Re: Power for Take-Off

Date:         08 Sep 97 02:03:42 
From:         don@news.daedalus.co.nz (Don Stokes)
Organization: Daedalus Consulting
References:   1
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Jack Sanders <Jack.Sanders@gsfc.nasa.gov> wrote:
>flight.  I expressed concern about losing power in one engine at a
>more critical point (liftoff), but he assured me that a fully loaded
>B757 would still have enough power to leave the ground.  Was he just
>trying to soothe my fears, since the event already passed, or is the
>plane actually that powerful?

Probably both.  8-)  Seriously, all twin-jet commercial transports have
more than enough power to continue a takeoff if there is not going to
be enough runway to stop.

The procedure is this: for each aircraft there is a speed, known as V1,
where the takeoff must be continued if anything goes wrong.  If the
aircraft can't get to that speed under normal takeoff conditions, and
then continue to flight speed (V2) with an engine out, or the aircraft
can't get to V1 and then stop, then the runway is by definition too
short for the aircraft to operate from.  (Both of these conditions
carry a safety margin.)  This is necessary since even though turbine
powerplants are highly reliable, they're not infallible, and takeoff
is the time when the engines are being asked to provide the most power
under the least favourable conditions -- this is particularly true after
rotation but just before actual liftoff when the airflow into the engines
is most off-centre.

Because of this requirement, and the desire to have twin-engine aircraft
able to operate from any reasonable runways, twins must not only have
enough power to operate on one engine, but enough that getting from V1
to V2 on one engine doesn't require too much more runway.

That principle is perhaps embodied most of all in the B757 which combines
a wing that performs very well in low speed conditions and (I believe) the
highest power to weight ratio of any large airliner.  The 757 not only can
take off on one engine, it wouldn't even be breathing hard to do it!  8-)

[Moderator's note:  Concorde with afterburners beats a 757, and probably
the Tu-144 as well.  Other than that, yes, the 757 is tops going by
thrust/weight ratio.  -- Karl]

-- don