Calculations (rib, span, cg)?

[jgore]

How do you calculate the space of the ribs in a wing?

I have just started to fiddle with the idea of designing my own gas plane, and I have no idea where to start.

My main concern now, however, is to calculate the center point of a rib in a wing, currently of an unknown span.

Let's assume 40 inches just for say.

How do I space an x number of ribs in that span with an equal amount of space in between each one. This can be odd or even.

Is there a simplified calculation that can help me out here?
Something that may not have anything to do with trig or calculus <=spelling?

Any and all help is certainly appreciated.
Thanks,
Jamie Gore

[Sparky Paul]

It's not a calculation generally, it's observation.
What works for one 40" span plane might be completely inappropriate for another.
The purpose of the plane for starters.
Gentle or active?
Some gentle performers use 4 ribs on each side for that span. Some actives use fewer or more...
A glider style might have a lot, for the airfoil, an funflying aerobat just a few.
A scale type might have 20 in that span.
See what other planes in the category use.
Try that.. if you like it, keep doing it. :)

[natoquick]

I'm actually thinking about developing some sort of formula that one could use to find the number of wing ribs. I figure that there is a way to do it, but I don't have the time right now to fully pursue it. I'm sure that the TLAR is the best approach in this case, but I want to develop the formula more as a personal challenge.

[steve lewin]

I don't really understand what you're trying to do.

Can you say what you expect the known values to be and what values you are trying to calculate ? Initially you said spacing for a known number of ribs and span. Now it's calculate the number of ribs (given what ?).

But anyway, as Paul says, you can't really calculate from scratch the number of ribs. That's a design decision which depends on what type of wing you're building and what type plane it's for, not on a simple formula.

Steve

[vintage1]

The purpose of the ribs is to hold the leading and trailing edges apart, and the top spars off the bottom spars, and the top covering off teh bootom covering.

How many you need depends entirely on what you are making the other things out of, and how good you want the final shape to be.

[jgore]

I have built a single wing gutter airplane of solid foam that had 5 ribs in it.

Another plane with 20.
It was a trainer not an ARF that was covered with MC.

Both of which look good. However, I am looking into building an all fiberglass model.

The outside will have a hard gelcoat similar to a boat covering.
This will give it a wet, sleek appearence.

If in the plane with 20 ribs I would not have to worry about the MC sagging as I would in the 5 rib plane with the foam.

Get rid of the foam and I have a seriouse sag problem with the monocoat(MC).

I need to be able to calculate rib count and space per area of wing.

Wing length of 40 inches, 20 ribs each of which are 1/8 th of an inch in width.

How do I find the center point of each rib.

There needs to be an equal amount of space between each rib, be it odd or even amounts.

Jamie

[Ollie]

If your objective is a sleek appearance without covering sag, one of the easiest ways to produce it is by vacuum bagging a thin FG skin over a foam core with prepainted mylar molding sheets to give a glass smooth finished surface. No ribs required. See:
http://members.cox.net/dwfee/VacBagWing.htm

To calculate the rib spacing with a span of 40 inches and 20 ribs of 0.125 thickness, start by adding up the total thickness of all the ribs. 20 X 0.125 = 2.5 Subtract the total rib thicknesses from the span to get the total of the gaps between the ribs. 40 - 2.5 = 37.5 Divide the sum of the gap spaces by the number of spaces to get the gap between adjacent ribs. 37.5/19 = 1.97... To get the space between rib centers add the thickness of one rib.
1.97...+ 0.125 = 2.098...

Another way to avoid covering sag is to use an airfoil designed to perform well with facets. See:
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm
Scroll down to the AG3- series of airfoils.

You can learn a lot by studdying Mark Drela's designs. In my opinion they are at the pinnacle of the designers art.