Not-so-solid massive core wings: Lightening the core foam

[ragflyer]

Thanks BB !

Think of the KR's as a fabric covered wing, but with thick foam backing up the fabric. It does give us some hope that a reasonable amount of foam can be left on the ground without being glassed on the inside...

Billski

Given the rear spar is a single bolt ( as mentioned by BB), the skin (glass or Dynel) is infact carrying wing torsion loads unlike a classic fabric wing. Rand essentially replaced the plywood skin of a taylor monoplane with Dynel/Glass and foam leaving the underlying wood structure.

To be clear, the Rand scheme with unbacked foam is not ideal (Rutan was not a fan of it) but it has worked over a number of years.

 

[blane.c]

What about?

The blue outline structural skin, the red foam, and the black could be anything like a carbon fiber reinforced flat panel or fabric.

 

[ragflyer]

Nope, Roark's Seventh Edition Chapter 11 is about flat plates. Chapter 13 is Shells of Revolution, and could well be useful for this same purpose. Chapter 11 is for flat plates, not the gently curved plates in one direction that are our wing skins, but they are thin (3/8" on my roughly 12" x 30" panels) and constrained on the edges, so it seems a reasonable approximation. Chapter 13 is for shell of revolution, so they are curved in one axis similar to our wing skins, but there are no appropriate edge constraints. I picked Chapter 11.

Got it. Unfortunately I am working of the sixth edition where flat plates are chapter 10. What is the title of the table?

 

[stanislavz]

Got it. Unfortunately I am working of the sixth edition where flat plates are chapter 10. What is the title of the table?

Give me in pm your mail adress.

 

[BBerson]

Foam is attached to the primary structure and hotwired to shape.
The inner ace of the foam core is just raw foam.

Not hot wired. It was sanded to airfoil contour.

 

[BBerson]

I think the foam could be curved to the airfoil contour instead of sanding as in the previous post.
Just make the ribs curved 1" smaller than the airfoil.
I remember an accident from sanding the foam too thin in the ends. Curving would prevent that.

 

[wsimpso1]

Got it. Unfortunately I am working of the sixth edition where flat plates are chapter 10. What is the title of the table?

Table 11.4 Formulas for flat plates with straight boundaries and constant thickness.
Case 8 Rectangular plate, all four edges fixed
8a Uniform over entire plate, and 8d Uniformly decreasing parallel to side b

This seemed most applicable to skins bonded to spars and ribs.

Now if you had a space frame with skins attached with screws around its periphery, Case 1a and 1d, seems more appropriate, as case 1 is simply supported around the edge, 1a Uniformly loaded and 1d Uniformly decreasing parallel to side b.

 

[Sockmonkey]

How about this.
Carve out depressions on the underside of a solid foam wing, and have the fiberglass skin conform to those depressions instead of enclosing them. The solid foam fore and aft is the "spars" and you can generally get away with a lot of shenanigans on the underside of a wing.

Curve the edges smoothly and those sections are like a regular wing with a lot of under-camber.

 

[Hephaestus]

Can I ask about the "other" way to lighten a monolithic block wing...

Remember that little section on the rohr 2-175 project on the wing being made using green fiberglass skins then EPS added, and steamed to create the final composite structure?

EPS can be done in down to like 1/2lb density, if you cut an female mold in XPS/other, layed your shells on those and followed a similar process...Compress the halfs together, add EPS then steam.

I haven't really looked into what it takes to mold EPS, but curious if this isn't something a little more achievable for a homebuilder.

 

[ragflyer]

Table 11.4 Formulas for flat plates with straight boundaries and constant thickness.
Case 8 Rectangular plate, all four edges fixed
8a Uniform over entire plate, and 8d Uniformly decreasing parallel to side b

This seemed most applicable to skins bonded to spars and ribs.

Now if you had a space frame with skins attached with screws around its periphery, Case 1a and 1d, seems more appropriate, as case 1 is simply supported around the edge, 1a Uniformly loaded and 1d Uniformly decreasing parallel to side b.

Thanks found it. It is table 26 Art 10.11 in sixth edition.

 

[ragflyer]

Take a look at KR2 wing construction.
It's pretty much what you are describing, although through different techniques.

yep! You could replace the the spar boxes with solid foam spars with uni caps and the rest could be the same as KR. You will get massive holes between the spars and will be easier to make than an creating holes through solid foam.

 

[rotax618]

Have a look at Mark Langford’s KR2S wing construction, he shaped the inner skin and laminated it, before glueing the foam into place and shaping and laminating the outer skin.

http://www.n56ml.com/owings.html

 

[Vigilant1]

yep! You could replace the the spar boxes with solid foam spars with uni caps and the rest could be the same as KR. You will get massive holes between the spars and will be easier to make than an creating holes through solid foam.

I guess "easy" is in the eye of the beholder. My personal preference would be to start with a unitary block rather than make the ribs, fit foam planks between them, then sand the planks to match the ribs. But, the KR method does work, folks enjoy building them.
The KR has 1" thick (PU) foam at the skin, unfaced at the back, forms a simply-curved panel that is unsupported for about 18" (between ribs) x 18" (main spar to rear spar). The skin is 4oz Dynell in epoxy. As far as aero loads on that wing, the wing loading was moderate (10 lb/ft2), but the maximum speed 200 MPH.
The core I'm thinking about is approx 1" thick minimum at the skin, has much more frequent support, and the wing skin is (for starters) 12oz CF. So, as others have said, there's hope.

 

[rotax618]

Mark Langford shapes and faces the underside of the foam skin, glues it in place then shapes the top surface before covering it with carbon.

 

[BBerson]

Looks like three times the hours compared with a metal wing, to me.

 

[stanislavz]

Question on time for composites.

How many area of fabric you can cover in one session for one / two / three plies ? Per single person ? Simple mold configuration - wing skins / flat panel.

 

[stanislavz]

Question on time for composites.

How many area of fabric you can cover in one session for one / two / three plies ? Per single person ? Simple mold configuration - wing skins / flat panel.

 

[wsimpso1]

Stan,

I have routinely laid up an entire wing skin in 90 minutes. Yeah, we have learned what we are doing. My wing skins are roughly 34" by 120", with 22 oz Triax, a 3/8 Divinycel core, 18 oz Biax, some woven tape, three pieces of peel ply, a couple pieces of perforated ply, a blanket, and are vacuum bagged. Yeah, I spend time prepping all materials, filling the epoxy pump, labeling, organizing, and the like. But when my crew shows up, we just want to go! Oh, we do this with a minimum of six people, usually three who know what they are doing and three or more who are willing, conscientious, and can take direction. At 1 hour 45 minutes, shoes have been dropped at the door, clothes changed, and we are eating chips and salsa, drinking beer, and waiting on Cottage Inn pizza. I have no trouble getting a crew for these events. That's a hint son (said in my best Foghorn Leghorn voice).

Next up is the stub skins. roughly 48" x 28" each, same laminate schedule or thicker (wing walk on top skins), and I do two in three hours, solo. Works great. Billski's Fiberglass Bird | Page 2 | HomeBuiltAirplanes.com I have other parts you can see on my build log, none take more than about 90 minutes from first pump of epoxy to sealed bag under vacuum.

If you are too loner for this, you might go vacuum infusion. Lots of info on how to do it on hba.com and elsewhere. Work by yourself on the whole thing. No time table is enforced until EVERYTHING is right, then you mix the resin and open the resin valve. There is either software or a learning curve on getting the whole part to fill...

Billski

 

[stanislavz]

Billski. Thank you. No, i am not lonely on this, one or two chaps available on request + while doing some test samples it looks like tailoring temperature in shop is way to go. But you have to start with warm epoxy. And now getting 40/60 glue to fiber ratio due to this. 2 layers of 450gsm weights 1.6kg per m2. But - this was one fifth or sixts test. Foil, spread warm epoxy on it, lay you fibers and other foil. With some help of hair dryer and roller all extra epoxy is mowing where you want it be / new area. With heat off - it is quite thick at 12-15 celsius degree. Gel time was extended from 60minutes to three hours.

For me it is ok for stabilator. For wing will want one extra chap or slower hardener..

 

[Vigilant1]

Can I ask about the "other" way to lighten a monolithic block wing...

Remember that little section on the rohr 2-175 project on the wing being made using green fiberglass skins then EPS added, and steamed to create the final composite structure?

EPS can be done in down to like 1/2lb density, if you cut an female mold in XPS/other, layed your shells on those and followed a similar process...Compress the halfs together, add EPS then steam.

I haven't really looked into what it takes to mold EPS, but curious if this isn't something a little more achievable for a homebuilder.

Everyone is different, but I would find hotwiring a foam core and doing a layup on top of it, then removing the hot-wired unneeded channels as more achievable than building two female molds, fabricating the wing skins in them, procuring EPS kernals, and injecting steam into the void to get them to puff up exactly right. I don't have a large quantity of available process steam at home, and making that happen would be some high adventure.
There is EPS that is half the density of XPS, but it is much less than 1/2 the compressive strength of the "normal" XPS foam used for solid core wings. ASTM C578 Type V111 EPS has a minimum density 1.15 pcf, but it has a compressive strength of 13 psi (with 10% compression). It is very soft. You can get 60 psi EPS (it would be ASTM C578 Type XV), but it has a density of 3 pcf (about 50% more weight than comparable XPS).