Originally Posted by andrecillo76

I hope not in one piece. The surface of a Horten is Gaussian curved and cannot be covered with one sheet of plywood.

Of course, one bent piece of plywood would never work. I get that.

2 sheets, one top and one bottom could no doubt could be persuaded to conform to a twisty wing, but if a shear web would be needed, then there is no advantage over conventional construction.

Thanks,
Kent

Originally Posted by andrecillo76

Lothar's plans are now available at the SourceFroge repository (Release 0.2):

https://sourceforge.net/projects/hortenmanatee/

The plans are found inside the file "MANATEE_Lothar.DXF". In principle anyone with access to CAD and to a thread milling machine could build a prototype. The ribs and jigs have to be organized for thread milling. The file contains notes on the materials used. I think Lothar suggests 0.6mm Plywood for the D-Tube.

So, anyone in for a prototype?

Regards,

Andrés

Andrés,

As usual Lothars drawings are wonderful! He has done a fantastic interpretation of your basic design. I really like the fact he keeps his designs clear and to the point. They definitely match his no nonsense personality. Anyway, I was wondering if you have the time to answer some questions about the design.

I know from my latest Ho IIIe and Ho VII designs it's probably not obvious but I really like to build light. However, I'm interested in building a version as light as possible. How light do you think would be too light for this model? Also, although I know it's a very complex subject...but I was wondering as I reduce weight where I should first focus. I'm thinking of focusing on things like using balsa ribs and such near the wing tips though I'd like to hear your thoughts first.

Thanks!
Brian

@Kent: That is exactly what he did. He used four pieces, i.e. top and bottom per side. No chance, and his wing had only a span of 1.3m! Of course he did not use a mould. I do not have photos of his wing. But if you would have seen it, you would believe me right away that nobody wants to have that

I attached a photo of a bottom covering of a center section with 0.8mm ply - the thickness you suggested. That was the most largest piece I was able to use (4") keeping the contour, and I had to struggle quiet a lot to fix it right. So the only way is to split in smaller pieces and glue them again. Schäften in german; sorry I found no englisch translation. Essentially you make wedges (1:10 or 1:15) on both pieces and glue these togetgher. This approach is the best for a continuous stress transmission.

If only a shell is used to uptake all the loads, the "Schäftungen" would have to very precise (1:15 or more). Using a spar, the covering can be schäfted with less precission (I use 1:10 normally, i.e. for 0.6mm the wedge is 6mm). I guess for Hortens with a shell structure, the easiest way is to laminate fabrics in a mould.


@Brian: Good question and not trivial to answer . Normally people ask the other way around.That depends on many factors: aerodynamics, strengh in the air and for handling, etc. I do not think you'll manage to decrease that much the weight without making too many compromises. The outer wing sections are subjected to the smallest aerodynamic forces, but during landing and handlig you'll have to be carefull. Light wings do not like gusts, so if you want to fly at a slope, you'll might better have a higher wing loading.

Anyway, regarding the aerodynamics, well that takes me some hours of work. Maybe if I find time later. I did my calculations for a wing loading of 5.4 kg/m², which is a usual value achieved using Lothars plans. I'll make a calculation for 3 kg/m², which is almost "criminal" for such a large wing

Regards,

Andrés

The translation for connection of wooden pieces called "Schäftung" in German is the "scarph joint".

...vocabular I learned by building boats....

Regards

Lothar

Americans usually spell it with an "f", Lothar.

Of course we're not the only English speaking people and thos other guys don't always agree with us about spelling. A belt sander works pretty well on 3mm ply. I suppose you would want fairly fine grit paper for the thin wood used for these skins?

--Norm

Part 2 Scarfing the Panel Edges (9 min 11 sec)

Thanks for explanation Andres. Chatting with you is like being back in college. Your thorough comments are much appreciated.

PLYWOOD JOINTS
I have been using butt joints with backing for plywood joints and not too successfully, I might add. The Schäftung or "scarfed" (California speak) plywood joint had always caught my attention as extra work. Especially at 15:1 slope. I always thought that it was done out of habit rather than structural need. Clearly, the increased surface area of the scarfed joint makes it the strongest joint. But I asked myself if it was needed in all cases.
Driven by the desire to have 100% of the parts cut with CNC, some time back, I modeled (Rhino 3D model) the curvy plywood pieces with a simple butt joint, which is easy to model. I added plywood splice plates at the joint. You can barely see those splice plates in post #64. That's the picture of the broken wing. If I had used a scarfed joint there, would it have still broken? We'll never know, but it would have made this joint much stronger. Looks like I've got a new arrow in my quiver, the scarf joint.

TWISTING PLYWOOD
I understand that plywood is rigid in X and Y axis and that it cannot be formed into a compound curve, at least not without steam or chemical treatment. After all, that is what we use it for, it's rigid shear strength. But slight variations are ,no doubt, possible. It depends on the amount of compound curve and the material itself. The skinny wing guys use .8mm balsa sheets over a total twist of 4.5 degrees. The balsa is applied under pressure. The twist is concentrated at the foam panel joints. The maximum twist at the worst joint might be 2 degrees. We do know that this method works under these conditions, but we don't know if the balsa is fully bonded to the shape at the twisty part. Atleast we don't know without cutting a wing up. If birch plywood were to be substituted for the balsa, it may very well suffer the problems you described.

Thanks for the plywood story. You have saved me from myself.

Looks like Horten wings are crying out for the flexibility of FRP. Hollow molded Horten anyone???

Kent

Hi Brian,

my computer calculated the whole day and I made the graphs below. I do not think that it makes sense to make the wing much lighter than 4 kg. The performance is concerning the loss of height and glide ratio significantly less. Not only the glide ration decreases from 15 to 13 but also the height loss is slightly larger. The only good thing is that it stalls later. But IMHO everything less than 50 km/h can be managed by everybody. So why making it lighter and more susceptible to damage ?

@Kent: Twist is not the only problem. The wing is fully curved due to e.g. the Horten tail and the change in airfoil thickness. A mould for FRP has also its peculiarites. For example, the parting line is not straight

@Norman: Nice video!
@Lothar: thanks for the translation. I've read that word somewhere else before


Best regards,

Andrés

Originally Posted by Knoll53

The skinny wing guys use .8mm balsa sheets over a total twist of 4.5 degrees. The balsa is applied under pressure. The twist is concentrated at the foam panel joints.

Kent

Kent, I've had no problems bonding balsa to foam in the past, at least no problems caused by the washout (which is very gradual even for 4.5 deg). I can vouch for continuity of bond too, as I has a wing cleaved in half in a midair with a mouldie.

Putting it into context, imagine how easy it is to twist a standard length of 1/16 balsa even as much as 45 deg. We need the glue and balsa to maintain only a fraction of that force.

Hi Norm,

I use 120er or 150er grit paper for scarfing (american style). It's much easier than I belived before I did it the first time. The big advance of birch plywood is the glue between the three shifts. If You grind it for scarfing You vcan exactly watch, what You are doing. Just by hand holding the gritblock.

And dont think to much doing this.Just scarf every part You make from plywood, glue it on the wing and than start with the next part gluing it on the scarfed edge of the last one. It took me only some trys.

By the way, the scarfed joint may be the strongest part of the planks.... and duckworks is a great page! (for boatbuilders)

...and it makes a beautifull plane

Regards

Lothar

I've had very good luck making scarf joints on .4mm (1/64") ply with 80 grit sand paper by hand. It's like Lothar said the layers of glue help a lot.

Brian

Originally Posted by andrecillo76

Hi Brian,

my computer calculated the whole day and I made the graphs below. I do not think that it makes sense to make the wing much lighter than 4 kg. The performance is concerning the loss of height and glide ratio significantly less. Not only the glide ration decreases from 15 to 13 but also the height loss is slightly larger. The only good thing is that it stalls later. But IMHO everything less than 50 km/h can be managed by everybody. So why making it lighter and more susceptible to damage ?

WOW! Thanks for taking the time to do this. I'm not near any really good slopes so I was thinking of keeping it light to reduce the miniums skink rate for thermaling. I'll have to re-think that.

Brian

Originally Posted by andrecillo76

my computer calculated the whole day and I made the graphs below. I do not think that it makes sense to make the wing much lighter than 4 kg.

The sink rate and glide ratio graphs are fascinating. It confirms what I have been flying for years. That is that the "extra" weight harms the sink rate only marginally yet really improves the glide ratio. I have, due to my primitive flying sites, always built durable wings with killer spars, knowing that those wings COULD be lighter. Now, I think that I will add ballast boxes to those wings.

While flying in mountain environments, the winds can come up from nowhere during a flight and it is critical to be able to fly back to the landing zone. A good glide ratio allows you to do that.

Thanks Andres.

Kent

Originally Posted by Tijgerhaai

By the way, the scarfed joint may be the strongest part of the planks

Lothar: Do you use a big scarf like Andres? For example a 6mm wide scarf for .6mm plywood. I have always simply let the 3mm width of the rib set the width of the scarf.

Kent

Originally Posted by Knoll53

I have always simply let the 3mm width of the rib set the width of the scarf.

The standard woodworking procedure is to make the length of the wedge some multiple of the thickness of the material being scarfed not the width of the surface under the scarf joint. 8x is usually the multiplier I've seen to get a joint the same strength as the wood but I've never seen such thin material scarfed so it may be different than for thicker material. I certainly shouldn't hurt to make the joint wider but making it narrower will create stress risers and you already know what that will do in a crash.

--Norm
BTW Isn't it OK to add more detail than the you think questioner really needs when replying to a post because there are often less experienced people following the thread who may not understand a short but just adequate reply?.

Originally Posted by Tijgerhaai

I use 120er or 150er grit paper for scarfing (american style).

Lothar--

I'm very grateful that such talented people are willing to share their hard earned knowledge with us foreigners. I couldn't even order lunch in a German restaurant unless the menu had pictures

--Norm