Tilt wing V/STOL aircraft

Model Airplane News, Nov 1998 by Hunt, Tom

BOB KRESS, retired vice president of Grumman Aerospace, has been designing and testing V/STOL (vertical/short takeoff and landing) aircraft for over 30 years. His recent endeavor is a "feeder-line" tilt-wing aircraft capable of conventional and vertical takeoffs and landings. Feeder-line aircraft service smaller airfields with regular service to larger metropolitan airports.

The ideal aeronautical solution to this form of "people transport" is to be able to ascend vertically, like a helicopter, but also have the speed of a conventional turboprop, and practical aircraft to accomplish this have long been sought. The V-22 Osprey has great load-carrying capability (cargo or passengers) but is still relatively slow, and it cannot land conventionally. The rotors are much too large to remain horizontal without striking the ground.

The forward-swept wing (FSW) V/STOL concept shown in the pictures will provide rapid service -into and out of larger airports or remote areas, with or without runways. The propellers are small enough to clear the ground with the wings in the "down" position. The "all-flying" wingtips will provide pitch and yaw control. Roll control during hover will be implemented through differential power and/or thrust-spoiling devices on the nacelle.

Since complete control of the vehicle can be accomplished with aerodynamic devices, no tail rotors (horizontal or vertical) or cyclic-pitch main rotors need be employed. That alone makes this concept stand out from other tilt-wing and tilt-rotor-type V/STOL aircraft.

The test model was built to demonstrate the ability to hover and maneuver such an aircraft using aerodynamic devices alone. Construction with conventional model materials (balsa and foam) and use of off-the-shelf propulsion and guidance systems were also a requirement.

Originally conceived as a single, internal-combustion-engine model with the engine driving two large-diameter model propellers via belts, the vehicle was quickly converted to an electricpowered model when overheating of the engine became a time-consuming problem to solve. The model, in its current state, uses two Aveox 1415/4Y brushless motors and controllers. The motors are mounted to Robbe 3.7/1 planetary gearboxes, which in turn drive 22x10 (modified) Zinger props.

Thirty cells per motor provide the necessary thrust to lift this 20-pound model into the air. Hover current is near 30 amps, and maximum power is a little over 40 amps. Hover flight times on 2000mAh cells are a little under 3 minutes-sufficiently long to investigate flight qualities.

The model has gone through many "design" changes to optimize stability and control in the hover mode. Various-size outer wing panels, rollcontrol vanes, fences and even "tuft" studies have been done to fully understand the problems and advantages of this type of control scheme on a V/STOL aircraft.

-Tom Hunt