how to balance a Biplane canard

[D-Rock]

I am designing a biplane canard aircraft. The main wings are stagared and swept. I used the following page to find aerodynamic center of both the wings and canard http://www.palosrc.com/instructors/putte.pdf . My question is, can I use the half-way point between the two biwing's individual aerodynamic centers as the aerodynamic center for the total wing planform to complete the formula to get the CG location?

D-Rock

[Sparky Paul]

That should work.

[vintage1]

Accordng to Gordon Whiteheads article, the thing that is relevant is to total projected area of the wingpair: This can be considered as a single wing for the purposes of MAC calculation..however the total area needs to be applied to the overall CG formula.


You may find this of some use..if you unlock it and key in the biplane wing area directly (its a derivation from the span/chorsd as written), it should work..

http://www.shaman.co.uk/models/spreadsheets/index.html

[aussie]

Sorry, I haven't checked the links so this might not be what you're after BUT... If the two main wing AREAS are the same you can just take the halfway point between each of their aerodynamic centres (AC).

However, if their areas are different you'll need to create an arbitrary point ahead of them both and call it "X". The distance back from X where the combined AC lies =

[AC of first wing (measured back from "X") multiplied by the area of first wing] plus [AC of second wing (measured back from "X") mulitplied by the area of the second wing] all divided by the combined area of both wings.

Kye.

[Padesatka]

Any geometric centering will get you close to center of pressure. Center of lift should wind up about 15% forward of that.

c/g versus center of pressure should probably be considered if you haven't already. Viewing the canard bipe from the front, try to visualize where parasite drag is in relation to the c/g. Otherwise, you may not anticipate pitching moments coupled to airspeed.

Since you are staggering the wings, you can tweak the pitch and airspeed stability (and even reduce the required area of the canard if eliminating tuck) by rigging just the right decalage (leading wing slightly higher incidence) and sharpening the leading edge of the leading wing. If you can cause that wing to always stall first (as Curt Pitts did) then you will get much more performance out of the wings without compromising a predictable stall.

Give that canard an extreme and exponential deflection, move the c/g as aft as not insane, and it might be possible to pitch those wings in and out of inside and outside stalls at any airspeed.

Which would be interesting (doing reasonably well what canards are notoriously poor at). I've always considered them a bit wasteful, in terms of not getting the most out of the wing: Because the canard must always (conventionally) stall before the main wing, the main wing is never flown near critical AoA, lest there appear control departure. So designing any canard usually means one must insure that the wing is never allowed to approach an optimal AoA, because it's too risky in terms of stability: If the wing lets go of the sky before the canard, there is rapid pitch-control departure into a deep and possibly flat stall/spin. Which is only interesting until impact.

[D-Rock]

Its kinda difficult to see but the two black dots are the individual wings AC, the red is the midpoint. This was the midpoint that I measured to find the cg, the green dot. Also I averaged the wing area of both wings in the equation on the link. Should I have used the total area? Thanks for the help

D-Rock

[Sparky Paul]

I've seen this method used to find the mac for a biplane..
Determine each wing's individual m.a.c., then combine them considering the gap and the stagger..

[D-Rock]

One further question, would it still be a good idea to put washout on the main wings?. Thanks for all the help so far!

D-Rock

[CloudyIFR]

http://h1.ripway.com/cloudyifr/files.htm

Try my Sailplane Calc spreadsheet for MS Excel.

It won't do canards but if you have Martin Simons book on Model Aircraft Aerodynamics the back of the book has the formulas I used for the spreadsheet and there is only a change of a math calculation from + to - or vice versa. I could do this for you but I've never tested a Canard so never applied it to the spreadsheet. However, the calculations for a conventional airplane are spot on and have worked for over 10 years.

Let me know if you'd like the math change done in the spreadsheet and I'll try and find time to help you out.

Curtis

[chrisgood]

Washout (or prop location) can affect where the CP will be. Read this entire thread for my experience on exactly that.

http://www.rcgroups.com/forums/showthread.php?t=354292&pp=15

Chris