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Aviation History
1944
1944 - 1806.PDF
-236 FLIGHT " Indicator " Discusses Topics of the Day AUGUST 3IST, 1944 Artificial Gravity A Word or Two About "Q" and "Negative Q" : Loadings and Reversed Loadings : The " High-speed Stall" and its Minor Repercussions ; Erroneous Ideas on Trimming Tahs and C.S. Airscrews J UST as the most intelligent people can sometimes be temporarily (or even permanently) defeated by the problems involved in airscrew constant-speeding effects and trimmer-tab action, so these same intelligent and reasonable people can have some quite appallingly wrong- headed ideas about " G," high-speed stalling, and negative or anti-"G." There have been cases, for instance, of crash experts who have been unable to understand why an airscrew may be found to be in the fully-coarse position when the control lever is in the erroneously-described fully-fine, position. These experts presumably still have the idea at the back of their heads that the c.s. control movement has a rela tion to the actual airscrew pitch, and have never learnt— or have forgotten—that a c.s. control is an engine speed control and has, except in static conditions, no direct con cern with pitch-changing, which is done by a governor- controlled valve. The airscrews of crashed aircraft might quite reasonably be found to be fully coarse, because, in likely crash conditions, though the throttle might be closed and the c.s. lever fully forward, the c.s.u. would be doing its best to hold back the windmilling revolutions in a high speed dive. A c.s. airscrew is influenced just as well by '' over-drive " as by driving loads. Wing Warping As for tab-effects, I wonder how many riggers realise that an- aileron with an over-flexible wing can be just as much a "reversed" influence on trim as the apfual tab is a "reversed" influence on an aileron. T^ieffe have been cases, particularly with high-aspect-ra^do^ wings, when aileron action has, in fact, been revefasd; the torsional! loads of aileron application have beeiy'scf considerable that! they have twisted the wing and givQh i/moK lift than tt aileron had itself removed. And 1 v onJer riggers understand that a tab is only lomg a pilot, and that the stick or spectacles will/unde cation, be over just as if the p/lot wab^in fac up a wing by his own unaided efforts. \ But all that is by the way, ^nd will Be old stuff to the aerodynamically-minded person. Positive and negative "G" are a little more difficult to define and explain. Although he may not be able to give a concise, story, .the experienced aeronaut knows quite well what both the terms mean, but the man on the ground—particularly if he has fixed ideas about the reality of the stable world around him—is often less able to see the situation clearly. The term " G " itself, of course, is merely a simple way of describing the natural weight of any object or person; a force of 3G makes that person weigh just three times as much as he would normally, while one of a negative sign merely reverses the natural order of things and stands him, so to speak, on his head. It would all be simple enough if the force known as gravity were not being applied the whole of the time.* The business would then resolve itself into the calculated com ponent of forces resulting from any object's violent dislike of being moved and, in particular, once moved, of going in anything but a straight line. Maybe the Backroom Boys should never have started to use the letter "G," but should rather have used the letter " C " for centrifugal or centripetal forces, measuring them in units equivalent to those of gravity. A change of direction of equal radius in any axis produces the same artificial forces, but to these must be added or subtracted the continued gravitational pull—if Mr. Einstein will forgive me for using such an obsolete expression. In order to apply the same loading on oneself at, say, the top of a loop, the radius of the turn must be much smaller than at the bottom; similarly, the radius of the beginning of an inverted loop from level flight must be much smaller to produce a negative force of 2G. than it would need to be to produce a positive force of the same amount at the beginning of a normal loop. Half the troubles experienced by beginnezs in flicking out of the" tops of loops when flying airc^tt with comparatively high wing-loadings would be removed if they were not so deter mined to keep themselves pressed into the seat to the same extent all the way round. Obviously, the pressure should naturally vary according to attitude—a 2G pressure at the top of a loop is equivalent to a 4G pressure at the bottom of a loop, and the poor old thing is probably then straggling round at an already very low speed. Even if we imagine it passible to maintain the same speed all the way round, tire perfect circle would only be completed if the driver's weight in the seat was, say, 3G at the start, 2G when in the vertical position, and G at the top—thus maintaining an artifk/al 2G all the way round (2G plus iG and 2G and keeping the radius of the vertically exe- cutedifircfe'Xpnstant throughout. No doubt M. Pegoud woukf be hurt -if he knew what a dated and neglected mMceuwe a loop has come to be—except, perhaps, when combined with a half-roll; and there are no records to show h<Trn4ented the ipll. Some unknown, parachuteless hero rst Worlf War, no doubt—maybe our ingenious e^efls^ Immelmanij, whose patent heigjrft-gaining turn was, intact, a scrappy half-roll off the top. «. In the ordinar/ way, .so-called negative " G" is the affect of our oldjfriend inertia again, but the forces in this f- case must be igreat enough to defeat, temporarily, the memory of Isfec Newton, in that tfee aircraft's downward accelerationyHmust be greater than that of any body left, casually jn space to the mercies and incipient acceleration of 32 fp&t per second per second. The dashing pupil who suddenly depides to point the nose of his aircraft down at spme objaeft on the ground quickly learns all about the practical effects of this force. Not only does his body wish to carry on iri its original course (at least until such a- time as it has had a chance to accelerate under the force of gravity), but his instructor's goggles and all loose objects, including the fuel in the float-chamber, have much the same ideas. The engine stops, but not the angry voice through the inter-comm. The same sort of thing happens in a down-draught of more than 16 feet per second or so, and, less violently, in any such disturbance. High "G" Stall As for high-speed stalling, one can think of it merely as the effect of multiplied G in inqse&s'mg artificially the weight of the aircraft (and consequently increasing its natural stalling speed); as the effect of the large angle of attack in a tight turn ; or, more correctly, as the effect of both together. Once again the wrong expression has been used; the words '' high-speed stall'' suggest that the effect is due to speed rather than to excessive loadings. The other day I experienced an interesting example of the minor results of forcing the stall. The aircraft in question was a particular '' rogue'' with a tendency to buffet quite vio lently at the stall, and more so when stalled with the flaps
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