c++ - Rotating an object in OpenGL about it's centre

Im writing a game for which, one of the models are loaded from an .obj file.

It's a model of a plane, and I want to rotate the propeller. The object file is broken into groups, and the propeller is identified, so that's all good. I'm writing the game in C++ with OpenGl/GLFW The drawing function is:

int win_width;
int win_height;
glfwGetWindowSize(&win_width, &win_height);
float win_aspect = (float)win_width / (float)win_height;
glViewport(0, 0, win_width, win_height);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(90, win_aspect, 1, 100.0);


glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0, 0, 30.0, 0, 0, 0, 0.0, 1.0, 0.0);

glEnable(GL_DEPTH);
glEnable(GL_DEPTH_TEST);
glEnable(GL_COLOR_MATERIAL);
glEnable(GL_NORMALIZE); 
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);

glEnable(GL_LIGHT1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glColor3f(0.0f, 0.0f, 0.0f);
int vertexIndex = 0, normalIndex;
glRotatef(90, 0, 1, 0);
glPushMatrix();
for(int a = 0; a < (int)groups.size(); a++)
{
    if(groups[a].type == "prop")

    {
//Code for rotation
        glPopMatrix();
        glPushMatrix();
        float x,y,z;
        x = y = z = 0;
        int Count = 0;
        for(int k = 0; k < groups[a].faces.size(); k++)
        {
            for(int p = 0; p < groups[a].faces[k].vertices.size(); p++)

            {
                int _index = groups[a].faces[k].vertices[p];
                y += vertices[_index].Dimensions[_y] ;
                z += vertices[_index].Dimensions[_z];
                Count++;
            }
        }
        z /= Count;
        y /= Count;
        glTranslatef(0, -y, -z);

        glRotatef(angle, 1, 0, 0);
        glTranslatef(0, y, z);
    }
    for(int b = 0; b < (int)groups[a].faces.size(); b++)
    {
        glBegin(GL_POLYGON);
        for(int c = 0; c < (int)groups[a].faces[b].vertices.size(); c++)
        {
            vertexIndex = groups[a].faces[b].vertices[c];
            glVertex3d(vertices[vertexIndex].Dimensions[_x], vertices[vertexIndex].Dimensions[_y], vertices[vertexIndex].Dimensions[_z]);

        }
        glEnd();
    }
}
glPopMatrix();
glfwSwapBuffers();

Since I don't know the exact centre of the propeller, that's what the for loop before the rotation is for. It finds the average of the y and z co-ordinates.

After I find it, I translate to -y,-z , rotate it, and then translate back. This makes the propeller spin as I want it to, but it also rotates along the origin >.< http://people.sc.fsu.edu/~jburkardt/data/obj/cessna.obj <- This is the object file.

groups is a vector of objects and each object has a vector of faces. I'm sure that the vertices and faces are being loaded correctly, since the whole model renders correctly.

Answer

OpenGL uses a column vector convention, which means the matrices must be applied in the opposite of the order in which you actually want the transformations to occur. Just replace this:

glTranslatef(0, -y, -z);
glRotatef(angle, 1, 0, 0);
glTranslatef(0, y, z);

with this:

glTranslatef(0, y, z);
glRotatef(angle, 1, 0, 0);
glTranslatef(0, -y, -z);

and that should fix it.