Sopwith Baby Project

[Joe Perkel]

I've received my proposed re-design of the floats from a subcontractor. Some of it I like, some not.

I like the new composite layup of the bottom panel which includes a layer of Kevlar fabric sandwiched within. But, I'm not convinced regarding a complete deletion of all transverse frames (Main Floats), in favor of a single centerline longitudinal vertical panel.

The frame spacing was originally decreased by Sopwith from a 6" spacing to a 3" spacing as seen on my CAD page.

Issues on my mind....

1) adequate vertical transmission of bottom shock loads through these frames (eliminated by sub-contractor???)

2) Original float weaknesses likely caused by inadequate adhesives / hardware joinery,..thus the decreased spacing.

3) I have the advantage of modern Epoxy resins, (ie,..West System)

Questions I am struggling with.....

Should I increase frame spacing to original 6" in order to reduce weight penalty realized by epoxy / composite saturation?

What to do about compartments,.. replace Willesden Canvas with solid bulkheads?

Use Willesden canvas, but place removable access panels for maintenance issues?

Clearly, the concern here is weight gain. Any comments in this regard are most welcome.

[David Paule]

I kind of remember that a friend's Volmer Sportsman amphibian had bulkhead spacing about every six inches, more or less. The floor was 1/4" plywood.

Might be worth looking up that plane and the Spencer amphibian to see what they used. I'd expect that three inches was not necessary except for the reasons you suggested. And for your plane, I'd absolutely stick with Sopwith's plans and not the subcontractor's scheme of a center bulkhead, especially if the struts for the floats come to the sides - if they do, then there's no good load path to carry the loads out of a center bulkhead. Where would its vertical loads go?

With the Sopwith design, the transverse bulkheads can carry the load and transfer it through shear into the sides, and there's a valid load path there.

Some subcontractors tend to want changes for their own convenience. These ought to be resisted. This airplane is special and should remain stock except for safety-related changes which are clearly defined for a particular reason. That's my opinon, anyway.

I'd want watertight inspection panels to all sections of the floats. These are difficult to achieve, even using boating hardware....

I missed the details of the bottom of the floats. If the subcontractor wants simply to put a thin layer of Kevlar between two sheets of Sopwith-specified plywood, sure; if you snag something that punctures the floats, it could prevent the loss of the plane. If he's planning a foam and fiberglass float, then no.

[Joe Perkel]

David,

The bottom panel is also 1/4" (3 ply) per Sopwith. The contractor proposes 6oz Kevlar between two panels of 3mm Okoume Marine ply, and a layer of 6oz fiberglass on the bottom for abrasion resistance, I am fine with this for the same puncture issue you noted.

I'd absolutely stick with Sopwith's plans and not the subcontractor's scheme of a center bulkhead, especially if the struts for the floats come to the sides

- if they do, then there's no good load path to carry the loads out of a center bulkhead. Where would its vertical loads go?

When I have internet access at home again (moving) I will post the proposed layup. However, the transverse float crossbeams do span the entire float, then across to the other one. My concern mirrors yours with the single vertical panel, (which is point loading -vs- a nice even spacing of the loads along the outside frames as you noted). As per the text "Aircraft Float Design" Richardson 1928,.. I can expect severe shock loads (in chop) with this section.

However, the same text describes this section as the "first successful type", and gives it points for simplicity and negating "suction" effect with the straight lines. I have to watch out sailing stern first with the downturned shape, but the tailfloat should save the day! (They carried sea anchors for this reason).

This airplane is special and should remain stock except for safety-related changes which are clearly defined for a particular reason. That's my opinon,
anyway.

In full aggreement!

I'd want watertight inspection panels to all sections of the floats. These are difficult to achieve, even using boating hardware....

Sopwith used three hand-holes for each compartment, I will do the same. However, If I go with the original Willesden canvas bulkheads, I need to come up with a way to access these panels for replacement. There is no indication of this in the drawings as the canvas was likely thought to outlast the aircraft service life. Since posting I'm inclined to leave as is and cut the top deck panels if need be, replace and epoxy new panels via the top deck.

Thanks for the input David, I will look at those two aircraft a little closer. I have always liked the Volmer, (it could be made to look like a single PBY with
a little imagination!)

Thanks!

[Joe Perkel]

One Year this month since project inception, and I have learned a spectacular amount of information, in no small part due to input from individuals in this forum,...thanks to all!

Revisiting an earlier spar junction issue and I have discovered (via newly acquired drawings), that Sopwith did in fact rabbet the forward spars on the center section to receive the boxes and set the dihedral. I dont like it,..but there it is. The aft spar, has a rounded, (ball in joint) type of an affair. All very strange and quite different from the later Pup.



The lift tabs are an interesting feature that I will also incorporate.




It occurred to me today, that this would make a nice double check of W&B calculations from the scales. After all, if it hangs funny, something must be off!

With good luck, hope to have all wing ribs done by this time next year,...but then again...no hurry!

Happy Halloween!

[brinesharks]

Hi Joe - as always really enjoy your CAD work!

On the torque rod aileron set up - the N17 lower wing only had one spar so it was not rigid enough for ailerons. The upper wing was more like the Sopwith design so it was able to have quite large ailerons.

Regarding the floats, is it worth making up some templates so you can build a new set any time? Rather than expecting them to last the life of the airframe, you might find you need to replace them more often. Not sure how the finances work out though. It can be cheaper to replace some parts rather than constantly repairing them. If you want a reliability study done (pro bono of course!) just let me know

[Joe Perkel]

Originally Posted by brinesharks

Hi Joe - as always really enjoy your CAD work!

On the torque rod aileron set up - the N17 lower wing only had one spar so it was not rigid enough for ailerons. The upper wing was more like the Sopwith design so it was able to have quite large ailerons.

Regarding the floats, is it worth making up some templates so you can build a new set any time? Rather than expecting them to last the life of the airframe, you might find you need to replace them more often. Not sure how the finances work out though. It can be cheaper to replace some parts rather than constantly repairing them. If you want a reliability study done (pro bono of course!) just let me know

Bryan,

Thanks for the compliment, but I must admit, the software is really quite special. Someone recently gave me a copy of Autocad 06, but I am leery of starting to try to learn another application.

Ailerons.

As tempting as it was to downsize to 7/8, (would now nicely fit the Rotec's = likely nice performance), I have decided that my original reasons remain valid. As I build this in my little shop, it will feel like a trip in time. Not much of a stretch to see that many of us have a fascination with both new and antique technologies! CAD + Web Design = WWI airplane!

Floats

I expect and must build the floats to last the life of the aircraft. Anything less, (a sudden failure on the water), could result in a loss of the aircraft and probable injury or worse.

In reading the Gourgeon Brothers book on wooden boats (West System Inventors), I am sold on modern Epoxies and their properties. So long as the elastic limits of the base material (marine ply skins / Fir Stringers), are never exceeded, then they should last the lifetime of the plane. Certainly no corrosion issues. Water absorption of old is negated by epoxy saturation of the ply.

I am however concerned about weight gain. When I build the tail float, I will get a better handle on what to expect weight wise per square foot of epoxy saturated ply.

Thanks for the input and the offer of the study. I hope to see you started on the Nieuport soon as well. As I write this, I'm looking at my home project through the window of the rental next door. I am now within close (Pestering) range of the contractor!

Thanks Bryan!

[Joe Perkel]

Anyone know of some hard and fast rules for lightening holes in webs? How much material,..distance from edges,..etc?

Thanks!

[Joe Perkel]

My take on the subcontractor's proposal for the floats, is the addition of the transverse bulkheads.

Still have to do a weight cost analysis.

[j ferguson]

Originally Posted by David Paule

I'd expect that three inches was not necessary except for the reasons you suggested.

If you look at the sides of the Sopwith design as load carrying webs of a box girder, is it possible that Sopwith doubled the number of ribs, not due to glue failures, but because they were having problems with insufficient lateral support of the web? Buckling problems?

Since the panel spans on the design your sub is suggesting are greater and you still have loads across these panels, you may find yourself increasing the thickness, and alas, the weight.

if you ae going to redesign, have you thought of going to hexcel type skins?

best, John

[Joe Perkel]

Originally Posted by j ferguson

If you look at the sides of the Sopwith design as load carrying webs of a box girder, is it possible that Sopwith doubled the number of ribs, not due to glue failures, but because they were having problems with insufficient lateral support of the web? Buckling problems?

Since the panel spans on the design your sub is suggesting are greater and you still have loads across these panels, you may find yourself increasing the thickness, and alas, the weight.

if you ae going to redesign, have you thought of going to hexcel type skins?

best, John

if you ae going to redesign, have you thought of going to hexcel type skins?

John,

An interesting suggestion considering the weight to strength ratio of the of these modern materials. An experienced naval architect could have a field day with this stuff.

Dilemma....

What were the so called "weaknesses" mentioned in various publications? No pilot reports that I can find that say something like, "The Bloody bottoms stove in, one in four landings",..or, "The Bloody Seams gave way at the forward crossbeams",...etc. That would make this decision a bit easier.

Options....

1) Leave things as Sopwith designed, except add a layer of Kevlar cloth into the bottom panel.

Advantage, engineering specific to the application proved serviceable. Modern epoxies, increase the reliability of joints.

Disadvantage, What was failing? Would Weight gain of epoxy saturation offset water absorption issues enough to pay the penalty, so to speak.


2) Re-design

Advantage, possibility of significant weight reduction and increased performance with modern materials, (ie Core cell cores,..etc).

Disadvantage, unproven design, possibility of unknown failure points.

Hmmm,..interesting to see it in writing like this. Must think about this!

Thanks John!