In looking at either of the two "failing"
pitching moment graphs presented in the article (VGT 1 or VGT 2), a pilot might
be tempted to conclude, " Well, clearly I wouldnt want to pull in on
a glider like that. Look at how the pitching moment goes negative at lower angles
of attack!" And to so conclude would be a mistake, as it fails to recognize
the relationship between stability and center of mass location.
This relationship has often been illustrated
by noting that a paper airplane is normally folded so as to increase the amount
of paper, and therefore the amount of mass, in the nose. It will generally fly with
increased stability if you add weight to the nose, yet it will try to fly backwards
if you add significant weight to the tail. In other words, an aircraft is more stable
when its mass is distributed more towards the front. Airplanes have weight and balance
limitations so that the pilot does not inadvertently load the airplane so as to
move the center of mass too far aft for adequate stability.
With the computerized pitch test vehicle, we
have the option of plotting the pitching moment graphs around various center of
mass locations. (In normal certification documentation, the graph is plotted about
the reference point of the pilot hang point.) The first graph below is the graph
of the stable configuration (VGT 3) re-plotted about a center of mass location that
corresponds to the pilot pulling forward about 18" from trim (bar somewhat
below the pilots waist). The glider in this configuration is very stable.
It has a single trim point, (the place where the curve crosses the x axis) at about
seven degrees keel angle, which would correspond to a fairly fast flying speed.
At angles of attack below that, the nose up pitch pressure rises sharply, and continues
to rise as the angle is reduced. At angles of attack above seven degrees, the nose
tries strongly to pitch down. Note that the top of the scale on the pitching moment
axis in this graph is six times higher than on the original graphs - in other words,
had we plotted this graph on the original axes, the slope would have been much steeper.
Pilot Forward - Stability Click image for a higher resolution view
The second graph shows the stability plotted
about a center of mass corresponding to a pilot who has pushed out about 18"
(basically full arms extension). Note that the glider has a trim point at about
32 degrees angle of attack (this would be above the angle at which the entire wing
is stalled). Over the range of angles of attack down to zero, the glider has a positive
pitching moment, though below about 12 degrees positive it begins to decrease rapidly.
After zero degrees, the gliders pitching moment becomes negative and as the
angle of attack decreases further (or increases in the negative direction) the negative
pitching moment increases rapidly. Note that the scale on this graph is also magnified.
Pilot Aft - Instability Click image for a higher resolution view
It should be obvious which graph youd
rather be flying if you happened to be in the process of pitching down through zero
angle of attack. In the first graph, you would have a strong and strongly increasing
tendency to pitch nose up, which would become greater and greater the farther nose
down you pitched. In the second graph, you would have a diminishing nose up tendency
while still above zero degrees, but then experience a strongly increasing pitch
down tendency as you transitioned to negative angles of attack. This portion of
the curve is a classic illustration of what is termed pitch divergence. The glider
is pitching nose down and the farther it goes, the stronger is the tendency to pitch
farther nose down.
So the moral of the story is keep your center
of mass forward in rough air - fly with a little extra speed, and dont try
to max out the really violent thermals by pushing all the way out.