AGARD R-800 - FTP Directory Listing - Nato

1-2
were defined by a 2.0g requirement. Presently,
specifications MIL-A-8862 and MIL-A-8863 include
roughness definitions in terms of (1-cosine) dips or
bumps plus requirements for steps and other obstacles.
Transport aircraft designers usually concentrate on
maximizing step bump capability and operations from
semi-prepared surfaces. Because of the low landing
and takeoff speeds of large transports, only the shorter
(1-cosine) wavelengths (below perhaps 100 ft.) usually
produce a speed/wavelength combination near enough
to resonance to produce high gear and wing loads. At
these short wavelengths, amplitudes are low. There is
little published information available indicating how
designers have addressed the longer wavelength
roughness that becomes important at the higher landing
and takeoff speeds of modem fighters. It is
particularly perplexing how gears are designed with
enough damping to meet the MIL-A-8863 requirement
if the aircraft is assumed to operate through resonant
speed on a surface of "continuous" (l-cosine) bumps.
Conversations with procurement representatives, with
design and structures personnel, with test pilots, and
with potential users have not indicated a consensus of
opinions with respect to "design roughness" as
contrasted with "operational capability." In almost all
instances, current specification wording is generally
vague in this respect, in some places using the term --
"operate to and from --'I and in other places referring to
taxi only, without braking.
With this uncertainty as to operational requirements vs.
aircraft capability in mind, it was decided to select two
sets of landing gears, one sized for carrier operation,
and one sized for conventional land based operations
for studying potential roughness capability as a
function of strut stroke and design load level. The
ground rules for carrier based gears and land based
gears have been generally followed at a preliminary
design level, concentrating on vertical strut loads.
Thus other load sources such as turning, pivoting,
jacking, etc. have been ignored as they can be
determined in connection with establishing gear
strength, setting wall thicknesses, lug sizes, etc. No
study of launch bar or holdback mechanisms has been
made. Vertical struts have been assumed for
computational simplicity. Non-vertical or articulated
struts are expected to yield similar results for
equivalent vertical axle strokes.
2. SINK RATES AND LANDING GEAR
ENERGY ABSORPTION
The first step in sizing a landing gear is to establish the
stroke length. For vertical struts, Currie, (Reference 1,
p35) shows that:
W
To calculate the gear stroke required vs. sink speed and
load factor, the landing impact is assumed to occur at
lg wing lift, and the term:
goes to zero. If it is further assumed that a tire will be
selected to produce 40% as much deflection as the
shock strut, and that strut and.tire efficiencies are 85%
and 45% respectively, Equation 1 becomes:
W
V2
S = (3)
0.45
1 + 0.4(-)(2)(386)(0.85)N
0.85
V2
S = 795.16 N
Using Equation 4, a plot of stroke requirements for
several load factors and sink rates can be developed as
shown in Figure 1.
Figure 1 may be used as a guide in selecting strut
strokes; however, a carrier based gear with a sink rate
requirement of 20-25 fps. would not be designed for
the lowest load factor shown because the stroke would
be excessive, nor would a land based gear with a sink
rate requirement of 10 fps. be designed to the highest
load factor shown, because of the lack of sufficient
stroke to absorb runway roughness. In keeping with
current acceptable aircraft strut sizing, a preliminary
design 25 inch stroke for carrier gears and a 12 inch
stroke for land based gears was selected for further
study.
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