Re: Cruising speeds - 727, 737, 767, 747

Date:         04 Jun 97 13:04:01 
From:         "Mark E. Ingram" <markt@mickey.mo-net.com>
References:   1
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On 23 May 1997, Ilkley Bear wrote:

> In this book, the comment is made that the 747 cruises at what is,
> anyway, the optimum speed for its size. The 737 cruises faster than it
> should, because otherwise it would make difficulties for Air Traffic
> Control. Any comments?

Chas, I am no aerodynamicist, but having flown the 737-100/200 as a pilot,
believe that this comment has some validity.  For best fuel conservation,
we would use a cruise speed of Mach .74.

However, other airliners, including 727's and MD-80's, can cruise
comfortably at M .78 to M .82.  We used to cruise the DC-10 at M. 84.

So, if a 737 is "leading the pack" in a line of flights flying the same
route in the same direction, then the 737 (200) can be bumped up *almost*
to M. 78 to keep the others from having to slow too much.  I say "almost,"
because even though the 737's MMO (maximum Mach) is supposed to be up
around M .80, at M .78 there is some trans-sonic buffeting (from the tail
section, I think) that starts to make the ride uncomfortable for the
passengers *and* pilots.

Just shy of the magic M .78, the 737-200 burns a lot more fuel than at M
.74, and the wind noise in the cockpit becomes pronounced (IMO).

Mark E. Ingram

MarkT@Mo-Net.Com (also mingram@mail.orion.org)