Helicopters
Walt Schoonard
THE COLLECTIVE helicopter presents a very different set of trimming problems. Most collective helicopters operate at a much higher blade speed than fixed machines so in setting them up, the power curve of the engine must be determined. It is absolutely essential that the maximum blade pitch be reached on the up side of the power curve of the engine. It is also important that a near constant blade speed be achieved. Both of these factors are rather difficult to obtain so it will require some very precise adjusting.
First, we need to make a visual device to quickly determine approximately 5/8 carburetor barrel opening. On a Kavan this can be done with a dental mirror held up to the barrel and taking a look-see, but to accomplish this, we have to stop the engine. This has to take time and may have to be done numerous times. A much easier method is to bolt a small aluminum plate about one in. high and 1½ in. long behind the engine bellcrank. This can be 1/16 in. thick or less—whatever scrap you may have. Once bolted in place, it now needs to be calibrated.
Turn on the radio and transmitter and then set the collective control (motor control) at full low and the motor trim at full high. Now mark the plate indicating bellcrank position at full low speed. Then advance collective to full high and mark the position, next retard to half collective and mark this position on the plate and from here it is easy to mark 1/4, 1/2, 3/4 and full. In a Gazelle this can be accomplished by calibrating the motor pushrod where it goes through the cooling duct. Now it is easy to look through the window and see the carburetor barrel position. The motor pushrod should now be adjusted so that 5/8 barrel opening occurs with the transmitter collective control at half with full motor trim. The pitch setting should permit the helicopter to lift off at this point. To now determine that maximum pitch can be achieved on the up side of the power curve, you will have to lift the helicopter off. A Kavan should be at least 5,000 rpm at lift-off and a Gazelle at about 4,000 rpm. During this testing, make sure that the helicopter is facing downwind. The maximum pitch should now be adjusted so that tail-rotor speed remains as near constant as possible. It is a
Radio Control: Helicopters
this point that the "pet" engine will show its bugs, and you will have to tweak the needle valve in order to get enough power; and under sustained collective setting, it will heat up and start to drop off the power curve. If you have a good powerful engine, its power output will increase even during increased collective settings.
A trial and error process will have to be employed to get the tail-rotor pitch and mix settings proper. To start with, set the tail-rotor pitch as per manufacturer's instructions and tail-rotor mix at a 40-60 percent relationship.
The tail-rotor mix needs to be adjusted so as you add collective the proper tail-rotor pitch is added at the proper time. A little time spent here will be very rewarding later when you turn this machine loose, and it will go straight up without the tail swinging around. When you have achieved this, the end result will be the up side of the power curve, a constant blade speed, and a proper idle speed relationship. I have seen a lot of people and heard about a lot more that cannot control the tail rotor. They then revert to gyros and all kinds of devices that are not necessary. In fact, a gyro won't help much on an overpowered and out-of-trim helicopter. It may not become too popular making that kind of statement, but it is the truth! My helicopters have demonstrated rapid take-offs, rapid descents, and rock steady hands-off hovering even in the wind.
Lately I have been flying a Schuler Helicopter. This little marvel flies hands off in all kinds of weather — yes, even the tail will come off! I am really excited about this rugged toy. A couple of weeks ago, while showing off, I stalled mine, and it hit the ground really hard from about 60 feet. It bounced over and landed. I straightened these parts and replaced the main blades, and it flew again. This is really a tough helicopter and well worth the money.
Next month's column is going to deal with "Goodies and Gadgets." Such things as electric starters, both in flight and on the ground, and other equally exciting subjects. The column needs pictures of your creations, especially if it incorporates some unique features. Please direct correspondence to RC Helicopters, care of Walt Schoonard, 2080 Sharon Road, Winter Park, Fla. 32789.
Radio Control: Helicopters
The collective helicopter presents a very different set of trimming problems. Most collective helicopters operate at much higher blade speeds than fixed‑pitch machines. In setting up the power curve the engine must be determined; it is absolutely essential that maximum blade pitch be reached up‑side of the power curve. Engine response is also important in maintaining near‑constant blade speed. Both factors are rather difficult to obtain and will require some very precise adjusting.
First, you need to make a visual device to quickly determine approximately the carburetor barrel opening. This can be done with a dental mirror held up to the barrel and taking a look. To accomplish this you have to stop the engine; this takes time and you may have done it numerous times. A much easier method is to bolt a small aluminum plate about 3/8" high and 1" long behind the engine bellcrank. It can be 1/16" thick or less, whatever scrap you may have. Once bolted in place it now needs to be calibrated.
Turn the radio transmitter and set the collective control (motor control) full low and motor trim full high. Now mark the plate indicating bellcrank position at full low‑speed advance, collective full high mark position next, and half‑collective mark position. The plate is easy to mark.
On the Gazelle this can be accomplished by calibrating before the motor pushrod goes through the cooling duct. Now it's easy—look through the window and see the carburetor barrel position. The motor pushrod should now be adjusted so the idle opening occurs when the transmitter collective control is at half‑full (motor trim). The pitch setting should permit the helicopter to lift off. The next step is to determine the maximum pitch that can be achieved up‑side of the power curve.
Kavan engines should be at least 5,000 RPM for lift‑off; the Gazelle lifts off at about 4,000 RPM. During testing make sure the helicopter is facing downwind. The maximum pitch should now be adjusted so tail‑rotor speed remains as near constant as possible.
Transcribed from original scans by AI. Minor OCR errors may remain.
