How to make a fully static-unstable aircraft？

[Scrooge]

Recently, I start to focus on modifying an aircraft with a CoL far ahead of CoM, that means, the plane wont fly smoothly (of course).

To make the plane manually-controllable, I am going to try several methods recommended by some other players to reduce the negative influence of negative stability:

1.Use canards instead of elevons.

Since my fetish required fixed position of wings (like F-15 or F-22), I have to arrange the mass to the tail of the aircraft to make CoM behind CoL.

However, the original elevons at the rear of wings are too close to the rearranged CoM, thus cause a heavily-reduced controlling torque, which impairs steering and SAS.

But the canard has a positive feedback from AoA, a larger AoA will impair the effect of reversely steering canards,

Question: How could canards improve the performance of static-unstable aircraft?

 

2.Use negative feedback of AoA (negative AoA% of control surface) to limit the AoA of aircraft to prevent it from stalling too fast.

The negative AoA% actually prevent my aircraft from being stalled, which makes post-stall maneuvers become unavailable.

Question: How to set the value of AoA% for different control surfaces for after-stall recovery?

 

3.Use big and fully vertical (while YF-23 do not use that) tails to strictly limit the instability in yaw.

With this kind of setting, even the aircraft goes into stalling, it can be relatively controllable and make it possible to recover speed in some direction, otherwise, the tail spin is unavoidable in most of cases.

When I start to try this, I found the side-slipping derivative calculated by FAR dropped a lot after the plane becoming unstable, and the value dropped from about 7m/s^2 to 1.5m/s^2, no matter how I resize and rearrange the position of vertical tails, the value of side-slipping derivative won't exceed 4m/s^2.

Question: How to improve the stability in yaw?

[paul23]

A full calculation is found in this point at wikipedia.

 

However the jest of the topic there is that the center of gravity needs to be in front of some point "neutral point", which is based on the lift of both the tail and wing.

 

A canard type craft can never be stable, so you constantly have to correct. 

 

It's important to realize that stability is important, but one can fly without stability. An unstable craft is like balancing a pencil on your fingers: you constantly have to correct. Luckily we have something called "computers" - or "SAS" in ksp that can correct this to a certain point. If sas won't be able to, you probably also won't be and you'll have to increase the control surface volume (area * distance from center of mass).

 

I have never ever anyone use "negative angle of attack" (or negative lift when horizontal for that matter) so I can't talk about that. If you mean negative cl-alpha: well that's impossible even a simple plate has a positive cl-alpha coefficient.

 

Static jaw stability is reached simply by having a larger (non moving) vertical lifting surface behind the center of mass. Just like longitudinal stability, it's just easy since there is no main wing. Further stability can be achieved by giving dihedral (in case of wings below body) or anhedral (when wings are above the body) to the main wing.

 

If it's dynamically unstable it is probably causing a dutch roll. A combination of yaw and rolling movement. This is a particularly nasty eigenmotion as improving dutch roll stability reduces the spiral stability (and vice versa). Some aircraft (cessna citation) even opted to go for unstable spiral motion in favour of keeping dutch roll stable. The cause is that when ailerons increase a rolling motion one of them has more drag, thus creating a yaw effect (adverse yaw). The yawing motion (correction) causes a rolling motion as the angle of one wing more  perpendicular to the airflow than the other.

 

Dutch roll can be reduced by putting the wings below the center of gravity, reducing dihedral angle and reducing wing sweep.

 

 

Edited December 11, 2019 by paul23

[Scrooge]

6 hours ago, paul23 said:

A full calculation is found in this point at wikipedia.

 

However the jest of the topic there is that the center of gravity needs to be in front of some point "neutral point", which is based on the lift of both the tail and wing.

 

A canard type craft can never be stable, so you constantly have to correct. 

 

It's important to realize that stability is important, but one can fly without stability. An unstable craft is like balancing a pencil on your fingers: you constantly have to correct. Luckily we have something called "computers" - or "SAS" in ksp that can correct this to a certain point. If sas won't be able to, you probably also won't be and you'll have to increase the control surface volume (area * distance from center of mass).

 

I have never ever anyone use "negative angle of attack" (or negative lift when horizontal for that matter) so I can't talk about that. If you mean negative cl-alpha: well that's impossible even a simple plate has a positive cl-alpha coefficient.

 

Static jaw stability is reached simply by having a larger (non moving) vertical lifting surface behind the center of mass. Just like longitudinal stability, it's just easy since there is no main wing. Further stability can be achieved by giving dihedral (in case of wings below body) or anhedral (when wings are above the body) to the main wing.

 

If it's dynamically unstable it is probably causing a dutch roll. A combination of yaw and rolling movement. This is a particularly nasty eigenmotion as improving dutch roll stability reduces the spiral stability (and vice versa). Some aircraft (cessna citation) even opted to go for unstable spiral motion in favour of keeping dutch roll stable. The cause is that when ailerons increase a rolling motion one of them has more drag, thus creating a yaw effect (adverse yaw). The yawing motion (correction) causes a rolling motion as the angle of one wing more  perpendicular to the airflow than the other.

 

Dutch roll can be reduced by putting the wings below the center of gravity, reducing dihedral angle and reducing wing sweep.

 

 

Thank you for your guidance! I will take a time to think about your reply !!!