How to bevel Nomex Honeycomb

Industrially, they get the min weight for any required strength and sturdy repeatable parts with pre-preg tapes and fabrics, film adhesive, honeycomb cores, vacuum bagging and autoclave curing. Our small workshops drive us to wet methods that do not include autoclaves and prepregs and film adhesives. What looks like the materials and processes the big guys are using ends up no better than our smaller shop parts and maybe even heavier...

If you are contact wetting the honeycomb to attach it to wet facings, you likely will drive assembled sandwich weights to or above the weights for 6 pcf foam cored parts. There are several things working here:

• A vacuum bagged foam core part will physically squeeze all excess resin from the cloth that form both facings. With honeycomb core, only the facing against the mold will be debulked, and then only if you do a bag cycle of just the outer facing. The inner facing will only be pressed against the edges of the Nomex paper, and will likely retain most if not all of the resin used to wet and transfer the inner facing - little of the resin in facing will be squeezed from it by the bag;
• Wetting just the edges of the honeycomb for bonding to facesheets is tough to do. Generally, the resin will disperse itself over the area of honeycomb paper, thus requiring more resin and thus more weight to bond facesheets by this method;
• Yes PVC foams do take a little resin to bond fabrics, but in my shop the weight of the cloth, foam, and min resin for the cloth works out very close to weight of the assembled sandwich. I am not getting much resin loading the foam.

I urge you to do some sample pieces 30 cm on a side by both methods. You need to practice your process and makes sure that you have the bugs out before you go making $1000 skins. Make some with your Nomex process and build a couple with PVC foam cores. Make the cores exactly the same shape and size, use the same cloths and vacuum bagging, trim the finished parts to the same size, then weight them. Then you can decide which you should build these wing skins.

Billski

 

I am having a difficult time with this. It sounds like you are building a pretty much standard Pitts wing with a few adjustments - wood spars with a little carbon reinforcement, only a slight reduction in the number of ribs, and compression ribs? Are you trying to build a stronger wing by adding some carbon to the spars and ribs, then put on carbon fiber skins? This may get you stronger than the stock wing, but it will not take weight out. I expect it will gain weight...

Then there is the little issue of load is distributed according to the stiffness of the pieces. A wooden beam (spars and ribs are beams) with some of its most effective area (near the top and bottom) routed away and replaced with graphite fiber becomes a graphite cap beam with a wood core. Wood is heavy compared to a graphite web laminated on both sides of a foam core. Yeah, you will have to replace foam with plywood or phenolic at the places where struts and the like mount and add a couple plies to the laminates there too, but this is still way lighter than the wooden spar with graphite reinforcements.

In composite structures, many of us abhor what we call "aluminum thinking". That is where you design a composite structure as it it were sheetmetal. It makes for heavy, difficult to build structures. Much better to start with what the structure has to carry load wise and scheme out an efficient composite structure.

In the case of swapping the stock Pitts wings for new set of graphite wings, this drives a few constraints - The attachment points for hanging the wings and struts and flying wires need to be in the same places, which drives the spars to the same places. There will be openings in the skins for the various attachments, so you add a couple plies locally around the openings. You will have to add some access panels for inspection and assembly/diassembly.

Now for how I would design those wings.

The spars would go from wood to extruded graphite rod caps and graphite webs with foam web cores. Even designing the spar and skin set (together) to 24 g you will save substantial weight over the standard spruce spar with some graphite rods inlet into the spruce.

The wing skins will be structural - they will contribute to bending stiffness and strength and will provide most of the torsional stiffness too, so why not include those contributions and do the calcs with that in mind? You can do some optimizing trading wing core thickness against number of ribs. You likely omit a bunch of ribs. There is math to be done to check that the skins are OK with large rib spacing to make sure, but once you get stiff skins, most of that internal stuff can go away.

Compression ribs? What for? Once you have a stiffer spar and structural skinset, you wing is set. Compression ribs are there in a fabric covered wings to keep the spars apart and the wing stiff against wracking and torsion. You now have structural skins that do all that, so the cross brace wires and their mounts and the compression ribs are superfluous. Gone with them...

Once you have graphite sandwich wing skins that will stand the aero loads, graphite sparset/skinset that will stand the bending and shear and torsion, and ailerons hung, you may find you have raised your max g and lowered your weights. Then you can run the numbers with similar outer facings on a massive core wing with built-up sparset internal to the massive foam and see if that saves you some more weight.

By going with either of these options, I suspect you will add a lot of strength and stiffness while taking some real weight and roll-axis inertia out of the plane.

But to keep the sparset, most of the ribs including the compression ribs and then hope to not add too much wieght in the skins, well it seems like immense effort to buy a weight gain...

Billski

 

OK, now that I understand the direction you are taking and have a little insight into your background, maybe our help can be better directed.

Shifting wing area inboard will decrease roll damping and raise your max roll rate. Good plan. Did the engineer you talked with tell you if the issue with handling torsion from larger ailerons is that the wing is too flexible (excess deflections) or inadequate strength? I would also be really interested in how he came by this knowledge...

I do not know the limitations of the stock Pitts wing, but I know that the mods discussed will increase weight and are likely to increase stiffness. None of this would I embark upon without doing a bunch of structural analysis.

The idea of adding structural wing skins is how you get to higher torsional stiffness and torsional strength.

Once you put on these structural wing skins and bond them to the spars and each other, the drag and anti-drag wires become superfluous as the skins carry that loading easily and the wires cease to do anything useful. With the drag and anti-drag wire loads gone, the compression ribs that were needed to carry the wire loads are no longer needed either.

Then we get to the ribs - ribs carry the aero loads to the spars. You drive the wing through the air, the air going by the wing is somewhat faster than the the free stream air. TOWS is a good reference for this whole topic. See the FAQ for design and for composites for these books. Anyway, the air outside the wing is moving, the air inside is stationary, by what we know about fluids in motion, the air outside is trying to pull the wing skins from the wing. I call this effect inflation. The difference between the pressure over area of the top and the pressure over the bottom is the lift. In a fast airplane (Pitts Vne is up there in "fast" category), this inflation effect can be create quite a bit higher load than the lift. Anyway, the skins are being pulled away from each other by this effect. Curtis' rib spacing was set to what the fabric could stand in each bay with a substantial Factor of Safety. Change to a stiff strong carbon fiber sandwich skin, and the bay sizes can go up, decreasing the rib count. Then the difference between airloads on top and bottom are pulling the rib too - this is the lift and since you will have less ribs, each one may need to be stronger. Might as well be composite and sized for the loads. You can analyze the skins and figure out your required rib spacing and then your needed rib sizing too. When you make the skin cores thicker, the skins gain a lot of strength and rib spacing can be increased. You probably want to see how much the skins plus ribs weigh for several core thickness options and pick the one that is lightest overall. This whole topic is covered in the aero structures books.

Then we come to spars. The primary book will be any on Mechanics of Materials with chapters on beam mechanics (I used Timoshenko and Gere in college and like it, but there are others) and then one on Composites (I have Tsai and Hahn, Jones is good too). If that all seems overwhelming, get to Jim Marske's web site. His book takes a simplified approach to graphite spar design, but it will get you started and serve as a primer for design of practical graphite construction.

Come back here with more questions on the details, and we will help with the path.

Billski

 

Not missing anything. I almost suggested it in an early post. The big problem I have is with honeycomb core in a home shop laminate - it is likely to add weight, not subtract it over a simpler and higher integrity foam cored skin. The thread then drifted - with the OP's assent - into issues of wing structural design in the first place, and that particular option became way less important.

Billski

 

This aerobatic pilot and engineer knows (or says that he knows) these things about the Pitts wings. How do you know he is right and that it will work? Has this been done successfully by someone else? Have other folks knowledgeable on the Pitts said the same things about needing more torsional and bending capability in Pitts wings? I urge you to vet this information carefully...

These upgrades of the standard Pitts seem like small changes to the existing wing, except they are not. Tooling the wing skin molds can be a huge effort. Well, unless the wings are symmetric airfoils and can all be turned out from one mold or maybe two molds... Routing groves in the spars for carbon rods seems straightforward, but the ability to react loads between carbon and wood this way has me a little spooked - I would want to check that the shear loading between rods and wood is adequately low. I am not experienced nor have I run the numbers on anything but composite webs connecting to composite caps - wood unit shear strength is quite a bit lower than even E-Glass BID on the bias.

How do you know?

Your English is way better than my French (I have had some training, but it was one year and a long time ago), no need to apologize. When we refer to compression ribs, we usually mean the beefy ribs that have the drag- and antidrag-wires anchored to their intersections with the spars. With structural skins, the beefy ribs and wires are doing nothing...

I am not getting why you are restricted to 4mm cores. Is that the space between the fabric wing outer surface and the spar?

There are two ways to build sandwich skins over composite spars. One way is to use no core at all at the spar locations (see photos on my build log) and make the spar full depth of the wing there minus the skin (no core) thicknessed and another bit for the glue lines. The other way is to make cores continuous but use a structural core of same thickness at the spars, then design an adequate spar for the depth of wing minus the cored skins and glue line thicknesses. The deeper spar can be lighter, but there can be some issues requiring re-profiling with sailplanes. In both cases it is standard to bond the spar and skin together. If you feel you must maintain the standard spar clearance to the Outer Mold Line (the visible outer surface of the airplane), then you use whatever core you want away from the spars, and a 4mm core at the spars... Since you are tapering the wing, this usually means a new tapered spar anyway, so why not go full depth and shave some weight?

It sounds like you are considering your options. Doing mods to an existing wing will be flying sooner. A new set of carbon wings, tapered, will likely take longer. You get to choose or maybe do both. Tooling new wing skins is a big job, so you may still have trouble being ready in the spring with new skins.

For modifying your current wings, have you considered doing the spar beefup (after an analysis to make sure they will stay together) and covering the wings in 3/32 plywood? That might give you enough torsional stiffness without having to tool new wing skins.

A new wing is probably a project you start after stiffening the current wings to have ready for 2021.

France has an aerospace industry, engineering colleges supporting it, training etc, there must be textbooks in French for everything you are talking about. But then the German and Chinese engineers I know refer to their translations of Timoshenko, Shigley, and Juvinal for mechanics, design, and stress/strength... Unless you already have calculus, linear algebra, mechanics of materials, and beam design, learning all this stuff could be a very tall order.

Billski

 

What I do know about aerobatic airplanes is that monoplanes inherently have greater penetration on vertical maneuvers because monoplanes have less induced drag and less total drag. Induced drag is big during the pull to vertical, and total drag reduces speed at the pull and slows the airplane faster early in the vertical run. I can not imagine that biplanes can have an inherent disadvantage in roll rate though... Stout composite wings have to help with that.

If the French Team pilot is also the guy to approve or not approve your wings, then he would be a good guy to listen to, at least in how to do the mods. I would still get very analytical about making sure that whatever you add to the spars will stay in. As to the 3-4 mm thick skin restriction, that only applies through the spar areas, and you will be bonding skin to spars, right? You can still use a 6 mm core everywhere else, omit the internal wires and compression ribs, and go with foam cored graphite ribs in the places where you still need them. They will be tabbed in on one skin and you will use the flange transfer trick for the other side. This should go a long ways towards making a stiffer wing until you can go ahead with an all carbon set of wings and really bring the weight and drag down.

On the full carbon set, do you have room through the cockpit to go cantilever on the bottom wing. The upper wing can be built with continuous main and drag spars, but the lower wings I do not know about. Going full cantilever allows you to skip the flying and landing wires. A number of racing and airshow biplanes have what would otherwise be full cantilever wings with an interplane strut per side, but no wires in the wind. Big drag reducer to make accel on downlines better and allow better vertical penetration.

Billski