ENGINEERING: J. C. HUNSAKER 
279 
My present investigation of two different types of aeroplanes, one a 
standard military tractor with no claims to inherent stability in flight, 
the other a machine designed to possess some degree of inherent stability 
while departing as little as possible from standard practice as exemplified 
by the other, has for its object: 
{a) To determine the aerodynamical constants for the two aero- 
planes by means of model tests in the wind tunnel.^ 
{h) To apply the aerodynamical constants so found in the dynamical 
equations of motion for the full scale aeroplanes in free flight and to 
examine the stabiHty of the motion. 
(c) To compare the stability, both lateral and longitudinal of the two 
chosen types of aeroplane, with a view to tracing to the individual parts 
of each machine their effects upon the motion. 
{d) To attempt to formulate general qualitative conclusions which 
may assist constructors of aeroplanes to avoid instability or to provide 
any desired degree of stability. 
{e) To throw light upon the general problem of inherent dynamical 
stability. 
Stability distinguished as statical or dynamical. — An aeroplane in hori- 
zontal flight in still air must be driven at such speed and kept at such an 
inclination of wings to wind that the weight is just sustained. When in this 
normal attitude the aeroplane, if properly balanced, is in equilibrium. In 
a statical sense, this equilibrium is stable if righting moments are called 
into play tending to return the aeroplane to its normal attitude if by any 
cause it is deviated thereform. In general an aeroplane, if stable in a 
statical sense, will, when given an initial deviation, take up an oscillatioQ 
which either may be damped out or may increase in ampHtude. The 
aeroplane is dynamically stable if, and only if, these oscillations die out 
as time goes on, leaving the aeroplane in its original normal attitude. It 
is clear that statical stability must first be provided before the dynamical 
stability of a design can be examined. 
The righting moments which tend to restore the aeroplane to its nor- 
mal attitude are a measure of statical stability. If the statical stability 
is great, the period of the oscillations will be short and the motion vio- 
lent, whereas an aeroplane should have a gentle motion of slow period 
heavily damped. An ocean liner of too great stiffness (large metacentric 
height) is not suitable for passenger or cattle carrying service; it is pref- 
erable in ship-design to give only enough metacentric height to insure 
that the vessel in never unstable and to damp the roll by generous bilge 
keels. In a similar manner the theory indicates that, in aeroplane-de- 
sign, it is preferable to give just enough statical stability to insure that 
