Comprehensive guide to aerobatic aircraft

[cwojcik]

Flying precision or 3D aerobatics (or both) requires the correct airplane and equipment.  While you can certainly learn a lot with any aircraft (I started 3D on an advanced trainer), the right aircraft will help you progress much more quickly.  There are several manufacturers that I have found provide consistently great aircraft, and I’ll go over them a little later, as well as the different sizes of aircraft that are available today.  But first, we should touch on some basics.

Transmitter

One of the most important investments you can make is in your transmitter.  Unlike aircraft components, it takes a very long time to wear out a transmitter.  There are several features that will make learning aerobatics much easier.  It’s also a great time right now to buy a new radio.  There are some incredible deals currently on new 2.4 ghz transmitters and receivers, and the spread spectrum craze has also made the prices for used 72 mhz equipment plummet.  Here’s what you should look for, at a minimum, in a radio to be used for aerobatics:

-At least 6 channels, with dual aileron (or “flaperon”) capability
Most modern aerobatic aircraft have dual aileron servos (one for each aileron).  Larger models have dual elevator servos.  In these situations it is almost always undesirable to use a Y-harness (like you would have to do with a 4-channel radio).   Instead, each aileron servo should be plugged into its own channel and the dual aileron mixing should be done in the radio.  This allows for each aileron to be trimmed individually (making set-up significantly easier), as well as allowing for aileron differential to be programmed (this is the practice of making the ailerons move farther up than down, or vice versa.  It allows you to correct for unwanted pitching or yawing in roll, so that your rolls are “on a wire” rather than barrel rolls).  It also allows for flaperons, which is the ability for the ailerons to act as flaps (this is not usually used anymore, but it can be helpful on a fun-fly airplane).  It also adds a certain measure of redundancy.

-Dual rates and exponential
In order to fly 3D aerobatics, huge control surface movements are necessary.  In order to keep the aircraft controllable, exponential is an absolute requirement (on some of my aircraft I use as much as 90% exponential on the elevator).  Dual rates are also very helpful for landing, or if you wish to fly both precision and 3D with the same airplane.  Exponential should be available on the rudder, aileron, and elevator, and dual rates should be available on at least the aileron and elevator.

-Programmable mixing
There are very, very few aircraft that do not require some programmable mixing.  When rudder is applied, most aerobatic aircraft on the market will roll in the same direction as rudder, and almost all of them will pitch nose-down.  With just a few minutes of programming, this tendency can be significantly reduced (but not completely eliminated).  This is not an absolute necessity for 3D but it is very helpful.

-Digital sub-trims
Extremely helpful for achieving correct linkage geometry while setting up your aircraft.

-EPA (End Point Adjustment, aka ATV or Adjustable Travel Volume)
This is also extremely helpful for setting up your throttle linkage, achieving uniform movement on all surfaces in both directions, and eliminating binding.

There are at least 5 radios available with all of these features, and more, for less than 300 dollars right now.  There are even some for less than 200.  Don’t be afraid to think outside of the box, either.  While Spektrum, JR, and Futaba make great radios, Hitec and Airtronics also have some fantastic deals right now.  The Hitec Aurora 9 rivals the 1000 dollar radios in programming features, and it’s only 459 with a 9-channel receiver.  The Optic 6 Sport also offers all of these features, and it’s only 195 with a receiver.

Servos

Don’t skimp on servos!  They are the most important part of your connection to your airplane, and a servo failure can easily result in the loss of your aircraft.  I have enjoyed fantastic performance from Hitec servos, and their service is the best in the business.

Servos from manufacturers such as Bluebird, TowerPro, Waypoint, and PowerHD are extremely inexpensive, but you often get what you pay for.  Some people have reported good performance from them, while others have not.  Personally I believe that they are probably inconsistent. Quality control is one of the first areas cut when reducing costs.  I lost one aircraft to Bluebird servos and almost lost another to a Waypoint on my throttle (the burnt one shown here).  Many people have reported poor centering and resolution with cheaper servos, which will make precision flying extremely difficult.


3D puts enormous shock loads and stresses on the control surfaces.  You should strongly consider using metal-geared servos; often the cost increase is less than the cost of a replacement gear set.  You will also be much less likely to strip the gears if you bump your aircraft’s control surface during handling.

Another question that often comes up is whether or not you need digital servos.  This question is made all the more confusing by the fact that most people can’t even give you a straight answer as to what makes a servo digital.

All receivers, regardless of make, output the same thing to the servos: a position signal.  This signal comes in the form of a pulse of variable length delivered many times (about 50) per second, over the white signal wire.  The length of the pulse, from 900 micro-seconds to 2100 microseconds, determines the position of the servo.  With an analog servo, every time the servo receives the pulse, it checks its position and moves the output shaft if it is not in the correct position.  This is why you hear and feel a buzz of about 50 hertz when you try to force a powered servo out of its commanded position.

Digital servos are different in that they have a microprocessor on board that interprets the signal.  Rather than checking and moving the output shaft every time the servo receives a new signal (50 times per second), the microprocessor stores the signal on board and checks and updates the position of the output shaft at a much higher frequency (several hundred times per second) between the pulses from the receiver. This is why digital servos hum or whine when powered on: that noise is the servo continuously updating its position.  On some servos, such as Hitec, the microprocessor can be programmed to interpret the signal differently, so that fail-safe, rotation direction, speed, end points, and sub-trim can be programmed into the servo itself.  This is very helpful in larger aircraft where there are several servos on one surface and they must be matched to prevent binding.

So what does this mean?  Firstly, because the servo’s position is updated so much more often, it means that digital servos often have better holding power than analog servos.  In some cases power and speed are also improved.  However, because the motor in the servo is being powered so much more often, it also means that digital servos often consume more current than analog servos.  Make sure your receiver batteries or BEC are up to the task before upgrading to digital servos (more on that later).  So generally, digital servos are certainly a worthwhile upgrade, but they are not a necessity.

Batteries

An often overlooked part of any aircraft is the power system used for the receiver and servos.  These days we are flying aircraft with larger control surfaces (and consequently stronger servos) and flying them harder and with more power.  This results in more stress on the servos and consequently more power being pulled from the receiver.  For more servo power and speed, most modelers use a 5 cell (6.0 volts) battery pack with aerobatic models.    A 5 cell, 1200-1600 mah NiMh is fine for everything up to about 8-9 lbs flying weight.  Good practice is charging your batteries after you fly your first hard flight with your airplane and seeing how many amp-hours went back into your pack (for example, if you have a 1500 mah pack in your airplane, and after one hard flight you re-charge and the battery takes 150 mah, you can probably safely fly for 8 flights before recharging).  If you don’t have a charger that can tell you this information just ask me and I’ll be glad to lend you mine.  Another valid practice is to check your pack voltage after every flight.  When it falls slightly below nominal voltage (like 4.6V for a 4 cell pack or 5.8V for a 5-cell pack) then it’s time to recharge.  It’s also a good idea to cycle your batteries (if they are NiMh or NiCd) before every season a few times to see how much capacity they have and to eliminate any memory that may have formed.

Besides capacity and voltage, you should also consider the internal resistance of your battery (also known as “C” rating).  All batteries drop their supplied voltage a little bit when a load is applied to them.  Batteries with a higher “C” rating, or less internal resistance, drop their voltage less with the same applied load.  AA-sized NiMh batteries are usually incapable of supplying lots of current if they are over 1650 mah in capacity.  So, for example, a fully charged 5 cell 2700 mah NiMh battery might read 7.0 volts on a voltmeter.  When pulling 2 amps from it, this reading might drop to maybe 5.5 volts.  Under the same conditions, a 1650 mah NiMh battery would probably still be supplying over 6 volts.  The important thing to consider here is that you don’t pull your battery voltage down far enough to cause a reboot of the receiver (this is known as a “brown-out”).  In a brown-out, you will usually have no control over your model until the receiver has rebooted and re-connected with your transmitter.  On newer radios this usually happens quickly; on older radios it may take up to several seconds (long enough to lose the aircraft).  It is also less of an issue with 72 mhz radios because they can generally operate on lower voltages and the reboot time is quicker or non-existent.

There are other battery chemistries available now as well.  2-cell Lithium-Ion batteries generally have decent (but not exceptional) internal resistances and very low weight.  However, they have a higher voltage (7.4 volts nominal, 8.2 volts fully charged), which, unless you have specialized servos, requires the use of a regulator to drop the voltage to a safer level.  Lithium-polymer (LiPo) batteries are very similar to lithium-ion batteries (in fact they are actually a type of lithium-ion batteries), but they have lower internal resistance, are cheaper, marginally heavier, and must be treated more carefully due to fire risk (for example, you should never charge LiPo batteries when they are in the airplane, and they are more susceptible to shock or vibration damage, so they must be carefully mounted in foam).

My favorite battery chemistry right now is lithium-iron phosphate (also known as LiFePO4 or LiFe).  A123 batteries, if you have heard of them, use this chemistry.  They are lower in voltage than Li-Ion or LiPo (3.3 volts per cell versus 3.7), so most servos designed to be used with 5 cell NiMh or NiCd batteries will work fine with LiFe batteries with no regulator.  In most cases they have fairly low internal resistance and can put out more current than a lithium-ion battery but a little less than a LiPo (A123 batteries are an exception; they can put out more current than most LiPos).  In any case the amount of current they put out is more than enough for powering your receiver and servos.  Additionally, they are fairly inexpensive in smaller sizes, can be charged quickly, and are safe.  The only drawbacks are that they must be balanced like LiPo or LiIon (both cells must be at the same voltage) and they require a charger that is designed for their unique chemistry (since this type of battery is relatively new, not all chargers support it).

To make things simpler, here are my recommendations:

-40-60 size model (50”-65” wingspan, 4-8 lb flying weight, 5-6 servos): 5 cell 800-1650 mah NiMh battery like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVTE9&P=0
 Or 2 cell LiFe battery of the same capacity range, like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXYAS7&P=0

-90-30cc size model (65”-75” wingspan, 8-13 lb flying weight, 6 servos): 5 cell 1650-2000 mah NiMh battery like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNHS4&P=0
Or 2-cell LiFe battery of the same capacity range, like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXYAS9&P=0

35-50cc model (75-90” wingspan, 14-19 lb flying weight): 5 cell 2000-3300 mah NiMh battery (if the battery is over 2000 mah capacity please make sure the cells are sub-C and not AA or 4/5A sized) like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNHS4&P=0
Or this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVHY4&P=0
Or 2-cell LiFe battery of the same capacity range, like this one:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXYAT1&P=0
Or this one:
http://www.hangtimes.com/a123_packs.html
Or 2-cell Lithium Ion or Lithium Polymer battery of the same size range, like this one:
http://www.fromeco.org/products/01frc2600-102/default.aspx
Or this one:
http://www.allerc.com/product_info.php?cPath=3_4_93&products_id=4513
With a 10 amp regulator, like this one:
http://www.fromeco.org/products/02frcsah-231/default.aspx

Another important consideration is the battery switch.  Using a heavy-duty switch is a good idea on any model larger than about 10 pounds.  A standard servo or battery plug is said to only be capable of sustaining 3 amps (although I believe they can take more, it is a good conservative rule of thumb to follow), so on models larger than 30cc it may be a good idea to use a switch with dual outputs like this one:
http://www.smart-fly.com/Products/Switch/switch.htm
Simply plug the extra lead into a free channel in your receiver if you have one.

Or you can make your own by soldering an additional lead in parallel onto the receiver side of a heavy duty switch and using Deans connectors to connect the battery to the switch.

Power

Critical to your success in any form of flying (aside from sailplanes) is a reliable and sufficient powerplant.  Mistakes in 3D can usually be recovered with a quick stab of power, which puts air over the tail surfaces and gets you up to altitude quickly.  Better pilots can fly 3D with less power, but generally, having plenty of thrust available will make things much easier.  While this seems obvious for 3D, it’s just as important in precision.  Flying straight uplines is much easier with plenty of thrust and airspeed; the aircraft will be more stable and require smaller corrections.  Even more critical than having lots of power is having reliable power.  When flying 3D, you will be relying on your engine to keep your aircraft from falling out of the sky.  You will be putting enormous stress on the engine, as well: hovering provides very little airflow, and puts a large load on the engine.  After your first few 3D practice sessions check your engine to see how hot it is.  If your engine EVER starts sagging during 3D, fly along gently at low throttle to try to cool it off.  Richen the engine if necessary.  3D is nerve-racking enough; worrying whether or not your engine is going to keep running will really ruin your fun.  I have had excellent luck with OS engines for glow stuff.

I also urge you to consider electric power.  Electric performance now equals or surpasses glow, and battery costs have come down enough to make the cost of an electric similar to that of glow.  Newer LiPo batteries are capable of 5C charging for charge times as low as 15 minutes, making all-day nonstop flying a possibility. Electric power is quiet, reliable, responsive, and much less stressful on the airframe and servos.  Best of all, you don’t have to clean your airplane off when you put it away at the end of the day.  The only thing you give up by going electric is a little bit of flying time (7-8 minutes is typical for a high-performance set up, but you can get more for a slight weight increase), and you must take careful precautions when charging LiPo batteries.  I have been fortunate enough to have enjoyed nearly perfect performance and reliability from my glow engines, but even still, I plan on going to electric on all my future aircraft that aren’t large enough to be gasoline powered.

If you are planning on building a larger aircraft you should consider gasoline power.  Much cleaner and cheaper to run than glow, gasoline engines usually also have better reliability.  There is a higher initial investment, but flying all day on only a dollar of fuel will make it pay for itself.  There are a few new 30cc engines available that make ¼ scale gasoline performance viable.  I have had great luck with engines from Desert Aircraft, ZDZ, and Syssa.

[cwojcik]

Airframes

Before I start recommending specific airframes, here are some good things to know.

Firstly, you can design or set up an aircraft up for great precision, or great 3D.  The trend now is to design aircraft that are capable of both.  Generally, an aircraft will 3D well with a lot of control throw, a large diameter prop with low pitch, and a rearward CG.  An aircraft will fly precision well the CG further forward, smaller control throws, and more pitch on the prop for a little bit higher speed at the expense of low-speed thrust.  You can certainly get an aircraft to do both precision and 3D, but there will be a bit of a sacrifice.  If you are purely interested in learning precision, look into a pattern model (more on that later).  If you are only interested in 3D, you might consider a fun-fly profile airplane (more on that later).

Larger aircraft are slower, easier to see, and more stable.  Both precision and 3D are easier with larger aircraft  (this is why IMAC guys fly 40% airplanes).  Of course, they are more expensive (if you do not imagine yourself being comfortable flying your aircraft on a regular basis, then consider something cheaper), and they require better airspeed and airspace management on landing. You can get good performance out of smaller aircraft as well, but they will always be more affected by the wind.

Sizes:

30-40” Wingspan
Power: 50W-200W (2S-3S 500-1500 LiPo) Electric
Approximate cost, receiver ready with 2 batteries: $150-$300
There are an enormous number of aircraft in this size range.  Mostly made from foam (either EPP or Depron), they are durable, ugly, and cheap.  The durability means that you can get away with really cheap components such as servos and speed controls, making them even more affordable.  However, they don’t tolerate wind very well at all, and their flight characteristics can be radically different from what you would find with a larger aircraft (you might learn bad recovery habits with them).  Still a great way to get your feet wet in 3D without a large investment.

Recommended aircraft:
Any aerobatic aircraft from Airfoilz, Multiplex Parkmaster, Any aerobatic aircraft from DWFoamies, 3DEpp.com Yak 55

40-45” Wingspan
Power: 200W-400W (3S 1500-2200 LiPo) Electric
Approximate cost, receiver ready with 2 batteries: $350-$450
A great size, capable of high performance with low cost.  Also a great way to fly in a large back yard or park once you are capable.  However, they can be harder to see, and are much twitchier than larger aircraft.  Probably a better choice as a compliment to a larger aircraft than an introduction to aerobatics.

Recommended aircraft:
3DHobbyshop 42” AJ Slick, 3DHobbyshop 41” Edge 540, Precision Aerobatics Addiction (3D Only), E-Flite Extra 260 480.

45-52” Wingspan
Power: 400W-900W (3S-4S 2100-3000 LiPo) Electric
Approximate cost, receiver ready with 2 batteries: $400-600
This is an extremely popular size right now, mainly because batteries keep getting cheaper and cheaper.  A good performance improvement over 40-45” aircraft, these can still usually be flown in a large park such as a football field or very large backyard, but they also have the presence and stability to make flying rewarding and fun.

Recommended aircraft:
3DHobbyshop 51” AJ Slick, 3DHobbyshop 46” Aspera (precision only), 3DHobbyshop 47” Extra 300SHP, 3DHobbyshop 46” Vyper, ExtremeFlight 48” Extra 300EXP, Extremeflight 48” Edge 540T, Sebart Katana 30E, Sebart Sukhoi Su-29 30E

52”-60” Wingspan
Power: 700W-1200W Electric, 40-55 glow 2-stroke, 72-82 glow 4-stroke
Approximate cost, receiver ready with 2 batteries, if applicable: $500-$700
This size of aircraft is probably the most popular at your local field.  It’s what might typically be called “40-sized,” although that’s a bit of a misnomer now since the advent of good electric power and oversized motors in a 40-sized footprint like the OS 46AX, 55AX, and Saito 82.  It’s a bit of an awkward size because manufacturers often try to design these aircraft to accept both electric and glow power, so there are a lot of aircraft too heavy for good electric performance or too fragile for good longevity as a glow aircraft.  Still a good size that is easy to see and capable of great performance.  Better in the wind than smaller models.

Recommended aircraft:
3DHobbyshop 57” Extra 330SC (electric only), 3DHobbyshop 55” Edge 540 (electric only), ExtremeFlight 58” Extra 300 (electric only, requires separate glow conversion kit), Sebart Katana 50E (electric only, requires separate glow conversion kit), Sebart Sukhoi Su-29 50E (electric only, requires separate glow conversion kit), Great Planes Venus (precision only), Great Planes Reactor

60”-67” Wingspan
Power: 1000W-1800W Electric, 60-91 glow 2-stroke, 91-120 glow 4-stroke
Approximate cost, receiver ready with 2 batteries, if applicable: $600-$1000
Heavier, bigger, and a little more stable than the 52”-60” aircraft.  Easy to see, good in the wind, stable, and still affordable.  Not as many aircraft available in this size as others.

Recommended aircraft:
3DHobbyshop 65” Vyper (electric only), Great Planes Venus II (precision only), Aeroworks 60” Extra 260 QB ARF 60-90 (may be a little heavy), Aeroworks 60” Extra 300 QB ARF 60-90 (may be a little heavy)

68”-78” Wingspan
Power : 1500W-3000W Electric, 100-180 glow 2-stroke, 120-220 glow 4-stroke, 30cc-40cc gas
Approximate cost, receiver ready with 2 batteries, if applicable: $1000-$2000
This size has suddenly become very popular with the introduction of the new DLE 30cc engine, which is very reasonably priced and makes fantastic power.  For a little bit more money you can have the Syssa 30cc gas engine, which makes similar power and is made in the USA.  Still affordable, easily transported,  and capable of fantastic performance.  This is a real sweet spot for a new aerobatic aircraft, and I believe it will soon become more popular than 50cc models.

Recommended aircraft:
3DHobbyshop 68” Velox (although capable of precision, is more suited to 3D), 3DHobbyshop 70” AJ Slick (both gas/glow and electric only versions available), 3DHobbyshop 72” Extra 330SC (Available for backorder, not yet released), ExtremeFlight 78” Extra 300 (both gas/glow and electric only versions available),  78” AeroWorks Extra 300 QB ARF 35cc (probably a little bit heavier than aircraft from 3DHS or ExtremeFlight), Pilot RC 73” Extra 260, Pilot RC 73” Yak 54.

80”-90” Wingspan (50cc)
Power: 50-60cc gas
Approximate cost, receiver ready: $1500-$2500
In the past 5 years or so this size has exploded with the introduction of the DA-50 and the DLE 50 and 55 engines.  They’re fairly inexpensive, fairly easy to transport, and capable of great performance.  Unfortunately, because the market segment is so popular, it has created a lot of awful aircraft available as well.  Many are overweight and poorly designed, but if you choose carefully it’s a great size.  It’s a great market segment for used equipment, as well.  As an introduction to gas aircraft, many people sell these planes to finance larger stuff when they want something bigger.

Recommended aircraft:
3DHobbyshop 89” AJ Slick, 3DHobbyshop 87” Extra 300SHP, ExtremeFlight 88” Extra 300, ExtremeFlight 88” Yak 54

Specialty aircraft:

Pattern airplanes:
If you have no interest in flying 3D, you might consider a pattern aircraft.  They are designed to fly as true as possible, requiring as little mixing as possible.  The new 30cc market segment overlays nicely with competition pattern aircraft, which are, in top classes, limited to 11 pounds or 2 meters in wingspan.  However, because of the competitiveness of pattern flying, pattern airplanes are usually extremely expensive, and it’s not uncommon to spend as much setting up a competition pattern aircraft as a 40% IMAC aircraft.  There are, however, some budget alternatives, like the Piedmont Focus sport.  You could look for used pattern airplanes.  You might also consider smaller pattern aircraft like the Great Planes Venus II (66” Wingspan), 3DHobbyshop Aspera (46” Wingspan), 62” 3DHobbyshop Osiris (which will be out towards the end of the summer), or Sebart Angel (62” Wingspan).

Fun-fly airplanes
Consider a fun-fly airplane if you have no interest in precision and/or you’re on a budget.  These airplanes are characterized by very thick wings with a large chord and small span, huge control surfaces, and sometimes flat “profile” fuselages for lighter weight.  This makes very tight loops and slow speeds possible without stalling, but precision flying is very difficult.  Earlier fun-fly airplanes were so light that high speeds would often cause devastating flutter, requiring careful throttle management.  However, when set-up properly, they can be very easy to fly, and can recover from mistakes quickly.  They can even make a good second airplane.  It’s important to note, however, that fun-fly airplanes don’t really 3D in the same way that scale aerobatic airplanes or pattern-3D hybrids like the 3DHS Vyper do.  The thick wings make for awkward rolling harriers, and they usually don’t “slide” through harriers like an airplane with a thinner wing would.  Some of them also have awful wing rock in a harrier in certain situations.  Good profile airplanes are available from Ohio Model Planes, and ExtremeFlight offers a profile Yak 55 (it’s on sale right now too!).  Full-fuselage fun-fly airplanes like the Hangar 9 Twist, Sig Somethin’ Extra, and E-Flite Eratix are cheap, easy to set-up, and capable of some basic 3D with enough power and control throws.

If you managed to read all of that then congratulations.  Next on the chopping block is proper servo and radio set-up, which should apply to ANY airplane.  If you have any questions, or you are interested in an aircraft that I did not mention, feel free to ask.  Don't feel discouraged or go right out and buy a new airplane if you don't own one that I mentioned...you can probably learn a lot with what you have.  In fact, anything with a symmetrical or semi-symmetrical airfoil will be a fine way to get your start in aerobatics.  I learned to hover with a Northeast Aerodynamics Train-Air 40 (remember those?) and I flew my first IMAC contest with a Somethin' Extra.

[cwojcik]

   Setting up your new (or old) aerobatic airplane is just as important as choosing the right one.  Assuming you’ve researched and selected a good airplane, good servos, a capable radio, and a reliable and adequate power plant, you’re still not guaranteed success.  Take your time and maintain attention to detail as you assemble your aircraft.  While these techniques are especially important for getting a good aerobatic airplane, they apply to all aircraft.  If you have issues getting any airplane to fly right, read through this article and make sure that your set-up is done properly.
   Make sure everything is as straight as you can get it.  If you find a warp in an aileron, elevator, or rudder, twist it gently while going over it slowly and evenly with a heat gun or covering iron.  Often you can get it straight.  Twists like this can make trimming your airplane later very difficult or impossible.  Check your right thrust/down thrust if necessary.  An incidence meter is a good investment.  Plan your build so that you arrive at the proper CG without adding lead weights (don’t secure your battery until you know where you should put it!).  Tie down any cables that could be flopping around; failure to do this can lead to wire or connector failure from fatigue.  Here’s some cheap insurance: for every build, buy a new, high quality switch (heavy duty if the plane is bigger than 40 size is a good idea).  A switch can wear out after repeated use, and will almost always lead to the destruction of the aircraft.
   With everything installed, you can start programming your radio.  I will give you my method for setting up an aerobatic model, but unfortunately, I can’t give you a blow-by-blow description of programming the radio since they are all different.  If the manual that came with your radio doesn’t make everything clear, then check some forums…odds are good that somebody else has done before exactly what you are trying to do right now.  You may also want to simply sit down with the radio with a new model memory and play with it for a while to get familiar with it.  This is how I learn my new radios, chargers, and power tools, and I have yet to ruin any or lose any limbs.
   Before I explain how to program your radio, it is absolutely necessary to fully understand the concepts of servo resolution, servo torque, and servo speed, as well as how they are affected by linkage geometry and ATV.  Servo speed and torque are very simple: the speed at which the servo rotates (typically given in seconds per 60 degrees), and the torque that the servo is capable of producing.  This is typically given in oz-in, but if you are shopping for servos that are a little outside of the ordinary (like Hyperion or Savox) you may find the torque given in kg-cm.  In order to convert from kg-cm to oz-in, multiply by 13.9.  Servo resolution is the ability of the servo to make very fine movements, and unfortunately, manufacturers almost NEVER give you this information.  Due to the nature of our motor-driven, geared servos, it is impossible for them to match the exact position of the control stick; it must approximate it.


   The best way to find the resolution of your servo is to just do some research, and see if people say that the servo in question has good or bad resolution.  If a servo has bad resolution then you’ll probably hear about it, but in my experience, if a servo has good resolution, nobody says anything (probably because they don’t notice it).  A servo with bad resolution will make the airplane hard (or impossible) to trim, and make precision flying very difficult.  Some radio manufacturers will give you a resolution number for the radio itself (1024 resolution or 2048 resolution probably sound familiar to you) but it’s important to note that these numbers refer to the resolution of the transmitter and receiver, NOT the servo.  I have flown a lot of airplanes with 1024 resolution and I can tell a difference between different servos, so I would guess that the resolution of the radio is not a bottleneck, but rather, the resolution of the servo is.
   In order to understand how resolution, torque, and speed are affected by ATV, I’ll have to give an example.  I find that understanding this type of thing is much easier when I greatly exaggerate the qualities or adjustments in question.  Let’s assume you have a servo with really awful resolution; only twenty steps.  It is capable of travelling 100 degrees (so, 5 degrees per step, or 5 degrees resolution).  When you tell it to go to 6 degrees, it goes to 5.  When you tell it to go to 12 degrees, it goes to 10.  Tell it to go to 56 degrees, it goes to 55.  Now, imagine you cut your ATV to 50 percent in either direction so that the total movement of the servo is only 50 degrees.  The resolution is still 5 degrees, however.  That means that the total number of steps is down to 10!  Imagine how this would translate to the feel of the flying airplane if you had this servo, at 50 percent ATV, on the elevator, with only ten elevator positions available.  It would be impossible to fly in a straight line without diving or climbing.
   Imagine you set your linkage up mechanically so that you needed to cut your ATV to 50 percent in either direction.  Not only would you have only half of the resolution available, but you would also only have half of the torque available (think about it).  Blowback on the surfaces, stripped servos, and excessive current draw could all result.  In this situation, the only thing that would improve is your servo speed, because it only has to go half as far (but in reality, most people don’t notice the difference between slow and fast servos).
   You can see that the opposite of this scenario would lead to the opposite effect: by raising the ATV to 125% (and mechanically reducing the control surface movement 25%), you gain 25% more effective servo torque, 25% better resolution, and 25% slower control surface response time.  If you ever program more than 100% ATV, check to make sure that you still have control at the extremes of your stick movement.  If the servo stops responding before you have the stick all the way over, reduce your ATV.  You are over-driving the servo; commanding it to move farther than it is capable.  If you are able to get full response in one direction but not the other, it is probably due to sub-trim.   If you do not get full response after reducing your ATV, then your transmitter probably has worn potentiometers.  Send it back and have it fixed before the problems becomes worse.
   Mechanical set-up is pretty straight forward but very important.  Essentially, you want equal throws in both directions (both sides of neutral) with equal servo movement.  On the throttle, half-stick should correspond to the throttle barrel being half open.  This usually, but not always, means that the servo arm will be perpendicular to the pushrod at neutral position (or half throttle).  In some cases, the horn is offset a little bit so that the pushrod is parallel to the horn at full deflection.
   

   So, to program your new aerobatic airplane, start with a new model in your radio: make sure all ATV/EPA settings are at 100% in either direction, your normal trims and subtrims are at 0, and all mixing is disabled.  Enable any features or mixing that is necessary for the functioning of your airplane (flaperons/wing type, ailevator/tail type, etc).  Reverse channels as necessary.  More “advanced” functions like the flap function of flaperons or the aileron function of ailevators should be disabled for now (and with any airplane other than a fun-fly, probably forever).  Now, check your throws at full stick deflection in both directions.  They should match the 3D rates given in the manual for your airplane (or the high rates, if you are setting up a pattern model or other non-3D airplane).  If they do not, adjust the hole position on your horns first.  Get it as close as you can mechanically, then fine-tune with your ATV (not dual rates – more on that later).  You shouldn’t have to change the ATV by more than 15 percent.  When you are making these adjustments, first attach the linkages to the outermost holes in the control surface horn and the outermost holes on the servo horn.  If you need to increase control surface movement, move the clevis or ball link in on control surface horn.  If you can’t do this, install a longer servo arm.  If you need to decrease control surface movement, move the linkages in on the servo horn.  However, in most cases, one end of the link should be attached to the outermost hole on one of the horns.  This ensures that the link is under the least stress, and that slop is reduced as much as possible.
   Now set your low rates in the radio to match the low rates given in the manual.  If you fly the airplane and feel that your low rates are adequate but you need more throw on high rates, increase your high rates from 100% to 110%, or what you feel is necessary, and check to make sure that you are not overdriving your servos (as described earlier).  Do not adjust your ATV, as this will affect your low rates.  If you can’t get enough throw on high rates by adjusting your throws electronically, increase them mechanically (by installing longer servo arms, if necessary), and re-adjust your low rates electronically.  If your high rates are good but the low rates are too high or low, simply adjust your low rates electronically.  If the high rates are too high, adjust the throw mechanically, and re-adjust your low rates electronically to match what is recommended in the manual.  This ensures that you have the maximum possible resolution and torque available.

   Adjusting your center of gravity is also very important.  First, you should adjust your longitudinal center of gravity.  Having the airplane nose-heavy will make for more stability in regular flying, having it tail-heavy will make it more stable in 3D at the expense of stability in regular, unstalled flying.  First, try setting it towards the forward range of what it calls for in the manual.  When flying straight and level at about half throttle, roll the airplane inverted.  If it requires a little bit of down elevator to keep from gaining or losing altitude, it’s just slightly nose heavy.  This is the preferred CG for precision flying.  If it neither gains nor loses altitude, it’s what’s called “CG neutral” and this is better for 3D flying.  If it climbs when it’s inverted, it’s probably too tail-heavy and won’t be much good for anything but 3D (but if that’s your bag, and the airplane is still controllable on landing and takeoff, then go for it).  The important thing to remember is that there is no perfect CG; you’re always compromising a little bit.
   Unlike longitudinal CG, there is only one correct setting for your lateral CG: right at the center of the airplane.  This is often difficult to check statically because of right-thrust (there’s no good place to grab the airplane!)  Check it dynamically by first trimming the airplane to fly straight and level at half throttle.  Then roll the airplane inverted.  If it requires left aileron to maintain wings level, add weight to the right wing.  If it requires right aileron to maintain wings level, add weight to the left wing.
   You can also adjust vertical CG if the airplane does not roll axially by moving components up or down in the fuselage.  This is very rarely necessary and probably beyond the scope of this course.

   If you have a computer radio (and you should), then you will have access to programmable mixing.  With an aerobatic airplane, you will have to mix out coupling that results from application of the rudder, and maybe throttle.  There are no special tricks here…just program about 5% up elevator to rudder if your airplane pitches towards the belly when you give it rudder in knife-edge, and maybe 2% aileron to rudder if it rolls.  You’ll have to tweak these values a little bit…some airplanes require more, some airplanes require less.  Airplanes with larger props (electrics, and to a lesser extent four-strokes) will often have asymmetric coupling: rudder will have a greater effect on pitch in one direction than the other.  If your radio is capable of tuning the mix in either direction then go for it.  If not, don’t sweat it.  Some radios are capable of multi-point or exponential mixing, which can also be helpful, but that doesn’t start becoming necessary until you are in a very high precision class competition.

   That’s pretty much all I can tell you.  The rest you guys will have to learn at the field… the first class is on Saturday, June 12, at 9:00 AM.  We’ll be covering straight and level flight for the first half (excited!?) and for the second half, the first 3D maneuver to learn: the harrier.  Get your planes as ready as you can make them, make sure those engines are tuned right, and keep your batteries charged.  As always, don’t hesitate to ask if you have any questions.


EDIT: due to a scheduling conflict training will instead start on WEDNESDAY, June 16 at 5:30 pm.

Cody

[TeamBarger]

 Nice job Cody , good info there! Do you still have your Wed. night class at B street ?

[cwojcik]

Yes, every Wednesday night.

Loops and watefalls tonight.