GRandolph

I am building a fuselage for an Ugly Stik using 3/32" balsa sheet for the sides. I am doubling the 3/32 balsa with ply from the firewall to just behind the TE. I have both 1/8" lite ply and 1/16" aircraft ply (3-ply) available. Does any one know which is heavier or s.tronger? Both materials feel close to the same weight to me. The Stik has a 70" wingspan and will be powered by a 1.20 AX. I am building from plans.

Thanks.

Truckracer

My Feedback: (19)

I don't have any equal sized pieces here I could weight to give you an answer about the weight differences. Regarding strength, again I have no scientific methods to prove there is a difference in strength, but the 1/16" aircraft ply seems stronger to me. The aircraft ply is certainly less brittle than the lite ply.

karolh

Like Truckracer I do not have any scientific evidence to back up my opinion but working with both materials for approx. 30 years have given me a very good feel of both and IMHO 1/16"' thk. aircraft grade ply is much stronger than 1/8" lite ply and the weight difference is marginal with the lite ply being the lighter of the two.

GRandolph

Well, I decided to perform a test to see which was actually stronger. From the photo below, with two equally sized pieces under the same stress, it is obvious that the 1/8" lite ply is stronger than the 1/16" ac ply. I don't have a scale, but the 1/8" LP feels just a tad lighter than the 1/16". Guess I will go with the light ply.

Truckracer

My Feedback: (19)

Don't mistake flexibility for strength. Just because one sample sags more under weight doesn't indicate it is weaker. Try some yield to destruction tests.

Many years ago when I was still a teenager, one summer I worked for a house mover. I learned that some movers preferred wood beams to support the house where others used steel beams. Steel was a far stronger material than wood but a long steel I beam couldn't even support its own weight without support in the middle. OTOH, wood hardly sagged in the middle when supported at each end. Steel was stronger but required different engineering and practices to take advantage of its strength. I suspect your plywood experiments may demonstrate similar results.

Some years ago, there was a trend in kits to switch from all balsa fuselage sides to lite ply. Some kits were even re-engineered to utilize the much cheaper and much more consistent lite ply. The result in most cases was a fuselage that was easily snapped in half, usually just behind the wing in most any mishap and especially if one wingtip contacted the ground. The problem being that lite ply was very strong but also very brittle. When it suffered a shock load that exceeded its yield strength, it failed where even lighter balsa equivalents were much more tolerant to the same treatment. Don't take me wrong here, I like Lite Ply for many applications but sometimes there is just no substitution for a stronger, more durable material.

sensei

If you go to Aircraft Spruce & Specialty you will find basswood plywood, 3/32" thick is a perfect thickness for building model airplane fuselages. It is almost as strong as birch plywood and almost as light as as light plywood. Best of both worlds really. I have performed much testing with this material and built many airplanes with it to date.

Bob

karolh

Exactly so.

Rodney

How was the grain oriented in your tests? Were both the same; i.e. was the outside grain of each piece oriented the same, it makes a big difference.

countilaw

I believe that when laminating lite ply to a balsa fuselage side as a doubler is as good as using aircraft grade plywood. However, I would not use lite ply as a firewall. Grandolph seems to have the right mind set as using the 1/16 a/c ply versus the 1/8 lite.

The other consideration is the 1/8 Lite ply give a broader surface at the fuselage coupled with the balsa fuselage side where it contacts the wing. The 3/8 balsa side of the fuselage plus the 1/8 lite ply gives the wing a 1/2 mounting area the lenght of the wing cord.

Frank

GRandolph

The 1/16" ac ply is more flexible, the 1/8" litte ply snapped where as the ac ply flexed.

karolh

He never asked for advice as to which would be a better choice as a fuse doubler, he asked which one was stronger and lighter.

Zor

karolh,

True but the OP had a purpose to ask and that was for his decision of what to use for the end results.

Guys that showed him the end results really helped him better.

It appears quite obvious that the AC ply would be preferable to the lite ply.
It is better to flex than to break.

Zor

GRandolph

Yes, thanks everyone for help. I came close to using 1/8" lite ply because it felt firmer, but after seeing a test piece snap in two with slight flexing, I have decided to use 1/16" ply.

Now, is it better to use epoxy or gorilla glue to laminate the ply to the balsa sheet? I am thinking I will try gorilla glue.

sensei

G/G will work great as long as you apply sufficient pressure to keep the G/G from hydraulically pushing the two substrates away from each other during the curing process. Vacuum bagging and alignment pins come to mind.

Bob

Zor

I laminate using regular wood glue typically Titebond. I was using Sig-Ment when it was available here.

No need to apply the glue over the whole surfaces. Some essing S patterns arefine.

It is essential to apply flat pressure and work on a flat table (work bench).

Typically my set up is as follows from bottom to top.
my flat work table
a layer ofdry wrappingpaper
one layer being laminated
glue
second layer being laminated
provision to prevent shifting when applying the weight
a layer of dry wrapping paper
a layer of flat plank (a piece of flat wood 3/4" thick and larger than the laminations)
anything judge heavy enough on top of the flat plank.

Leave minimumovernight or a whole day.

Now do it as you judge best for you ___.

Zor

sensei

Now that I think about your question, the very best bond line you are going to get is with the use of a slow curing epoxy, alignment pins and a vacuum bag placed on a released flat surface. Now the lightest, cleanest fuselage construction you can do utilizing wood is still the use of 3/32" basswood plywood from Aircraft Spruce & Specialty, and guess what; no fuse doublers needed, I have been designing and building my motor boxes and other components for very large aircraft with big hp engines for years with just 3/32 basswood ply wood and bonded together with thin and medium CA only. With that said, I have never experienced a bond line, or structural failure with the use of CA glue and this 3/32" material, and we are talking about thousands of very aggressive flights. Here are a couple of pics.

Bob

ARUP

I noticed the fan. I'm a Houston grad and know TX weather!

sensei

Your sure right about that. I couldn't stand to be in the garage working in the summertime without a fan running.

Bob

GRandolph

Thanks for the tip about 3/32 basswood ply, Sensei. I will have to use that for the next build.

airraptor

My Feedback: (66)

do you really need the doubler?? When I build kits I remove those and just add balsa. I dont think its required on most kits even a stick