How is a transonic aircraft longitudinal stability different than a subsonic and supersonic aircraft?

Question

/I would like to know-how is the longitudinal stability is achieved between different aircrafts

Answer 1

At this level, there is nothing special about transonic stability.

Generally, airplanes become significantly more stable at supersonic speeds (due to aerodynamic center moving backward). At the same time, elevator efficiency drops. Transonic is simply the transition between the less stable subsonic regime and more stable supersonic regime.

Now, for an engineer as well as a pilot, transition can be a challenge, especially if it's quick. For example, most supersonic aircraft have variable stick-to-elevator ratio, and transition happens at transonic speeds. Apart from making sure that the aircraft is controllable throughout the transonic range, you need to consider what happens if the system fails at any stage.

As another example, modern fighters are often made longitudinally unstable. But this is only true at subsonic speed. They are all stable at supersonic regimes. Now you have transition between even more different states: unstable to stable (and back). Flight control systems must cope with that.

Thankfully, the transition is not too abrupt, especially for swept wings. (This, controllability, is in fact one of the main reasons to use swept wings. It's not just about drag).

Answer 2

It is not different.

Aircraft is longitudinally stable if increase in angle of attack causes larger increase in coefficient of lift of the aft horizontal surface than in the forward horizontal surface, which happens if the forward surface flies at higher angle of attack than the aft.

For most aircraft the forward surface is the wing and the aft surface is the horizontal stabilizer, but in canard layout the canard is the horizontal surface and is in the front. For tailless aircraft it is the forward and aft part of the same wing with carefully set sweep and twist.

The angle of attack of the horizontal stabilizer is adjusted to control the aircraft—note that changing camber by deflecting the flap (elevator) at the trailing edge is equivalent to changing the zero-lift line and therefore principally similar. In most aircraft, the horizontal stabilizer actually flies at negative angle of attack to provide sufficient range for adjustment while staying in the range for static stability, but the condition is that it is less than for the forward wing, not negative.

The only difference for transsonic and supersonic aircraft is that when using supersonic wing, the centre of pressure shifts from about quarter-chord to mid-chord when the flow speed over the wing exceeds speed of sound. This causes strong nose-down moment (known as “Mach tuck”) and requires significant change in the incidence of the horizontal stabilizer (decrease for conventional tail, increase for canard). It also creates irregularity in the control behaviour—you push down/trim nose down to accelerate, but then you need to pull up/trim nose up when you reach the critical speed.

However this depends on the flow speed perpendicular to the wing, so sweeping the wing increases the speed at which the Mach tuck occurs. And transsonic, and even most supersonic aircraft have the wings swept so that it does not happen in their design speed range.

So for modern aircraft you don't need to bother and there is indeed no difference. It was significant for older aircraft with straight wings when they tried to break speed limits, X-1, the first aircraft to “break the sound barrier” that still had straight wings, and some fighters with supersonic wings like the F-104.