Planform Design

[space_case]

How do you guys go about designing planforms?

I mean the taper ratio and span of each panel in a wing.

OK, I want X wing loading so I will need Y wing area. I (or contest rules) want to limit myself to a Z span. Due to build complexity and deminishing returns, I want to limit myself to N panel breaks over the semi span.

How do you get your numbers for a 2 panel? 3 panel? N panel?

Do you arrive at some solution by iterating over a a range of values while trying to meet some optimum lift distribution or L/D? What sort of metrics or combimation of metrics do you use and which ones do you weigh more heavily?

[Tom Harper]

Space,

I set up some equations in an Excel spread sheet. The different scenarios are a series of formulas in the rows. Each row is a possible configuration. The variables are all fed from the same cells. So as I vary the parameters in those cells I get an array of results.

For guidance I use articles by Andy Lennon or (my favorite) Laminar Aircraft Design by Alex Strojnik. Strojnik gives induced drag typicals for various straight line approximations of eliptical planforms.

I like a straight center section with double taper at the tips. The taper is chosen so that the spar remains straight.

[Ollie]

Depends on the type of aircraft and size:

A. Sailplane
1. Thermal
a. Trainer
b. Contast
c. Scale
2. Slope
a. Trainer
b. Aerobatic
c. Racing
d. Scale
e. DS
B. Power:
1. Electric
a. Trainer
b. Aerobatic
Etc....
Etc....
2. Glow
a. Etc.....
Etc....
Etc....

[space_case]

I like a straight center section with double taper at the tips. The taper is chosen so that the spar remains straight.

Is part of the reason so that you can avoid joiners at the root? I'll check on some of this material.

Ollie, most of the planes I build are TD type sailplanes so I would choose that from your list.

Size, say 3m for the sake of argument.

[Tom Harper]

Space,

Yes, I use a straight through spar.

[Ollie]

I would follow Dr. Mark Drela's designs for TD. His are the best that I have seen. Learn from him.

[BMatthews]

I lean heavily on plagarism with a small dash of originality. My designing draws heavily from the amalgamation of countless magazine construction articles and pictures of planes similar to what I want to build. This is aided by the odd article or book on aerodynamics of the models I'm interested in.

For example a few years back I would never have even considered using a flat airfoil on an aerobatic model. But the unique way the flat foamies fly has got me thinking that there's a place for such airfoils in larger models provided they can be kept light.

As for your glider it sounds like you are doing a RES design since you are considering poly breaks and how many. One thing I have learned is that you want to equalize and minimize the angles of the dihedral breaks in order to minimize the drag producing turbulence at the breaks. But putting the majoritiy of the dihedral out at the tips is going to provide the yaw roll reaction more leverage and a snappier response. Based on this part of me says that a 4 panel wing is best and just live with the drag off the fewer but sharper angle joints. But another part of me says that the 5 panel wing has smaller angles and puts the dihedral out further from the center line. So does having two diheral break angles compared to 4 smaller angles result in the same, less or more drag? *shrug* I don't know.... But the 5 panel wing should provide a snappier turn for less overall dihedral angle. How much less? Probably 20% less than what is more or less "normal" for a 4 panel wing. At least that's what the pictures of the various successful desings make it look like.

If all this sounds like a "definite maybe" you're right. The successfull designs are those where all the bell curves just happen to coincide. Just keep the tail moment arm relatively long. That's a win-win in every way other than transporting.

[raptor22]

I agree that, fi results are your goal, you should probably go to the drela designs. They are very good.

But, if you want to do your own design, check out XFLR5. Its a freeware program that does a pretty complete wing analysis; its really an amazing peice of software.

--Alex

[HELModels]

Space Case,
Definately listen to the advice given thus far, but I would also suggest 2 additional books: Sailplane Designer's Handbook and Drag Reduction and Structures Handbook. They were self-published by my late father, nothing fancy, typos are included, but both provide for stable designs. The SDH has tables for combinations of planform considerations, and formula based stepwise procedures. The DRH has a similar style stepwise procedure for determining balsa/spruce spar requirements, so you dont fold 'em.

If you or anyone is interested in obtaining these books, let me know.

Harry

[ji_blanco]

Space,

I set up some equations in an Excel spread sheet. The different scenarios are a series of formulas in the rows. Each row is a possible configuration. The variables are all fed from the same cells. So as I vary the parameters in those cells I get an array of results.

For guidance I use articles by Andy Lennon or (my favorite) Laminar Aircraft Design by Alex Strojnik. Strojnik gives induced drag typicals for various straight line approximations of eliptical planforms.

I like a straight center section with double taper at the tips. The taper is chosen so that the spar remains straight.
Hi Tom, could you please send me your excell file?

I´m more interested in designing a motor plane to carry a photocamera which is able to rotate by the use of servos.

Thanks