NEW CALCULATION METHODS OF AIRCRAFT CENTRAGE USING THE CAD TECHNIQUES

Marian BOBE, Doru LUCULESCU

Air Forces Academy “ Henri Coandă” Braşov

Abstract: Autogyro has become again a field of study on which the manufacturers of ultralight aircraft focus on due to the involved flight safety and relatively low costs of production and maintenance.The present paper describes a precise calculation method of the autogyro centrage using the possibilities offered by the 3D parameter-based CAD design software. The Solid Works 2001 software is used, emphasizing on the calculation method of weight and position of the center of gravity for flight controls of an autogyro.

Key words: autogyro, flight controls, center of gravity, inertial moment, parameter-based 3D design.

1. Introductory considerations

An important stage in designing aircraft is determining the weight of the components and the position of the center of gravity, in other words the centrage calculation. The accuracy of centrage execution influence the results subsequently obtained in calculating the static and dynamic stability as well as the results of the performance calculation. A significant error in determining the position of the center of gravity leads to significant errors in the results previously mentioned.

The classical method used for centrage determination involves the estimation of the weights of different mechanical and electrical components and equipment which is to be assembled on board the aircraft, considering the initial design data represented by the total aircraft weight and using the approximation formulas for the weight of these components and equipment. As a consequence of the wide range of aircraft shapes and sizes, these calculation methods imply significant errors. It is not possible to estimate accurately the control weight of a medium helicopter (e.g. IAR 330) compared to the

control weight of an ultralight helicopter by using similar calculation methods.

Another error derives from the fact that, once the weight of some equipment or installation has been estimated, the position of the center of gravity for these sub-assemblies is calculated by means of other approximations.

2. The suggested solution

The solution that offers the most precise results is based on the 3D parameter-based CAD design software. This software offers the possibility of 3D-shaping of the parts which form the sub-assembly whose weight we want to calculate, and of attributing each component a certain material as well. After the operations previously mentioned have been performed, a simple click of the software results in calculating the weight of the whole assembly, the position of the assembly’s center of gravity, the inertial moment, etc.

The operation method is the following: a) each part is shaped separately as follows: - the form of the basic part is 2D-drawn and sized

(Fig.2.1.).

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Fig. 2.1. The drawing of the basic part

An operation of transforming the 2D-draft into a 3D-body by one of the commands: extruded boss,

revolved boss, extruded cut, revolved cut, sweep or loft. In this case we use extruded boss (Fig. 2.2).

Fig. 2.2. The extrusion of the basic part

− a cut-up is made in the part (Fig. 2.3). − the finished shape of the part is made by boring

and broaching (Fig. 2.4).

b.) the density of the material used for making the part is selected form the menu Tools-Mass Properties-Options (Fig.2.5).

Fig. 2.3. The part is cut up by using the command “cut extrude”

Fig. 2.4. The finished shape of the part c.) after all the necessary parts have been shaped, the parts are assembled in a sub-assembly similar to the one presented in the figure (Fig. 2.6). d.) all the necessary sub-assemblies are assembled and the desired 3D-object is obtained (Fig. 2.7).

e.) After all the operations have been completed, the 3D-shaping of the parts, the density attribution, and the assembling of the parts, the position of the center of gravity, the weight

of the whole assembly, the inertial moment can be calculated from the menu Tools-Mass Properties.

Their values are displayed in a window similar to the one presented in Fig. 2.8.

Fig. 2.5. Attributing a density to the part material

Fig. 2.6. Constituting a sub-assembly

Fig. 2.7. Obtaining the desired 3D-object

Fig. 2.8. Displaying the position of the center of gravity

Finally, the image can be knurled and transformed in order to obtain a high quality

image of the assembly (Fig. 2.9).

Fig. 2.9. The final image of the assembly

References 1. Biellawa, R.L. Rotary wing structural dynamics.

Ed. A.I.A.A., Washington, 1994. 2. Donaldson, B.,K. Analysis of Air craft

Structures. McGraw-Hill International Editions, Aeronautical and Aerospace engineering Series, 1993.

3. Marinescu, A., Anghel,V. Aerodinamica şidinamica elicopterelor. Editura Academiei Române, Bucureşti, 1992.

4. Postelnicu, A., Deliu, G., Udroiu, R. Teoria, performanţele şi construcţia elicopterelor. Editura Albastră, Cluj Napoca, 2001.

5. * * * Jane’s All the World’s Aircraft 2000-2001. Edited by John W. R. Taylor, Jane’s Publishing Company Ltd., London, U.K.