Saito FA-72

AIR POWER

If power-to-weight ratio means anything at all to you (and since we in the airplane world are talking wing loading and thrust, it certainly should), then I want you to take a close look at this latest offering from Saito: the FA-72. According to factory specifications, this new engine weighs in at 16.6 ounces with muffler, which is very light for a

.72ci displacement 4-stroke engine. Consider this: the weight of the FA-72 is a mere 1.1 ounces more than the FA-56; 2.8 ounces less than the smaller displacement Saito FA-65, and an impressive 3.7 ounces less than the smaller displacement O.S. FS-70 II Surpass. On my electronic measuring equipment, I found the weight of the FA-72 even a bit more favorable at 16.27 ounces. Either way, this engine is light for its size; moreover, it will fit into the spot where an FA-56 once sat (since it has the same mounting-bolt pattern as that smaller engine). This also means that you can easily stuff the large-prop-turning power of a .72-- class 4-stroke engine into a .40-size airplane. That's what I plan to do with my FA-72.

POWER-TO-WEIGHT WIZARDRY

So how did Saito accomplish this breakthrough in power-to-weight ratio? I've had excellent luck with Saito engines manufactured over the past six or seven years; I own many of them. One thing that has slightly puzzled me over the years, however, is the seemingly arbitrary use of both chrome-plated brass cylinder liners and chrome plating applied directly onto the aluminum of the cylinder casting. Both methods have worked well for Saito, but I give the nod to applying the chrome plating directly onto the aluminum, as I feel this supplies better heat transfer and cooler running. Eliminating the brass liner also saves weight. Maybe using both lined- and non-lined-cylinder technologies gives Saito considerable marketing and manufacturing versatility with a single cylinder/head casting. Perhaps the company has had its reasons for using both technologies all along. Accordingly, Saito has taken its FA-65 cylinder/head casting, removed the brass liner and directly chrome-plated the inner finished surface of the aluminum cylinder casting. In doing so, Saito has instantly increased the FA-65's bore from 24.8mm to 27mm.

THE STROKE STORY

The other half of the story-the stroke-- was not as simple a task as the bore. To keep the FA-72's crankcase small enough to fit in the FA-56's bolt pattern and to arrive at a size targeted right in between the FA-65 and FA-80 (.72ci), the FA-65's stroke had to be reduced from 22 to 20.6mm. Had Saito stayed with the FA-65's stroke, the engine's displacement would have been very close to .77ci. To leave the stroke as it was would not only have left the engine as heavy as the FA-65 because of its larger crankcase size but would also have put the engine's displacement far too close to the existing FA-80 engine. As it turns out, the FA-72's stroke is 1.6mm longer than that of the FA-56, which is 19mm. What this all adds up to is an engine whose bore and stroke can be characterized as quite "over-square." That means the bore is markedly bigger than the stroke-27x20.6mm.

RUNNING AND PERFORMANCE

I found the FA-72 to be a rather low-- vibration engine, considering it's a single-- cylinder design, and this may be due, in part, to the relatively short-stroke configuration. Over the years, I have noticed that many short-stroke engines tend to fare a bit better in terms of vibration than longer-stroke engines. In a single-piston engine, the degree to which you can perfectly balance the engine is limited. For example, if you run a long- and a short-stroke engine at the same rpm (both having pistons of equal mass), the long-- stroke engine's piston will have to move faster than the short-stroke engine's. The greater the piston speed, the greater the inertia. To offset this increased force of inertia, a bigger counterbalance is required to keep things smooth. And this is where you can run into problems with single-- cylinder model engines; you can run out of room for the counterbalance. I may get letters about the preceding statement, and that's OK; that's part of what this magazine thing we do is about. However, the fact that the FA-72 has a larger piston but a smaller counterbalance than the FA-65 is certainly strong evidence that I'm not completely out to lunch here. In addition, short-stroke engines have lower rod angularity, which can further smooth things out.

At any rate, the FA-72 is one nice-- running engine with very non-critical main-needle characteristics. I was able to get a solid 1,900rpm idle on a 13x6 prop using an O.S. "F" glow plug. The very best I was able to do with the supplied glow plug on the same prop was 2,300rpm. Needless to say, I tossed the stock plug in favor of the O.S. "F" plug. All break-in (two 12-ounce tanks at 4,000 to 5,500rpm) and high-speed rpm readings were done running on Wildcat 15-percent-nitro Premium Xtra fuel, which contains 18 percent oil (80/20 synthetic/castor blend). I still like a bit of top-grade castor for guarding the low end against corrosion; call me old-fashioned.