Can airflow reverse over the wing of a uav in high tail winds and low forward airspeed i.e 25knt fwd airspeed in a 35 knt tailwind? What would be the overall effect on the elevator and rudder? I suspect the rudder would be remain the same but operate at a reduced efficiency however, wouldn't the function of the elevator reverse and/or become ineffective, especially at high angles of attack? As a point of note, this uav does not have ailerons, I believe the roll effect is achieved as a combination of pitch and yaw.
It is possible, but only if there were a very sudden and dramatic change that the aircraft was unable to overcome. However, it would be only temporary.
If the airflow direction was actually reversed the control surfaces would likely be blown to a full deflection position one way or the other depending on which side of neutral they were.
Otherwise, the aircraft would still move forward at 25kts relative to the moving airmass, with the additive effect of the wind giving it 60kts of ground speed.
If you're taking about a steady 35kt tailwind it would make no difference. The surfaces only react in relation to the air mass whether that air mass is moving or not.
No, at least, not in the way you seem to think.
When any aircraft flies, it does so relative to the air, not the ground. So, when you're watching from the ground, the air acts like a treadmill.
So, if you've got a 35kt South* wind, and a UAV that's flying North at 25kts, then, from the perspective of the ground, it will be traveling North at 60kts. It will still have 25kts of wind over its control surfaces, which will perform as normal.
If this UAV then turns around, it will appear from the ground to be flying backward, going North at 10kts even though it's facing south. But it will still have 25kts of wind across its control surfaces, which will perform as normal.
There are two ways to actually have a reverse airflow across its wings. You can reverse the thrust of the engine, or the wind can suddenly switch directions (as detailed in other answers). In either case, all control surfaces would function in reverse (assuming, of course, that whatever servo system moves the control surfaces is strong enough to fight the tailwind and actually move them).
*Remember, when discussing wind direction, you specify the direction that the wind is coming from. So a "South wind" would be blowing out of the South, toward the North.
You can test this with a simple balsa model and a desk fan. Reversing airflow on both rudder and elevator will reverse the direction of their torquing force. In the air, this has actually been seen with a model plane at around 15 mph hit with a strong gust from behind. The elevator, pulled up to create downforce on the tail (to pitch the nose up), reversed, sharply pitching the nose down. Needless to say, the wing lost lift as well.
A complete reversal in airflow in flight is extremely rare, requiring a huge gust. Keep in mind the plane moves with the air mass in a constant wind. But a gust need not drop your airspeed to negative numbers to cause a dangerous loss in altitude, as can happen in a microburst. The remedy is to pitch down immediately and add power to regain airspeed. Close to the ground, recovery may not always be possible.
But there is one practical application to your thoughts, positioning of ailerons while taxiing. Is wind is quartering from behind, you want that aileron DOWN to help hold the wing tip down. As you turn into the wind, having the aileron UP pushes the wing down (as in normal flight). Same story with the elevator. And a crossing wind can certainly prompt a rudder input.
Far more likely to experience these issues on the ground, and maybe not a good day to fly if they are happening in the air.