mpriestley

My Feedback: (1)

I need help men,
I need balancing advise....

I have a biplane (type unknown) with a forward stagard wing on top. I do not have any other information other than what it looks like and what I would guess would be the required engine size/plane size.

it looks similar to a stearman (ie tail/body built of ribs), ailerons on bottom of wing only and it looks like it would require a 61 2stroke type engine. (kinda heavy if you ask me but oh well)

I have put a 3 cyl. Saito Radial 90 on it.

Please help.

FHHuber

Take the view of the model from directly above with it sitting level. However much area of the lower wing shows behind the upper wing... add tothe upper wing. Now use that to calculate the MAC as if it were a monoplane. Set the starting CG at 25% to 28%.

This will work. and have it balanced adequately to fly if the wings are in proper alignment. (normally the top wing is at the same incidence or -1 deg from the lower wing's incidence.)

DBCherry

My Feedback: (4)

FH has it.

Graphics make it easier to "see" (usually).

The first sketch shows what he meant by, "Take the view of the model from directly above with it sitting level. However much area of the lower wing shows behind the upper wing... add tothe upper wing."

The second sketch hopefully explains the rest, "Now use that to calculate the MAC as if it were a monoplane. Set the starting CG at 25% to 28%."

(Unfortunately, the second sketch does not use the "wing planform" that I show in the first sketch, so you'll have to substitute your actual planform. The second sketch is actually based on a low wing monoplane, but it works the same way.)

I know it looks confusing, but take a couple minutes to really look it over.
"Root cord" is the edge of the wing that does (or would) meet the fuselage, "Tip Cord" is the wing tip.
Note the colors in the sketch, and take it one step at a time. It really is easier than it looks.

And once you "see" it, you can calculate the CG of ANY wing without ever looking at it again. Honest!
Dennis-
Dennis-

daveopam

My Feedback: (9)

The plane sound like a Super Sportster Bipe from Great Planes. Can you post a pick? I have the blue prints and could give the CG. Maybe I'm wrong. Is the whole tail built up or just the turtle deck?

David

DBCherry

My Feedback: (4)

Dave,
It could be, but the "kinda heavy" certainly doesn't sound like a Sportster Bipe.
Dennis-

BasinBum

My Feedback: (2)

Use a sling and a plumb bob. Sling the front and back of the fuse (two slings) from a ceiling hook. Hang a plumb bob from the hook and it will point to the CG. Then you can use the above mentioned formula to determine where you need the CG.

mpriestley

My Feedback: (1)

Good morning guys, thank you for the advise and all of you were correct.

my computer was down on sunday so I could not respond but I might as well tell you about the experience flying this piece o #%&*.

I still do not know what plane it is or if it was a kit or not. I can tell you that it is horribly designed/built. I think I see more plywood than balsa on this thing.

I found the CG as per your instructions and just could not believe that it was correct. for the simple reason it was still tail heavy after I put a Saito 90 radial, mcdaniels rc on board glo driver (big box and set up for 3 cylinder) 2- 4.8volt batt. and some lead weight and all of this is as far forward as I can get it.

I never did get it to balance, I was just too concerned that she would not be able to lift off. and it was strange adding so much weight to the nose with all that was there already. (I think one prob is that the cg is over the fuel tank in the nose!)

So after adding as much as I could feel comfortable with and tried it out.

what a flight that was, the plane was extremely tail heavy and required negative elevator and all the trim I had to just get her around and determine what all my problems were. When I did land I had to land by slowing decreasing the amount of negative elevator and chopping the throttle.

the plane is yellow with blue stars on it.

The turtle deck is the built up part (long ribs like a stearman) tail felt like it was solid. anyway, crappy plane what ever it is and it weighs a ton!

Thank you for the help!

BasinBum

My Feedback: (2)

A tail heavy plane will not need tons of down elevator to fly. It will be overly pitch sensitive and hard to control, generally pitching up and down with each control input. You probably have an incidence problem along with to much equipment wieghing that thing down. While it may never be a great flyer you won't know untill you get it straightened out and any heavy plane will fly like crap.

mpriestley

My Feedback: (1)

Basin bum,

I appreciate your input but I have to disagree, this plane while in flight would go strait up and stall if I did not give it down elevator, as well I know for a fact that the plane was tail heavy because I found the CG and while on the ground the plane was clearly tail heavy (it could also have an incendence problem but I could not tell due to the other problems I was experiencing).

I have had a tail heavy warbird and it did the same thing until I took the weight out of the tail, (granted it too had an incendence problem but that just made it fly bad.) when it was tail heavy it did the same thing, shot for the sky and I had to decrease throttle and use negative elevator to keep it in control.

anyway, thank you for replying...

I do have one more question, although I got the concept and was able to find the CG and verify the CG.
I still do not know what MAC stands for. (mean avg. cord?)



and your right about a heavy plane... fly like crap!

BasinBum

My Feedback: (2)

Take a plane that is flying correctly and make it tail heavy and see if all it needs is down elevator. You will quickly see the differance between an aft CG and an incidence problem. If you don't understand what the MAC is how did you conclude the correct CG location?

You need to first put an incidence meter on the wings, horizontal stab and engine to see what is going on. Then I would sling the plane and use a plumb bob to find where it's balanced and figure 25-30%MAC using this calculator [link=http://www.palosrc.com/instructors/cg.htm]MAC[/link]. For a Bipe you will have to add the sum of the two wings while looking down from above to figure the area.

Get back to me and let me know if I'm still wrong.

DBCherry

My Feedback: (4)

MAC = Mean Aerodynamic Chord, and I agree with BB assessment. A tail heavy plane will not require down elevator to fly. Being tail heavy does not mean the tail will want to hang low when in the air, it simply means the plane will be extremely unstable in the pitch axis.

While it MAY BE tail heavy, the down elevator has nothing to do with it. As BB said, it's most likely an incidence problem.
Dennis-