RADIO CONTROL HELICOPTERS
Eric Hawkinson
319 Yellowstone Ave., Billings MT 59101 E-mail: [email protected]
It's another great month to be enjoying the sport of radio control (RC) helicopters! I'm going to start out with a discussion about a new "big-block" for our wing‑spinning Tim Taylor types to get excited about.
O.S. .91 SX‑H C‑Spec — First Look
The newest helicopter engine to ship in any quantity is the latest offering from one of the oldest and most respected RC-engine manufacturers: O.S. Engines Mfg. Co. This is actually its second big-block .91 helicopter engine, and it has the same SX‑H designation as the .91 that came out a year ago.
However, almost everyone will refer to this new .91 as the "C‑Spec," which is the tag that O.S. put on the end of the model name to differentiate it. Following is a first look at the O.S. .91 SX‑H Ringed C‑Spec engine.
If you have been reading this column for the last year, you know that the original .91 SX‑H had a great deal of power but turned out to be a bit lacking in manners at times. Some (maybe most) would not run at all well without the addition of a Perry pump, and even then they were a bit on the rough side in terms of vibration.
They were also a bit difficult to turn over, requiring a good-quality starter and a fully charged starter battery. The addition of head shims or the lower-compression Hyper Head alleviated the starting problem, but the Hyper Head cost $70–$90. And nothing we tried made the engine smooth like the O.S. .50 or the O.S. .70.
Realizing that it had an engine that needed refinement, O.S. spent time and money working to retain the power and torque of the .91 while adding manners and smoothness to make the helicopter crowd happy. The result is the new .91 SX‑H C‑Spec "Competition Special" version.
This is still a ringed engine, and it still fits the 61 mounting dimensions as far as engine mounts and crankshaft. And it's still .91 cubic-inch displacement. Other than that, you will be hard-pressed to find any common ground between the original .91 and the C‑Spec.
The C‑Spec comes with a new version of the Hyper Head installed as stock, with a gold finish. The crankcase has a black finish. The head and case are treated with Alumite, which is not a good conductor for your glow-plug battery, so O.S. recommends using a backplate bolt as the ground‑wire‑attachment location. The taller head may cause installation problems in some helicopters, such as the Raptor 60, and in others you may have to do some cutting on the fan shroud for clearance.
The major changes to improve smoothness (besides the head) include a lighter piston, a lighter connecting rod, and considerably different balancing on the crankshaft itself.
My greatest excitement about the new engine is that it comes with the same three‑needle carburetor as the O.S. .70 SZ‑H: the 60K‑C. The O.S. .70 is one of the most perfect helicopter engines I have ever owned, and a big part of that is the carburetor's flawless performance. Idle, transition, and full-power settings are easy to tune, and there is little fiddling to be done after the initial settings are established. If the carburetor works as well on the C‑Spec as it does on the .70, there will be many happy pilots.
The C‑Spec's published specifications include:
- Weight: 19.3 ounces
- Practical speed range: 2,000–16,000 rpm
- Power: 3.0 horsepower at 15,000 rpm
Thus you will need to run a lower gear ratio than the standard 60, which is true of all of the other .80–.91 engines. Although the muffler‑mounting spacing is identical to that of the O.S. .61, most of the 60‑size mufflers lack the area to deal with the exhaust volume of the .91 engines. You'll want to make sure that you have a muffler designed for the bigger engines.
At the shop we've run three different exhausts that work well on these large engines:
- The inexpensive Mavrikk 90 muffler
- The one-piece Hatori 90 muffler
- The Muscle Pipe II from Curtis Youngblood (adds a definite power boost)
The new two-piece tuned muffler from Hatori also fits the O.S. and YS engines. Dubbed the "Todd Bennett Special," the new Hatori SB‑16 is designed to be smooth and quiet while allowing more power than a standard muffler. The SB‑16 uses the stub header with O‑rings sealing the joint and requires that you mount the muffler body to the helicopter frame.
If the weather cooperates, I'll have some time on the C‑Spec within a month or so, and I'll let you know what I find out.
Flying‑Site Discussion
Now I'll return to the flying-site discussion I started two issues ago. You may remember me mentioning that you should not expect to be welcome if you plan to take your new helicopter out to the airplane field on a busy weekend and fly your first five tanks of fuel trying to hover in the middle of the runway. Blocking the runway (effectively closing it to use for all of the airplanes) to practice with the only type of aircraft that doesn't have the slightest need for a runway will not make helicopters welcome.
Nor will most clubs be happy about your model flying around behind their backs. If your club has a good working arrangement, by all means fly there. Otherwise, proceed with caution and respect for your clubmates.
Since you don't need a runway to fly a helicopter, you may decide to fly somewhere other than the local airplane field. When we talk to RC airplane fliers at the shop, many of them want to get into helicopters because of the ability to operate at places other than a prepared landing strip.
Often these pilots own, or have a friend who owns, a pasture or undeveloped field somewhere that would be perfect for helicopter flying. But we also hear plans to fly at other locations that make me a bit nervous. The least comforting thing I hear is someone talking about flying at a local park or school yard. Sometimes these places work out fine for helicopter flying, but there are usually serious safety issues involved.
The obvious problem is that many other people—who have no particular knowledge of RC helicopters and the potential dangers they pose—use these places. This is especially true of the younger crowd, who are the natural inhabitants of any school yard. And even if the area is vacant when you arrive, don't expect that it will stay that way for the duration of your flight.
While they are flying, our helicopters hold an almost universal attraction for people of all ages who happen to be in the area or passing by. A few years ago some of us local RC helicopter pilots started flying at a partially finished soccer complex in the winter months. It seemed like a great place to fly, with paved parking and more than 80 acres with minimal obstructions.
There was seldom anybody else there—that is, until we would start flying! Suddenly there would be anywhere from two to two dozen spectators of all ages surrounding us, sometimes parking directly below the airspace in which we were flying. Other times they would park behind us then walk out beside—or even in front of—where we were standing!
We kept flying there for a while, but only when there were two or three of us so that someone could keep the spectators out of the flying area. This was better than having no place to fly, but was not a great arrangement. We were uncomfortable "bossing" people around, even though it was obviously for their own safety. And since there was no formal flying site, some people occasionally grumped about us telling them they couldn't walk out on "their" park property.
Add to that the fact that many city and county parks actually have rules prohibiting operation of any motorized vehicles, and you can see that there are many possible obstacles to being able to fly safely in these areas.
Safety and Etiquette
One thing that airplane pilots occasionally don't realize when transitioning to helicopters is that just because the helicopter can hover in a small area doesn't mean that it will! I advise new helicopter pilots not to try to do any flying in the back yard.
First, an RC helicopter accelerates much faster than most people expect when it transitions into flight, so that big back yard gets small quickly. Second, yards tend to have far too many obstacles: fences, trees, houses, garages, decks, and power lines.
There are two reasons why these things are bad. The obvious one is that your helicopter can hit something and explode. The other reason is that if something goes wrong and your helicopter misses all of the obstacles and heads for you, those same obstacles can impede your ability to move out of the way. I don't know if it's ever wise to operate an RC helicopter in your average-size yard, but I am certain that it is a bad idea for your first attempts.
Until next time, fly safely and happy rotoring! MA
Transcribed from original scans by AI. Minor OCR errors may remain.
