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549796.  Thu May 07, 2009 4:42 pm Reply with quote

Thanks, Mike.

I was also right about at "lighter than air" aircraft, (except when they're parked), at least some of the time.

If only I knew anything else even remotely interesting...


549804.  Thu May 07, 2009 4:48 pm Reply with quote

You weren't exactly wrong* ("airship" can mean an aeroplane**), but around here*** you**** need about half a page***** of disclaimers****** to make any sort of statement*******.

*wrong here means factually inaccurate and does not imply any sort of moral******** judgement
**aeroplane is here intended to mean a vessel designed primarily to transport goods or people by air
***here being this internet forum rather than the physical location of the poster
****not you specifically, but you in general - ie one
*****page being a generic term of no specific size but indicating about 40 lines of text
******disclaimers to include clarifications
*******or assertion
********for definitions of moral speak to Bertrand Russell

549807.  Thu May 07, 2009 4:49 pm Reply with quote

bob - the perfect QI post.

549810.  Thu May 07, 2009 4:50 pm Reply with quote

Hmmm - watches and learns...

549993.  Fri May 08, 2009 5:27 am Reply with quote

An airship can *fly* wilst heavier than air, but it cannot take-off or land in such a condition, with the possible exception of airships with vectorable thrust like the Airship 600 series (I say "possible" exception because whilst it can physically do it the regulations prohibit flying in such a configuration for safety reasons).

It is possible for an airship to fly whilst heavier than air because its hull develops aerodynamic lift when moving. It can get into this condition as a result of a gas leak, an extreme change in ambient humidity, airframe icing and one or two other more obscure conditions. But as the "wing" planform of an airship has an extremely low aspect ratio it has a very flat lift slope (dCl/d(alpha) can be down below unity ra5ther than the 4-6 region for most aeroplanes and 2Pi for an infinte aspect ratio) which means that it cannot generate useful boosts of lift coefficient by hauling the nose up. The means that an attempt to land whilst heavier than air would require a very long, fast and flat approach and a very long runway.

Even using the vectored-thrust systems of the Airship 600 it is not practicable to attempt to take off and land whilst heavier than air simply because once stationary the vehicles would fall over; they have no undercarriage or struts to support themselves.



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