Moldless vs molded weight

[zolotiyeruki]

I've been watching the RevelAero posts on Facebook, and have been wondering: Could you make a Long-EZ/Cozy/whatever even lighter if you used molded parts for the wings and canard?

I understand the biggest benefit of moldless construction when RAF was selling plans--it makes composite construction accessible to the average Joe, without the need for expensive tooling, or selling complete assemblies, etc.  But if you had molds for the wings/winglets/canard, could there be some weight savings to be found, compared to moldless foam core construction?

This is just spitballing--I recognize that you'd need to do some proper engineering to determine rib spacing, wing spar design, layup schedule for the skins, etc.  But roughly speaking, would you expect to see much of a weight difference?  I imagine you'd have less filling and paint on the outside, and less weight on the inside due to the absence of foam, but more weight due to the ribs. And I'm ignoring for now any difference in the time required to use one or the other technique. 

[Kent Ashton]

Yeah, I imagine you'd have a lighter airplane, but also a lighter wallet and a 5% chance of flying compared to a person that builds from plans.  You propose to save time on filling and painting  but the time expended in mold-making to get such parts is enormous.  If money is no object, try Aerocanard   https://aerocad.com/16-aerocanard-fg    or just build a Velocity.   Ronenberg's molded Berkut did not sell enough to stay in business.  Here is another molded dream:  "A-solution"    https://www.canardzone.com/forums/topic/21972-sales-ive-seen/?do=findComment&comment=63573     Bob spent months--maybe years--making  beautiful molds that never produced an airplane as far as I know.   RevelAero, with their beautiful computer-cut, anodized and carbon fiber parts, looks very expensive to me, especially for a one-seater.   They may sell a few but I doubt the market will support them.

 

 

Edited January 26, 2022 by Kent Ashton

[zolotiyeruki]

Yeah, I acknowledge the realities you bring up, in terms of cost, time to build, likelihood of finishing, etc.  I'm not seriously proposing tackling such a project, I'm simply trying to get a feel for what the weight impact would be.

[Marc Zeitlin]

1 hour ago, zolotiyeruki said:

Yeah, I acknowledge the realities you bring up, in terms of cost, time to build, likelihood of finishing, etc.  I'm not seriously proposing tackling such a project, I'm simply trying to get a feel for what the weight impact would be.

In a perfect world, if you could magically build exactly the same aircraft but mold/VB it, I've estimated that you'd save maybe 70 lb, max. The reason is that out of the 1150 - 1200 lb that a COZY MKIV weighs empty, or the 850 - 1050 lb. that a Long-EZ weighs empty, first you have to remove EVERYTHING that isn't composites, and see what you have left, and THEN see what you've  got left that's moldable, since all assembly layups are not (and the plane wasn't designed to be assembled in molds or with features that use molds or VB'ing. Then you've got to remove the composite landing gear bows and nose strut.

And once you do that, you realize that maybe you've got 300 lb. of composites, all told, that's modifiable without a redesign of the aircraft (which even this would need, to some extent). You're not going to reduce the weight of any of the fiberglass, so what you've got less is fill (but at 12 lb. / gallon for epoxy, MAYBE you can save 10 - 20 lb. of fill) and layup epoxy. If you assume that you can cut the glass/epoxy ratio from 45/65 (decent contact layups) to 55/45 (decent VB'ing), you can get a 100 lb. layup down to 82 lb. So for 300 lb. of composites, you'd save 3 x 18 = 54 lb. Add the 10 - 20 lb. of fill, and you're in the 60  - 70 lb. range.

As Kent says, clearly not worth the time and effort unless you're in business manufacturing them.

And don't expect hollow wings to weigh substantially less than solid core wings, even with bagging - they won't. Bill Simpson on the Homebuiltairplanes.com forum has posted regarding the comparative weights, and until you get to very large wings, solid core is at least as good from a weight standpoint.

[zolotiyeruki]

Thanks for taking the time for such a thorough reply, Marc.  60-70 lb on a 900-1200lb aircraft is nothing to sniff at, but not a game changer, either.  I lurk on the HBA forums as well, but hadn't seen this particular discussion.  I'll go do some more searching to see if I can dig it up.

[zolotiyeruki]

On 1/27/2022 at 12:26 PM, zolotiyeruki said:

Thanks for taking the time for such a thorough reply, Marc.  60-70 lb on a 900-1200lb aircraft is nothing to sniff at, but not a game changer, either.  I lurk on the HBA forums as well, but hadn't seen this particular discussion.  I'll go do some more searching to see if I can dig it up.

For the benefit of anyone coming along later with a similar question, a relevant post by the aforementioned Billski is here: https://www.homebuiltairplanes.com/forums/threads/wing-design-for-lightest-possible-wing.31780/page-2#post-477756

[court]

Is there a good summary of what was said? That forum prevents non-members from seeing the text. The $12.99 (per month) doesn't make sense just to read one post.

[zolotiyeruki]

I'm pretty sure you can sign up for a free account at HBA, although there are paid options to help support the forum.

Below is a quote from the linked post.  The short version: for the size airplanes we're talking about, the wing weight is a few (~3%) lighter using a massive foam core and moldless construction, vs a molded wing with the same dimensions.

So if you're thinking of doing it for weight savings, hollow wings won't do it, and as Marc says above, vacuum bagging the other parts will save some weight, but not a ton.

If you're looking to make more than one copy of a part, and want to reduce the amount of time for filling/sanding/paint prep, then the molded approach may be more attractive.

Quote

I was not talking equivalents for a 60 knot Vne airplane. The OP talked about a replacement wing for the Luscomb. Luscombs have a Vne much higher than the Aluminum Dragon and have been flown upward of 180 knots. No one is admitting to this illegal flying, but a rewinged Luscomb should probably be designed for that. When q is 9 times higher, you need a lot more torsional stiffness to keep flutter at bay. When you design for the upper end of an improved Luscomb, you must have a much stiffer structure and much stiffer skins. For a 900 pound airplane with a 1500 pound gross weight that must survive at three times the airspeed and 9 times the dynamic pressure, the wing areas may be close, but there is a lot more glass in spars and skins than you might use in your foot launched glider. In the OP's airplane range, he will usually save weight with massive foam cored wings.

Let's run some numbers. Let's say a 48 inch chord, 15% thick, one foot long piece, for a fast airplane. Flaps and ailerons are 25%, so the wing proper is about 70%, let's say 33.6" with a 7.2" thickness. Total cross section area is about 219 in^2 and our foot long piece is 2632 in^3.

Spar is the same for both. Let's say 8.16 in^2, 97.92 in^3, and 4.70 lb.

The rest of the approximations: Full cross section is about 80% of spar depth times chord of the piece. Perimeter of the piece is about 2.2*chord of the piece - we have an overlap at the leading edge for bonding and a drag spar to close out the back edge. Glass-epoxy and adhesives are about 0.048 lb/in^3. Blue foam is 2 lb/ft^3. PVC foam is 8 l/ft^3.

Skin of the solid cored section is 3 UNI, about 0.022 thick, so 1.63 in^2, 19.51 in^3, and 0.94 lb;

Foam is about 183.75 in^3, 2205 in^3, and about 2.55 lb;

So the solid foam wing is 4.70 + 0.94 + 2.55 = 8.19 lb/running foot.

Now for foam cored skins. The outside is still 3 UNI, the inside is 2 BID, total thickness is 0.045", 3.33 in^2, 39.92 in^3, and 1.92 lb.

Foam cores are about 18.48 in^2, 221.8 in^3, and 1.03 lb.

Next we have adhesives. Each side of each spar has a glue line that does not exist in the solid core wing. I get about 0.36 in^2, 4.32 u\in^3, and 0.21 lb.

Flanges are applied at the leading edge and at the drag spar. I get 16 plies 2" wide and 0.009" thick for 0.29 in^2, 3.46 in^3, and 0.17 pounds.

Ribs, are still needed, let's say one is used every three feet. Each will have about 171 in^2 of area, and they run about 0.5 lb/ft^2, or about 0.59 pounds. Each one foot section has about 1/3 of that or about 0.20 pounds.

Each rib also has flanges, I count 10 plies per rib 2" wide and 0.009 thick, at about 0.75 pounds per rib or 0.25 pounds per running foot - Yeah the flanges weigh as much or more than the ribs themselves. Seen it over and over.

Hmm, 4.7 + 1.92 + 1.03 + 0.21 + 0.17 + 0.20 + 0.25 = 8.46 lb/ running foot.

I have run these sorts of numbers over and over. In faster airplanes - where you can justify a 100% composite structure - massive foam is lighter. In an airplane with much lower wing loadings and tiny q, the composite amount drops, but the foam amount won't and yeah, the foam might weigh more than the internal parts to replace it. But not in faster airplanes.

Billski

As an aside, I've also wondered what would happen if you got really ambitious and built giant molds for fabricating a monolithic wings-and-spar combination. You'd lose trailerability, but also a bunch of overlapping spar length, the wing attach bolts, etc.

Edited September 12, 2023 by zolotiyeruki

[zolotiyeruki]

5 hours ago, zolotiyeruki said:

As an aside, I've also wondered what would happen if you got really ambitious and built giant molds for fabricating a monolithic wings-and-spar combination. You'd lose trailerability, but also a bunch of overlapping spar length, the wing attach bolts, etc.

I couldn't get the idea out of my head, so I dug into the plans and made a first pass at determining how much weight could be saved.  It's...not much.  Between the LWA's, extra spar caps, extra shear web, reinforcing layers of glass for the LWAs, hardware, etc, it looks like you'd save a whopping 10 lbs.  And in the process, you'd have to do some design rework--the spar caps on the wings are aft of the spar caps on the center section spar, so you'd have to rework the wing design to move the spar caps forward of the shear web, and that sounds like an awful lot of design and fabrication work to save 10 lbs.

And further into the weeds, the wing's top spar cap is 7 layers of UNI tape, compared to 5 on the bottom.  Is that connected with the g limits of +3.8/-1g listed in the POH?

[Marc Zeitlin]

1 hour ago, zolotiyeruki said:

And further into the weeds, the wing's top spar cap is 7 layers of UNI tape, compared to 5 on the bottom.  Is that connected with the g limits of +3.8/-1g listed in the POH?

Yes. Upper spar caps are always thicker than those on the bottom for non-aerobatic aircraft, since the bottom cap is in tension (no buckling) and the top is in compression (and will have buckling limitations, rather than just strength limitations). Caps would only be the same if the negative "G" limit was the same as the positive "G" limit.

[thseng]

Something like this?  Spar cap only shown.  

[zolotiyeruki]

21 hours ago, thseng said:

Something like this?  Spar cap only shown. 

Yes, in part.  You'd have to increase the sweep of the center section spar, but that's not all.  The stock wing has the shear web forward of the spar cap, while the CSS has the shear web aft of the spar cap. You'd have to relocate the wing's shear web to be aft of the spar cap, in order for the shear web to be continuous between the CSS and the wing.  I'm no airplane designer (but I'm learning!), but moving the shear web aft sends up red flags--I don't know the proper terminology, but I'd be worried that moving the "center of stiffness" aft in relation to the chordwise lift distribution could lead to structural instability.  Think Tacoma Narrows Bridge.  It seems to me that you'd be better served by leaving the CSS's sweep alone, and moving the wing's spar caps to be forward of the shear web.