How would a fighter be changed from having static stability to having relaxed stability?

Question

A good portion of aircraft, especially those designed before the 1970's, possessed static stability. However, modern fighters (and some airliners) have been build with relaxed static stability.

This question was aroused by the book Designed for the Kill by Mike Spick. On page 119, it states (in reference to the Shenyang J-8 II Finback B):

In 1991 improvements were stated to be in hand to improve subsonic manoeuvrability, and one of the obvious moves would be to introduce variable camber by using leading edge slats. There can be no doubt that FBW and relaxed stability would also benefit it considerably, while weight reduction could be achieved by the use of composites.

How would a fighter such as the J-8 physically be changed to change the centre of gravity in relation to the centre of lift? Would the wings be resized/moved? Would the tailplane be made smaller? Would internal items be shuffled around to put heavier ones at the back? Or could some sort of clever aerodynamic device be added to modify the airflow somewhere?

Answer 1

The first and most important step is to move the center of gravity back relative to the neutral point of the configuration. This is done either by shifting masses on the airframe back or by shifting the wings forward. This needs to be complemented by the addition of a multiple redundant and fast flight control computer.

Now the tail will add lift instead of downforce, so the wing area required for manoeuvring and take-off/landing can be reduced. Or performance improves - compare the minimum speed of the Mirage III to that of the Mirage 2000.

To see how much the wing area could be reduced, here is a comparison drawing of the regular SEPECAT Jaguar with one designed for the same requirements, but with relaxed stability (copied from this answer). The forward shift and reduced area of the wing should be obvious.

Comparison of conventional Jaguar (top half) and CCV version (bottom half) by Ray Whitford.

What is done to a specific airframe depends on the objectives for the move to relaxed stability: This could be much better supersonic range or better short field performance. For the first you will shrink the wing, for the second you better keep wing size but move it forward and heavy items back. The change from the one engine of the MiG-21 to two more powerful engines certainly helped to shift the center of gravity of the J-8 back.

Usually all of the methods you mention are used in combination (except for the clever aerodynamic device, unless you mean a sophisticated FCS):

• The wings are resized/moved
• The tailplane would be made smaller
• Internal items will be shuffled around or exchanged to shift the center of gravity further back.

Answer 2

I don't have access to the book, but the improvement mentioned in the quote is

... leading edge slats ...

Slats wouldn't change the center of gravity noticeably. Your Wikipedia reference doesn't currently mention slats. Perhaps you intended to say "change center of lift/pressure". The main goal is improve maneuverability, which can be achieved various ways.

This reference: https://en.topwar.ru/24868-shenyang-j-8-ii-finback.html includes the following text:

Acceleration characteristics and maneuverability were enhanced by the use of maneuverable slats, as well as more powerful engines WP13B.

You also asked:

How would a fighter be changed from having static stability to having relaxed stability?

Of course, the aircraft can be designed that way from the start. To increase maneuverability, negative-dihedral, or anhedral can be included in the design, but that is not adjustable during flight.

Other options would include:

• Speed/Dive Brake:
• Vectored thrust:

Next Question:

How would a fighter such as the J-8 physically be changed to change the centre of gravity in relation to the centre of lift?

I don't see any documentation that says it could. (Burning of fuel in tanks laterally displayed will do that a bit). More effective and easier to change camber with controls like slats.

Would the wings be resized/moved?

They can: https://en.wikipedia.org/wiki/Variable-sweep_wing But the J-8 isn't part of that club.

Would the tailplane be made smaller?

Dynamically? not that I'm aware of.

Would internal items be shuffled around to put heavier ones at the back? Or could some sort of clever aerodynamic device be added to modify the airflow somewhere?

Moving heavier objects around, no. Aerodynamic device, yes. As mentioned slats. Besides flight controls, my reference also said more powerful engines. Your reference mentioned composites, which reduce weight and can allow tighter turns.