C.G.Question

[Marion]

I have a biplane with identical upper and lower wings. They have 6 inch cords, and are spaced vertically 6 inches apart. The lower wing is staggered 1.5 inches aft.

How far aft of the leading edge of the upper wing should this plane be balanced?

I have to note the horiz stab (including elevator) is smallish on this semi scale Sopwith Camel.

[Sail 'n Soar]

Assuming the wings are of equal span, 3" back from the top wing leading edge will put you at the combined quarter cord. ~3.3" back will be at 30%. I'd suggest balancing someplace between the two.

[steve lewin]

What am I missing ? Surely if the horizontal stagger is 1.5 in then you have a combined chord of 7.5 in. The 25% point is at 1.875 in from the front wing LE, the 30% point at 2.25 in.

Steve

[Marion]

Hmmm..... I get a diferent result than either of you. If I did it right, the 25% is at 2.25 inches aft of the top wing leading edge. When I figure out how to add a drawing, I'll show you how I came to that conclusion... :confused:

[Sail 'n Soar]

Don't know where my head was with my previous reply. Basically, for a constant cord wing a good place for the CG is 25% to 30% behind the LE. For the 6" constant cord wing that is between 1.5" and 1.8" ifrom the leading edge. For two equal span constant cord wings, the CG should be at the average position of the two. That will be at 1/2 the 1.5" stagger, or .75". Adding .75" gives 25% at 2.25" and 2.35" for 30% measured from the top wing leading edge. :o .

Basically, previously I forgot to divide the 1.5" stagger by 2 to get the wing average.

[Sail 'n Soar]

What MarionDK said :o

[Marion]

Steve ain't no dummy, and he got a different answer than I did. Are you there Steve?? Help me understand this properly, please.

[steve lewin]

Unfortunately aerodynamics is far from my strong subject ;(. For many years I've done it by projecting the 2 wings onto a surface and calculating the CG as though I had a single wing. E.g. 6" chord, 1.5" stagger looks like a 7.5" chord wing from above. However that gives you real problems if the wings are not identical and the more I think about it the less I like that method.

I think I prefer the idea of effectively calculating CG separately for each wing then "averaging" it. Presumably where the wings are very different areas e.g. like a Nieuport 11/17 then you should average with regard to the relative areas. I suspect my version only works because if anything it's a safer option for conventional biplanes, giving a more forward CG than really needed.

Steve

[Sparky Paul]

I went thru the exercise of finding the c.g. for a biplane with another aeroguy on another forum, and we failed to agree :(
I use the plan view method. All the wing you see counts. That which is covered up doesn't.
I haven't been disappointed yet.
Research found nothing on the subject on the 'net.
Almost got to the stage of building a flier to test the theory, but the plane looks too goofy to proceed with..
The chuck glider flew OK though..
The idea is to see where the flying c.g. is with a monoplane, a no-stagger biplane, a full-stagger biplane.
The glider liked the plan view points.
The other idea was to use only one wing's area, positioned at the average location for both. ???

[Marion]

Soon as the weather permnits, I will go fly the model and see how it performs. I will set the c.g. per my drawing, between 25 % and 30% -- depending on how my batterty packs makes it balance.

Thanks to all for the ideas posted here.....

[Sail 'n Soar]

Saprky,

The average position relative to the two wings makes more sense to me from a phenomenological basis as long as the wings are on the order of a cord length or greater apart. While the what-you-see-is-what-you-get approach probably works because it comes close to the same area (moves the CG slightly forward - more stable as long as the horizontal isn't stalled), the only thing I see supporting it is if the two wings are very close together they may start looking aerodynamically like a single slotted surface, which could also result in the what-you-see approach coming closer to the effective wing cord. Another problem I have with the planform approach is there is no impact at all of the relative area or cord of the lower wing.

Gerry

[steve lewin]

Yes the relative areas thing is what is putting me off the pure plan view approach. For sesquiplanes like the Nieuports it just doesn't seem to make much sense.

OTOH I've never actually believed in any form of CG calculation in the sense that I expect it to be perfect. Any calculated CG is just a place to start and the old plan view seems to have worked pretty well over the years. OTOH I'm not sure how well it works for extreme stagger (approaching tandem wings) or reverse stagger. I've never tried either of those.

Steve

[Sparky Paul]

The lower wing on the Nieuport contributes almost nothing to the plan view area, and can be ignored.
MOF, my biplane Kadet uses a Seniorita wing for the lower, set so its trailing edge is on the same line as the upper wing so it's not visible in the plan view.
The plane will fly with it, or without it, the only difference being the weight of the wing itself. I don't adjust the c.g. for either condition.
With it and the flaps down, the plane will turn in just about its own span! Without it, it's just another Kadet.

[mosquitowings]

Hi guys, what about swept wings like on the tiger moth? Do You just average the distance from center LE back to tip LE and move the CG back to that point or what?

Jim

[Sail 'n Soar]

Hi guys, what about swept wings like on the tiger moth? Do You just average the distance from center LE back to tip LE and move the CG back to that point or what?

Jim

That's basically it for a constant cord wing. For a tapered wing the wing aerodynamic center will be biased toward the larger cord location, typically the root cord. Martin Simons', "Model Aircraft Aerodynamics" offers a simple geometric method of finding the aero center of swept and tapered wings.

[BMatthews]

Sparky, nice test glider. I often wonder why folks just refuse to cut a bit of balsa and do what you did. It just makes SOOOO much sense.

I also like your rule of thumb about the exposed plan view of the area. Seems to make perfect sense or certainly it's close enough. Well done.

And... er....[hushed voice] you forgot to change the sticker... [/hushed voice] :D

[Sail 'n Soar]

Sparky, nice test glider. I often wonder why folks just refuse to cut a bit of balsa and do what you did. It just makes SOOOO much sense.

I also like your rule of thumb about the exposed plan view of the area. Seems to make perfect sense or certainly it's close enough. Well done.

And... er....[hushed voice] you forgot to change the sticker... [/hushed voice] :D

Sparky, I believe in scrap stuff flight test models, but yours appears to be a stacked deck. Just noticed your horizontal stab is 25% of the total wing area. Using your 15" -Ceff/4, the tail volume you get no matter how you calculate the equivalent 25% cord is going to be a stable flyer - since you should still use 4" cord for the wing moment calculation.

[Sparky Paul]

I cut the stab area down to 15%... 3x7..
With the 144 in wing, it made no observable difference.
Drew up a flyable plane using a couple of my Air Cruiser wings... 9x48.. but it looks just too goofy to try to assemble. A realistically lengthened fuselage gets too long.
There is the opportunity to investigate several configurations though..
Monoplane
Biplane, no stagger
Biplane, full chord stagger lower wing
Biplane, full chord stagger, upper wing..,
Monoplane, both wings in line.