Aerodynamic characteristics of ribbon stabilized grenades

Abstract

An extensive experimental program has been conducted to determine the aerodynamic characteristics of grenade ribbon stabilizers. While data has been acquired from vertical and horizontal wind tunnel tests, free-flight drop tests, and freeflight gun tests, this paper presents only the data from the horizontal wind tunnel test. During this test, both static and dynamic free-yaw data were obtained at speeds ranging from 90 to 180 feet/second. The test obtained axial force, side force, yawing moment and pitching moment while the grenade was free to yaw, or while statically fixed at a given yaw angle. Analysis of the static and dynamic forces and moments indicates that there are four types of ribbon induced oscillatory motion. These types are presented, as is the zero-yaw drag as a function of ribbon length and ribbon width. NOMENCLATURE Svmbols: AoA Angle-of-attack, degrees. AF Axial force, Ibs. CM Pitching moment coefficient, = PM/QSD. CY Side force coefficient, = SF/QS. CLN Yawing moment coefficient, = YM/QSD. CAF Axial force coefficient, = AF/QS. CD Drag COeffiCient (derived from CU. CAF & Theta) CL Lift COeffiCient (derived from Cy, CAF & Theta) D Reference length, = 1.515 inches. PM Pitching moment, in-lbs. S Reference area, sq. in., = nD2/4. SF Side force, Ibs. YM Yawing moment, in-lbs. Theta Yaw-angle, degrees. * AIAA Member-Aerospace Engineer This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. C. Wayne Dahlke* David C. Purinton* Dynetics, Inc Huntsville, AL FORWARD An effort has been undertaken to reduce the hazardous dud rate of rocket-dispensed grenades to less than 1 percent. As a part of this effort, the current study was initiated as a low cost solution to this problem. The objective being to eliminate side impact, the largest single cause of hazardous duds. Following dispense the grenades experience large oscillatory motion until the ribbon stabilizer is deployed. After the stabilizer is deployed, the grenade motion quickly damps out and slows to terminal velocity. Once the grenade has reached terminal velocity it should not experience any oscillatory motion in excess of 20 degrees. Previous wind tunnel tests of ribbon stabilized grenades may be classified into three types of tests. These are static force and moment tests, dynamic “free-yaw” tests, and vertical flight tests. Static force and moment data presented by Dahlke”‘*’ were collected utilizing a pyramidal balance to obtain the grenade axial force, side force andyawing moment. Dynamic “free-yaw’ tests presented by Dahlket” I measured the grenades yaw angle as a function of time and from that data the dynamic derivatives (CMa, CMq) were calculated. Analysis of the static data alone would lead the analyst to believe that the current ribbon provides adequate stability. Furthermore, incorporation of the static data into a 6-DOF simulation would no doubt indicate that grenade oscillations imparted during dispense quickly damp out and the remainder of the grenade trajectory is at relatively small angles-of-attack. However, data taken during the “free-yaw” tests and observations made during the vertical wind tunnel tests indicate that this is not the case. This discrepancy may be attributed to the ribbon dynamics. While the flapping ribbon does provide the drag needed to stabilize the grenade body (and arm the fuze), the flapping ribbon also acts to destabilize the grenade. The task of designing a test that truly captures all the flight dynamics of the grenade is not currently possible. A “perfect” test would require a breakthrough in a miniaturized