Apparatus and method for measuring gyroscopic stability

The apparatus and method are designed to allow a projectile to be supported n an external air bearing and spun up to a desired spin rate. When the desired spin drive is disengaged the projectile reacts freely to the gyroscopic forces in three degrees of angular motion. Spin loss, coning angle and nutation motion are measured as they vary with time.

BACKGROUND OF THE INVENTION 
This invention relates to experimental apparatus and method for measuring 
gyroscopic stability of gun launched spinning projectiles. The present 
invention permits simulation of a spinning projectile in free-flight 
conditions for the purpose of providing real-time motion rate results 
without extensive data manipulation. 
In the past, spin stability measurements were frequently made in fixtures 
employing the forced-mode principle. In this technique a fixture, having 
provision for holding and spinning a projectile and forcing a simultaneous 
coning motion, is used to determine the projectile's stability 
characteristics by computational means. In this procedure the projectile 
is permitted to despin freely while being held at a constant coning angle. 
The problem with this method is that inconsistancies in the friction of 
the supporting ball bearing races have led to questionable test results. 
In the prior art devices which utilize spherical air bearings to test a 
spinning projectile model in a supersonic wind tunnel, it has been 
necessary to locate the support bearing inside the model and with a rear 
oriented strut to support the bearing and to minimize aerodynamic flow 
interference. The problem with the aforementioned apparatus is that the 
strut support severely limits excursions for most realistic model 
configurations. 
SUMMARY OF THE INVENTION 
In the present invention a projectile is supported in an external spherical 
air bearing, and spun up to a desired spin rate. When the spin drive is 
disengaged the projectile is left to react freely to gyroscopic forces as 
it is allowed to despin. The freely developed spin loss, coning angle and 
nutation motion are measured as they vary with time. An electromagnetic 
repulsion device is used to develop large initial yaw. 
An object of the present invention is to provide an apparatus and method 
for measuring gyroscopic stability of gun launched spinning projectiles 
which substantially eliminates support bearing friction. 
Another object of the present invention is to provide an apparatus and 
method for measuring gyroscopic stability of a spinning projectile having 
three degrees of angular freedom of motion. 
A further object of the present invention is to provide an apparatus and 
method for measuring gyroscopic stability of a spinning projectile in an 
environment which will allow the projectile to respond substantially in 
the same manner as it would in free-flight. 
For a better understanding of the present invention, together with other 
and further objects thereof, reference is made to the following 
descriptions taken in connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The spin fixture for measuring gyroscopic stability comprises a spherical 
shaped support body member 10 which fixedly holds a projectile 12 in a 
radially extending holding slot 14. Projectile 12 is located in holding 
slot 14 so that its center of gravity 16 is operatively disposed so that 
it is coindicent with a radial axis of ball body member 10. Body member 10 
is supported by a lower female pole piece support members 18, 18' and an 
upper female pole piece support members 20, 20' having spherically 
contoured air bearing surfaces 22 and 24 respectively. A plurality of air 
passages 26 are located in the lower support members 18, 18' and a single 
air passageway 28 is located in the upper support members 20, 20'. 
Passageways 26 and 28 carry high pressure air from air supply source 30, 
at a controlled rate by an air pressure regulator 32, to exit orifices 34 
and 36, respectively. Orifices 34 and 36 supply clean, dry pressurized air 
to the air bearing clearance space 38 located between the ball body member 
10 and the spherically contoured air bearing surfaces 22 and 24. Air 
passages 26 in lower support members 18, 18' are designed to have larger 
flow capacity than passage 28 in the upper support members 20, 20' because 
the lower part of the air bearing support member 18, 18' must support the 
weight of the projectile 12 and the body member 10. The smaller air flow 
to the upper bearing surface area 24 can be considered as a cooling 
provision with limited usefulness as a stabilizing adjunct if the spinning 
projectile develops gyroscopically induced radial motions. The lower and 
upper pole piece members 18, 18' and 20, 20' include precisely meshing 
dowels 40 and locating holes 42. This permits upper and lower pole piece 
members 18 and 20 to be readily disassembled and reassembled when required 
by the insertion in the fixture of a new projectile for testing. A teflon 
lined element 43 is fixedly attached to a ball bearing race 45 which in 
turn is positioned on top of upper pole piece support member 20. The 
teflon lined ball bearing race 45 is used to minimize any damage to the 
projectile 12 by the fixture in the event the projectile gyrates wildly. 
Vertical frame 44 is used to support pole piece members 18 and 20, an 
electro-magnetic solenoid type horizontal repulsing device 46 and 
vertically restraining upper and lower hoops 48, 48' which have teflon 
lined inner member 49, 49' respectively are fixedly attached. An electric 
motor 50 is mechanically coupled to a gripping clutch type device 52 which 
releaseably engages the base 54 of projectile 12. 
In operation when a projectile is ready for test, the support assembly 18, 
18' and 20, 20' are supplied with air to raise the lower part of the ball 
support body member from metal-to-metal contact with lower support 18, 
18'. The ball 10 is centered in supports 18, 18' and 20, 20' first by hand 
spinning the projectile 12 as the air supply pressure is adjusted by 
control of regulator 32 so that the ball 10 makes very slight or no 
contact with the lower and upper spherical pole piece surfaces 22 and 24 
respectively. Pressure regulator 32 and manually-adjustable micrometer 
position-indicating throttle valves 56 are then adjusted to maintain the 
ball member 10 approximately midway between the lower and upper support 
members 18 and 18' and 20 and 20' respectively. 
Spin-drive motor 50 and the gripping clutch device 52 are activated to 
engage projectile base 34 so that the projectile is brought up to a 
desired spin rate. If the projectile 12 does not immediately begin to yaw 
and nutate, when the clutch 52 is disengaged from the projectile 12, the 
projectile is given an initial yaw by the rubber wheel 58 of the 
electro-magnetic actuated repulser 46. The spin, coning and nutation 
motions are recorded versus time to provide the desired rate information 
related to gyroscopic stability. The teflon-lined race 45 and the 
restraining hoops 48, 48' are both utilized to help restrain the gyrating 
projectile nose 60 whenever there is severe gyroscopic instability. 
The foregoing disclosure and drawing are merely illustrative of the 
principles of this invention and are not to be interpreted in a limiting 
sense. I wish it to be understood that I do not desire to be limited to 
the exact detail of construction shown and described for obvious 
modifications will occur to a person skilled in the art.