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Aviation History
1950
1950 - 1424.PDF
128 FLIGHT, 27 July 1950 SIDEWASH and STABILITY A Study of Wing/Fuselage Interference Effects IN the course of various projects it has often occurredto the writer that an article destined to clarify certainstability problems might be of some general aero- nautical interest. During the initial project design of a given aircraft, the proper determination of wing/fuselage junction, and the size and shape of fin and rudder, are both of paramount importance to the stability and controllability. The apparent differences between a good and a bad design might be very small indeed ; but, nevertheless, it may well be detail design which eventually determines whether the aircraft is to be a success or a failure. To cite a hypo- thetical case, a few extra degrees of rudder travel might be necessitated by take-off safety-speed demands, but those same few degrees of movement might well be conversely detrimental in producing fin stall. If, on the other hand, the shape of the fuselage and its canopy were properly determined in the first place relative to the fin and its moment arm, the subsequent expenditure of much time and thousands of pounds in test flying and modifications would not be neces- sary. Nowadays, probably more than ever, it behoves designers and their staffs to think very carefully indeed before committing any lines to paper. Before continuing this discussion it should perhaps, be statedthat the subject-matter has purposely been limited to give a brief explanation of (i) why the fuselage effect is the mostimportant in lateral stability, and (ii) how the wing/fuselage interference affects stability; in addition, there are a fewdesign do's and don'ts relative to the attainment of good lateral stability for a given aircraft. From the very earliest days of flying, aircraft stabilitycriteria have always been referred to the wing—a logical enough practice in the past, when aircraft had more wingstructure than anything else. Nowadays, however, when fighter types resemble projectiles, and airliners have bodieslute monstrous whales, and in each case the amount of wing is kept to a minimum, it is, surely, high time to stop usingwing size and shape as data for proportional coefficients and, • instead, adjust the empennage to the proportions of thefuselage. It is a matter for arithmetical proof that, if the area of ageometrically similar wing is halved—thus doubling the wing loading for a given aircraft—the tail volume coefficient isnearly trebled (2V2 to be precise). This, however, entirely neglects the influence of the fuselage as a main destabilizingfactor. The whole thing is essentially a matter of sensible proportion, and with the common trend toward the use ofever greater wing loadings there must, inevitably, be. a con- current increase in tail surface areas relative to the diminishingwing area. Some of the-comparatively recent monsters might have been designed to Picasso requirements instead of thoseof (P)I.C.A.O. There is also much cause for wonder at the way in whichthe air somehow manages to follow their detail contours. It is worth remembering that, once the flow has broken away—for example, on the hard edge of an optically flat windscreen— nothing save boundary-layer suction will make it adhere tothe surface again. Such discontinuities can have far-reaching results, and it is frequently disruption of the airflow in frontof the aircraft e.g. which is responsible for such unpleasant "after-effects" as snaking and rudder locking. On one par-ticular aircraft, to the writer's knowledge, the directional stability coefficient (a,) for small angles of yaw turned outto be only one-third of its proper value at large angles of yaw, owing to an aerodynamically bad canopy design.Another aircraft with a sharp-cornered windscreen was found By— H. K. MILLICER, Dpi. Ing* M-Sc, A.F.R.Ae.S. Failing this, and when an optically flat panel must be used,it is the writer's contention that a 15-deg three-dimensional intersection-angle is about the maximum that the airflow canfollow. Sharp-sided bullet-proof panels have caused many a high-speed aircraft to snake, and many of the faster moderntypes are now being fitted with curved fairing panels. Generally speaking, the smoother the airflow around thefuselage nose, the better will be the stability and drag: after all, the fuselage can justifiably be regarded as an aerofoilsection with a large nose overhang. On this basis, the area of the fuselage forward of the e.g. is equivalent to an area infront of the hinge of a control surface, and is equally de- stabilizing, i.e., over-balancing. Extending the analogy givesthe conclusion that the sharper the fuselage nose the less over- balance is likely .to occur. Whereas the fuselage with abulbous nose—more particularly those of parabolic derivation —will reach the instability threshold and. thus " over-balance '*. if the e.g. is more than one-fifth of the total length, a fuselage with a sharper (elliptical)nose profile can be stable with a e.g. position at one-quarter of the total length. That bothweight and drag of tail surfaces can be saved by a clean front fuselage design can, therefore,safely be assumed. As to the size of the vertical tail surface(s) asdetermined by the fuselage dimensions, a valuable formula was initiated by Gilruth. He stated that, in order to avoid fin stalland aerodynamic lock-over by the rudder, and also to ensure good all-round lateral stability, the product of fin and rudderarea times its arm should not be less than half the product of the fuselage length times the square of its depth. On applica-tion of this formula to various modern aircraft, however, the writer found it inadequate, and the required coefficients forsuccessful aircraft turned out to be something over 0.6 for single-engined types and 0.8 for multi-engined types. Further-more, the Gilruth formula does not take into account the shape of the front fuselage. There is ample evidence to show that most of the damageinflicted on the airflow is done at the front of the fuselage and, in addition, sidewash is most active in front of the wing, -j-, being two or three times greater in front than behind it.dp On the basis of this theory, the writer derived the following formula for the size of the fin and rudder relative to the size TABLE I : FUSELAGE/FIN RELATION, AIRSCREW-DRIVEN AIRCRAFT to have a fin and rudder efficiency (2\ of iess than 50 per cent at some stations, and an average of but 70 per cent for thewhole. On this basis, about 30 per cent of the fin and rudder area and weight was useless, and on the aircraft concerned thiswas equivalent to roughly 2 per cent of the payload. There is a simple recipe for canopy shapes which do not .spoil the drag and stability of an aircraft (ref.: N.A.C.A. T.R. 730). The canopy corners should be rounded off at aminimum radius of one-quarter the canopy height or width. •-'•••' • * Aircraft 1 m+hrml <>mt«narifin Avro York Bristol Brabacon .. Handler Page Hast Vickers Viscount.. Boeing B-50 Baumann Brigadier Bristol Wayfarer.. Convair 240 tags . Airspeed Ambassador . Martin 2-0-2 Douglas Super DC-3 Perdval Prince ... Lockheed Neptune Short Sturgeon ... Consolidated Vultee B-3 North American Navkm Aircraft llfrom Ji N~A.CA.~l \ TN- \ 3 ... . Aeronca Chief ... BeHano. Cruoair... Cessna 120 Piper Vagabood ... D.H. Beaver Saab Safir ... Sokol Hraz MacchiMBSOB ... Douglas Skyraider D.H. Chipmunk ... Avro Athena Bouhon Paul Baffiol Hawker Sea Fury FairchiM T-31Fieseler Stored ... CnitftlnlitMl Vnlr 6 '. ee L-13 m Eng*. 4 4 4x2 4 4 4 2 2 2 2 2 2 22 2 g, H»(») Rudder(s) 3 3 • ; ; ; i i i WineFosn. low high low low low mid mid high lowhigh low low highmid mid high low high low high high high high low high high high lowlow high low low low low low lowhigh high SnxLaXLn SirXSn 3.952.39' -•-:. 3.31 "2JI U 6J2 2Ji 2.15 :2.5 3.342.91 2.44 3a4.64 5JS 3.462JI 2.431.99 3472.32 X9I X35 2.112M 3.18 3.153M 2.98 . 2442.66 342 2.0.1.35before modificMMn 3J22.98 2.124.25 2.6
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