2 Replies Latest reply on Jun 16, 2015 7:05 PM by Andrei Popov

# Model rocket center of pressure/location of aerodynamic forces

Hi,

I am new to flow sim and am attempting to use it to determine the location of the aerodynamic forces on a model rocket.  I've been attempting to figure this out using a simplified model, which is attached.  If all of the aerodynamic forces are simplified into a single vector, the point from which said vector originates, sometimes called the center of pressure, is what I am looking for.  Extensive searching has suggested using a torque goal, but I haven't been able to figure out how to use that to identify a point.  Also, I am able to get the force, either total or by component, but not the point it acts at.

How do i find this point?  Thanks for any suggestions or resources.

The position and magnitude of the force vector will vary with velocity and angle between the rocket and the fluid, the assembly attached has the rocket at 5 degrees and the fluid at 10 m/s, once I figure out how to do this I plan to test a number of values to get data over the expected operating range

• ###### Re: Model rocket center of pressure/location of aerodynamic forces

Is your main concern to create a plot for Center of Pressure vs angle of attack? To determine stability?

This is what SOLIDWORKS has to say on the subject:

It is impossible to determine the point of application of a force from its Force and Moment components. All that can be obtained in the line (not point) of application of the force: the lien of application is the infinite line passing through the actual point of application and parallel to the force vector.

Extra information from another source has to be used to determine the point of application on the line.

The Line of application of a force can be determined by using the torque (M) definition: M=[r*F]. Here r is a vector to force application point from the origin O of the coordinate system, F – force vector, M – torque vector. In obtained calculation results (Surface Parameters for instance) you get components of force and torque. So, you can solve the equation regarding r. The equation can be rewritten regarding coordinates as following:

Mx = y * Fz - z * Fy

My = z * Fx - x * Fz

Mz = x * Fy - y * Fx

Here (Mx, My, Mz) – torque components, (Fx, Fy, Fz) – force components, (x, y, z,) – coordinates of force application point. So, we have three equations and three unknown values (x, y, z) which can normally be found from these equations.

In fact, one of the system of equations is undeterminate and cannot be solved (it is as if you had 3 unknowns and only 2 equations).

To determine the line, you can for instance assume two values for z (0 and 1 for instance) and calculate the corresponding values for x and Y. this will give you two points and the line of application pases through these points.

To determine the point of application from the line requires extra information which may not be possible to obtain.

In some special cases, it will be possible. For instance if you are trying to to determine the point of application of a force on a flat face. In that case, the point of application will be the intersection of the flat face with the line of application.

In the case of the force on a whole body with nothing but flat faces, you can calculate the point of application the force on each face separately, and then find the equivalent force and point of application for the entire body.

• ###### Re: Model rocket center of pressure/location of aerodynamic forces

I have never done this before but this is how I would do it:

1. I would define an axis or a point on an axis with a dimension distance from CG or from a fixed point ( rocket nose) and I would use this point as a reference for the further calculations.

2. In flow simulation you can set a surface goal the total force on the rocket surfaces, this force would produce a torque about the above axis. Also you can set a torque goal, I am not sure if you can select the axis to be the reference of the torque but you can try and see.

3. Then you can set a parametric study where the control parameter is the position of the axis w.r.t. the CG, you can select the dimension that defines the position of the axis. The goal of the study should be the torque goal = 0, Solidworks will make several iterations by changing the axis position until the torque is zero.

SolidWorks Quick Tip - Parametric Studies in SolidWorks Flow Simulation