Vintage 118 inch Viking glider
 

A friend brought out a 118 inch Viking glider that was built 40 years ago and had survived well, we would like to fly it and he says he still remembers seeing it fly but it appears to be very tail heavy at a 25% C;G it takes 12 ounces of lead to bring it anywhere near balance, so does any any one have any info on this plane Mike

Mike, 25% is way to forward for a sailplane. Try more like 35% to 40%.

I'd agree fully that 25% is way too far forward. Hopefully someone that flew a Viking back in the day will chime in with a proper number. In the meantime perhaps run the model dimensions into one or more of the online CG calculators to get a better idea.

Generally for contest gliders you'll want to run the calculations at a little lower range of stability margin. I'd run the calculations for a 7% and a 2% stability margin to get a good idea of the most forward balance and the most rearward and use that as the range of optional balance. The final location is best determined from flying. Rusty old timers or lower hour pilots will want a little more stability at first until they sharpen up as the air time rises. All in all the further back the balance the more efficient the glider is. But there's a trade off where you reach a balance between airframe efficiency with the rearward balance point and the ability of the pilot to fly it smoothly and well when it's just a dot in the distance. Some amount is still wanted to aid on that.

Or the new fangled auto stability modules with fancy algorithms can be employed on a model that is set up to be much closer to neutrally stable.

Thanks guys , we fly 3 Bird of Times and several other Gentle Ladies etc and pack them up on top of a Telemaster , I was going by the C/G on those to get some idea of where is should be, I will move it back a bit and so some hand launches before we take it up , did you get hit with the winds yesterday in Chilliwack ? It just brushed us here on the Island Mike

Please excuse me for hijacking this thread, but I'm a little confused about this percentage thing. Where exactly is the "XX% from the leading edge" supposed to be figured at? If the LE is swept back (or forward) or even tapered that XX% is going to be in a different place at the wing's tips than it will be at the root.

Mike, as rule of thumb I set up my sailplanes so that the elevator trim will not change regardless of speed. If the CG is in the forward range then the sailplane will carry some up elevator trim. As speed increases this elevator trim will be more effective and the sailplane will balloon up. Some find this preferable some don't. I have also found that while circling in lift with the more aft CG the sailplane will show signs of being in lift more and will circle in the thermal slower and tighter.


Fly, when referring to these percentages it is the amount back from the average cord. Example is with. 10" cord at the root and 5" at the tip you have an average of 7.5" so the CG at 25% would be 2.5" back from the LE at the point along the span where the cord is 7.5". Note that usually these are staring points and can be fine tuned to suit personal preferences.

OK Flywheel, it can be BIT complicated but here is an easier way to set the C/G and it WORKS. measure the wing chord at the fuse and at the wingtip and add them together, divide by 2 eg the chord is 12 at the fuse 6 at the tip 12+6 = 18 divided by 2 = 9 take a tape and move along the wing until you find the 9 inch width, at that point measure back 25 %( or what ever % you want,) from the leading edge 2 1/2 inches, do the same on the other wing and draw a line between those two points and where it hits the fuse will be your C/G , you may have to fudge at little after you test fly Mike

Or just balance it under the main spare where it contacts the fuselage.

Maui

http://www.rcuniverse.com/forum/atta...mentid=2117701

Hope this helps.

Joe

Actually not that complicated at all, I had no problems visualizing the method. Thanks MM and everyone. And may I Ass-U-Me that the "point on the wing which measures @ the average chord width" would be used regardless of wing 'sweep' (back or forward)?

That seems logical, but I want to be sure. :)

I've got a Viking I built in 1984 or so. Wonderful flyer. I believe I balance on the spar. I'll double check.

Thermaler posted his picture where the cg is suppose to be, and it looks a little behind the spar.
Maybe, sometime next winter I might take a shot at scratch building this beautiful bird.

Maybe get a jumpstart with this Viking partial kit ...
http://www.skybench.com/index.html?h....com/home.html

We balanced it just behind the spar it took 8 oz of lead in the nose, we packed it on top of the Telemaster and hit a bit of a boomer and it was incredible, it just floats he had intended just to just do the one memorial flight and then hang it back up but it flew so well that that he decided to fly it a few more times and it never let us down it is a wonderful glider with no bad habits Mike

I'm not at all surprised. They were a floater and great flyer back then. No reason why they won't fly well now.

Why hang it up? There's no reason why an RC glider can't hang from a "thread thermal" between flying days and still put in lots of air time on the weekends. It's not like there's a lot of oil to ruin it.

Hey Flywheel, if you read this did you figure out how it works? There's a term and method for finding the Mean Aerodynamic Chord (MAC) which is largely done with geometry. It takes into account any wing taper and wing sweep to arrive at an equivalent wing chord and position as if it were a simple straight constant chord wing. It's this MAC that you want to use for locating the balance point on a swept wing. You then extend the line in to the fuselage where you're likely going to put your fingers.