Inertia Relief Analysis of Nastran

[alinalin]

Can I use Nastran Inertia Relief to analzye an unconstrained system with unbalanced froces and moments?
The system is not supported and not constrained (for example, an aircraft in flight), and have translational/rotational accelerations due to nonzero total forces and moments?

I am not able to find examples of Inertia Relief analysis.

Thanks for any input!

 

[Onda]

You can run the analysis with PARAM, INREL, -2
(I think work only with sol101 but not sure).
I suggest you to run the model with a system of equilibrated forces in any case.

Onda

 

[alinalin]

Thanks for the reply.

For PARAM, INREL, -2, no constraint is needed.
For PARAM, INREL, -2, constraint need to be specified.

My problem is that I need to analyze critical flight conditions-the forces and moments are not balanced. I don't know if it is working for inertia relief.

 

[Onda]

The inertia relief apply an acceleration to the mass of your model in way to balance the force applied.
The normal way to use it is to apply to your model all the force and acceleration that is subjected and let the inertia relief to correct the imbalance in force applied.
What I suggest is to apply on wings / control surfaces the right pressure due to the operating condition. To the body the acceleration due to critical flight condition.
The inertia relief create a fictious force on each grid point associated with a mass. Is exactly the same things as an acceleration.
So if you apply to your model only an acceleration and the inertia relief you will not see any deformation because the two system of force are equals and opposite!
The inertia relief method is for structures in static equilibrium!

 

[alinalin]

Thanks a lot, Onda!

I only apply a gravity acceleration to my model. Except that, I also have aerodynamic and dynamic loads. My question is that the sum of external forces and moments are not zero, which means that rotational/translational acceleration exist for the system (considering freebody analysis).

You said inertia relief works for static equilibrium. What if the system undergos a constant acceleration?

 

[Onda]

A free falling object (loaded only by gravitational acceleration) will see no load because the inertia relief will be exactly the same and opposite to grav load.

On an airplane loaded by 1g grav, pressure loads on wings and tale the inertia relief is taking only the spurious loads (non equilibrium) and the airplane is normally loaded.
You could also let the inertia relief equilibrate the airplane but in this way you have less control on loads. For this reason I prefer always apply a system of equilibrated forces on the model analyzed.

If your system is subjected to a constant acceleration, you should equilibrate the system with that thrust that create the acceleration. If you let the inertia relief equilibrate your acceleration the system doesn't see any load!

 

[alinalin]

Onda,

Thank you again!

I understand what you said. My point is that I need to analyze the condition, in which the forces and moments are not in a equilibirum state. I know that I can change thrust, control surface loads to balance forces and moments, which is a steady state. As we know, there are various flight maneuver conditions. I am interested in studying the deformation of some critical states, when the moments or forces are not balanced. For example, a Dive condition.

My result shows that when I apply inertia relief to the system with a constant acceleration I can obtain deformation. I am not sure about Nastran capability in handling this since no errors and warning are generated.

 

[johnhors]

"in which the forces and moments are not in a equilibirum state"

- may be not in "static" equilibrium but then they must be in "dynamic" equilibrium where applied forces and moments are balanced by inertial loads. Whether static or dynamic an equilibrium exists.


"My result shows that when I apply inertia relief to the system with a constant acceleration I can obtain deformation"

- you appear to have broken the laws of physics! As Onda correctly states the sum of inertia relief and gravity loading is zero. Nothing happens. In the absence of any other loading how can an object deform when undergoing constant acceleration????

 

[alinalin]

Thanks for the reply.

It is common that there are many flight maneuver that the air vehicle is not in static/dynamic equilibrium. By "equilibrium", I mean that sum of all the external forces and moments are not zero. For example, to obtian a Diving condition, there is a pitching down moment, although steady-state diving condition.

The inertia relief can generate an acceleration to counterbalance the acceleration of vehicle due to all external loads. That doesn't mean there is no deformation of the vehicle. The vehicle is not considered as a rigid body. It is a complicated structural system. The balance of total forces can remove rigid body motion but individual forces at different parts of the vehicle do generate deformation.

 

[johnhors]

"the air vehicle is not in static/dynamic equilibrium"

So:-

F = m.a + k.x (ignoring damping)

doesn't apply then ??

 

[alinalin]

There are some misunderstanding.

By equilibrium, I refer to the state that forces and moments are balanced and no accleration due to total loads. Nothing to do with "damping".

Is it "common" that air vehicles in flight have translational/rotational accleration and the forces and moments are not balanced?

 

[frluga]

hi there,
i'm also balancing the model with INREL, -2.
My question is; where i can see the INREL output?.
It is in the f06 file .... but what exactly i have to watch to understand the ammount of unbalance, the applied acceleration, to my model with the INREL param -2.

Thx for any answer,
Frluga

Quote: Alinalin

"I understand what you said. My point is that I need to analyze the condition, in which the forces and moments are not in a equilibirum state. I know that I can change thrust, control surface loads to balance forces and moments, which is a steady state. As we know, there are various flight maneuver conditions. I am interested in studying the deformation of some critical states, when the moments or forces are not balanced. For example, a Dive condition."

The model will be allways in equilibrium, but you can simulate the unbalance just with increment the applied loads, so the result will be a higher applied loads to the model. My oppinion though.

 

[alin09]

The sum of forces and moments is given in
"OLOAD RESULTANT".
The acceleration is given in "INTERMEDIATE MATRIX ... URA".

You can read NASTRAN output manual to have underdtanding of the output items.

I am alinalin. I can not use the previous user name, just created another user name.