A flight test method to determine the level flight performance of propeller driven aircraft is reported that measures the amount of power it takes to overcome a known increment of added drag to maintain steady state flight conditions to determine overall drag and propeller efficiency of a general aviation aircraft

Cross, E. J.

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11627
8.5 Determination of the Level Flight Performance
of Propeller-Drlven Aircraft
E. J. Cross
Mississippi State University
A flight test method to determine the level flight performance of propeller-
driven aircraft is currently being investigated at Mississippi State University. By
measuring the amount of power it takes to overcome a known increment of added
drag to maintain steady state flight conditions, it may be possible to determine the
overall drag and the propel ler efficiency of a general aviation-type aircraft.
Propeller efficiency, rip, is defined as the ratio of thrust horsepower to
brake horsepower, or
TV
qp = "550BHP (1)
Equating thrust to drag by the thrust inclination angle Y gives
* DV
np = 550BHP
where ri; = rip cosY.
If an increment of drag AD is added and power is increased such that the
alrspeed remains constant, then
(2)
• * (D + AD)V
np + Anp = 550(BHP + ABHP)
Using o propulsive efficiency factor
np + _np An
E = = i+ -p
p * *
np np
and eliminating drag gives
* _DV
np = 550[ (Blip+ ABIIP)Ep - BHP]
Expresslng BHP as the product of torque, Q, and propeller rpm, n, and
substituting for np in (2) gives the basic incremental drag equation
(3)
(4)
(s)
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https://ntrs.nasa.gov/search.jsp?R=19760003939 2020-03-20T01:23:33+00:00Z
AD
D = (6)
(I +-_)E - I
V P
If there is little or no change in propeller efficiency, Ep 1 and
AD
D = Q _--_ (7)
Thus we have expressed the total drag of the aircraft as a function of three
easily measured parameters. A standard propeller torquemeter will be used to
measured Q and AQ, while a load cell attached to a drag chute will measure AD.
By also measuring ¥, the propeller efficiency np can be computed directly from
(2).
Equations (6) and (7), however, neglect changes in induced drag and
profile drag. It is expected that the profile drag will remain constant, but it may
be necessary to consider AD;, the change in lift-dependent drag. By using a
parabolic polar for the aircraft and including an amount of lift _L, it is possible to
express the drag as
2a0AaW + _ ARe ADD
D = ^_ (8)
AN e [(i+ L'-_)E - i]
V P
where a0 is the slope of the llft curve, Acxls the change in angle of attack, W is
the aircraft weight and AD D is the incremental drag of the parachute. Further', e
is the Oswald efficiency factor and AR is the aspect ratio.
Note that this equation requires measurement of flight test variables and
aircraft parameters that are not included in the simpler forms. If the added drag
is small, it is expected that equations (6) and (7) will be sufficient. However, the
final form of the drag equation which will be most suitable remains to be determined.
The incremental drag method of determining aircraft performance appears
to offer an excellent alternative to current flight test practice. The aircraft lift,
drag and thrust values and propulsive efficiency are readily determined from a
simple flight test procedure. It seems reasonable to expect that sophisticated data
acquisition and processing systems will be unnecessary since the flight test is con-
ducted under steady state conditions. The current practice of determining aircraft
drag by the gliding flight method is tedious and provides no information concerning
404
propulsive system performance. The incremental drag method appears to provide
the potential for significantly greater flexibility and utility ct reduced cast and
complexity with increased safety.
405
9. ADDITIONAL PAPERS RECEIVED AFTER THE CONFERENCE
9.1 Possible Applications of Soaring Technology to Drag Reduction
in Powered General Aviation Aircraft
J. H. McMasters and G. M. Palmer, Purdue University
9.2 Minimum Vertical Tall Drag
E. E. Larrabee, Massachusetts Institute of Technology
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Determination of the level flight performance of propeller-driven aircraft

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