FROM AIR2 to CHOPPAIR … our technical quest for  CHOPPAIR CR4


The program of the demonstrator MOSQUITO AIR 2 is accomplished . It is a technical and human success, which opens wide the door to the pre-industrialization program of the “new born” :  the CHOPPAIR CR4.

In order to succeed in this next challenge, we are integrating additional financial shareholders and our current technical advisers into the new capital of Mosquito Europe, which becomes HELYXIR S.A.S.

The CHOPPAIR CR4 , a  two-seater Ultra Light helicopter from  HELYXIR S.A.S , integrates changes, developments,  technical improvements , updates and fine tunings achieved on the demonstrator AIR 2, throughout the conceptional work and its flight tests.

The architecture of the AIR 2 drew heavily from the MOSQUITO AIR designed and manufactured by John Uptigrove,  a Canadian engineer, who managed to live from his  passion by selling nearly 400 MOSQUITO kits worldwide.

From the beginning, the initiator, program director, co-designer, developer and test pilot of AIR 2 and now CHOPPAIR, comparing his flight experiences in ROBINSON R22, R44, R66, ECUREUIL BA, T2, BELL 206, and Ultralight helicopters as CH7, DYNALI H3, LH 212,in the end opted for the MOSQUITO AIR as the basis for following reasons:

– Exceptional stability of the helicopter in hover (ease of piloting)

– Large inertia of the rotor allowing very long and very high flares without jeopardizing the integrity of the aircraft

– Simplicity and accessibility of the mechanics allowing a fast control and easy maintenance –hence economic.

– Ideal tool for teaching and understanding flying

His cost conscious approach consisted not in  reinventing  what works perfectly, but just mixing  the best of the existing, aiming for the tightest price in terms of materials, mechanical assemblies and simplicity of production lines.

The slow adoption of the MOSQUITO AIR2, a helicopter originally designed for amateur builders, led to the  CHOPPAIR, which incorporates numerous changes and improvements as well as further decisive proven industrial solutions, such as a large-displacement 4-stroke engine cooled with air like in the ROBINSON R22 but mounted vertically like in the BELL 47.

The CHOPPAIR is above all, the result of  perfect collaboration  of generations of aviation enthusiasts, young technicians, engineers, experts in design, motors, computer science, programming and power electronics, with experienced entrepreneurs and creative designers.


Why will this pre-industrialization program result in success

1 / Because, its initiator and main financier did not start by approaching the banking system considered as too cautious and suspicious at the first stage of this kind of long term development . The program is designed to reduce expenses to the bare minimum – cost control together with no time constraint .

2 / Because among the team of young people, everyone is planning their own future as a helicopter manufacturer. There is a full identification and dedication of the team to the program.

3 / Because this same team of young (ingenious) engineers convinced some friendly investors to bet financially on them. Financial support eases quicker achievements of the defined steps.

4 / Because this helicopter offers a very reasonable price,  optimal manoeuverability,  technical simplicity and exceptional roominess, it  will become a sensation in the ultralight helicopter world, similar to one when ROBINSON launched the R22 in the world of certified helicopters.

The concept of “affordable flying”, the dedication of people and the technical improvements were the key factors for this success story.


The rest of this article  is more dedicated to aviation enthusiasts and details the evolution of thoughts , concepts , steps,  technical and economic hurdles and choices that flow from it, the intellectual and operational progress of the response team to the hurdles of such a creative program.

7 years ago the DGAC published for the first time rules authorizing flight for ultralight helicopters, class 6 in France. Before that the pilots of these machines flew with foreign equipment , foreign licenses or for some of them completely  “outside of the law”.

Five years ago, the French dealer of single-seat helicopters, after 2 years of experience in this class 6 helicopter business segment , realized that in France there is no real market for single-seat helicopters and that the two-seater aircraft on the market, given their price and sophisticated technology, were only reserved for a small number of elite enthusiasts. Also, schools couldn’t afford to buy 2 aircraft to allow their students to fly alone, once they have obtained  their pilot’s license. The student is obliged to buy his own helicopter. This reduces significantly the development of this activity and eventually of this market.

From this observation is born a strong will to democratize helicopter flying by designing a helicopter that would be  the simplest possible to manufacture, from a solid and proven basis and respecting a simple rule of common sense “Minimum financial impact of the development program must always take precedence over its speed of realization. The Leading factor is cost not time.

You want to blow up the cost of a program !!! Easy, hire several engineers and put them in expensive offices. They will find the best computer and software; most reputable subcontractors; they will implement the most modern ideas that will work for sure because their software has calculated everything and finally they will do everything to compare the performance of the 3 or 4 demonstrators they will have built to validate the best solution. This is called the CADE (Computer Aided Deathly Expenditures).

We didn’t do it that way. We have created a company dedicated solely to this program where only the chairman is operational. Beside it, we have helped brave and committed young people to start their own aviation business. They have an income independent of the program and have started this successful entrepreneurial experience.  On the day the industrial manufacturing starts, they are ready. We are well connected in the regional industry –  near the aerodrome Bourg Terre Des Hommes  in Bourg en Bresse where we are installed. We have gathered around this program, experts who share their expertise and know-how, aware that their interest in the program will pay in the long term. Finally we have sown interest with the local politicians responsible for the airfield where we are based.

So that’s the concept – how did we get here?

The initial objective was to size a two-seater MOSQUITO AIR 2 while keeping the overall architecture of the one-seat MOSQUITO AIR. So we first kept the American materials, the vertical 2-stroke engine, the centrifugal clutch, the toothed belt reductions and especially the fact that the entire bearing structure is fixed on a tube, the tube of mast. The main reasons were the minimum of technical changes and the maximum savings in development time.

1 / Technically a vertical motor does not use a gearbox. The main helicopter gearbox is a large pinion/crown reduction gearbox under enormous continuous loads. The only solution to last a lifetime is to oversize this big block of steel. This implies an overweight hardly compatible with the weight limits of the Ultralight rules. The life of this mechanical assembly is very short on ULM, making prohibitive maintenance costs.

2 / the vertical engine allows to connect all the masses on the mast tube, reducing  to its simplest form the design of the chassis.

3 / 2-stroke engine is the engine that offers, by far, the best power-to-weight ratio, theoretically ideal for ultralight specifications.

4 / the centrifugal clutch is the cheapest and lightest system and can possibly serve as a second freewheel in case of engine failure.

5 / toothed belt reductions do not transmit motor/rotor and rotor/motor vibrations. Belts reduce maintenance time, slip lubricant consumption and absorb transmission noise.

6 / The aluminum blades, compared to carbon blades, are much cheaper, lighter and especially their non-twisting facilitates the autorotation of the helicopter.

The  first building year and the second flight test year proved that the concept Mosquito Air was the right choice .  The two-seater demonstrator behaved in flight in the same way as the AIR (high stability in stationary and high efficiency of the rear rotor).

However it also highlighted serious problems to solve which emerged over time and forced us to question some “given facts”.

The 2-stroke engine is not at all adapted to the specific constraints of the helicopter. The power is there but the fact that a helicopter engine runs at constant speed requires it to receive more or less gas, according to the power required during the different phases of flight. The cylinders are cooled by water but the piston is cooled only by petrol. If you want to reduce the gas without damaging the piston, you need to inject more fuel than needed. This “overdosing” floods the spark plugs, causing random stopping of a cylinder. Losing power, the pilot reopens the gases to recover power. If the unlit cylinder reignites, the power goes up from 70 hp to 140 hp instantly. This makes a shock of extreme violence throughout the chassis and flight controls. If we reduce the mixture to avoid this effect, the piston is deformed under the effect of heat causing  motor clamping. We have gone to the end of our working hypothesis, using more and more sophisticated injection mapping software, increasing the size of the radiators, multiplying the fans, redesigning the exhaust pipes, leading to our conclusion  that the only engine suited is the 4-stroke engine.

All these issues have shown us that the chassis could handle huge shocks and vibrations of a motor wrongly settled. During these tests, only the aluminum structure around the Sprague clutch and the engine reduction had to be reinforced after breaking during a flight test. The test pilot could observe the extreme effectiveness of the rear rotor which enabled easily to contain departures when he either  lost a passing cylinder, 140 HP to 70 HP, or then in the other direction, of 70 HP to 140 HP, when the cylinder turned on again. As far as  stationary failures are concerned that  remained simple formalities that never caused the slightest deformation of the landing gear .


Looking for the best engine solution, the decision of a 4 stroke engine was obvious ..but which one ? We spent almost a year visiting manufacturers and pseudo aircraft engine manufacturers, analyzing the serious options available. We eliminated the turbine engines, rotary engines theoretically best suited to operate a helicopter and also water-cooled engines. 4-stroke water-cooled engine runs at high speed, installing a turbo requires oversize radiators of water and oil. Theses complex options were going against our concept and adding weight.

The final choice is an air cooled engine with a large displacement. Due to the dimensions and the weight/power ratio, Lycoming engine was not usable so we chose the UL Power engine. This company doesn’t modify existing engines but manufactures its own engines. They were able to verticalize their motor and designed a motor mount perfectly suited to our chassis.

The choice of this engine resulted in many essential peripherals changes:

1 / It was necessary to modify the frame on the engine side and its links to the mast.

2 / It was necessary to modify the whole drive from the engine to the main rotor since this engine , in order to deliver the requested power, needs to run at 3200 rpm. It had to be mounted directly on the secondary reduction without primary reduction. To reach this , we had to rethink freewheel and clutch . At this step we made a major technical choice : we chose to avoid the clutch. We have developed a system of pre-launching the main rotor by an electric motor powered by a separate battery. The weight is almost identical but the source of clutch failure in flight is avoided.

3 / After flight tests (engine speed at 2650 rpm ,power 112 HP) to check the suitability of the engine with this helicopter, it was necessary to redraw the rotor head wheel to increase the original reduction, change the belt size to reach the 533 rpm of the main rotor and full power that (130 HP) that the engine delivers at 3200 rpm. Interestingly it turned  that above 533 rpm the extra speed of the rotor causes the destruction of spindles and consumes a lot of engine energy. The performance drops dramatically.

4 / It was necessary to reduce the size of the blades of the tail rotor because previously, they ran at 2650 rpm with the 2 stroke engine, and now at 3200 rpm. The tests showed us that the driveshaft bearings of the tail rotor were less stressed at 2650 rpm than at 3200 rpm. We have redesigned the angle gear box from the main rotor to the tail rotor. The perfect reduction would have been 1.2 but to our surprise the customized spiroconic gears are extremely expensive. Following our concept of “economic standard solution first” we found standard gears ! But the standard reduction is 1.5 which greatly reduces the speed of rotation of the tail rotor and therefore its efficiency. Another issue was then to be solved.

We decided to design and build larger blades keeping the technology in place. This solution is much cheaper than the customized gear. As a positive point the rear shaft rotates even less quickly than originally. Even more importantly, we could source the entire manufacture of a tail rotor blade, from European standard aluminum sheets and machined parts locally. We had “bought ourselves” our autonomy.

5 / The sizes of the gear box created a slight misalignment of the main rotor drive shaft. With a perfect alignment the belt wasn’t tight. So we designed and made a belt tensioner.

6 / The choice of prerotator electric motor made us dive into the world of mechatronics, computer science and power electronics. To operate a new generation electric motor, we have to manage the battery technology, power controllers, servo motors and the programming phases of acceleration. The first prerotator allowed us to validate the principle. We could start the engine but the laps speed was not high. The starter ring gear was destroyed after 100 engine starts. This led us to upgrade the ring gear, we  found a more powerful electrical engine that gives enough rotor speed to start the petrol engine without stress.

7/ Maybe, like us, you think that we had solved all the issue, but NO! The motor starter shock absorption system that protects the engine cooling fan was made with rubber pins.  The first set held about twenty starts, the second one hundred. We had to find another solution, to reinvent a super system. Well no, we have adapted the same principle that has been running for more than 40 years on ROBINSON helicopters, i.e. a fan/crankshaft connection per cone.


In summary, over the past 2 years we have adapted the existing chassis to a new engine, redesigned a large part of the drive, developed the prerotator, the belt tensioner, the fan cone, the rear rotor blades. Who would have believed it ! we are there !  and it flies perfectly!

But it’s no time to sit back and relax. It’s just the start of the next exciting step. In parallel we have already designed the series prototype CHOPPAIR. The chassis is designed around the engine. The solutions validated on the AIR 2 demonstrator are retained, the dashboard looks like the cabin and the cabin is sliding so that the old experienced pilots, the great gifted pilots and the more robustly built pilots all can sit in comfort. There are no more doors that one could forget to close and the lock of the cabin is a central lock. . The rear cover is more enveloping, the tail boom is bigger for a better look, the rear-wheel drive shaft will be better guided, and the main blades will no longer be American, but French with a much longer  lifespan going from 500 h to 2000 h. The raw materials will be European, only the screws will be American because Europe is unable to provide us the equivalent price/performance.

The nomenclature and design folder are being finalized.

The molds cab, rear cover, dashboard and canopy are almost all finished. We still have to assemble the CHOPPAIR series prototype to fly in early 2020. To enhance safety, the CHOPPAIR will be equipped with a safety parachute, an assisted roll axis control and a flight recorder (black box). We will accept to take the first orders in early 2020, while drafting the manufacturer’s identification file for the DGAC.

My special thanks to the first shareholders who bet, (seen from their perspective) on a very risky superhuman project. We’re going to show them that they were visionaries – not crazy. I would also like to thank, AEROLIGHT company, its boss, its employees and its trainees, without whom the whole program would not have been so technically efficient, and who represent the industrial future of CHOPPAIR. And finally, I thank on behalf of all present and future operational staff, new shareholders who still take the risk of an accident happening during the test flights of the prototype. They allow the program to start its pre-industrialization phase and makes it possible to take pre-orders.

You can see the evolution and evolutions of the MOSQUITO AIR 2 demonstrator:

. You tube 4 years ago with a 2-stroke engine

. You tube 2 years ago with the 4-stroke engine

. You tube this summer with all modifications and flight check before launching the pre-industrialization program.

Future vision :

When commercial success, the next CHOPPAIR (HELECOPTER) will be equipped with an electric motor powered by a French-made turbogenerator.

If huge commercial success, the devices could enter certification program!!!


Didier Lagneaux, Aérodrome Bourg Terre des Hommes on September 2, 2019