PROJECTILE S DRAG COEFFICIENT EVALUATION FOR SMALL FINITE DIFFERENCES OF HIS GEOMETRICAL DIMENSIONS USING ANALYTICAL METHODS

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1 HENRI COANDA AIR FORCE ACADEMY ROMANIA INTERNATIONAL CONFERENCE of SCIENTIFIC PAPER AFASES 015 Braso, 8-30 May 015 GENERAL M.R. STEFANIK ARMED FORCES ACADEMY SLOVAK REPUBLIC PROJECTILE S DRAG COEFFICIENT EVALUATION FOR SMALL FINITE DIFFERENCES OF HIS GEOMETRICAL DIMENSIONS USING ANALYTICAL METHODS George Surdu*, Ioan Vedinaş**, Georgică Slămnoiu*, Şomoiag Pamfil** *Research Center for Nay, Constanta, Romania, **Military Technical Academy, Romania Abstract: In this paper is described an algorithm for ojectile s drag coefficient ealuation for small finite differences of ojectile s geometrical dimensions. The algorithm is esented in ojectile design bibliography as ojectile s shape index ealuation using empirical relations. The study is useful for engineers who work in research and ammunition design when is necessary to ealuate the eliminary ojectile shape index and his implication on trajectory. In the same time based on ojectile s shape index the drag coefficient is calculated using the reference law s Siacci, 1943 and The paper offer an ealuation of ojectile s drag coefficient/shape index in accordance with the small finite differences of ojectile s geometrical dimensions of ogie and tronconical part. This study can be easily implemented in a standalone application as a module of ojectile s drag coefficient ealuation. Keywords: drag coefficient, ojectile s shape index, ammunition, aerodynamics, ojectile, aerodynamic configuration, reference drag laws 1. INTRODUCTION Analytical methods are ery useful for eliminary ealuation for aerodynamic parameters of ojectiles and ojectiles ballistic design. In this case is necessary a simple and expedite instrument to ealuate the influences for small differences of ojectile s geometrical dimensions in drag coefficient ealuation or ojectile s shape index. In addition, these small differences reesent the accepted tolerance or accepted ojectile s dimensions tolerances in ballistic design. These types of studies offers to engineers or field test specialist of ammunitions an important standalone instrument which seres in: ojectile geometrical modification ealuation, ballistic table ealuation, ojectile s trajectory ealuation using drag coefficient modifications caused by small differences of ojectile s dimensions. This small differences/ tolerances of ojectile s dimension are inherent and the only way is to accept them and ealuate the implication of them on ojectile s aerodynamic coefficients, ojectile s trajectory or ojectile target impact. Some of these studies can reesent a cheap and handy alternatie for field-testing but cannot replace the experimental tests. Specialist for eliminary ealuation and implications can use these instruments. This study esents ojectile s shape index and drag coefficient ealuation using the method of the French researcher Hélie that has

2 established a relation for the shape index for ojectile based on his ogie dimensions. In addition, this relation is corrected using the ojectile shape index correction made some relations obtained based in aerodynamic tunnel experiments made in U.S.A. This study put together these relations for ojectile s shape index and also ealuate ojectile s drag coefficient in accordance with the standard drag law s Siacci, 1943 and This study also contains and exemplification of how small differences in ojectile s geometry can affect the alue of ojectile s shape or drag coefficient. The study has two main objecties: ojectile s shape index and drag coefficient ealuation using the oposed analytical algorithm and the influence of ojectile s small geometrical tolerances on ojectile s shape index. The ealuated drag coefficient is for Mach alues between 0.1 and 4.0. The ojectile aerodynamic configuration is similar to the 30 mm ojectile esented in Fig.1. Fig. 1 Aerodynamic configuration of 30 mm caliber ojectile used The study uses some of the ojectile s geometrical dimensions and his flight conditions.. MATHEMATICAL MODELS USED The study has two main objecties as we mentioned before and for these objecties are reached by using simple empirical relations to ealuate the ojectile s shape index and drag coefficient. In this case, the mathematical model for the analytical ealuation will be esented in the following. The mathematical model [1,, 3] for drag coefficient estimation uses ojectile s geometrical dimensions (Fig. ). L L D Lp Fig. Projectile s dimensions used These dimensions are: p D p L ojectile s total length, L - ogie length, tronconical length, D Lp - - transersal section diameter, D p ojectile back side diameter, p angle for ojectile s tronconical part. For the algorithm of shape index estimation, we use the following relations [,3]: L (1) D p D Lp tg( p) () p 1 (3) D Where is ogie s relatie length, p reesents a geometrical coefficient for tronconical part, is a geometrical coefficient for ojectile shape. The eliminary ojectile s shape index, taking into account the ogie s length, is estimated using relation [1]: 1 i T (4) Using the aboe relation for ojectile shape index ealuation we can obtain the final alues for i [1]: i it (5) alid for ojectile s elocities less than 50 m/s, i it 1 (6) alid for ojectile s elocities greater than 05 m/s and less and equal with 400 m/s and V (7) 0

3 HENRI COANDA AIR FORCE ACADEMY ROMANIA INTERNATIONAL CONFERENCE of SCIENTIFIC PAPER AFASES 015 Braso, 8-30 May 015 GENERAL M.R. STEFANIK ARMED FORCES ACADEMY SLOVAK REPUBLIC For ojectile s elocities greater than 400 m/s we use for shape index ealuation following relation [1]: i it (8) Once we ealuate the ojectile index, i, we calculate drag coefficient for ojectile using the reference drag coefficient law s Siacci, 1930 or 1943 based on ojectile relatie length. The relation for ojectile s drag coefficient is: CD i CDref (9) In which C D reesents drag coefficient calculated and C Dref reesents the alue of drag coefficient used as reference. Based on relations (5) to (8) we can ealuate the influence of ojectile s small geometrical tolerances on ojectile s shape index using the small differences hypothesis [3,4,5,6] or in mathematical terms, the shape index functions differentiation as follows: i Lp p (10) In which: i (11) i L p L p Lp (1) i p p p (13) L D L D (14) 1 L D (15) L D D (16) i In which, i i, Lp,, are p L D partial deriaties. As an example of ojectile s shape index ariation caused by we hae for index shape ariation the following relations: i i (17) where is from relation (14) and i i 6i (18) Using relations (14), (17) and (18), we can ealuate the influence of ojectile s ogie length on ojectile s shape index and drag coefficient. In similar way can be ealuated, too, the influences on ojectile shape index and drag coefficient caused by p and L p ariations. 3. NUMERICAL RESULTS Initial data used to make the calculations for ojectile shape index and drag coefficient are esented in Table 1. Table 1. Initial data for calculation Parameter Value D [mm] 30 L [mm] L [mm] L p [mm] 5. Mach number [-] 0.1 to 4.0 p [deg] 15 Drag coefficient alues obtained for this initial data are exposed in Table. These drag coefficient alues are calculated using the mathematical model esented in chapter.

4 Table. Numerical results for shape index and drag coefficient Mach number alue Shape index alue Drag coefficient after Siacci law Drag coefficient after 1930 law Calculated drag coefficient graph is esented in Figure 3. Results are obtained using the exposed analytical method. Fig. 3 Drag coefficient alues s. Mach using the algorithm esented Results for ojectile s ogie relatie ariation,, caused by ogie length ariation, L, and ojectile diameter ariation, D, are esented in Table 3., Figure 4. to Figure 6. Results for ojectile s shape index ariation with ogie length ariations and ogie length and ojectile s diameters ariations are esented in Table 4., Figure 7. to Figure 9. Table 3. Numerical results for ogie relatie length D caused caused by L by D L Fig. 4 for L and D in the same time

5 HENRI COANDA AIR FORCE ACADEMY ROMANIA INTERNATIONAL CONFERENCE of SCIENTIFIC PAPER AFASES 015 Braso, 8-30 May 015 GENERAL M.R. STEFANIK ARMED FORCES ACADEMY SLOVAK REPUBLIC Fig. 5 only for L Fig. 7 i only for Fig. 6 D only for Table 4. Numerical results for shape index i L i D i Fig. 8 Fig. 9 i only for L i only for D As we can obsere ojectile s diameter ariation introduce the highest ariation of ojectile s shape index.

6 We also can conclude that we can use for eliminary calculations this method to calculate the ojectile s shape index and drag coefficient for aerodynamic configurations of ojectiles. As a plus we can ealuate too drag coefficient ariation caused by ojectile s geometrical ariations. 4. CONCLUSIONS & ACKNOWLEDGMENT The drag coefficient was calculated, see Table and Fig. 3 using the method esented in chapter. On the other hand, the drag coefficient was calculated using the algorithm esented in chapter, and the results obtained for it were in accordance with eliminary design requirements. In addition, reference [1] recommends this method to be used for eliminary ojectiles design ealuations. This kind of study can be used to implement the esented method as module for an exterior ballistic. The software module is a good ready-made instrument for ealuations instead of sheet eliminary calculations. The usefulness of this type of study can be seen in teaching purposes, experimental testing and design of different type of oducts. This paper has been financially supported within the oject entitled Horizon 00 - Doctoral and Postdoctoral Studies: Promoting the National Interest through Excellence, Competitieness and Responsibility in the Field of Romanian Fundamental and Applied Scientific Research, contract number POSDRU/159/1.5/S/ This oject is co-financed by European Social Fund through Sectoral Operational Programme for Human Resources Deelopment Inesting in people! REFERENCES 1. Antonescu, I. Bazele oiectării oiectilelor, Editura Academiei Militare, Bucureti (1977).. Niţă, M.M., Moraru, Fl., Patraulea, R., Aioane şi rachete. Concepte de oiectare, Editura Militară, Bucureşti (1985). 3. Moraru, Fl., Elemente de teoria estimaţiei cu aplicaţii în balistică. Editura Academiei Tehnice Militare, Bucureşti (1979). 4. Surdu. G., Slămnoiu. G., Moldoeanu C., Ciuculin. A., Şomoiag P., Metode statistice şi instrumente software realizate in house pentru aplicaţii în balistică cu scopul determinării alorii adeărate a parametrilor determinaţi experimental. Sisteme energetice şi Balistice. Bucureşti (013). 5. Şomoiag, C.E. Moldoeanu, Numerical Research on the Stability of Launching Deices During Firing, Defence Technology Elseier Journal, Volum 9, Issue 4, decembrie 013, pag. 4-48, ISSN C.E. Moldoeanu, A. Gioannini, H.C. Boisson, Turbulence receptiity of longitudinal ortex-dominated flows, Lecture Notes in Computational Science and Engineering, Vol. 69: BAIL Boundary and Interior Layer Proceedings of the International Conference on Boundary and Interior Layers - Computational and Asymptotic Methods, Limerick, July 008, Singer Verlag, Berlin, pag. 7 36, ISBN: , 009.