Re: Do DC-9s exhibit a nose-down attitude under power?

From:         morse@mprgate.mpr.ca (Daryl Morse)
Organization: MPR Teltech Ltd., Burnaby, BC, Canada.
Date:         13 Jan 93 01:30:19 PST
References:   1
Followups:    1 2
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Daryl Morse                     | Voice  : (604) 293-5476
MPR Teltech Ltd. 		| Fax    : (604) 293-5787
8999 Nelson Way, Burnaby, BC    | E-Mail : morse@mpr.ca
Canada, V5A 4B5                 |        : mprgate.mpr.ca!morse@uunet.uu.net

Date: Tue, 12 Jan 93 11:56:08 -0500
From: sharpes@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes)
Return-Path: <sharpes@c-17igp.wpafb.af.mil>
To: morse@mprgate.mpr.ca
Subject: Re: Do DC-9s exhibit a nose-down attitude under power?
Newsgroups: sci.aeronautics.airliners
References: <airliners.1993.41@ohare.Chicago.COM>

In sci.aeronautics.airliners you write:


>I have been told (by someone who had a lengthy career maintaining
>large military transport aircraft) that the DC-9 family of airliners
>exhibit a nose-down attitude under power. This supposed behaviour is
>attributed to the angle at which the engines are (or appear to be)
>canted.

  I have flown on many DC-9s and have not noticed the nose-down attitude
you describe.  I question the engine cant angle causing it.  In steady,
level flight, the pitch attitude equals the angle of attack.  If an
aircraft was flying nose-down, it's because the wing would generate too
much lift for level (constant altitude) flight at nose-level or -up
attitudes.  An example of this would be putting out the flaps near the
max flap speed limit.  In that case, I would expect to fly nose-down.
The B-52 on takeoff and approach is a good example of flying nose-down
(although I don't mean to imply B-52s are near their max flap speeds
during those manuevers!).

>  At the outset, I have to admit that I find it somewhat improbable
>that an aircraft would have that characteristic built in, at least if
>it is pronounced. 

  One reason the engines might be canted could be to minimize trim drag.
This is a bit far fetched, though, because there are so many other
factors that influence trim drag that thrust angle is, IMHO, a secondary
consideration.  (I'm only applying this to high-speed cruise conditions.)

>to the fuselage.  Is that just an appearance or is it a result of the
>nacelle or the engine actually being canted upward at the inlet? Was
>that done to place the inlets in less turbulent airflow?

  The nacelle inlets were most likely canted upward to optimize inlet
performance.  Designers want the air going into the nacelle to be turned
as little as possible.  The more the air flow is turned, the more likely
it is to separate.  This is just as true for inlet lips as it is for wing
leading edges.  Since the inlets are located about 1.5 diameters behind
the trailing edge of the wing root, the air flow will have been turned by
the wing - downwash.  If I had to guess, I'd say the inlets are optimized
for cruise downwash conditions with the landing/takeoff downwash falling
within the design tolerances.
  BTW, if you look closely at the transports with engines under their
wings, you'll see the engines are canted inward (by about 2 - 4
degrees).  This accounts for the effect the fuselage and sweptback wing
has on the upwash flow.

>--
>Daryl Morse

  Dan Sharpes

p.s. - my server won't let me post due to a bug in the initialization
software.  If you'd like to post this to the net for others, I'd be
grateful.

--
Daryl Morse                     | Voice  : (604) 293-5476
MPR Teltech Ltd. 		| Fax    : (604) 293-5787
8999 Nelson Way, Burnaby, BC    | E-Mail : morse@mpr.ca
Canada, V5A 4B5                 |        : mprgate.mpr.ca!morse@uunet.uu.net