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Unequal inboard/outboard panel length v equal wing panel len...

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PeterH · Aug 06, 2001 09:42 PM

Unequal inboard/outboard panel length v equal wing panel length#0 source
Dear folks,

I have drawings to several veteran c/l stunters that I intend to build. The design fashion at the time they were designed was to shorten the outboard wing panel by a couple of inches compared to the length of the inboard panel. Discussions of authenticity aside, is there merit to building as the drawing shows, or does forty-plus years of experience suggest that equalizing panel length, most likely by stretching out the short one to match the long one, is a good idea? If I persist in what some might see as heresy, are there any compensatory 'rules of thumb' as applies to other adjustments?

Many thanks.

Peter Havriluk

LNeumann · Aug 06, 2001 11:06 PM

RE: Unequal inboard/outboard panel length v equal wing panel length#1 source
I happen to like equal panels as it allows more tip weight which gives better tension overall--including the overheads.

There are a number of people who still prefer some assymetry and a number of modern designs that use it. It has been found that half to three quarters of an inch is all that is needed, however, and the 2 inches (and sometimes a LOT more) of assymetry used in some designs of the past causes all sorts of trimming problems and "banging" in the corners.

However, for authenticity's sake you will need to build as per the design. Some have "bent the rules" a bit by adding a "trim tab" to the outside wing, and I suppose a little bit of "mismeasuring" can go a long way, too.

Leonard Neumann

Scott Riese · Aug 06, 2001 11:11 PM

RE: Unequal inboard/outboard panel length v equal wing panel length#2 source
PETER. The Old time and classic planes were thought up when stunt flyers were trying to get the upper hand. Engines were less powerful and reliable. The outside wing tended to be shorter to compensate the lift of the longer inside wing. Moderen PA plane also have this unique feature,But not as much difference in lenght of wing panels(not all pa have the offset).
IF it were me I WOULD build the way the plane is desinged. Enjoy the past. These planes are speical. Classic is fun to watch and fly. Old time even more. You'll be surprized on how well they fly!

Ted Fancher · Aug 07, 2001 12:39 AM

RE: Unequal inboard/outboard panel length v equal wing panel length#4 source
NO. 1, for sure, if you're building a classic or OTS ship for competition, follow the plans. The rules say so and your competition will likely remind you if you don't.

Having said that, however...

there is value in moving the fuselage outboard on the wing. I kind of like that way of expressing it better than saying one wing is longer than the other. The old timers were quite right in their assessment that because we fly in a circle/sphere, the further from the center of the circle a given segment of wing is the faster the airspeed. Since lift increases as the square of airspeed (as does drag!), every unit of wing will produce a different amount of lift depending on how far from the center of the circle it is. Thus, the "center" of lift is not located at the geometric center of the span.


As a result, unless some compensation for the difference in lift is made the wing will always fly with the outermost tip higher than the inner due to the fact it is making more lift. Since we can't equalize the speed difference, we must somehow adjust the lateral center of gravity to a point in line with the lateral center of lift.

It is important to realize that we can achieve that equalization in several different ways...each with positive and negative consequences. The easiest way (as Leonard suggests) is to simply add weight to the outboard tip until the two forcs co-incide. Two good things result, the lift and CG are aligned leveling the wings and the airplane looks better because it is symmetrical...both wings are the same.

One bad thing results, you've got to add a lot of weight and the gross of the entire ship goes up as a result. If you are of the opinion that three or so ounces of dead weight is disadvantageous you might want to look elsewhere to solve the problem.

The lateral CG can also be brought into line with the Lateral center of lift by merely moving the fuselage and its weight outboard until you achieve the desired result. Again, two good things result: the two forces are in alignment and, this time, you've saved three full ounces of weight? Of course, it does look a bit weird although not so much as you might think since many many beautiful airplanes from the Classic era were built with as much as two full inches difference. Sounds like a lot but when you think of it as moving the fuse outboard only one inch on a 54-60+ inch span it is actually fairly modest.

However, we're not done yet! Another bad result is a potential for a very serious problem on take-off. By definition, when the fuse is offset enough to move the lateral CG outboard enough to balance the center of lift its centerline will actually be slightly outboard of the lateral center of gravity. With a straight ahead thrustline this will result in the thrustline passing outboard of the center of gravity and causing *moment* about the CG which tends to turn the ship into the circle. Once airborne and at speed this isn't much of an issue. On take-off, however, that moment can be enough to make the ship light on the lines and take-offs in bad conditions can become a problem.

The solution is to have the engine offset enough to insure that the thrustline is at worst in line with the lateral CG thus resulting in "0" moment and a straight ahead T.O. roll.

Many designer, myself included, opt for a compromise on these realities (much like Leonard suggested). We use a smaller amount of assymmetry coupled with a lesser amount of tip-weight and little or no engine offset is necessary as a result.

It may be valuable to realize that there is *no* correct amount. As I've demonstrated, you can achieve a proper trim with either of the extremes as long as you deal with the special requirements of each. The compromise approach is just that, a compromise and flight trim will determine the precise amount of tip-weight required to compensate for whatever compromise assymeetry one selects. for what it's worth (probably not much), I use about 5/8" fuse offset (1.25" assymmetry) on my 60 inch span ships. I use 1.5 degrees of engine offset and adjust tip weight through flight trim trial and error...it ends up around one to 1.5 oz depending on such things as aircraft weight, line length, line diameter and line sweep.

whew! that ought to be enough.

Ted

Scott Riese · Aug 07, 2001 01:13 AM

RE: Unequal inboard/outboard panel length v equal wing panel length#5 source
DANG Ted what a super responce. I wish that I could of been there went these planes were flying. I've ONLY been flying stunt for four years, and I want to hear all of the stories from the past. For those who have'nt seen TED fly the chilizer you've missed alot.
Someone today asked me the BEST stunt flight that I've seen. Sure the B-17, But TED'S flight with the Chilizer at the 2000 NW Regionals was the best I'VE ever seen. Scott(xoxox)Riese

Peter Hess · Aug 07, 2001 08:05 AM

RE: Unequal inboard/outboard panel length v equal wing panel length#6 source
>DANG Ted what a super responce.
Ted:
I agree with Scott. Thanks for a very concise, clear, and helpful discussion of this subject.


LNeumann · Aug 07, 2001 09:03 AM

RE: Unequal inboard/outboard panel length v equal wing panel length#7 source
One nice things about these forums is the ability to share different approaches to the same problem, and then you can make your decision as to which way you wish to travel. And neither side is completely wrong (unless you get way overboard on assymetry, and then you are on your own.) As Ted says, it is a compromise, no matter what you do.

And, as Ted mentions, he uses assymetry, and he has done quite well with his flying using that approach. And David flies the same plane, so... But as Ted once said (I am paraphrasing now) so many of the top planes are so well designed that any of the top flyers could probably swap each other's planes, get in a few practice flights, and do very nearly equally as well. (Did I quote that pretty close?) Still, there is nothing like flying your own plane. Like a comfortable shoe you know what it can and cannot do.

But we are never done trimming. Never! Matthew discovered this past week end, after some 800 flights or so on his current plane, that he had too much tip weight in. Todd Lee had noticed a slight tendency to hinge on the bottom that neither one of us had noticed.

On the subject of tip weight, however, we aren't carrying that much. I guess we will have to go back and weigh this one "just to see", but, as Ted said, you do add by trial and error to get it right. And it will vary with line length, line rake, aircraft speed, and so on, in addition to design. And when you make changes in one area, you will often need to go back and make changes in others. And Matt has currently gone to a larger prop and increased the turn rate on the plane, all of which will affect other things.

We measure tip weight, not by the amount we add, but by holding the plane upside down by the prop and the tail wheel and placing the outboard tip on a gram scale. Since you add a tip weight box to the outboard tip (I have built some boxes so heavy I didn't have to add ANY weight to them) and have a leadout guide and leadouts in the inboard wing, plus the differences in material weights used in the construction--and, maybe even, size--the weights of the individual panels are different anyway. And flaps, well, that is the next story.

I do remember on the Stuka 97, we weighed it this way after 400 or so flights when we had it pretty well in trim, and it came out at 1.7 ounces of additional weight in the outboard tip--this with equal panels.

I agree with Ted (and others) in their assessment that the outboard wing, flying faster in the circle, will develop more lift. Such additional lift will depend on line length, wing span, etc., but I calculate the "average" difference to balance this out to be about 3/4 of an inch if you are adding assymetry for this reason. Since we are, also, trying to balance out the weight of the lines, some people will go for more. It sounds like an easy solution to keep from adding additional tip weight, but...

Tip weight can be beneficial. When you put a rock on a string and swing it, the rock swings to the outside of the circle. If you put an airplane on the line and weight in the tip and swing it, the airplane will hold out. You can swing the rock overhead, and with enough momentum it will hold straight out (up). What would happen now if you put the weight on the inboard tip of the airplane and swung it in the same way? Building the inboard wing longer and adding the leadout guide and leadouts, etc. does just that. It is minor, admittedly, very minor. But it is there.

But there is one area where assymetry does not work. Since it is supposed to be the additional lift that balances things out, what happens when the airplane is pointed straight up? We go into the wingover, squares, etc., and the plane is pointed straight up, and suddenly there is no lift balancing out the differences. And the inboard wing weighs more than the outboard wing. And the line weight is still there pulling down on the inboard wing. And line drag is now added to all this pulling down on the inboard wing. And there is now no lift difference and no tip weight to compensate. Did you ever wonder why some airplanes get loose in certain areas of the pattern?

Line drag is, also, a factor that needs to be reakoned with. And sometimes in strange ways. One thing that Ted did not mention, but that he and a number of others have discovered, is that, ragardless of which style you choose, you should have more outboard flap than inboard. This can be accomplished with longer flaps outboard (if you don't take them all the way to the tip) or with a wider tip in the outboard flap or adding a "wart" as Ted did to the outboard flap.

The purpose of this is to help prevent the "banging" in the bottom corner of the maneuvers (mostly the triangles and hourglass where it is the most noticeable). Different theories as to why this becomes necessary have been presented, but I think it happens primarily from line drag.

Since it is most noticeable at the bottom right corner, I will explain it from there. When the airplane is coming straight down, gravity is working on the plane, there is no lift on the wing to compensate for anything, we have tip weight on the outside, but, most noticably, we have drag on the lines pulling the inboard wing UP! Since the leadout is moved back to compensate, this pull has no effect and the airplane flies straight down. (That is the reason we need to move the leadouts back, and by an amount that will vary with airplane speed and line thickness and line length.)

Now, when you snap the corner hard, think of what happens. The plane rotates rather quickly, but the lines don't immediately catch up. The plane that was aiming straight down is now aimed to the left and ready to move in that direction, but the lines are still pointed up from the drag and momentum and all. Our leadout guide that was aimed back to compensate is now ineffective--for just a brief moment--but in that moment the line drag pulls the inboard wing up and the outboard tip then drops.

So, if we add just a little more flap to the outboard wing, the added lift generated will compensate for the pull of the lines, and we now eliminate (mostly) the tendency to bang in the corner. Of course, if we still have too much tip weight, the outboard wing will drop. Or it will rise if we don't have enough. We still need to trim, but this is just one more tool to help.

And you need that extra flap area whether you are using equal panels or lots and lots of assymetry. The same forces are still at work. The amount needed will vary, however, with design, speed, etc.

And in the rounds, the forces are less, but so is the amount of flap deflection, so this still works.

And, by the way, the "banging" affect mentioned above is at work in all corners, but the forces of gravity affect the plane differently, so it is most noticeable in the bottom right corner.

There, more than you asked for, but a lot to chomp on.

There is a lot of agreement, but, also, a lot of diversity in this hobby. We chose our weapons, and then learn to handle them. And each chooses what is most comfortable to him.

Leonard Neumann

Mikey · Aug 07, 2001 03:39 PM

RE: Unequal inboard/outboard panel length v equal wing panel length#8 source
Leonard,
I have used both equal and unequal wing panels on models for as far back as I want to remember. I have had great results with both. There is a slight advantage going to the unequal panels. Uncle Teddy is correct in his statement that the outboard wing is traveling faster than the inboard. Doing the math and considering that a model is traveling a radius of 70’ and the model has a wingspan of 60”. It was easy to calculate the difference between the inboard tip and the outboard tip in the rate traveled and then covert those numbers into mph. The velocity difference between the centers of the inboard and the outboard flaps is only 1.8 mph or less than 3%. I do not think we can even measure our lap times that accurately with a stopwatch. However, there is no doubt that there is an effect on the model. The unequal wing panel models I built had equal span flaps with the tip of the outboard flap 3/32” wider. The wider outboard flap tip allows you carry slightly more tip weight.

One other thing to consider, and it may have more of an effect than the shorter outboard wing, is how the relative wind is acting on the model. Since we are flying in a circle the relative wind is actually coming from an angle about 3° over the inboard wing panel. This may blank out a portion of the flap at the root end of the outboard wing and thus cancel or at least limit any excess lift in the outboard wing. Hmmmmm1 I am getting a headache. What we really need is young aerodynamicist to test these models and theories in a wind tunnel (yea! right, file that under never).

This is a great discussion and merits the time and thought put into it.

Mike

currell · Aug 09, 2001 10:54 AM

A quick calculation for panel lengths#10 source
Ted: Thanks for your part of this thread.

As a quick back-of-the-envelope calculation, I tried to figure out what the IB/OB split should be to get equalized lift. Starting with the statement that lift is related to the square of velocity, I assumed, for reasons of simplicity, that the aerodynamic center of lift is right smack in the middle of either panel. (This is not strictly true, but it would be fairly close).

Using your hypothetical model as an example (a 60" span), I assumed 70 foot lines. So the two aerodynamic lift points are concentrated 71 feet 3 inches (IB) and 73 feet 9 inches (OB) away from the handle.

So square both of these, and divide: (73.75 x 73.75)/(71.25 x 71.25) = about 1.07, meaning: the outboard panel would create about 7% more lift than the inboard, if they both were equal.

Now, set up a ratio equation:
X/60-X = 1.07

What you end up with is about 31 inches for the inboard, 29 inches for the outboard. Two inches difference. Not terribly distant from what you use.

I know that other factors come into play, and some of my assumptions are approximate, but if someone is designing a model around 60" span from scratch, and wanted unequal panels, these values could serve as a starting point. currell

LNeumann · Aug 09, 2001 01:48 PM

RE: A quick calculation for panel lengths#11 source
There are a number of aerodynamic factors that come into play that leave the simple equation a starting point, but not a conclusion. If we have a tapered wing, the center of lift on the two wings is going to be closer together. If we have a tapered flap, the center of lift under deflection is going to be coming closer together (this is a good thing). If we have a flap that is less than full span, the center of lift is going to be coming closer together. If we have part of the outer wing blanked by the fuselage because of the constant inward turn, then part of that outboard wing flying faster may not be flying at all. It is these differences that bring the actual difference much less than the original equation.

When I ran my own figures through the equation and worked all the factors I could think of along with it, I came up with a difference of 1/2 inch to 3/4 inch maximum.

As has been pointed out, however, longer than necessary inboard panels and equal panels can all be made to work. But the process of trim and the amount of tension you have on the lines will vary, and that is also a part of the choice that each modeler must make for himself.

Leonard Neumann

Preston Briggs · Aug 09, 2001 02:01 PM

RE: A quick calculation for panel lengths#12 source
>If we have
>part of the outer wing
>blanked by the fuselage because
>of the constant inward turn,

I wondered about that, but have always
figured the area near the root was hugely disturbed
on both wings due to flow around the fuse and vortices
coming off the prop.

Preston

Howard Rush · Aug 07, 2001 12:38 AM

RE: Unequal inboard/outboard panel length v equal wing panel length#3 source
I have been building combat planes with equal-span wings, but with more tip chord on the left wing than the right. I have pretty much decided that this is a dumb idea, but heck, it's an excuse to make twice as much tooling.

mogren · Aug 07, 2001 09:26 PM

RE: Unequal inboard/outboard panel length v equal wing panel length#9 source
I think that tip weight's effectiveness changes with speed. IE. the faster you turn the less # you need to get the tip to hinge or not. The faster you spin the rock on the string, the harder it pulls. Same in a loop. If you loop at 45mph, you need more than at 75. I trim my combat planes to just hinge a bit at 70. When I retire them and bolt on a Fox 35 to play with or train kids, they need a lot more tip #. They go from 6 pennies(15gr.) to 30gr. This is with 3/4 to 1" span diff on 44" plane. (22oz total)
The wing lift should be more linear, more speed doesnt change the the % from inboard to out board much. If you fly at the same speed all the time , the trim shouldnt change.
Damage to the wing, wrinkles, tears. etc cause a big diff in tip # needed. I dorked a plane , wrinked the covering on the outboard panel and it hinged so bad I tore the tip# out for the next round.
The roll at the hard bottoms talked about prior is most likley the combination of drag, gravity on the lines. The lines want to continue to the ground and pull the plane with it, thus the inner tip drops and strts the death roll in to the circle. At the uppers, the gravity stops the lines faster. Try the same craft on lighter lines and it will diminish. The combat craft with surface mount leadouts do the same thing at the bottom of wing overs. I turned my engine upright to reduce this ( by moving the vertical CG) and got the retro look besides!!!
The best thing about this sport is that a bunch of crafts can be designed and all can fly fairly well. The common #s seem to be a decent airfoil, light weight, proper Cg, linear stiff controls, and enough power.
The best ones fly straight when you need em to and turn exactly the same way each time, combat or stunt.
IMHO .03 MM