Published data on the Su-24 stall speed is difficult to find, but we can make an educated guess using a typical estimate of maximum lift coefficient for combat aircraft that ranges between 1.2 and 1.8 -- in the so-called clean configuration, ie., no high lift devices like landing flaps extended. In normal flight the aircraft would be flying clean to reduce drag, but might deploy flaps in a tight turn to increase its lift and help prevent stall.
If we assume the CLmax is between those given values, say 1.5, then we can work out the rest since we know the airplane's wing loading is 133 lb / ft ^2 (wikipedia) and we know the airplane was flying at about 6,000 meters altitude, about 19,700 ft. At this altitude air density is only about half of what it is at sea level: in metric the air density on a "standard" day at that altitude would be about 0.66 kg per cubic meter, or in English units, 0.00128 slugs per cubic foot, where 1 slug equals 32.2 lb; (32.2 feet per second being the acceleration of gravity on earth).
So if we plug in the numbers we get:
sqrt(133 * 2 / 0.00128 / 1.5) = 372 feet per second.
To convert to mph we multiply by 15 and divide by 22 and get 253 mph as our stall speed at straight and level flight at 6000 meters altitude, about 19,700 ft. Now any turning or banking maneuver would obviously increase stall speed further, so we can think of 253 mph as a minimum flying speed at that altitude for the Su-24 (again assuming standard day conditions).
Now that wing loading assumes a fully loaded aircraft, which probably was not the case. The Sukhoi would likely not have been carrying full fuel of about 11 tons (22,000 lb), although it might have been carrying its complete payload of over 3 tons of ordnance (6,000 lb). However reports of the sortie described the pair of bombers had already made one bombing run, so full ordnance was probably not on board either.
We can take a stab at estimating the operating weight of the aircraft at that time. The Su-24's empty weight is published as just under 50,000 lb (wikipedia). If we assume half fuel, that would be about 11,000 lb, and if we assume half ordnance load, that would be about 3,000 lb, giving a total aircraft weight of 64.000 lb.
The published wing area is 594 square feet, so the wing loading under this flying condition would be 64,000 / 594 = ~108 lb/ft^2.
If we now plug that into our stalling equation that takes into account the air density, we get:
Vstall = sqrt(108 * 2 / 0.00128 / 1.5) = ~335 fps, which equals ~227 mph.
Again, this is an approximation of the Su-24 stalling speed at part-weight and straight and level flight at 19,700 ft altitude. Any turning maneuver would increase stall speed as described above. Also a warmer than standard temperature at that altitude would lower air density (and increase the effective altitude). This is known as density altitude and all pilots are trained to work only with density altitude, since non-standard meteorological conditions are not uncommon.
Note also that the actual CLmax value of the Su-24 may be something different than our guesstimate; a higher value would decrease the stall speed, but a lower value would increase it. We are making an educated guess. However, this puts us in the ballpark and hopefully sheds some light on the technical issue under dispute.
On a practical level, it is outside the realm of possibility that a bomber aircraft would be flying at the edge of its stall at any time during a sortie, other than hard evasive maneuvers to avoid enemy action. Since the rescued co-pilot reports that they did not even see the missile coming, nor had time to take evasive maneuvers, it can be assumed with strong certainty that the airplane would not have been flying near the stall at any time.
More than likely the aircraft would have been flying in the so-called "transonic" regime where airliners usually operate, which is about 550 to 650 mph.
In light of this brief and inexact analysis, there appears to be reasonable doubt as to the Turkish version of the Sukhoi's incursion into sovereign airspace under the conditions described.
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