FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0866.PDF
864 FLIGHT WITHOUT VISIBLE MEANS OF SUPPORT . . . The Bruche A piloted ground-attack fighter powered by an afterburning Atar. In addition to the data given on p. 862, this design is to have a minimum radius of turn of 3,280ft. employed spoilers located within the jet-pipe of the engine, and/orlips close to the discharge orifice. In both cases, control in roil was effected by inducing rotation of the jet.When the annular wing serves as a ramjet, a deflection of the trailing-edge flaps directly affects the thrust-producing jet. Bymoving these flaps in opposite directions the exit area is varied without deflecting the efflux. Such area restriction is helpful foravoiding the formation of any dangerous low pressures within the wing immediately after the ramjet is switched off at high speed.It may also help as an air brake. Coleopter Propulsion. As this aircraft class, like all jet-liftersand helicopters near the ground, cannot be landed normally when the propulsion has become inoperative, manned coleopters musthave either dual propulsion or duplicated propulsion units, each of which will be sufficient for safe descent. For most coleopter projects, "combination propulsion" is pre-ferable. For short periods of high speed flight, ramjet propulsion proves the lightest. This, however, demands that the aircraft isaccelerated to operational speed by other means, e.g., by a rocket motor or by a turbojet, and that sufficient static thrust is availablefor ascent and descent. For manned or recoverable coleopters, ramjet propulsion alone is, therefore, considered impracticable.It is employed solely for boosting to high supersonic velocities. For ascent, the static thrust should be at least 1.25 times thetake-off weight; for landing it may be less, and a value equalling the landing weight would appear acceptable. Most currently usedturbojets weigh around 0.3 lb/lb static thrust. This means that the weight of airframe, disposable load and fuel tanks should beequal to, or less than, half the static thrust. This is achievable, since a coleopter airframe weighs only about half of that of a com-parable standard aeroplane. Lighter turbojets render the scheme even more practical. The latest Atar 101E turbojet, of 7,700 lbstatic thrust, weighs only 0.235 lb/lb thrust, and Dr-Ing. H. Oestrich, the designer of this engine, holds further weight reduc-tions to be possible for coleopter installations. The expendable Rolls-Royce Soar turbojet weighs only 0.15 lb/lb static thrust. S.N.E.C.M.A, investigations show that turbojets with reheat andwith additional ramjet propulsion guarantee coleopter perform- ances which are unattainable by any standard aircraft, quite apartfrom the take-off and landing advantages. The ramjet would be put into operation at speeds in excess of about 500 m.p.h. As far as ramjet operation is concerned, the cross-section of theduct is oversized; this leads to relatively low combustion tem- peratures, structurally most desirable. Since, however, the velocityat which air enters the combustion space is high (of the order of Mach 0.3, in present projects), special burners have been de-veloped, with shields and flamehplders, designed to achieve sufficiently reduced local flow velocities for reliable combustion.The entry of these patented subsonic burners is shaped like a diffuser for a ram intake; high compression efficiency and lowburner drag are claimed. Up to 33,000ft altitude, the pressure in the burner diffuser exceeds one atmosphere, decreasing to not lessthan half that pressure up to 66,000ft (where the internal flow velocity corresponds to about Mach 0.1). This safeguards theramjet operation and makes it economical. S.N.E.C.M.A. experimentation is at present concentrated uponthe turbojet. A vertically ascending Atar engine with all flying controls is under test. As its tailpipe is located within the coleopterduct,, forward of the wing trailing edge, the hot gas jet is sur- rounded by a cold air stream as long as the ramjet is inoperative(i.e., during launching and descent). By virtue of injector action, this airflow is present at any speed. Ducted airscrews can produce a substantially higher staticthrust than the free-running type, provided that the duct exit area is suitably chosen. Seibold claims more than 25 per cent static-thrust increase as attainable. Heating the airscrew slipstream might produce some acceleration; at present, such a method is notutilized, and it is scarcely economical. Structural Features. The annular wing has 1.57 times thesurface of a plane wing having the same span. Wing weight might therefore become prohibitive, and a high airframe weight wouldrender the whole coleopter scheme impractical. H. Hertel investi- gated the structural possibilities from this aspect. He found that,in fact, coleopters can have much lower airframe weights than comparable standard aeroplanes. The annular wing cannot be conceived as a plane wing bentinto a ring shape, since closure to a ring renders the system static- ally three-fold indeterminate, thus completely modifying the stress distribution which is characteristic for plane-wingstructures. Basically, the annular wing is a tube structure, with the central body anchored to it byfour radial arms or struts, which also help to retain the ring shape. Besides these concentrated loads,distributed loads act upon the ring-wing structure —such as the outer-air loads, pressures arising fromflow through the duct, pressure differences within the wing, hydraulic pressures from integral fueltanks, and mass forces during manoeuvring. All these could be resisted by the tube without second-ary moments as long as the loads were ideally dis- tributed. This, of course, cannot be assumed; stiffening-ringframes are therefore needed to preserve the circular cross-section of the stressed tube. Where tanks, radial struts and control flapsare located, substantial members are required for the transfer of loads; these correspond to the spars of a plane wing. The aerofoil of a coleopter wing is only about 3.5 per cent chord(= length of annular duct) thick. In spite of this, and partly owing to relief by the radial strut bracing, the skin sheets required arevery thin. In certain unfavourable circumstances, however, the use of such skins may lead to critical stiffness and structuralinstability. According to H. Hertel, minimum weight and satisfactory stiff-ness and stability (as well as easier manufacture) are obtained when one only of the two wing surfaces is constituted by a structurallysubstantial stressed skin; the other surface can be an unstressed fairing. This interesting development leads to the smgZe-walledshell structure, which is characteristic for all coleopter wings. In the case of ramjet propulsion, the outer wing skin forms the shell,whilst for slower coleopters the inner duct surface is constituted by the stressed-shell skin. In all cases, the stressed skin is rein-forced by numerous light ring frames, in accordance with the load distribution, so as to retain the local stability of the skin. Inaddition, more substantial ring-frame structures are provided, as mentioned. Since the structural parts of such a single-walled shell areperfectly accessible, manufacture, inspection and maintenance become easy; also, installations buried within the thin wing areconveniently housed. Tanks and other containers form an integral part of the annularwing. For the assembly of such items Hertel proposes a rather neat solution by sliding two finished halves over each other, withfreely accessible final joints. To resist internal loads arising from high-pressure feeds, rib-like stiffeners are incorporated. Envisaged,too, is local support of the stressed skin by foamed stabilized plastics bonded to it. This practice is already adopted for largeresearch models and for non-recoverable missiles. H. Hertel has proved that such an annular wing structureweighs less than 40 per cent of that of a plane wing having equal strength and span, in spite of the larger surface. Full use is madeof magnesium alloys. General Assessment. The drag of the annular wing is, atoptimum cruising speed, 1.25 times that of a plane wing having the same lift and equal aspect-ratio. The higher cruising thrustrequired demands more fuel and would, a priori, handicap the coleopter on account of greater take-off weight.In fact, however, the airframe weight of a coleopter is only 24 per cent of the launching weight (compared with 33 per centfor the corresponding conventional aeroplane having the same aspect ratio and the same specific powerplant weight. Thus thedrag penalty is well compensated until long ranges or durations are considered. For airscrew drive, the ducting would allowreduction of the take-off power against that needed for V.T.O. aircraft of the "pogo-stick" variety. Over ranges in excess of about 2,000 miles, the plane-wingaeroplane attains better economy in cruising flight. In other respects, however, superiority is claimed in favour of the coleopter,provided that the powerplants have a reasonably low specific weight. Since the annular wing is aerodynamically better than the cross-wing configuration, it should be preferable for missiles designed for high manoeuvrability. The influence of low specific powerplant weight is easily seenfrom the single-seater intercepter project "Charancon IV" (see table). With a specific turbojet weight of 0.3 Ib/lb static thrust,the launching weight is about 3 tons for 60 miles radius of action. A specific powerplant weight of 0.20 lb/lb thrust leads to only
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
