Replace a single device with two devices that are each subjected to half the demand that the single device is subjected to.


"The simultaneous failure of both devices is as rare as the failure of a wing or structure. The principle of associating two random events is fundamental in the statistical evaluation of improbability and can easily be understood on reflection." The Helicopter: by Jacob Shapiro


This extra weight and initial cost can be reduced somewhat by decreasing the time between failures and the safety factor on each of the two small devices. The two devices will still have a higher safety factor and longer time between concurrent failures than that of the single larger device.

See also: ~ B335.html

Comments by CRAN on PPRuNe:

Dave Jackson:

Just an idea

The use of twin reciprocating engines is another alternative. This will significantly increase the reliability of the power source; even if each of the smaller engines has reliability that is lower then the single engine, which they replace.

Integrating the two engines into a single 'package', in respect to timing, power division, etc., has been done in other applications.


That make matter even worse!


The reliability point is valid (ish), but the weight consideration once again rules it out.

Say we need 224hp, we could use

(a) 2 x Lycoming O-235L producing 112hp @2600rpm and weight 99kg each (198kg and 224hp total).


(b) 1 x Lycoming O-540J producing 235hp @2400rpm and weighing 165kg. (This is a derated engine aswell while the O-235's are screaming!)

This is why it hasn't been done!

It's expensive, heavy and more complicated....

Dave Jackson:

There is no disagreement with what you are saying. The Lycoming is already considered a reliable engine, so to have two of them would be over-reliability.

I am suggesting going in the other direction. For example, 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.


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!

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Last Revised: August 24, 2003