Hi!
I am trying to adjust the aircrafts longitudinal “neutral static stability” with CFD simulation ON.
How can I do this?
I want the plane to be more neutral in static stability.
Best regards.
In general, you need to find the sweet spot of distance between the center of gravity (CG) and center of lift (CL), as well as the length of the arm between the CG and the elevator/tail section. Efficacy of the tail also plays into it.
The farther forward the CG is from the CL, the more stable it will be (and vice-versa). But it can be too stable in that if it’s too far forward, the tail will not have efficacy in raising the nose. At some point, there will be a sweet spot of neutral stability. Get it too far back and it will be unstable. You just have to play with it in the wind tunnel to see what the output will be in the sim.
How does that play in?
And lets say I load the plane front heavy, move CG forward, (lower CG value?), and trim the plane up. Should that give the same effect as you describe.
If it dose not what could be the reason for that?
With the CG being ahead of the CL, there is a constant rotational (torque) moment around the lateral axis in a nose-down direction. The horizontal stabilizer (we can just colloquialize it as the “tail section”) in a conventional aircraft is designed to provide lift downward, imparting a tail-down/nose-up force that counteracts the nose-down force described above.
Several factors go into how much tail-down force (lift) is produced, including airflow, size of the control surfaces, and angle of attack. Take that force and multiply it by the arm (distance) between the CG and those aforementioned control surfaces and you get the total torque (moment).
Let’s say you have a really small distance between the CG and the tail (maybe the aircraft is loaded severely aft of CG limits). In this case, you may still have enough (tail down) lift being produced by the tail, but the shortened arm drastically reduces the moment (torque) available to raise the nose, and the nose falls.
In your nose-heavy (forward CG) example, it may still fly, it may not. It simply depends on the final numbers. However, the more you have to trim the tail down (nose up) from neutral to remain in a desirable vertical path, you are adding to the down force already provided by the aircraft’s weight, so performance suffers. Additionally, you will have to fly at a higher angle of attack to overcome that extra apparent weight, so you’re closer to the critical angle of attack and the likelihood of stall is increased. You’re also that much closer to running into of physical trim limitations.