How to create airfoils?

[Erfolg]

I am building a Fokker V29, which had a airfoil u/c wing.

 

I wish to 3d print this small wing.

 

I am sure that a number of methods have been used by 3d modelers to produce a print.  Rather than try and re-invent the wheel, I thought a number of you guys will have been there and now have the "T" shirt, So what are my options?

 

I use Fusion to model (if it matters).

[Ron Gray]

For a simple airfoil use the fit point spline. Define the length, the airfoil thickness then the 'high' point on the top surface. Draw your fit point spline through the start (LE), high point (spar) then the end point (TE). You will now have a nice airfoil section. If you want to then print the U/C wing, extrude that airfoil section for the length that you need then 'shell' the object to make it hollow.

 

 

 

 

[Erfolg]

Thanks for the reply Ron.

 

I will certainly try this method.

 

I had thought of putting the upper surface points from a set of aerofoil co-ordinates. I thought that this would be time consuming and there would be a better way for something that is essentially cosmetic.

 

I will keep others informed as to progress.

 

An aside I have printed out the Thingverse "red baron pilot figure", I suspected it would be necessary to resize, for once my luck was in. The size, dimensions seem perfect.

[Ron Gray]

If you wanted more accurate airfoil sections then you can always go tot the Autodesk App Store and get a 'proper' tool for them:

 

Autodesk Apps

 

[Erfolg]

I have now used the concept Ron has suggested.

 

The first result has just come of the printer. I have modified a model to increase the extrude by 2, So that I will have 3 prints to join instead of 4.

 

I see printing as being essential part of aero modeling, now. The next picture is of a thingverse Pilot and a design and print I have produced of a BMW 3a engine.

 

I really would like to see much more in the mag on 3d design and printing. In my case i am using Fusion and cura. An obvious question, are all programs essentially the same in functionality and user interface?

 

 

[Ron Gray]

1 hour ago, Erfolg said:

are all programs essentially the same in functionality and user interface?

Functions - similar, UI - no!

[Erfolg]

Ron, the method has worked.

 

Having produced one set of prints I decided that the item was to heavy. On that basis I have redesigned it with thinner walls, as the the earlier format was poorly designed, in that changing some dimensions did not achieve what was intended, also some features could not be amended as there was on direct dimension. Yet another lesson learned, although how to undertake the task more affectively needs some thought and a strategy, also a change in methodology may or will possibly be required.

 

Anyway a picture of the finished item, along with a section of a trimming from cutting to length.

 

I cannot but think there are more than a few mag articles on how to design and print model items for model aircraft, by 3d printing. That is by someone who knows what they are doing.

 

 

[Simon Chaddock]

Erfolg

Very neat and entirely practical but I think you may have been a bit generous on the section 'depth'.

I understood that Reinhold Platz tended to use the same section on the U/C fairing as the wing. He certainly did on the Triplane. 

 

[Erfolg]

Simon, I estimated the depth for a photograph by scaling the chord to depth ratio. I do not remember which aircraft the axle detail applied to, nor from memory, what the function of the under wing supposedly performed. Particularly that at the time it was claimed that no Wind Tunnel was owned by Fokker and that access had to be gained from others, on limited baisis.

 

Even so, I would make no great claim to accuracy. The model is semi scale by dedicated scale modellers standards. I can see them now pulling their hair out (I do not have that problem, as you know).

In the Putam book, it certainly identifies that the plan forms and other details were done by ratios from one aircraft to another. Much the same was done with the welded structures

[Erfolg]

I now have a better understanding with respect to the thick UC fairing/wing.

 

Towards the end of WW1 the German Procurement branch, required that all German Aircraft had to incorporate means to avoid fire that could endanger the crew.

 

Some responded by use of fire extinguishers. In the case of Fokker they responded by transferring the tank into the under carriage. Previously the wing had been under cambered. It now became a thick flat bottomed shape. The UC axle ran through the middle in a slot.

 

I have not found yet how Fokker went from the thin profile wing (that most if not all) manufacturers were using, to a thick section. A clue could be that Platz was developing unbraced wings. The question still remains, how did he know the section would work and work very well. He had no windtunnel and Antony Fokker apparently starved Platz of technical data. We know that the camber could have been similar to the then fashionable thin undercambered sections, he woul presumably not known anything about Profile drag, Stall characteristics and only guessing that total drag might have been reduced.

[Simon Chaddock]

I believe the wing section used was more or less forced by the relatively thick cantilever wing spar used by Platz.

I understand the general view held, including by Fokker himself was it would create unacceptable drag compared to the very thin sections used in fully braced wings. However the attraction to "business man" Antony Fokker was the cantilever wing would be simpler to produce being built up around a simple rigid constant section box spar. The short span of a triplane was considered a practical means to limit the wing span and thus the bending loads.

The aerodynamic qualities of the thick wing was a surprise to all but showman and excellent pilot Fokker quickly demonstrated it abilities. 

The spar design was quite advanced for the period being structurally efficient but it relied almost entirely for its strength on the glue used in its construction.

 

As you can see the front and rear element of the "box" were themselves box spars made up of square spruce? sections joined by ply shear webs front and back. These sub boxes were structurally joined together at each rib station and finally the whole structure ply covered top and bottom to provide torsional stiffness.

Note the thin ply covering over the leading edge did not contribute directly to the wing's strength but to give a smooth transition where the top surface curvature was at its greatest to the rest of the wing's fabric covering. 

 

Apparently the sophistication of the construction evident in a captured Triplane amazed the members of the Air Board such that a detailed report was quickly commissioned although its content was largely ignored by the UK plane designers at the time.

That is quite enough about that! 

     

Edited October 12, 2022 by Simon Chaddock

[Piers Bowlan]

Fascinating stuff Simon. Box spar similar to the Jodel, ahead of its time indeed (torsionally very stiff).

[Erfolg]

It certainly was a brave decision on the part of Platz, to even contemplate a thick wing section.

 

I am sure it was an iterative process, similar to what came first the "chicken or the egg", still get the idea past A Fokker, yet another obstacle.

 

I have now tried Rons methodology on proper airfoil sections by using the "old style" plots that used to accompany airfoil data. It works very well. It is only the nose area that really needs any number of points' did not know the Fokker details.

 

All I am reasonably certain of the thickness ratio is above 10%, seemingly an incredible 16%, a massive nose radius, or entry. It is probably the nose radius that contributed to the very angles of attack ( or alfa, that is so fashionable today). It also contributed significantly in reducing the camber.  And yes, as we know that the high depth produces a stiff beam, particularly with the wide flanges (when will "I", second moment of inertia become fashionable?).

 

I have found that compared to balsa, PLA seems to be heavier by an amont that has not encouraged me to use it for wing ribs, as the relative strength and properties (impact resistance, deformation recovery) seem to be inferior. What are peoples thoughts? particularly how to optimise 3d design of such items?

[Simon Chaddock]

I do agree that PLA although plenty strong enough ends up pretty heavy if used to make a conventional wing rib.

I did spend some time exploring various layout to see if I could arrive at an acceptable printed rib. A situation made more complex as I wanted to use them with  light weight sheet foam skin.

I used this rib design on my Antonov AN2

 

Printed with a single 0.2mm bottom layer and a single 0.3mm wall 3mm high with a specific low density grid pattern infill exactly positioned to give a warren brace effect.

The AN2 wings have a parallel chords so there are a lot of ribs and printing is so much easier then cutting ribs out. It weighs about 15% more then a solid 3mm Depron rib.

Note there is no provision for a spar as the AN2 simply does not have any relying on the strength of the foam wing and the full biplane bracing. 😲

As a rib design is is plenty strong enough for its purpose but is rather sensitive to "squashing" due to handling finger pressure.

 

A bit more successful was this design I used on the similar sized Big Dragon. It also has a constant chord single cantilever wing.

 The design does include provision for a top and bottom spar so it is rather more tedious to set up and is slightly heavier than the warren brace type rib but as the circles just touch the rib outer wall it does give a better handling resistance.

 

By far the most complex rib design so far was used on the Antonov AN124. It huge centre section wing chord far exceeded the printers bed size so the rib had to be made in pieces and glued together. 

Still a single bottom layer but the rib and diagonal bracing achieved using relatively substantial "U" channel sections. Even more time consuming to set up but printing remained attractive as the wing section profile remained constant out to the outer engine nacelles. Only the physical size of the rib changed (by nearly 40%!) which could be achieved by scaling in CURA from a single master for each rib part.

 

All the ribs described above were designed to be used specifically on sheet foam based wings, however for me the biggest benefit was simply the cost.

You can print an awful lot of light weight ribs from a single kg roll of PLA! 🙂