Conference back40::soapbox

    helicopter rotors do not stop when the engine fails.  they are designed
    with a feature called autorotation - the clutch disengages, and the
    rotors keep right on spinning.  to stop the rotor when the engine stops
    would require a failure of the clutch, too.  something else went wrong
    over the charles.

Yep.  I've been aboard a helicopter on a test flight where the pilot
killed the engine to test autorotation and recovery from it by
restarting the engine just a few feet off the ground.

/john

    
    
    	re: autorotation
    
    	I've been told by many helo-junkies that they'd rather have be in a
    helicopter with a dead engine than a plane with the same. Autorotate
    all the way home......
    
    
    jim

    This is old now (always is when I show up) but..... A controlled 
    landing using autorotation is part of the exam for a PPL (h)

    
    
    	Just heard this morning on NPR that witnesses who watched the
    copter crash said the rotor was NOT turning on the way down.
    
    	I'd say fell like a rock is probably an accurate description.
    
    jim

re: .1223

Jim,

Witnesses to aircraft accidents are notoriously inaccurate, especially among
the non-pilot public.  You'd be amazed at how many 'witnesses' report fixed
wing aircraft falling out of the sky with 'engines on fire' and the physical
crash scene directly contradicts the report.

Bob

    
    
    RE: .1225...copter crash...
    
    
    Actually, one of the key witnesses to the crash was a 
    university professor that knew all the 'technical' terms
    for what the copter was experiencing, so I tend to assume
    it is accurate that the top rotors were not turning.  He
    also said that the copter was turning to compensate for 
    the rotors not turning.
    
    
    Terrie
    

    > turning to compensate for
    > the rotors not turning.
    
    this would happen if the transmission froze up.  the rotational inertia
    of the rotor would be largely transferred to the chopper's fuselage -
    quite suddenly and with devastating effect.

    
    
    What Dick said.  The only way the rotors would have been still is if 
    a) the transmission locked, or b) the rotor blades were at 0 pitch
    (which they wouldn't have been if the chopper was in flight).
    
    
    

re: .1229

>    Actually, one of the key witnesses to the crash was a 
>    university professor that knew all the 'technical' terms
>    for what the copter was experiencing, so I tend to assume
>    it is accurate that the top rotors were not turning.  He
>    also said that the copter was turning to compensate for 
>    the rotors not turning.
 
This doesn't make sense.  The copter tries to rotate in a direction opposite
to the rotor rotation due to the equal-and-opposite reaction law of physics.
The purpose of the tail rotor is to apply a force equal to, but opposite
in direction to the turning of the copter.  If the main rotor stops, there is
no force to cause the copter to turn.

One question is, what powers the tail rotor?  I wonder if there is some failure
mode that would cause the tail rotor to continue to run if the main rotor is
somehow stopped?  This could potentially cause the copter to rotate if the
main rotor was stopped. 

What kind of chopper was it?

Bob

    Jeesh, it all seems so simple on my little gas-powered RC
    helicopter... how come the big ones aren't as simple!!!
    :-) :-) :-)
    
    -b

    .1238
    
    > If the main rotor stops, there is
    > no force to cause the copter to turn.
    
    wrong.  if the engine fails, the clutch disengages, and the main rotor
    autorotates.  it does NOT stop.  as long as the craft is airborne, the
    aerodynamic forces set up by autorotation will keep the rotor turning. 
    
    if the transmission freezes, the main rotor is suddenly LOCKED to the
    fuselage, and all of the kinetic energy inherent in its inertia of
    rotation is distributed to the combined, larger, "single" object.  the
    fuselage will suddenly begin rotating slowly in the same direction that
    the rotor is going, and the rotor will appear "stopped" to a witness
    because it is not rotating with respect to the fuselage.

re .1238:

>This doesn't make sense.  The copter tries to rotate in a direction opposite
>to the rotor rotation due to the equal-and-opposite reaction law of physics.
>The purpose of the tail rotor is to apply a force equal to, but opposite
>in direction to the turning of the copter.  If the main rotor stops, there is
>no force to cause the copter to turn.

During normal flight the rotors have significant angular momentum while
the body has none.  The torque from the engine is countered by the tail
rotor.  The total momentum is that of the main rotor.

Now assume the transmission seizes suddenly.  The total momentum is unchanged
(conservation of angular momentum) but the body and rotor are essentially
attached together so the whole bird will rotate as a unit, slower than the
main rotor was originally but not zero.  This is a sudden change and is much
larger than what the tail rotor could counter, not that it would do any good
doing so.

    .1241
    
    see .1240.  :-)

re: .1240

>    > If the main rotor stops, there is
>    > no force to cause the copter to turn.
    
>    wrong.  if the engine fails, the clutch disengages, and the main rotor
>    autorotates.  it does NOT stop.  as long as the craft is airborne, the
>    aerodynamic forces set up by autorotation will keep the rotor turning. 

Dick,

I did not say "if the engine fails", I'm accepting the statement that the
main rotor did in fact somehow "stop".  I am aware of automatic clutch
disengagement and what an autorotation is.

>    fuselage will suddenly begin rotating slowly in the same direction that
>    the rotor is going, and the rotor will appear "stopped" to a witness
>    because it is not rotating with respect to the fuselage.

I agree.  The question is "For how long?"  The mass of the fuselage and
everything in it, combined with its rather non-aerodynamic shape (relative
to the rotor blades) would seem to suggest that this rotation wouldn't last
very long.

Does anyone know if the transmissions are NOT designed to shear upon some
massive lockup to free the rotor to autorotate?  Other aviation things
are designed in this manner, i.e. vacuum pumps and almost any accessory
attached to the aircraft engine.

>main rotor was originally but not zero.  This is a sudden change and is much
>larger than what the tail rotor could counter, not that it would do any good
>doing so.

Mike,

I wasn't suggesting that the tail rotor could counteract it, but rather
was trying to account for the "rotor was stopped and the copter was turning"
statement.  After some unknown time (at least unknown to me), the fuselage
will stop turning as it has used up all the energy transferred to it from
the main rotor.  At this point, if the tail rotor was still turning, it
could possibly cause the fuselage to yaw (turn) since the torque it was
designed to counteract, is no longer present.  At this point in time, it also
is useless to try and use the tail rotor to control the aircraft as it has
now become a rock.

Bob

    Not that it's particularly germane, but what the hell that's never
    stopped me before :-), a few years ago I was driving down a 4
    lane highway, minding my own business, when suddenly my air
    conditioning compressor decided to sieze up, and the car basically
    stopped dead in the left-most lane (in fairly heavy traffic).
    The belt that was around the compressor did break, but not
    before the car stopped dead in its tracks, without using brakes.
    This caused the people behind me a great deal of displeasure,
    which they freely expressed with various gestures common to
    the PRM driving experience.
    
    Could a similar sitation happen with a helicopter, where whatever
    was supposed to sheer did not do it fast enough to keep up
    with the pace of catostrophic events?
    
    -b

    (P.S. The car was an 88 Ford Tempo... the worst piece of $%!^ I've
          ever had the pleasure of owning).

    .1246
    
    the "not fast enough" phenomenon you cite is a known corollary of
    murphy's law.  in electronics, it's expressed along the lines of
    "semiconductors will blow to protect fuses."  it's not impossible that
    a similar thing could happen in a chopper.

    
    
    RE: Autorotation....that's the wording they used!  I could not
    remember it before!  Thanks.
    
    
    Terrie
    

	notes moved here from the news briefs topic.  of course,
	now all the reference numbers don't make sense, but i'm
	sure you can deal with it.

	di

  

re .15:

>>main rotor was originally but not zero.  This is a sudden change and is much
>>larger than what the tail rotor could counter, not that it would do any good
>>doing so.
>
>I wasn't suggesting that the tail rotor could counteract it, but rather
>was trying to account for the "rotor was stopped and the copter was turning"
>statement.  After some unknown time (at least unknown to me), the fuselage
>will stop turning as it has used up all the energy transferred to it from
>the main rotor.

The helicopter was close to the point it took off from.  It may not have been
that high in the air.  By the time the fuselage had stopped rotating from the
air resistance the craft may have already completed its rock imitation.

    i find the irony of this topic's title vastly amusing.  i assume that
    the basenoter intended as much.

    suggestion:
    
    You folks are answering each others questions with a "No that would
    never happen" or a "Thats not possible, laws of phyysics" and then
    asking more questions and around we go again.......
    
    There is a note VMSZOO::FLYING which will provide you with the meaning
    of life and a few answers on Hunkypropters problems. Please have an 
    adventure on me.

    y physics. :*)

    re: .19
    
    Thank you Lady Di.
    
    re: .18
    
    Terri,
    
    Do you know the context of the use of the word 'autorotation' by the
    'expert'?
    
    re: .??
    
    I'm sorry, I forgot who wrote that the aircraft had just taken off.
    Given that description, it seems very likely that the aircraft was
    spinning when it hit the ground.  Also, there are points in all
    helicopters flight envelopes where a successful autorotation to a safe
    landing is not possible.  Most chopper pilots avoid/minimize their time
    in this portion of the flight envelope.  This is why, for example, that
    even though helicopters are capable of vertical takeoff and climb to
    various heights, a typical takeoff involves a short vertical climb
    followed by forward movement and then a climb to cruise altitude.
    
    Bob

My son was on Memorial Drive, riding with my in-laws to the
Museum of Science, after the crash.  Josh got to see "four
dead bodies under white medical sheets."  He found that
incredibly fascinating.  :^(

    If he thinks death is fascinating, let him watch Schindler's List as
    soon as he's old enough. It'll change his mind rather quickly.

>>    If he thinks death is fascinating, let him watch Schindler's List as
>>    soon as he's old enough. It'll change his mind rather quickly.

	I watched it, and I still think death is fascinating.

    
    *********************************************************************
    *                                                                   *
    *             In Appreciation for Your Dedicated Service            *
    *                                                                   *
    *                     Massachussetts State Police                   *
    *                                                                   *
    *                          Trooper Paul Perry                       *
    *                                                                   *
    *                        Trooper James Mattaliano                   *
    *                                                                   *
    *                           February 22, 1995                       *
    *                                                                   *
    *        "Remember them not for the way by which they died,         *
    *                       but for the way they lived"                 *
    *                                                                   *
    *********************************************************************
    

    .27 Di', i agree...it's fascinating aslong as it's happening to
        someone else:-)
    
    
        Chip 

    
    
    In order for a helicopter or gyrocopter to successfully autorotate it
    must have sufficient altitude and air speed. Attempts to autorotate
    below 1000ft with speeds under 90knots or so will not be sufficient for
    the operator to control his/her rate of descent and angle of descent.
    Autorotation is acheived as a result of the forward airspeed causing
    the rotors to turn. If the machine crashed shortly after takeoff it's
    quite possible that the pilot didn't have enough forward speed and
    altitude to successfully pull this off. This condition is known as the
    "dead man's curve". It is possible to successfully land in the curve
    but the operator would have had to have sufficient time to recognize
    the problem and take appropriate actions. I don't beleive that he did
    given the reports that I heard regarding the incident.
    
    Jerrt

    .30
    
    the craft crashed approximately three minutes after takeoff.  i should
    like to believe that any competent chopper pilot would get through the
    dead man's curve into a safe flight condition in three minutes, unless
    there was something very wrong with the aircraft.
    
    i'll look forward to the ntsb's report on this one.
    
    on the other hand, who remembers gyrocopters any more except the idjit
    or two who built a benson?   :-)  does anyone besides me recognize the
    name juan de la cierva?

I remember gyrocopters, but I never built one.

Bob

    
    
    anybody know where those troopers were from (as in hometown) or their
    ages???
    
    
    (just asking cause i grew up with a paul perry...)
    
    

    
    The troopers were 32 and 39, I believe.  The engineers were 40's.
    
    Lots of orphans.  bb

    
    
    Re: 33
    
    Massachusetts State Police
    
    Trooper Paul Perry         Age 39  Salem
    
    Trooper James Mattaliano   Age 33  Sandwich
    
    State Police Air Wing Section out of Norwood, MA
    
    

    
    
    thanks...too old to be the one i know...
    
    

The crash site was almost two miles from the Nashua Street helipad.

Plenty of time to achieve appropriate altitude; since he would have had
to have cleared the Longfellow Bridge.

/john