RE: Saito 130 twin info
The Saito twins with dual cam shafts have, lacking only a second crank shaft, twice as many moving parts as the equivalent size single cylinder engine. As a result they also have almost twice the friction losses. Seventy to seventy-five percent power of the single is s good approximation.
There are two classes of Saito twins, the even fire, and the odd fire. The even fire engines have one firing point every 360 degrees of crank rotation, the odd fire engines will fire one cylinder, then after 180 degrees of rotation the second cylinder will fire, then there is 540 degrees of crank rotation before the first cylinder fires again. The different feel is obvious when turning the propellor, and looking from the top the even fire engines have the cylinders directly across from each other, while the even fire engines have the cylinders slightl;y offset.
From an engineering standpoint the odd fire engine is easier and less expensive to manufacture, it has only one crank pin on the crankshaft, and uses a â€śFork and Bladeâ€ť assembly for the connecting rods. One rod has a much wider big end with a slot in the middle, the con rod for the opposite cylinder fits into the slot, both rods riding on the single crank pin. This also allows both con rods to be made in one piece, no bolted joint in the big end.
The even fire twins have two throws on the crank, with one con rod on each. There are several ways to assemble the crank and rods for this type. Using one piece rods the crank can be built up from several pieces with field service capability, several pieces supplied only as an assembled unit, or a one piece crank shaft with the rod big ends bolted together. Saito even fire engines use the bolted big ends.
Letâ€™s compare the FA-91 single, the FA-90T odd fire twin, and the FA-90TS even fire twin.
FA-91 single. For reasons I wont get into here it is not possible to have a single cylinder engine fully balanced mechanically. The mechanical vibration can be minimized with a heavy crank shaft and flywheel, but this weight is not good in an aero engine, mechanical balance will always be a compromise. Also, since the engine is developing power (in a four stroke engine) only ÂĽ of its running time and coasting for the other Âľ of the time, there is a dynamic imbalance as well. The resultant vibration can not be eliminated, there will always be an amount of vibration we canâ€™t get rid of.
FA-90T odd fire twin. The odd fire twins have both pistons going to the left, they stop, then both pistons go to the right, stop, and the cycle repeats. They are, from a mechanical balance standpoint, the same as a single cylinder engine. With the two power strokes overlapping the peak torque is less than the single cylinder equivalent, the dynamic imbalance is lessened.
FA-90TS twin. This is an example of the even fire engines. With both pistons going in opposite directions they each balance the other. The rotating parts can be brought to almost complete balance, the mechanical vibration is almost totally eliminated. With the power strokes evenly spaced the power-coast ratio is now 1:1. This basic design is the smoothest common model engine, but being more complex mechanically itâ€™s more expensive to make.
Summary: If you can stand the weight (and price) of the larger twin for the equivalent power of the smaller single, the even fire twin is the best engine to use. The odd fire twin is a compromise for reasons stated.