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IT is remarkable how even in these days quite an important device can creep into common use practically ”unbeknownst,“ and become a regular feature almost as soon as it has first been noticed. Typical of such little causes from which great tilings spring is the ”tab.“ The word is not one of which we are enamoured, but it seems already to have become established, so there is little use in kicking against the pricks. Actually, it is difficult to find a substitute, ”flap“ having already been appropriated to the movable surface of which the tab forms an appendage. There is justification for the adoption of the term in the definition, which according to one dictionary is, “a small flap forming an appendage to something”—in this instance, the larger, main, flap. So we must try to forget its more familiar connotation of a label attached to something for identification purposes and become accustomed to its use in the more general dictionary sense.

DURING the past year or so there has been a growing tendency towards the use of small servo flaps on elevators and rudders either for trimming or for balancing. The object of this article is to review broadly the aerodynamic principles which govern their design, their advantages and disadvantages.

IT has for a long time been realised that if a complete exploration is made of the wake behind a body and the loss of momentum suffered by the air flowing past it measured the drag on that body can be deduced. In 1925, Betz suggested a method whereby this principle could be conveniently applied to the measurement of the drag of wings in flight and a little later, in 1926, Schrenk published results obtained by the method. At that time interest in wing profile drag was not great and the results Schrenk obtained for wings covered with surfaces of different roughnesses were mainly of academic interest. At the time when aircraft had overall drag coefficients (CD based on wing area) of 0·040 to 0·050 a change of wing profile drag coefficient of 0·002 meant only 4 per cent or 5 per cent change in total drag or 1·3 per cent to 1·6 per cent change in the top speed of the aircraft. With the rapid improvement of performance which has taken place in the last 5 to 7 years due to a general “cleaning up” of fuselage shapes, elimination of strut and undercarriage drag, overall drag coefficients of 0·015 have been achieved and a saving of 0·002 in wing profile drag now means a 5 per cent increase in top speed. For this reason designers are now much more interested in accurate data regarding the effect of wing thickness and surface roughness on profile drag.

TEN years ago most aeroplanes, certainly nearly all those of the R.A.F., had open cockpits and pilot's view was not the problem it is to‐day. Occasionally a pilot would be heard to murmur that “in a so‐and‐so you couldn't see a thing straight ahead when you were on the ground,” but apart from a few such complaints very few were made about the view once the aeroplane had got into the air or when it was flying in bad weather. The pilot might complain if he had to move his head out of the shelter of the front screen when travelling at, say, 70 m.p.h., but he could at all events do this when wearing goggles and thus safely approach the aerodrome and land.

TRAILING edge flaps, it seems, have come to stay for some time; the devices that looked almost too good to be true when first investigated have proved invaluable, not only as aids to landing more heavily loaded aircraft, but more recently they have been shown to be extremely useful in helping take‐off. Evidence of the importance they have lor designers is the large amount of experimental work done on them in all the research establishments of the world, and as a result a lot is known of their aerodynamic characteristics so that it is not difficult to choose, from all the types available, a flap to serve any particular purpose. The object of this article is therefore not to discuss the effects of flaps on lift, drag or stability and control, but briefly to examine them, from one particular aspect that has not received a great deal of attention up to the present, that is—balance.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Committee, Reports and Technical Notes of the U.S. National Advisory Committee for Aeronautics and publications of other similar research bodies as issued

WITH tailless aeroplanes, all known aerodynamic control devices possess the peculiarity of not only producing moments about one axis, but of also causing secondary moments about one or both of the other axes. Horizontal controllers forming part of the wing near the tips in wings having sweep‐back or sweep‐forward, for instance, do not produce rolling moments alone, when differ‐entially deflected; they also cause yawing and pitching moments. Similarly, wing‐tip disk rudders operated on such wings not only produce yawing moments, but may cause rolling and even pitching moments.

The orientation of a rigid body is described by a position‐tensor, composed of three unit vector axes fixed in the body; rotation is effected by tensor transformations in which a rotational operator is a Cartesian matrix, formed from the co‐ordinates of the pivot axis and components of the angle of rotation; rotational sequences are represented by matrix products. Three practical applications are discussed: sequences of aircraft manoeuvres, which include composite rotations of roll and pitch, or roll, pitch and yaw, applied simultaneously; the variation of sweep, incidence and dihedral of a wing moving on any axis fixed in the aircraft; the direction of the pivot axis and the angle of rotation in the motion of a retractable undercarriage between two specified positions.

IN a series of articles entitled “Tailless Aircraft and Flying Wings”, concluded last month, the evolution of the tailless aeroplane and the flying wing was treated. The different trends of the development were classified, and a short discussion of the difficulties which had been experienced during experimental work given.

THE complexity of the problems which are associated with the lateral stability and directional control of tailless aeroplanes was not realized until rather late.