2d Helicopter simulation (gravity vs. force question)

Looking for suggestions for gravity and Y axis forces for helicopter game…
I am creating a helicopter game and it and looks to me that the default gravity force for the 2drigidbody is too strong.
in my code when pressing the down keyboard key I apply force to the gameobject 2dVector.y, and when releasing the key I gradually decrease the applied force until it reaches 0.
my problem is that it seems that the default gravity value is pulling the object too strong.

any suggestion for the best values to apply to the gameobject ?(gravity,Force Increase by value , Force decrease by value)
or maybe even a change in the Project - “Physics2DSettings”?

and if I want to apply constant force against the gravity factor, how can I calculate the force that should be applied to keep the chopper from falling down or rocketing to the sky?

using UnityEngine;
using System.Collections;

public class testSCript : MonoBehaviour {
public Rigidbody2D r_2d ;
public float POWER_FACTOR = 0.5f;
public Vector2 power_vector = new Vector2(0,10);
public float power_dec = 0.99f;
public float MAX_POWER = 10.9f;
public float MAX_VELOCITY = 3f;
public float current_velocity;
// Use this for initialization
void Start () {
r_2d = this.GetComponent();
}

// Update is called once per frame
void Update () {
	if (Input.GetKey ("down")) {
		power_vector.y += POWER_FACTOR;
		if (power_vector.y>MAX_POWER) {
			power_vector.y = MAX_POWER;
		}
	} //else {
	if (power_vector.y > 0.01)
		power_vector.y *= power_dec;
	else 
		power_vector.y = 0;
	//}
	current_velocity = r_2d.velocity.y;

	if (current_velocity<MAX_VELOCITY) {
		r_2d.AddRelativeForce(power_vector);
	}
}

}

looks to me that the default gravity force for the 2drigidbody is too strong

The default gravity is set so that all objects should get the acceleration of 9.81 m/s^2 just like they in real life (so 1 world unit == 1 meter by default). If your chopper object is smaller than a real chopper would be, then it will appear to fall too fast in relation to your scene’s scale with this gravity setting (like a small toy chopper).

Also

when releasing the key I gradually decrease the applied force until it reaches 0

Remember that forces cause acceleration, so decreasing the force gradually will not slow down the chopper’s downward movement, it will just slow down the downward acceleration; its downward speed will still increase every frame (even if you turned off gravity altogether).

Did you take a look at the ForceModes ? You can add them as the second parameter of your AddRelativeForce call. Especially if your chopper consists of only 1 rigidbody, it should be fairly easy to control it using ForceMode.Acceleration or ForceMode.VelocityChange

You should increase your drag value to simulate air resistance. Without drag objects will fall like they would in vacuum. Unity uses a simple linear drag force. This has nothing to do with real world air resistance (which is quadratic in relation to the velocity). Though in most cases the linear model is just fine to simulate air resistance.