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Aviation History
1956
1956 - 1111.PDF
FLIGHT, 17 August 1956 257 AIRCRAFT CONTROL SYSTEMS . . . theorists have been in their assessments of the air load problemsas applied to balanced control surfaces. The first practical air transport to be fitted with a power boost system was the Lock-heed 49 of 1939. The innovation did not occasion excessive comment at the time. That was partly due to war-time securityand partly due to the then accepted fact that such a huge aircraft (75,000 lb originally) travelling at the "fighter-like" speed ofnearly 250 kt, had passed the limit at which manual forces could be practicably applied.Some 20 years later, the Boeing 707-320 is being ordered at the all up weight of 295,000 lb and with a cruising speed of 500 kt.Yet the control system is basically a manual one, aerodynamically balanced.Whatever the theorists may say, the 707 not only flies but has been slow-rolled. As a breed of practical men, pilots prefer theevidence of their own eyes to that of someone else's slide-rule. From the pilot's point of view, the suitability of a controlsystem is first assessed by considering its reliability. The yard- stick is the reliability experienced in 50 years of flying with adirect mechanical linkage between the control column and the control surfaces. In addition, a control system is assessed on itsability to feed back to the control column an accurate representa- tion of the loads on the control surfaces such that the pilot cannotinadvertently overstress his aircraft. The designers, however, whilst accepting all this and the viewsof the slide-rule theorists, are of the opinion that for any aircraft which has to cruise at high Mach numbers it is much easier todevelop a powered control system than it is to develop a balanced manual system. Power systems can be developed that areirreversible. Manual systems could result in both nutter and control reversal. In fact, until Boeing proved otherwise, designerswere in general agreement that, for the immediate future, control without power was a practical impossibility for Mach 0.7 andabove—beyond the means of normal manufacturing tolerances and beset with servicing problems which no normal operatorcould accept. The customer is very often right. So, before proceeding withdescriptions of the various types of control systems, a look into the future orders books might be enlightening. Fig. 2 shows the extent to which the two major classifications of CRUISING SPEED (KNOTS) 50JO Douglas DC-8, Boeing 707 4C Viscount and Britannia 30B DC-7C Metropolitan ^Q Q MANUAL CONTROLS (Aerodynamic Servos) 10 D Comet Caravel le Electra L.1M9 POWERED CONTROLS (Hydraulic/electric Servos) 200 100 0 100 200 - NUMBERS OF AIRCRAFT ON ORDER FOR 1960 Fig. 2. "Balance of power" in numbers of aircraft on order for I960- controls, power and manual, will be used in 1960. The index ofinfluence has been derived from the sum of the numbers of the different types of modern transport aircraft expected then to bein service. The index is plotted aganist maximum cruising speed to show at each band of speed the "balance of power" betweenmanual and power systems. It is interesting to observe that the top speed section of thetable is dominated by the DC-8 and the Boeing 707. It could be argued that the DC-8 should be placed in the power ratherthan manual division, since the only elevators are all manual and the ailerons and rudder are power assisted. However, Douglasemphasize that "power is not necessary to fly the aeroplane." When comparing British and U.S. control trends, it is necessaryto consider the differences between the resources of the two coun- tries. The limited resources in this country might well be thedominant factor accounting for the different systems now being fitted to the latest Vickers and de Havilland aircraft. When theComet was first being constructed, there was no experience at hand for an aircraft of that size and speed. The Vanguard, how-ever, being produced at a later date and designed for less critical speeds, can draw upon the experience of the V-bombers, theViscount, the Britannia and the Comet itself. The technical barrier of time can only be conquered by havingadequate experience and resources. Otherwise revolutionary pro- cesses become gambler's throws. This situation should then be compared with America'sadvantages of having quantitive superiority in money, military orders and brains (not necessarily better brains) with which toevolve a logical manual system for a high-speed transport. The question then arises: why do not Douglas go for an all-manualsystem in view of their unrivalled experience in this field? The answer might be that the firm had to have a competing transportto prevent Boeing from sweeping the world markets and had, therefore, to develop a control system quickly. In other words,they had to build a system which could be made perfect on ground-rigs without the necessity for extensive flight-testing.It must be remembered that Boeing have accumulated a mass of experience with high-speed jets, such as the B-47, which, althoughfitted with power controls, has a manual reversion system. It is generally believed that their present manual system, as fitted toboth their 707 and B-52, was evolved from the efforts made to refine the manual side of the B-47's system. It is possible that,in the years ahead, for follow-up aircraft to the DC-8, Douglas will be able to do the same, once sufficient flight experience hasbeen accumulated on the early DC-8s. Let us consider now the effect of speed on a control surface.The elevator will be given the most attention as it is the surface which can cause the pilot the most trouble, either 'through alack of elevator control or as the means, inadvertently, to over- load the structure in some manoeuvres.As with the airspeed indicator, the predominant effect of the airflow is found from the square of the speed, as defined in thewell-known expression ipV3. Therefore, as the speed increases, the airloads upon a control surface will increase as the square ofthe speed. With the older aircraft, the designer could evolve a systemwhich operated successfully throughout the limited speed range, with the result that the aircraft of 1920 with direct manual con-trols and a 100 kt speed range could have its control characteris- tics decided by the rigger by rule-of-thumb procedures. With alarge speed range the problem is very much more complex. At low indicated speeds the effort required at the controlcolumn might be ten pounds to alter the elevator angle by one degree. Assuming the designer had, by the introduction of apower boost system, kept the effort per degree of surface move- ment constant throughout the speed range, then a nonchalant tenpound tug on the elevator control would pull the aircraft apart. If the designer arranges the characteristics of the control systemto suit the mid-range of speeds, then the extreme speeds will suffer. At low speed, when effective control is essential duringthe approach, the controls would be "sloppy" and at high speed "hard." As speeds increased, the designer modified the control systemand surfaces to maintain the effectiveness of the controls through- out the speed range. As has been seen, at first the task was easy,as the pilot could be expected to make any adjustment to the amount of control displacement, or to the push or pull heexerted upon the stick. Later, an aerodynamic balance was added which made the airstream do some of the balancing work. Forobvious reasons, only a proportion of the airloads on the main part of the control surface would be balanced; therefore, withincreasing speed, the aerodynamic balance had to be abandoned along with mass-balance weights which moved proud of the mainsurfaces. Also aerodynamic and mass balances added to the ice problems associated with modern flying. Ice formed on the lead-ing edge of the balance and upset the calculations. Ice also formed in the large hinge-gaps left exposed by aerodynamicbalancing. The next control refinement carried the designer.up a bigstep in speed. This was the servo and booster tab. The servo tab came into use early on in aviation and was a feature of mostlarge aircraft, some of which had as many as three rudders to overcome the resistance to turning of a forest of struts, wiresand nacelles, not to mention wing-mounted stability fins. The servo tab was essentially a simple solution to the earlyproblems of control loads. As the surface displacement angle increased the airloads increased. At the same time the servo tabwas moving in the opposite direction, thereby balancing out part of the surface loads. From there it was a logical step to connectthe pilot's control wire, or rod, to the tab and not to the main control surface. The pilot now displaced the tab in the oppositedirection to the intended control surface movement, thereby unbalancing the system so that it moved in the required direction.This system had the advantage that as the angle of displacement reached the limiting position the servo tab was blanketed by the
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