FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1913
1913 - 0943.PDF
AUGUST 30, 1913. since these increasing vectors are inclined more forward than those in the more advanced portion of the wing, the resultant, besides travelling back, also takes a decidedly more forward inclination, and becomes somewhat as shown in Fig. 14. Instead of the retarding component, D, of Fig. 12, we now have the propelling component, T, and this is just enough to carry the machine over the crest of the phugoid, and save the steep plunge down the other side. As a result such descent to recover speed as may be necessary is done at a decent easy angle, somewhat as shown by the dotted line in Fig. 13, instead of in a highly dangerous combined pancake and plunge. In brief, this reserve tangential affords an ever- available and entirely automatic means of temporarily increasing the speed in emergencies without the immediate necessity of diving. The effect this has on the smoothness of the flight-path in high winds is quite amazing when first experienced. One curious result of this reserve tangential—curious, that is, to the spectator—is the ability of the machine to maintain itself under full control at apparently impossibly large angles of incidence. These four longitudinal devices, utilising as they do the entire aeroplane surface, probably confer the maximum longitudinal stability obtainable, and prima facie considerably greater than that which can be obtained by the manipulation of small subsidiary surfaces attached to a main unstable surface. The directional stability also we have seen to be very considerable. Let us now examine the effect of these two properties in a state of affairs where the machine is tilted sideways.* Since the days when Wilbur Wright first showed us what banking really meant, every student of aeronautics knows the old diagram shown in Fig. 15. Here AB is a front or back view of the tilted plane, P is the air-pressure, G is gravity, and R the resultant of P and G. The force / is the centrifugal force necessary to balance / Fig. 17. R ; or, looking at it another way, / and P together have the resultant L which balances gravity. If that force /can be obtained, the system is in equilibrium, and the necessary support against gravity is maintained. Lacking that force, there is neither equilibrium nor a continuity of support, and the machine must fall, no matter what the reserve of power. Recovery there may be, due to a magnificent sideslip bringing some fin- or dihedral-angle effect into operation, but our object is to maintain steady equilibrium throughout the flight and adequate support at all times, and we particularly want to avoid these falling, slipping, rolling evolutions. Therefore, the equilibrating centrifugal force f must be obtained at all costs, which is equivalent to saying that the machine must be made to revolve about some such axis as N. (See Fig. 16.) Now the revolution about the axis N, with radius r, and angular velocity w, and centrifugal force f, can be resolved into two simultaneous revolutions, one about the axis X with radius r,, and angular velocity <•>„ and the other about the axis Y, with radius rn, and angular velocity ain. The centrifugal forces due to these simultaneous revolutions are respectively fi and/u, which combined have the resultant f. The revolution about the axis Y is in this case evidently a purely directional manoeuvre, due to tail or fin or some equivalent. The revolution about the axis X is a movement of elevation involving a longitudinal manoeuvre. In order, therefore, that the aeroplane shall revolve round N following the path a, a„ au, and so produce the requisite force/, it is necessary that it shall have sufficient directional stability to follow with its bow the lateral deviation in the trajectory of its mass induced by the lateral component of R (Fig. 15), and sufficient longitudinal stability to follow with its bow the upward deviation in the trajectory of its mass induced by the upward component of * As if, for example, it were deliberately banked over by the use of the ailerons, which were immediately afterwards returned to their normal position. IfiiGHf] R.+ The machine under discussion fulfils, as we have seen, both these requirements. This resolution of the turn into two components is not necessary unless we want to understand in detail exactly what is happening. For a general comprehension of the idea, Mr. Berriman's lucid statement that, granted " weathercock " stability, a machine will follow with its nose any deviation in its trajectory induced by R, is less complicated. But I have shown the full analysis, in order to introduce to aeronautical students this method of resolving banked turns into components in the planes controlled respectively by rudder and elevator. It enables you to see what is actually happening, without the usual necessity of assuming some complicated interchange of the relative functions of these organs. We have it then established that if the machine be in a tilted position with one wing lower than the other and all controls normal, it will commence to circle towards the depressed side. Fig. 17 shows the aeroplane circling thus, seen from above. I have shown the machine as heading straight into its curved path, and it will presently be obvious that such a position cannot be maintained. Xi tj„ «: t. _ y ; 0 : .- ' is* .*•'" _ Gc •• * ! Fig. 16. In the first place the outer wing in order to keep up would have to travel through a greater arc than the inner wing in the same time, and so overcome greater resistance. Obviously this outer wing will tend to lag behind. In the second place the outer wing would tend to be depressed, and this for two reasons. First, the pressure increases gradually towards the outer tip, because the arc described by each portion of the wing increases in length as you travel out towards that tip. And, as the pressures increase, so are they applied to less and less positively inclined portions of the wing, till at the outer part, where the pressures are greatest, the tip is actually negatively inclined. Now, in the early days of my experiments, I used to think that this negative inclination of the outer and faster-moving tip was sufficient to account for the fact that the machine—if turned by an ordinary rudder—depressed the outer wing. In this I was wrong. Later quantitative analysis of the positive and negative couples showed at once that, with the small amount of negative surface we use, the counterbanking couples were utterly insufficient to balance the banking couples due to the increased pressures on the lifting parts of the outer wing and the shortage of pressure on the inner negative tip. In this I believe I have the support of Mr. Hume Rothery, who calculates that in order to obtain a counterbanking result one would require to have a quarter of the negative surface inclined so as to come under negative pressure. I think it would be impracticable to do this with the class of machine and motor we have at our disposal at present. But if a practicable negative tip does not produce sufficient counterbanking, we must look for something else in the construction which supplies the deficiency. t i.e. It must elevate as the angle of incidence is reduced by this upward deviation. [To be concluded.) 969
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
