A New Twist on Retractable Motors

Dean Sigler Batteries, Electric Aircraft Components, Electric Powerplants, Sustainable Aviation Leave a Comment

GP Sailplanes in Poland recently partnered with MGM Compro of the Czech Republic to add electric power to its line of small, sleek sailplanes.  Concentrating initially on the 13.5 meter racing class (44.29 feet), GP’s craft provide high performance with light weight and compact dimensions.

A Long Reach on the Antares

Putting a retractable motor and folding propeller into such a tight space required some clever engineering.  Think of the tall mast on the Lange Antares or the Arcus two-seater.  These are large, heavier machines, so the propeller, mast and motor can be accommodated (tightly) in their fuselage.  The video gives a good view of just how close the quarters are on the Antares at about the 3:14 mark.  With a 118-square-foot wing area and 59-foot wing span (the 18T model), the average chord of the Antares is two feet.

Tucking the Motor into a Tighter Space

GP’s aircraft are much smaller, with narrow chord wings, part of their high aspect ratios.  That gives a smaller center of gravity range.  RecreationalFlying.com suggests not exceeding a CG range of 20 percent – less than eight inches on a typical trainer.  For the Antares, that range is less than five inches.  The GP14 SE has a mean aerodynamic chord of about 20 inches, so the CG range is closer to four inches.  Constrained by a need to keep CG travel to a minimum, the designers came up with an ingenious solution, something they label RESLS – retractable electric self launching system.

Pivoting the motor on the mast and folding the propeller allows retraction into the fuselage through a small opening – easier to seal for lower drag.  Having the motor in its most rearward position during takeoff roll and climb-out also probably eases trim requirements.  When the motor, mast and propeller fold into the fuselage, their mass is close to the center of gravity, keeping pitch moments tight.

A Lightweight, Compact Package

Weighing only 374 pounds (170 kilograms) empty and 924 pounds at maximum takeoff weight, the GP14 should perform well with its 25 kilowatt (34.5 horsepower) Rotex motor.  The four kilowatt-hour Sony Li-ion battery pack is 44 pounds of the empty weight, and it’s possible to add four kW-hr or two kW-hr packs to extend endurance.  Since these fit in the wings, the center of gravity is not affected.  Wide-body pilots can fit into the standard fuselage, but those lucky enough to be of lower heft can have a slim fuselage option, making the plane perform even better.

The GP14 E with standard equipment costs 82,900 euros ($89,500 USD) and should be a nice addition to the number of electric flying machines now quietly filling the skies.  A 15-meter standard class machine is on the way, and will have an RESLS available.

Thanks to Martin Dvorsky of MGM Compro for alerting your editor to this news.

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