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Aviation History
1950
1950 - 0028.PDF
FLIGHT, 5 January 1950 The four barrels of the rocket motor offer 6,000 1b of extra thrust. Murocketeer Gene May, Douglas test pilot (seen here), is making a routine practice of riding the transonic hump at Muroc from desert level to 40,000ft. TONIC SONICS . . . tankage for the jet unit is only 250 U.S. gallons of ordinary aviation gasoline (not kerosene), and so the overall endur- ance is limited to about 25 minutes. Incidentally, one of the penalties of the high-compression axial-flow J-34 seems to be a fairly high drop-off in thrust at high speed, a fact which makes the supersonic performance of the airftame all the more remarkable. To boost the take-off on the turbojet unit, and thus con- serve the jet fuel and the net time for altitude test runs with both jet and rockets operating, provision has also been made for carrying a pair of 1,000-lb jettisonable Jato bottles on the fuselage sides. These were not fitted at the time of our visit, since the main purpose of the demonstra- tion was to show off the low-level supersonics, but never- theless there must be many occasions when the extra 2,000 1b Jato kick is worth having up the pilot's sleeve, so to speak. Fully loaded, the Skyrocket grosses around 16,000 1b, which means that on the net wing area of 133 sq ft, the wing loading is 120 lb/sq ft. In the arrangement of the power plants and fuel systems, extreme care has been taken to distribute all the fuel uni- formly about the aircraft e.g., in order to reduce the trim changes during consumption of the fuel. Exhaust outlets also are arranged so that the thrust reacts thiough the e.g., thereby eliminating the effect of offset thrust moments on the aircraft trim. (With the high jet powers now available, this factor can have a powerful effect on stability.) Apropos which, it will be noted that the rocket battery discharges from what normally would be the tailpipe nozzle at the fuselage extremity, while the turbojet exhausts through an elliptically shaped ventral opening in the after part of the fuselage belly. Wing Transonics Apart from the tell-tale sweepback, there is nothing particularly supersonic-looking about the wing ; the profile, for instance, is patterned along high- speed, sub-sonic lines, with rounded leading edge, well-aft maximum ordinate and reflexed trailing-edge contours; moreover, the thickness/chord ratio is only moderately thin—approximately 9 per cent, we should say, from optical inspection. The reason, of course, is that the de- sign objective behind the Skyrocket was a research aircraft capable of the highest possible level speed without sacrifice of relatively normal low speed characteristics, and the eminently successful fulfilment of this broad-ranging specifi- cation is precisely why this aircraft is such a significant contribution to the design art. The sweep angle, measured along the quarter-chord axis of the wing, is the standard N.A.C.A. configura- tion of 35 deg, while in front elevation the droop anhedral of 5 deg is quite noticeable. This negative geometric di- hedral has been found necessary in order to reduce the excessive " dihedral effect" of the sweepback at low speed and high-wing incidence, an effect which is known to cause the so-called "Dutch-roll" instability—a form of lateral oscillation akin to the snaking motions (roll-plus-yaw) of a Dutch skater Longitudinal Control.—For adequate longitudinal con- trol at high Mach Number, the tail-sweep angle is 40 deg, the extra 5 deg over the wing having the effect of placing the tail on a higher rung of the Mach ladder—with a bit in hand, as it were. (The thinner section of the tail would also contribute another rung.) One other noticeable feature of the horizontal-tail design is the large angular change provided on the stabilizer portion, in order to accommodate the large changes of longitudinal trim throughout the transonic zone. This trim adjustment is performed through movement of the rear spar at a fairly rapid rate by electric motor. Changing the stabilizer set- ting instead of the elevator angle is obviously a more effective tail-trimming device, since it alters the pressure distribution over the entire tail surface, rather than the local pressure over the elevator. And so, by an odd design quirk, we return to the standard practice of well over 30 years ago! A nice blend of science and art is clearly evident in the tail ensemble, particularly in the treatment of the vertical surface, which escapes that depressing linear angularity so often seen. In this regard, although the Skyrocket tail displays a geometric similarity to that of the Messerschmitt P-IIIO previously mentioned, it has gone one better, in that the horizontal tail is mounted higher and, conse- quently, the rudder is vastly more effective throughout the whole speed range. It should be obvious by this time (but, oddly enough, is not, if you take a look round at the current scene) that, in order to avoid buffeting of the tail surface, the latter must be mounted high enough to clear the wing wake at high Mach Numbers. A satisfac- tory relationship recommended by Douglas is to locate the horizontal tail at least 13 deg clear of the zero lift line of the wing, as seen in the accompanying sketch. Because of the vulnerability of this Achillean heel in HQEUZflNTAL TAIL-ddCATION -ZERO-LIFT L1NF
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