Re: Landing speeds

Date:         13 Jul 97 19:52:42 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1
Followups:    1
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On 18 Jun 1997, Peter Coe wrote:

> I was hanging out near SFO at the weekend, and had the opportunity to
> watch a United 777 and a United 737 doing a parallel landing.  The
> thing I found surprising was how much faster the 737 was going.
> I would have said the 737 was at least 10% faster on the approach.
>
> When I first saw them coming in, the 737 was a way behind, and
> much higher, but by touch down the 737 was quite a way ahead,
>
> So what's the deal?  Logic says the bigger they are, the harder
> they fall, but apparently this isn't true with aircraft.

   Other things being equal; if an aircraft were simply to be scaled up
and its density were to remain constant, they, as you say, the larger
one would have a higher minimum speed. However, in this case, all other
things are not equal.
   Wing area is enlarged roughly in proportion to weight for larger
airliners so that wing loading stays about the same, and wing loading
rather than weight itself determines minimum flying speed..
   A modern long range jetliner like the 777 cruises at up to 45,000
feet to reduce drag, and so is designed with a lower wing loading than
a short range plane, and this lower wing loading tends to give lower
speeds at low altitude. The 777 also needs a high fuel fraction for long
range, and at the end of the flight with the fuel consumed it will have
an even lower wing loading.
   Further, Boeing airliners 757, 767, and 777 are of newer design than
the others and use new airfoil sections, what I think are called
"supercritical" wings, which give lower drag at cruising speeds than
older wings, but also serendipitously are thicker and perform better at
low speeds than the very thin wing sections that used to be needed at
high subsonic speeds. These wings have far simpler flap systems than
needed by the 707, 727, and other older planes, and still fly slower.
I do not fly commercially very often, but the difference in landing
performance between a Boeing 767, the first time I rode in one, and a
727 was very noticeable. The 767 was much more responsive to gusts and
floated in ground effect much longer.
   The next model of the Boeing 737, now in development, will have a
new technology wing which will presumably give it similar landing speed
to the other newer jets.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"