research,, the answer depends on what YOU want from your plane,, its a package,, no single component of design can be altered without affecting others,,

spend more time using the search engine feature,, the answers there,,, and less starting new topics,,

the REAL problem is,, what you want when you design depends on these other factors,, so if someone tells you "make it longer" then it begs the question,, what about the other parts of the design,, if you want to design,, then study,, read,, look at lots of plans,, then figure out what you want,, and make your desicion based on what YOU want

Chris,

Thanks for that, it's exactly what I was looking for.

Here's where Chris' link goes, I copied this exceptional read for the benifit of anyone interested in model design. As I am.

Couple of items Ted Fancher mentions in his article that relate to my designs. One is wing speed and how lift relates to a wing traveling in a circle.

Both my scratch built designs, the ARGO and the Mig-3, have larger inboard wings. Nothing excessive, but some. The ARGO, has more. Weight box in every model as an added feature and adjustable lead outs.

Second topic Ted Fancher mentions is fuselage "offset." I do this with all my models, at least 1 degree or 1.5 degrees.

All engines are offsete one degree with my designs also.

Thanks to the article provided by Chris, and Ted's comments in this article, I'm sure I will be confident with my new wing design AND the rest of the model that goes with it.

Thanks Chris

Thanks Ted, exactly what I needed, it's a jewel!   

Here's Ted Fancher's article.
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           "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"

 

Geoff,

Thanks for the reply.

I understand most of that fully, all the models I design and scratch build have assymetry.

What I don't understand is the concept of having both flaps the same size with assymetry?

One would think the inboard flap would be larger, if only a half inch. What it would be in my case with a 52" span wing. The inboard wing is only .675" larger.

I'm staying with the one degree fuselage offset and one degree with the engine. Others do this?

I'm also considering dialing down the size of the model to 48" instead of 52"-55".

Thanks for getting me thinking.

Charles

What I don't understand is the concept of having both flaps the same size with assymetry?

Unless I am misunderstanding something here the point of an asymmetrical wing span is to make the effects of that lift symmetrical in LEVEL flight whereas in maneuvers you don't need added asymmetry.

If you had added lift asymmetry in maneuvers the model would roll necessarily.

Unless I am misunderstanding something here the point of an asymmetrical wing span is to make the effects of that lift symmetrical in LEVEL flight whereas in maneuvers you don't need added asymmetry.

If you had added lift asymmetry in maneuvers the model would roll necessarily.

What Howard said.  It is definitely not just for level flight.

   I would also note that as long as the asymmetry is in the ballpark, you can trim it to about the same overall performance, if you know what you are doing.

     Brett

What Howard said.  It is definitely not just for level flight.

   I would also note that as long as the asymmetry is in the ballpark, you can trim it to about the same overall performance, if you know what you are doing.

     Brett

Perhaps I was not clear back there, I view wing asymmetry as a balancing tool for level flight taking into account the weight of the lines as well as any speed differential in level flight.
(That was to the original post.)

But ADDING to that already determined amount of wing asymmetry with different sized flaps seems not to be needed.

I was addressing the flaps here viz -
"What I don't understand is the concept of having both flaps the same size with asymmetry?"
(That was to the above quote and a separate subject.)

What is the school of thought in F2B with different sized flaps? I take it that the flap areas should be the same despite the wind areas being unequal.

Perhaps I was not clear back there, I view wing asymmetry as a balancing tool for level flight taking into account the weight of the lines as well as any speed differential in level flight.

Maybe you weren't being clear.  You prefaced your statement with "Unless I am misunderstanding something here" .  You are, but it appears that you meant that as a rhetorical device, rather than as a possibility.

It is very easy ... try to trim the model to have optimal tip weight in round figures ... then retrim model for squares, you will see that optimal tip weight is not the same. Usually if model has symetric flaps, you will need more tip weight in round figures and less in squares. Little extra outboard area (small tab on outer flap) will carry that extra tip weight optimized for rounds also in squares, that is how I trim size of that my tab ... it is simply another trimming tool. And it will also tell you how much flap asymetry it needs :- ))

The trick is, that effect of that tab size (or flap chord asymetry) is only small if flap is in neutral (practically nill tip weight difference ... ratio = tab chord / wing chord ), little more effective if flap is deflected for rounds and much more in squares (ratio = _almost_ tab chord / flap chord).

I've liked this method for a long time.  I like to make the tab from .030 acetate clear canopy sheet glued into a slot in the flap.  It's almost invisible and can be easily trimmed at the flying field.  I also use equal span wings.  My newest renditions didn't need the tab- the outboard flap is 3/32" wider full span.  Tip weight is just over one ounce.

Dave