Newsgroups: sci.aeronautics.airliners
From: David Lednicer <dave@amiwest.com>
Subject: A320, MD-80, 727
Date: 01 Dec 93 03:16:29 PST

	In reply to the question as to why MD-80s and 727s have their ventral
stairs down on the ground: it keeps them from tipping over (tail down)
should someone goof when loading them!  In the AIAA "Case Study in
Aircraft Design: The Boeing 727", Mark Gregoire relates a story about
when the first 727-200 was delivered to National Airlines.  "As it rolled
to a stop near the National hangar, amid the expectant dignitaries, the
pilot touched the brakes and the airplane nose went down and then recoiled
up and lifted the nose gear off the concrete approximately 6 to 8 inches.
The gasps in the crowd were heard 3,000 miles away in Seattle.  Bill Clay
put a team together and, armed with weight and balance data, toured the
airlines outlining the entire spectrum of configuration control, ground
handling, ballasting, and precautionary measures from sloping ramps to
heavy snow loads on the tail.  As far as we know, no 727-200 has ever sat
on its tail and maybe we over reacted to the National incident, but that's
why, you will nearly always see a 727 with its rear airstairs down when
parked.  There are some rare cases where we attach lead to the radome
bulkhead for extreme loading conditions."

	As to why the Mercure looks so much like the A320 - yes it is
related and no, it isn't.  The Mercure was built by Dassault, which is a
(nominally) private French aero company, while Aerospatiale, which is part
of Airbus is the publicly held French aero company.  Back in the gestation
of the A320, Dassault was involved in one partnership which looked at a
growth Mercure with CFM-56s.  This partnership split, but begat other
partnerships that lead to the A320.  You might say that the growth Mercure
genetically led to the A320.  On the other hand, Dassault was not involved
(directly) in the design of the A320, so any relationship is distant.


-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052         |   fax:     (206) 746-1299

Newsgroups: sci.aeronautics.airliners
From: ehahn@fairlite.mitre.org (Ed Hahn)
Subject: Re: DC 10-ski topples on to its rear when unloaded!
Date: 23 Feb 94 12:05:49 PST

   I remember hearing about the need for ballast in the nose on some British
   air-craft due to union out-cries when the planes were outfitted with
   mechanical stairs in the front.  After trying to simply remove this option
   it was found that due to ensuing balance problems they had to put concrete
   in the nose to solve the problem.  Could this be related to this discussion?
   ______________________________________________________________________

   Christopher Hutton
   dmitri@acs/acs2/cgl/hendrix.bu.edu
-------
I don't know about the VC-10 or the Il-62, but on the DC9-1X series,
you can't remove the forward airstairs without carrying the same
weight in ballast.  I understand that TWA, for example, is currently
removing the airstairs from all of its DC9s EXCEPT for the DC9-15s it
operates to free up some weight.

I also seem to recall that the MD80 requires ballast in the forward
cargo compartment when being towed around empty to prevent inadvertent
wheelies on the taxiway.  These were on aircraft without the airstairs
installed.  I think that the ballast only had to be added when there
was less than a specified amount of fuel in the center tank...

BTW, there is a tail skid on the B727, but I think it is only there to
help if there is over rotation on takeoff.  Also, the aft airstairs
now have an airspeed switch that prevents the aft airstairs from being
lowered in flight.  Maintenance crews occassionally call this the "DB Cooper"
switch.

ed
////////   Ed Hahn | ehahn@mitre.org | (703) 883-5988   \\\\\\\\
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: A320, MD-80, 727
Date: 28 Nov 93 16:39:03 PST

In article <airliners.1993.764@ohare.chicago.com>,
Brandon Walts <bwalts@lamar.ColoState.EDU> wrote:
>I have 2 completely unrelated questions that I've always been
>curious about.  Perhaps someone here knows the answers...
>
>1)  Why do 727s and MD-80s always have their ventral stairs down
>when parked at the gate.  I never see any service or maintinence people
>using them.  Is it for faster emergency egress in case of a refuelling
>accident?  Does it somehow hold the tail up when there's nobody in the front
>of the plane (ala DC8 and IL-62)?  It seems like a lot of wear on
>the hydraullics and mechanical systems, so there must be a good reason...

Typically it is to keep the airplane from tipping back on its tail.  Rear
engined airplanes have chronic problems with CG location empty.  They have
other chronic problems, but I won't go into that.  :-)

The CG problem stems from having the CG of the empty airplane well aft
of the CG of the payload (the passengers and baggage).  When summed
together, the CG of the airplane system must be within a relatively small
range defined by the stability and control requirements and tail power
available.  The landing gear like to be pretty close to the CG of the
loaded airplane in order to allow easy rotation at takeoff.  So, when the
airplane is NOT loaded, the CG moves aft - very close to the main gear -
and someone walking around in the back, or loading cargo into the aft
cargo compartment, can easily cause the airplane to tip back.

In fact, if one pays close attention to cargo airplanes with the main deck
cargo door aft of the wing you will notice the ground crew using a tail
prop (called a 'pogo stick') to keep the airplane from tipping back.  This
is regardless of the engine configuration.  Shoving a heavy pallet onto the
main deck gives a dynamic loading of twice the actual pallet weight, and
can cause an empty unpropped airplane to tip.  In this particular case, tip
back is quite serious with an 8,000 lb (or heavier) pallet running the length
of the cabin to impact the aft structure.  This is considered poor form by
most crews.  :-)

No doubt this is more than anyone really wanted to know about tipping
airplanes back on their tails.  :-)


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

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: Aircraft not toppling onto rear (VC10)
Date: 11 Feb 94 03:55:05 PST

In article <airliners.1994.922@orchard.chicago.com>,
Gregory R. TRAVIS <greg@bronze.ucs.indiana.edu> wrote:
>In <airliners.1994.918@orchard.Chicago.COM> julian@gatwick.sgp.slb.com writes:
>
>>c) Overly strong fuelage and wing construction (Comet legacy) so lots of
>>   weight ahead of the main wheels.
>
>Comet legacy?  I distinctly remember several Comets coming apart in
>mid-air.  Or are you saying that the redesigned Comets were over-engineered
>to avoid embarassment?

I rather doubt that the Comets are over-engineered for their time, the
1950s.  Granted, they missed the stress buildup around the window holes and
forgot to put in a flange to give themselves more area (P/A you know), but
the big issue in that time, IMHO, was fatigue life vs weight (come to think
of it, that is still a big issue driving gobs of materials work).  Look at
the DC-8 and the 707, and the Lockheed Electra, all designed in that same
era with essentially the same alloys.  Tremendously long lives.

>>d) 90% + of payload located ahead of main wheels (I think that's about right).
>
>Is this true?  How the heck do you rotate, or flare, a VC10?  I find
>this hard to believe, considering they have four aft-mounted engines!

Um, I don't think so either.  Typically, you'll find the main gear a few
tens of inches behind the aft CG limit.  Therefore, you shouldn't expect
more than 55% of the airplane's weight in front of the main gear.

But, after another millisecond's thought, I note that julian wrote 90% of
the PAYLOAD is forward of the gear.  Not that I agree with that either,
even Douglas puts less than 70% in front of the mains.  That number is
eyeballed, by the way, and is strictly my uninformed opinion, your mileage
may vary.  Karl probably has better numbers than I do.

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."

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: Vickers VC10 (4 aft engines+ T tail)
Date: 23 Feb 94 12:05:41 PST

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."