FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1914
1914 - 0579.PDF
MAY 29, 1914. positive rolling moment L is produced, the machine tends to bank as required for the turn, and also at first we have a positive pitching moment M, the angle of attack, is increased, and the nose of the machine rises. fig. 7 gives the curves of pitching moment of a biplane model for various settings of the elevator in the tail, the wind speed in this case being 40 ft./sec. Without the tail plane the angle of attack for horizontal flight would be slightly negative, but the machine in this state would be unstable, any increase or decrease in the angle of attack causes a moment tending still further to increase or decrease the angle respectively and so to disturb the machine. With the tail plane the machine flies though with no great longitudinal stability when the elevator is not raised. The effect of raising or lowering the elevator is shown in the curves. Positive angles correspond to an elevation of the elevator tending to raise the nose of the machine. Curves were then given in which the longitudinal moment about the e.g. was plotted against the inclination of the machine to the t/UGHT] * 8 1 3 t — 5 8 •T 1 ' s ' s i 1\ 3 ' -/- rr- , ' •J \ T ' \ ' 1 — s_i \ • : 3 i • - .'/ - — . l./l , _ : .•/' \\ i . • N \ tV ' _ •tor* ' • \ — - •, s 1 ' w.m "•" 1 \ A " ~, .in, c* * / . lj°t!^i' ". ' 1 «±. *4- reFjio J,.,, 1 '"" We can go further than this, as Mr. Bairstow has shown m the beautiful model experiments he carried out recently before the Society, and determine the aherations which must be made in the machine to produce modifications in the conditions of its flight. Now we have already seen in Kigs. 5 and 6 the curves ol forces and moments from which the constants can be determined for the model monoplane which has been exhibited. Mr. Bairstow and Mr. Nayler have recently determined the coefficients for this machine and used it to determine its motion under a variety of circumstances, and I propose to give some account ot their results. Let us suppose that the machine is flying horizontally against a uniform wind, and at a certain moment the horizontal velocity of the wind changes by a known amount, tt0 say, and remains per manently at its new value. Fig. 8 gives the solution of our problem ; the longitudinal motion only is affected and the results on the com ponents of this motion are shown in the curves. The quantities we have to deal with are 11, the change in the horizontal velocity relative to the surrounding air ; w the normal velocity, « the pitch angle or angle of attack, and q the angular velocity of the machine in the plane of symmetry. At a certain moment the velocity of the wind changes by an amount ;<„. At that moment, therefore, the velocity of the aeroplane relative to the air is increased by an amount «„, but this increase rapidly dies away, and after five seconds has become zero ; the velocity goes on decreasing for five seconds mote, and at the end of some ten seconds has reached its minimum value, which is less than the original steady value by about $«„• Tne velocity then ft NGU Oil' T c H Fig. 5. -Variation of forces and moments on model Ble'rlot type monoplane with angle of pitch. Fig. 8 —Ditto with angle of yaw. Fig. 7.—Pitching moment for complete machine.—Wind speed 40 ft. per second. wind. These indicated that the effect of the tail on the longitudinal balance is much less than would be anticipated if it were supposed that the air current is not deflected by the main planes and is free to act on the tail as though the main planes were absent—the moment as measured being only one-half of that calculated on the latter assumption. This required further investigation. So far it had been supposed that the aeroplane is in steady flight in a uniform wind. The author next dealt with some aspects of disturbed flight in a non-uniform wind. He said in the first place we may deal separately with the motion of the machine in the vertical plane of symmetry, the longitudinal motion, as it is called, and its lateral motion or motion perpendicular to that plane. If the longitudinal motion be slightly disturbed and the machine be stable, two kinds of oscillations are set up ; one kind consists of rapid oscillations, both of the machine about its centre of gravity and of the centre of gravity itself. Ordinarily, in a stable machine, these are quickly damped down and give no trouble. But besides these the centre of gravity of the machine itself describes an un dulating path ; the period of these undulations is much longer than in the first series, though for a stable machine the amplitude of the oscillation is gradually reduced and the machine, if no other changes have been made, gradually recovers its normal flight path. Mr. Lanchester has called these slower oscillations the phugoid oscilla tions If the machine is not stable, when disturbed from its normal path its motion will diverge more and more from the condition of steady flight, and this divergence may take place continuously or it may happen that a series of oscillations of increasing amplitude is set up which ultimately become so large as to cause disaster. It is not possible to predict exactly what will happen in a given case without a knowledge of the relative values of the constants or coefficients already referred to, but if these be known the equations can be solved numerically and the details of the flight can be predicted. 5?9 increases again for some ten or eleven seconds and the changes follow a regular periodic curve of rapidly decreasing amplitude, and after some 40 or 50 seconds are completely damped out. Dealing next with w, the normal velocity, we find it starts from zero, but rises rapidly in a fraction of a second to its maximum amount, about 0-2 of «, and then dies away rapidly in the fame manner asw. The angular velocity q is small, so small that the curve is drawn to give 100 times q, and this follows practically the same law as w, while 6, the pitch angle, increases for five seconds and then decreases to pass through periodic changes of decreasing amplitude and of about 22 seconds period. The motion is stable ; the complete calculations show that the rapid oscillations in this case only affect w and q, and that they die out in less than a second, while the other disturbances are those arising from the phugoid motion of the machine. Thus, in this case the machine when struck by the horizontal gust loses longitudinal speed at first, and after passing through a series of changes of velocity, settles down after a few oscillations in less than a minute to its original speed relative to the wind. This loss of speed is accom panied by an initial increase of normal velocity ; the machine rises for a fraction of a second, acquiring a rapid positive angular velocity, but these motions soon change sign and die away like the horizontal velocity. The nose of the machine rises for five seconds, at first rapidly, then more slowly, and this oscillation dies down in the same manner as the others. The next figure, Fig. 9, gives the changes due to a downward gust, w. Relatively to the air the machine acquires an upward velocity, wa, which dies down in about one second and is followed by the slow phugoid changes as before. The changes in the other quantities are shown in the curves, and the motion of the machine can be traced as before. By combining the results of these two diagrams we can find the effect of a steady gust striking the machine in any direction in the p'ane of symmetry. In a similar way we
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
