Nose-down attitude

From: (Paul Raveling)
Organization: Unify Corporation (Sacramento)
Date:         26 Jan 93 23:47:07 PST
View raw article
  or MIME structure

	Last week, while various combinations of our news server
	and network were in shambles, Geoff Miller wrote...

> The fuselage of the DC-8 has a pronounced nose-down attitude on the ground,
> which is especially noticeable with the stretched Super 60 and -70 models.
> Does anyone know the reason for this?  It occurs to me that the designers
> might have had the possibility of a fuselage stretch in mind, ...

	Without knowing actual engineering history, I'd note that the
	DC-8 is among the more long-legged airliners and would speculate
	that these could be reasons for that choice:

	1.  Safety:  Having a nose-down attitude on the ground reduces
	    the wing's AOA on landing rolls.  This puts more weight on
	    the gear, which enables better braking action...
		[braking force = coefficient of friction * vertical force]
	    and the difference probably is most significant on wet,
	    icy, or snowy runways.  The same effect helps directional
	    stability as well as braking.

	2.  Economics:  A shorter nose gear is lighter.  In the DC-8's
	    case it probably translates to either (a) ability to carry
	    a couple extra paying passengers or (b) leaving behind a
	    proportional amount of fuel, which reduces gross weight
	    some more.

	3.  Safety (minor):  A nose-down attitude improves visibility
	    over the nose slightly for taxiing, admittedly a very minor
	    effect though.

Paul Raveling