Rib Spacing vs Load Distribution

[Mark Scarborough]

Man oh Man. This guy Frank has got a cool looking plane, he looks young, AND he speaks English for crying out loud!!

What he is not telling us is the plane weighs 85 ounces. I'm sure of it-it has to-it must.

yeah I think I heard 58 ounces,,
I know,,
I know,,

[Tim Wescott]

The commercial tubes I've seen have fibers all parallel to the axis.  They are also a bit heavy.  We make them with fibers at +/- 45 degrees.  They are good for torque, but flexible in bending, which my stress guy demands.  Frank's are prettier than mine.

There's commercial tubes and commercial tubes.  Here's what I know, both from growing up in a high-zoot fiberglass shop and from some intensive web searches for spar material:

• The cheap tubes are extruded, with all the fibers parallel.  This gives them great rigidity in bending, but not so great in torque. Also, when they break they'll fail with long breaks along the length of the tube as the fibers pull apart.  They'll break quicker under torque, of course
  
• Cheap composites (regardless of the fiber/resin combination) pretty much always tend to be resin-rich.  The best strength/weight ratio comes when the resin is almost, but not quite, too sparse -- then the strength drops off rapidly as there's not enough resin to hold things together.  And processes that really allow you to be resin-lean (like vacuum bagging) are expensive.  So if you want to sell a decent but not super product, with few defects, you need to be more resin-rich in the first place, and you need to adjust your machines and your processes to add enough extra resin to account for process variations.
  
  An example of this is shower stalls vs. the stuff that my dad's company makes.  Shower stalls are made with a dingus called a "chopper gun", which can do great things but only in the hands of an experienced operator, and with secondary steps.  Normally the gun is adjusted for short fibers and lots of resin; this allows the guy holding the gun to just wave it in the general direction of the mold and lay down an acceptable batch of material.  My dad's company, on the other hand, uses a strictly hand-layup process.  Fiberglass mat is laid into the mold by hand, resin is brushed on by hand, and the air bubbles are rolled out by hand.  Because of this both the thickness of the part and the fiber/resin ratio can be much more closely controlled so the part is stronger and lighter than chopper gun layup (but still not as good as if all the above were done, then the whole shebang were vacuum-bagged and tossed into an autoclave).
  
  Careful hand layup, with just enough resin, followed by vacuum bagging (or molding in rigid matched molds) is probably best.  And most expensive.  (And I haven't vacuum-bagged anything yet -- I've done quite enough hand layup of polyester/glass composite for any ordinary person, but no vacuum bagging).
  
• The criss-cross tubes are expensive for the reasons noted.  Either you know someone, or you make them yourself.
  
• There's no reason you can't make a tube with some fibers laid criss-cross, and some laid lengthwise.  This would give you both bending and torque strength, at the expense of weight and complexity.  There's also no reason you can't put more fibers on the "tops" and "bottoms" than the "sides" (which is hard to keep track of on a round tube, but if it's rectangular...)
  

[Tim Wescott]

The commercial tubes I've seen have fibers all parallel to the axis.  They are also a bit heavy.  We make them with fibers at +/- 45 degrees.  They are good for torque, but flexible in bending, which my stress guy demands.  Frank's are prettier than mine.

Someone mentioned that you (or some other guy named "Rush" -- do conservative talk show hosts fly model airplanes?) make tubes by vacuum bagging the CF onto aluminum tube, then etching the tube away with acid.

Can this be done more cheaply by using styrene tube, or do you just lose too much precision when you do?

And how in heck to you dispose of your aluminum-laden acid without bringing down the Enviro-Nazis on your head when you're done?

[Larry Cunningham]

But fellas, is torque deflection a truly serious (or measurable even) problem with a common CF arrowshaft or tube (extruded or otherwise)? You're twisting through a simple control horn arm, right? Not some worm drive or torque multiplier, true?

Aren't we talking serious overkill here? How much torque can you apply to an elevator on a stunt model in flight? Won't you be limited by the Netzband Wall?

I see the elegance and workmanship and skill, but question actual mechanical need.

Just sayin'.. 

L.

"Therefore, you will.. NARFLE THE GARTHOK!" -High Master, to Beldar Conehead

[Howard Rush]

Someone mentioned that you (or some other guy named "Rush" -- do conservative talk show hosts fly model airplanes?) make tubes by vacuum bagging the CF onto aluminum tube, then etching the tube away with acid.

Can this be done more cheaply by using styrene tube, or do you just lose too much precision when you do?

And how in heck to you dispose of your aluminum-laden acid without bringing down the Enviro-Nazis on your head when you're done?

"Rush" and "acid" were mentioned together, so it took awhile to determine that the person being discussed was I.  

I use aircraft prepreg which cures at 350 F, and the shrink tape that provides the pressure can distort the mandrel, so styrene probably wouldn't work. Even small 7075 arrow shafts distort, so I put drill rod inside.  

Spent etching material is disposed of per JCT regulations.  

[Frank Wadle]

Hello,

as I previously mentioned, the tubes that I used were made using the filament winding method.
It is possible to do that "at home". All you need is a mandrel. Something like a steel rod with a smooth surface. Now you have to apply a coat of wax, lets say 0,1mm thick. This can be done by dipping the mandrel into liquid wax. Now put in in your lathe and let it rotate very slowly. Next you wrap a carbon roving that is already soaked with Epoxy around it. You can do it in any angle you want (5° for bending; 45° for torsion; 54° for internal pressure). Finally a layer of Peel-Ply is wrapped around the whole thing. This squeezes out all the excess resin. Now cure it at RT. During the post curing at ~100°C the wax gets liquid again and the mandrel can be removed easily.

Or you just go out and buy this:
http://shop.r-g.de/en/New-Products/Carbon-fibre-tube-pulllwinded.html
 

Frank

[Howard Rush]

But fellas, is torque deflection a truly serious (or measurable even) problem with a common CF arrowshaft or tube (extruded or otherwise)? You're twisting through a simple control horn arm, right? Not some worm drive or torque multiplier, true?

Aren't we talking serious overkill here? How much torque can you apply to an elevator on a stunt model in flight? Won't you be limited by the Netzband Wall?

I see the elegance and workmanship and skill, but question actual mechanical need.

Indeed, it's overkill for torsional stiffness.  The +/- 45-degree tubes provide the requisite stiffness at less weight (.014" wall) and are flexible in bending, which may keep the controls from binding at high wing loading. I use pretty big tubes-- some were .375" ID-- because they form the flap LE.  For me, it's the easiest way to make flaps. Another big reason for my choice of materials is because it's what I had around the house.  A local surplus store, now extinct (boo hoo) had lots of cool composite materials and supplies.

[Howard Rush]

Here is the Kahuna at the 2008 World Champs, where I had the privilege of seeing it in person.  I was impressed by how perfect it was.  There wasn't even any dust inside.  Frank, was someone else flying it there?  I didn't see your name on the roster.

[Avaiojet]

I have some of those Dave Brown pushrods,  5/16" stuff, the brown ones with the thin black line.

Stiff enough to be used at the LE of flaps. I can get a photo.

Anyone know what they are made of?

CB

[Frank Wadle]

In 2008 I was not qualified for the German Team.
But I competed in the Worldcup event that took place right before the Worldchamps. And I did some of the calibration flights for the judges.
The rest of the time I was trying to pull out other peoples bellcranks 
I did the pulltest at circle1 (paved).

[Howard Rush]

Dangerous work.  There were many bees on the Lavender Path.

[Avaiojet]

Here's a thought.

I purchased some aluminum tubes I was going to use as torque rods. They are as long as flaps.

They could be used as leading edges.

I see no reason why they cannot be notched where each rib will be placed. You could insert a snug balsa wood dowel into the aluminum tube the entire length of the tube. The notches would expose the balsa wood dowel and allow for gluing rips at each exposed area.

Trailing edge could be aluminum also. They have flattened airfoil type tubing. Once everything is in place, covered, doped and painted, I'll bet you would have an interesting control surface.

Concept worth developing? Wishful thinking?

OK, the aluminum tube could also be used as a LE for solid flaps and elevators also. As I was going to do. I was going to glue them to foam covered balsa.

CB

[Tim Wescott]

Also, there's aluminum and aluminum.  It can range from dead soft to quite strong, with different alloys having different fatigue properties, etc.

AFAIK there isn't an aluminum that has nearly the stiffness/weight ratio of CF, but if you were going to use aluminum tube for a leading edge, I'd just epoxy straight to the aluminum (aluminum takes epoxy really well).  I'm not sure what I'd do about any cutouts for hinging -- hold my breath and wait for fatigue cracks, most likely.

[Howard Rush]

Lancaster Archery (and hundreds of other places, no doubt) sell nice 7075 arrow shafts in all sizes.  They are light and straight.  I'm sure they'd work dandy as flap leading edges.  I got some 3/16" OD shafts from them, intending to use them as mandrels to make carbon flap torque tubes for a local guy.  These tubes were to go through the middle of the flap ribs.  That diameter was too small for me to make a 3/16" ID carbon tube the way I do it.  The guy eventually used the aluminum mandrels, rather than the carbon tubes, for torque tubes.  There wasn't much weight difference.

As Ty says, aluminum tubes might lose too much strength if they're notched.  I tested the carbon tubes with hinge slits in them, and they are still plenty strong.

[Frank Wadle]

Well, the idea for the flap construction on BK1 and BK2 came from an old AeroModeler magazine from the 70's.
The original design, a RC sailplane, used aluminum tubes.
So I guess aluminum tubes would just work fine.
I would also use Epoxy to glue the ribs right to the aluminium tubing.

Frank

[Avaiojet]

Frank,

Bet I have that magazine someplace.

You must have experience gluing balsa/plywood to aluminum using Epoxy.

I don't have the corrage. To feel comfortable, I would have to insert pieces of hard balsa and drill a few weep holes in the AL so the Epoxy can be wood to wood, even if it's through small holes.

Did I explain this well?

CB

[Frank Wadle]

I would not drill holes!

Yes, I do have a lot of experience with Aluminum glue joints. That's part of my job :-)
The problem with gluing wood to Aluminium is the aluminum oxide. One solution is to sand the surface to be glued right before applying the glue.
The better solution is to use anodized aluminum tubing (chromate anodized). The anodization improves the strength of the glue joint and, to a small degree, improves the stiffness of the tube.
I any way, it is very important to carefully clean the aluminum using acetone prior to gluing.
Then, don't just use laminating epoxy. It's always better to use a structural adhesive.

But we are going to run into another overkill here :-)
The way the flaps are designed they would almost work without any glue. The covering film holds all the parts in place :-)
That means: I would use CA 

Frank

[Avaiojet]

Frank,

OK, I'm fine with that, but what about hinges? Gotta be a hole someplace, or you wrap the aluminum with a thin narrow strap. That would be hokie.

CB

[Lauri Malila]

 Hi.

 Why are you talking about an overkill in here? I don't think it is. It is basically allways so, that the stiffer and stronger construction you use, the better the end result is. A rigid plane behaves more logically and is easier to trim & fly. Ans I'm not talking only about Stunt but basically every category of aeromodelling.
 BUT, when converting a known design with a better structure, it may be necessary to alter the geometry/dimensions of components like flaps. Stiffer structure allows you to make the flaps smaller than original or make them turn less. Don't break the harmony, you know.. L

[Frank Wadle]

Hi Lauri,

I was talking about the glue joint of the Aluminum tube and the ribs. Maybe the use of chromate anodized tubes and structural adhesive is overkill. CA glue will propably do the job, too.

But you are right, there is not such a thing like overkill for modelairplanes.
My definition of "hobby" is:
Having the least possible benefit with the biggest possible effort :-)
We spend hours and hours building the perfect plane, "waste" weekend after weekend practising. And after all the best we can hope for is a ugly metal thing with "walker Cup" engraved in it :-)
I'm just joking    

Frank

[Lauri Malila]

 Yes, Frank. But I was referring to an earlier posts in this thread. I have a feeling that many people comment these things without having seen and "felt" things like your flaps. We are not talking about a small inprovement here. It's like when I started to fly free flight, circle hook would open at 3kg tension, and wing would break at 6. Now, my hooks open at 12..13kg and I haven not tested my wings to the point of breaking yet. And I know that I pull the line quite hard, up to 30kg..
 About structural adhesives, do you mean stuff like Loctite Hysol? The E-120HP is my new favorite glue! L

[Frank Wadle]

Yes Lauri, thats the kind of stuff I'm talking about.
There are many others like that, some specialy developed for aluminum joints.

Frank

[ash]

While I mostly agree with the idea that more stiffness will never go to waste in aeromodelling, there are times that it does step into overkill territory.

1. When it results in excessive weight

2. When the flap is much proportionally stiffer than the wing TE and it causes the hinges to bind under load

3. When the structure sacrifices resilience for stiffness, resulting in cracks from fatigue in normal use and from being knocked around in transit

I've experienced the first and second of those, but not the second. The thing about Frank's model is that it seems to successfully avoid all three.

[proparc]

Hi Lauri,

 And after all the best we can hope for is a ugly metal thing with "walker Cup" engraved in it :-)
I'm just joking    

Frank

Frank,
Don't try to fool us. You do it for the women and the money just like the rest of us.

[Allen Brickhaus]

Well, the classic plane I'm building didn't worry about that. The ribs are 7/8" apart ... all the way to the tip. 28 ribs inboard, 27 outboard.   

Mirage perhaps?