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Aviation History
1964
1964 - 1916.PDF
FLIGHT International, 25 June 1964 1059 Aerodynamics Any question as to why the B-70 was designed the way it was must be related to the time it was designed and the problems to be overcome. All the early American Mach 3 SST designs sprouted canards, but in the final FAA competition entries only North American retained them, in a more pronounced double-delta design than the XB-70A. An obvious advantage of the canard is the long moment arm forward of the e.g., so that trim deflections of the canard can be less than with elevons or elevators. Critical instability occurs at transonic speeds, when the c.p. moves rearward. The movement can be countered by trimming with an elevon, but the higher angle of attack increases drag. With a canard arrangement, the foreplane angle of attack is increased (from 0 to +6° in the XB-70A) to coun- ter c.p. shift without changing the angle of attack of the wing. Another advantage claimed for the canard configuration relates to the nose-up landing attitude of a swept-wing aircraft. On the XB-70A the rear section of the canards is hinged to droop 20° to form a plain flap. When the canard flap is selected down at low speed, the nose pitches up. This change in trim is compensated for by moving the control column forward, thus drooping the elevons. At this point all vectors are lifting, and nothing is subtracting from the basic wing lift. Thus the landing attitude is shallower and L/D higher—permitting landing speeds comparable, in North American's view, to current jet airliners. Canard designs are not without their difficulties: pitch and direc- tional instabilities at high angles of attack; side-flow effects on the fins; and flow disturbances affecting the engine inlets. However, North American Aviation engineers point out that B-70 models spent 14,000 hours in the wind tunnel, and claim that their design actually increases stability at high angles of attack. e.g., thus minimizing the trim correction required from the canard surface. High-drag shock waves originating from a conventional wind- shield cannot be tolerated in a Mach 3 design, yet pilot vision in the approach must be preserved. Several schemes are possible to reconcile these requirements. North American chose a relatively simple method of folding the upper nose surface in front of the canopy. At low speeds, a long section of the forward ramp hinges down to give forward vision; when high speed is anticipated the canopy is streamlined into the forward fuselage. At Mach 3 the air at the lip of the engine inlet system is pre- compressed aerodynamically through a ratio of about 36 : J and consequently heated to 630 F. Since the engine compressors cannot accept a Mach 3 airflow and remain efficient, the intake system must be designed to decelerate it to subsonic speed and higher pres- sure. It is generally considered that a series of at least five con- trolled shock waves are required to lower Mach 3 air down to Mach 1. A variable throat at the end of the plane of the last shock then reduces the air velocity to slightly below Mach 1, after which it is diffused to the acceptance level of the engine compressor. Air for the XB-70A*s six engines is split into twin rectangular intakes, about 7ft high at the splitter, and is led back some 80ft to the three engines each duct serves. The plenum chamber in front of each powerplant is large enough for four men to sit round a table. The first ramp in the inlet system is fixed; thereafter there are three hinge points and one slide making three movable panels in the variable throat. Each throat-panel is positioned, according to the airflow required by the engine, by two dual hydraulic jacks. Perforations in the panels bleed off boundary-layer to ambient pressure behind the throat walls. This provides an even air distri- - COMPRESSION LIFT HIGH SUBSONIC - LOW SUPERSONIC THROAT: MAXIMUM BYPASS-CLOSED INTERMEDIATE SUPERSONIC THROAT NEAR MAXIMUM BYPASS-CLOSED HIGH SUPERSONIC THROAT.-NEAF? MAXIMUM BYPASS:f(MA,MDE) THROAT/ ((MA)BYPASS: VARIES TO POSITION SHOCK Comparison of B-70 compression-lift L/D ratio with that of a conventional design, plotted i angle of attack Modes of operation of the XB-70A engine air inlet system They also emphasize that compression lift made the B-70 possible. The wedge-shaped engine box is so located under the wing that the positive static pressure from behind the main shock wave (where free-stream Mach number is reduced from 3 to 2.3) acts on the large wing undersurface. There is no corresponding force on top of the wing to neutralize the effect, which is said to add 30 per cent to the lift with no drag penalty. The aircraft can cruise at a lower angle of attack, again minimizing drag. The increment in L/D is shown non-quantitatively in a sketch above. Wing tips on the XB-70A can be folded downwards in cruising flight. North American adopted this feature to increase directional stability and reduce trim drag at Mach 3. It was found easier to solve the mechanical problems of folding the tips, than to overcome 'he stability problem in the basic design of the aircraft and its con-tr ol system (the XB-70A has three-axis stability-augmentation). The ''PS have three preset positions: up, at all subsonic speeds; lowered to an intermediate 25° position for low-altitude supersonic flight; and lowered all the way to 65' for high-altitude Mach 3 cruise. Folding the tips downwards reduces the lift generated aft of the bution across the three engines. Primary and trimmer by-pass doors in the top of the wing enable the pilot to exercise some con- trol over the location of the terminal shock in the throat. If, due to gust disturbances, the shock moves away from the optimum posi- tion in the throat, it can become unstable and produce an inlet "unstart"' and compressor stall. Therefore, as an inherent feature of the design, the terminal-shock position is focused as far aft as possible to preclude instability. The pilot can choose one of three possible positions on a performance selector. Farthest forward is considered the most .efficient for air pressure recovery and aircraft range. Manoeuvring and turbulence would bring a choice of inter- mediate or full aft. The computations necessary to achieve proper inlet-duct operation in varying flight conditions are performed by a complex system of probes feeding information to computers which generate signals instructing an elaborate electro-mechanical system to assume optimum throat and by-pass positions [diagram overleaf —Ed]. This is as critical a system-design as anything yet fly- ing, and as likely as any to be troublesome. This is one aspect of Mach 3 design which it is vital to perfect before starting SST operations.
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