biplane tail volume coefficient

[Richard]

How do you calculate the tail volume coefficient for a biplane?
Richard

[Sparky Paul]

I've never done that.
I've flown a Kadet variation as a biplane with the normal kit horizontal, and a normal Kadet with 50% of the kit horizontal.
And a Krafty 25 both with and without the horizontal.
It's not a critical feature.
.
PJB's Seriously Aeronautical Stuff:
http://home.earthlink.net/~pjburke1/aindex.html
Sparky Paul:
http://www.angelfire.com/indie/aerostuff

[DICKEYBIRD]

I think calculating the T.V.C. is a worthwhile process if for no other reason than to exercise your mind doing the math. See, 'ol Sparky, being a gin&wine, er, genuine trained engineer type guy, is just tired of exercising his mind, having used it for so many years at Lockheed!;)

Seriously, it's hard to go too far wrong if everything looks "about right" to you....if you're like most of us, you've spent so much time looking at this stuff that it'll do just fine if it looks OK to you.

Having said that, I do calculate the T.V.C. in my designs so that I'll have the confidence that they should fly OK. I designed a little 250 sq. in. .020 glow biplane and calculated the T.V.C. using the formula on Al Lidberg's site. I just figured the MAC of the wings looking at the "footprint" or projected view of the wings as viewed from directly above, plugged that number in using the combined area of both wings in the normal formula. I then computed the CG with Lidberg's CG formula and she flew GREAT, hardly any trim needed. This was checked after I had made the moment & area decisions, based partly on what "looked right" to me. As someone else here says: "Your mileage may vary.":)

[Sail 'n Soar]

Haven't seen Lidberg's site. But the moments generated by a wing are a function of the product of the wing cord times the area. Accordingly, I would assume that for a biplane you should calculate the product for each wing individually and then take the sum for the denominator of the TVC. The only thing that would change this would be due to some interaction between the two wings. Don't know what that would be. If both wings have the same cord, then this is the equivalent of what Dickeybird did for his .020 bipe.

[DICKEYBIRD]

Originally posted by Sail 'n Soar
Haven't seen Lidberg's site.

Here it is, look under free stuff.:)
http://www.aalmps.com/

[Sail 'n Soar]

Thanks, found the site. Rearranging the terms of tail volume coefficient as given on Lidberg's site you end up with two moment related terms:

Tail Volume Coefficient = (Tail Area * Tail Arm)/ (Wing Area* Wing Average Cord)

Hence my reference to the wing moment denominator term. For a multiplane model the denominator to use would be the sum of the individual wing area - average cord products.

Lidberg defined the Tail Arm as the distance between the wing and the leading edge of the stab at the average cord. I always measure my moment arms from the respective quarter cords - probably no significant difference in the actual number. In any case, the wing reference point will be at the weighted average between the two wings for a biplane; that is, between the two but closer to the wing with the higher Area *Avg cord product.

[Richard]

Sail N Soar, so based on this then the hoizontal tail area needs to be approximately twice that of a monoplane with same wings in order to maintain the same tail volume coefficint. What brought this up was that I noticed that the Bantam and Bantam Bipe appear to have identical tails. Just something to think about or forget about it if it flyes ok.

[Sail 'n Soar]

Not really. The biplane of a given area and span will have an average cord of 1/2 that of a monoplane with the same area and span. That would mean for the same TVC it would only require 1/2 the tail area. That's why gliders can have so much smaller horizontal stabs - small wing cords for a given area. The big player is the wing cord. In terms of TVC, think of a bipe as a high aspect ratio, i.e., glider wing, cut in half. Don't the Bantam and Bantam Bipe have about the same cord? Design the tail for the bipe and it will be slightly more stable and still look about right for the monoplane.

[Larry @ SR]

Hi Guys...

There's nothing like going to the horses mouth! <G> Or, is it the horse's A**!!! <VBG>

In a biplane there is always interaction between the two wings so in calculations it's always best to figure on an effective wing area that's 80% of what ever the measured wing area is. Use that 80% figure in your calculations and things should work out well.

I designed the Bantam before I did the Bantam Bipe. In so doing, I considerably over sized the vertical and particularly the horizontal stab. I wanted huge amounts of built in stability so that the Bantam would fly easily hands off by inexperienced flyers and be fun for the more experienced ones.

When I decided to do the Bantam Bipe, I re-ran the numbers and they came out very nicely without changing anything because of the extra I had designed in to begin with. The proof of the pudding as they say is the way both ships fly. The December issue of Model Airplane News has the latest review of the Bantm and Bantam Bipe and I understand that there will be one here on the Ezone very soon.

Larry

[Sail 'n Soar]

Different sequence but same conclusion. Design adequately for the bibe and you'll have a more stable, but not unreasonable monoplane. Start with the conservative, oversized stab for the monoplane and you can still fall within a reasonable range for the bipe.

Even if the bipe might come up undersized, you can always move the CG forward a little. In fact, that illustrates why it's best to measure the tail moments from the quarter cord and not the leading edge. Away from stall the aerodynamic center of a subsonic wing lies around the quarter cord - on the quarter cord for ideal flow thin airfoils. Move it far enough forward and the wing, itself adds static pitch stability, hence the flying planks.

Larry @ SR, thanks for the 80% area effectiveness factor. Makes sense in terms of lift, I guess it should in terms of moments as well.