RC Helicopters
Byline
Paul Tradelius, 4620 Barracuda Dr., Bradenton FL 34208; E-mail: [email protected]
I am very thankful to Model Aviation and to the readers for allowing me to write this column for the past several years. I have tried to incorporate practical information, aerodynamic theory, and helpful products to make your helicopter building and flying more enjoyable.
However, there comes a time to step aside and let someone else take over; I feel that time has come for me.
I hope that in the future you or someone you know would like to take over these writing duties to further help and improve this wonderful hobby. I have requested that I be involved in the process of finding a new columnist, so I invite you to contact me to discuss your ideas, desires, and goals for future columns, and so that I can answer any questions you may have.
More Helicopter Theory
In the May 2000 column I discussed the basic theory of a helicopter in a hover, so that you could better understand what your helicopter is doing and why.
I would like to continue that train of thought by taking the helicopter into flight—not so much in the theoretical sense, but to describe some of the flight characteristics you see in every helicopter, from beginner to expert.
Once you understand and expect these flight characteristics, you will better be able to successfully maneuver your helicopter in any phase of flight.
From Hover to Climb: Torque and Collective
After the hovering phase is somewhat under control, it's only natural to want to get farther away from the ground and do some real flying. But in the process, the helicopter does some strange things that must be prepared for. One of those things is the changing of torque produced by the main rotor system.
The increase in collective needed to make the helicopter climb also produces an increase in torque, which in turn tries to spin the helicopter around the main shaft. Most helicopters have a clockwise-spinning main rotor system, and the result of the increase in torque is an equal and opposite reaction, resulting in the helicopter tending to rotate to the left while climbing.
This reaction is very predictable and dependent upon the amount of increase in collective, and can therefore be easily compensated for by making adjustments to the tail rotor with the radio. Older radios will have up-and-down knobs; computer radios will have a revo-mix adjustment as part of the electronic menu.
These adjustments are rather easy to make if you take one step at a time.
- Set the up-and-down adjustment approximately 50%, and start a gentle climb from a hover.
- If the nose swings to the left, more up-setting adjustment is needed.
- If the nose swings to the right, take out some adjustment.
- In a similar manner, get the helicopter well away from the ground and start a descent, making slight adjustments to the down-setting until the nose stays straight.
With the helicopter on the ground and the engine turned off, move the collective stick through its full range while you are looking at the tail rotor, and you can see the adjustment needed to compensate for this change in torque.
It is nice to have these helicopter radios to do this type of mixing.
Forward Flight: Rotor Efficiency and Disk Imbalance
Once the helicopter is in forward flight, the entire rotor system becomes more efficient because of the increased air-mass flow rate through the rotor disk; this is why your helicopter will maintain nice forward flight with the same power setting required to hover.
However, this change in aerodynamics will also throw off the balance of forces you so carefully trimmed the helicopter for during a hover.
As the rotor blade advances from over the tailboom to the front of the helicopter, it is rotating into the oncoming wind. Therefore the rotor blade sees an increase in wind that is equal to its rotational speed, plus the wind that is produced by the forward flight. And since lift of the rotor blade (or any airfoil for that matter) is dependent on the speed of the air going over it, the lift is increased during this portion of blade rotation.
Conversely, as the rotor blade goes from the front of the helicopter to back over the tailboom, the blade is going "with the wind," so the speed of the air over the blade is reduced by the amount of forward-flight speed; the lift is reduced during this portion of blade rotation.
Taking the rotor disk as a whole, lift is increased on the side of the helicopter with the advancing blade and reduced on the side with the retreating blade. Most helicopters use a right-hand rotation; that means more lift on the left side, and less on the right side.
This effect can be seen in flight (requiring slight left cyclic trim to keep the rotor disk level), but is even more pronounced in a turn.
Turning Characteristics and Tail-Rotor Effects
The next time you are flying, try making level turns to the right and to the left using only cyclic controls, and see which direction the helicopter tends to turn best. Most fliers find the helicopter definitely tends to turn to the right.
Why? The answer has to do with the tail rotor. Remember, you used the revo-mix to trim the tail rotor and bring the nose to the right when increasing collective. In addition to the increased efficiency of the tail-rotor system because of the forward flight speed, this makes the tail want to go to the left (or the nose to the right) in a nice right turn.
Therefore, most helicopters require little or no tail-rotor adjustments while turning right, but need a fair amount of left nose to make a coordinated left turn.
This is a mistake I see many fliers make—not adding enough left nose with the tail rotor while turning left, leaving the helicopter in a nose-high attitude as it tries to make a left turn.
If this is happening to you, try adding a little left nose on your next left turn and see the improvement in the way your helicopter performs.
Engine Care: Demon-Clear™
Demon-Clear™ is a model engine and muffler cleaner that should be available at your dealer by the time you read this.
Fuels with castor oil do an excellent job of lubricating engines at extreme temperatures, but cause a varnish buildup on the outside of the engine and muffler, acting as a thermal insulator to reduce the cooling effect of the engine fins and muffler.
The dark varnish buildup can be removed with Demon-Clear™ in 30–45 minutes, leaving your engine and muffler looking like new.
Unfortunately, most engines are hidden well inside the helicopter, so you don't pay much attention to what the engine looks like. But if you look at the engine from the bottom where the fan air exits, you can see why these engines need a little extra attention.
Because of the adverse glide characteristics of helicopters, having a good, running engine with adequate cooling makes for a much more enjoyable day at the flying field.
Transcribed from original scans by AI. Minor OCR errors may remain.
