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Aviation History
1940
1940 - 0283.PDF
FEBRUARY I, 1940 105 STALL WARNING C.M.W. Stall Indicator Tested in Australia : Light or Hooter? Reserve Airspeed IndicatorT HE accidental stall is still a grave danger and causes a large proportion of the accidents which happen to private aircraft. Stalling of large aircraft in the transport class is, of course, very rare, but it cannot be said that even this lias been entirely eliminated. A device which indicates the approach of the stall has been designed and made in Australia by Mr. C. Murray Waller, of Sydney. It works by means of a venturi placed in the airstream, and is arranged to operate a warning light on the dashboard before the stall occurs. By an adjustment, the margin of warning given to the pilot may be set to any desired figure in terms of airspeed. The light may be made to come on when flying speed has fallen to within 5 m.p.h. of stalling speed, or 10 or 15 m.p.h. warn- ing may be desired. Once set, the margin will not alter. In external appearance, the device consists of the venturi connected by tubing to a small cylinder, of about 4m. diameter and the same length, which is set inside the wing of a lafge aeroplane, or outside it if the aeroplane is small. In the latter case it is suitably faired to reduce drag. From . the cylinder electric wires go to the battery and dashboard light. Though details of construction are not disclosed, it is obvious that pressure variations in the venturi (caused by varying speed of flight) operate an electrical contact in the cylinder and so light the warning lamp if airspeed drops below the predetermined safe figure. The adjustment of the margin of warning is made by removing a cover on the cylinder and setting and fixing a pointer. With the cover replaced, unauthorised persons cannot tamper with the setting. Tested in Flight A test flight in a Puss Moth with the C.M.W. Stall Indicator fitted was an interesting event. With the aero- plane stationary no light showed. The glass in the warning light was dark orange in colour—why this had been chosen in preference to the conventional red for danger is not known. In the take-oft the light came on soon after start- ing the run (at about 20 m.p.h.) and stayed on till past the stalling speed. While it still showed, the pilot knew his flying speed was insufficient to pull off safely. Having attained about 3,000ft. of height, a straight stall was tried. Throttling back, the pilot kept the nose high and speed fell and fell. Then, with a couple of preliminary flickers, on came the light. As yet there was no sign of stalling, although any pilot would have felt the aeroplane was "limp." Then a little slower still, and the pilot was obviously tensed up waiting for the stall and keeping the aeroplane on an even keel by rapid, jerky sideways move- ments of the stick. Finally, as though the struggle had rather wearied it, tht Puss sagged away completely and attempted to put its left wing down. This the pilot pre- vented, and a recovery to flying speed in the glide was made. The light had come on at about 10 m.p.h. above the stall, at which speed it had been set to come on. After several more straight stalls, medium turns were done and speed reduced to stalling while keeping bank constant. Stalling speed on the turn is, of course, increased by the bank. In these turns the light came on at the same A.S.I, read- ing as when flying straight; so, of course, the margin of warning was decreased. But up to a 45-deg. banked turn the device still gave its warning before the stall. Theoretically, increases of stalling speed in banked turns are: 30 deg.,'7 per cent. ; 45 deg., 19 per cent. ; Cx> dog., 41 per cent. If stalling speed of an aeroplane is 50 m.p.h., these increases amount to 3! m.p.h., o£ m.p.h. and 2o| m.p.h. respectively. Stalling speed is a difficult thing to determine accurately in the air, but the impression was quite definite that, up to 45-deg. bank, the light gave warning before the stall. It is, of course, a limitation of the C.M.W. Stall Indicator that it is a true stall indicator only for stalls in straight flying, but pilots who are going to do turns over 45 deg. should not need an indicator or should do the turn at a height such that they can get themselves oat of trouble. The device is really an indicator of dynamic pressure (that is, \ pV2) and so indicates the straight stall at any altitude or temperature. (Straight stalling occurs at the same mark on the A.S.I, independent of these two variables, as every pilot knows.) Supplementing the A.S.I. The device seems to have a field of usefulness. On first thoughts, one would think that the modern airliner and its highly .trained pilot were not in need of it. But "Indicator," in the issue of January 18, pointed out that aircraft with high wing loadings have very little '' feel'' and require " an almost hundred per cent, faith in such instruments as the airspeed indicator. You cannot feel the approaching stall," he says. He advocates two A.S.I.s and two pitot heads, having had his one let him down by registering some 40 m.p.h. too high. If a stall indicator were fitted, this could act as a reserve A.S.I., indicating, as it does, the critical value of flying speed. The case of a Lockheed 14 crash in Australia which took four lives because it was pulled off the ground too early in the take-off run springs to mind as another unfortunate occurrence which could have been prevented by a stall indicator. It is not thought that student pilots learning to fly should have a stall indicator—they should be taught to '' feel'' their aeroplane. But when that student pilot goes off for his solo practices, after having three or four solo hours, no doubt his instructor might have fewer grey hairs if he knew a light was going to flash in the pupil's face should he get down near the danger point. It has been argued as to whether a light or a hooter is the best warning. Mr. Waller prefers the light. As he says, the device starts acting just before touching down in a landing and continues till the run has slowed to about 20 m.p.h. It would not be steadying to a pupil's nerves to have a hooter start just as he wras breaking the glide and making those delicate backward movements of the control column which hold her off until she is ready to sit. The designer also says it has been suggested to him that his device should pour out a whisky-and-soda to fortify the pilot for the approaching stall, but he feels that this might tend to popularise stalling practice too much. The device has been tested by the Civil Aviation Depart- ment of Australia, and it is believed that their pilots and engineers are favourably impressed by it. No official pro- nouncement has yet been made. AIRCRAFT CONSTRUCTION NUMBER Thursday, FEB. 8th, 1940. NEXT week's issue of " Flight" will be aspecial number mainly devoted to asurvey of structural progress in the field of aircraft design, and a discussion of future possibilities. The development of airscrews, undercarriages and other components will also be embraced. A review of present-day tendencies in aero-engine design will be a feature. ORDER TO-DAY, THIS SPECIALLY ENLARGED JSSUE OF VV/DE INTEREST. Out Thursday next, Feb. 8th Sixpence as usual.
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