FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1956
1956 - 1110.PDF
256 FLIGHT, 17 August 1956 AIRCRAFT CONTROL SYSTEMS A Review from the Pilofs Angle By CAPT. A. SPOONER AND L. F. E. COOMBS OF the new airliners now on the market, the Comet, L-1649, Caravellcand Electra have powered or power-assisted controls. The DC-8 is a marginal case and the remainder have manual systems. Why suchwidely opposed schools of thought? We feel that the authors of this article present points of view which—although we do not necessarilyagree with them ally-help to put this interesting and widely debated topic into some kind of perspective. NO two design teams appear to agree about controls.Consider the DC-8, the Boeing 707 and the Vickers 1000.These aircraft, all of comparable size and speed, vary in their control systems from the all-manual aerodynamicallybalanced system of the 707 to the split-surface, all-hydraulic power system of the V.1000. The DC-8 method, whilst similar inprinciple to the Boeing system, is dissimilar in detail. The Comet 4, although smaller and lighter, has single surface, all hydraulicpower-operated controls. The Britannia and Lockheed 1649 Super Constellation pro-vide another complete contrast. Whereas Bristol favour the all- servo-tab system, Lockheed prefer their well-established hydrau-lic system with last-ditch manual (and muscular!) reversion. Several systems have been reported for the Electra but it isbelieved that that aircraft will have a basically hydraulic system [as described on July 6—Ed.]. The Vanguard, by contrast, willhave all manual controls. Approximately 1,000 Douglas DC-6s and DC-7s, 700 Lock-heed Constellations and Super Constellations and 900 Boeing Stratocruisers have been built for civil and military use. Surelysufficient aircraft to provide a proving ground for the three control systems in use? Yet in their next design, the DC-8, Douglashave added to their conventional system the Boeing boosted rudder and, in addition, have boosted the ailerons. Boeing, incontrast, have abandoned in their next design their well-proved boosted rudder and gone over to the Douglas all-manual system,adding to it separate low speed ailerons. As for Lockheed, they retain their hydraulic system and continue to pay a weight penaltyof about 1,000 lb per aircraft. The conflicting Electra reports indicate some degree of uncertainty about Lockheed's futurepolicy. An outsider may well be permitted the observation that, inspite of the importance of the subject, insufficient is known about it. It is certainly all too obvious that the division of opinion isone which cuts across international barriers. And the manual- hydraulic controversy cuts back and forth across the speed line;vide the 1649 and the 707. Clearly a problem exists. As with many other mechanical advances, aircraft controlshave tended to become more and more complex. Complex demands for effective control over a widening speed range resultin complex equipment to handle those demands. The aircraft of the first 30 years of powered flight operatedwithin a speed envelope of 100 kt and control problems were relatively simple. There were no compressibility problems,buffeting or overstressing, and if controls iced up that was just bad luck—or bad piloting. Today, by contrast, modern transports will soon have a speedrange of 450 kt and will operate so near their limiting factors that momentary dives may too easily result in structural failure.Vital balance may also be upset by small ice formations in the wrong places. The clean shape of modern designs is such that they behavelike fighter aircraft (the modern jet airliner is essentially a scaled- up Hunter) and the excessive acceleration loads induced in theairframe soon reveal structural weaknesses. It is important to note that, contrary to what might otherwise be expected, the jettransports of tomorrow contain less structural material than die older, slower types of comparable weight. This is equally true ofthe Comet 4, the Boeing 707 and the DC-8, all of which have empty weights considerably less than half their maximum all-upweights. Here, then, we have our modern transport: a slim fuselage,lightly constructed, stuffed with compressed air, supported by thin wings, speeding through the air balanced upon an invisibletrajectory—departure from which, unless done slowly and care- fully, could bring catastrophic disaster. The means of change of trajectory is accomplished by deflectingthe trailing edge of the supporting or stabilizing surfaces without much more refinement than the schoolboy's quick twist of thetrailing edge of his paper dart. Fig 1 shows the development of control systems from the techniques of the 1900s to the presentday arrangements of all-moving tailplanes and asymmetrical lift spoilers. It is appropriate at this stage to reflect how pessimistic the FIG. 1. Simple manual control—aerodynamic balance is achieved by setting back the hinge. Aerodynamic balance with a mass balance added to overcome flutter. TORQUE TUBE Spring-tab assistance. The tab is directly coupled to the stick; the elevator is coupled to the tab through a torque tube. MASS BALANCE REDUCED POWER ACTUATOR Manual control with power boost—the direct linkage is retained and part of the hinge moment is fed back to the pilot. POWER ACTUATOR Fully powered elevator—the powered control is irreversible and feel is supplied artificially. TAILPLANE INCIDENCE ACTUATOR POWER ACTUATOR "Follow-up" tail — power is used to alter the tailplane incidence in relation to the displacement of the elevator. Operation of the trim control alters this relationship. An artificial feel unit is still required.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
