Tow Plane CG
by John Derstine
John is the designer of the Pegasus tow-plane, and a US importer for EMS sailplanes.TOW PLANE CG
I wanted to comment on Paul Kopp’s excellent article regarding the CG on tugs, and especially the Pegasus, (with which I have some experience). Not knowing what the best way to do this, I chose this forum to hopefully access as many “tuggers” as possible. The general consensus is, as Paul suggested, that a forward CG is desirable for towing.
Two reasons: one as stated, which is better tracking and tug handling in turns, and the second which was mentioned as possibly a detriment to towing, having the tug nose down at low throttle. This second characteristic I would count as a plus also, (given two requirements having been met). They are: excess power, and plenty of elevator throw. I find, and this is a personal preference, that having the tug point towards the ground after throttle reduction helps establish the descent from high altitudes without pilot input, and two, acts as a safety feature if in fact you lose sight of the tug, cut power and you know it is coming down.
Certainly there is a possibility of way too much nose heaviness. The danger lies where you do not have the elevator authority to flair sufficiently for landing, which I believe Paul referred to in his comments. I try to strike a balance so that I get one (a nice downline at low throttle without any stick input), without the other, (insufficient elevator authority). The Pegasus has very interesting aerodynamic characteristics, belied by its meat and potatoes look. The aerodynamic center for those interested in such things is about 70% of chord. Which means the CG range is phenomenally wide. This is due to the effect of a long tail moment and relatively large tail volume, as well as a narrow relative chord. So flying at 40% percent as Paul suggested, is still very much in the middle of the controllable flight envelope. However, don’t take this as a recommendation to move the CG to 65%!!
I generally would recommend a CG within the width of the 1.5″ wing tube. I try to set up the Pegasus’ I fly so that when I land after a no power dive from altitude, I can flair at the end of the runway, and the sheer surface area of the plane, and prop disc will act like an air brake allowing the plane to settle in without flaps. The new Pegasus II with slightly longer wings tends to fly out a little more in windless conditions. The rule of thumb, then, would be to make sure you have control of the plane in all flight modes, with sufficient elevator throw to control the plane at low throttle settings, and a CG that gives you the flight characteristics you are comfortable with, without sacrificing safety. Happy towing.
EMS WING CONNECTORS
P.S. I am hoping to be able to supply complete wire harnesses for all EMS planes in the future, these to include a plug in wing 9 pin connector at the root. This then, would be the easiest way to install your servos in EMS scale sailplanes.
Thanks for your comments John!