cwatters & Mechta:
Do you mean, first stall and then dive? Or is it, one has to imagine an “up-side-down stall”, against a positive buoancy, which means, “stalling” towards the ground, and not a dive?
Firstly, Airlander is not lighter than air. It uses the buoyancy of helium to reduce the amount of lift required from other sources, namely, vectored thrust at takeoff and aerodynamic lift when in forward flight.
To me it’s very hard to cope with overimposed static and dynamic forces at the same time, especially, if there is no constant level or reference, neither for one nor for the other.
You're not the only one!
Let’s look, what the photographs and the video could tell.
First we can see the ship passing by in what should be the downwind leg of the pattern, in a rather normal attitude.
Then we see the ship in the final approach, may be some 400 meters out, but with an overly nose-down attitude in respect to the glide path (and with the mooring line hanging down).
Last we see it arriving over the airfield, one ship-length inside the perimeter fence, and with the same nose-down orientation as before.
That suggests, that the approach was made all the time nose-down. Why could it be?
Assuming the pilot was performing a conventional aircraft type approach, this would consist of:
Descend with a sufficient margin of speed above stalling speed to allow for any wind gradient and possible loss of thrust.
Convert speed into height (rotate) so descent (potential energy) is converted into flight parallel with ground (kinetic energy).
Maintain nose-up attitude until forward speed & kinetic energy decays such that aerodynamic lift is insufficient to keep aircraft airborne (flare & landing).
That's what it would have to do if gliding in, however Airlander has four engines, which can deflect their airflow downwards to some extent. These allow the pilot to replace some lost aerodynamic lift at low speed with their thrust, permitting a slower airspeed at touchdown.
The load-distribution of the airlander is: cockpit and cabin in the front, projected payload in the mid – and the fuel tanks in the aft! And then we know, it just made a 1h 40’ flight, feeding the 4X500 hp drives. That must have made it lighter and lighter at the rear end.
Ballonets, which are variable size, fan-filled bags of air inside the helium-filled envelope, displace the helium and therefore the buoyancy it generates, allowing the centre of gravity to be adjusted for changes in fuel load.
At the begin of the video and before starting the dive, we see the ship in this very situation. Nose down by 15 deg, GS less than 5 kt, no vertical speed visible. And the drives are running, providing a little forward and a very little downward momentum.
At a guess, either the pilot was positioning the Airlander to start the descent, or air turbulence tipped it into this position.
In the very next moment it gets out of control – but I can see no initial situation to develop a stall.
Low initial airspeed combined with a loss of energy due to wind gradient could cause this despite the nosedown attitude.
It seems more, it has to work to come down than to stay aloft.
As mentioned earlier, Airlander is not lighter-than-air. unless it was in a thermal or other rising air, its going to come down.