Quantcast

Foam ribs with plywood capstrips?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

ToddK

Well-Known Member
HBA Supporter
Joined
Jan 13, 2016
Messages
414
Location
The Real Texas
I remember that seeing that Brian Austein's Woodpecker was built with foam ribs and wood cap strips. That turned out to be a fairly fast and tough little bird. Unfortunately there have never been any photos or details about the inner structure of the wing made available. I have widely circulated tests from a 1999 sail plane group where a member found that a solid 1" foam rib with a cap strip turned out to be incredibly strong and very light.

What I am wondering is if wood ribs are ever designed to bear a portion of the horizontal compression loads? My particular airplane has 3 1x1 spruce members to handle compression loads between the spars, and I am wondering of the stick build ribs are also factored into loading, and if foam ribs would screw all that up.

The obvious thing to do would be to build a wood rib, and a foam rib and see how each handles compression loading. Just curious if any one has already done that or had any thoughts on the topic.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,461
Location
Rocky Mountains
If the attached file is the rib testing you are thinking about - The testing method could use some more thought.
The typical capped foam rib either needs to be rib stitched or if using the glue on method should have the equivalent of rib stitching incorporated into the rib, like the Lazair ribs:
Lazair6.jpg
The standard method of load testing, piling weighton to an upside down wing or rib, doesn't similuate the tension load trying to separate the caps from the foam - both top AND bottom. This is a similar situation to certified aircraft that either depend on rib stitching or PK screws/Martin fabric clips to hold the fabric in place.
 

Attachments

ToddK

Well-Known Member
HBA Supporter
Joined
Jan 13, 2016
Messages
414
Location
The Real Texas
Thanks for info. So, if I am correct in what you have written, what you are describing are forces trying to separate that plywood cap strip that has been epoxied to the 1" (DOW) foam rib, and that the typical load test does not cover that. What is the difference between say, a Kitfox rib that has a thin plywood web glued to a cap strip? Both seem pretty different from using contact cement to glue aluminum to foam, which seems like it would not adhere as well as wood, foam, and epoxy. They would be subjected to the same forces, and require the same kind of testing right? What kind of test(s) could simulate that? Thanks for you help.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
7,995
Location
Fort Walton Beach, Florida, USA
I am not sure if this is useful for you, but some French builders have integrated foam ribs, plywood capstrips, plywood leading edge wrap, and box spars to make quite elegant hybrid construction wings for Mignet HM.293s and simlar types. The article -- Pou-Guide - Nervures en polystyrène extrudé -- is in French, but the images below are pretty self-explanatory. Cheers, Matthew

 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,641
Location
US
I am not sure if this is useful for you, but some French builders have integrated foam ribs, plywood capstrips, plywood leading edge wrap, and box spars to make quite elegant hybrid construction wings for Mignet HM.293s and simlar types.
Thanks, Matthew. What has been the method of attaching the fabric, and has it worked in practice? I know some Sky Pup builders have used FastBond to glue the covering to the capstrips.

Separate subject, but it seems to me that a cap of fiberglass/epoxy with flanges folded down over the XPS ribs would be light and also provide good resistance to having the capstrip debonded from the foam by avoiding a "peeling edge" there.
 

poormansairforce

Well-Known Member
Joined
Mar 28, 2017
Messages
1,086
Location
Just an Ohioan
I've always thought it would work great if you wrapped a number of strands of CF tow around the ribs at 45°. It would add strength and keep the caps on.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
7,995
Location
Fort Walton Beach, Florida, USA
My understanding is that adhesive was used for the fabric given the much greater surface area compared to stick ribs but I don't know what kind exactly. I agree that this would be an excellent application for Oratax though I would want to experiment first to be sure that the heat of the process doesn't damage the foam through the plywood. IIRC, polystyrene melts at about 250 C but starts to soften at 100 C, Oratax cures at 70 C using a 100 C iron, so I *think* it would be OK.
 

Geraldc

Well-Known Member
Joined
Nov 12, 2011
Messages
456
Location
nz
My understanding is that adhesive was used for the fabric given the much greater surface area compared to stick ribs but I don't know what kind exactly. I agree that this would be an excellent application for Oratax though I would want to experiment first to be sure that the heat of the process doesn't damage the foam through the plywood. IIRC, polystyrene melts at about 250 C but starts to soften at 100 C, Oratax cures at 70 C using a 100 C iron, so I *think* it would be OK.
Some of the new pvc foams designed for prepreg would be suitable.
They are stronger and can be heated without shrinking like some did.
I use Corecell M by Gurit GURIT® CORECELL™ M
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,015
Location
Memphis, TN
Most of the homebuilt biplanes have compression ribs. Slightly different internally than regular ribs with ply covering both sides. Pitts, Acrosport and pretty sure Skybolt are done that way. I want to say three per panel.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,461
Location
Rocky Mountains
What kind of test(s) could simulate that? Thanks for you help.
I've been giving this some thought because I want to use this method of rib construction. The only thing I've come up with is kind of a wiffle tree with the attaching links glued at intervals to the the rib cap. The group on the top would need to be run through a pulley system if simple weights were used.

My thinking at the moment is to just copy the 'internal' rib-stitching similar to the ribs in the Lazair photo. I have yet to decide if I'll use Spectra/Dyneema or just plain old Dacron. I'm leaning to Dyneema at the moment even though it has higher creep. From what I've been able to find the creep rate is not linear and a simple doubling up should 'solve' the problem. Dacron has been proven to work for years on conventional ribs but I'm not sure it is stiff enough to prevent cap separation on a deep foam rib.

Archery types seem to have decided that the creep with their Dyneema bow strings is not as pronounced as theory would suggest. There is also a bow string available that is made from a 'creepless' Dyneema.

Spectra fishing line is available almost everywhere in strengths appropriate for rib stitching.

Edit: It tought I posted this back around post #4..... so It's kind of out of order.
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,822
I wouldn't use plywood cap strips. Half the grain is running the wrong way, so they'd have to be thicker and heavier.

Some extruded polystyrene foams are a lot stronger than others.

The Sky pup ribs are 3/4 inch thick with 3/4 inch by 3/32 "softwood" cap strips glued on in the back, plywood sheeting on the front. Keep in mind that the Sky Pup is a slow plane. Also that it only has the one spar.
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,822
Vigilant1:
I think light glass epoxy over the cap strip and down the sided of the ribs a short distance would make for a stronger attachment, but I don't think glass cloth would make a very good cap strip by itself. If a design was already using wood, then it might be hard to beat for the cap strips.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,015
Location
Memphis, TN
Originally the Woodpecker had the fabric glued to the ribs. No rib stitch. That’s why Brian had wood strips on the ribs, for the support area for glue. The fabric started pulling off the ribs; he just sank screws in the wood. He made it look a little better with some tape; I think to stop the questions on the ramp. After he got everything sorted out, he cut all the covering off and did it up like seen in Sport Aviation. It was pretty plain before.
 

TiPi

Well-Known Member
Log Member
Joined
Aug 25, 2019
Messages
345
Location
Peeramon (AUS)
I'm leaning to Dyneema at the moment even though it has higher creep
Check the durability of Dyneema or any other plastic type product. The Dyneema ropes for rescue shutes have a limited life (mainly due to some sections exposed to UV rays).
Some interesting reading attached
 

Attachments

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,461
Location
Rocky Mountains
Check the durability of Dyneema or any other plastic type product.
Actually one of the 2 reasons I'm favoring Dyneema/Spectra at the moment. Stiffer than Dacron and better UV resistance as well. Dacron has proven to be quite acceptable in service so I don't see a reason Dyneema would suffer more from UV than Dacron?

Carbon would be better but the trade for ease of use seems worth it to me.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
7,995
Location
Fort Walton Beach, Florida, USA
My understanding is that polyester fabric material is itself a little more UV resistant than nylon, and ultra high molecular weight polyethylene (aka Dyneema/Specrtra) is better still. For control or bracing "cables" I would simply design for 50% strength to allow for UV exposure, set the "sell by date" of the cables based on 12 hours per day UV exposure to get down to that 50% strength, and then keep the plane out of the sun to be sure I am miles away from that point. I don't really see the point of using exotic fabric covering, however, unless you are doing something very exotic like human-powered flight. Oratex is well-proven, very durable, lighter and easier to apply than traditional covering, that seems plenty good enough for me.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,461
Location
Rocky Mountains
I don't really see the point of using exotic fabric covering,
I'm interested in using the UHMWPE for is the rib stitching function. Cover would be normal Dacron processes or Oretex. The only reason not to use plain old Dacron is that it stretches more under load which my lead to failure of the capstrip/foam bond when used with a cover system that depends on glue rather than conventional through the fabric rib stitching.

The load paths are different.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
7,995
Location
Fort Walton Beach, Florida, USA
OK, so what you're concerned about is the capstrip-foam bond, not the fabric-capstrip bond? I have never heard of that being an issue in Sky Pups or other designs that use plywood over foam.

My suspicion is that even thin plywood has enough stiffness to diffuse the peeling force such that you'd peel the fabric off the plywood before you'd peel the plywood off the foam. It might be worth getting an Oratex sample kit and trying it out a test piece of plywood-foam-plywood sandwich cut into slices to simulate ribs so you are not coming up with a solution to a problem that doesn't exist.

Regardless, Oratex instructions are basically to use rib stitching or rivets or what have you, whatever the plans call for with conventional covering, so presumbly that would also mean no stitching if the plans don't call for it.

I would also think that it may be a false economy to integrating compression and airfoil profile ribs. There are plenty of designs out there that use two- or three-bay wing bracing, which suggests three or four compression struts/ribs per wing panel. Separating the compression strut function from the airfoil profile ribs seems a lot easier in terms of math and then each can be optimized for their specific function.

To give you an idea of how simple compression struts can be, the Evans VP-2 uses three 1" diameter birch dowels in each wing panel with 1/2" diameter holes in each end to fit over the bolt heads for the drag/anti-drag wire fittings. They are not attached in any other way, just held in place by...compression. ;-)
 
Top