Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.4 Reciprocating Chemical Muscle
The forces from each wing pair are resolved across the fuselage. If each wing is producing an
upward lift vector on both the down- and upstrokes, then a net lift for the entire vehicle will be
realized. By separating the pair of wings at either end of a fuselage, longitudinal (pitch) stability
is increased over a single flapping wing pair.
Now, if the fuselage separating the fore and aft wing spars is made of a material with a torsional
memory, then the wing spars can become integral to the fuselage and can be torqued out of phase
such that the fuselage acts as a torsional spring with a resonant frequency depending on the inertia
of the wing spars, the flapping drag of the wings (damping), and the spring constant (K) of
the torsional fuselage [185].
This implementation achieves two important things. First, it allows the entire system of fuselage,
wing spars, and wing ribs to be constructed as a single piece with no hinged parts. This greatly
simplifies the construction of the Entomopter and facilitates it mass production. If further guarantees
a higher level of consistency of performance.
More importantly, this construction forms a totally resonant structure. The torsional resonance is
essential to make a flapping wing vehicle viable. All insects have resonant structures that store
potential energy in either the muscles themselves, or in their exoskeletal parts. Many insects use
a substance called resilin to store mechanical potential energy. In order to flap wings, energy
must be added incrementally in order make flight viable. Any attempt to flap wings using brute
force without a tuned resonant structure will require too much energy. This is particularly obvious
when one considers that actual insect muscles (wasp) have been shown to expand and contract
only 2% of the overall muscle length during each full beat of the wing [104], while the
muscle efficiency and level of resonant energy storage has been estimated for the fruit fly
Drosophila hydei to be only 10%, with the energy stored elastically for resonant release estimated
to be somewhere between 35% and 85% [59].
When first started, the RCM will drive the wings, but if it is sized for autonomic flapping cruise,
it will be underpowered for start up. The result will be that the wings flap more slowly at first
and with less than the designed flap angle. Since the system is tuned for resonant flapping and
the torsional fuselage can release energy back into the wing system upon each flapping cycle,
the speed of flapping will increase as will the flap angle until the full range of motion is
achieved. Upon starting, the Entomopter will not attempt to lift off (nor will it be able) until the
wings have had a chance to reach optimum (resonant) flapping frequency. What this means is
that the RCM can be sized for autonomic cruise speeds (resulting in weight savings and optimized
fuel consumption for maximum endurance) rather than blindly sizing it for the maximum
force required to instantaneously flap the wing over its maximum excursion on the first flap.
3.4.3.2 RCM for the Mars Entomopter
The RCM is comprised of a fuel tank, reaction chamber, actuator, fuel metering, and control circuitry.
The actuator is the primary component of the RCM system that affects the vehicle sizing,
then to a lesser degree the reaction chamber with its fuel-specific catalyst bed. Of course the fuel
tank size affects endurance and overall vehicle weight (when full). The reaction chamber is contained
within the fuel tank to preheat the fuel since the highest system temperatures are evolved
at this point in the RCM system. Gas generated within the reaction chamber, the quantity of
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