Fuselage design

I have several designs in my head that I’ve jotted down, configurations, engines, airfoils, even predicted performance specifications for a multitude of designs but while I’m an engineer I’m not a mechanical engineer and are more familiar with turbine engines than fuselage design and don’t really have a design methodology or know where to start with fuselage structures. I’ve looked through stickies but was wondering what yalls experience was like learning the ropes and what helped you learn fuselage structures design?

Thanks!
I’ve always greatly appreciated this forum.

 

If you really start by zero, I would recommend reading "General Aviation Aircraft Design: Applied Methods and Procedures" by Snorri Gudmunsson.
It is easy to read and contains all basics you need to start the design of a general aviation aircraft.

Despite that, are you interested in how to design the "form" of the fuselage, or the "strength"?

As for structural strength, you either need FE analysis or knowledge. For our plane, the fuselage is built from carbon and honeycomb, so the minimum thickness required for touching it without creating a hole is already stronger than what you expect from load in heavy turbulences . Still we perform a lot of FE to be sure.

 

kudo, Think of a fuselage as a 3 dimensional truss. the loads
pass thru the hard points of engine, wing, tail surfaces, and
on some designs the undercarriage. These loads are calculated
by solving moments into three axis forces. There are several
flight conditions to look at and for each the G forces such as
3.8 for general aviation, +10,-8 for aerobatic or 2.2 for
jet transports. The loads are reacted by the shortest load paths
practical for strength and weight savings. Tensile strength is one starting point to calculate the cross sectional area of a load
path but often buckling of a section determines the cross
sectional areas. Then you choose a type of structure such as
tube and fabric, or aluminum sheet metal 6061 or 6063, or
the wood with many gussets approach. I do S2 glass and epoxy
primairly for the ease of making compound curves and flat bulkheads as bracing. With composites the thickness of the layup more
often than not is determined by withstanding handling
damage of ramp rats or the general public at airshows who
have a hard time with not touching. The chrome moly tubing
can develop numerous faults due to rust, overloading, less
than optimal landings and still not kill you if you inspect a
few things before flight. Lest I forget, the moldless composite
often called Rutan moldless like a Varieze,or Cozy also
sometimes referred to a surfboard construction is an option
of stressed skin with a little monoque flavor thrown in the mix.
I would suggest you first do a design study of a tubular
engine mount to get a feeling for the triangulation and cross
sectional areas required. It uses the same boring moment
equation as calculating a highway bridge. Then start attaching members for wing hardpoints, cabin loads, fuel loads, tail
surfaces. From there it is a short jump to wooden structure
and a little more for sheet metal. Composites add in the use
of core between 2 layups of glass with solid chunks of layup
or phenolic plastic at the hardpoints. In any event calculating
and reacting load forces is common to all constructions and
that in my view is clearest when designing tube structures.
I have a bookshelf full of textbooks which seem more
directed to expressing everything in calculus notation. Very
little practical use formulas are presented and the examples
are a struggle if only calculus is used. I took all the higher
forms of math in engineering school and decided it gave
good insight into how things are related and where and how
formulas are created. However you can either bone up on
infintessimly small elements, things taken to infinity, and
integral this or differential that or.... you can design the
structure. Remember the Spitfire, P51, B29, SR71, ME109
Spruce goose and the list goes on were all done before the
Univac computer in 1955 and CAD system I use today. I did
finite element analysis on my corrugated spar wing done
entirely with S2 glass and it showed 40G load capability. The
loads were from a wing analysis software which plots forces
for the entire wing surfaces rather than assume every load
is at 1/4 of the chord and they get transferred there
by some magic. Also loading gets reduced approaching the
wing tips.
wizzardworks

 

There is a LOT of good info crammed in Wizzardworks' post.

Hey Wizzardworks, could we convince you to let the line wrap function work and then use paragraph breaks?

 

In an attempt at increasing the knowledge in the thread:

Home building an airplane is already a massive thing. As a mechanical engineer with a structures and analysis background, I can tell you that designing your own is insane. I should have bought plans for a Cozy MkiV and built it. I am far from done with my design and the Cozy would have already toured the country a few times... If you are looking to build, select a design and build that airplane to the plans. You will have a far higher likelihood of flying it that way.

Next, do you have background in any particular material sets? How about any serious likes or dislikes in material sets you have worked in. I know that I would never complete a sheet metal airplane. Pick your material set based on what you like working in. If you have no experience, I would recommend visiting the shops of builders in your local EAA chapter and doing the workshops on wood and fabric and welding and fiberglass at Oshkosh. If you find you do not like building wooden ribs, a wood wing ship will never be finished...

Then you can go about build choices. If you still want to design your own ship, have at it. It is keeping my brain engaged in retirement. Most of the skin of composite and even aluminum skinned airplanes is set not by strength or stiffness, but mostly by being thick enough to survive construction and the real world, and spacing of ribs, bulkheads, and longerons to allow panels to stand the airloads on them.

Welcome to the Monkey House...

Billski

 

Pazmany recommends starting with the crew and design the cockpit around them.
Once you know the cockpit external dimensions and mission, you can start sizing the wings, engine, tail, etc. Raymer’s small book is great for initial sizing.
Then do the first weight and balance study to determine length of aft fuselage, etc.

 

Is the mission defined? Will it be a lazy two place beach cruiser or an all out aerobatic? Or an very efficient airplane like the Lightning? What is the load to be carried, in fuel, people, luggage and instruments? I think you need to start with these very basic questions and then the fuse design will just follow on.

 

Books rarely show the insides of an aircraft fuselage. You will need to stick your head inside fuselages during inspection at the airport or under construction or rebuild to see how they are designed, as a start.