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National Advisory Committee for Aeronautics, Technical Notes - The Use of Multiplied Pressures for Automatic Altitude Adjustments

The efficient performance of an airplane requires that cer—
.rin adjustments be made as the density of the air through which
the airplane passes, becomes changed. Hence,'there is the halti—
tude control" of the carburetor which enables the pilot to mains
tain the desired air-fuel ratio. Recent developments have in—
creased both the number and importance of such adjustments. For
example, the safety of the over—dimensioned engine depends upon
careful manipulation of spark advance and throttle opening and
a proper control of a variable pitch.propeller, if the maximum
performance of the supercharged engine is to he obtained.. It is
evident that there is a real need for satisfactory devices to
make such adjustments automaticallf. The following discussion
deals with a method of automatic compensatiai which Ceserves, but
never to the writer‘s knowledge has received, consideration in
the design of such devices.

Prior to this discussion a few remarks concerning existing
schemes for automatic compensation seem desirable. The majority
of these schemes depend upon some ccntrivance which functions
simarily because of changes in atmospheric pressure. Though
uninfluenced by temperature, these devices are usually arranged
so that the adjustment effected by a change in pressure compen-
sates also for the change in temperature by which it_is normally
accompanied. Hence the inability of these devices to alter an
adjustment solely because of a change in temperature is of minor
importance except in rare instances where the temperature is far
removed from its normal value.' Nearly all of these automatic
.untrcls are essentially the same in.principle. Their chief ele—
-ent is a tight chamber containing a gas (usually airi. When the
atmospheric. pressure changes, the resultant change in the differ—
ence between it and_the pressure of the gas within the chamber
causes motion of some wall of the chamber. This motion effects
the desired adjustment.

In such devices a leak in the supposedly tight chamber means
utter failure. The elimination of this source of danger is one
of the aims of the method of altitude compensation about to be
described. To this end the most important step is a change in
the-method of operating-the automatic‘device° The change suggest—
ed is to make the source of operation the difference between at—
mospheric pressure and some multiple of atmospheric pressure
instead of the difference between atmospheric pressure and.that
Jf a gas confined in a tight chamber. Following are tabulated
values of the difference between atmospheric pressure and twice
and ten times atmospheric pressure respectively.