replacing plywood construction with cored composite panels

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BobbyZ

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I'm currently looking at a plywood/foam design that uses a VW engine for a first build.While it is a great plane it's a little over 103 weight and I think it could be made to comply with the use of lighter materials and a different engine.

Now before this turns into a thread on the pitfalls of redesigning a plane and all that is involved I want to say that I'd most likely pay to have someone go over the changes.But before I proceed and spend the money I want to do some rough calculations on my own as a learning exercise.

I know they also have different characteristics but I've worked on boats where we've substituted honeycomb panels for structural interior bulkheads that originally were designed for marine ply.So I figured it wouldnt be a massive undertaking.

So in essence would it be feasible to replace the older style ply construction with a newer sandwich style composite?

Ideally I'd like to redesign the plane to lose some weight and gain a little interior volume and I feel its possible.But I have no clue as to where to look to get started.
 

Vigilant1

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I'm currently looking at a plywood/foam design that uses a VW engine for a first build.While it is a great plane it's a little over 103 weight and I think it could be made to comply with the use of lighter materials and a different engine.
Just some initial thoughts:
-- Does the present design meet the PArt 103 stall criteria? If not, you'll probably need more wing, and that will add weight. Do the calcs for the present wing/stall speed modified for the all-up Part 103 weight.
-- As a quick back-of-the-envelope calculation, you could start by finding the weight per sq ft of the major plywood components in the present plane right now (bulkheads, skin, spar, etc) and then figure the weight of a cored sandwich with similar strength in the direction of greatest importance in each application (compression, tension, etc). Now, this won't be optimized--it's quite possible that the plywood chosen was selected just because that thickness of plywood is readily available, and it is considerably overstrength for the application, so "duplicating" it will result in an overstrength composite part. Be aware that there's a minimum thickness for the external composite skin surface that is driven by handling requirements (hangar rash, careless look-e-loos at airshows, etc).
-- Just to say it--I think you'll find that modifying an existing wood design to meet Part 103 criteria won't work. Overall, aluminum light aircraft construction tends to be slightly less heavy than composite conventional designs, and even these aluminum Part 103 aircraft can barely make the weight requirements with very close attention to every detail in order to reduce weight. To get the wing area needed to meet the Part 103 stall requirements, it requires a construction material that is very light per sq ft.
 

TFF

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Usually the modifications, although intended to be better, almost always come out worse. If you want to use a full VW is going to be just about impossible. What design and how much weight does it need cutting?
 

Topaz

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I'm currently looking at a plywood/foam design that uses a VW engine for a first build.While it is a great plane it's a little over 103 weight and I think it could be made to comply with the use of lighter materials and a different engine.
What design? Does it meet the rest of the requirements of Part 103?

Now before this turns into a thread on the pitfalls of redesigning a plane and all that is involved I want to say that I'd most likely pay to have someone go over the changes.But before I proceed and spend the money I want to do some rough calculations on my own as a learning exercise.
Learning exercises are excellent. :)

I know they also have different characteristics but I've worked on boats where we've substituted honeycomb panels for structural interior bulkheads that originally were designed for marine ply.So I figured it wouldnt be a massive undertaking.
In terms of design, your experience on boats will not be applicable here. Simply substituting material on most boats is non-critical so long as the strength and weight of the materials involved are broadly comparable. This is not true of airplanes, where the structures are much more refined and weight-critical.

So in essence would it be feasible to replace the older style ply construction with a newer sandwich style composite?
Feasible? Yes. How's your matrix algebra?

Ideally I'd like to redesign the plane to lose some weight and gain a little interior volume and I feel its possible.But I have no clue as to where to look to get started.
Here's your one-two punch. Read Peery first, then Niu.

Aircraft Structures Peery, David J.

Composite Airframe Structures Niu, Michael C.
 
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ragflyer

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The design challenge with part 103 is not so much the 254lb limit- this can be easily met with any of the traditional construction techniques- but meeting the combination of weight and wing loading. A part 103 UL will need about 300- 325 sq.ft of wing wetted area (give or take a little depending on use flaps). Covering this large area with anything but fabric makes it pretty much impossible to meet part 103.

Plywood is in fact very efficient when used as a skin(comparable to carbon). Unfortunately what you need is fabric. In a part 103UL the high tech materials are best applied in spars and ribs and leave the skin to good old fabric.

Finally if the design you are looking at is very close to part 103 weight with plywood then almost certainly you are not meeting the stall speed requirement of part 103.
 

BobbyZ

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Thanks for the excellent replies.
One thing I should add is being UL legal isnt necessarily a deal breaker.I'm looking at it as a added bonus as it will get me back in the air easier.It's a long story but when I was young and dumb I voluntarily surrendered my ticket.It was the right thing to do at the time but I have yet to see how bad it's going to be to sort it out now.

As for the design I'm leaning towards the JN1 or one of it's siblings like the Flying Squirrel etc.Both can be seen here and I'll check on the speeds as I forgot to look as I was focuses solely on weight. http://flysquirrel.net/JN1/JN1.html

Topaz,
I agree with you 100% on most boats its a hand grenades approach is close enough where any weight saved helps sell more boats and save a drop of fuel. But these aren't most boats and i should have clarified that.The boats I'm referring to were IOGP F1 Champs/APBA F-1's and the like.Essentially they're 20ft hydros with a hull weight around 5-600lbs and usually a hair under 900lbs with fuel and motor minus driver capable of 150+ mph and 4 G's in the turns.
Over the last 10+ years we've rebuilt quite a few plywood hydros to ply/composite boats and built some classic plywood designs mainly in composites.Years ago they changed the rules to keep the sponsons from spearing guys and added crumple zones up front.So for a while the shop I used to work for was quite busy retro fitting older designs to the new rules.

As for the books,I'm looking on Amazon now.Thanks

Motor wise I have a VW I could use from my Ghia build but I'd prefer to use something else,but I want the option to run this one if the budget requires it.

Lastly please keep in mind anything I do will be gone over by a pro for safety reasons at the very least.But I still want to learn as much as I can design wise.
 

ragflyer

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on the contrary, Dana, the Hummel is a perfect example of what I am saying. Per spec meets the empty weight limit but does not simultaneously meet the wing loading requirement: 452/112 = 4.03, in other words the Cl of the wing needs to be a tad over 2. Show me (or the FAA) how that (unflaped) wing can can have a lift coefficient that high.
 
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StarJar

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Looks like an excellent plane from reàding the specs, but you'd have to shave off 66 lbs to make it an ultralight. My guess is that you would shave off less than 15 lbs. by substituting composite for plywood. I could be wrong though.
In any case it looks like a really cool plane.
 
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Dana

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on the contrary, Dana, the Hummel is a perfect example of what I am saying. Per spec meets the empty weight limit but does not simultaneously meet the wing loading requirement: 452/112 = 4.03, in other words the Cl of the wing needs to be a tad over 2. Show me (or the FAA) how that (unflaped) wing can can have a lift coefficient that high.
Hmmm, yes, it would seem so. The UC is so generally spoken of as a "legal" ultralight that I'm surprised at that. Of course the AC103-7 appendix charts that are accepted as "proof" are very conservative, but even so the UC doesn't quite make it without flaps. Flaperons, as long as they're present even if not effective (like the Minimax) would do it.

Dana
 

BobbyZ

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The plans claim to stall at 28mph and cruise at 60 so it surely goes faster than the regs allow.But that can be easily dealt with.My question is I'm assuming the listed stall speed is not engine off.So besides the weight issue it looks to be a stretch for a UL legal bird.
Would reducing weight help the stall speed issue at all?
Also is it close enough that some STOL tricks could help or is it just too hot a wing period.

Last but least is who checks these things on a UL?I've flown a few in the past that friends built/restored but never asked a lot of details nor do I think they cared.They just built them and flew and thats all there was too it,but they always stayed close to home........I guess it's time to look around the UL forum ;)
 

wsimpso1

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Most composite birds have a minimum amount of composite skin to keep them from being broken in build and ground handling. Faster airplanes with glass skins usually have 20-22 oz/yard of glass fabric, graphite fiber around 10-12 oz/yard. Then remember that the foam has to have something on the other side too. Get into slower airplanes, and you could probably get lighter, but you ought to build some sample panels and see how easily they are damaged. Once you have a weight you think you can tolerate, see if you saved any weight over the original design.

Then you can start checking out hardpoints. Anyplace you attach one piece to another you need to either bond and do some layups, or you will need a hardpoint that will carry fasteners, pins, etc. Composites do not end up looking so light once you have looked at the entire airplane.

Best advice I can give you is that if the airplane, as designed meets your mission and has a good safety record, just build to plans and be happy. Designing a new entire airplane (and that is really what you are talking about) will multiply your build time, and not by a little either.

Billski
 

BobbyZ

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wsimpsono1,
I've worked with panels before so I'm no stranger to bedding in hard points for mounting things and the like.Although I'm sure there is more to it for aircraft and that is part of why I want to do this redesign is to learn.

We actually used a product called Honeylite inside the bulkhead panels.That was aramid fibers and paper of all things impregnated with a resin that was shaped in a honey comb core.So while being super light and strong we had to be very careful about how things were formed in order to keep water out of the core.But even with the draw backs it made for a super light strong and rigid panel.The company also had some really nice aluminium honeycomb cores they wanted us to try but we were afraid of the chance of galvanic corrosion and never used it. I actually still have quite a bit of the sample material that they gave us.Granted I dont know if it'll work for this project either but I'll try.

I appreciate the caution about redesigning a project and it is great advice for someone looking to go flying.But I'm interested in this more for the project aspect over the finished product.I actually have a project plane that needs very little to be airworthy.It's been recovered but it's still in silver dope and needs paint and a annual.Chances are once I figure out the hoops the FAA has in store for me to get current,I'll get a nice coat of white on her and go flying.

The more I look into things the more I see a completely new design coming about and I'm fine with that.I'm thinking to start with the wing and go from there.As it sits now I've got a LOT of reading ahead of myself ;) Along with some testing using some old unused materials I ended up with.It's not anything I'd use to make critical parts from since I dont know exactly what it is but I think I should be able to see if my ideas are feasible.
 

lr27

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In no particular order.

What kind of plywood is specified? In areas that are designed for stiffness, I could see a lighter type of plywood that was slightly thicker. Okume? (sp??) Of course you'd have to evaluate things carefully before doing that, since it still counts as a redesign.

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Regarding plywood skin, I have seen a couple of ultralight plans where 1/32" ply is used between the spar and the leading edge. The VJ23, according to the plans froma magazine, used a 1/32" shear web! (Ok, that's a REALLY light aircraft, but, in flight, over halfway to the weight of an ultralight. Assuming 170 lb pilot) Still not light enough for everything! Definitely not something to leave unattended at the airshow.
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Even a 1/2 VW is rather heavy. I hear a stripped down Briggs and Stratton of 20 or 25 hp can be under 60 lbs! According to the drawings, the Jn-1 uses a 440 Kawasaki engine with a rather optimistic power rating. I'm sure that's much lighter than a 1/2 VW. If you make a part 103 ultralight with a long wing, with a redrive that swings a long propeller, you will need much less horsepower to climb. If it's really long you can probably use a paramotor. Bailey has a 4-stroke one, though I can't say I'm familiar enough with them or the industry.
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Long wings may be easier if you use a thick airfoil. Fortunately, there seem to be suitable airfoils that are 20 percent thick! Or, at least, Xfoil likes them. I found some with fairly high lift and with very moderate pitching moments. I went nuts and dug up a bunch that appear here: https://www.homebuiltairplanes.com/forums/showthread.php?t=25967&page=7 (Sandlin Goat or Similar thread in Light Stuff, page 7)
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There ARE some airfoils which have been tested to a Cl over 2. Others look good for that in Xfoil. If you don't mind a huge pitching moment and a thin trailing edge, you can get this at Reynolds numbers as low as 300,000, but I think ultralights would be in the 1 to 2 million range when they're flying slowly. (S1233, it's a little wacky, but it gets there). Just now, I found this paper where they tested the Eppler 423 up to a Cl of 2 at a Reynolds number of only 140,000. But I lost the URL! Xfoil thinks the Icarus 5 airfoil, which was used on a rigid wing hang glider, gets up over a Cl of 2 at a Reynolds number of 1,500,000. It doesn't look hard to build, either, and the pitching moment is pretty small. Build in a little bit of flap and Xfoil goes into paroxysms and awards it a Cl of 2.3. The gnu-v airfoil appears to be almost exactly the same, to the point where you can't see its polar behind that of the Icarus V.
icarus v polar with gnu-5 s1223 icarus with a little flap.jpgs1223rtl-il_l.jpg

A bit of background on the glider itself:
http://www.nestofdragons.net/rigid-wings-overview/footlaunch-listing/icarus-5/

I think some of the info I have on the Icarus came from Norman in his Kiceniuk Icarus V airfoil thread.

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BobbyZ,
Aren't there some existing part 103 designs that meet your requirements? I'm sure you could find one in your material of choice.
 

Victor Bravo

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In order to make something significantly lighter in composite than a well designed wooden aircraft structure, it usually takes a tremendous effort. A university, or a significant team going for a specific goal. Not an average homebuilder.

You can also save a significant amount of weight on a Part 103 airplane by using a lighter engine. The old standby Rotax 277 and 377 engines have been surpassed in weight and power by the recent developments in paramotor engines. You stand a much better chance at cutting 20 or 25 pounds off the airplane this way than you do with exotic materials.

The big exception to this is if you a re able to swap a steel or aluminum spring landing gear for a properly made carbon gear. That will gain you some ground.

The JN-1 and/or M-19 Squirrel are going to be pretty difficult to get down into Part 103 IMHO.
 

ToddK

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The M19 is impossible to get into part 103, its heavier then then JN-1 which is already too heavy for 103.
 

DangerZone

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I'm currently looking at a plywood/foam design that uses a VW engine for a first build.While it is a great plane it's a little over 103 weight and I think it could be made to comply with the use of lighter materials and a different engine.

Now before this turns into a thread on the pitfalls of redesigning a plane and all that is involved I want to say that I'd most likely pay to have someone go over the changes.But before I proceed and spend the money I want to do some rough calculations on my own as a learning exercise.

I know they also have different characteristics but I've worked on boats where we've substituted honeycomb panels for structural interior bulkheads that originally were designed for marine ply.So I figured it wouldnt be a massive undertaking.

So in essence would it be feasible to replace the older style ply construction with a newer sandwich style composite?

Ideally I'd like to redesign the plane to lose some weight and gain a little interior volume and I feel its possible.But I have no clue as to where to look to get started.
It is possible to replace the older style ply construction with newer composite materials, yet it often comes at the cost of either building time or money. For example, a guy in Argentina substituted plywood for glass fibers and epoxy in his KR-2S, his KR2 won a reward for this at OshKosh. Such a fuselage is substantially lighter and stronger yet needs many more hours of building time.

It might be wise to write down all your wishes and desires, check them for feasibility, check the time&money requirements and then see whether it would fit into your financial plan and reality. I've seen a guy build a very light and performant homebuilt but it took him years to make it. He used to fly a Pitts and a couple of other aerobatic aircraft, so he built a superfast and superlight experimental pusher. By the time he ended the project and started flying, the aircraft was too performant for his age.
 

Lendo

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Building lighter than timber is difficult but can be done with extensive use of Carbon. The Blackshape prime is a prime example HOWEVER they had a hell of a time getting the cofG right and I believe there are still issues - read the test flights. I would suggest that it could be done to the same weight (as timber) if done well. As for foams their good Divinycell and kleggecell are just two, research the densities and strength to get the right weight that has to have some impact resistance and Stress transferring qualities.
I would find a good set of Composite plans similar to what you want and use that as a start point to evaluate plies and strengths. I've been to converted to Billski's assessments, he's done a tremendous amount of research on composites and what he doesn't know, doesn't matter!
George (down under)
 

lr27

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I think cored plywood construction could be pretty light. I have a workbench top that I can pick up, but if you support the ends, the middle is capable of holding at least a ton. That's based on strength of a little over 2,000 psi, which is meant to account for the cross plies. It would weigh more than twice as much in solid wood than it does as foam sandwich. 5/16" core with 1/32" faces would be 95 percent as strong in bending as 1/4 inch ply, and much stiffer. However, it would be less stiff in shear. It would weigh slightly over half of the solid sheet if using 2.4 lb core. Maybe Highload 60 or something like it. Honeycomb would, of course, be lighter. A somewhat thicker core would give even better results. Of course you might have to use a denser core at stress points, and any exposed edges should probably have a bit of core routed out and some wood glued in. Some attention to detail and improvisation might be necessary to make things fit and function properly. Probably easiest to keep the inside dimensions and let the fuselage get slightly wider. I wonder how much you could lighten a Volksplane this way? The panels wouldn't be hard to make. If I was making them, I might set up a robust vacuum bag and make a bunch of panels first. I suspect this might still be slightly heavier than a truss fuse out of aluminum, wood, or steel tube. But it depends. And you could do some kind of foam cored truss. For instance, you could make square, foam cored beams to cut up for trusses. In this case it would probably be best to use thin regular wood and not plywood. You could cover both sides of a foam sheet, and then saw it into strips. Other strips could be glued to the sides.

Plywood can be as thin as 1/64, but of course it's in particular sizes, which might not always fit the load.
 
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