R/C F.A.Q.

RC Airplane Home

If you are just starting out in R/C (radio controlled) flying, or just thinking of it, be sure to read at least the Introduction and the sections on Building/Buying, Learning to Fly and one of Gliders, Power or Electrics. We have tried to address all questions a beginner may have; if your question isn't here, please send it to us and we will get back to you.

Remember, NO amount of FAQ reading can substitute for an INSTRUCTOR !

Contents

Introduction

Should I start with powered flight, or with a glider?

Depends on your preferences.The 2m (6 foot wingspan) class is a good sizelarge enough to be easy to fly but small enough for easy transport. Beginner gliders are lighter, fly slower and are more acceptable to the non-flying community than powered planesno noise or mess. Gliders are also cheaper (at least the trainers are) than powered planesno fuel, batteries, starter etc. to worry about. Electric Flight is silent and clean so finds greater acceptance from neighbours etc. at the flying field, although some people feel that electrics are not robust/easy enough for beginners. There is a little more paraphernalia - you may need spare battery packs, but you can fly from smaller fields. Power (with engines that use a fuel) will let you fly longer, and your model doesn't need to be as light as with electric (so it's likely to be easier to build); however, you may have to go to a field far from populated areas.

Whatever you pick, stay away from those sleek fast scale jobs! They look nice and fly hard, and those are the last things you want in your first plane. Stick to the trainers - they might not look as glamourous, but they will make you a much better pilot. And the most important point - no matter what else, try to find an instructor! This is the one thing that may make the difference between a rewarding experience and endless frustration. R/C pilots are friendly, and most will gladly teach you for free.

Are there any organisations etc. I can join for information?

Ask at your local hobby shopthere may be a club in your neighbourhood. This is the best way to meet other pilots and find an instructor. Most pilots will more than glad to help you out. If you can find a club, for an instructor, choose someone who is smooth in flying his/her plane and that you get along with. Remember, the ones best at flying (hangar or otherwise) may not always be the best instructors.

For U. S. residents, an organisation well worth joining is the Academy of Model Aeronautics (AMA). They are the modellers' main voice where it mattersthey liaison with the FCC, the FAA and Congress. It is an affiliate of the National Aeronautic Association (NAA) and is the US aeromodeling representative of the Federation Aeronautique Internationale (FAI). Membership in the AMA also gets you $1,000,000 of liability insurance, without which most fields will not allow you to fly. You also need to be an AMA member to participate in contests. Besides, you also get a magazine, `Model Aviation' which is rather good in itself, and it keeps you informed about the state of the hobby. So JOIN AMA!!! You can write to: Academy of Model Aeronautics, 5151 E. Memorial Drive, Muncie, IN 47302-9252. Membership is $40 per year (and well worth it). Their phone number is 317 287 1256.

Radios

Since a trainer needs only 2 or 3 channels, should I get one of the cheap radio systems?

Don't bother with the cheap 2 or 3 channel setsget a 4-ch system. It will come with NiCad rechargeable batteries and (usually) 3 servos; this is the most popular and most cost-effective kind of system. You can put the main pitch control (elevator) and the main turning control (in this case the rudder) on one stick, which is how most people (and thus most instructors) fly. The cheaper systems come with the controls on separate sticks and you will have tough time finding someone willing to teach you with that setup. They also use non-rechargeable cells, which can get very expensive, and sometimes have corrosion problems at the terminals.

What is a "1991" system?

Strongly recommended! A "1991" system is so named because in 1991 the radio control frequency regulations changed, which effectively made the "old-style" radios unusable. The "old-style" radios have a separation between channels of 40 kHz. Today, a separation of 10 kHz is needed, even though R/C channels will still be 20 kHz apartbecause the FCC in their infinite wisdom have created channels for pagers and such between the R/C channels, i.e. 10 kHz away from our frequencies. The Airtronics VG4 FM series is an inexpensive example, and is about $120 mail order. [U. S. specific]

If you can afford it, a system that has a "buddy box" is a really good idea. This is an arrangement where the instructor's radio is hooked up to yours, and he just has to release a button on his radio to take over control, rather than wrestling the radio from your grip. If you do this, be aware that you need to get the same (or compatible) radio as your instructor.

What about R/C rocket powered airplanes?

Rocket power for R/C aircraft is strictly regulated by the AMA; however, R/C rockets are a popular hobby. You should contact the National Association of Rocketry (NAR), and read the rec.models.rockets newsgroup. The FAQ list for that group may be obtained from sunsite.unc.edu by anonymous ftp; get /pub/ftp/pub/archives/rec.models.rockets/RMRFAQ/rmrfaq.*. Part 3 contains information on R/C rockets.

Buying and Building

Should I start with plans and build my own plane from scratch, buy a kit plane with wood and plans included, or go with one of those everything included ready to fly planes.

There are a few good trainers that are ready to fly (or almost ready to fly, aka ARF). ARF planes are usually heavy and hard to repair. The new generation of ARF kits is all wood and better built but more expensive. The better kits have parts that are machine cut, the somewhat cheaper ones are die-cut. You'll probably have to so a little more work with a die-cut kit, mostly in separating parts and sanding them.

ARFs vs. kits: this is a matter of opinion, but more people seem to think that kits are a better idea for beginners. Pro kits: you get valuable building experience and are able to do repairs. Moreover trainers are good planes to learn to build as well as to fly, and most of them are cheaper than most ARFs. Pro ARFs: you can be flying sooner, and you have less emotional investment in the plane so when you crash you don't feel as bad.

However: regardless of what you chose, your chances of a painless education are greatly improved if you have an instructorboth for building and for flying.

Remember, the plane you buy doesn't have to be good looking, it just has to teach you to fly! Many pilots after building a beautiful model are so afraid to crash that they never fly. Far better to have a scummy looking plane that you don't mind crashing again and again and learning to fly than to have a slick model that you can only mount on a stand! After you are proficient you'll have plenty of time to build good-looking planes.

It seems to be the general consensus that there are enough decent kits around that building from scratch is not really worth the effort unless you are into design or obscure scale models. If this is what you really want, you may find the "plotfoil" program (available from the rec.models.rc ftp site and from comp.sources.misc archives) useful.

The most important thing you can do while building is to make sure that everything is straight and square. This will result in a plane that flies consistently, predictably, and according to what you do at the transmitter instead of constantly trying to turn! This means: make sure the fin and the stabilizer are at right angles; make sure the wing and stabilizer are at right angles to the fuselage (viewed from above); looking at the side view, the wing, stabiliser and engine (if any) are all at the angles specified on the plans; and that the wing is built on an absolutely flat surface, to make sure it doesn't have any warps or bends.

Covering
For now, stay with Monokote. It's reasonably easy to apply, not too heavy, and fuelproof. (The label gives directions.) Also, if you screw up a bit and find that the wing is warped, sometimes you can fix it by twisting it and re-shrinking the covering to hold it in place.

Hinges
There seem to be as many opinions on this as there ways of hinging! The important thing to watch out forthey should be strong enough so they won't pull out, and the gap between the surfaces should be as small as possible. This is yet another place that an instructor is invaluable.

Learning to Fly

The most important point, one which cannot be overstressed: GET AN INSTRUCTOR! Here's what one beginner had to say:

You probably won't have any really bad (i.e. irreparable) crashes. (Of course, you'll still crash.) Also make sure you have your instructor check your plane thoroughly before the first flightas someone said, "it is much better to go home with no flights and one airplane than go home with one half a flight and many little pieces." This is really, REALLY important.

Pre-flight Checklist
When your model is ready to fly, make sure it is thouroghly checked over by someone who has done alot of building and flying. When I say thouroughly, I don't mean just picking it up and checking the balance and thumping the tires a few times. Every detail of setup and connection should be gone over in detail. If your instructor doesn't want to spend this much time checking your plane, find a new instructor.

The importance of this pre-flight check cannot be overemphasized! Many planes are lost due to a simple oversight that could have been caught by a pre-flight!

Here's a checklist:

Before the first flight:

After repair:

The checklist should be gone through again, with particular attention to the areas that were worked on or repaired.

Before EVERY flight:

If you can't find an expert that is willing to teach you, it is best to start with a 2-3 channel model with a long wingspan and alot of dihedral. The ideal thing to start with here would be a 2 channel glider. If you MUST start with a powered plane, a 6' foot powered glider, like the Piece O' Cake from Dynaflite is a good way to go.

Gliders

Introduction
A 2-channel (rudder+elevator) polyhedral glider is just about the easiest way to learn to fly, and is highly recommended. Once you can handle that you can move up to rudder - elev. - spoiler - flap competition ships or (on the slope) rudder-elevator-aileron-spoiler aerobatic ships.

Sailplane plug (aka religious sermon): ... don't think glider flying is just "launch, glide back"It's very easy to get 30+ minute flights and about 1000' altitude. Remember, power flying is limited by the size of the fuel tank (about 10 minutes) and gliders are limited by the receiver batteries (about 2 hrs). And glider flying is much more challenging (my opinion, of course), while at the same time being easier to learn. And no fuel costs, no starting hassles, no cleanup afterwards... Also, many cities have ordinances prohibiting model engines, which means the flying fields are outside city limits. BUT, since sailplanes don't have those nasty, messy smelly things, we can fly in any large enough area!

Are most gliders hand launched, or do they have a small engine to get them up?

Gliders are usually launched by a "hi-start"a section of rubber with about 500' of line. It is hooked to the plane and pulled backthe rubber then pulls it up kite-like. (It is NOT like a catapult launch!). Launches of heights up to 500' can be obtained on a good day. The launching procedure is simplekeep the elevator neutral (for now) and keep it flying straight. At the top the glider will just fly off the line. A small engine can also be used but it creates a lot of drag which is very detrimental to gliding performance. Hi-starts range from around $20 for light-weight models to $100 for heavy-duty ones.

Winches are also usedit's very similar to a high-start except that you (the pilot) always have control over the line tension so you can usually get better launches. It does take more skill, though; as a beginner you don't need to worry about winches just yet.

Flying at the slope you just chuck the plane into the windthat's it!

How do sailplanes stay up?

Since a sailplane has no engine, it follows that it must always sink through the surrounding air. The trick then is to find some air that's going up faster than you'll sink through it... and for our purposes, there are two kinds of such air:

Recommendations
Sailplanes:

Costs

What kind of cost am I looking at for a solid training glider or powered plane, with all gear?

$200 is in the ballpark. $120 for a 4-ch radio, $60 for a 2m glider, covering and other supplies. ARF Gliders$100 + radio + supplies.

Powered planes: about $350-400$120 radio, $70 plane, $120 engine + supplies.
(By "supplies," I mean things like rulers, knives etc.)

Where can I mail-order stuff from?

Try Tower Hobbies (1 800 637 4989) and Sheldon's (1 800 228 3237) [Inside CA, 1 800 822 1688]. Also look in modelling magazines for ads. Try Radio Control Modeler, Model Airplane News, Flying Models, Model Aviation.

Powered (gas)

Even though "wet" power is called "gas", it's not the same as car gasoline. Model fuel is usually a mixture of a lubricant (synthetic or castor oil), methanol and nitromethane. The power plants are usually called engines, as opposed to electrics, which use motors (see below). Engines are available in 2-stroke (louder, cheaper, and more powerful for the same displacement) and 4-stroke (a more scale sound, less vibration, but more expensive). Engine displacements are usually measured in cu. in. the US (A 60 engine = 10cc [actually 0.61 cu. in.]).

Compared to beginner's gliders, powered trainers are more difficult to master. This means that everything about instructors and equipment checks goes DOUBLE for powered planes. There are many, many ways a beginner can make mistakes and destroy a model that he/she has spent alot of time and money on. With the typical powered trainer, going it alone is foolhardy and will likely end with a destroyed model and a very disappointed modeler.

If you can't find an expert that is willing to teach you, it is best to start with a 2-3 channel model with a long wingspan and alot of dihedral. The ideal thing to start with here would be a 2 channel glider. If you must start with a powered plane, a Sig Kadet is one of the more docile trainers.

If you have an instructor, but have not flown R/C before, you can start with something a bit more advanced. In general, the larger the plane, the easier it is to see and to fly; but at the same time, the more expensive it is. The most popular size is the so-called "40-size" plane, with about a 50" wingspan and .40 cu. in. engine. The Great Planes PT-20/40/60 series are good. You can build these with ailerons, but due to their large dihedral, they can also be flown without ailerons. It won't hurt to have them built-in. Even though they will not be very effective, they will get you used to using them. Other recommended planes are the Midwest Aerostar and the Goldberg Eagle. Something with a "tricycle" undercarriage, that is one with a nosewheel and two main wheels, is the easiest to learn on.

If you have an instructor, and have flown R/C gliders, you might want to start with something still more advanced, say a Great Planes Trainer 20/40/60 or the like. These have a fully symmetrical airfoil and less dihedral. They are capable of more in the way of aerobatics, but are trickier to fly due to higher speed and less stability.

Electric Flight

I didn't know that you could put an electric motor and batteries in an airplane. Isn't that kind of heavy?

Modern NiCd batteries are pretty amazing. You can charge them in 15 minutes, take power out of them at up to 50 amps or so, and do it all day. That capability is what makes electric flight possible. Electric power can be used for any kind of flyinggliders, aerobatics, even racing. It's an excellent choice for sport flying.

What are the advantages and disadvantages of electric flight compared to wet power?

Electric power systems are heavier for a given power output. This means that planes must be built lighter, which may be more challenging (especially for the beginner). That's really the only significant disadvantage. The big advantages are that electrics are quiet and clean.

To me, the biggest advantage of all is that electric flight is unusual and interesting.

What is the best way to get started in electric flight?

That depends on what you want to do and where you're starting from.

If you've never flown RC before, and you want to start with an electric plane rather than a pure glider, I recommend an electric glider like the Airtronics Eclipse. This will give you the best chance to stay ahead of the plane. In the sport/trainer category, I hear a lot of good things about the Leisure Amptique.

If you know how to fly RC, you have a lot of choices. The simplest and most available electric power systems use six or seven cells. These are called "05" systems, and are very similar to the power system of an RC car. You can find all sorts of planes in kit or plan form which will work well with these systems. Outstanding examples are the aforementioned Eclipse and Amptique, old timers such as the Leisure Playboy and Astro Viking, a variety of semi-scale kits from Davey Systems, all sorts of gliders, and the aerobatic ElectroStreak from Great Planes. Any two-meter glider kit can be easily adapted to six or seven cell electric by a moderately competent builder. Just stick a motor in the nose, battery under the wing, and go.

If you want more performance, good ground handling, or just like larger planes, there are larger power systems available, all the way up to systems which will handle a 60-sized power plane. The cost and complexity, of course, go up with size. Any reasonably well-designed power plane kit or plan can be adapted to an appropriately chosen electric power system. The first step is to leave out half the woodall power planes are grossly overdesigned. Electric motors generate very little vibration, which helps you get away with lighter structures.

What are the elements of an electric power system?

The power system includes a battery, a motor, a control, and wiring. The battery is almost always made up of Sanyo NiCd cells in the appropriate number. Motors vary from the simple, cheap "can" type (otherwise known as "540" or "550" style), through more sophisticated styles adapted from RC car motors, up to the cobalt powerhouses.

Controls can be a simple on-off switch controlled by a servo, a directly controlled on-off switch, or a proportional electronic control.

If you are going to fly a glider or old-timer type plane with less than a 500-watt motor, think seriously about getting battery packs made of Sanyo 900 SCR cells. They are significantly lighter than the more usual 1200 mAH (sub-C) cells and give excellent performance.

What do the various letters used to refer to NiCd cells mean?

SC is the basic cell. SC cells will take fast charging and have reasonably low internal resistance. SCR cells have lower internal resistance and a somewhat flatter discharge curve, that is, they put out nearly the same voltage from beginning to end of the discharge. SCRs are best for high current drain applications. SCE cells have somewhat more capacity for the same physical size, but also have higher internal resistance. They are best for low current drain use (less than about 10 Amps.) The higher capacity of SCE cells will not be realized at high current drains, and they will heat up more than SCR cells.

Many kits nowadays come with a power system. In most cases, these systems are adequate for the application. It won't hurt to try what's there to start with, you can always experiment later. If the kit you choose doesn't come with a motor, of course, you'll have to choose one. If you are a beginner, go with the recommendations of the kit manufacturer. If you are an experienced RCer, you probably don't need my help.

For a six- or seven-cell glider or old-timer with a cheap motor, an on-off switch is sufficient control. For anything else, you will have much greater enjoyment with a proportional throttle. Get a high-rate control, they are much more efficient at part throttle. There are several good brands out there, but I like Jomar for good controls at good prices.

What support equipment do I need?

You need a charger of some sort. If you are using six or seven cells, any RC car charger will do the job. You don't need peak detecting or any of that fancy stuff to start with. For larger packs, there are good high-voltage chargers around. Check out Astro Flight and TRC, among others. Remember, the biggest enemy of NiCads is heat, so try and keep those batteries cool when charging. Expect to pay about $40.

How are motor sizes specified?

Motors are traditionally specified by a system which attempts to equate them to wet engines. There are significant problems with this, but they probably aren't of concern to beginners. An "05" motor takes a six or seven cell battery and puts out 75 to 120 watts, and so on up to a "60" which takes 28 cells and puts out 1200 watts. Incidentally, there are about 750 watts in a horsepower.

The actual power output for a given voltage (number of cells) depends on the load. Unlike wet engines, electric motors put out more power with more load. If you don't like the performance you get from your plane, you can try a bigger propellorup to a point. More power, of course, means less run time.

In the ideal world, motors would be specified by the total power they are capable of supporting and by the number of cells (or voltage) with which that power is produced.

What's a cobalt motor and why would I want one?

Rare-earth magnets, of which the most common type is samarium cobalt, are stronger for a given weight and volume than ferrite magnets. Perhaps an even more important reason for getting a cobalt motor is that they also have better brushes, bigger shafts, better bearings, are built more carefully, and so on. For the serious electric flier, they are worth the extra expense.

Where can I get this stuff?

Electric equipment is somewhat specialized, and most hobby shops aren't yet sufficiently enlightened to carry very much. You can use RC car equipment for a lot of things (after all, they developed this stuff in the first place) and your local hobby shop will have lots of that. If you want to get more sophisticated, get the catalogues from Hobby Lobby and Hobby Horn (both have ads in all the usual magazines.) Both catalogs contain a lot of detailed information that I can't fit in here. Hobby Horn has good prices on mainstream stuff. Hobby Lobby sells the lines of several European manufacturers, and tends to have higher prices for fancier (or at least more unusual) stuff. I haven't dealt with CS Flight Systems on the East Coast, but I read good things about them.

Helicopters

Getting started

Controls on a heli

Radios

Flying

Getting started

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.

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.

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.

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.

What accessories should I get?

There are a million accessories that you can buy. There are a relative few that are indispensible, 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.

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.

Controls on a heli

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 refered 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).

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.

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).

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.

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 outmoded 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.

Radios

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 propellor, 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.

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

Exponential: can be used to make cyclic less sensitive in midrange.

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.

Flying

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.

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.

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.

Some Aerodynamics

The aircraft can rotate around three axes: the fore-and-aft axis (or the roll axis); the spanwise (nose-up/nose-down) axis or the pitch axis; and the nose-left/nose-right, or yaw axis.

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.]

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.

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 Cli.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 roadif 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).

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:
(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.

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 fallingthe 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].

The rec.models.rc ftp site

Nur Iskander Taib has been kind enough to establish an ftp site for the use of the rec.models.rc community. Use anonymous ftp to log in to "bigwig.geology.indiana.edu" and go to the directory called "models" . You will find subdirectories called "airfoil", "faq" and "circuits". These contain, respectively:

Another site is CRL. These files (among others) are available:

Other FTP sources:

Sometimes people have trouble getting to bigwig. Plotfoil is also available from comp.sources.misc, which is archived at many sites, including sites in France and Australia. Get Volume 31, parts 28-30 (archive name: plotfoil). Contact your sysadmin, or read the periodic posting in comp.sources.misc for more information on how to reach the nearest one.

This FAQ is available from rtfm.mit.edu, the news.answers archive. It is in /pub/usenet/news.answers/RC-flying-FAQ/part*. These two sources are guaranteed to be up-to-date, since it is all done automagically.

Other Information

Tuned Pipes

Tuned pipes are a means of boosting the power of two stroke engines. They are not for all engines, but when properly set up they can be very effective.

If you have ever played a note by blowing over the end of a piece of tubing, you are using the principle involved. This is that any tube has a natural resonant frequency, usually dependant on its length, and the speed of sound in air. This means that some oscillations will die away quickly, but one in the right range will resonate, and be strengthened in force, when the wavelength matches the resonant length of the tube. As a pressure wave in the sound reaches the end of the pipe, a reflection is set up, and moves back up the tube. This occurs at the end, whether open or closed, and at changes of section or taper. Now, if we arrange a length of pipe as a muffler for a two stroke engine, we will find that at a certain rpm, the pipe will resonate, and boost the engine's rpm up. This is because the reflected pressure wave arrives at the exhaust port just in time to push some fuel/air mixture that was about to be lost out the port (due to timing overlap), back into the cylinder, where it will be burnt, producing more power than without the pipe. All we have to do is arrange the length of the pipe so that the boost in rpm occurs at a rev range that is useful to us with the relevant load (propeller). It may be that the engine cannot produce enough power to turn the fitted prop at a useful speed. Some engines have port timimg that cannot benefit usefully from any pipe.

The major factor in setting up a pipe is the length for a given propeller and rpm rangesome examples are given later. Some different designs of pipes will produce different lengths, because of the effects of diameter, taper angle and type of end reflector. Many pipes also have a muffled section which hides the rear cone or reflector's shape. Here are the basic questions to ask yourself before trying a pipe:

Let's get started. Record the static rpm on the prop of your choice with a muffler before doing anything else, so we know where we are starting from. Try to get a starting point for the length from a similar set up if possible, and fit your pipe. If you have a choice, get a header that is a bit (1") longer than you think you needit is easier to shorten than lengthen the header. Now start the engine and tune for slightly rich from peak revs. Note that this may require a richer setting than usual, as we (hopefully) are producing more power than before. If we have fewer revs than with a muffler, something is wrongif your mixture is correct, the pipe is probably too long. Try shortening the header (or pipe if more convenient) in 1/4" increments until the revs start to rise. If the pipe is too short, the motor will run harshly, and the needle setting will be unstable and criticaladd 1/4" spacers between the header and the pipe. Now to fly it. If it is not visibly faster in the air, try a shallow dive. If there is a distinct jump in revs and speed, the pipe is too short, and the `coming on' is caused by the prop unloading in the dive and coming up to a resonant rpm. If however the dive produces no change, but the vertical performance is better, the pipe is too long. Note that the references to `short' and `long' are relativethe pipe cannot improve the speed over all rpm ranges, and you will have to decide what the most appropriate compromise for your case is yourself. Most fliers do not need to have the engine speed up while descending, only to slow down in level and upward flight, so most adjustments will be aimed at improving level and upward flight.

Remember that pipes will vary in their boost and tolerance of non-optimum length.

The lengths given below are from the exhaust port face to the high point of the two cones of the pipe, or if muffled, usually to the point where the muffled section joins the first cone.

These examples used a variety of pipe makes, but I have found that MACS pre-tuned pipes are hard to faulti.e. they will come up straight away. Some other types and makes of pipes will differGRAUPNER pipes will give bigger boosts, but are MUCH more critical on almost every parameter length, prop, plug, fuel etc.

Dont forget to record what you try so you don't repeat mistakes or dead ends in your trials. I have found good muffled pipes, properly set up, frequently are quieter than mufflers, especially when set up long with big propsthe best result I have had was an OS 46 SF with a 12x6 and a pipe about 40mm(1.5") longer than for a 10x6, measured about 85db (at 3m/10ft) over grass, and in the air it was inaudible if there was anything else in the air.

Sources

Materials:

Composites - carbon-fiber, glass, epoxy and other composite materials are available from:

Aerospace Composite Products: P.O.Box 16621, Irvine, CA 92714. +1 714 250 1107 Carbon fiber tow, rods and strips; glass cloth, kevlar, Spectra, rohacell.

Fibre Glass Development Corporation: 800 821 3283. Glass fabrics, resins etc.

Composite Structures Technology: P.O.Box 4615, Lancaster, CA 93539 800 338 1278 CF, glass cloth, rohacell, kevlar. Recommended by some for good prices and lighweight materials.

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