トラックボールっぽい回転（慣性付き）

RotateTest.cs

using UnityEngine;

public class RotateTest : MonoBehaviour {
    public Vector3 axis = Vector3.up;

    public float rotSpeed = 0;
    public float power = 0.1f;
    public float friction = 0.98f;

    public float stickPower = 100;

    Vector3 prevPos;
    
    Vector3 rotVec = Vector3.zero;  // 回転ベクトル

    Vector3 rotKeyVec = Vector3.zero;

    float startTime;

	// Update is called once per frame
	void Update () {

        // マウスフリックで操作
        if (Input.GetMouseButtonDown(0))
        {
            prevPos = Input.mousePosition;
            startTime = Time.realtimeSinceStartup;
        }

        if (Input.GetMouseButtonUp(0))
        {
            Vector3 pos = Input.mousePosition;
            Vector3 diff = pos - prevPos;
            AddRotateVector(diff / (Time.realtimeSinceStartup - startTime));
        }

        // 十字キーで操作
        rotKeyVec.x = Input.GetAxis("Horizontal");
        rotKeyVec.y = Input.GetAxis("Vertical");
        rotKeyVec = rotKeyVec.normalized * stickPower;

        AddRotateVector(rotKeyVec);

        // 摩擦
        rotVec *= friction;
        rotSpeed = rotVec.magnitude;

        transform.rotation = Quaternion.AngleAxis(rotSpeed * Time.deltaTime, axis) * transform.rotation;
    }

    /// <summary>
    /// 回転ベクトル加算
    /// </summary>
    /// <param name="vec"></param>
    private void AddRotateVector(Vector3 vec)
    {
        Vector3 diff2 = transform.position - Camera.main.transform.position;
        diff2.Normalize();

        float p = vec.magnitude * power;    // 回転速度計算

        vec.Normalize();
        
        rotVec += vec * p;  // 足す

        rotSpeed = rotVec.magnitude;    // 新しい回転速度計算
        axis = Vector3.Cross(rotVec.normalized, diff2); // 新しい回転軸計算
    }

    private void OnDrawGizmos()
    {
        Vector3 pos = transform.position;

        Gizmos.color = Color.red;
        Gizmos.DrawLine(pos, pos + axis * 10f);
        
        Gizmos.color = Color.cyan;
        Gizmos.DrawLine(pos, pos + rotVec * 10f);

    }
}