project pattern
T
Redux
he February installment of Project Pattern Redux finished with a throttle > aileron programmable mix (Pmix) that, when properly set
up, would reduce pilot workload. The throt- tle > aileron Pmix counteracts torque pro- duced by single propeller propulsion sys- tems. Torque is not the only troublesome byproduct of a single propeller; gyroscopic precession, propeller factor (p-factor), and spiral airflow are additional gremlins that adversely affect the trim condition of an aer- obatic airplane. Delving further into these adverse effects and how to mitigate them is the focus for this month.
While a great deal of information on torque, gyroscopic precession, p-factor, and spiral airflow exists in aerodynamic texts, the applicability to radio controlled aerobat- ic planes (whether 5-ounce indoor foamies, or 40% sized 50-pound scale planes) is com- monly debated. To the best of my knowl- edge, my self-taught trimming methods/ap- proach do not contradict any classic aerodynamic theory.
Specific to the aforementioned topics, my knowledge benefitted greatly from a pair of identically built and trimmed Bravos. The #2 ship was converted from a traditional single prop drive to the Brenner Sharp Contra Drive (Contra), with no other changes. Brenner’s Contra uses a single motor to power a pair of counter rotating propellers through a geared differential system, balancing the torque be- tween the propellers. I have never noticed any trim issues typically attributed to torque, gy- roscopic precession, p-factor, or spiral airflow. Post conversion, the #2 ship initially flew quite poorly, requiring substantial re-trim- ming (or un-trimming) to fly properly. The “un-trimming” process validated the various offsets, mixes, and asymmetries I had imple- mented to achieve the best state of trim with the single propeller.
As the vast majority of aerobatic aircraft are powered by a single propeller, the bal- ance of this text is specific to single prop air-
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
Even a well-used stick plane is a valuable aid for evaluating and deciphering trimming interactions. PHOTOGRAPHY: DAVE LOCKHART
planes. I am not the most adept at producing schematics and diagrams, so a bit of web searching on the topics discussed may be helpful to provide visuals. Alternately, a live, interactive 3D model could be even more helpful, a sample of which is pictured. Gyroscopic precession describes the reac- tion of a spinning object when a force is ap- plied. When a force is applied to a gyroscope, or spinning propeller disc, the reaction of the gyroscope is rotated 90 degrees from the applied force. Gyroscopic precession has no effect unless the aircraft is maneuvering in pitch or yaw, and is greatest during high-G maneuvers. For aerobatic aircraft, this means a positive corner will result in “pull” to the right, and a negative corner will re- sult in a “pull” to the left. Similarly, yawing with right rudder will result in a pitch to the belly while left rudder will result in a pitch to the canopy.
The magnitude of force attributable to gy- roscopic precession is relatively minor and opposite to the effects of p-factor, so in prac- tice it does not need to be addressed. The ef- fects of gyroscopic precession can be mini- mized by reducing the rotating mass; primarily the propeller, but the spinner and motor as well. Yes, the very expensive super light carbon fiber props and spinners not only save weight, they fly better. P-factor is the term used to describe the asymmetric thrust produced by a propeller disc when it moves through the air at an an- gle. With classical aerodynamics, the overall positive attitude of the airplane to create lift in straight and level flight results in the top of the propeller disc being tilted aft. The tilt results in the descending propeller blade (on the right side) having relatively increased pitch compared to the ascending propeller blade (on the left side). The increased (effec-
Many elements of the trimming process for single prop propulsion systems (above) were validated when the #2 Bravo (at right) was converted to a Brenner Sharp Contra Drive system.
38 MARCH 2014
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