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AirLander take off then 2nd Flight Mishap

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AirLander take off then 2nd Flight Mishap

Old 29th Aug 2016, 06:28
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Now that we brought up Jeremy Clarkson I can't help but compare this Airlander to the Snobine Harvester of Top Gear fame. A wonderful and impressive piece of engineering marvel but useful like breasts on a boar.
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Old 29th Aug 2016, 08:09
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I see that the Airlander has been moved back into its hangar.

With it being only 60% buoyant, that would imply that the dead weight is 8 tonnes - or is it the full 20 tonnes?

Do you measure deadweight the same way as a ship?
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Old 29th Aug 2016, 11:08
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Is it possible they just flew too slow and stalled it?
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Old 29th Aug 2016, 20:29
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Originally Posted by cwatters
Is it possible they just flew too slow and stalled it?
Well, I wouldn't think that the "second" test flight would be conducted at a gross weight higher than the craft's buoyancy, but even if it were, presumably the device has enough vectored thrust to lift any weight in excess of buoyancy. I don't see how it would work otherwise.
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Old 30th Aug 2016, 10:26
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Quote:
Originally Posted by cwatters View Post
Is it possible they just flew too slow and stalled it?
Well, I wouldn't think that the "second" test flight would be conducted at a gross weight higher than the craft's buoyancy, but even if it were, presumably the device has enough vectored thrust to lift any weight in excess of buoyancy. I don't see how it would work otherwise.
I'm with cwatters, A stall was my first thought as soon as I saw the video. Although the wind appears to be light, any wind gradient would exacerbate the problem and make stall recovery less likely in the height available. A 500 foot operating height with a 300 foot long aircraft does not give a lot of room to play with.

If the gross weight were less than the craft's buoyancy it would be operating as an airship, not a hybrid, so it would not have come to a stop after the arrival, but would had to be held on the ground by the engines or the suction 'undercarriage; if that is working or even fitted yet.
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Old 30th Aug 2016, 10:35
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Do you measure deadweight the same way as a ship?
Maybe not. I've zoomed in on the pic but cannot make out any plimsoll line.
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Old 30th Aug 2016, 14:42
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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?

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.

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 primeter 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?

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 4X350 hp drives. That must have made it lighter and lighter at the rear end.

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.

In the very next moment it gets out of control – but I can see no initial situation to develop a stall.

It seems more, it has to work to come down than to stay aloft.

Last edited by minimum clean; 2nd Sep 2016 at 07:33. Reason: poor language/orthorgraphy
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Old 30th Aug 2016, 15:48
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Originally Posted by compressor stall
Maybe not. I've zoomed in on the pic but cannot make out any plimsoll line.
An airship is not like a ship; it's not floating on the interface, so it doesn't have a load water line, so no Plimsoll line, so no deadweight. It's like a submarine, but in a compressible fluid
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Old 30th Aug 2016, 22:56
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Did it occur to you why I posted the smiley face?
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Old 31st Aug 2016, 03:26
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Unlike a heavier than air aircraft, Airlander 10 can presumably be stood on its nose or tail if mismanaged, with gravity tugging downwards on everything attached to the envelope. Must make for an interesting engineering problem where every component must be braced and stressed for gravitational pull in all directions. It also suggests that all loose objects will need to be secured firmly before and during flight, just in case.

Passengers might not be to happy driving towards the ground at a very steep nose down angle, even if it is in slow motion.

As this incident has demonstrated, the craft may be lighter than air, but the is still all the inertia of mass when it comes to making a sudden stop against a solid object. I can't help feeling that an inflatable airbag towards the front of the cockpit would have allowed a more gradual deceleration and prevented any damage at all. Something similar to a large car air bag might do the trick but staying inflated for much longer. That wouldn't upset the aerodynamics and need not inflict too much of a weight penalty. Big red panic button on the pilot's panel to deploy the airbag when necessary.
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Old 31st Aug 2016, 09:28
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Passengers might not be to happy driving towards the ground at a very steep nose down angle, even if it is in slow motion.
Happy or not, on an statically lighter than air airship (especially going in to a temperature inversion) that's exactly what's going to happen. In my experience of giving pax rides in airships, the pax do not expect it to behave like a fixed wing, so no issues there.
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Old 31st Aug 2016, 10:15
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Nothing on the Airlander website refers to it as "lighter than air". In fact, they seem to go to great lengths to say that it is not. Additional lift is created from the airfoil shape + forward speed and vectored thrust.

Although how it uses this airfoil shape to get airborne from a static start is a question I have not seen answered.
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Old 31st Aug 2016, 10:19
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Vectored thrust.
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Old 31st Aug 2016, 11:02
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How does 60% buoyancy plus 25% vectored thrust get it off the ground, exactly?
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Old 31st Aug 2016, 11:46
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I would assume that the immense size of the aircraft coupled with vectored thrust creates a bubble of air underneath that just lifts it clear of the ground and allows acceleration enough for aerodynamic lift to be generated. A super ground effect if you like. The whole thing is designed as a lifting body.
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Old 31st Aug 2016, 12:20
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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.

Last edited by Mechta; 31st Aug 2016 at 12:32.
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Old 31st Aug 2016, 12:34
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minimum clean cwatters and Mechta

To me it looks like the CofG got in front of the centre of buoyancy. Possibly because of the fuel used up,as mc suggested. I'm guessing they must trim it by pumping gas into and out of multiple gas bags inside the envelope. So, I'd speculate the crash occurred because the trim mechanism failed, or pilot error, pumping gas the wrong way. Is it fbw?

Didn't see Mechta's last post. Does the Airlander certainly use ballonets? I would have thought it more practical to use pumps to transfer helium to a high pressure storage tank when reducing buoyancy.

Last edited by Rob Bamber; 31st Aug 2016 at 12:41. Reason: new information
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Old 31st Aug 2016, 13:55
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Rob Bamber wrote:
Does the Airlander certainly use ballonets?
Yes.

Hybrid Hopes: An Inside Look At The Airlander 10 Airship | Technology content from Aviation Week

The compartments fore and aft and either side also house ballonets, or airbags, that are used for pressure control of the vehicle. The ballonets are inflated with air on the ground, reducing the volume available for the lifting gas, making it denser. Because air is also denser than the lifting gas, inflating the ballonet reduces the overall lift while deflating it increases lift. In this way, the ballonet helps to adjust the lift as required. There is also a septum diaphragm in the ballonet compartment to prevent mixing of the helium in the upper section and air in the lower part.

“As you go up in altitude, the air wants to expand and you can’t cope with trying to contain it with the strength of the hull, so the helium pushes down on the ballonets and pushes air out through valves,” says Durham. “When you come back down the helium wants to contract, so the ship would go soggy unless you push air back into the ballonets. So each ballonet has a big valve and fan in it so can vent air in and out and run the ship at a constant delta p. It’s the one system on the vehicle that’s got no parallel to any other aircraft or helicopter,” he adds.



I would have thought it more practical to use pumps to transfer helium to a high pressure storage tank when reducing buoyancy.
Rob Bamber, The Aeros Aeroscraft, uses a system along the lines of what you describe. Technology copy - Aeros
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Old 1st Sep 2016, 10:15
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Mechta et.al.

I agree, it was not lighter than air. Had made this impression for a little time while hovering above the Airfield. But when it came down, it sat on the cushions and made no attempt to lift off again. So it shoud be proven.

On a closer look, it is visible that it came to a rest on the cabin/cockpit and only the front half of the cushions, with the aft half cushions not on the ground. That make me think, it was indeed out of trim, as the nose-down orientation in the approach indicated before.

The attitude then was excessively downwards, and I have never seen a plane come in with a negative AoA. Hence my problems to accept the stall hypothesis.

Many thanks for the Aviation Week article. It teached me a lot about the Airlander.

By one detail I was a little shocked, otherwise. The lateral engines are pivotable only by +/-20 deg!!? Despite the vanes with their additional angle, to me it seems not sufficient to make it fully maneuverable.

Last edited by minimum clean; 2nd Sep 2016 at 10:56.
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Old 1st Sep 2016, 18:14
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I have been trying to follow the aerodynamics but my head hurts, can anybody check my understanding and questions:
• The airlander is heavier than air
• It takes off by vectored thrust (like a Harrier?)
• Once airborne, it transitions to forward airspeed which creates lift over the body keeping it airborne.

Questions:
1. If it slows down sufficiently will it actually stall or simply sink; or are they the same thing?
2. If the above answer is stall, does it therefore have a minimum airspeed? If so what do you think it would be? Obviously I assume it depends upon weight and CG etc but a ball-park figure.
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