CafeenMan

I'm curious how many people actually believe that building a wing that has 3 ribs in each panel actually saves weight.

Fewer ribs is an illusion and it doesn't make for the best wing. I'll explain why.
The advantages to having more ribs are that the wing has a more accurate airfoil, is more durable, is stronger and there is no significant weight gain. Sure, gluing in 10 ribs instead of 3 may add an unacceptable 3 to 7 minutes building time to the project. The plane will also *look* heavier, but that's all it is.

The ribs are for my latest design which this is as far as I've gotten. It's a bipe, but I don't know what the span will be, whether the wings will sweep or not or anything at all about the fuselage. I'll build the wings and then build a fuselage and tail surfaces to match.

The ribs have a 15% symmetrical airfoil and a 7" chord including ailerons (both wings).

There are 42 ribs and 32 half-ribs. The ribs will be spaced approximately 2" apart with a half rib in between.

Simple math tells us that the most weight that can be saved can not exceed the total weight of the ribs.

So if the stack weighs 5 oz, then the most weight I could save would be 5 oz if I could find a way to not use any ribs at all and achieve the same end.

I'm busy cutting holes in the ribs and will check back in when I'm finished. It's taking a while because I'm doing it the hard way.

Look at this photo and tell me how much you think these ribs weigh.

How much weight would I save by using fewer ribs?

the-plumber

Enough so that hauling trash bags to the crash site would not be burdensome ?

CafeenMan

LOL... yes indeed! What you obviously don't realize is that 80% of my models are destroyed by means other than crashing. In fact, it is normally a catastrophe in the home that results in the demise of my models.

Nevertheless

Build to fly - not to crash.

CafeenMan

Well, no guesses. Too bad. To anyone who guessed the correct answer of the weight of the original ribs within 2 grams I would have given a Thwing! completely ready to fly with all equipment installed. No guts, no glory.

Anyway, the original set of ribs (all 74 of them) weighed a mere 30 grams (1.06 ounces).

After removing the inner portion the weight dropped to 21 grams (0.74 ounces).

By the way, about 2/3 of the ribs are from contest balsa and the rest are from medium or heavy balsa. I will use the heavier ribs in the center section for additional strength there.

The savings amounts to 1/4 of an ounce or approximately 25% less. When you consider that the entire set only weighed an ounce to begin with you can see that it doesn't really make a whole heck of a lot of difference.

So why did I just spend 4 hours cutting out the ribs? Because I will do similar things throughout the structure which will make the overall gain more significant. Also, I just think the wings will look better with the transparent covering I will use.

Lastly, I think there is some psychology at play here. If a model looks light, the pilot is more likely to fly it "light."

But the problem with modern designs - especially kits - is that many companies take a lot of shortcuts to save on their bottom line at the expense of proper engineering which more than negates the advantage of any weight savings of having just a few ribs in the wing.

For example, lite ply fuselage sides are much heavier and weaker than properly built up sides. A profile fuselage built from a slab of balsa with plywood slapped on the sides is far heavier than it needs to be or should be.

The point being that there's no sense in saving a fraction of weight on one component and then building another far heavier than it needs to be.

Joe Ortiz

My Feedback: (75)

Those are some nice looking ribs, what did you use to cut them out?, I'll be building a plane from plans soon.
Joe

CafeenMan

I drew a rib, made copies, spray glued it to 1/16" plywood, cut it out, glued sandpaper to the back and used a hobby knife to cut the ribs from 1/16" and 3/32" balsa. I used a single edge razor blade to cut the front and back of the spar notches.

In the middle, I used a sharpened piece of brass tubing to cut the corners and then connected them with a hobby knife using the template as a guide.

From my website:

How to Make a Set of Ribs for a Constant Chord Wing

Joe Ortiz

My Feedback: (75)

Thats pretty slick, thanks.
Joe

CafeenMan

You're welcome.

Tall Paul

For lots of ribs, I generally cut rectangles out of the raw sheet, the size + a bit of the chord and thickness.
Stach these together with a spot of glue between each sheet, and saw the ouline and spar notches on a bandsaw.
A tad less labor intensive than cutting out each rib.
For a tapered wing, the rib outlines on paper are cut off the 8-1/2"x11" sheet + a bit for chord and thickness, sprayed on the back side with contact glue, then two (four) sheets of the proper wood are stacked, with each rib layed out to maximize the use of the wood, then cut out again on the bandsaw.

CafeenMan

I've tried that and have always had problems with the stack of ribs flexing enough to ruin the stack. Normally I nail them together tight. Whenever I have something that is taller than it is wide, it's a problem for me. What's your secret?

Tall Paul

The cutting table on the saw MUST be 90 degrees to the blade.
The stack should be wide, so it doesn't tilt going thru the blade, and don't force the blade thru the stack.
Sometimes pins more than 1/2 the stack height are pushed in from both ends for nailing the parts.
The glue dab, if it's white glue serves to hold the stack well, and it's easily seperated if the stack is cut as soon as it's assembled.

MinnFlyer

My Feedback: (4)

Mmmmmm...

Ribs.....

CafeenMan

mmm... those do look good. I wonder if the chef used genuine balsa wood chips to add smokey flavoring.

The Ghost

CafeenMan,
I agree with you. If you build it right you can build it lite. I must be lucky as I have access to a Lazer cutter. So cutting out parts is as easy as setting the Lazer and pushing a button. The advent of foam wings in the late 70's and 80's people were building quicker, but the were heaver and personaly not as strong as a built up wing.

Cheers.

hookedonrc

So MinnFlyer, what did you use to cut out your ribs?

JoeAirPort

My Feedback: (41)

Drool !!

Balsa Master

i agree 100%, skimping on ribs is NOT the way to save weight!

ballgunner

My Feedback: (1)

Being an old experimental aircraft tin bender (Northrop) I can cut a pretty good pattern from aluminum or stainless. I have time and I love to spend it doing the little things right. I cut the ribs one at a time using the pattern, fasten them together and block sand. Your sure right about leaving out every other rib. The weight saved can usually be put in your eye with little discomfort. If you really want to save weight go to work on the engine compartment and the landing gear.

Mike James

Good points, Paul. Balsa ribs are almost weightless.


And... This may prove that I'm a geek once and for all, but:

"The weight of any material, (in ounces) including balsa, ply, and foam is .009259 X Density, per cubic inch."
For example, a 4 inch X 36 inch sheet (144 square inches) of 1/16th balsa, (.0625 inches thick) at 6 pound density, is:
(144 X .009259 X 6) X .0625 = .49 ounces ...Ta da.

I did a truly geeky article on this weight-prediction stuff, at http://homepage.mac.com/mikejames/rc...rediction.html

damifino

As an auto racing designer and fabricator my rule is to lighten the heaviest, most dense components first and so on down the line until every component has been involved in weight reduction. In my opinion too, the ribs are at the bottom of the list.

CafeenMan

Ok, here's the wing. It still needs a few very small pieces of plywood to mount the cabanes to. The way I'm doing it is not pieces going from one rib to another. I will glue a 1/4" wide piece plywood to a rib sticking straight down. It will also attach to the upper and lower spars. I'll put some fill around it for the covering to adhere to.

I used 1/8" x 1/4" spruce spars which weighed 1.2 ounces for all of them (top and bottom wings). I haven't decided whether I'm using interplane struts yet, but I probably will because the wing is flexible. But covering will stiffen it up considerably. For example, My Gonzo designs use the exact same construction except having larger 3/16" x 3/8" balsa spars. The covering prevents the wing from twisting unacceptably.

The top wing weighs 48 grams (1.7 ounces) as shown. It also needs ailerons. The bottom wing has some sheeting in the center so it will weigh a little bit more. Bottom line is I expect both wings to weigh less than 9 ounces completed and covered. Wing span is 36", wing area is 504 square inches.

The structure can easily handle a .40 4S, but I plan on something smaller than that. I want this plane to weigh no more than 40 ounces complete which will be a wing loading of 11-1/2 ounces per square foot.

CafeenMan

How well does this work for you in practice? I mean in predicting the weight of a completed model before you begin building?

SoonerAce

My Feedback: (1)

sorry to go off thread, but on the website, is it just me or was the date on it like um March 2002? But also, nice site.

CafeenMan

That's the "born on" date. I've been asked about that a lot. In fact, one guy said something to the effect that there must be nothing new since then (which isn't true). I should probably just remove the date from the home page. Look at the "What's New" page and you'll see a lot has been added since 03/02.