The Concept

The Katroli helicopter concept brings together four existing technologies, which have been successfully developed and implemented independently, and could now be combined to create a new generation of helicopters.

Origin of the Idea

Recently, a mechanical & aeronautical engineer named Ruben Maier came to me with his vision to develop a helicopter without the problems of conventional helicopters (instability, unsafe tail rotor, vibration & noise level; low cruising speed; command complexity, rpm reduction systems, low pay load; low service ceiling etc.). To solve all those issues, Ruben proposed combining four technologies that have been developed and tested in the past, but never combined:

  1. Tip Jet (TJ)
  2. Circulation Control (CC)
  3. Advancing Blade Concept (ABC)
  4. Command Controls by Mast Tilt

The Katroli concept unites those ideas, and we would like to present them to you.

How It Works

First of all, the Katroli’s power source is electrical; a brushless motor turns a radial compressor feeding air at low pressure, but relative high flow. The air is injected at the bottom of the stationary (non-rotating) mast. The air flows up inside the mast and out along the inside of the blades, with most of the air exiting from a nozzle mounted on the tip of each blade, allowing the blades to rotate. Each blade is attached to a hub equipped with sealed bearings to reduce air loss.

This technology is called ¨Tip Jet¨ (TJ) and it’s already been implemented in the Djinn 1221, the Voljet helicopter (see video), and currently by Zummo Flight Technologies.

The remainder of the air exits from thin slots along the trailing edge of each blade, modifying the air flow and pressure distribution around the blade and potentially yielding at least double the lift of a similar airfoil. This principle is known as “circulation control” (CC).

In the past, TJ had high energy consumption due to loss of dynamic pressure because of blade length and inner section ducts. Today, TJ and CC would be made to work in symbiosis by using short-span blades.

The third technology would be the Advancing Blade Concept (ABC), using two coaxial contra-rotating rotors to make use of the aerodynamic lift of the advancing blades. At high speeds, most of the load is supported by the advancing blades, offloading the retreating blades and eliminating retreating blade stall. The ‘ABC’ technology was used by Sikorsky on its “S69and recently on the “X2” and “S97 raider’’, where it reaches high velocity and approaches the speeds of fixed-wing aircraft.

The fourth and final technology involved in the Katroli is ¨Command by Mast Tilt¨ (CMT), already used on Delta glides, Autogyros, and the Nolan light helicopter.

The stationary mast is linked at the fuselage frame through a gimbal that allows tilt both longitudinally and laterally. This allows the elimination of the traditional cyclic command. Collective command is achieved by changing the pressure discharge of the compressor, which acts simultaneously on TJ and CC, modifying RPM in TJ and lift on CC.

The Market

All these technologies combined might help build helicopters that would use less parts and reduce most common problems. However, a lot of research and testing will be needed to prove all the above hypotheses. The end result could be a new breed of efficient helicopters.


I would like to thank Ruben Maier for submitting this idea to the Future of Aviation ICAO contest in January 2019; David Scorer, who helped improve this description; and Olivier Peraldi for providing feedback on the concept. I would also like to thank Jorge Ciprian, an Industrial Designer from Argentina who designed the Katroli concept. Jorge graduated with a degree in design from the University of Buenos Aires, and he currently works as freelance designer.