# How much force do I need to add to a rigidbody with a certain mass to hit a certain speed ?

Hello. I’m making an Aircraft simulation with Unity based on the cessna skylane (or 182). The maximum speed of this plane is 296 km/h and his weight is 894 kg. I need to know how much force( and by force I mean the thrust) i need to add to the rigidbody which represents the plane to hit those 296 km/h. I wasn’t able to find this info on Google. So I need to search it myself but I have no idea how to do it!
I thought I could use a code like that :

``````Rigidbody x;

if(x.velocity.magnitude == 296)
{
// Save the thrust force at the rigidbody hit that speed
}
``````

Knowing that I use this code to add Thrust:

``````if(Input.GetKey(KeyCode.UpArrow))
{
thrustForce = thrustForce + 15f;
x.AddRelativeForce(Vector3.forward * thrustForce * Time.deltaTime, ForceMode.Acceleration);
}
if (Input.GetKeyUp (KeyCode.UpArrow))
{
thrustForce = 0;
}
``````

(Sorry if there are some mistakes I’m not a native speaker)

The answer is: Without drag it doesn’t matter. Any non zero force will get you there since there are no “counter forces”.

If you have set a drag value for your rigidbody you can calculate the max velocity for a given force / acceleration.

Read my answer over here carefully. Unity doesn’t simulate realistic air friction / air resistance. Unity’s “drag” is just a generic linear counter force while real air resistance is a quadratic force. Real air resistance depends on may factors.

I’ve also added a method that can calculate the required drag for a given acceleration and max velocity.

Just in case that’s not clear:

``````force = acceleration * mass
``````

likewise

``````acceleration = force / mass
``````

i would use some log statements and Rigidbody.velocity just to have a look