NO. 5 STABILITY OF AEROPLANES HUNSAKER AND OTHERS 4/ 



the use of fornnike in which the (hfference between products of ob- 

 served quantities is involved. Naturally, the precision of the result 

 is poor when we are left with a small difference between large 

 quantities. 



The measurements / >, My., I' p, J'ji, Mn, Mc are probably correct 

 within 2 per cent. L involves no difference and may be taken as 

 equally precise. 



Since iV-, = A'' — a)' we may make the distance a very small in 

 setting- up the api)ara(us and so keep the precision of N about 2 per 

 cent. 



From 



^, _ Vji — Mg cos ifz — Mn sin ij/ 



y __ ^ , 



we note that (iUr cos 1// + .I//; sin i//) is from three to five times as 

 large as I' r. The precision of ]' should then be between 2 and 6 per 

 cent. 



From similar reasoning, we may expect Z and X to be precise 

 within lo per cent, but in s]iecial cases, where wc must take the 

 difference of quantities of nearly e([ual magnitude, the precision is 

 not so good. 



The cjuantity ^1/ is a small moment which should be nearly zero if 

 the aeroplane is balanced properly. Obviously, no estimate of the 

 precision of M as a per cent can be given in such a case. Where M 

 is large, as in the 12° condition, the measurement is precise to about 

 10 per cent. 



Fortunately, for a study of lateral stability, we are concerned with 

 Y, L, and A'^ only, and these quantities are determined with fair 

 precision. 



The values computed for X, Z, and M for zero yaw may be com- 

 pared w^ith A^, Z, M, determined independently in the tests on lift 

 and drift discussed in Part I. 1die latter are probably precise within 

 2 per cent. Consequently the computed A', Z, and ^1/ obtained from 

 the asymmetrical tests have been adjusted to make them agree with 

 A', Z, M obtained from the symmetrical tests. 



The change of X, Z. and .1/ \vith i// is not important, and A', Z, 

 and M are not used in the theory of asymmetrical or lateral stability. 

 Since by our equililirium conditions, the pitching moment M^ must be 

 zero for normal flight, we must assume that the pilot makes M^ zero 

 by slight adjustment of his elevator tiaps. In the tables below, the 

 small value of A/^, observed when the angle of yaw xjj is zero has been 



