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Ship landing attempt during Storm Ophelia

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Ship landing attempt during Storm Ophelia

Postby StuartC » Mon Oct 16, 2017 7:33 pm

Took the latest Merlin HM1 development version out to a Type 45 Destroyer Sailing in Luce Bay during Storm Ophelia to see how well things went. All in, the choppers stability is superb even in those conditions, but the Ship itself killed me due to the severity of the waves.

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Re: Ship landing attempt during Storm Ophelia

Postby wlbragg » Mon Oct 16, 2017 10:24 pm

So, did you hit to hard and cause a "crash" condition or did the pitching after you "landed" cause a "crash" condition?

I thought it was looking pretty good until you fell off into the drink!
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Re: Ship landing attempt during Storm Ophelia

Postby Thorsten » Tue Oct 17, 2017 6:29 am

So, did you hit to hard and cause a "crash" condition or did the pitching after you "landed" cause a "crash" condition?


Looking at the end of the video, probably the latter. The ship is moving pretty wildly, and the FDM gets this as four to six times the kick due to FDM vs. framerate - that's easily over the acceleration limit of most craft.
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Re: Ship landing attempt during Storm Ophelia

Postby StuartC » Tue Oct 17, 2017 7:55 am

yep the severity of the pitching got me, although I'm not sure if it was the gear/airframe registering and impact due to that, or it might have been the ship went down so far enough to dunk the back of the chopper into the sea, registering a crash. Ether way, The situation would never have been attempted in real life, the conditions were just too severe.
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Re: Ship landing attempt during Storm Ophelia

Postby Thorsten » Tue Oct 17, 2017 3:33 pm

Actually, how do you determine wave motion?

I've looked into this and came as far as deriving a set of transfer functions based on Ship geometry which should be multiplied with the Fourier spectrum of the current wave motion - to find out that the wave model we have is not only very simple (okay, I could deal with that) but clamped to cut high and low amplitude waves off and also create only a very limited frequency spectrum.

At which point I decided to drop it, because I didn't feel like coming up with a proper wave excitation model from scratch.
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Re: Ship landing attempt during Storm Ophelia

Postby GinGin » Tue Oct 17, 2017 4:29 pm

Nice Video Stuart, That is a challenge :)
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Re: Ship landing attempt during Storm Ophelia

Postby StuartC » Tue Oct 17, 2017 5:16 pm

Thorsten wrote in Tue Oct 17, 2017 3:33 pm:Actually, how do you determine wave motion?

I've looked into this and came as far as deriving a set of transfer functions based on Ship geometry which should be multiplied with the Fourier spectrum of the current wave motion - to find out that the wave model we have is not only very simple (okay, I could deal with that) but clamped to cut high and low amplitude waves off and also create only a very limited frequency spectrum.

At which point I decided to drop it, because I didn't feel like coming up with a proper wave excitation model from scratch.


Another FGUK member came up with the calculations. Its something to do with the ship mass + windspeeds but beyond that, its like black magic, it just work and works quite well. FG needs actual visible big waves to go with it though.
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Re: Ship landing attempt during Storm Ophelia

Postby Thorsten » Wed Oct 18, 2017 9:29 am

Its something to do with the ship mass + windspeeds


Hm, windspeed is a poor proxy for wave size, because that depends much more on the wind history and the distance upwind where there's still water.

FG needs actual visible big waves to go with it though.


I suppose the problem is that the shader utilizes a simple bump map, so from low altitude you're going to see massive artifacts for 10 m waves, though they work fine from high altitude (that I tested).

So we'd need a solution modifying the actual geometry of the mesh - which is also not so easy, because the existing ocean mesh is very coarse and would need massive tesselation. Anders G has a solution for ships where the ship brings a wave mesh at close range, but then matching the visuals with the distant parts is an issue. It's kind of a nasty problem.
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