FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1949
1949 - 1130.PDF
- JUNE I6TH, 1949 FLIGHT 715 Fig. 2. Airscrew horsepower grid plots for I.C.A.N. atmosphere conditions. although approximately 50 hours of flying were then com- pleted in order to build up some endurance time. During this period, readings were taken at 25,000 and 30,000ft. The accurate measurement of jet thrust in a turboprop installation offered formidable difficulties, as the effects due to airscrew thrust entirely masked the much smaller jet thrust. This difficulty affected the accuracy of specific fuel consump- tion, since at cruising altitudes the jet thrust was quite a significant contribution to the total power developed. The use of a variable-area jet pipe in flight testing had accentuated this difficulty and, in fact, loss of total power was recorded in flight when the nozzle area was reduced, although the aircraft actually increased its speed and the piston engines had to be throttled back to restore the original conditions. The general impression of the progress made up to the time of the first flight trials could be gained from the fact that, with practically no maintenance, the first pair of engines ever to fly had a total life of 100 hours before being removed from the aircraft for strip and inspection. Mr. Owner then went on to deal with the use of the Theseus- Lincoln by R.A.F. Transport Command and, after reviewing personnel training, the necessary modification to the airframe, and the clearance of the aircraft for operational use, he stated that the original proposal was to fly over the Transport Com- mand main route to Singapore. It was ultimately decided, however, that the route from England to Fayid (Egypt) and return, with an intermediate stop at Malta, would provide the required conditions. It was found that the aircraft could fly the 1,330 miles from London to Malta at 10,000ft with a reasonable payload and with sufficient fuel to cover all normal contingencies. Flight Conditions The Theseus engines were to be operated at conditions laid down by the Bristol Company, but in view of the nature of these tests it was not thought possible for Transport Command to fly to a strict schedule comprising a number of different engine conditions, and, accordingly, recommendations with regard to desired engine conditions were made and discussed with Transport Command, from which they were able to formulate a flight plan. A cruising speed of 7,200 r.p.m. was suggested, and this was followed during the first "leg" of the first route flight. Later on, however, the normal cruising speed of 7,800 r.p.m. was adopted and used for all subsequent flights. It was evident that the altitude conditions at which the aircraft was to be flown would depend largely on the type of weather encountered, and it was left for Transport Command to use their discretion in this matter. Most flying had been done at altitudes between 10,000 and 15,000ft. Since the main object of these trials was to secure as many Theseus flying hours as possible under conditions obtaining in the R.A.F., the Bristol Company was content with en- durance flying at conditions within the engine operating limits as laid down, detailed flight plans being left to Transport Command entirely. Turning to control problems and improvements, Mr. Owner stated that a remarkable stability was noted at all r.p.m. conditions, very little interference with the throttle lever for speed adjustment being necessary throughout the entire trip. This rceived very favourable comment from the pilots, who found' the continual resetting of boost pressures on piston engines to be somewhat irksome by comparison. They had previously accepted this matter of continual adjustment as being part of the job, but the use of turboprops in the same aircraft now showed that this point was one of no small magnitude. However, notwithstanding the excellent stabilityo! the Theseus engine, it was considered that to obtain n 10 maximum consistency in r.p.m. and fuel con-sumption on very long-range flights some form of governor should be employed, and work wasproceeding satisfactorily in this respect. For take-oil, with the aircraft brakes on, theTheseus engines were first set at 5,000 r.p.m.. then the Merlins and Theseus were opened up totake-ofl r.p.m. simultaneously or separately as required, the brakes being released at the sametime. With the lower idling speeds permissible on later engines, it would not be necessary to usethe brakes in this mariner. In cold weather and with low oil temperaturessome airscrew r.p.m. surge might be encountered on starting, due to the presence of viscous oilin the airscrew hub cylinder, and, although this surge had never been sufficient to effect directionalcontrol, it was wise to take the precaution of feathering and unfeathering the airscrews once ortwice to distribute warm oil around the airscrew hub whilst the engine was idling. After this, the normal take-off, routinecould be followed. This ability to circulate warm oil through the airscrew system whilst the engine was idling was a uniqueadvantage of the fiee power turbine engine. The required cruising power was set by positioning throttlesonly, this being a considerable advantage over the piston engine where a precise relationship between boost and r.p.m.had to be kept, and boost resetting was usually accompanied by an adjustment to the r.p.m. When throttling back the Theseus for approach and land-ing, the airscrew pitch coarsened, and consequently the drag was considerably less than that experienced during the sameoperation on a piston engine. The absence of drag on throttling back gave the pilot the feeling experienced in a car fitted with afree-wheel mechanism, and the approach technique adopted utilized a lower approach speed and earlier closing of thethrottles at touch-down. Mr. Owner went on to say that although the maintenancecrew had to " learn their way about" the new units, the turbines had already thoroughly established their claim toneed far less maintenance than the piston engines alongside which they had been working—indeed, maintenance had beenvirtually negligible, such time as had been recorded against the engines having been spent almost entirely on inspection.Throughout the second and subsequent route-flights, the Theseus engines were entirely satisfactory. The original"life" of 100 hours was extended to 150 hours and, in view of the satisfactory performance and conditions at this stage,it was further extended to 200 hours. After 212 hours' flying time it was decided to remove the engines for examination.One of the engines removed was submitted for bench check RATING CURVE PRIOR TO FLIGHT SHOWN PERFORMANCE CURVE AFTER 212 HRS FLYING SHOWNO O 40OO 5DOO 6OOO 7OOO AIRSCREW R.P.M CORRECTED 80OO Fig. 3. Comparative curves of performance before and after 212 hours' flying.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
