Emergency Procedures Information Supplement - Piper PA-24-250 COMANCHE Pilot Operating Handbook

AircraR Publications
Pipcr Colnnnchc 250 (Early Modcl)
Section
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3
Emcrgcnq
Procedures
EMERGENCY PROCEDURES INFORMATION SUPPLEMENT
Engine-Out Glide Speed:
During the era when Piper Aircraft was producing the Comanche, no flight tests were conducted
to determine the best engine-out glide speed for the airplane. The one exception to this was the
Turbo 260C, and it is estimated that only two dozen were built. The figures in this Handbook
have been determined by the following method:
Use of the term "best" is a misnomer, however, best glide speed is most generally referred to as the
optimum, or maximum-range glide speed, and results in the best glide ratio.
Best glide ratio is obtained when the wing is operated at an angle of attack that will produce the
best lift-drag ratio, or L/DmzY. This is basically true of the airplane's best rate-of-climb speed also.
Theoretically, optimum glide speed will be close to the best rate-of-climb speed, but included
among the variables in the mathematical formulas related to the best rate-of-climb speed are the
elements of thrust and drag. Because efficiency is reduced by a dead engine (thrust is now zero),
and airplane drag is increased (due to the windmilling propeller), optimum glide speed can be
expected to be a value somewhat less than Vy.
The generally accepted formula for estimating best engine-out glide speed in a typical
reciprocating-engine, propeller-driven, light airplane when it is not provided by the aircraft
manufacturer is to multiply 1.4 times Vsl.
Vy for this model of the Comanche is 105 mph, and 1.4 times
Vsl
is 99.4 rnph. Therefore, for the
purposes of this Handbook, the best engine-out glide speed for the Comanche 250 at 2800 lbs
maximum allowable gross weight has been established to be 100 mph IAS. Actual glide tests
conducted in a Comanche 250 support this figure.
Glide testing done on sub-sonic aircraft by the military has produced graphs which show that a
five-percent deviation from best glide speed will not cause a significant reduction in glide ratio.
This means that if this figure is not exactly correct, the error is not enough to produce a
measurable difference.
In addition, since optimum glide speed decreases as the airplane's gross weight decreases, this fact
also allows the specifying of glide speeds for a range of gross weights. An example of when use of
a lower glide speed applies would be a solo pilot who is totally out of h e l . In this case the
airplane would be several hundred pounds below maximum allowable gross weight, and use of an
airspeed below 100 mph IAS would be appropriate.
Airplane Gross Weight
Suggested Glide Speed
2800 lbs ...................................................................................................
100 mph (87 kt)
2600 lbs ....................
. . .
.................................................................................. 96 mph (83 kt)
2400 lbs
..........................
. . . .
...........................................................................
92 mph (80 kt)
2200 lbs ................................................................................................................ 88 mph (76 kt)
Copyright:
1993
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