wing/fuse ratio

[flyingdogtwo]

:( I posted this query in different forum. I have to apologize, cuz, being a new subscriber I'm still learning the ropes at this website. Anyway, I am planning on extending my wing length on one of my 2M sailplanes from 79" to aprox 110" I read somewhere about the length ratios between wing and fuse, but can't find it. Was wondering if I would run into any ill effects by lengthening the wing and not the fuse. I talked to a fellow who did this to his "Laser Art's Majestic" and it flew and handled much better. thanks for any input

[Ollie]

Much depends on how you extend the span. From a structural point of view, the bending moment at the center of the wing will be almost doubled. If the 2-meter wing is a two piece arrangement and you add a 31 inch center panel, then the spar caps should be twice the crossection of the spar caps in the two existing panels. The new center panel will also be subject to higher torsional loads.

If the existing wing is three piece ala the Gentle Lady and you add two new panels between the center and tip panels, then the old center panel will be quite flimsey in the new application.

The best glide angle may be improved roughly 37% and the best sinking speed may be reduced on the order of 45% if you maintain an efficient lift distribution by the design of the wing planform and twist. In addition, the sinking speed and stall speed may be reduced by the square root of the wing loading reduction.

The moment of inertia in both yaw and roll will be increased and the damping will be reduced unless you increase the tail moment arm and/or vertical tail area to maintain the tail volume coefficient.

The neutral point will move forward unless the horizontal tail moment arm and/or horizontal tail area is increased to maintain the horizontal tail volume coefficient. When the neutral point moves forward the CG must be moved forward to maintain pitch stability.

Unless the dihedral is increased or the tail moment arm is increased, the spiral stability will be reduced.

Design is a challenge because one change affects so many other things. Almost any configuration can be made to fly. Making it fly better is a much more involved question.

[flyingdogtwo]

:eek: Hi Ollie; I agree with you on all factors. My little note pad here has all kind of arrows, numbers, and words pointing to just about any square millimeter on the plane that would be changed, aerodamnically speaking. I just didn't have the bottom line. I realized up front that it would be squirrelly cuz of the longer wings vs. no change in boom length. But since I only do thermal flying, even with my electric, I figured I could over-come the compensation factor. I was just looking for a few more sq. in. of wing area vs. the heavine ss and less sleek box-style fuse. Very simply put, I've got a few of the same style wing panels not doing anything, thought I'd stickém together and go for it. Thanks again for the input.

[Ollie]

PC Soar will calculate what some of the performance and handling effects are so that you can see them before you actually make the changes. See:
http://my.athenet.net/~atkron95/pcsoar.htm

[BMatthews]

Originally posted by flyingdogtwo
[B..... But since I only do thermal flying, even with my electric, I figured I could over-come the compensation factor. ..... [/B]

:confused: A bad handling model is a bad handling model regardless of where you are flying it so this "I only do thermal flying" isn't being fair to yourself or the model.

There is a chance that it'll work. There are a few models that have gone up in size with success..... or at least a minimum of bad side effects. You're buddy's model is apparently one of them. And Monarch has a 60 inch HLG that grew to a 2 meter and flies well I hear.

I'd cut my risk on a project like this and only go up to 100 inches from the 2 meter 79 inches. You'll get your extra area and the risk of any bad handling will be somewhat more minimised.

Selecting an airfoil that has a lower pitching moment will also reduce the effects of the smaller stabilizer. As a bonus most of those types of airfoils have a lower camber value and will give you a better speed range.

[thepilotbruss]

Generally speaking a fuse on a glider should be between 50-65% of the wingspan. However this amount can be drastically changed by the size of tail. If you use the same size tail on the bigger plane I would go with 65%. If you increase the area of the tail the same % as your wing area increase then I would go with 55% fuse to wing length. You may run into CG problems if you only lengthen the tail and not the nose.
I scratch built a hand launch with a 1/1 fuse to wing length and was able to get away with a very small tail that still had good control.