Re: Vickers VC10 (4 aft engines+ T tail)

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         23 Feb 94 12:05:41 PST
References:   1
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In article <airliners.1994.933@orchard.chicago.com>,
 <julian@gatwick.sgp.slb.com> wrote:

>Re: My point (D) about the position of the payload. On reflection this does 
>sound a bit daft but if you look at the VC10 nearly all of the passenger
>compartment is ahead of the main wheels and the aft cargo bay is tiny. The high
>'T' tail plane is a long way back and those 4 Conway engines must weigh a bit.
>I would guess about 2 tons each. As you suggest the CofG must be close to the
>wheels or you have major difficulty rotating. Presumably the CofG does not move
>much with a full load so tipping isn't an issue. Can anyone comment further? 
>I wonder how the MD80 manages to rotate????

I still have yet to locate a photo of a VC-10.  :-)  I know what they look
like, and you'd think someone in this office would have one, but no!  Lots
on all grades of shoot 'em up bang-bang airplanes (even as far back as Der
Grosse Krieg), but nothing past 1978 otherwise.  And Jane's says that the
VC-10 was last listed in the 1970-71 edition.

Never mind.  On with more interesting technical discussions.  :-)
Movement of the CG does take place in flight, particularly on swept wing
aircraft with integral wing tanks.  Aircraft with aft-mounted engines tend
to have problems loading out the forward CG limit.  That is, with full
passengers, fuel, and cargo the actual CG of the airplane can easily be
forward of the most forward allowable position.  In that case, the aircraft
does not have enough tail power to rotate at the normal speeds.  And it
doesn't get better in flight; as the fuel burns off, the CG will typically
move forward (it gets more complicated with tail fuel and transfering fuel
out of the center section into the wing tanks as *they* burn off, but in
general...) making it more difficult to flair.  To make up for this, the
aircraft must come in faster, using more field length, making the brakes
absorb more energy (and they will only absorb a specific amount, after which
the airplane is on its own).  BTW, it is not legal to fly an aircraft
loaded beyond either CG limit.  Loading beyond the aft limit is
particularly dangerous as you affect the aerodynamic stability of the
aircraft.

Tip back on this type of configuration is generally a concern when the
airplane is empty, not full.  It is designed to be safe when fully loaded,
but in the past not much attention was paid to the empty condition.  This
is changing.  I've sat in meetings where obscure ferry conditions were
discussed, so we as an industry are learning.  Anyway, the problem is that
the airplane balances acceptably on its gear with anything like a full
payload, but once that mass is removed from the front part of the airplane
the balancing mass aft (engines mostly on the VC-10) stays put, and like a
teeter-totter with only one child on it, the airplane can settle down the 
wrong way.

One further word on the 727 airstair, the previous poster was correct in
that the stair was not intended to prop the airplane; however, in actual
operational use, the stair is put down to prevent the airplane from
tail-tipping.

>Julian
>
>PS: Gentle with those flames - I'm not an aircraft engineer  8-)

Gosh, what happened to our reputation as a genteel group?  :-)

Terry


-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."