News Letter 1941 Jul-Dec - Air Force Historical Studies Office

Et:H~IQlIE
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An epidemic of broken cylinder heads bas conditions the time lag in the propeller
indicated a necessity for a review by the pitch changing mechanism prevents failures
Materiel Division, Wright Field, of the in that the slowing of the engine is not
technique of correctly operating the rapid enough to produce excessive cylinder
throttle and propeller control. The sequence
of events is important. While this upon to prevent failures because it depends
pressures. This feature cannot be relied
information is published in various technical
orders, it is consolidated here for the viscosity of the oil supplied to the
upon the clearances in the propeller and
ready reference and emphasis.
propeller.
In the training type airplane equipped To reduce to a minimum failures oaused
with a two-pod tion propeller and an engine by excessive cylinder pressures, it i. recommended
that the procedure used when re-
having no altitude rating, the technique
of reducing engine power ~ differ somewhat
from that required for satisfactor,y
ducing engine power be:
1. Close throttle or supercharger control
to obtain desired manifold pressure.
operation of high performance tactical type
airplanes.
2. Reduce eZ1€inespeed by operating
Following take-off in a training type the propeller control.
airplane, the throttle may be left in the 3. Lean mixture to obtain the desired
advanced position while the propeller is values.
shifted to the high pitch position without When increasing engine power, the procedure
should be accomplished in the re-
subjecting the engine to any adverse operating
condition. In the tactical type airplane
engine, with its constant speed proverse
manner:
peller and variable altitude horsepower 1. Set mixture control "rich.-
ratings, operating procedure on the above 2. Increase engine speed by operating
method, when redttcing engine power, may
the propeller control.
cause serious overstressing and failure of 3. Open throttle or supercharger control
to obta.inthe desired manifold pres-
engine parts. .
sure. If necessary, readjust the mix.-
When power is reduced as outlined above, ture control and throttle to obtain the
the throttles are wide open, supplying all exact value desired.
air possible to the internal supercharger,
so that the cylinder obtains a heavy charge It is aga.in desired to emphasize the
of mixture. In attempting to burn this point that the above holds true in airplane.
dense charge at a slow engine speed, preignition,
and often detonation, is expericharger.
equipped with or without the turbo superenced.
Failures resulting from these conditions
are cracked cylinder heads, stuck
and broken piston rings, burned pistons A streamlined aircraft maintenance system
and scored cylinders.
is being introduoed at Gunter Field, Montgomery,
Ala., following the efficient
In airplanes equipped with an exhaust production methods of automobile and airturbine
supercharger, the excesd ve strains plane factories throughout the country.
on the engine are accentuated because of
the attempt of the turbine to maintain a Prior to entering the hangar to be startconstant
exhaust back pressure with a 41- ed on their course down the production line,
minished amount of exhaust gases. airplanes will be subjected to a thorough
cleaning process. The aircraft radio equip-
Instances of 50 to 60 inches of mani- ment and other instruments will then be
fold pressure have been reported when cyliil- checked; engines tuned, or replaced where
der heads have fe~led following take-off. necessary; bolts and brakes adjusted, and
Under conditions of excessivemanifold pres- minor repairs effected. Each operation
sure immediate failure or overheating are will be performed at one of a series of
usually experienced. Sometimes the final stations set up along the production line,
failure is noted several hours later when where there will be on hand groups of topcruising
at very moderate power. Under some flight, experienced mechanics who have had
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