Re: An-124 and wing dihedral

Date:         06 Jun 98 15:38:57 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>>I imagine that the 747 wings are stiffer than the 777
>>wings because of the extra bending moment from the outboard
>>engines.

>	I am open to correction  but I thought that one advantage
>of the L1011 over the DC-10 was that its engines were further outboard

I understood it to be an aerodynamic advantage -- having engines on the
wing which are closer to the fuselage increases the interaction between
the two airflows in undesireable ways.  (I'm sure the aerodynamicists
on the group will correct my oversimplification of this.)

But you also have to think about where the center of lift is for the
wing.  It's reasonably close to the fuselage, but not too close.  It may
be the case that the L-1011's wing engines are very close to this spot.
By moving them inboard of this spot, the DC-10 requires more structure
to support the now "off-center" weight.  Similarly, the outboard engines
on a 747 are probably well outboard of the center of lift, again
requiring more structure.  (Aerodynamics are better, though.)

>(caveat: because the tail engine on the DC-10 was off the centreline viz.
>one engine out scenario)

The L-1011 has the same issue.  Are you thinking of the fact that the
DC-10's #2 engine is so high above the centerline, whereas the L-1011's
is right on it?  That would affect pitch, not yaw, which is what the
placement of the wing engines influences.  If the #2 engine on either
aircraft is shut down, it simply because a sluggish twin.

My understanding was that the DC-10's wing engines are closer to the
fuselage than those on the L-1011 because they didn't want to put an
enormous tail and rudder on the thing.  This is at least partly an
artifact of the #2 engine placement -- the L-1011 appears to have
greater rudder area, without significantly different tail height,
because the S-duct allowed them to get the engine out of the way.

>Relatedly I am pretty sure the `clean wing' VC-10 needed a stiffer/heavier
>wing than the 4-engine 707  for the same sort(s) of reason(s) back in the `60s.

Perhaps because it needs to support a relatively heavier fuselage --
which is heavier both because its weight includes the engines but also
because it must include the structure to support those engines, neither
of which is necessary on the 707.

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Karl Swartz	|Home	kls@chicago.com
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"The average dog is a nicer person than the average person." - Andrew A. Rooney