Subject: Re: Are we close to a Lisp boom ?
From: (Rob Warnock)
Date: Tue, 03 Jun 2008 06:20:56 -0500
Newsgroups: comp.lang.lisp
Message-ID: <>
[Drifting *way* OT...]

Nicolas Neuss  <> wrote:
| Probably good pilots, good maintenance, and the fact that it is
| possible to land it with zero horizontal speed (note that a
| helicopter can perfectly land even in case of a motor failure).

*IF* the craft has enough speed *AND* altitude and *IF* the pilot
recognizes the engine failure *AND* initiates autorotation within,
oh... ONE TO TWO SECONDS!! Really. Else you're toast.
    Learning to Fly Helicopters
    In a helicopter, by contrast, if the blades spin down more than
    10-15% from their normal velocity, there is no way to convert potential
    or kinetic energy into spinning such that the helicopter will start
    to fly again. If you can't restart your engine, therefore, your
    helicopter can very quickly become a rock.

    In a turbine-powered helicopter like a Bell 206 JetRangers the
    blades are heavy and the blades won't slow down for several seconds
    after an engine failure. In the flyweight Robinson, however, after
    an engine failure you have no more than 1.2 seconds to take exactly
    the right actions or the helicopter cannot be recovered. 
    Rotor RPM - This is the single most important ingredient of a
    successful autorotation. Without sufficient rotor RPM you will
    become something like a brick. In the event of an actual engine
    failure, you have a very limited amount of time to get the
    collective fully down to regain any lost rotor RPM, and there
    *will* be some to gain.
    Common Mistakes
    Failure to Lower Collective all the way down
    If the pilot forgets to lower collective and this is a real
    engine failure, it's a fatal mistake. Lowering collective is
    the most important part of doing an autorotation. ...
    Autorotations: The physics of the autorotation explained
    This is the transition from powered flight to autorotative
    flight, and is the most critical of the four phases. Not the
    most difficult, but definitely the most critical. If you don't
    get the collective down quickly enough to prevent the R/RPM
    decaying to a point where its recovery is impossible, it's all
    over, regardless of your skill levels in the remaining phases.

Also, a true zero-forward-velocity landing in autorotation is
extremely difficult, since you have to maintain sufficient forward
velocity (typ. ~60 kts) until *just* before landing (when you perform
the "flare" and trade forward velocity for rotor speed and vertical
slowing). A "good" autorotation landing is any one in which your
forward velocity is less than ~20 kts when the skids touch down.

Finally, autorotation requires that you either be high enough *and*
fast enough (e.g., >500' & 60 kts) or else low enough *and* slow enough
(<10' & ~0 kts) to enter autorotation and still land safely. There's
a rather large schmoo of "deadly" height-velocity combinations for
which autorotation is impossible, and unfortunately some kinds of
helicopter operations are done routinely in this regime [e.g., heavy
lifting of equipment onto buildings]. Again quoting Philip Greenspun:
    This all sounds good until you look at the "deadman's curve".
    The marketing literature for helicopters says "if the engine
    fails, you can autorotate down to a smooth landing." The owner's
    manual, however, contains a little chart of flight conditions
    from which it is impossible to landing without at least bending
    the helicopter. Unfortunately these conditions are the very ones
    in which nearly all helicopters seem to operate. If you're above
    500', for example, you're pretty safe. But TV station helicopters
    are often lower than that when filming. Flying along at 65 knots
    at any altitude is safe, but if the camera needs the pilot to
    hover the helicopter slows to a crawl and may get under the
    "height-velocity diagram", as the FAA calls the deadman's curve.


Rob Warnock			<>
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