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Aviation History
1950
1950 - 2267.PDF
FLIGHT, 28 December 1950 CLEAR-AIR GUSTS A Digest of Dr. G. S. Hislop's Notable R.Ae.S. Lecture ONLY since aircraft in increasing numbers have regu-larly been flying at the higher altitudes has clear-airturbulence become of note; but the ability of modern types to fly not only higher still, but faster as well, has steadily brought the matter into prominence. In the July 15th, 1948, issue of Flight appeared an article dealing with the pioneer work of the British European Airways Gust Research Unit This formation, under the direction of Dr. G. S. Hislop, Ph.D., B.Sc, A.R.T.C., M.I.Mech.E., A.F.R.Ae.S. did valuable work, and in his paper, Clear-Air Turbulence Over Europe, given before the Royal Aeronautical Society on December 14th, Dr. Hislop reviewed the mode of opera- tion of the B.E.A. Gust Research Unit, and thereafter gave an interpretation of the statistical results obtained. The B.E.A. results had shown that, at around 25,OOOft, on average, about eight miles had to be flown to meet a 4ft/sec gust; about 60 miles to meet an 8ft/sec gust; about 400 miles to meet 12ft/sec, and so on (Fig 1). Most results had been obtained between 20,000ft and 30,000ft, and it was probable that, statistically speaking, the results were most reliable between those height limits. The apparent increase in distance to be flown for heights less than 20,000ft was possibly due to the smaller number of results available at 15,000ft. The increase in distance at altitudes above 30,000ft when gusts greater than 12ft/sec were considered, was also a reflection of the fewer results available. It was possible that the presence of the tropopause was a factor which made large gusts less frequent than at the lower altitudes; on the other hand, the number of small gusts increased at higher altitudes, i.e., the average dis- tance to meet one diminished. This was probably a genuine effect, as the flight-crews frequently reported slight turbulence when passing through the tropopause. On the whole, and bearing in mind that a sample of 92,300 miles (the B.E.A. work) was relatively small, the variation with altitude could hardly be classed as of great significance. In Fig. 2 is given a comparison between the B.E.A. and U.S. data, depicting the lelative frequency of occurrence of gusts of various magnitudes irrespective of altitude. In this figure, the B.E-A. data have a threshold value of 4ft/sec, whereas the U.S. data have a much lower threshold value which, however, had been corrected to the British gust standards. It was extremely interesting to see how the B.E.A. curve not only lay between the two limiting curves but had the same slope at higher gust velocities. This confirmed that there was little difference between the frequencies of occurrence of gusts of moderate intensity at low and at high altitude in clear air. It also suggested that an extrapolation could be made on this diagram, with fair confidence, to velocities of about 36ft/sec. In dealing withthe question of ex- trapolation, Dr.Hislop considered it both justifiableand statistically more satisfactory toneglect the slight variation with alti-tude and to present all the B.E.A. dataas a plot of miles- to-fly to meet agust of given inten- sity versus gustvelocity. This was done in Fig. 3, to* oz Mi 3 s a \ V V.S CURVES BASED ON 9XtO*MILES CLOUD AND CLEAR AIR 6 12 16 2O 24 28 32 34 46 44 46 CUST VELOCITY (tt/jee EAS.). Fig. 2 (left). Relative frequency of occur- rence of gusts of various magnitudes. f'S- 3 (right). Miles- to-fly to meet a gust of given (or greater) magnitude, plotted against gust. ZO 25 3O HEICHT » lOOO ft 5S JM Fig. I. Miles-to-fly to meet a gust of given (or greater) magni- tude, plotted against altitude. which also includedsome results of V-g recorder analysesconducted by the RJLE. on Constel-lation and Libera- tor operations, to-gether with samples derived fromN.A.C.A. research. It had been suggested that a law of the " negative binomial>:type might fit the B.E.A. results, and it was clear from Fig. 3 that it did so very well. The negative binomial extrapolationmight be a little optimistic, but the lecturer thought it a reason- able compromise to offer at this stage, at least for gust velocitiesup to about 36ft/sec. Some confirmation that the negative binomial distribution might be optimistic was suggested by theComet encountering severe turbulence at high altitude during its early development trials (Flight, January 12th, 1950). Theactual equivalent gust velocity at the e.g. of the Comet was not recorded in this instance, but it was probably around35ft/sec. Even allowing for the probability that the mileage flown at the time of the occurrence (about 6,000 miles at above15,000ft; had probably trebled since then, and no further turbu- lence of this order had been reported, yet the correspondingpoint lay between the negative binomial extrapolation and the V-g data. The broad conclusion was that the Comet episodecould not be regarded as a freak. To put the matter into perspective, it was of interest toconsider what would be involved in operating a fleet of 20 aircraft, broadly of Comet type. At an annual utilization of3,000 hours, the total distance flown in a year might be about 27 million miles. On the negative binomial extrapolation, a36ft/sec gust would be met, therefore, on average, once a fortnight, and a 50ft/sec gust once in four years. These gustvelocities corresponded approximately to acceleration increments of 1.5g and 2g respectively at the assumed cruising conditions.[A car stopping in 30ft from 30 m.p.h. experiences lg.—Ed.] O-O-O B.E.A., 92.300 miles, 15.000 37,00<Ht. "Negative binomial —_ Typisal " normal error V-K record!, ConlteUation, 15,00011, 3,360 hr. Vj records. Liberator, 10.000ft, 3.000 or. * Vl records. 3,480.000 miles V-g records, 1.850,000 miles. Q Q SJi.c.A iM^erommer, 47,800 mile*. S.AX.A. accelerometer, 17.400 milei. TO 3O 4O SO GUST VELOCITY ( ft/sec E.A.S.) to
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