stream tending to maintain the rotation. This explanation in the present case is to some extent a conjecture, but the author believes that it will in due course receive confirmation.[1]
There is a further difficulty in judging the effective form of a boomerang, owing to its want of symmetry. If we suppose a two-limbed boomerang rotating truly about its principal axis, it is evident that there is nothing to balance the unsymmetrical distribution of the pressure reaction, and in practice, therefore, the boomerang will not spin exactly about its principal axis, but about an axis making a small angle with same, so that the resulting dynamic and aerodynamic couples[2] will be in equilibrium. It is for this reason that the author has recently adopted the three-legged form; such a form appears to be in every way equal to the Australian type, and has the advantage of being perfectly symmetrical so that it will spin true, and measurements of angles, etc., relatively to the containing plane may be relied upon.
The torque that gives rise to the gyroscopic precession is a variable depending upon the relation that exists between the velocity of rotation and that of translation, and upon the total pressure reaction. The latter is of necessity equal to the apparent weight i.e., the resultant of the weight and the centrifugal force of the boomerang at every instant; for any given value of the apparent weight the torque is proportional to the distance separating the centre of gravity and the centre of pressure, and hence depends upon the position of the latter. It
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- ↑ Certain screw forms are very capricious in their behaviour. The aerial tourbillion described in Vol. I. (§ 30) may be looked upon as a windmill capable of behaving either as right or left handed. There is a certain form of screw that, mounted in a similar manner, will act as a windmill until a critical speed of revolution is reached, and then it will suddenly change its effective pitch and become a screw propeller eating its way up to windward until its velocity is reduced to a lower critical limit when the windmill conditions again supervene.
- ↑ The aerodynamic couple here referred to is not that to which the processional motion is due, but one due to the want of symmetry of the boomerang; this couple is mobile, its axis rotates with the boomerang.
