Rib extrapolation / interpolation

[Stevo]

The drawing I am working from (Stuka) gives me almost every other rib. My Technical Drawing days are 40 years passed I'm afraid.

Anyone got any techniques for drawing the ones in the middle... ?

[onetenor]

Use the rib stacking technique Sandwich all the ribs you've got with blanks in between. Carve and sand the the blanks down to conform to the other ribs Voila .A full set of ribs Yes it does work with tapered wings.

Good luck John

[Stevo]

Yes indeed I have used that approach before thanks John. I made the ribs a little larger as you inevitably sand a diagonal on them

Just wondered of there was any drawing techniques?

[Phil 9]

if you have it profili would work well for this

Edited By Phil 9 on 22/05/2017 20:30:10

[Piers Bowlan]

Stevo, to ask a silly question, is the rib section used different for the tip rib and the root? If it is a straight taper and the same section is used can't you use the enlarge/reduce function of your printer to produce an image of the missing rib images. Yes the spar slots will be the wrong size and in the wrong place but you should be able to work that out from the plan view of the wing. Just a thought.

[Bob Cotsford]

The way I was taught too many years ago was to select a point near the centre of area and project radial lines out to the outline to be reduced/enlarged, ideally placing additional lines through any hard corners. The number of radial lines used will vary depending on how complex the shape to be scaled is. Now measure the length of each line and apply the required scaling factor, plotting the resulting length along the line. Once you've done this for each line join up the plotted points and you should have a replica of the original outline to the desired scale. This works best where the scaling factor is not close to 1:1, i.e the bigger or smaller the better. If you have many ribs to plot this could keep you busy for a week or two

[jrman]

+1 for Bob's method which is long established but as he says is better when the scaling is not close to1:1. The use of a "good" set of proportional divides can make it quicker but a good set are not cheap.

[Broken Prop]

Crikey Bob, I had forgotten all about that method of graphical enlargement (which I originally learnt at school too many years ago for comfort).

I could feel the ancient cogs grinding as I read your post. Thanks for the reminder!

Pete

[Bob Cotsford]

There was a method of drawing a triangular scale so that you could read of the scaled measurement using a setsquare if I could only remember how to do it. 50 years have gone by and I've never needed the knowledge, nor that of log tables or calculus

Slide rules - that's another matter, those I did use a lot at one time.

Edited By Bob Cotsford on 23/05/2017 15:49:44

[Broken Prop]

Ah, that one I do remember. It is used to produce a ruler to the scale that you require.

Draw a horizontal line to the length of the original item. Mark off any intermediate points, say inch divisions.

Draw a line at an angle up from the origin. Any angle will do, but if the item is not too large, 30 degrees is OK. Mark the new required length at the far end

Draw a line from the right hand end of the horizontal line up to the mark you have just made.

Set your set square to the angle of that line. Draw intermediate points from the original line up to the new line to produce a ruler to the scale that you need.

This method works best if the line that you draw from the end of the original line to the new line forms an acute angle.

[Erfolg]

I was also thinking along the lines that Bob suggests.

Having the ribs either side, reduces the work required dramatically, just apply Bobs method and perhaps a little logic if the ribs are not equally spaced etc.

Even for me, it is not a big deal job.

[kc]

The radial technique is well illustrated in Gordon Whiteheads scale book ( Stevo - send me a PM if you need a copy of the relevant page)

However a much easier way now is to photo copy and reduce to 95 percent, 90 percent - whatever to get the missing ribs. Or to use the  Profilli software to print out a new set of ribs at the correct size .

Edited By kc on 24/05/2017 09:46:43

[TIM Shaw]

Profili will only work if you know what the section is - or are otherwise able to create a .dat file for it - but it is a super tool if you do have that information.

[Clive Hall]

Steve, I have a software solution to this problem - did you get my PM, with details?

[Stevo]

Well thanks guys!

Sorry as I've been busy at work the past few days.

I shall experiment... I'll PM you, Clive

[Clive Hall]

Steve, I have generated two possible rib sets for the Stuka. The lower one has a symmetrical tip section, the upper one a non-symmetrical tip. Also there are slight differences in the root section; any combination or adjustment is possible. These ribs may differ slightly from the drawing, but they will fly the model.

[onetenor]

Re sanding a diagonal on ribs in the sandwich method. I think this angle creates a smoother profile when covering. Just my opinion of course .

[Clive Hall]

I changed a few input numbers slightly - I think this is closer to the original drawing:-

The ribs can be printed individually on a larger file with skin thickness and spar slots, which can then also be used for laser cutting.

[Stevo]

All duly noted!
Am at work now and will process over the weekend!@

[Stevo]

All processed in my head!

I've printed off the above graphic on A4, but will experiment with a larger printer next week when my son gets back to his job (that involves a large printer).

I'll get him to blow the drawing up so the largest rib is he same chord as my drawing and go from there. For spar positions, I usually cut the inner and outer ribs, position all the ribs in between and place the spar over the top and mark and then cut, using a rib slotter from permagrit. With a tapering wing (L.E and T.E) it's probably the most efficient way. The wing is sheeted over that lot anyway!

Thank you to all!

[Clive Hall]

My post on 26/06/2017 was only to give you an idea of what that rib family looks like, I did not think of it as having any practical use. What you need is a paper print of this file, or even a .DWG or .DXF file by email. What you suggest is really doing it the hard way.

Here is a revised version of the ribs, all separated and including a 2mm skin thickness to reveal a true shape for cutting. Or I can post you a full size print on paper – very easy to do.

[Stevo]

OH!

I see... I'll email you my address

[Gordon Whitehead 1]

Here's another method of drawing up a set of ribs. It shows an elliptical wing, but works for any wing planform. It's easiest to use if you have CAD or some other graphics program that will let you draw and print to scale. Otherwise a large sheet of paper and long ruler is needed.

 

You can add washout tabs too, if needed.

Gordon

Edited By Gordon Whitehead 1 on 29/05/2017 20:12:46

[Jonathan M]

Thanks for posting that Gordon. I always wondered how it was done!

Are there any resources to help (a beginner to designing) decide which aerofoil to choose for any given RC or FF application?

[Peter Miller]

If you are serious about getting into designing I would suggest investing in Compufoil. THis program is fabulous as you have a huge range of airfoils to choose from and you can create your won NACA sections.

You can change from one section to another over the length of the wing.

You can install all your spars etc where you want them and holes in ribs etc. plus masses more

IT takes a little practice to find out how to do everything but you do get a really good manual with it which is in the program, not a hard copy although I did print mine out.

It is extremely reasonable in price too.

**LINK**

I have no connection with it, just a very, very happy user

Edited By Peter Miller on 30/05/2017 08:22:38