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Re: Photograph



Brian Whatcott wrote, in regard to equipment tipping under torques:

Some large cargo planes are fitted with jury struts while loading
is in progress to avoid such curtseys.
Some large modern passenger planes are fitted with loading indicators
which may sense gear leg compression.

Boy, you guys can sure bring back memories. Year ago I was flying a
DC-4, which had a fitting under the tail for just such a jury strut.
It was part of the aircraft equipment. When the plane stopped the
first thing the crew chief did was install the pins in the landing
gear struts to prevent them from accidentally retracting, and the
second thing was to install the jury strut on the tail to keep the
plan from tipping.

The DC-4 was designed with the CG near the landing gear (actually, it
was the other way round--the landing gear, for convenience of storing
them in the engine nacelles, were located near the aircraft CG), so
it was fairly easy to momentarily get the CG aft of the landing gear
while the plane was being loaded. I remember sitting in the cockpit
on more that one occasion when the plane started to rock back and
rested on the jury strut under the tail. In fact, I think at one time
I had a picture of the plane with the nose wheel off the ground and
the jury strut firmly planted there, keeping the plane more or less
level.

When changing engines on those old planes, we used to routinely wedge
a maintenance platform under the tail, because removing an engine
moved the CG far enough aft to allow the plane to tip if it wasn't
supported.

Of course, that's not the only concern about the CG with regard to
airplanes (or boats, for that matter) Their stability depends
critically on the relative position of the CG and the center of lift
(center of buoyancy, in the case of boats). Get the CG above the
center of lift (or the center of buoyancy) and the plane (or boat)
has a tendency to flip over. This is a bigger problem for ships than
for aircraft, because ships don't normally have controls to correct
for that condition.

The main problem for airplanes is to keep the CG close enough to the
center of lift that the plane's controls can compensate for the
torques introduced. The job of the loadmaster (or whatever the
equivalent is in the civilian world) is to make sure that the plane
doesn't leave the terminal unless the CG is well within the allowed
range. And it is the job of the pilot (or flight engineer, on those
planes that still have them) to manage fuel use so that as the fuel
is consumed during the flight, the CG doesn't move outside of the
safe limits. With large aircraft, they often have fuel tanks located
all over the place, and so the fuel system frequently include pumps
in the tanks which allow fuel to be pumped from one tank to another
to keep the load properly balanced.

We often wonder why those large aircraft have two or sometimes three
people in the cockpit. It turns out they have plenty to do beyond
just keeping each other awake (although that is a factor as well).

A nice classroom demonstration of stability (and the close
relationship between static friction and rolling friction) is to take
two (large--6-8 inches long, so they can easily be seen) toy cars and
put them on an inclined plane. Tape the front axle on one and the
rear axle on the other, so that the taped wheels don't turn. Then get
the class to guess which car will roll straight down the ramp and
which won't. My experience has been that most students will guess
that the one with the taped rear wheels will be stable and the other
one will not be.

They are quite surprised when the opposite proves to be the case. The
explanation is that once the rear wheels are locked, they are
providing less of a retarding force that the front wheels, and so any
misalignment of the front and rear wheels will cause the car to start
spinning about a vertical axis unstably, so that the small
perturbation gets magnified. This illustrates why two features of
many modern cars, front-wheel drive and anti-lock brakes are
extremely helpful in keeping cars under control in reduced friction
situations.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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