COMMISSIONING THE NEW PITCH DAMPING RIG AT THE CSIR WIND TUNNEL FACILITIES

Transcription

1 3 RD INTERNATIONAL SYMPOSIUM ON BALLISTICS TARRAGONA, SPAIN 6-0 APRIL 007 COMMISSIONING THE NEW PITCH DAMPING RIG AT THE CSIR WIND TUNNEL FACILITIES F. Dionisio, L. Vn Zyl nd A. Stockenström 3 CSIR DPSS, PO Box 395, Pretori, South Afric Phone: , Fx: , emil: fdionisi@csir.co.z CSIR DPSS, PO Box 395, Pretori, South Afric Phone: , Fx: , emil: lvzyl@csir.co.z 3 DYNAX, PO Box 70336, The Willows, Pretori, 004, South Afric Phone: , Fx: , emil: dynx@ifric.com Session: Exterior Bllistics ABSTRACT The first phse in the estblishment of pitch dmping cpbility ws presented t the nd ISB (reference ). Following functionlity checks nd nlysis of the cquired dt from tests performed t M0.3 it ws decided to re-evlute some spects of the rig design. This pper describes the commissioning of the first rig t the CSIR s wind tunnel fcilities. In ddition, the spects for reevlution re discussed with some bsic design criteri required for follow-up work by second rig. 7

2 7 EXTERIOR BALLISTICS DESCRIPTION OF EXPERIMENTAL DESIGN In order to respect the requirement for performing pitch dmping tests t nominl incidence ngles of up to 0, test rig development focused on the concept of n oscillting frme, fitted with sting nd blnce such s to provide controlled, sinusoidl rottion of the model in the pitch plne. Motion ws effected vi wing, mounted on this frme, controlled from n externl hydrulic ctutor through drive trin system. The min technicl prmeters describing the pitch dmping test rig re s follows: Mximum Dynmic pressure = 5 kp Typicl Reynolds number =.0 x 0 6 Mch number limit, M 0.9 Commnded frequency 4 Hz (limited by rig nturl frequency) Commnded mplitude = ±.0 (structurl limits) Rig nturl frequency = 7. Hz Model mss 5 kg Mximum offset incidence 0 CALIBRATION Sensors Dt from three mesurement sensors ws cquired: reference gyro-on-chip (clibrtion shown in Figure ), 4-component lod blnce (clibrtion shown in Figure ) nd n ccelerometer with nominl sensitivity of 50mV/g.

3 Commissioning the new pitch dmping rig t the CSIR wind tunnel fcilities Sensitivity [mv/(deg/s)] 4 3 mesured mgnitude theoreticl mgnitude mesured phse theoreticl phse Phse [deg] Frequency [Hz] Figure : ADXRS300EB gyro-on-chip clibrtion NF A PM A NF B PM B Blnce output [V] Lod [kg] Figure : Lod blnce clibrtion

4 74 EXTERIOR BALLISTICS Inertil chrcteriztion The bsic ssumption used in processing the dt ws tht the inertil lods could be described by constnt reltionship: NF = PM gyro ccel The elements of the mtrix re trnsfer functions. The mtrix ws determined by oscillting the model in two independent motions with ir-off. With these results one could define: NF PM or NF PM NF PM NF PM = gyro ccel gyro ccel gyro ccel = gyro ccel Without loss of generlity, ll the mesurements were normlised by the gyro output. The results were recorded s trnsfer functions for the output signls of the mesurement sensors. The dt ws then processed by subtrcting the inertil lods, pplying the sensor trnsfer functions nd converting the blnce output to lods. The first motion nlyzed ws the sting verticl bending mode while the second ws the pitching mode. The frequency of the sting bending mode obtined ws 7.5 Hz, which mde it impossible to obtin sufficiently pure pitching motion bove 6 Hz. TEST DATA ACQUISITION AND DATA REDUCTION To check the full functionlity of the rig, low speed test cmpign ws first performed t M0.3 in the Low Speed Wind Tunnel fcility (LSWT) (see Figure 3).

5 Commissioning the new pitch dmping rig t the CSIR wind tunnel fcilities 75 Figure 3. Pitch Dmping Rig in the LSWT using the Stndrd Dynmic Model (SDM) Test results The pitch dmping derivtive results for three testing frequencies re shown in Tble. These represent the erodynmic pitch dmping moment t the instnt of mximum ngulr velocity nd minimum (or men) incidence. Tble. Pitch dmping, Nm/(rd/s) Frequency Repetition Repetition Averge One would expect resonbly constnt vlue of pitch dmping moment over the frequency rnge of to 4 Hz (k=0.0 to 0.04), which is not the cse. This is likely due to combintion of poor dt qulity, prticulrly from the built-in inertil sensors, high model mss nd low dynmic pressure. The pitch dmping coefficient pproches

6 76 EXTERIOR BALLISTICS the experimentlly determined vlue s the frequency, nd therefore the signl levels, increse. RE-EVALUATION OF THE DESIGN The wing-driven oscillting frme potentilly provides n elegnt solution to the chllenge of cquiring dynmic derivtive dt t significnt ngles of incidence. The design did, however, pose the following problems during implementtion in the Low Speed Wind Tunnel nd design review. Model inerti The model mounting to the blnce is designed for multi-functionlity, llowing xil position djustment nd controlled roll motion. A lrge mss penlty is incurred by the berings, ctutors nd structures required to provide this functionlity. The high model inerti negtively impcts the ccurcy of the dt reduction result, becuse the blnce redings re dominted by the inertil forces. Rig stiffness A reltively low nturl frequency is chieved, pproximtely 7.5 Hz. The first mode consists of the model mss pitching ginst the stiffness of the sting in bending, the cross-bem in torsion nd the two side-rms in bending. This vlue is difficult to improve upon without significntly reducing the model mss nd thereby removing the multi-functionlity. Another design restriction with respect to rig stiffness is the cross-bem running through the wing. The wing size, internl structure nd berings, limit the mximum cross-bem dimeter, resulting in excessive flexibility. Similrly, the presence of the wing behind the model determines n dequte clernce distnce. Incresing length of the sting nd side-rms decreses the inherent

7 Commissioning the new pitch dmping rig t the CSIR wind tunnel fcilities 77 stiffness of the frme. These fctors present inherent design limittions, tht link chievble stiffness to wing size. Aerodynmic pitching moment nturl frequency An erodynmic pitch plne frequency is identified by considering the nturl frequency of the rig inerti bout the centre of oscilltion with the wing erodynmic restoring moment s the spring fctor: f n = (/π) (d.q.s. C Nα /I)½ where C Nα ( = 6.3) is the wing norml force slope with respect to incidence, in rdins, d is the distnce between the wing erodynmic centre nd the rig centre of oscilltion nd I ( 5 kg m ) is the inerti of the rig bout the centre of oscilltion. It follows tht the term (d.q.s.c Nα ) yields the erodynmic stiffness resisting rottion of the rig inerti (I). The high vlues for chrcteristic length (d = 0.8 m) nd surfce re (S = m ) results in uncommonly high vlues for f n, when compred to typicl vlues for bllistic rockets, for instnce. At dynmic pressures pproching 5 kp, vlues for f n 7.5 Hz, the erodynmic nd structurl first modes converge nd resonnce occurs. This potentil filure scenrio cnnot be tolerted. In ddition the potentil dynmic pressure llowble for testing would lso be seriously limited. Flutter potentil The combintion of lrge wing nd low frequency structurl modes poses the wkwrd possibility of flutter. With most of the frme flexibility residing in the sting, there is lso the possibility of the SDM / sting system pproching flutter mode.

8 78 EXTERIOR BALLISTICS GENERAL SAFETY CONSIDERATIONS The high potentil energy ssocited with the lrge wing leds to sitution where it is difficult to gurntee sfety. Potentilly criticl scenrios include controller runwy, wing instbility nd ero-mechnicl resonnce. Conclusion of review It is concluded tht the design, lthough inherently promising, should be modified to ddress sfety concerns nd, if possible, extend the currently limiting prmeters of operting frequency, dynmic pressure nd model inerti. DESIGN CRITERIA FOR SECOND RIG The oscillting frme is still regrded s the nswer to chieving pitch dmping testing t significnt ngles of incidence. A redesign of the frme nd ctution method is being conducted with the following in view: Investigte direct ctution, possibly hydrulic or electricl, to eliminte the need for lrge wing inside the test section. Increse the frme nturl frequency by optimising with respect to frme member stiffness, frme shpe, frme mss nd model mss. Achievement of pure sinusoidl motion is priority. It is n importnt criterion in the selection of n ctution method nd / or peripherl systems to fcilitte this function. One option under considertion is to implement mechnicl spring resistnce, to mke use of the nturl sinusoidl motion resulting from spring-nd-inertil system. Due to the poor qulity of cquired dt from the build in sensors, lser mesurement techniques re going to be investigted.

9 Commissioning the new pitch dmping rig t the CSIR wind tunnel fcilities 79 Decrese model nd blnce block mss to the fullest extent, s limited by structurl sfety nd mnufcturbility. The design im is for mss of less thn 4 kg. The structurl design gol is to chieve 8 Hz operting frequency nd 30 kp dynmic pressure (double the current test limits, for both prmeters). The offset incidence requirement is 0, with n oscilltion bout the nominl of ±3. Preliminry design results indicte fir level of fesibility. The frme inerti, ctuted t the stted frequency nd mplitude levels, become driver design prmeter. With the wing-driven design, the erodynmic force is vilble t such n excess tht it ws not concern. The frme needs to be optimised for low mss (inerti) nd drg (frontl size) on the one hnd nd stiffness on the other, in order to converge on solution tht stisfies the contrdicting requirements. REFERENCES. F Dionisio, Estblishing Pitch Dmping Testing Cpbility t CSIR Defencetek. nd Interntionl Symposium on Bllistics, 4-8 November 005, Vncouver Cnd