Re: Early flap retraction

From:         cpeters <cpeters@sparc1.castles.com>
Organization: North Bay Network, Inc. news server - not responsible for content
Date:         20 Apr 96 14:02:47 
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Steve Howie wrote:
> I could have sworn I saw the flaps on the trailing edge of the wing fully
> retracting about 30 seconds or so into the climb, which seems very early
> indeed.

> Is this a normal occurence under such "favourable" take-off conditions?
> Would the extra lift afforded by the headwind make flap use less
> significant? Or were my eyes maybe playing tricks on me?

This is certainly possible, especially if the plane was light. First, let's
deal with the headwind. The plane does not get extra lift from a headwind -
it only changes the ground speed at which takeoff occurs (and takeoff roll
distance). The plane cares only about its speed in relation to air. So if
takeoff is at 130 knots, and you have a 20 knot headwind, the groundspeed
will be 110 knots.
The second issue regards weight. If an aircraft is light, it needs less lift
to become airborne, thus less speed rlative to the air, and uses less runway
to reach the lower speed. If the 737 was light, it may have a takeoff speed
40-50 knots less than max gross. It will reach takeoff speed in a shorter
distance, and accelerate quickly. In your flight, I imagine the plane was
light, reached takeoff speed, and then accelerated in the climb quickly
beyond the flaps up speed.
For every flight on any airliner, the aircraft weight (empty+cargo+pax+fuel)
is added up, then takeoff speeds calculated or looked up from a table based
on that weight. Thus a 747 at 600,000 pounds may have a rotate speed of
140knots, V1 (liftoff and climb) of 150, whereas at 650,000 it may be 146 and
158. Flaps up will differ,too. After the cruise, the weight is estimated
after subtracting fuel consumed, then landing flap settings can be looked up,
with the same principles applying.
Hope this helps,

Carl Peters