ORIGINAL: skulboep
Alright so I'm beginning building my 3rd plane (Sig Four Star 60 ARF) and after searching the forums for a couple days I have discovered a trend that I thought was supposedly a BAD idea: overpowering an aircraft. Now I'm not talking about going up an engine size if one lives at high altitudes in order to compensate for the lower pressures and decreased performance, I'm talking about strapping 1.20-sized 4 stroke into a 40-sized aircraft and running an extremely low pitch prop. Every instruction manual that I've ever read also cautions about overpowering an aircraft, claiming that the aircraft can become unstable in flight, the firewall can become weak, and if no catastrophe actually happens in flight, the warantee will at least be voided. However, in looking for an engine to pair with my Four Star 60 ARF, I've become interested in Super Tigre's line, especially the .75 2-stroke (a .75 engine in a .60-sized plane sounds pretty good, right?). According to many forums, most pilots that fly this plane recommend the Super Tigre 90 over the 75, claiming the 75 is barely enough engine (furthermore the 90 is has the exact same weight as the 75, meaning that it's performance increase over the 75 is probably pretty dramatic). How can this be, especially if Sig claims that the plane is a 60-sized plane? A 90 sounds insane!!! Do you guys have any experience with a particular engine in this plane or about the topic of overpowering in general? Let me know what you think! Thanks!!!
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These are, indeed, confusing times.
Are you after unlimited vertical without a stall (still visible, of course)? Then you will need more than a typical .61 sized two-stroke up front. No, the .75 or .90 probably won't provide unlimited vertical either, in their stock form.
Today's .75 and .90/.91 engines are made from smaller crankcases that are often shared with their baseline .61 engine (but big for a .61). As a result, the new .90's do not offer the staggering amount of power that such a displacement would provide had the engine been designed as a .90 to begin with. Why? Because there is not enough material in the block after boring it for the larger piston/liner to provide the necessarily larger port ducts to feed such a displacement at its highest capability for producing the most power. Yeah, kind of a rip-off. You (the manufacturer) call it a .90/.91, but it doesn't produce the power of a dedicated .90/.91. To add insult to injury, the .61 variant is so large and heavy that it too turns out to be best used as a paperweight.
Were I forcing myself to buy the Super Tigre engine for this model, I would go with the .90 too. If you are going to carry the weight, you might as well have what little extra power the .90 produces over the .61 or .75.
If you just want to fly it like a normal sport model with no 3D or infinite vertical in mind, any .61 to .90 will do the job.
The model was designed in a time when the term .60 meant something different than it does today. The 4*Series were designed for serving as a gentle low wing model for the student pilot that had just graduated from a Senior Kadet or Seniorita. They were designed to be low wing floaters that flew slow and stable in order to help the newbie get the hang of flying a low wing model. They were not structurally suited for overpowered (eggcrate wing design) and quite often they would have their wingtips get into oscillation when flown very fast. Some even experienced wing failures due to overpowering (kit versions).
This model shines best when flown at moderate speeds with an engine intended to power it in such a manner. If you want to go fast and hotrod, buy something else.
The only four-strokes that I would mount on such an ARF are the very light weight and small, relative to other four-strokes, Saito .80 thru .91. It would probably fly well on a .72, but I haven't tried it as yet. The .80 to .91 four-strokes will provide about the same power as the originally intended .61 two-strokes, maybe a tad less, but will not burden the model with the extra nose weight of say an OS.91 or Magnum .91. Not that the latter two engines do not do a good job of powering the model. Their extra weight can be problematic in some instances. Still, a stick-on lead weight near the tail post of the model can often compensate adequately for the heavier .91 sized engines.
Ed Cregger