Why do I fly pusher props?
1.) I do all of my practice flying on a hard surface circle, w/o an assistant or stooge retaining the aircraft on take-off. With electrics and using tractor props, the airplane noses in when the motor spools up at the beginning of the take-off run. Depending from motor spool-up/propeller/RPM setup, the effect is anything from unpleasant to aggressive and dangerous. Reversing rotation and using pusher propellers solved the problem as it made the nose go out during acceleration, thus keeping line tension time safe during acceleration to lift-off speed. As a welcome side effect, the CW (pusher) rotation allows slow RPM spool-up which in turn makes initial ground roll and lift-of control quite easy.
3.) Due to the added mass of the electric motors rotor, gyroscopic forces being generated by electrics are higher than those of IC drive trains. From this and with tractor props, the resulting force when doing outside loopings/corners yaws the nose of the airplane in. This reduces line tension in the upper looping of the vertical eight and in corners two and three of the hourglass manoeuvre. Reversing rotation and using pusher propellers inverts the effect and consequently increases line tension in the critical same segments of those two manoeuvres.
From the above, I’ve built and flown quite a number of (electric) F2B airplanes being equipped with cw rotating pusher propellers over the past 8 years. All of their fuselages were laid out conventional, i.e. with the motor something like ¾ to 1 ¼ in above the wing centerline and the empennage up to 1 ¾ in up, too. Some had their motor thrust axis approx. 1 ½° down and/or a stab incidence angle of 1° up, too. All flew well in manoeuvres but suffered, some more and others less, from the same weakness:
Altitude holding in level, and more so inverted flight was tricky and required constant pilot attention. From what I have read and observed, other flyers have occasionally experienced the same and I believe to understand that this phenomenon was then called “e-hunting”.
In 2016 I have then built an in-line airplane, and, which in order to get in-line drag, was equipped with a retractable landing gear. See “List your setup”,”Symmetria” post 171, Feb. 25 2016. It flies very well and altitude holding is much easier the ever before.
Encouraged by ”Symmetria” and in order to quantify the effect of CW gyro force and define compensation fuselage design parameters, I’ve teamed with Wolfgang Nieukamp. From his much appreciated calculations the resulting measures are:
Motor thrust axis below wing centerline: 10 mm (3/8”)
Motor thrust axis up: 1.2°
Empennage above wing centerline 20 mm (3/4”)
Empennage LE radius 0.5 mm (0.018")
Stabilizer incidence down: 1°
Opposite direction operating (out with elevator up) Rabe rudder
Fixed landing gear
A new airplane (Aerodynamics as per Igor’s Max Bee) was then built and I have just done the last of the first couple (20) flights today.
It seems to perform very good, doing “Shark” like corners with clean exits. Altitude holding in level and inverted flight is effortless. See “List your setup”,”C.29” post 179, May. 25 2017.
It is of course much too early to draw further reaching conclusions but I thought it might be interesting to share this with the community.
Edited July 13 2017:
Reasons for pusher added
C.29 now regularly flies in competition and proves to fly fully symmetric, w/o "e-hunting" in level and inverted flight
Peter Hofacker's (SUI) new airplane built around the same numbers as C.29 functions as expected.
July 31st 2018:
Flying "My Way" (In-line pusher design) at the 2018 W/C in Landres reassures above findings. 5.2 sec/lap. 9'400 logged constant speed RPM, Fiala (Wood) 13 x 6 E3 Pusher, 1'770 Grams,