R/C Flying: rec.models.rc FAQ


Chapter 11) Helicopters (Getting Started, Controls, Radios...)

(From Greg Johnson)

Getting started

  11.1) How hard is flying heli models?

Getting the hang of flying an R/C heli is a fairly challenging undertaking. It's like riding a bike: when you first start trying it seems impossible, but with enough practice it starts to seem easy, like second nature. It may take 5 or 10 sessions to get to the point of being able to hover with some consistency. Helicopters provide a long sequence of challenges, and the corresponding satisfactions of mastering them. After hovering, there is forward flight, nose-in hovering and flight, auto-rotation, aerobatics, inverted flight, etc.

  11.2) What are some good helicopters to consider?

There are several good helicopters on the market. It's a bit like Ford people versus Chevy people: different people develop preferences for different helis. Good ones to learn on include the Hirobo Shuttle, Kyosho Concept .30, and Kalt Enforcer. An excellent although somewhat more advanced heli is the X-Cell .40. Also, Shluter makes first-rate R/C helis. Check out the local hobby shops to see what the well-supported helis are in your area, and if possible find where the locals fly. Hang out at the flying field for an afternoon or two, and see what the locals are flying.

  11.3) Price to get going?

The helicopter itself will cost from $250 to $400 for a good starter heli. A radio will cost $200 to $450 or so. Gyro is about $70. Engine is about $130. Starter box, starter battery, etc. will probably be at least another $100.

  11.4) What are some good books?

There are two excellent books. Paul Tradelius's book (available through Model Airplane News) is particularly good for beginners. He presents the material in an order and a depth that is well suited to getting started. A more encyclopedic book is the one by Ray Hostetler. This book goes into great detail on all topics, and is a book to grow into. Ray's book mixes beginner info and info necessary only for advanced pilots, and consequently can be a bit overwhelming at first. There's a lot of stuff in there that you won't need to delve into for quite a while. I would recommend getting both of these books.

  11.5) What accessories should I get?

There are a million accessories that you can buy. There are a relative few that are indispensable, or almost so. I'd put the following items on the short list: a prop balancer, a pitch gauge, a pair of ball link pliers, and a receiver battery tester. You will need a standard assortment of tools such as needle nose pliers, screw drivers, hex wrenches, etc. You'll also need a starter and starter battery.

  11.6) What about electrics?

There are a couple of pretty good electric helis on the market. One is made by Kyosho (the Concept EP), and one is made by Kalt (the Kalt Whisper). These machines are small, light, delicate, and squirrely. Not the thing to try to learn on. They are more novelty items for experienced R/C heli pilots.

Chapter 12) Controls on a heli

  12.1) What is cyclic? Collective?

On most R/C helis (and full-scale helis for that matter), the main blades can change their (so-called) pitch angle. What this means is that if you sit the heli on a table and look at the tip of one of the main blades, the chordline of the blade can be tilted through a range of angles by the servos. In this sense, the rotor disk of a heli is a bit like a variable-pitch prop on an airplane. If the heli is hovering and you wish to make it climb straight up, you increase the pitch of the main blades, and increase the throttle so that the engine can overcome the increased drag and keep the blades turning at the same speed. The increased blade pitch results in more lift, and so the heli climbs. (With R/C helis, unlike R/C airplanes, engine RPM's are supposed to stay the same over (most of) the throttle range. At high throttle the engine puts out more power, but there is a corresponding increase in the load on the engine due to increased main rotor blade pitch, and so the engine stays at the same RPM's.) This overall increase in pitch that makes the heli climb is called collective control.

To get the heli to pitch forward or back, and to roll left and right, there are controls that are analogous to airplane elevators and ailerons. These controls are referred to as cyclic controls. The idea is to set up asymmetric lift on the rotor disk. (This is similar to what ailerons do to an airplane-one wing can be made to generate more lift than the other, and so the airplane rolls.) If there's asymmetric lift on the rotor disk, the plane of rotation of the rotor disk is going to change. For instance, the rotor disk (and the heli that is attached to it) might go a bit nose-down. In that case, the heli will transition out of a hover and start flying forward. Similarly, the heli can be made to lean back (nose-high), left, right, or any combination of these. The way this asymmetric lift is set up is to vary the pitch of each blade as it goes around. For instance, say you push forward on the cyclic control stick (the right one on your transmitter, which does the same thing as an aileron/elevator control stick on an airplane radio). This will make the blade pitch down as it travels through the forward-moving part of the rotor disk (usually the left side of the rotor disk), and it will make the blade pitch up as it travels through the backward-moving part of the rotor disk (usually the right side of the rotor disk).

  12.2) What is gyroscopic precession?

This is a counter-intuitive aspect of helicopters, that even many advanced pilots don't clearly understand. In order to get the helicopter's rotor disk to tilt (for example) downward at the front, you increase the lift on the right side of the rotor disk and decrease the lift on the left side of the rotor disk. (This is assuming the standard clockwise main rotor rotation.) To see why this is so, consider the following example. If the heli is in a nose-down attitude, the forward moving blade travels downhill, and the aft-moving blade travels uphill. The blades travel level at the front and back. To get a hovering heli to go into a nose-down attitude, you need to encourage the forward-moving blade to start going downhill and the aft-moving blade to start going uphill. Hence, pushing the cyclic stick forward causes lift to be killed on the forward-moving (left) part of the rotor disk and increased on the aft-moving (right) part of the rotor disk.

  12.3) What do the servos control?

There are usually five servos on an R/C heli. One controls throttle, one controls collective, one controls fore-aft cyclic (analogous to elevator), one controls left-right cyclic (analogous to aileron), and one controls tail rotor pitch (analogous to rudder).

  12.4) What is the use of gyros and how do they help?

The gyro is positioned so that it senses yaw. It then feeds small inputs to the tail rotor servo to counter the yaw that it detects. This keeps the helicopter from yawing to the left and right when you don't want it to. Left-right movement of the left stick also supplies input to the tail rotor servo; so you and the gyro are both giving control inputs to the tail. A gyro is a MUST. It's probably not an exaggeration to say that gyro-based stabilization of the tail rotor made R/C heli flying feasible. It is possible to fly an R/C heli without a gyro, and it's also possible to juggle seven balls. It's just darn hard! Furthermore, it's definitely not something you want to try tackling when you're just getting started. Without a gyro, the heli can begin to whip around wildly as soon as the skids leave the ground. The heli will do a 180-degree turn and you're looking at an angry helicopter coming right at you before you know what happened. Definitely not something for a beginner to tackle.

  12.5) How about fixed-pitch versus collective helis?

Helicopters with collective are now inexpensive and reliable. Every reasonable modern heli, from beginner-trainers up to FAI world-beaters, has collective. In a fixed-pitch heli, lift is controlled by varying engine RPM, just as in an airplane. This is an outdated technology, and you will outgrow such a heli very soon. Virtually no aerobatics, no auto-rotation (if the engine quits at altitude, the heli becomes a brick), not as much fun.

Chapter 13) Heli Radios

  13.1) How many channels do you need to control a heli and why?

You need five channels to control a heli. You need one each to control pitch, roll, and yaw. You need one to control throttle, and you need one to control collective. You might think that one servo could control both throttle and collective, since they are related. There are several reasons this wouldn't work, however. The main rotor disk of a heli is huge and generates a correspondingly huge amount of drag compared to an airplane prop. (If you think of the heli rotor disk as a big propeller, its actually pretty amazing that a tiny little .32 engine can turn it at all. There's about a 10:1 gear down from the engine to the main rotor, which makes it possible for the engine to turn the main rotor.) So, you have to have fairly fine control over the relationship between the collective pitch (and corresponding drag) and the throttle setting. If you get it wrong, the engine bogs badly or races wildly. Also, auto-rotation is an important maneuver, and this entails control of collective pitch while the throttle is set to idle. Finally, for inverted flight you want to have full throttle both at maximum up collective and maximum down collective.

  13.2) What are the radio options?

Pitch curves and throttle curves
You can adjust the amount of servo travel at 0% stick, 25%, 50%, 75%, and 100%, both for throttle servo and collective servo. This feature is a must.

Throttle hold
Flip this switch to practice auto-rotation; the throttle is reduced to idle. All the other controls still work normally.

Idle up
This is an alternate mode, usually used for aerobatics. You can set throttle and pitch curves, mixes, etc., and change over to the different setup at altitude or whenever.

Programmable mixing
This neat feature lets you establish a relationship between channels. One channel is designated as the input or master channel. As the master channel varies, it causes small changes to the output channel. This is an advanced feature.

Revolution mixing
This feature causes increases in tail rotor as throttle and pitch increase. This is useful to compensate for the increased torque the engine produces. I feel that this is a somewhat over-rated feature, and that it only really comes into its own when you're doing aerobatics. Even then, a programmable mix may be better.

Electronic trim adjustment
Similar to and augments mechanical trim

End point travel adjustment
Sets where servos go at max stick displacement

Can be used to make cyclic less sensitive in midrange.

  13.3) Can I use my airplane radio?

It is possible to control a helicopter with a 4-channel airplane radio. You can master hovering and move into elementary forward flight this way. For anything beyond that, you will need a helicopter radio. If you do try to use a 4-channel airplane radio, build a Y-connector, and control two separate servos (collective and throttle) off the throttle channel. Then adjust control arms to get a form of mechanical throttle and pitch curve adjustment. It's not too hard to set a heli up so that it will hover tolerably well at mid-stick this way, and you can contrive to increase lift above mid-stick and lose lift below mid-stick.

Chapter 14) Heli Flying

  14.1) What's the deal on auto-rotation?

If a heli's engine quits in flight or you simulate this by going to throttle hold mode, it is still possible to glide the helicopter down safely. As the helicopter descends, the wind flows up through the rotor disk from below. At a low or negative collective pitch setting, the wind flowing up through the rotor disk keeps the blades spinning. Heli blades usually have lead weights epoxied into the tips, so as the blades spin they build up a fair amount of rotational inertia. When you are near the ground and ready to land, you add in collective to increase lift, and the inertia maintains head speed sufficient to execute a controlled landing. In theory. ;-) Auto-rotative glides and landings are beautiful to watch. A helicopter can sustain as much as a 4:1 glide ratio in auto-rotation.

  14.2) What about aerobatics?

Helis can do awesome aerobatics: loops, rolls, pirouettes, you name it. My personal favorite is inverted flight. If looks 'way cool to see a helicopter hovering inverted right above the grass. I've seen guys do aerobatic routines flying the whole thing BACKWARD. With a helicopter you have unbelievable versatility.

  14.3) How high do they fly? How fast do they go?

Helicopters can go so high they are out of sight. Being able see the thing in order to control it is the only limit on how high they can fly. R/C helis can go 60-80 MPH or more.

Chapter 15) Some Aerodynamics (Speed, Turning, Stalls)

(From Shamim Mohamed)

The aircraft can rotate around three axes: the fore-and-aft axis (or the ROLL axis); the span-wise (nose-up/nose-down) axis or the PITCH axis; and the nose-left/nose-right, or YAW axis.

  15.1) Aerodynamics - Speed

The cross-section of the wing has a shape called an "airfoil". It has the property that when it meets the air (usually at some small angle, called the "angle of attack") it generates an upward force (lift) for a small backward force (drag). The amount of lift (and drag) depends on the airspeed and a value called the "lift coefficient" (and a few other things like surface area and density of the air). If the plane is in unaccelerated flight, the upward force (approximately equal to the lift) is equal in magnitude to the weight of the plane, which is a constant. It thus follows that the total lift generated by the wing is always constant (at least in unaccelerated flight). [One example of accelerated flight is turning - see below]

The above mentioned "coefficient of lift" (abbreviated Cl) depends on the angle of attack. Usually, as the A-of-A is increased, Cl increases; to keep the lift force constant, speed can decrease. So to fly fast, we decrease Cl (and A-of-A); to slow down, increase Cl (and A-of-A). Since the wings are fixed, we alter the A-of-A by pitching the entire plane up or down. This is done with the elevator. The elevator is thus the speed control.

  15.2) Aerodynamics - Turning

To turn a body moving in a straight line, a sideways force must be applied to it. For a plane, the best method for generating a force is to use the wings. To get them to act sideways, we roll the plane: now part of the lift is acting sideways and voila! a turn. To roll the plane, we use the ailerons (the movable surfaces at the wingtips). Also, notice that now since part of the lift is acting sideways, the lift force in the upward direction is reduced; but the upward component of the lift needs to be equal to the weight of the plane i.e. we need a little more lift from the wings, which we can do by increasing Cl - i.e. by pulling a bit of up-elevator. That's why to turn in a plane you push the stick sideways in the direction of the turn and then pull back a bit to keep the nose level.

What happens if you try to turn with the rudder alone? The application of the rudder will cause the aircraft to yaw, and it will continue to travel in the same straight line (more or less), skidding. (Think of a car on a perfectly slippery road - if you try to turn just by turning the wheel, you'll skid but won't turn). So we need a roll to turn.

But most of the trainers we see don't have any ailerons! How do they turn? They use a configuration of the wings called "dihedral" (or, for most gliders, "polyhedral").

     Flat                  Dihedral                     Polyhedral
					       ~-_                     _-~
-------O--------     ~~~----___O___----~~~        ~~~~~~~----O---~~~~~~

       ^                       ^                 ^           ^         ^
 0 angle between       small angle between        small angle between 2 wing
 2 wing panels         2 wing panels              panels and also small angle
					      within each panel (Gentle Lady)
						 0 angle between 2 wing panels
						 and small angle within each
						 panel (Olympic 650)
When we apply rudder (say left rudder) to a plane with dihedral, what happens? The plane yaws; the right half of the wing then sees a greater angle of attack than than the left half:

		      / / / / / / <--- airflow direction
		left wing    right wing
(You can try this out if you don't believe it: take a piece of paper and fold it slightly, like dihedral; then look at it end on, but slightly off-center, i.e. from the point of view of the approaching airflow. You will see that you can see more of the underside of one half than you can of the other.) And what does an increased angle of attack do? It increases the Cl and the lift generated by that half! So we now have the right wing generating more lift and the left less; the result is a roll to the left. With polyhedral we get the same effect, only to a larger extent.

  15.3) Aerodynamics - The Stall

If you try to fly slower and slower by pulling back on the stick (i.e. applying up-elevator) you will reach a point where the plane "falls out of the sky" or the stall. What happens is that an airfoil will only "work" up to a certain angle of attack. When that angle is exceeded, the airflow above the airfoil breaks up and the result is an increase in drag and a drastic decrease in lift, so that the wings can no longer support the plane. The only remedy is to reduce the A-of-A i.e. to push the nose down. This may be a little difficult to do when you see your plane falling---the natural tendency is to pull back on the stick, to "hold the plane up."

A development of the stall is the spin. Volumes can be written about it, and have been; go to the library and check any book on introductory aerodynamics.

If you want to know more about Aerodynamics as it applies to Model Aircraft (the small Reynolds' number regime, as it is sometimes called) check "Model Aircraft Aerodynamics" by Martin Simons [Argus Books, ISBN 0 85242 915 0].

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