FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1946
1946 - 0812.PDF
FLIGHT APRIL 25TH, 1946 METRO-VICK AOOO 3OOO British Axial-flow Unit with Efficient Ducted' fan Thrust Augmentation : High Thrust, Low Specific Consumption WHEN it is appreciated that Metropolitan-Vickers,Ltd., have a virtually unsurpassed background ofsteam turbine experience and, further, this is allied to what is, perhaps, the foremost mechanical sciences re- search organization existing in the commercial world, then it will not occasion surprise that such a company should take its place in the front rank of gas-turbine progress as applied to aircraft propulsion. The axial-flow type gas turbine is so basically at an advantage in the critical factor of thrust/unit frontal area that on this score alone its development is imperative. Another advantage is the low specific fuel consumption. . The unit which forms the subject of this survey is a straightforward axial-flow compressor and gas turbine with which has been incor- porated a ducted-fan thrust augmenter. By this means the thrust delivered by the unit has been increased by 67 per cent., for an un- changed fuel consumption and for an increase in weight of only 33.3 per cent. In specific terms, the plain jet part of the unit produces a static thrust of 2,400 lb for a weight of r,65O lb, so giving a thrust/weight ratio of 1.455: 1, but the unit when fitted with the aug- menter gives a static thrust of 4,000 lb for a weight of 2,200 lb, i.e., a thrust/ weight ratio of 1.818: 1 which shows a percentage specific gain of 25. It must also not be forgotten that this achievement is paralleled, if not eclipsed, by the reduc- tion in specific fuel consumption. The great thing about the embodiment of the thrust augmenter is that the gain in thrust is realized without any increase hi fuel consumed. This results in the specific consumption coming down to 0.65 lb/ hr/lb thrusf, which is some 35 per cent lower than that hitherto regarded as best obtainable, viz., 1 lb/hr/lb thrust. The static thrust rating of 4,000 1b is, of course, for sea level conditions, and that proportion of the overall thrust 2000 1000 4OOO These curves show graphically the respective thrust valuesof the pure jet and augmented units, together with fall in specific consumption. The low end of the curves is notthe idling condition, this being at approx. 2,500 r.p.m. GAS TURBINE contributed by the augmenter will decrease with increase in flight speed—very much in the same way that conven- tional airscrews lose tractive efficiency with increase in air- craft speed. Further, the fall-off in augmented thrust occurs with increased altitude, again in accordance with those same laws which cause an airscrew similarly to lose efficiency. However, the thrust drop of the augmenter is not parallel with that of an airscrew and can, in point of fact, be largely modified by the incorporation of a variable- area outlet orifice. Perhaps the best way of visualizing a ducted-fan thrust augmenter in general terms is to regard it as something of a half-way measure be- tween pure jet propulsion and airscrew propulsion. It has been shown that smaller losses are occasioned by moving a large mass of air rearward fairly slowly—as in airscrew propulsion—- than is the case when a small mass of air is moved rearward at high velocity— as in pure jet propulsion. ,By including ducted fans in conjunction with jet propul- sion, the thrust obtained from the jet alone is some- what reduced, but a tre- mendous gain in overall thrust is provided by fans entraining a relatively large mass of air and. driv- ing it rearward fairly slowly relative to the jet velocity. The streams from the gas turbines and the fans are separated and do not mingle until both have left the jet pipe. Thus at the orifice there is, in effect, a hot high- speed jet from the turbines, and a cold lower-speed annular jet from the fans, the latter stream cylindrically enclosing the jet stream. This has an effect on noise suppression which is most marked, and the unit is, without doubt, the quietest aircraft propulsion engine of any kind ever de veloped. There should be a very useful future before it on this count alone for passenger aircraft, aside from the inherent lack of vibration and extremely good performance. Flow through the unit is uncomplicated and simple to 5000 6000 • 7000 COMP/TURBINE - R.P.M 8OOO
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
