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AS ULTRALIGHT WAVERIDER VEHICLES |
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A CONCEPTUAL STUDY
BY
GORDON J. ROSS
Waverider Should you wish to contact the
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refer to references. In a previous concept paper to the
1990 Waverider Symposium at Maryland,4 work carried out by
amateur experimenters was presented. Subsequently, the
author began examining the concept of variable geometry, in
relation to Waveriders. This study revealed a very close
analogy between the seemingly unconnected fields of membrane
aerodynamics at very low speeds, and the construction of
Waveriders. It would appear, from preliminary studies
(7),
that a Rogallo-derived shape could function adequately as a
Waverider, given the existence of appropriate materials
technology. Also, a wide variety of Waverider
"aero-shells" may be derived from the basic leading
edge/keel boom Rogallo configuration. If it were possible to design a
Flexwing Waverider, why would one want to ? Consider first
the drawbacks of Waveriders as aircraft. One of the most
obvious must be the relative inflexibility of the Waverider
flight envelope, i.e. its small speed range for
shockwave-riding with attached shocks.(8)
If intended for non-terrestrial use, the mass of the vehicle
would be an important factor. Another potential problem is
the large airframe, and the need to have thermal protection
for high velocity flight. In short, Waveriders could turn
out to be specialised and very expensive to build and
operate. If Waveriders could be more flexible, lighter and
perhaps even change the shape, then their utility for
planetary exploration would be greatly enhanced. The use of flexwing technology and
design practises may enlarge the flight envelope by allowing
the Waverider to change its shape during flight. This
ability to change aerodynamic shape, may also be used to
effect control in the roll axis, using a system common in
flexwings, called "Billow Shift". Hang gliders are quite similar to
Waveriders in several fundamental respects, as mentioned
earlier. The double concavity produced an inherent dihedral,
which imparts roll stability to the flexwing. Roll control
is achieved by weight shift, where the pilot moves his body
weight to one side, causing the inboard wing to twist under
load. The incidence of the twisted wing is
reduced, causing a reduction in lift. The outboard wing
incidence increases, as the sail tightens on that side. The
billow or twist effectively moves from a state of
equilibrium, where both the wings have the same incidence,
to a state of imbalance, where the wing produces more lift
on one side than on the other. Since modern Rogallos have
the very flat, tight sails, this twisting of the wing
requires the keel boom to pivot slightly from the nose.
This, combined with the coupled leading edges and cross
boom, enhances the movement of twist in the sail, thus
reducing control input force required. Another useful feature of the Rogallo
wing, is its almost unique ability to "dump" excessive
dynamic loads during high G manoeuvres, by bending and
twisting at the wing tips. This property allows some hang
gliders to take 7 or 8 G's, both positive and negative, and
because of this flexibility, washout of the wingtips means
that roll control is preserved even at high angles of
attack. How then does this background
information on flexwings relate to the problems of
Waveriders ? Firstly, from a purely financial viewpoint, a
rigid alloy airframe, possibly clad in thermal insulation
material made from carbon/carbon, can be very expensive to
manufacture (cp. the space shuttle). The low speed performance would be
marginal at best, since low aspect-ratio deltas are
notoriously difficult to fly near their stall
speeds. A Waverider Flexwing, in its simplest
form (Fig.1)would
look something like a Rollago wing, but with an additional
upper surface (not shown here, for clarity). The main
structure would consist of: A: carbon/carbon nose section;
C: an upper surface centreline strut; D: a lower surface
centreline floor pan; E: a flexible sail, woven from
cardonfibre extrusions (see later text for details); F:
three hydraulic actuators to effect change in geometry; G:
rear mounted flaps for pitch control; H: payload palette and
control system. As can be seen from the drawing, the
"Ultralight" Waverider is fairly simple, although this
apparent simplicity belies the true subtlety of the concept.
The shockwave is generated by the conical section of the
nose and the floor pan, in the same manner as a
conically-derived Waverider. The leading edges may be
drooped and swept to the required angles for any design Mach
number, and any given angle of attack (fig.4). The sail may
also be "reefed" in or out to obtain the best shape of
concavity for the desired performance. This could be maximum
area for a rapid deceleration, or perhaps a minimum angle of
attack, with maximum sweep for a fast low drag aero-gravity
pass around a planet. Page
Three
Page
One
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