bird of time wing building

[mothflyer]

i have been working on a dynaflight bird of time and have the center section done and i am very unhappy with how it is going. i was thinking about building the wings out of foam, sheeting the top and bottom leading edge and cutting out the foam in the open bays and putting cap strips on where the ribs would be. has anyone done this or seen it done. will it work for a 118" wing span. i was also going to use a 3/8 spar top and bottom. the reason i wanted to do this is so i could get a much more accurate airfoil. and what would be the best way to attach the sheeting to pink foam? i want it to last along time. i have seen sheeted wings with the sheeting coming up and i don't want that to happen.

tell me what you think

thanks

tim

[Ollie]

The Dynaflite version of the Bird of Time is not a high quality kit by today's standards. The wood selection is haphazard and the die crunched ribs lack a well defined edge. However it is a very beautiful and well mannered plane in the sky. Finish it, put your best effort into it and fly it even if you aren't happy with the precision of the results. it will probably fly bet5ter than you expect it to. You will have have paid your dues about what not to buy in the future and you will improve your building skills in the process.

You have several choices for your next project. You could save the plans and build another BOT from scratch, with better wood selection. Another possibility would be to buy and build a quality laser cut kit like the Big Bird. A third possibility would be to build or buy the foam cutting gear, vacuum pump etc that would permit building bagged foam wings. You could then attempt a design like one of Harley Michaelis'. Cutting your own cores and bagging your own foam core wings is another new technology to learn. Your first attempts may be less than satisfactory.

My recommendation is to concentrate on developing your building skills by building another BOT from scratch.

I wouldn't recommend trying to design and build in a new technology. First learn how to build with it. Then you can attempt design.

[BMatthews]

I've seen that method used for power models to save a little weight but for gliders you still need a very good spar system. When you factor that into the picture this partial skinned foam method is more trouble than it's worth.

This method wouldn't work on the BOT anyway. The tip panels use compound curves on the upper surface around the "swelling" in the first portion. Good luck cutting foam for that.

If you want to go for foam then look into the proper engineering for it's application to gliders. There's a LOT more forces from launching than you realize.

[mothflyer]

i have a foam cutter and am very familiar with composites. i do it for a living on real airplanes. the swell in the wing outboard panels are just created with the trailing edge stock and so all i will have to do to create that is cut three cores. one inboard and two sections for the outboard panels. i wanted to keep the open bay look in the wing when it gets covered. i may just build the wing up and make a second wing out of composites. will 3/8 top and bottom spars and foam be strong enough along with 1/8 wing joiners. the built up wing uses 3/8 bass wood spars so i was going to use 3/8 spruce. thanks for all the input. any more would help.

[RCheroske]

Moth, If the center section didn't come out right, I would do over again.

I had a BOT that I built from scratch once and it was one of my most favorite planes ever. I tried one time to make a foam composite BOT once but the wings never looked right. Also be aware that foam composite wings are usually heavier than built up wings so if your BOT comes out heavier, you may not get the sweet flying BOT that we all enjoy.

[Ollie]

A cambered airfoil like the BOT has can develop more lift upright than inverted. Wood is about twice as strong in compression as in tension. That means for the best strength to weight ratio, the root panel top spar cap should have about twice the crossection of the bottom spar cap. The inner panels should have 3/16X3/8 on top and 3/32 X3/8 on the bottom. The bending load at the polyhedral break is less than 1/4 the bending load at the wing center. That means that the tip panel top sparcap only has to be 1/8X1/4 and the tip panel bottom spar cap only has to be 1/16X1/4. The vertical grain shear webs between the spar caps carry the shear load which is zero at the tip and tapers linearly to a maximum at the wing center. The front to back thickness of the shear webs should taper to match the load.

Tapered, unidirectional carbon spar caps from Aerospace Composite Products and CST have over ten times the strength to weight ratio of spruce spar caps and they are much stiffer too. They have the additional advantage that they are thinner than the stock wood spar caps. That allows more room between them for a larger diameter joiner rod to allow zoom launches on a winch. Consider these for a foam cored wing.

The compression force in the top spar cap of a ten foot span wing with a maximum thickness of about an inch can easily reach 2,000 to 3,000 pounds during a zoom launch from a winch. Spruce will only carry about 5,610 pounds of compression per square inch of crossection. The best carbon fiber spar caps can carry 275,000 pounds per square inch under compression. The shear webs at the wing center should be able to carry half the breaking strength of the winch line or about 80 to 100 pounds of shear.

BTW, the top of the BOT tip panel at the widest chord is a compound curve because that airfoil is thicker than at any other station along the wing. That fact will make it very difficult to smoothly skin a foam core wing panel for a BOT planform.

[JimTrainor]

I ordered the RCM Magazine BoT plan.

Does it differ in any substantial way from the Dynaflite kit?

What is a reasonable minimum weight one should shoot for when scratch building this plane, assuming optimal wood selection and building technique?

Thanks!

[Ollie]

An all up weight, ready to fly, of 42 ounces or less would be outstanding, 45 ounces would be very good and 48 ounces would be normal.

You can save two ounces of radio gear by using a battery pack of high capacity 600AE cells instead of the typical AA cell square pack and mini servos instead of standard servos. Those changes will allow a narrower fuselage, the front of which can be well glassed for durability. You can save weight in the tail with 1/16 inch dia. carbon fiber pushrods in teflon tubes and avoid or minimize metal fittings where possible. The rudder can be built up instead of solid balsa. Any weight saved in the tail will reduce the inertial loads on the fuselage in a hard landing. The 3/32 music wire stab joiner and brass tubes can be replaced by a 1/8 inch diameter hollow carbon joiner rod epoxied into 5/32 OD aluminum tubing and mating 3/16 inch diameter aluminum tubing can be used in the fin and stab halves. The increase in tubing size will reduce the bearing loads in the fin and reduce the chance of the stab developing a wobble. This stab joiner set up will also be a bit lighter than the stock arrangement. Making oval cut outs in the ends of ply dihedral braces will reduce stress risers and unnecessary weight. The wood spar cap resizing in my previous post will increase wing strength by 50% while reducing weight a little. Tapering the spar caps will save even more weight without compromizing strength.

[BMatthews]

.... and at the other extreme end of the spectrum.....

My BoT started out as a buddy's model. It was modified for contest flying at the time with fully sheeted center panels, flaps and the, at the time, new Selig 4061 airfoil. He wasn't the lightest builder and the model came out at about the mid 50's for weight. With that airfoil it was still a real floater.

During a severe basement cleaning bout a few years later I became the new proud owner. Some water damage required a full covering strip and refinish job. The weight climbed to 68 or so oz as a result. KEEP THE TAIL LIGHT.... Even at that weight the airfoil carries it well. Penetration is NOT an issue. I can fly it in high winds against the early 90 vintage crop of super sailplanes very competively and better than most. Low speed low altitude thermalling has suffered but with enough concentration it can still ride a small bubble and spec out. Done it on a number of occasions in bad conditions rather than take a 3 or 4 minute with landing points flight on a 10 minute task. But then I've lost the bubble and the landing points many a time as well. The weight doesn't make it tolerant to even the slightest mistake.

If you're building from plans then this may be a fine chance to update the model to incorporate much that has been learned in the past decade or two.

Check out my thread on the Millenium Issue BoT. There were lots of good ideas and opinions in there....

http://www.rcgroups.com/forums/showthread.php?s=&threadid=79545

A lot of my concept was aimed at going to ailerons in that one and I still like the idea of that but Ollie hit the nail right on the head with his talk of dihedral. And truth be told I have no doubt that the slower than desireable response at lower speeds has a lot to do with the weight of my version. Keep your's light in the extremities and have fun. There are two mods that I'd do to the original even if building as a polyhedral RES model. I'd still lengthen the tail moment by 1 to 1 1/2 inches and the nose by 3/4 to 1 inch to compensate and I'd consider using Mark Drela's Allegro-Lite airfoil with it's extended upper sheeting. I believe this is also the one being used on his Bubble Dancer.

The only other caution I can think of is that Dave Thornburg in the original magazine article mentioned keeping the fuselage as narrow as possible. Just be sure you can get the controls in OK. Much of the extra weight in my version came about as a result of a "custom" control rod install to get past the flap servo. If you build it as per the plans this wouldn't be an issue but if you make it a squeaky tight fit for the radio you could run into contol system problems. Just think ahead a little is all I'm saying.

Good luck. The sky will be a prettier place with another BoT... :D

[RCheroske]

I ran across this link today in case you're interested:

http://www.soaringspecialties.com/birdoftimeglassfuselage.shtml

BTW I built my BOT from the RCM plans and had the best RES plane I've ever flown. Died due to a dead receiver battery from flying it too long in one day.:rolleyes: :(

[mwtrull]

Ollie:

You noted that:

"Wood is about twice as strong in compression as in tension. That means for the best strength to weight ratio, the root panel top spar cap should have about twice the crossection of the bottom spar cap."

I would expect the highest load the wing would see would fold the wing up putting the top spar in compression and the bottom spar in tension. Since wood can carry twice as much load in compression (the top spar), shouldn't the bottom spar have twice the cross section to match the load carrying capacity of the top?

As always, thank you for the time and effort you put into sharing your knowledge and experience!

Sincerely,

Michael

[Ollie]

I'm sorry for my mistake. I meant to say that wood is about twice as strong in tension as in compression. In fact all fiberous materials are stronger in tension than in compression. In the case of pultruded unidirectional carbon fiber in epoxy, the strength in tension is only about 16% more than its strength in compression.

Using a difference in cross sectional area isn't the only way to allow for the difference between wood's strength in tension and the strength in compression. You could do so by using the same crossectional area but different materials. In tip panels, for example, you could use rock hard balsa for the bottom spar cap and spruce for the top spar cap. For the root panels you could use spruce in the bottom spar cap and ramin in the top spar cap. BTW, ramin is about 80% stronger in compression than spruce but is only 60 percent denser. The advantage of using dissimilar materials is that the size of the rib notches are not affected.

Ramin is a very pleasant tropical hard wood to work with. It has a close, straight grain and machines well. I have seen it available in pool cues and dowels (imported from its native country) but not as dimensioned lumber.

If you combine spar tapering with dissimilar woods, you can get about a four times improvement in strength to weight ratio of the sparcaps.

[mwtrull]

Ollie:

Thank you for the additional information.

Can you recommend a source for material property data for the materials used in modeling?

Models do not use a lot of materials, but the ones we do use tend to be diverse - wood, composites, plastics, and metals along with various fabrics and adhesives. Since these all come from various sources, the material data tends to be scattered about. Has anyone pulled these together into a a few references?

Sincerely,

Michael

[Ollie]

www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/Ch03.pdf -
www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/Ch04.pdf -

http://www.cstsales.com/material_data.htm

If you look under carbon rod, you will see that not all carbon rods have the same properties. The properties of carbon fiber in various resins, fabrics and tows is quite variable in physical properties too.

Polystyrene Extruded Foam (From a post on R/C SE):

Foamular:
Type Compressive Strength Compressive Modulus
Flexural Strength Density

FOAMULAR 150 15 psi min. N/A
60 psi min. 1.4 pounds/cubic foot (approx.)
FOAMULAR 250 25 psi min. N/A
75 psi min. 1.8 pounds/cubic foot (approx.)
FOAMULAR 400 40 psi min. 1400 psi min.
115 psi min. 2.04 pounds/cubic
foot (avg.)
FOAMULAR 600 60 psi min. 2200 psi min.
140 psi min. 2.4 pounds/cubic foot
(avg.)


HiLoad:
Type Compression Strength Minimum Density

High Load 40 40 psi min. 1.8 pounds/cubic foot
High Load 60 60 psi min. 2.2 pounds/cubic foot
Grayboard 15 psi min. 1.35 pounds/cubic foot

From my limited experience with High Load 60, the above density for seems low, and I might better agree with the following data found in the same article:

Type Weight Compression Strength
High Load 60 2.8 pounds/cubic foot 60x60x60 psi
Spyder foam 2.3 pounds/cubic foot 45x15x15 psi
Interesting to note the better three dimensional properties of High Load vs. Spyder foam.

White expanded polystyrene is available in several densities and strengths. Type I is 1 pounds/cubic foot @ 10 psi. Type II is 1.5 pounds/cubic foot @ 15 psi. Type IX is 2.0 pounds/cubic foot @ 25 psi.