Why no plywood monocoque homebuilts?

wrote:
It is called "complimentary structure". Most GA aircraft have a
structure consisting of a skin (alum or plywood) that carries most of
the load. However, the stringers and longerons have an important
function, that is, they provide out of plane stiffness to the skin,
thereby preventing it from buckling under load. Each element of the
structure has an axis about which it is weak, and it needs the other
elements to provide strength in that direction. They need each other
very much.

Bud

Semi-Monocoque is the term, not "complimentary." Your
average CessBeeMooPip is semi-monocoque in the aft fuselage. Most of
the rest has heavy structural members and the skin is used just to keep
things square.

Dan

wrote:

Semi-Monocoque is the term, not "complimentary."

Whatever. My professor in graduate school (a Stanford Ph.D.) called it
complimentary, since the stringers and longerons compliment the skin in
that, as I said, they provide strength in a direction that the skin
does not have, which is out of plane stiffness. Since a true Mono
(meaning a single) coque (shell) structure has only a shell for
structure (an egg is a perfect example), any deviation from this is
often called semi-monocoque, even when the skin carries no load, which
is an incorrect way of describing such a structure.


Your average CessBeeMooPip is semi-monocoque in the aft fuselage.

The fuselage skin from the firewall back is the primary structural
member everywhere except at the wing spar attachments, and the landing
gear on Cessnas.


Most of the rest has heavy structural members and the skin is used just to keep
things square.

Dan

Well, I agree that many aircraft are much heavier than they need to be
because the designer couldn't or wouldn't do the calculations and
design that would eliminate excess weight (i.e. the Cirrus airframe,
even though it is supposedly made of modern high strength composite
structure, is actually about 300 lbs heavier than aluminum planes in
its class). Perhaps the longerons and stringers are heavy enough to
take the necessary loads, But the skin serves as the aerodynamic shell,
or Loft as it is called everywhere on the aircraft, and in places where
the skin is in tension, such as the bottom of the wings, the skin is
again a major structural member. In planes that have heavy wing skins,
such as private jets, the wing skin is again a major structural member.
Since the OP was inquiring about wooden aircraft, it is worth
noting that the reason wood still is a wonderful material even though
its tensile strength is much lower than metal (the strongest wood is
Hickory, which has a tensile strength of less than 2000 psi), is
because its weight versus volume is lower, resulting in a thicker
section for the same weight. This means that the thicker section has a
much better buckling load than metal for the same weight. Plywood skin
aircraft carry much more load in the skin (which is the best place to
carry it) due their superior buckling properties.
Aircraft skin, in a properly designed airframe, does much more
than just hold everything square.

Bud

In article .com,
wrote:

wrote:

Semi-Monocoque is the term, not "complimentary."

Whatever. My professor in graduate school (a Stanford Ph.D.) called it
complimentary, since the stringers and longerons compliment the skin in
that, as I said, they provide strength in a direction that the skin
does not have, which is out of plane stiffness. Since a true Mono
(meaning a single) coque (shell) structure has only a shell for
structure (an egg is a perfect example), any deviation from this is
often called semi-monocoque, even when the skin carries no load, which
is an incorrect way of describing such a structure.

You mean "complementary," meaning "completes the function," not
"complimentary," as in "offerimg compliments."

Yes, you are correct. Thanks.

Bud

Orval Fairbairn wrote:
In article .com,
wrote:

wrote:

Semi-Monocoque is the term, not "complimentary."

Whatever. My professor in graduate school (a Stanford Ph.D.) called it
complimentary, since the stringers and longerons compliment the skin in
that, as I said, they provide strength in a direction that the skin
does not have, which is out of plane stiffness. Since a true Mono
(meaning a single) coque (shell) structure has only a shell for
structure (an egg is a perfect example), any deviation from this is
often called semi-monocoque, even when the skin carries no load, which
is an incorrect way of describing such a structure.

You mean "complementary," meaning "completes the function," not
"complimentary," as in "offerimg compliments."

wrote:
Your average CessBeeMooPip is semi-monocoque in the aft fuselage.

The fuselage skin from the firewall back is the primary structural
member everywhere except at the wing spar attachments, and the landing
gear on Cessnas.


The forward fuselage on the Cessna has heavy structural members.
There are hat-section channels to which the engine mount is attached,
and these run back to the doorposts, which are the primary lifting
members in the fuselage, since the wing's front spars, the spar
carrythrough and the strut attachments are all part of that big
bulkhead. The skin contributes much less in the way of tensile strength
in that area, and it's not a true semi-monocoque. There are sturdy ribs
under the floor and fuselage top, another bulkhead at the rear
doorpost/aft spar attach and carrythrough, and more frame members
behind that, especially around the windows, until we get to the aft
passenger compartment bulkhead. Past that point it's mostly skin. The
framework around the doorframes is fairly heavy to keep them square;
even at that, we find some distortion when we jack the R182 to swing
the gear. If those doors aren't set right, they end up taking flex
loads from the airframe and the hinges eventually break.

Dan

I refer the original poster to Low Power Laminar A/C Design by P.
Strojnik... A series of three books... Fascinating reading... The EAA
should have them at the book store...

But, to answer the question, the biggest impediment to the home builder
for making a monocoque fuselage in wood is the need for a plug to cold
mold or laminate the wood onto... If the fuse is symmetrical, a half
plug will work and join the two halves later... The favored material is
cement for making the plug, mostly for cost reasons I suspect - it
certainly would hamper portability...
The Mosquito was done this way, as was (I believe) much of the Spruce
Goose... Much of the Cirrus airplanes are plug molded as semi-monocoque
structures, but I don't think they use cement plugs

denny

"Denny" wrote in message
oups.com...
I refer the original poster to Low Power Laminar A/C Design by P.
Strojnik... A series of three books... Fascinating reading... The EAA
should have them at the book store...

So, I take it that the cold molding and/or laminating process is extensibly
covered in these books?

I might be interested in a read, if that is the case. I'm a carpenter/cabinet
maker, but I have to admit to being clueless about the cold molding process.
--
Jim in NC

Saying that something is the primary structural member doesn't mean it
is the only structural member. Aircraft structures typically have
several members sharing the load, hence the term "complementary
structure", as they complement each other in carrying the load. The
forward belly skin on my Cessna is .040 in thick, and 44 in wide. This
is approximately 1.75 sq. in. The forward side skins are .032 by 24 in
tall and has approximately .75 sq. in of area. The structural channels
you mention don't have anywhere near this much area, and are themselves
mounted to the forward skins. They are major structural members and
carry significant load, but the skin is still the primary load carrier.
The load paths and stress distribution of aluminum skin versus fabric
covered aircraft are very different. This is why fabric covered wings
require internal diagonal wire bracing, and aluminum skinned wings do
not. Fuselages behave in the same manner.

Bud

wrote:
wrote:
Your average CessBeeMooPip is semi-monocoque in the aft fuselage.

The fuselage skin from the firewall back is the primary structural
member everywhere except at the wing spar attachments, and the landing
gear on Cessnas.


The forward fuselage on the Cessna has heavy structural members.
There are hat-section channels to which the engine mount is attached,
and these run back to the doorposts, which are the primary lifting
members in the fuselage, since the wing's front spars, the spar
carrythrough and the strut attachments are all part of that big
bulkhead. The skin contributes much less in the way of tensile strength
in that area, and it's not a true semi-monocoque. There are sturdy ribs
under the floor and fuselage top, another bulkhead at the rear
doorpost/aft spar attach and carrythrough, and more frame members
behind that, especially around the windows, until we get to the aft
passenger compartment bulkhead. Past that point it's mostly skin. The
framework around the doorframes is fairly heavy to keep them square;
even at that, we find some distortion when we jack the R182 to swing
the gear. If those doors aren't set right, they end up taking flex
loads from the airframe and the hinges eventually break.

Dan

Earlier, Denny wrote:
...Much of the Cirrus airplanes are plug molded
as semi-monocoque structures, but I don't think
they use cement plugs...

I'm pretty sure that the Cirrus airplanes are female molded, which
seems to be the standard in modern composite aircraft construction. I
don't know of any modern male-molded composite aircraft. The only
composite male-molded production aircraft I can think of offhand is
Gerhard Waibel's ASW12 sailplane, and that was just for the fuselage.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24

Denny wrote:
I refer the original poster to Low Power Laminar A/C Design by P.
Strojnik... A series of three books... Fascinating reading... The EAA
should have them at the book store...

Thanks! I'll take a look.


But, to answer the question, the biggest impediment to the home builder
for making a monocoque fuselage in wood is the need for a plug to cold
mold or laminate the wood onto... If the fuse is symmetrical, a half
plug will work and join the two halves later... The favored material is
cement for making the plug, mostly for cost reasons I suspect - it
certainly would hamper portability...
The Mosquito was done this way, as was (I believe) much of the Spruce
Goose... Much of the Cirrus airplanes are plug molded as semi-monocoque
structures, but I don't think they use cement plugs

denny

I saw a concrete mold like that in a picture of lockheed factory taken
when the vega was being produced.

I've been very interested in sorting out what manufacturing techniques
would be most appropriate for mass production of light aircraft given
modern tooling.

Robotic welding is of high enough quality to handle steel tubing these
days. Obviously filiment wound composits present very high levels of
automation as well, but a much higher material cost. Aluminum obviously
has a reasonably high material cost, good workability, but the size and
flexibility of the sheets would concern me somewhat. I'd be inclined to
guess aluminum requires more skill in jigging than the alternatives.

I wonder whether filiment winding could be used with non-standard
materials. Could you filiment wind a fuselage with say... twine? Sounds
bizarre, but if it is encased in epoxy it might have a hope of
achieving a certain level of strength. If course most of the cost is
probably in the epoxy and not in the filiment.

Suprisingly I keep coming back to wood as material for mass production
since the whole of the structure could be made of one material. There
are obvious logistic benefits there, and I think most wood techniques
could be practically achieved robotically.

If you designed an aircraft to leverage modern production lines, what
would it be made of?

Thanks!
Matt