Bot

[mhmitchell333]

I am considering a Bird of Time from scratch. Since these plans most likely havent been upgraded is there any problem areas I should know about.

[Ollie]

The original BOT hasn't been improved appreciably by modification over the years if you accept the original goals of the designer Dave Thornberg. The problems lie in the fact that the standards of performance have evolved over the years. To know how well the BOT meets your objectives you first have to think them through and articulate them. Then you will have a basis for asking judgements that fit you. Without that, you will get judgements based on priorities that fit other people rather than yourself.

[rpd]

You might check this web site for ideas-
http://isoar.ca/~andrewm/rc/bot/
I have a Skybench Big Bird and really like it.
Patrick

[mhmitchell333]

I just like the look of the BOT, Not interested in flying contest as there is no soaring club nearby. Just want a lazy type flyer that looks like a sailplane.

[BMatthews]

I've got a BoT that I fly that was modified from the plans. It was actually built by a buddy of mine but with a lot of consultation between us. The original is a fine flying model but I think it's possible to add to it with all that we've learned since those days.

I had a thread here a while back to discuss the "Millenium Edition" of the BoT. Have a read of THIS THREAD (http://www.rcgroups.com/forums/showthread.php?s=&threadid=79545&highlight=bird+of+time)

[mhmitchell333]

Thanks for the BOT thread,

I have ordered the BOT plans from RCM magazine should be here soon and will start building as soon as I finish my problem infested Bobcat. I think I am the major problem.

Anyway I love floaters that are able to penetrate I might build the Apogee ? wing as a spare wing. ( Since I bust wings alot mostly by my infamous low altitude knife edge followed by cartwheel trick)

[R. Carver]

FWIW, Soaring Specialties has a FG fuse for the BoT- www.soaringspecialties.com

[mhmitchell333]

Thanks for the fiberglass fuse link, however being the cheap person I am I will stick with the stock concept. But it would certainly save building time.

I have read that carbon fiber on top of the spars is a good idea but how do you stick the carbon to the spar. Before assembly or after wing is built up?

I have also read that fiber tape works just as well as the carbon fiber, any comment?

[Ollie]

A wet layup of unidirectional carbon fibers in tow or uniweb form has a compression strength of about 50K to 100K pounds per square inch of crossection. Precured carbon strips of various thicknesses and widths are available that have compression strengths of 275K PSI. For the good stuff see:
http://www.rcgroups.com/forums/showthread.php?s=&threadid=208264

Glue it to the wood spar caps with epoxy and then dress the wood side of the assembly to the original thickness with a plane. Strengthing the spar caps is wasted money and effort if you don't have shear webs to carry the higher loads allowed by the spar caps. To make a chain stronger all the links have to be strengthened.

[mhmitchell333]

Seems easy enough thanks.

[BMatthews]

Originally posted by mhmitchell333
...I have also read that fiber tape works just as well as the carbon fiber, any comment?

Horsepucky. Fiber tape works great for reinforcing foamies and other items like that but it can't hold a candle to carbon fiber used as you would use it for a glider.

Also it's just as important to put carbon on the lower side of the lower spar as it is the upper spar. What you're doing is actually making an I or box beam out of the bits and pieces with the carbon being the star performer in the matrix. Ollie's prodedure describes what to do but may not have told you WHY you should do it that way.

To get the best out of the carbon the spars should be as far apart as possible. This means the spars should go all the way to the outside of the airfoil at the thickest spot. The sheeting then butts up to the front and back faces. The carbon being glued to the outer faces of these spars. For the upper spar cap I would suggest a bit of 1/32 or 1/16 well sanded to thicness to form a fairing over the upper carbon spar cap. And as Ollie says the webbing system is VERY important. At least as important as the spar cap material. Without support the caps are nothing. Without the caps the webbing is nothing. Together than can survive tremendous loads.

Or if you're using pure carbon and full thickness wide webbing then you don't actually NEED wood spars. Check out the Allegro Lite at www.charlesriverrc.org for a seriously engineered full carbon spar SYSTEM. Overkill? Perhaps. Effective? Without a doubt.

Best of luck with your Bird.

[Ollie]

Here are some of the "whys." the best, precured, unidirectional carbon strips are about 49 times stronger in compression than the best spruce and about 20 times better in strength to weight. Wood is more variable in strength weight and stiffness than carbon. Because the croaaestional area of the needed carbon is so small compared to the lengths, it needs to be held in column (straight) in order not to fail in buckling long before its load is near the maximum compression capability. The shear webs not only must carry the shear loads but they also must prevent carbon spar cap buckling so that it can carry the compression loads indicated by its compression strength. The shear web characteristic that resists spar cap buckling is shear web stiffness. End grain balsa is both stiffer and stronger than side grain spruce. Therefore spruce should not be placed between the carbon and the vertical grain balsa shear webs. if you are using carbon spar caps, the only reason I can think of for including spruce is to fill up the notches in precut ribs and that really doesn't justify the weight of the spruce. If you are going to use carbon fiber then you are better off leaving the spruce out.

The usual untapered spars are not structurally efficient in the sense that at least 2/3 of the material in the spar caps and 1/2 of the material in the shear webs is just along for the ride because it is everywhere too big except at the wing root. The spar caps should be linearly tapered in both width and thickness and the shear webs linearly tapered in width so that their strengths more closely match the spanwise distribution of the bending loads. Both the shear and bending loads are zero at the wing tip so not much spar is needed there. Using tapered carbon spar caps not only saves weight but money too.

Really good spar designs like Dr. Drela's are far more complicated than the simple outline in my previous post. The best I can say about the "simple" approach is that it will get you started in the right direction.

[pixelotis]

I scratch a BOT back in 1980? from RCM plans. After several hundred flights the right wing folded. I rebuilt the wing and added a second drag spar about two inches back from the main spar. This spar had 1/8' x 1/4" spruce spar caps and 1/4" end grain balsa shear webs. The spar went out to the dihedral break. No carbon fiber was used. It made the wing extremely stiff, and I'm still flying it. I would guess it has over 1,500 flights on it by now.

The original wing was very weak due to its extremely shallow depth. I would guess it was 8 or 9%.

If you don't want to go through the trouble of a second spar, the least you should due is use carbon top and bottom and go easy on the winch.

Marty C.
Woodstock, NY

[mhmitchell333]

Thanks I will use carbon