wood ? why wood for the main frame and body ? scroll down...
because that's cool and a nice material to work with !
yet convinced ? a trivial example:
still nowadays many light aircrafts, even acrobatic ones, are using wood
as a structural material (including the main wing spar/longeron)
first step was to carefully examine how light structures made of wood are
built. A lot was to be learnt from the past: how engineers solved the problem
of load transfert from a concentrated area to a wood frame. Typically in
our design, how to transfer cycling torque, brake effort, seat moment, etc...
to the frame without introducing stress risers, in a durable manner ?
second step was to understand how to design a light frame, to predict its
stiffness, strength and safety margin.
where a "Do-It-Yourself" bicycle just need to be stiff enough to withstand the abuse of its user and builder, a mosquito velomobile cannot be designed on the same basis. Being stiff is not a suficient condition, we need to make sure that the frame won't brake under ultimate loading conditions.
first we have used the composites material theory to predict the stiffness and strength of any laminates and beams made with plywood and timber. This has been compared with the experimental results provided by the specialised literature.
we have been building simple finite element models to simulate the behaviour
of various beams configurations (I beams, box beams, girders; trusses),
those have been compared with the theoritical results provided by sound
analytical solving. the intention here was to calibrate the relative (in)accuracy
of a finite element model; it depends on the software, the computer, but
most importantly the level of ignorance of the designer. Here, we have tried
to minimize the last point, although being a lifetime mission.
Analytical calculations, experimental design guidelines and FEA simulations have been compared all together with our real world. The left photo shows a box beam under torsionnal load, the angle deformation is being checked against our theoritical models.
it comes to computer simulations, we have learnt the hard way that "push
buttons black box" softwares coming with CAD packages are just not
doing the job.
we have spent time following a more robust approach.
it is necessary to know all the possible load cases scenario a velomobile
will live in a 20 year period of time. We want our velomobile to be an
utilitarian vehicle, the load cases are much more severe than for a racing
bicycle, especially due to the large spectrum of weather and road conditions
over such a long period of time.
|Finite Element Analysis model of the main frame. It has yet to be "adjusted" by comparaison with the real mosquito. By corrrecting the overall stiffness of the model we can make sure the predicted stress is very close to the reality.|
|FEA post process|
|and the real work...the main front spar has to be within very close tolerances compare with the model.|
|what is the point of calculating if we are not able to build what we have designed ? sounds like a silly question, but what about plies orientation tolerance ?, unconsistant material properties ? unconsistant material and joint thicknesses, unconsistant geometry ?, etc... in one word: discrepancy between the virtual world and the real one.|
the solution to minimize the unknown ?
only materials of consistent properties have to be used;
only "square" and simple beams have to be built, max 1 curvature, no funky shapes, compound curved panels and beams are banned from our workshop;
of the art" assembling methods have to be used.
|for instance, any plywood meeting the G.L. requirements exhibits certified minimum mechanical properties.|
from being carefully selected, all timber and laminated wood are being tested
to evaluate their strength
3 point bending test according to an old french "air" standard
short beam test and 4 points bending test are part of the plan as well.
Ash tree is used were high stresses and large deformations are expected.
All the beams are designed to fail under compressive load, it's a safer failure mode compare with a failure under tensile load (instant break into 2 pieces with sharp splinters).
|from laminated spruce...|
|...to the completed resulting frame.|