Scale Glasfugel Libelle 201 - in Depron!

[quorneng]

I restored a moulded foam Easy Fly ST330. Of course it is not scale but it could have been moulded in any shape, although it might not fly so well.
I was always impressed by the Libelle. One of the first production glass fibre gliders at the time it had an exceptional performance for a standard class 15m glider.

It was technically clever too being very light and incredibly easy to put together with all the controls but the ailerons being self connecting. The tailplane was secured by a single tiny bolt and the wings by a single pin.
The problem with scale model based on a high performance glider is that the wing chord tends to be very small. Wing chords less than about 75mm (3") tend to to be rather inefficient.
My intent was to build a scale glider using the lightweight balsa/Depron mix I had used successfully on several 'own design' electric gliders.
To save weight these all had one piece wings but to achieve a reasonable chord the Libelle would have to rather bigger 2.8m or more.

To get an idea of the construction I built a short root section test section of a one piece 2.8m wing.

All 3mm Depron it has a step in the skin top and bottom to accommodate
the wide 1mm balsa spar flanges. Only a part of the flange has been added.

The wing across the fuselage is flat with dihedral at each wing root.

The Wortman section is really quite thick (18%) for a model glider.

But it is nevertheless very strong and stiff yet incredibly light.
This rather tipped my decision in favour of a one piece wing but limited to 2m and suffer the penalty of very narrow wing chord.
The wing and fuselage profiles 'tile' printed to 2160mm span and 896mm fuselage length.

Now I have just got to work out how to make it!

[quorneng]

Start with the fuselage as it is almost certainly the most complex to make.
It wil be built as a half fuselage over the plan and then the other side added.
A start of the fuselage formers.

14 in total. Each is cut to shape and a duplicate made at the same time for the the other half.
The outer profile is not a conventional keel but actually the first plank.

As and when I get the fuselage completed after the amount of time and effort involved I will almost be obliged to complete the rest of the plane!

[TM4197]

Good luck with this! I love this sailplane..Will be watching your build!

[JetPlaneFlyer]

It will be interesting to see how it goes with the scale airfoil. As I guess you know it's normal to use much thinner airfoils on model gliders (except maybe for the really giant scale stuff) so they work better at low Re numbers. Really thick airfoils like an 18% Wartmann would normally be considered a total 'no-no' for glider models of this size, but if anyone can re-right the rule book you can!

[JetPlaneFlyer]

I thought it would be interesting to run a simulation of the Wartmann FX-66-17AII-182 (the airfoil used on the Libelle 201) against a purpose designed sailplane airfoil for models, I used the Drella AG24 as an example.

See screen shots attached. It makes pretty horrific viewing. The Wartmann is basically stalled all the time as the air cant turn around the high curvature of the thick airfoil, this leads to low lift and huge drag. Drag is about 4x greater for the Wartmann. As you know for a glider lift:drag ratio is all important. For the Drela it peaks at about 40:1, for the Wartmann its about 5:1

Simulations run on Profili which is based on X-foil code. Re number = 60k

Only advantage of the Wartmann other than scale looks is that it's thick so much easier to build strong.

[JetPlaneFlyer]

here's an extra couple of airfoils added to the simulation

• SD7037 which is a bit thicker so easier to build compared to the Drela but performs very well.
• The 'trusty' Clark-Y which is added just for comparison as a average middle of the road non-high-performance airfoil (but it still soundly trounces the Wartmann)

[quorneng]

JPF
That is very interesting - thank you very much.

I had intended to use the Wortman FX-66 only at the root (for strength) and quickly thin it out and eliminating the under camber along the way, although I suspect as a model it will have more looks than performance!

The only other element is the Reynolds number. At say 6oz/sqft is even 60k appropriate?

It still amazes me just how good the Clark Y is or is it perhaps that at very low Reynolds numbers the thickness to chord ratio of the wing becomes the predominant factor rather the actual characteristics of a particular section?

[JetPlaneFlyer]

At this Re it's mainly about thickness and camber, you dont want too much of either.

If it's as light at 6oz/sq ft then you're right, 60k is too high. I've run some more polars at 40k. At 40k old reliable Clark Y has fallen off the cliff and the true low Re airfoils start to show their metal.

[quorneng]

The planking continues.

At this size some of the former radii are quite small so the planks have to be appropriately narrow, just 6mm at their widest.
After 2 days work the last plank goes in.

There are 22 planks in each half.
A great sense of achievement but then it slowly dawns that the same has to done all over again for the other side and even then it will only be the basic fuselage shell!
After leaving it weighted down over night for the glue to fully harden the other half of the formers can be added along with the cockpit walls.

This not only provides a location for the battery and ESC but by integrating it with the fuselage skin it greatly increases the strength and rigidity of the cockpit area.

[pmullen503]

Nice planking job!

[TM4197]

Was wondering if you guys could look at the eppler 201 foil??

[JetPlaneFlyer]

Originally Posted by TM4197

Was wondering if you guys could look at the eppler 201 foil??

Probably wouldn't do well at the very low Re we are talking about here.

[TM4197]

Understand!

[quorneng]

The radio box.

The tapered end is to guide the 2.4 aerial as the radio is slid in. I have mounted above and behind the wing to leave the space under the wing mounting clear so that the battery box can be extended there is required.
The elevator servo.

My usual micro type but more than adequate if you consider the size of the elevator!
The wing hold down bolts to organise and then is just planking!

[quorneng]

It was about time I started seriously thinking about the wings to ensure the method of fixing them to the fuselage actually worked.
To maintain a scale fuselage and canopy the section has to the scale (18%) Wortman at the root.
I only have 36" length 0.8mm balsa for the spar flanges so it made sense to build the wing in 3 pieces, two 914mm (36") outer panels and a one piece root section of 312mm (12.4"). Once built the 3 sections would be permanently joined creating a 2140mm wing.
I wanted to use a thinner (10%) wing section on the outer panels with the root panel incorporating the change in wing section.
To my eye the scale Wortman thinned out to 10% looks quite reasonable and allowed me to simply resize the section on my PC.

The bottom 4 are the actual rib profiles for the root panel. The top 3 give the sections root, mid point & tip of the 914mm outer panel. I expect the slight under camber, particularly towards the tip, will get 'lost' during the build!
I will build the root panel first. It will allow me to organise the fuselage to root panel join before the fuselage planking is complete.

[quorneng]

The root panel under way.

The Depron box spar is shallower by 1mm top and bottom than the rest of the wing skin. This will provide a step for the the 1mm balsa spar flanges to be glued in.
The bottom skin added.

It is hard to imagine that nearly 1m of wing will be added to each side of this modest centre panel.

[Duffy]

Hi, great project...

What plan are you using, or is it your own design? I've been looking for fuse profiles for a 1/4 scale libelle for ages :

[quorneng]

Duffy
It is my own plan, actually just a 3 view scaled up.
I did build (and made the moulds for) small resin solid models so I do know the shapes and profiles quite well!

The centre section mounted on the fuselage.

The wing hold down 'nuts'.

Actually small pieces of cross grain hardwood drilled and tapped to take a 4mm nylon bolt.
The fuselage planking can now be completed.

Time to start the 'tail feathers'.
First the tailplane.

2mm Depron skins over a tapered hard balsa spar.
The elevator is divided as it passes through the fin.

[quorneng]

The tail feathers.

It looks like the Libelle will weight no more than 15oz so it no longer really needs the 150W motor and 1500mAh 3s I originally proposed.
A rather lighter set up.

It will save 1.5oz yet will still deliver well over 100W/lb.

I will use lighweight filler to achieve a smooth surface and then paint it overall white. A scale gloss finish on Depron will be too heavy. White emulsion does give good coverage for little weight and the soft matt finish can be sealed and given an almost silk finish with extra hold hair spray.

And it smells nice while you are doing it!

[quorneng]

The all Depron motor mount.

The skin provides all the necessary rigidity for a 100W motor and terminates at a 40mm diam spinner.
Complete with the spinner

It is not quite the correct scale profile for the glider but its not far off.
The prop is a 7.5x4.

[quorneng]

The canopy under construction.

I was impressed by the 'solid' canopy I made for my ST330 so I decided to do the same for the Libelle.
Light and strong but a bit tedious to do - although not nearly as long winded as making a 'plug' and moulding you own!
Completed and painted a delicate green!

I used it to test out the matt emulsion/hair spray finish.
It weighs 8g.

[quorneng]

After a bit of head scratching on how best to actually make it, I made a start on the wings proper.

It is built up entirely off the lower skin which already has the 0.8mm balsa flange let into its surface.
With the leading edge skin and top spar flange added it gives some idea of its properties.

The wing section really is small for its span and raises a question as to where it really will be strong and rigid enough in Depron & balsa.
The rear skin will be added once the aileron servo is installed,
At least it does not deflect under its own weight.

But then it only weighs an ounce!

[quorneng]

Waiting for the aileron servos before I can complete the wing panels. I fear the Christmas delivery is holding things up.
In the meantime I have painted the fuselage.

Two thin coats of 'Cotton White' matt emulsion and 2 of hair spry!
In places you can still tell the fuselage is 'planked' but you certainly wouldn't see it from the air.
I could have gone on filling and sanding but on such a light structure weight is always an issue.
Hopefully the servos will arrive tomorrow - but I have been saying that for at least a week!

[quorneng]

With the aileron servos delivered I can atlast continue.
The wing is so thin at even the inboard end of the aileron that the micro3.7g has to be fully inset into the wing and even then a small part of the case will protrude.
Flush on the underside.

Part submerged on the top side.

The aileron itself is very long and narrow (18"x5/8"x1/8" thick). Making it anything like stiff enough will be a challenge!

[quorneng]

I spent some time finding a solution to the very long narrow aileron.
A carbon rod worked but the Depron tore away long before the ultimate strength of the carbon was reached and it more than tripled the weight.
A larger diameter hollow glass fibre tube was nearly as stiff and about the same weight but it was almost impossible to retain the required nose profile of the aileron.
The simplest solution was not only the lightest but the easiest to do, cover the Depron aileron completely with tissue stuck on with PVA.
This in effect gave the whole aileron a monocoque skin over a foam core, which is pretty close to what they did in the full size.


It may all sound a bit flimsy but although the Libelle has a 2.1m span it is actually quite a small and 'petite' plane in comparison to say the ST330.

The ST330 wing looks almost 'elephantine' in comparison.
Just as well the Libelle should be just 1/3 the weight!