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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 (2) 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 • * (D + AD)V (3) np + Anp = 550(BHP + ABHP) Using o propulsive efficiency factor np + _np An E = = i+ -p (4) p * * np np and eliminating drag gives * _DV
(s)
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 AD
D = (6)
(I +-_)E - I V P 1 and If there is little or no change in propeller efficiency, Ep 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 AD D D = ^_ (8) AN e [(i+ L'-_)E - i] V P where a 0 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 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.
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