I have just seen a dark long aircraft fly over my car followed by a long helicopter. I am not an expert at this but it is 8pm, i am on a hill with a few houses, there is a little street light on the clouds, The clouds are not too high or too low and are broken allowing some stars through. I was sitting in my car listenin to music when I heard a helicopter quite low and loud sounding. Not unusual as we have a police helicopter out a lot and we are near Lympstone Commando base In Devon. I looked up through the window and saw a long black aircraft fly quite fast 50-90mph? But it had no lights on at all. It looked like a shadow but it blocked the stars as it went past. Then 2-3 seconds later a helicopter passed, a but further away going the same direction, Almost like it was escorting it? This helicopter had a light flashing at each end but it must have been huge. Army for sure I should think. I only heard one helicopter - but i was in the car with some music one to begin with. But my question is no doubt it is an aircraft but what aircraft would be allowed to have no lights on at night? Or do you think I was under the belly of a military helicopter and just couldn't see the lights?
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It's likely not a stealth aircraft due to the speed it was flying at. Your speed estimate of 90mph is unrealistically slow, particularly for military aircraft. However, here is the likely scenario:
You misidentified the aircraft: At that speed, or anywhere near that speed, it was likely to have been a helicopter. In the world of VTOL and Tilt-Rotor aircraft, which private and military industry are both pushing heavily, these are becoming more and more common. In fact, the United States Marine Corps trains quite often with the Royal Marine Corps - and we have tiltrotor aircraft! The MV-22 Osprey, pictured below, looks like a plane, but tilts its rotors to achieve a helicopter-like flight model. And let me tell you, these are loud and sound exactly like helicopters. They're also quite big, and will look doubly so when flying low over your head.
In the United States, our military do so-called "dark" flights quite often. They're practicing the skills required for these flights, and it is perfectly legal for them do so, with the proper safety precautions.