|
|
|
|
The Truth about Engines Weight on aircraft is a double or triple whammy. Every ounce added to an aircraft has to be lifted in the air by horsepower. To get more horsepower you need a bigger engine. The bigger engine itself weighs more so now you need even more horsepower to lift the weight of the bigger engine. Bigger engines burn more fuel so they are more expensive and you need to carry more fuel. More fuel means more weight so you need even more horsepower. It is a never ending quest for diminishing returns. On traditional aircraft they have no choice but to continually increase engine size as they add more bells and whistles to the airplane. Continental and Lycoming make certified aircraft engines of all sizes but of course each time you add horsepower you add weight and cost. Light sport and amateur built aircraft are not required to use the very expensive certificated engines required in Cessnas and Beechcraft but we still want to fly with the safest engines we can, thus Rotax aircraft engines are the overwhelming choice for small aircraft. There are tens of thousands of them flying worldwide. These engines meet ASTM standards and are specifically designed for light aircraft. The largest engines Rotax makes are in the 100 horsepower range or up to 122 HP with Turbo boosting. As noted above, if an aircraft design is too heavy or too aerodynamically inefficient the designer has no choice but to chase the diminishing returns of more horsepower. However if he needs more than 122 horsepower he now faces a significant comprise in his aircraft's safety as there are no reasonably priced aircraft engines in the 120-200 horsepower range. Most designers caught in this difficult bind are forced to use Subaru car engines scavenged from car junk yards. In some cases these junked car engines are rebuilt; in other cases they are merely compression checked and sold with gyroplane kits. No one knows what kind of care they have received. While Subaru engines are generally reliable they have several drawbacks. First they were designed to operate around 2,000-3,000 rpm and running them continuously at 4,000 to 5,000 rpm as aircraft do is very hard on them and clearly not what they were designed for. The second drawback is that they generally utilize a rubber drive belt to connect the engine to the propeller. If the rubber drive belt fails, the propeller stops. The Rotax engines on the other hand are designed to operate at 5,000 rpm and utilize a fully enclosed gear drive that is far more robust than a simple rubber belt. When reading aircraft specifications it is illogical to be impressed by high horsepower; this only means the designer did not do his best possible job to eliminate weight. What you should be impressed with are performance specifications. If two aircraft both climb at 1,200 feet per minute and cruise at 70 knots, they perform equally regardless of the horsepower required. However, if one is doing it on an engine that is lower horsepower, he is burning less fuel for the same result thus operating less expensively. If at the same time, the smaller engine is a Rotax with a gear drive being operated at its designed rpm, it is safer and will certainly last longer. I do not wish to imply Subaru engines in aircraft are unsafe as there are many of them flying. However, it is clear that the preferred engine would be the Rotax if the aircraft's design and weight allow it to deliver good climb and speed performance. Lastly in terms of value it is worth mentioning that a new Rotax turbo engine is approximately $25,000 while a used Subaru car engine is in the range of $3,000. Thus all other things being equal, the aircraft with the Subaru should cost on the order of $22,000 less.
For more information about Engines, Design and Stability read: Gyroplane Design and Stability Considerations
|
|
|
