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Base pressure at subsonic speeds in the presence of a supersonic jet
No full textThis paper presents the results of an experimental investigation into the effect of supersonic jets upon the base pressure of a bluff
cylinder in a uniform subsonic flow. The ratio of jot diameter to base
diameter was 0.1875.
Jet stagnation pressures giving slight under-expansion of the jet
cause an increase in the base pressure but for larger jet stagnation
pressures the base pressure is again reduced.
A simple theory, based on a momentum integral, shows the dependence
of the base drag upon the jet and free stream speeds and upon the dimensions
of the jet and the base
Base pressure at supersonic speeds in the presence of a supersonic jet
No full textThe effects on base pressure of jet Mach number, free stream
Reynolds number and jet to base diameter ratio have been investigated
experimentally.
It was found that, for jet stagnation pressures greater than that
required for the nozzle to reach its design Mach number, an increase
of jet Mach number reduced the base pressure. Similarly the base
pressure increased with increase of the ratio of jet diameter to base
diameter and, at nigh jet stagnation pressures, base pressures higher
than free stream static pressure were found. The base pressurewas
independent of frek: stream Reynolds numbers greater than 2 x 10[to the power of 6] per
foot but increased with reduction of Reynolds number below 2 x 10[to the power of 6] per
foot.
Unsteady wave patterns were found when the jet Mach number did
not differ markedly from the free stream Mach number and the jet had
just reached its design conditions
The characteristics of a two-dimensional supersonic sail
No full textThe two-dimensional supersonic sail is analysed using Busemann's
second-order theory. It is found to have a universal shape, which is part
of a Sici spiral. Aerodynamic characteristics are calculated for a few sails.
The tension in sails flying at Mach numbers of 2 and 3 at altitudes of 20,000
and 70,000 feet is large and suggests that wire sails will be needed for flight
under these conditions
An improved law of cumulative damage in metal fatigue
No full textAfter a brief account of theories currently put forward on damage accumulation,
a modification of the best of these, the linear rule of damage accumulation, is
presented. This theory is based on a new approach to the problem wherein the
material behaviour in each of three different stress regions is analysed, in contrast
to current approaches which endeavour to fit fatigue behaviour over the whole stress
range into a single fundamental relation.
In the three-region approach, evidence is given to show that the linear rule is
effective in the intermediate stress levels of the S - log N relation, and that this region
appears linear due to an exponential crack initiation theory, as opposed to present
power function theories, which is based on prior history effects in plastic yielding.
Non-linear damage activity is seen to modify this fundamental behaviour outside this
stress region.
To substantiate these theories, an experimental programme was undertaken using
plain L.65 bar specimens. After establishing the S - N relation, a 'traverse' of the
alternating stress range was made, cycling specimens at eight 'prestress' levels,
each conforming in size to an envelope spectrum for a projected airliner, and then
cycled at a datum level in the intermediate-stress region. From these results, the
non-linear functions were estimated, and the final modified linear law developed.
Specimen life under the spectrum was then calculated and compared with unmodified
linear rule calculations.
The results of the experimental tests, the background of the statistical methods
used in assembling the data, and an assessment of the relative merits of three
fillet designs are presented in Appendices
Aerodynamic characteristics of a hypersonic parachute
No full textNewtonian theory, both in the form of the Modified-Newtonian and the Newton-
Busemann pressure laws, is used to find the shape, cloth area and drag of the
axisymmetric canopy of a hypersonic parachute, whose only load-carrying fibres
are longitudinal ones. As an example, an estimate is made of the size of canopy
needed to give a drag of 20,000 lb. in flight at a Mach number of 10 at 100,000 feet
altitude
The effect of curvature on the stress concentrations around holes in shells
No full textAn experimental investigation has been carried out to investigate the effect
of curvature on the stress concentrations around holes in shell structures. Two
methods have been employed: -
(I) Araldite cylinders, containing holes of various shapes, subjected to axial
tension, internal pressure and torsion were examined by the photoelastic
frozen stress technique.
(2) Aluminium alloy curved panels and hemispheres were used in conjunction
with miniature electrical strain gauges.
The results are compared with the theoretical solutions and suggest that
the curvature effect can be significant, particularly for the case of shear or
biaxial loading
Ignoration of distortional co-ordinates in the theory of stability and control
No full textGates and Lyon have proposed to treat theoretically
the stability and control of deformable aircraft by a method
in which the distortion co-ordinates are ignored and the
influence of distortion is allowed for by suitable modifications
of the derivatives and other coefficients. in the 'present
paper an exact method for eliminating the distortion
co-ordinates is given and the conditions in which the true
eliminant conforms with the simplification of Gates and Lyon
are examined. In general the simplification is not justified mathematically, but in certain circumstances it provides an
acceptable approximation. it will not be practically valid
unless the structural distortions occur so relatively slowly
that the associated inertia forces are negligible, i.e. the
distortions must be quasi-static
Temperature effects on material characteristics
No full textSome of the physical properties of the main elements of interest in
high temperature technology are reviewed. Some general trends emerge
when these properties are viewed as a function of melting point, but there
are a few notable exceptions. Titanium, zirconium, niobium and tantalum
all have disappointingly low moduli; chromium is excellent in many ways,
but has a limited ductility at lower temperatures; molybdenum oxidises
catastrophically above about 700° C, and niobium suffers from severe
oxygen embrittlement. Beryllium and carbon (in the graphitic form) both
stand out as exceptional materials, both have very low densities, beryllium
a very high modulus but an unfortunately low ductility, while graphite has
a relatively low strength at the lower temperatures, although at temperatures
of 2000° C and above it emerges as a quite exceptional (and probably as the
ultimate) high temperature material. Some of the fundamental factors
involved in high temperature material development are examined, in the
light, particularly, of past progress with the nickel alloys. If a similar
progress can be achieved with other base elements then a considerable
margin still remains to be exploited. Protection from oxidation at high
temperatures is evidently a factor of major concern, not only with metals,
but with graphite also. Successful coatings are therefore of high importance and the questions they raise, such as bonding, differential thermal expansion,
and so on, represent aspects of an even wider class covered by the term
“composite structures". Such structures appear to offer the only serious
solution to many high temperature requirements, and their design,
construction and utilization has created a whole series of new exercises
in materials assessment. Matters have become so complex, that a very
radical and fundamental reassessment is required if we are to change, in
any very significant way, the wasteful and ad hoc methods which characterise
so much of present-day materials engineering
Aircraft design studies - vertical take off and landing airliner
No full textDuring the 1970 academic year the students in Aircraft
Design worked on the design of a vertical take off and
landing airliner. The aircraft is intended to be capable
of carrying up to 118 passengers over stage lengths of
500 n.miles. The maximum cruise speed is Mach 0.83 at an
altitude of approximately 20,000 ft and the predicted take
off weight is 125,000 lbs. Vertical take off is achieved
by using 12 fan lift engines, each of 1.500 lb thrust,
which are based on the Rolls Royce RB 202 design. The lift
engines are housed in two large nacelles which are mounted
on the high, sweptback wing. The installed thrust/weight
ratio of 1.4 makes allowance for hot and high operation,
control requirements, and lift engine failure. An unusual
feature of the design is the location of the two propulsion
engines on either of the vertical fins.
Some indication was gained of the penalties associated
with this type of aircraft, but the weighing of these
against the realisable advantages was outside the scope of
the work