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Waverider Should you wish to contact the
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email: The notion of a confined region of
compression - implicit in my statement that "the shock wave
... remained attached to the leading edge" - is fundamental
to the definition of a waverider and is responsible for its
advantages over other wings. Even so, it is quite an
unrealistic concept. It depends on the leading-edge being
sharp (razor sharp if you like, but the qualification is
redundant) and the shock wave being a surface of zero
thickness. The perfect 'sharp edge' has never of course been
made, and because of the effects of viscosity, all shock
waves have a finite thickness. The "seal" between edge and
shock wave can never therefore be complete. I don't get
worried greatly by the shock thickness, because it is
usually much smaller than the nose radius, and there are
other effects of viscosity that have a greater significance
(although still very small). However, if you think about
'sharp' leading edges, turn to ant picture of the USS
Shuttle Orbiter. I can't believe it would make the slightest
difference if its underside were recessed as if it were a
waverider. Surely, with such gently rounded edges, it could
never behave like one. That is of course no kind of attempt
to denigrate the Shuttle, which is an amazing and effective
aircraft. But it belongs to the class of lifting bodies that
rely on heat insulation to keep the interior structure and
payload cool in hypersonic flight. The round edges are there
to reduce the peak heating rate and so reduce the
temperature reached to the insulation. The edges produce
extra air resistance and therefore much of the heat
generated at high speeds escapes into the wake, instead of
being radiated away from the hot surface. Waverider wings have indeed been
tested with rounded edges. However, so far as I know, it is
not a matter of common knowledge what the effects are, nor
how these may be related to (say) the ratio of leading-edge
radius to cord. Does that mean that we are merely amusing
ourselves with a theoretical idea that has no reality - at
least as applied to hypersonic flight? Put another way,if
relatively sharp wing edges need to be used, how do we get
rid of the peak in aerodynamic heating that occurs at that
edge? Maybe if we are considering an aircraft that carries a
cryogenic propellant, perhaps this could be circulated along
the edges to remove the heat, though no doubt this would
prove an expensive remedy. Yet is there really any
other? I have myself for a long time
canvassed another possibility - that the edges should be
constructed as a solid wedge of material that conducts the
heat downstream. This spreads it over a wider area of the
wing,from where it can be radiated away at a reduced
temperature. There's also a device called a heat pipe that
might be used. This achieves almost infinite conductivity by
using a liquid that evaporates and recondenses in a closed
cycle.The advantage of such methods of cooling is that they
are passive, as distinct from active methods that depend on
pumping coolants. My own interest in
conduction-assisted cooling in fact predates my work on
waveriders. From time to time, I have published sets of
calculations of what can be achieved and other people
confirmed these quite independently. At Glasgow University
we undertook a set of laboratory experiments that
demonstrate the principle. Yet I have to admit that, despite
this, most people still simply do not seem to believe in the
idea. Certainly at very high flight speeds, this form of
cooling can only be successfully applied to the highly swept
edges of lightly loaded wings. In my study of the reentry
vehicle last year, I purposely chose a low wing loading of
15lb/sq.ft. to reduce the heating rate so that conduction
could cope. It would not have been adequate if the wing
loading were several times this figure (as it is for the
Shuttle Orbiter). In many applications of hypersonic flight,
there might be good reason why it might not be either
practical, or desirable, to use a low wing loading. Yet
sometimes the only 'reason' lies merely in a habit of
mind. I hope I have said enough to show
that there is more to the choice, or indeed rejection, of a
waverider design than first meets the eye. A great deal of
work has been performed over the years to prove the
effectiveness of the idea, but much - of great interest, but
difficulty - still remains.
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