall. This creates a serious amount of rolling turbulance even in the smallest of wind. 1 to 3 MPH can be the worst conditions. The kind that can slap your plane around 3 feet in level flight. Care must always be taken when flying in winds from the east. getting good practise this time of year is tough. This type of turbulance makes practising for accuracy tough. I have been flying the plane in 8-15mph winds from the east. 8 and it really seems good and handles the wind with some bobbles here and there. 15 and it is tough. See the wind tricks you. Below 45 you do your manuvers with the wind from the SE above 45 you so your manuver with the intersection dead east. This makes it very tough to nail the wingover for sure. The winds should start turning west soon and I will know for sure just how bad this turbelant wing really is.

>
>My experience with high A.R.s has
>been that they have a
>much greater tendency to wind
>up in maneuvers. They
>can be spectacular performers in
>calm to ideal conditions but
>when the wind blows they
>start chasing their own tails
>in consecutive maneuvers. Careful
>attention to wind placement and
>careful seleciton of props and
>power delivery become much more
>critical.

This is where the 65 comes in handy and some really good props are in demand. So far the usual stuff seems to be working well as far as speed up and the ability to control the plane but I am always searching for more.

>
>Let us know what you think
>after some of your well
>known Texas wind days.

I will let you know how it turns out.

>
>Ted

What about a wing fence or a tiplet that extends past the wing to the rear? My thinking here is a wing fence on just the leading edge just past the high point even with tip rib will help trap air on the wing tip and clean up the flow. The tiplet like you see on large airliners would help keep the flow even across the tip all the way aft the flap just a bit.

Just thoughts.

>I think Serge suggested the CG
>might be aft of the
>Aerodynamic Center (or center of
>lift, if you prefer) because
>of your description of the
>CG location at the fuse.
> Once again, I caution
>that common practice (and pragmatic
>reality) requires that we make
>our observations regarding wings based
>on the Mean Aerodynamic Chord
>(MAC) which, for our purposes,
>can usually be considered as
>halfway out the panel.
>Thus, if your ship has
>a significant taper to the
>leading edge it is entirely
>possible that the CG in
>relation to the Aerodynamic Center/center
>of lift might be right
>in the ballpark.
>
>I bet if you measure the
>length of the chord at
>the MAC and check the
>location of the CG at
>that point it'll work out
>that the CG is right
>around 24-26% aft of the
>leading edge at that point.
> Check for us and
>see if that isn't about
>right. I'll go out
>on a limb here and
>guess you'll find the length
>of the chord at the
>MAC to be about 11-3/8"
>long and the CG will
>be located about 2-3/4" to
>2-7/8" aft of the leading
>edge.

Well how did he know that? At mid length of the wing panel the chord is 11 3/8" at the CG is at 2 7/8". Pretty much spot on.

I know you have said it before but how did you go about figuring this out?

Cool!

Mr. Moon,
Is this "new Bear" the same size as the larger-sized Bear that Bob G. Flies?
Sunday, I saw one of your kitted Bears fly (and quite well!), but it appeared about 90% sized of the one you describe.
Keep up the good work!

Ted-

Sorry, forgetting that 1/4-chord lines are often swept on stunters, I should not have chimed in with such an absolute statement. I just looked again at Doug's posted photos this morning, and the root chord surely is a poor gauge for where things fall along the MAC.

Another thing that is important though is that the MAC position is weighted by area (involves integral of c^2 dy), which shifts its position inboard toward the area of greatest chord for tapered wings. For instance, it would only be 1/3 of the way out for a triangular wing. With that in mind, I thought I'd try a computation of Doug's MAC, its position, and where along it the c.g. lies. I used your data to extrapolate a tip chord of 9 5/8" but had to assume (oh, Boy!) symmetrical taper, for want of better info. Of course this is a very important assumption which can invalidate everything if it is very far off. My taper includes flaps, another uncertain area for me because of the discontinuity of airfoil surface at the hinge line.

(It remains a mystery to me how spanwise high-point progression and sections pinched to straight hinge lines affect all of this in reality. Doug's high points SEEM to be aligned spanwise, making the chord-wise point of maximum thickness further forward near the tips than at the root. Does this actually move the c.p. forward? I know that a.c. moves forward with increasing thickness.)

Anyway, being lazy, I went to the Palos Verde R/C site, where they have an MAC calculator that I had tested a couple years back (It works!). Naturally their program does not "know" about varied wing sections and hinge discontinuities.

here's the URL: http://www.palosrc.com/instructors/mac.htm

Results:

M.A.C. = 11.46"
M.A.C. Distance from Root: 14.94" (47.4% semispan)
c.g. % of M.A.C.: 27.6% - possibly a LITTLE far aft?

For what it's worth.

Sunday morning and Sunday night I was flying the Big Bear. That was the first real session of patterns on it. It is exactly the same evrything that Bob G. has. Numbers and everything down to the smallest detail. They are as close to identicle as I could get them. Only difference is his weighs 57oz and mine weighs 59oz. Were you the one that watching while your wife was in the car waiting? Because the tail is so large it doesnt look quite as big as it really is. When I put it next to my kitted Bear it is much larger span wise but the fuse length is only about 2" difference because my kitted Bear has an extended rudder.

The Bear we saw was from one of your kits being flown at Ned Brown Woods near Chicago by Dale Josephson. Dale was putting it through some of its initial flights with a Jett 50 for power.
He had mentioned that Bob G. had upscaled his version by about 10%. So when reading this thread we thought that you might be flying something close to a 110% Bear. And you are!
Nice kits, by the way.

>>
>
>Well how did he know that?
> At mid length of
>the wing panel the chord
>is 11 3/8" at the
>CG is at 2 7/8".
> Pretty much spot on.
>
>
>I know you have said it
>before but how did you
>go about figuring this out?
>
>
>Cool!
>
>DMoon

Hi Doug,

While I'm tempted to ignore your question and just let everyone think I'm some sort of stunt pyscho...er, uh, psychic...it was really pretty easy. If the span is 63" and the area is 714 squares you simply divide the span into the area and the result will be the average chord.

As Serge suggested, the average chord isn't precisely the same thing as the MAC but for wings with modest taper such as most stunt ships it is so close as to make the distinction moot. If it was a delta as Serge mentioned it would make a big difference but for a typical 70 t0 75% taper ratio stunt ship the difference is fractional inches.

The CG location I predicted was simply my pragmatic method of determining a "good" starting point for CG at the MAC derived by dividing the tail area by the wing area. In your case you told us that the tail was 26% of the wing. My experience and data collection over a lifetime of stunt design observation has lead me to believe that ideal response for a normally configured stunter is going to result from a CG at an MAC location within a percent or two of the ratio of the stab/elevator area to the wing area.

Your description of the manner in which the airplane flew was pretty much exactly what I'm looking for in a ship which is well balanced in pitch response. So much so that I was confident in "prognosticating from afar" about the CG location. Your tail is 26% of the wing area; the MAC (average chord to be absolutely correct) is 11-3/8"; 26% of 11-3/8" is just under three inches; a CG at 25% MAC is pretty much co-located with the center of lift which essentially eliminates negative pitching moment from that source. Ergo, it was a pretty safe bet that your CG was between 24 and 25% of the MAC.

Don't for a minute think I'm suggesting this is some sort of NACA WIND TUNNEL DERIVED ABSOLUTE! Just the result of careful observation of many well trimmed stunt ships over a lifetime of being interested in such things.

I can only reitereate what I've said in several articles in the past. You can use this ratio as a starting point for CG location on just about any stunter with confidence you will produce a good flying plane. Your personal preference may result in minor changes foreward (or even a tiny bit aft if you fly a ton or have the reflexes of a young Paul Walker), but if you feel it is way off you may want to consider whether or not the perceived problem isn't the result of an ill considered handle line spacing (control sensitivity) rather than an aircraft balance issue.

I was mistaken. I though you meant you were at my field watching us fly. There was a guy watching from out of town and talking stunt and stuff. My mistake I am sorry.

Thanks for the compliment! UHP has sold 35 of those kits and they are really nice.