DESIGN: SynchroLite ~ Engine

Outside Helicopter


Information on engines for Helicycle



The 2-engine arraignment must be able to run with their crankshaft vertical; output up and output down. The vertical crankshaft resulted in problems getting oil to the upper crankshaft bearing on the Sportcopters that used the Hirth F-30 and 2706 engines and they had to come out with an 'H' version of the engines.

The craft is limited to 5 gallons of fuel and therefore the flight times are quite short. Could the engines' upper bearing have a small wicked reservoir of oil above them? Every time the fuel tank is filled the reservoir on both engines could also be filled.

The two engines are just about horizontally opposed. If the engines can be made to fire in synchronization then it will probably be best to hard mount them to the frame and have them cancel out each others oscillations.


Hirth has come out with a 3-cylinder engine for the Helicycle.

The following was removed from the UniCopter and placed here. It might be relevant, then again...

1 seat & 1 engine:

Consider the Hirth 2706 65 hp. Fuel injected. Crankshaft mounted vertically.

Second consideration; the Rotax 852 UL 65 hp. Fuel injected. Crankshaft mounted vertically?.




Hirth 2706

Rotax 852 UL



625 cm

580.7 cm



56 ft-lb @ 5700 rpm

53.3 ft-lb @ 6000 rpm



65 @ 6200 rpm

65 @ 6500 rpm



79 lb inc. electric start, fan cooling and exhaust.

79.2 + approx. 8 lbs for radiator etc.

Hirth F-30 2-cycle - 85 hp. Crankshaft mounted vertically. Fan cooled, Fuel injected. Ultrasport mounts the F-30 vertically.


2 seats & 1 engine:

Hirth F-30/2c - 85 hp. Crankshaft mounted longitudinally. For use with transmission item 0814.

Hirth F-30/2c ES Fuel Injected - 110 hp. Crankshaft mounted longitudinally. For use with transmission item 0814.

Engine - Reciprocating - 4-stroke - Continental O-200, 100 hp. WIDE ~ 1098

Engine - Reciprocating - 4-stroke - Lycoming - O-235, 116 hp. WIDE ~ 0935

Comparison between Using 1 Rotax 912 and 2 Hirth 2703's:



1 Rotax 912s:

2 Hirth 2703's















98 ft-lbs



@ RPM:






1352 cc

1042 cc



Weight (with gearbox):

160 lbs (approx.)

192 lbs



Overall width






$ 16,000.00 (Can)(approx.)

$ 9,000.00 (Can)(approx.)


Note: The 912 will be far more reliable engine than the 2703 but the two 9703's together .........

1 seat & 2 engines:

Consider two Hirth F- 33. This may be too light.

Consider two Limbach L 275 E. Two 20/25 hp @ 7300 rpm engines. These may be too light. The opposing cylinders will reduce vibration.

Consider two Zanzottera MZ 202's or 201's. It looks like they can be modified to rotate CW and CCW. See DESIGN: UniCopter ~ Power Train , arraignment 'C' with crown gears swapped as per 'D'. Note that the cooling fans will not allow the engines to butt foot to foot.


Use two lightweight 2-stroke engines. They will have less reliability than a Lycoming, but the probability of the simultaneous failures of both engines should be much less frequent than the failure rate of the single Lycoming. Hopefully, this would offer a greater time between complete failures, plus a reduced total weight. The second engine need only support the craft at the minimum power setting, and possibly only until a safe landing site was found.

The following example is not going to deliver the horsepower that CRAN is talking of but it does convey the basic concept.. Hirth has developed a
2-cylinder inline engine for the single seat Ultrasport helicopter, and 4-cylinder opposed engine for the two seat Ultrasport helicopter. Most of the components are common to both engines.

The idea would be to position the 2-cylinder engines in an opposed arraignment. This would cause these engines to be very similar to a single opposed 4-cylinder engine, except that there are two crankshafts. Just as Hirth has modified two engines specifically for helicopters, they might be willing to develop a lower crankcase housing for two crankshafts. Of course, they would have to synchronize the engines etc., but this should not be a significant problem.

Power Train: Locate a small spur gear on the end of the crankshaft. These two gears are offset from each other so that they drive separate larger spur gears, which are located on a common axis. The ring of the large gear delivers power to an inner hub via a vibration absorption device. This hub then delivers the power to a shat, which is common to both engine halves, via an overriding clutch.

If shafts on the small gears were to extend out of the gearbox then sheaves could be put on them to drive pusher props. If one engine were to suddenly fail there might be an excessive yaw due to the dissimilarity of forward thrust.


I agree with your point in principle but these engines have such short overhaul lives by virtue of the extreme rpm's that they operate at they would only really be ECONOMICALLY viable for experimental aircraft where they will only do a couple of hundred hours total time. For a commercial application then the overhaul costs must be considered. Also 2-stroke engines typically have poor fuel consumption so what you gain in reduced engine weight you loose in additional fuel weight. I have got the data on these engines somewhere, I think a typical SPC for hirth engines is about 0.65 lb/hphr, therefore that 102hp for the four cylinder engine is going to cost you 11 US/Gal/hr and 30kg in fuel weight per hour of endurance required. The lycoming will operate at 0.4 lb/hphr giving 6.8 US Gal/hr and 18.5kg in weight per hour. Therefore, we have to carry 12.5kg per hour extra fuel.

Based on a 3hr mission:

2 Hirth C32 (120hp max together) each 32kg = 64kg
3 hrs of fuel 90kg (@ 102hp)
Total = 154kg

Lycoming O-235 (118hp max) @ 99kg
3 hrs of fuel 55kg (@ 102hp)
Total =154kg

So there is not a weight saving in this case, you'll burn more fuel AND loads of OIL. Furthermore, given the demanding duty cycle of helicopters after 500hrs or so those screaming Hirths will need rebuilding! Cost cost cost!

I'm the first to criticise the Lycoming engines as they are too heavy for helicopters really....but with current available engine options they are the best that's available for the price. Especially the US were fuel is so b100dy cheap!



2 seats & 2 engines:

Consider two Hirth 2706 65 hp

Consider two Limbach L 550 E. Two 50 hp @ 7500 rpm engines. The opposing cylinders will reduce vibration.

Idea re 2 engines for the 2 seater:

Both the F-23 (40 hp - 2-cylinder opposed) and the 2703 (55 hp - 2-cylinder inline) have the same bore, stroke and cylinder displacement as the F-30/2c (80 or 110 hp - 4 cylinder opposed) engines. The F-30 ES is fuel injected.

This might mean that it could be possible to build two 2-cylinder engines, one with CW rotation and the other with CCW rotation. These engines could then be mounted with the two pair of cylinders opposed. Perhaps the fuel injection system may provide the means for synchronization between the two engines.

This should also allow the primary reduction to be belt or gear, by basically switching the engines.


Date: 26-Feb-01 18:36
Author: RR

I see little advantage to using two 2703s over an F-30.

Two engines would be heavier, have more parts to break, would cost more, would require a more complicated and heavier power transmission system, and would have some fancy vibration problems to solve.

What advantage do you see to "symmetry of torque?" I wasn't aware of this as a significant problem.

Having two engines may or may not offer additional safety. The chances of a motor failing are twice as high with twice as many motors. Depending on the aircraft, flying on one motor will probably only increase the glide ratio, rather than providing true redundancy. Keep in mind that, due to engine failure, Senecas crash more often than Lances. A Seneca is basically a twin-engined Lance.

BTW, 2703s can be had with fuel injection.

Date: 26-Feb-01 19:09
Author: LG

The 2706 is rated for 65HP with the same block as the 2703 - more hp/lb. The fuel injection options on the Hirth engines have RS-232 data out on engine parameters!

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Last Revised: December 23, 2007