FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1974
1974 - 0038.PDF
34a FLIGHT International, 10 January 1974 No. 20 By T. E. FORD Direct lift control WHEN THE PILOT moves the control column of a conventional air craft there can be a significant delay before the aircraft responds and begins to move in the required direc tion. For control in pitch, movements fore and aft of the column result in changes in elevator angle which in turn produce relatively small aero dynamic forces on the tailplane. The design is laid out so that the resulting pitching moment is sufficient to control the angle of attack of the aircraft and therefore the lift generated by the wing. The force on the tail is, in fact, in the opposite direction to the change in lift required from the wing and the aircraft may initially begin to move in the wrong direction. Under the in fluence of the pitching moment the aircraft changes its pitch attitude and continues to rotate until it reaches a new state of equilibrium. As aircraft increase in size and pitching inertia, the response to elevator also becomes slower and it is possible that there may be an upper limit beyond which conventional con trol techniques will not allow a safe and efficient approach and landing. In a report published in 1968 W. J. G. Pinsker of the Aerodynamics Depart ment of Royal Aircraft Establishment, Bedford, proposed that these difficul ties could be removed through the use of direct lift control (DLC). This tech nique uses spoilers on the wing to modify the total lift generated by the aircraft without altering its angle of attack. The spoilers are raised to an intermediate position and variations about this position used to modulate the total lift and therefore the rate of climb or descent of the aircraft. With controls of this nature, aircraft size has no effect at all on response. In stead of the lift being generated by the angle of attack, it is now defined by the aerodynamic efficiency of the primary lifting surface. As well as the removal of the size limit there are a number of other im portant benefits of DLC to be explored. These include more accurate touch downs and steeper approaches—and reduced noise levels. Following early studies, proposals for research into the benefits of DLC were made by British industry. These were mainly con cerned with assessing how effective DLC can be in maintaining adequate standards of longitudinal performance during manually controlled landings of large and slender aeroplanes like Concorde. These studies assumed that no extra control task was presented to the pilot and that the controls for DLC and the elevator had to be inter linked. It was realised that the optimum connection of these two con trols would be greatly assisted by the advanced types of automatic flight control systems then planned for future aircraft. During 1969 a Comet was fitted with a Smiths Industries SEP 5 autopilot at RAE Bedford. It was suggested that the investigation of the control laws associated with DLC could be undertaken by an aircraft equipped with a flight control system at least as versatile as that in the Comet coupled with more advanced comput ing facilities and sensors. Further work was undertaken by BAC Warton in 1970 on the simulation of manual DLC on the VC10 and at BAG Weybridge in 1972 on a similar system for the One-Eleven. In the same year an investigation was carried out by Hawker Siddeley and Smiths Industries into automatic DLC for the Trident. In February 1972 the RAE's Series 200 One-Eleven registration G-ASJD was flown to Cranfield where a SEP 5 autopilot and a new flight system was installed to fit it for its new role for the investigation of DLC. The conversion was completed during 1973. Initially, flying of the aircraft will be shared with Aero Flight of the RAE studying the manual use of DLC during the approach and landing phase and the Blind Landing Experi mental Unit (BLEU) considering the benefits to be gained from advanced automatic control with and without DLC. The feasibility of making auto matic landings from steeper than nor mal approaches as a means of noise reduction will also be investigated. On the standard One-Eleven there are two pairs of spoilers at mid-span which are used as speedbrakes at high speed. With the modified aircraft it is hoped to assess the potential bene fits of DLC by using these fast-acting spoilers for control at slow speed. With DLC operative, the aircraft is re stricted to a flap setting of 45° and a maximum speed of 160kt, 297km/hr IAS to prevent the design load on parts of the flap mechanism and attachments being exceeded. For manual DLC the only input is from movement of the control column (see block diagram below) which is fed to the computer, the spoilers and the elevator. The drive element of the system is based on the yaw damper actuator of the 200 Series One-Eleven and is capable of accepting control signals from either the BLEU "versatile autopilot" (an analogue computer) via an electronic interface, or from signals generated by a separate DLC/pitch compensation computer. The actuator is mounted in series with the existing speedbrake control system and when DLC is not in operation it is held in a fixed position by an integral lock. Prior to engagement of the DLC system, the pilot uses the speedbrake lever to deploy the spoilers to a pre determined position. The DLC drive actuator—which takes its signals from a pick-off on the elevator control run —then drives the spoilers indepen dently of the speedbrake lever posi tion. An adjustable gate is used to pre-select the position of the spoilers within the range 20° to 30° while the DLC drive actuator modulates the position of the speedbrakes in re sponse to its computed input signals. Normal manual speedbrake control can be regained by operation of a dis connect button situated on the pedestal between the pilots. To pre vent selection of DLC other than at a flap angle of 45°, an independent inter- X^Spoiler^ Left-hand aileron Actuator Right-hand aileron
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
