Pettinger,
Happy to help explain here.
The F-35B's powered lift system has two 'propulsors'. One is the main engine in the rear of the aircraft - it delivers thrust downwards via a swivelling nozzle at the rear of the aircraft. Two large doors open up under the rear of the aircraft to allow that nozzle to swivel down. There are also two doors on top of the aircraft, located just aft of the large 'vertical panel' you described, which open to form an auxiliary air intake for the main engine at zero and low airspeeds, when the normal engine air intakes would not be sufficient.
The 'large vertical panel' is the intake door for the second 'propulsor', which is a shaft driven lift fan mounted vertically just aft of the cockpit. There is a pair of doors under the lift fan, just aft of the nose landing gear doors, through which the lift fan exhaust exits.You're quite correct - the lift fan door does create significant drag, and its opening angle is automatically adjusted from fully open (around 85 degrees by memory) to part open (around 35 degrees) as the speed builds up. And yes, there are significant forces on that door. Design of the door was a huge challenge: the original 'bifold' system used on the X-35 had some serious problems, and a large number of options were looked at before settling on the eventual design. There were also a number of almost invisible but very significant changes to the shape of the lift fan intake lips and duct.
Development of the lift fan, its intake and exhaust system, and the vectoring system for it (a vane box located just under the lift fan) was a tremendous technical challenge. Between the X-35 and the F-35B, the lift fan system was almost totally redesigned, and called on a huge range of technical skills including advanced aerodynamics, propulsion design, software driven flight controls, acoustics, advanced structural design and so on. PPrune readers should know that this magnificent effort was led by extraordinarily talented UK engineers. It was my privilege to be able to watch them at work.
Best Regards as ever to all those talented Brit engineers still working hard on the programme,
Engines