294
FLIGHT International.
|4E
OUTBO I MID
SPOILERS
NO1 ENGINE
20 February
1969
•
INBOARD SPOILERS
Hydraulic system schematic showing the
four basic systems charged by engine
driven pumps, and the supplementary
air turbine motor pumps (ATM) driven
by engine tapped air
XZ
!
L O C K H E E D 1011 T R I S T A R . . .
boundaries and initial cruise altitude. The Tri-Star can tolerate
a gross weight increase to 470,0001b and still maintain a 1.35g
buffet margin at 31,000ft and M 0.85 (Fig 3). If the operator is
willing to accept initial cruise altitudes of around 30,000ft,
then gross weight could exceed 500,0001b. Buffet boundaries
were investigated in the wind tunnel by two methods: liftcurve slope break; and a root-mean-square root-bending
moment method. Variations in initial cruise altitude for varying weight and Mach are indicated in Fig 4.
Engine Location and Nacelle Development
With considerable
design and operational data to go on with respect to rearengine locations for two-, three- and four-engined designs, it
is significant that both Lockheed and Douglas calculated very
early in their studies that two under the wings and one in
the tail was the optimum arrangement. The preference points
were reduced wing structure weight (bending and torsion
moment relief from forward slung pods); a longer tail arm
giving an empty e.g. nearer the centre of the cabin and hence
more flexibility in loading; a better chance of locating the
tailplane out of the wing wake and hence of providing more
predictable pitching characteristics with less probability of
pitch-up; beneficial effect of pylons at high incidence through
flow-constraint and enhanced inboard flap effectiveness; and
less aircraft weight. System complications were not so great
as the designers of the Trident and 727 had anticipated, and
debris ingestion and ground clearance has not proved to be a
particular problem with low-slung pods. The major point of
difference between Lockheed and Douglas is in the method
of installing the centre engine—of which more later.
Comprehensive studies of underwing pod design and location
were undertaken in collaboration with all three engine manufacturers who were submitting in the early stages trade-off
studies of various nacelles, considered engine performance,
weight, drag, aircraft performance, noise and maintenance.
Six nacelles were studied. Two were integrated with the wing
—one located at mid-chord and the other cantilevered aft of
the trailing edge. Both were considered excessively heavy—
particularly the latter which, despite having the lowest drag
of all, was not able to offset the weight disadvantage. The
mid-chord type was further criticised for having a high interference drag. The other four nacelles were on conventional
forward cantilevered pylons. A long-fan-duct nacelle was found
to offer the best overall range efficiency through good
mixing of fan and exhaust flows, but there was a weight
penalty of 1,4901b. It was believed that the layout would need
intensive development to achieve the result and so there was a
development risk. A short-fan duct was found to be lightest,
but that this was insufficient to offset engine efficiency losses
caused by close-coupled flow turning and internal pylon interference and increased scrubbing drag. Three-quarter-length
fan cowls emerged as the optimum. With core-mounted gearbox and accessories it was found that thrust loss due to access
door seal leakages gave range losses more significant than the
weight savings. Fan-case-mounted accessories were therefore
chosen in conjunction with a three-quarter-cowl.
The nacelles are as far outboard as possible consistent with
ground clearance and directional control—this reduces wing
structure weight through bending moment relief and flutter
damping. Tunnel tests with simulated exhaust showed least
interference-drag with the nacelle 0.95 of a diameter below
the wing chord, and 1.85 of a diameter forward of the leading
edge. A 2° toe-in provided best conformation with the cruise
flow, and a 4° declination optimised the wing positive pressure
field acting on the nacelle aft body.
Centre Engine Location involved one of the most critical and
involved trade-off studies undertaken. Four layouts were considered and tested in the subsonic and transonic tunnels; two
were "S duct" intakes and the others of a straight-through
type. The final choice of an "S duct" is substantiated by
Lockheed on five points relative to the straight-through type.
First, S-duct inlet recoveries were considered equal to those
of realistic straight ducts (compared to the 727 duct, that of
the 1011 has a proportionately less severe double bend, is
shorter, circular throughout, and has a ^-diameter straight
section before the fan). Tunnel tests suggested that the S duct
will have acceptable pressure distortions in cross winds of up
to 50kt. Aft-body drag with an S-duct intake and tailcone
exhaust is less, and permits more aft cabin floor area for a
given fuselage length. The low engine position with an S duct
gives greater rudder area and effectiveness for a lower overall
height, and the lower e.g. means reduced rolling inertia and
better control characteristics at all speeds. Despite the need
for a longer duct and extra fire protection, the S-duct was
calculated to be 8001b lighter because the tailplane and rudder
were simpler, the wing engines could be spaced further outboard due to superior directional control, and more of the
rear fuselage could be used for accommodation. With an S
duct the engine is 10ft closer to the ground and is clear of the
airframe for easier removal of engine modules or complete
units. AH of which, Lockheed estimates, adds up to a 1.2 per
cent advantage in d.o.c.
Built-in Growth Potential
Considerable thrust-growth potential without change to the basic configuration is a major