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Aviation History
1946
1946 - 0373.PDF
FEBRUARY 2IST, 1946 FLIGHT 197 TYPE III CONTROL BOOSTER control-surface angle. This results in having a varying amount of aero- dynamic balance ahead of the hinge line. The comparative data are based on wind-tunnel tests of models. Fig. 3 shows the four types of control sur- faces considered; all surfaces are assumed to be equipped with com- bined servo-trim tabs. Type I is the plain, unbalanced con- trol surface of the- type used on the Constellation. Type II is the conventional, highly developed, elliptic-nose, overhanging- balance widely used on many aircraft of all sizes. Type III is an effective internally balanced unit. The balance may be symmetrically located or, for elevators and ailerons, displaced to obtain more travel one way than the other.- Type IV consists of a bulge or modi- fication to the elevator contour behind the hinge line. Without attempting to present a mass of graphic data on the advan- tages of each type of surface, the comparison of the basic four is given in Table III. In order to clarify* the reasons for using such light con- trol-booster ratios, some data will be presented as an example. A study was made of the more promising types of aerodynamic balance compared with the unbalanced type. Exhaustive wind-tunnel tests were undertaken to evaluate the advantages of all the basic means for reducing hinge moments. The four control surfaces shown in Fig. 3 were tested in" comparative tests by a large model. To determine the interaction of all the tail components, a one- sixth scale model was constructed. Flutter models having the proper deflection and fre- quencies were used and tested to destruction at high air- speeds. With information from the research noted, Table III was prepared. Conditions for the comparison in the table are: (1) A given control surface section, platform, and area (except as noted). (2) Equal Control surface chords behind the hinge line. (3) Maximum amount of aerodynamic balance without Fig. 3. Control surface balances com- pared in Table HI. Data are based on Lockheed tests on 20 in. chord models. over-balance at any angle of the con- trol surface or angle of attack. (4) Use of a combined servo-trim tab. (The use of aerodynamic over- balance plus an anti-servo tab is not considered.) To simplify the comparison further, the use of the control surface for an elevator will be considered. In view of the fact that both the rudder and ailerons require a higher degree of hinge moment balance than the eleva- tor (as noted by the boost ratios), this comparison is conservative in showing the advantages of the mechanical booster combined with Type I control surface. (Refer to Fig. 2 to see the design condition under which elevator control is critical.) Type 1 control surface is similar to that used on the Constellation. It gives the highest lift, highest hinge moment, and lowest drag. Struc- turally, it is simple and strong. Type II is the widely used over- hanging-nose balance. It cannot de- velop as high a maximum lift because oi leakage across the balance and an early stall due to the balance project- ing. The surface gives a drag-increase of 12 per cent, or more over Type 1. On a laminar-flow tail section the drag increase would be much more, has an internal balance with a cloth or rubber balance is sensitive to coutour of the surface, While the drag is good, the maximum Type III seal. This gap and leakage. control angle is severely limited, resulting in a low maximum lift. An asymmetrical balance-location can be used for elevators and ailerons but not rudders. The hinge moments obtainable are low, but the lack of effectiveness is serious. This can be improved by using a thicker tail section at a sacrifice iii drag. Type IV has a bulged trailing edge. This is effective on small, fast aircraft where stick forces in high-speed manoeuvres are important, but it is not useful for the land- ing conditions. For the case where maximum control is sacrificed and equal control surface areas are considered, Type I control with a booster on the Constellation saves 100 b.h.p. for the design cruising condition at 17,000ft. when compared to Type II. For equal overall control, where the area of Type II must be increased to get the same lift, the gain is approxi- mately twice this amount. This saving of 200 b.h.p. results in an increase of 880 lb. for a trains-continental TABLE III COMPARISON OF CONTROL SURFACE TYPES. Control surface type Maximum down-load comparison on landing with maximum elevator angle. Control surface angle for maximum lift. Relative drag at cruising Hinge moment on landing (tab setfor glide). Hinge moment with area changed to get equal landing control. Required boost ratio for 80 Ib. control landing force on Con- stellation. Effect of manufacturing tolerancesand abuse. Compressibility effects at high,speeds. Approximate relative tail structural strength Remarks on icing Surface area varied to get equal landing control. Surface area kept constant. I 100 per cent. •40° 100 per cent. 100 per cent. 100 per cent. 9.33 Little effect. Most stable type. 100 per cent. Control surface practically unaffected. II 88 per cent. 30° (balance precipitates stall) 112 per cent. 47 per cent. x 47 — 53.5 per cent. 5.00 Very sensitive to balance contour. Subject to over-balance in manoeuvres. 91 per cent. Subject to ice formation on balance and therefore over- balance. Ill 78 per cent. 20° (limited by clearance) 102 per cent. 25 per cent. -=5 X 25 = 32.2 per cent. 3.01 IV 100 per cent. •40° 102 per cent. 96 per cent. 96 per cent. Extremely sensitive near gap Very sensitive to contour of for highly balanced surface. control surface. Greatly subject to changes of j Definitely unsatisfactory if highly balanced.aerofoil press, distribution. 86 per cent. Subject to many adverse effects due to ice ahead of gap and water freezing in seal. 100 per cent. Probably unaffected by ice.
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