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Toolbox Essentials
Setup for F3C
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JR Challenge 2003
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Helicopter Resources
Comparing the Webra 91AAR and the YS 91ST
Engine Tuning
Curtis Youngblood in New Zealand
Futaba GV-1 Governor
Pilot Profile - Malorie Zastrow
Scale: Flybarless Heads
Pilot Profile - Jason Krause
JR 10X
Pilot Profile - Mark Christy
Futaba 9Z WCII
Pilot Profile - Alan Szabo Jr
163km/h with a Vigor CS!
Raptor 60 V2
Low cost, high camera!
TSK & the Squirrel Part (V)
Follow up - Hirobo Freya
Follow up - Hirobo Shuttle RG
Sceadu 30 update
Hirobo Shuttle RG
Vigor CS - My thoughts
Bye bye little Ergo
Kyosho Caliber 30
OS 91
JR Voyager 50
Hirobo Sceadu
TSK & the Squirrel Part (III)
NZ Team Returns from Heli World Champs
Hirobo Freya
OS 50 Review
Millie vs CS (Part III)
Living with the CS
TSK & the Squirrel (Part II)
Promoting the Hobby
Ergo Z230 Gasser
Millie vs CS (Part II)
Millie vs CS (Part I)
TSK & the Squirrel
TSK & the Squirrel (Part IV)

Nothing beats cubic inches!
Simon Lockington

One of the latest developments in model helicopters has been the introduction of the 'monster motors'. These engines have been introduced mostly as a result of the relaxation of the 61 size engine capacity limited imposed for FAI competitions. Now that limit has been increased to 90 size and so the manufacturers have responded accordingly.

YS have their 80, Webra has the 75, Thunder Tiger has the 70, Rossi has the 65 and OS has the 91. The YS and OS have taken off in the states where the 3D enthusiasts have embraced the new found power with a passion.

When it came time to decide what was going to power my new Vigor CS, I was in a bit of a quandary as to whether to stick with the tried and tested OS 61 with Muscle Pipe, or try the new OS 91 with one of the mufflers. After listening to a lot of American pilots going on and on about how much power the 91 had and how smooth it was I decided to hell with it, get the 91!

These big engines require big exhaust systems. Some of the most popular exhausts are provided by Hatori and KSJ. These things are huge! Word on the street is that Curtis is designing a new Muscle Pipe for the big engines. Don't skimp when it comes to exhausts for these things. A friend of mine put a 60 size muscle pipe on his new OS 91 (one of the first in New Zealand) and it chewed it up within 10 or so flights. When you think about it, it makes sense, after all, the 91 is 50% bigger than the 61. I decided to go with the KSJ exhaust after hearing lots of good things from pilots in the states. These big exhausts are none too cheap in themselves. If you're thinking of putting a 91 in your machine, remember that in addition to the engine cost itself, you're going to have to stump up with another NZ$~300 (US$139) for the exhaust. Then you've got gearing ratios etc….

Now the 91 produces it's power at lower RPM than the standard 61, so regearing is recommended. Fellow Vigor owners had found that by replacing the drive pinion with a 10 tooth pinion, a gear ratio of 8.4:1 was obtained which many found to be nicely suited for the 91's power output. JR makes this 10 tooth pinion as an aftermarket part so it was easy to obtain.

From the outside, the OS 91 looks very much like an OS 61. The bolt pattern is the same and no modifications were required to fit it to the CS. It just slid right in.

Externally, the carburettor looks much the same as the OS 50's although ofcourse it is much bigger. It has only two needles, a low and high speed needle which seem to work together very well in achieving nice smooth mid range power for hovering. The machine idles very well on the ground and once the needles have been set transition from idle to power is smooth and steady.

With the KSJ exhaust, the engine sounds quiet and smooth. On 10% nitro, the smoke content is more than I expected when the engine is running at what I believe is optimal settings, but I like the fact that it runs rich.

Stabbing the collective into a vertical climbout results in rocket like performance even at twelve (that's twelve) degrees pitch. I'm running MS 710 FAI blades on my CS and 12 degrees is no problem. I would dare to say that with my setup and my needle settings, 12 degrees is probably about it, but really, who wants more than that? There's no point. Looping with this setup is something else! However, I think I'll back the pitch off a little to maybe 10.5 or 11 degrees to keep the engine howling.

I'm really impressed with how smooth the engine runs. It's quite happy to sit there chugging away in the hover and then instantly transition to full power without any fuss.

If you like the OS 50, you're going to love the OS 91, tonnes of power and very smooth.

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