Gear Mesh & Belt Tension

Every model helicopter, outside of the RTF realm, requires the user to line up the gears, adjust a belt, or both. To the new helicopter pilot, proper gear mesh or belt tension can be difficult to attain. If you have asked an experienced pilot about this subject before, chances are they have used the term "feel" to describe how they set gear mesh or belt tension. Knowing how to "feel out" these settings takes time and practice. Don't worry; there are some tips and guidelines that will help.

Gears first: What is gear mesh?
First, we will focus on gears. Before you can set gear mesh, it helps to know what it is. Gear mesh, in simple terms, is the relationship between gears where their teeth engage (or mesh) together. More specifically, gear mesh measures how tightly these gears are pressed together.


Why is proper mesh so important?
The area where two gears meet is crucial because all the power from one gear is transferred to the other at that point. Proper gear mesh allows this transfer of power to occur without adding unnecessary friction to the process. Gears that are meshed too tightly add drag to the system. This added drag strains components, heats things up, robs power, etc. Electric helis are especially susceptible to the adverse effects of tight gear mesh. I have seen instances where failed motors, ESCs, and puffed batteries were traced back to gears that were too tightly meshed. On the other side of the coin are gears that are too loosely meshed. If too much slack is allowed, gears wear down quickly and can strip out and fail in flight.



What types of gears are usually found in a model heli?
With all this talk about gears, we should briefly go over the more common gears relationships found in a model heli.

• Pinion to Main Gear: The pinion gear is attached to the motor or engine, either directly to the shaft or as part of a clutch assembly. This gear drives the main gear, which is the gear attached to the helicopter's main shaft (either directly, or via a one-way bearing).



• Tail Drive Gear to Counter Gear/Torque Tube: The tail drive gear generally sits underneath or on top of the main gear. In helis with a counter gear assembly, the tail gear drives the smaller counter gear. In models where a crown gear is used, the main gear directly drives a bevel gear at the front end of the torque tube.



• Counter Gear to Torque Tube: In many helis, the small counter gear that is run by the tail drive gear, shares a shaft with a bevel gear, which meshes with another bevel gear at the end of the torque tube.


• Tail Box Gears: At the tail end of the torque tube, a bevel gear meshes with another bevel gear that is attached to the tail rotor shaft.


The above list describes the most common gear relationships found in today's models. Your heli may have a few gears that are adjustable, or it may have none. In some helis, all the gear spacing is set in the factory. When this is the case, just put it together, check it out, and fly.

Setting Gear Mesh
Adjusting gear mesh is more of an art than a science, meaning that you do not really measure the mesh. You use sight, touch, and hearing to set it. What you are looking for, in most cases, is to get the gears lined up so that there is just the slightest amount of play between the two.
One tried and true method of setting gear mesh between straight-cut gears, which the car and truck guys have been using for years, is the "paper method". Simply take a small strip of notebook paper and feed it between the two gears you are meshing. Once the paper is between the two gears, push the gears together and tighten the bolts. Turn the gears to remove the paper and look it over. What you are looking for is a nice crisp zigzag shape in the paper. If there are cuts in the paper, you need to loosen the gears a little. If the V shapes in your zigzag look more like U shapes, you need to push the gears tighter together.



To check out your final gear mesh, hold on to the smaller of the two gears and move the other gear back and forth. There should be a very small amount of give. This movement of the teeth of one gear inside the gaps of the other is called backlash. The backlash should be barely perceptible; anything more will be too loose. If there is no backlash, the gears are too tight.

Note that, since some gears are not a true circle, you need to check the gear mesh all the way around. If you find a point on the gear where it meshes more tightly, this is your high point. Mark this spot on the gear and set the gear mesh to this point. I like to set my mesh a little tight at the high point so that the rest of the gear is meshed properly. With a little breaking-in during flight, the high point should wear itself down to a good mesh. After a few flights, recheck the mesh and adjust if necessary.


You usually want to run bevel gears (such as those running a torque tube) tighter than straight cut gears. There should still be some backlash (unless the manufacturer recommends otherwise), but only the smallest amount, just enough to free the gears up. Shims are often used to adjust the mesh on bevel gears or crown gears.


Another way to check mesh is to spin the gears around and listen. With proper mesh, the gears will spin smoothly with little noise. If there is a grinding sound, the mesh probably needs to be looked at again. With time and practice, a pilot will know what the proper amount of backlash feels and sounds like. Have an experienced pilot check the gear mesh on your model before flying it. If they make a change, move the gears a little afterward to familiarize yourself with what proper mesh feels like. In most cases, using the paper method will give good results until you are more experienced at setting gears.

How to spot and avoid gear wear
Gears give off clues both during and after flight that indicate how well they are meshed:

• Look on the helicopter's frame for large amounts of dust of the same color as the gear material. A little gear dust is normal during wear in, but large amounts may point to an overly tight gear mesh.

• Look in between the gear teeth for marks that may indicate that the gears are meshed tightly enough to bottom out on each other. If they are, you will need to loosen the gear mesh a little.

• When gears are too loose, wear is generally indicated by the gear teeth rounding off or stripping out. Loose mesh can also be indicated by excessive slop between the components. It can also manifest as gears showing wear only on the outer ends of the sides of the teeth.

• Gear wear can be avoided by setting the mesh properly and by regular checking of the gears to ensure that they are still lined up.

Belts: What and Where
Belts are often used in model helicopters. The most common use is to connect the tail rotor to the main shaft. Belts are also sometimes used for the main drive train. They are usually made from reinforced rubber or polyurethane, much like the serpentine belts found on a car engine. These belts usually have gear-like teeth on them.


Setting Belt Tension
The proper belt tension is usually defined in a heli's manual using a measure of deflection. The manual will direct you to press a spot on the belt and will give the general distance that the belt should deflect when pressed. The tension can be changed for a tail belt by moving the boom in and out of the frame, while adjusting the tension on a main drive belt involves moving the motor mount. I have found that main drive belts are usually run tighter than tail belts.


Be careful when setting belt tension, if it is run too loose, the teeth can skip, or be stripped, the belt will rattle in the boom, and it can move off its guide pulleys. Too much tension results in more drag in the system and increased wear on the belt, pulleys, and guides. A good test for checking whether your belt tension is too loose is to take the tail rotor with one hand and the main rotor with the other and turn the main rotor while holding the tail firm. If the belt is too loose, you will feel it give way and skip teeth when moderate pressure is applied to the main rotor.


Finally, keep in mind that a belt's tension will vary depending on the temperature when you fly. Check your belt tension each time you go flying and adjust as necessary, you will be surprised how much the tension can change with the weather.

Tail belt tension - when to run it loose or tight
There is no "correct" setting for belt tension. You may need to adjust the belt for a few flights to find a setting that works for your flying style. Generally, a sport pilot will run a looser belt than a 3D pilot. A looser belt uses a little less power to run and puts less stress on the drive train. Pilots that do many autorotations generally run looser belts as well, as a tight belt robs power from the main rotor during an auto.


3D pilots need to run their belts pretty tight. They are much harder on their tail systems than sport pilots and need all the tail authority they can get. With all the heavy pyro moves and sudden stops that come with hard flying, a loose belt would skip teeth and flex too much.

Regardless of your flying style and tension setting, inspect belts often, as they are a part that wears out over time. Replace a belt if it shows cracks, rounded off or missing teeth, or if the strands inside are broken or frayed.

Conclusion

Knowing how to properly adjust gear mesh and belt tension is a skill that you will use often in this hobby, as every model heli out there uses gears, belts, or both. With time and practice, you will be able to feel out how to adjust these settings. Following the simple guidelines above should help to get you in the ballpark. Gears that are meshed properly and belts that are adjusted correctly last a long time and are very dependable. Take the time to perform these procedures properly and you will be rewarded with a better running helicopter. See you at the field!

Adjust Gear Mesh & Belt Tension

Topics like getting a proper gear mesh and good belt tension can be very confusing for new helicopter junkies. The term "feel" is used a lot when an experienced pilot is attempting to explain either of these procedures, and the problem is that a new heli pilot has nothing to base this "feel" on. Not to worry; with a little understanding and practice, setting gear mesh and belt tension can be made easy.

Gear Mesh. What is it?
Gear mesh is the relation between two or more gears where their teeth engage. The point where these gears join together is very crucial, as that point is where all power is transferred from one to the other. Basically, gear mesh measures how tightly two gears are pressed together as they operate.

Where would I worry about mesh in a helicopter?
There are numerous types of gears and locations in a model helicopter. Here is a short list of the most common:

• Pinion Gear: The gear that is attached to the motor shaft in an electric helicopter and the gear attached to the clutch assembly in a nitro one.
• Main Gear: This is the large gear found below the main shaft, receiving power from the pinion gear and driving the main rotor head.
• Tail Drive Gear: This gear is found underneath or on top of the main gear, and is driven by the main gear or by the main shaft via a one-way bearing. This gear can be of numerous types: a standard gear, a crown gear, or a belt pulley, depending on the helicopter it is in.
• Secondary Tail Gear: This gear is not found in all helicopters, but is present in many. When a helicopter's tail drive gear is a crown gear or a belt pulley, this gear is omitted. This gear meshes with the tail drive gear. This is usually done in one of two ways: In belt-driven tails, this gear shares a shaft with a belt pulley which will run the tail belt. Or, in shaft- driven tails, this gear shares the shaft with a bevel gear which will mesh with the bevel gear at the end of the tail shaft.
• Tailbox gears: In shaft-driven tail systems, the tailbox will have two bevel gears (one at the end of the drive shaft and one on the tail shaft) that mesh together to run the tail rotor.

Please note that in many modern helicopters, the mesh has already been set for some of these gears and the factory mounting holes are already set for them. In these cases, just bolt and fly!


Why is proper mesh important?
When any set of gears in a helicopter is not meshed properly, performance can greatly suffer. When gears are meshed too tightly, drag is induced in the system, which will rob power from the system, wear out the gears prematurely, and will put extra strain on the components that the gears are attached to (especially in electric systems, where the added strain on the motor can burn it or the power system out in a hurry). When gears are set too far apart, the teeth can strip in flight, there will be play between components, and the teeth will wear off quickly.

How to Spot & Avoid Gear Wear
There are a few clues that a heli will give you when a gear is wearing poorly. Look around a gear for large amounts of dust that is colored the same as the gear material, which would be actual material coming off the gear during flight. A little gear dust is normal during break in, but large amounts may point to a gear mesh that is far too tight. Also, look in between the gear teeth for marks and material which may indicate that the gears are meshed tight enough to bottom out on each other. When a gear is running too loose, the wear is generally indicated by the gear teeth rounding off. When a gear's teeth have lost their sharp edges you can be sure that the mesh is loose. Also, a loose mesh can often be indicated by missing teeth and slop between the components. All these types of wear can be avoided by setting the mesh properly and by regular checking of the gears to make sure they are still lined up.


How to Set Proper Mesh
So here it goes, there are a number of ways to set a good gear mesh. Car and truck drivers may have heard of the "paper method," in which a piece of notebook, printer, or even cigarette paper is cut into a small strip and placed between the two gears being pushed together. With the paper strip pressed tightly between the two gears, tighten the adjusting screws down and remove the paper. The paper should be pressed into a zigzag shape from the teeth, but not be cut through. If the paper is cut through, the mesh may be too tight. This method works for many applications and has been proven as a tried and true system. But it is important to check the gears regardless of the method used. To check gear mesh, hold the smaller gear of the two and move the other gear back and forth. A small amount of movement--very small--should be present. This movement is called backlash, the movement of the teeth of one gear inside the gaps of the other. This movement should, in most cases, be very small and just perceptible; anything more will be too loose. If no movement is present, the gears are too tight. This is where "feel" comes in. With time and practice, a pilot will know what the proper amount of backlash feels like. It is very helpful to have an experienced pilot set it the first time so that you can get a "feel" for it by moving the gears. Using the paper method will get very good results until this "feel" can be obtained.
It is also important to note that since most gears are not perfectly round, the mesh needs to be checked at various points around the gears. If there is a high point in a gear where the mesh is tighter than the rest of the circumference, it may work best to set the mesh tighter than normal at that point, so that mesh will be right around the rest of the gear, the high point should wear in over a short time.
All gears should be set with a small mesh, with as little backlash as possible (but with no drag or tightness). Another way to check if things are too tight is to just spin the gears and listen; they should spin freely with little noise. If there is a grinding noise when the gears are spun, the mesh may be too tight. Use shims or spacers wherever necessary to get gears nicely meshed together.


Belt Tension
Belts, where are they used?
Belts can be used in helicopters in two places: the tail drive and the main drive. Using a belt as a main drive (in place of a pinion and main gear) is pretty rare in a modern helicopter, but can still be found. Belt drives being used to run the tail rotor are more common, and in fact are becoming more popular than their shaft drive counterparts due to their ease of installation, lower parts count, and inexpensive repair costs. The belts in a helicopter have teeth in them as do the pulleys that drive them.


How to Spot & Avoid BELT Wear
Belts can wear just as gears do. Some telltale signs of a belt that is wearing out are a fine dust or powder coming from the belt, missing teeth on the belt, strands of the reinforcement bands coming out of the belt or stringing off of it, or teeth that are rounded on one side or the other. While it is difficult to avoid belt wear, as they do wear out quicker than gears usually do, it is possible to prolong their life by doing some of the following actions. Make sure that the belt is neither overly tight, nor overly loose. One way to check for a belt that is too loose is to grab the head block and tail hub at the same time, and while holding the tail hub tightly, turn the head block. The tail belt should not skip, even with considerable force. If it skips, tighten it up. Also, make sure the boom is straight and that the belt does nut rub against anything in its run. Once a belt has started rounding off at the teeth, it needs to be replaced.


Belt tension, how to set it?
Setting belt tension is fairly straightforward. The manual for the helicopter should have a general recommended tightness, usually measured by how far the belt can be pushed, or deflected, in before it stops. Start with the recommended setting. Setting this tension is simple; with a main drive belt, simply loosen the motor mounts and move the motor back and forth until the desired tightness is found (main drive belts like to be pretty tight to prevent skipping during use). With a tail belt, loosen the boom and pull it from the frame until the belt is at the desired tightness. Too tight of a setting will stretch the belt, rob power, and wear out quickly. Too loose of a setting can cause the belt to skip while running, strip teeth, rattle in the boom, and run off the pulley.

Why would I run the tail belt loose?
The tail belt can be run tight and loose, and unlike gear mesh, can be run either way and work fine (within limits). The main reason a pilot would want to run a relatively loose belt is to maintain power during autorotations. A tight tail belt will rob a bunch of head speed in a driven auto, making the auto more difficult to perform. A looser belt will use less power to run and creates less wear on the shaft bearings that support the pulleys.

Why Would I Run The Tail Belt Tight?
3D pilots generally run a tighter belt than sport pilots. The reason to run a tight belt is for greater tail authority. A tight belt has very little play and will not skip or flex during hard maneuvers.

Conclusion
Learning to set proper gear mesh and belt tension will help you throughout your time in this hobby. When the gears and belts are set correctly in a helicopter, the machine runs smoothly and efficiently, not to mention that the gears and belts will last a very long time as well. And who doesn't like that?

Repair Your Broken Landing Gear

It happens all too often: a minor or severe crash breaks off the mounting tab for your landing gear from the main frame. Often the frame is not badly damaged. However, without a means to mount the landing gear, you may be forced to replace the entire frame. This how-to was written to provide you with some ideas for mounting the landing gear in the event that the tab breaks off, and it will hopefully save you from having to replace the whole frame.

This author has two methods for approaching the repair. Every model will require a different approach because of variations in both design and crash damage. When I damage a mounting tab, I try the first method listed here, and if that is not practical I move to the second.

Method 1
If the model has a bottom support that is strong enough to be mounted to, I mount the landing gear on the inside of the frame.


1. Make sure there is enough material to mount to on the inside of the frame. If it seems too weak to support the landing gear, move on to Method 2.


2. Hold the gear firmly in place with the frame upside down and drill a new mounting hole on the inside of the frame. Keep the hole as close to the outside as practical while still maintaining the strength in the landing gear. Be cautious of anything mounted above the landing gear.


3. Use a machine screw inserted from underneath with washers and a self-locking nut on the inside of the frame.

Method 2
If there is nothing to mount to on the bottom of the frame, and you don't want to replace the entire frame, you will have to build a replacement mounting tab.


1. You will need a smooth mounting area for your new bracket. If part of the old mounting tab is left over, use a cut-off wheel to make it flush and smooth.


2. Make your new mounting tab bracket out of a small piece of aluminum or an angled piece of plastic. All you need is a simple 90-degree "L" bracket. Select the material you are going to use based on the size of your model, and cut it down so it is just large enough to serve its purpose.
3. Use a machine screw with a self-tapping nut on the inside of the frame to mount your bracket to the side of the frame. Mount it so the bottom of the bracket lines up flush with the bottom of the frame.


4. Drill your new mounting hole as close to the frame as possible. You might not be able to get the mounting hole close enough to the frame to allow you to use the original mounting hole in the landing gear. If that is the case, drill a new mounting hole in the landing gear.


5. Use a machine screw with a lock-nut to mount the landing gear to the new bracket.

TECH TIP
Mounting Tab
Keep pieces of scrap aluminum or pieces of angled plastic in case you ever need to make a replacement mounting bracket. Sometimes broken parts can live on to fly another day and save you time and money.

Conclusion
It's really annoying when you do minor damage that forces you to replace major components of your helicopter. If you only break the mounting tabs for your landing gear (which is often the case) you can use these tips to get your bird back in the air without all the time involved in replacing the whole frame. There aren't very many parts on a helicopter where it's safe to make your own replacements, but the landing gear mounting tabs can be replaced and work fine as long as they are sturdy. Get a little creative and dig into your parts bin for scrap aluminum, and your heli can keep flying with ghetto fabulous style!

Replace Your Clutch Liner

After reading a few posts on internet message boards, and talking to several fellow pilots, I seem to hear the same thing: "It's too hard to replace a clutch liner," or "I've tried, but can't get it correct, so I just replace the complete clutch bell." Well, this is okay, but expensive. Hopefully after reading this you'll be changing your own just like the pros.

There are several reasons clutch liners go out, but here are the 3 most common:

1) The hot start.
If not stopped very quickly, this is a sure clutch killer.
2) A crash with idle up on.
Come on, we've all done this at least once.
3) Worn out; great!
It takes a considerable amount of normal flying to wear out a clutch liner, and if this is the case, you're doing well!


NEW CLUTCH INSTALL: STEP-BY-STEP
1. Inspection
So there you sit with a box full of perfectly good clutch bells, with little or no liner. The first thing we want to do is clean and inspect the clutch bell, as there is no sense in putting a new liner in a bad clutch bell. Clean it thoroughly with alcohol, and wipe it dry. Now place the clutch inside it, and look around the complete circumference to see if the gap is the same all the way around. It doesn't have to be perfect, but any more than about .010" out of round is going to give you a less then perfect clutch engagement. If the old liner is still inside, and is in really bad shape, you will probably need to remove it before evaluating the bell.


2. Out With the Old
To remove the old liner and glue--and this is important--you must get the bell back down to bare metal. First, I like to protect the bearing, so if it's in place and in good condition I choose to leave it there and simply place a small piece of duct tape over it. Next, if all or most of the liner and glue are gone, I simply take coarse sand paper (I use 80 grit) and sand out any remaining liner and glue. If there is a considerable amount of material to remove I have found the following to work very well.
Gently chuck the pinion gear into a variable speed drill (I use my cordless), turn it on very slowly, and check that the bell runs true. If not, simply loosen it, rotate it slightly and try again. I have always been able to get it to run true after a few tries. DO NOT OVER TIGHTEN or you will damage your pinion. Now while slowly turning the clutch bell, sand the inside with the coarse sand paper. This usually only takes a few minutes, and will leave behind a clean and roughened surface to glue your new liner to.
NOTE: Never try to remove the liner with anything other than sand paper while spinning the bell in a drill.


3. In With the New
Start by test fitting your new liner. Some brands come pre-cut to the correct length, and some will need to be trimmed. I like to see a tiny gap, maybe 1 /32" or so. Now take your clutch and apply several wraps of electrical tape around it (6 to 8 should be fine), being careful to stay flush or close to flush on the side that goes into the bell first. On most clutches this will leave the tape hanging over the opposite side, and this is fine. Take a razor knife and cut a line through all the layers of the tape, right down to the clutch.


4. Easy Does It!
With the liner just lying in the bell, gently push the clutch into the bell, which should press the liner against the bell. DO NOT FORCE IT! It should go with just a light push. If it does not, simply go to the line you cut through the tape, remove one layer and try again. Keep doing this until you get a light push that is holding the liner to the bell without undue pressure being needed to insert the clutch. If on your first try it was too loose, remove all of the tape and re-wrap it using more layers. This may also indicate that either your clutch or clutch bell is excessively worn, and may need replacement.


5. Sticky Situation
We want to glue the liner into the bell. For this I use slow, medium thick CA glue. I brush it on the inner surface of the bell with one of those cheap throw away brushes, this way I am sure to get 100% coverage with a thin coat of CA. Now carefully put the liner in and push it against the glue. Next, push your clutch with the tape back in, and it should hold the liner in perfect position. Give it a good look--this is why I use SLOW CA--and adjust if necessary. Once you get it in place correctly, set it aside overnight.

6. Check Your work
Okay, it has sat overnight, now you want to see your work. Gently remove the clutch, and remove the remaining tape from it. Now wash the clutch with alcohol to be sure there is no sticky tape residue left on it. Place the clutch back into the bell and check clearance; the recommended clearance is slightly different for different models. However, I have found that if you can put 1 to 3 pieces of regular paper between the clutch and bell, it will work fine.


All Finshed!
That's it, you're done, and usually for less than 1/10 the price of a new clutch bell. After a few tries this entire procedure takes about 30 minutes, and you'll be doing it like an old pro.

Understanding Autorotation

The first step in learning about autorotation is to understand that rotor blades are rotary wings. We can all imagine an airplane flying without a motor (glider), but it's a bit more difficult to visualize a helicopter doing the same. By recognizing that each blade is a wing that acts like the wing of a glider, autorotation is more comprehensible. The blade element diagram can be used to understand the forces acting on the blade during an autorotation.

Blade Element Diagram
Normal Powered Flight

In May's edition of RC Heli, we defined the components of the blade element diagram and how they work during normal flight: The blade sees a combination of rotational flow (1) and downward induced flow (2) called relative wind (3). The angle of attack (4) is the angle formed between the relative wind and the chord line, and the pitch angle (5) is formed between the rotor plane and the chord line. Lift (6) is the total aerodynamic force perpendicular to the relative wind. For a helicopter in hovering flight, the lift vector is tilted aft. The lift vector can be broken into two components: a vertical component (7), which is the total force that generates vertical lift, and the rearward component called the induced drag (8), formed by the acceleration of air mass (downwash) and the energy spent in the creation of trailing vortices. Induced drag must be overcome to develop lift, and power is required to the rotor system to overcome this drag. The remaining vector on the blade element diagram is profile drag (9), a result of air friction acting on the blade element.

Blade Element Diagram
Autorotation

To keep the blades turning during normal flight induced drag must be overcome by engine power. During an autorotation, the power that turns the rotors comes from another source-potential energy (gravity), as the helicopter loses altitude. The rotor head will initially slow down, feeding on its own rotor inertia. Lowering collective will stop the decay. The increasing up flow of air through the rotor system reverses airflow. With
the lift vector always perpendicular to the relative wind, induced drag reverses and the lift vector is tilted forward providing a Pro-Autorotative force that turns the rotor head. A component of profile drag (In-Plane Drag) acts in opposite direction to the Pro-Autorotative force.

The Autorotative Regions of the Blade

As the rotor blades travel around the arc, each part of the blade sees a different relative wind--from lowest velocity at the hub, to highest velocity at the rotor tip. This is because the rotor tip has to travel farther in the same period of time as the part of the blade near the hub. During an autorotation the rotor blade sees three different regions: prop, auto, and stall region.

Auto Region • Rotational speed combines with induced flow, shifting the relative wind below the rotor plane. Notice that the lift vector is tilted forward, providing a pro-autorotative force. This region is increased and shifted toward the blade tip at higher pitch settings, decreasing rotor rpm and slowing the sink rate.

Prop Region • Relatively high rotational speed at the outer portion of the rotor disk combines with induced flow, shifting the relative wind towards the horizontal. Notice the lift vector is tilted more vertical than forward, providing less pro-autorotative driving force. In this region, the profile drag is the largest and causes greater anti-autorotative force. This region is increased with lower pitch settings and higher rpms, thus reducing the auto region, resulting in a faster sink rate.

Stall Region • The stall region is at the blade root, where rotational flow is reduced to the point where lift is not generated and profile drag dominates. As pitch angle is increased, rotor rpms are reduced and the stall region increases across the blade, reducing the auto region and prop region.

Putting it
all together
As blade pitch and rpm changes, the three regions change across the blade. It is very important that during an autorotation, pitch angle and rpm are controlled for the most practical use of the blade regions.

Autorotation Entry
When the motor decides to quit at the most inconvenient time, the lift vector is pointed aft, quickly slowing the rotor head without the motor to overcome the induced drag. The pilot quickly lowers the collective to decrease the stall region on the blade.

Descent
During the descent, the pilot starts to control the pitch of the blades in order to adjust the amount of prop and auto region. If the pilot wants more lift, he adds collective, slowing the rotor head and increasing lift. For a higher sink rate, the pilot decreases pitch, increasing rotor rpm. This balancing act is optimized in order to find a suitable rate of descent that will get the pilot to the desired landing point.

Flare
Getting close to the ground, the pilot trades airspeed for rotor head power by flaring, increasing the reversed induced flow through the rotor head, increasing the lift vector, and tilting it more forward, causing a higher rotor rpm.

Touchdown
As airspeed is traded, the helicopter starts to settle. At this point the pilot trades energy from rotor inertia into greater lift. Hopefully the pilot didn't go too deeply into the bank account of rotor energy. Before the blades stall, the helicopter should settle safely on the ground.



Conclusion
I hope this took some of the mystery out of autorotation. By adjusting pitch you control the direction and magnitude of the relative wind, lift vector, and pro-autorotative force. Proper collective management through the decent, flare, and touch down will save you from buying a new set of skids and will result in a magnificent crowd-pleasing auto.

Mastering They Gyro

So what exactly is a gyro and why do I need it? A gyro is a device that senses angular movement and generates feedback to the control system in order to stabilize it. We use a gyro in our helicopter to stabilize the rotation around the main rotor shaft (yaw axis). The gyro helps reduce the pilot workload associated with keeping the helicopter pointing in a given direction by counteracting any unintentional yaw caused from the forces exerted on it by wind and torque. A sensitivity (gain) adjustment on the gyro determines the amount of effort put into corrections. When the gain is adjusted too low, the gyro will not effectively stabilize. When adjusted too high, the gyro will overcorrect, resulting in oscillation (wagging).

What about the different modes of operation?
We know the gyro will sense and correct, but how it does this varies depending on its mode of operation. The two common modes are "rate" and "heading hold."

• Rate
A rate gyro senses angular velocity (see sidebar) and counteracts it with input to the rudder servo. It detects when the helicopter rotates and will reasonably attempt to offset it. I say reasonably because a rate gyro has no sense of heading; it simply provides correction based on velocity and timing. It has no way of determining if it actually moved back to the original position.

Unfortunately, rate gyros exhibit an unwanted side effect: They may actually resist (or dampen) rudder commands initiated by the pilot.



• Heading Hold
As its name implies, a heading hold gyro (a.k.a. Angular Velocity Control System or tail lock) will provide the correction necessary to maintain a heading. Any deviation to the helicopter's heading not initiated by pilot command causes the gyro to apply correction until it's returned back to its original heading.

Unlike a rate gyro, the heading hold gyro does not resist pilot command.


What about the features?
In addition to the gain adjustment, a gyro may feature additional functionality and adjustments. The majority of gyros offered today contain most if not all of them.



Where and how should I mount it?
Some believe that the gyro must be as close to the main rotor shaft as possible. This is simply not true, as I explained in the angular velocity sidebar. There are, however, some other things to consider when choosing a mounting location.

Protection — The gyro should be protected in the event of a crash. It's for this reason that I don't advise mounting it on top of the tail boom mount. It can be easily whacked by the flybar or the flybar paddles. Good candidates for location would be under the tail boom mount, inside the frame, or up front.

Accessibility - You'll most likely need to make adjustments to controls on the gyro itself. If you mount it inside the canopy, you'll have to remove the canopy every time you need access to the gyro.

Interference - In electric helicopters, you'll want to keep it away from sources of electrical interference, such as the motor and ESC.

Secure - Mount it to a clean, solid surface with one layer of double-sided vibration damping tape. As an additional precaution, it can be further secured with a Velcro strap or a nylon cable-tie.



Bench setup
Initial Transmitter Settings
Dial the following settings in for the rudder channel:
• Servo Reverse: Check for proper direction.
The helicopter's nose must turn in the same direction as you move the rudder stick. Tail rotor pitch should increase when the rudder stick is moved to the right. Set servo reverse accordingly.

• Trim: 0 (centered)
This ensures that you start with a good mechanical setup.

• Sub-Trim: 0
Once again, this ensures that you begin with a good mechanical setup.

TIP! With the rudder stick centered, you may observe that the rudder servo walks to one end when the gyro is in heading hold mode. If it does AND you're sure there isn't any trim or sub-trim dialed in, your gyro does not sense neutral but rather expects a specific neutral pulse width. Should this be the case, just dial in opposite direction sub-trim until the walking stops. You should also note that it may be temperature sensitive and could require subsequent adjustments.

• Adjustable Travel Volume/End Point Adjustment:
Maximum (in both directions)
Normally, this would control the amount of servo travel. However, because of the gyro's design, this controls the pirouette rate—not the travel limit. We'll adjust travel later on mechanically, and if available, with the gyro's limit adjustment.

• Exponential: +30%
This compensates for the non-linear pushrod movement that occurs due to the circular path of the servo arm (see sidebar on pushrod geometry). It will bring the pushrod movement closer to linear with respect to rudder stick movement. Some advanced pilots prefer the feel of no exponential on the tail.

• Dual-Rate: 100%
We may adjust this later. If the gyro's electronic travel limit is used to reduce servo travel, sometimes you'll notice that the servo will reach its limit without full rudder stick deflection. Dual rate can be used to bring the servo's limit and the rudder stick's full deflection in sync with each other.

• Revo Mixing: Inhibited
This will be adjusted later if you're using a rate gyro or intend to operate a dual mode gyro in rate mode.



• Gyro Mode & Gain/Sensitivity: 70%
If you have a dual mode gyro, one side of neutral puts the gyro in rate mode, and the other side puts it in heading hold mode. The further away from neutral you are, the higher the gain in its respective mode. If you use a two-position auxiliary channel (such as the gear channel), you'll make the adjustments with the channel's ATV/EPA feature.

TIP!You can use a spare proportional channel to control the mode and gain, thus having an infinite on the fly adjustment. The downside of this is that once it's set to your liking, if the control is bumped, you must re-determine the correct position.

Initial Gyro Settings
Where available, dial in the following at the gyro:
• Gain/Sensitivity — 70%
This is a good starting point. Some radios have a gyro menu that set the values differently. Follow your radio's manual to set up the gyro sensitivity appropriately.

• Limit — 75%
If your gyro provides limiting, you want to make sure it won't produce excessive travel and cause binding prior to adjusting it.
• Delay - 0 (no delay)
No delay is required initially.

• Digital Servo or High/Low Frame Rate - Set accordingly

IMPORTANT: An incorrect switch setting here can potentially destroy your servo. Make sure your servo can support a high frame rate BEFORE putting the gyro in that mode. If in doubt, use the low frame rate.

Mechanical Setup
Here comes the meat of the setup procedures. I cannot overstress the importance of proper mechanical setup. In fact, gyro performance depends on it. There are five things we're looking to achieve:

1. Smooth operation of the pushrod and tail rotor pitch change mechanism
2.Formation of pushrod at a 90-degree angle with the rudder servo output arm at neutral
3.Tail rotor pitch control at its center of travel with rudder servo at neutral
4. Maximum tail rotor pitch travel with nominal rudder servo travel of approximately 80 degrees
5.Correction provided by the gyro in
the proper direction

Begin by detaching the pushrod from the servo arm. The pushrod and pitch change mechanism must move freely in order for the servo and gyro to efficiently and accurately do their job. Move the pushrod by hand and make sure there is no binding in it or in the tail rotor pitch change mechanism throughout its entire range of movement. It should be silky smooth.

Turn on your transmitter. If you have a dual mode gyro, make sure to set it in rate mode. Now turn on your receiver. With the rudder stick at neutral (or centered), the rudder servo output arm should be such that it forms a 90-degree angle to the tail rotor pushrod. It may be necessary to remove the arm from the servo and reposition it on the output shaft. Get it as close to 90 degrees as possible without using any trim or sub-trim.


To mechanically set the pushrod's travel range, we'll need to measure the distance the pushrod travels when moving the tail rotor pitch control mechanism from stop to stop. Now we'll need to do a little math (see sidebar on pushrod geometry). This is how you'll determine how far out on the servo's output arm to attach the pushrod.

With the rudder stick centered, adjust the length of the pushrod so that the tail rotor pitch change mechanism is at its center of travel when the servo end of the pushrod is held over the attachment point. Attach the pushrod to the servo's output arm.

Now set the servo's travel limit. This is done via the limit adjustment on the gyro, not from the transmitter. Move the rudder stick and observe the movement of the tail rotor. Increase the limit until the pitch change mechanism is just short of hitting the stops with full rudder stick deflection. Check it in both directions. Once the limit is set, you may notice that the servo reaches its limit before the rudder stick reaches maximum deflection. The rudder dual-rate can be used to correct this. Decrease the rates until the servo reaches its travel limit, just as the rudder stick reaches full deflection.

[sidebar]NOTE: If you don't have a limit adjustment on your gyro, measure the full servo deflection. Use that angle in your calculation.

Finally
To complete the bench setup, you'll need to verify that the gyro provides correction in the proper direction. Observe the direction in which the pushrod moves when you move the rudder stick to the left. While looking at the helicopter from behind, bump the tail to the left. The pushrod should move in the same direction as it did when you moved the rudder stick to the left. If it didn't, switch the reverse switch on the gyro.

NOTE: It may be necessary to change the
transmitter's servo reverse on the rudder channel if you reversed he direction of gyro correction. Always check for proper operation.

On to flying &
the final adjustments
The remainder of the setup will need to be done at the flying site.

Final Trim - If you have a dual mode gyro, verify that it's in rate mode. If you don't have a rate mode gyro, move on to the gain adjustment. Hover the helicopter with the nose pointing into the wind and trim as necessary using the transmitter's rudder trim lever. If you added trim, land the helicopter and mark the position of the tail rotor pitch arm with the rudder stick at neutral. Re-center the rudder trim lever and adjust the pushrod length so that the tail rotor pitch arm is back at your mark. Operate the control and check for binding. You may need to redo the limit and dual-rate adjustments.

Gain Adjustment — If you don't experience a tail wag in a hover, keep increasing the gain until you do. Then, decrease it just enough to stop the wagging. If you have a dual mode gyro, you'll need to do this individually for both modes of operation.

Delay — While hovering, initiate some relatively hard rudder input. If the tail wags when you stop, you can increase the delay adjustment until it doesn't do this anymore.

Revolution Mixing — Although this has nothing to do with the gyro, per se, you can use revo mixing to help keep the helicopter straight with changes in collective/power. There are individual adjustments for "up" (correcting for increase in power with positive pitch), "down" (correcting for decrease in power with positive pitch), and "negative pitch" (correcting for increase in power with negative pitch).

REMEMBER
Revo mixing should only be enabled while operating in rate mode.

Angular Velocity
The definition of angular velocity is "the rate of turning of a body about an axis expressed in angle turned per unit." The axis of concern is the yaw axis, which is the helicopter's rotation around its main rotor shaft.
Visualize the main rotor shaft as a point and the helicopter's frame as extending out from that point. You'll notice that if you rotate that line around the point, any position on that line will traverse the same angle.
It's for this reason that regardless of where you mount the gyro in the helicopter, its ability to sense angular velocity will be the same. It's only important that the gyro's sensing axis is perpendicular to the main rotor shaft.
Pushrod Geometry
The adjustable travel volume (ATV) or end point adjustment (EPA) on the radio allows you to specify the travel limits on each side of neutral, typically from 0% to 150%. This translates to a servo rotation of 0 to 60 degrees from center. Using this information, 100% servo travel would produce a servo rotation of 40 degrees each way and a total travel of 80 degrees.
The pushrod's travel is not linear with respect to degrees of rotation. As it moves further away from neutral, the pushrod moves less for each degree of servo rotation. Therefore, it's desirable to get the control deflection required with the travel limits at 100%.
Looking at a servo wheel (or arm) with a pushrod attached, you can use basic right triangle trigonometry to calculate how far out from the center or rotation to attach the pushrod in order to get the control deflection you require.
Your ATV/EPA values on the tail affect the maximum pirouette rate, the actual ATV/EPA value for your tail must be adjusted on the gyro.
The formula we use is Sin( ) = Opposite/Hypotenuse. The opposite side of the triangle is half the distance we want the pushrod to travel. The hypotenuse is the distance out from the center of rotation that we need to attach the pushrod, which is what we're looking to determine. Solving for the hypotenuse, we can use the formula Hypotenuse = Opposite/Sin( ).
As an example, let's say your total pushrod travel needs to be 20mm. That would be 10mm travel from neutral on each side, the opposite measurement in our formula. In our case, the angle we use is the maximum servo rotation of 40 degrees, and Sin(40) = 0.64. If we plug in the numbers Hypotenuse = 10mm/0.64, or 15.6mm, this is the distance out from center that we must attach the pushrod to the servo arm.


Some final advice
A heading hold gyro wants to maintain a heading. It doesn't know that you carried it to another spot. If you move the helicopter and happen to put it down facing a different direction, the gyro will provide correction and return the helicopter to its original heading as soon as it lifts off. This could be quite unnerving.

If you need to move the helicopter after you turn it on in heading hold mode, simply do one of the following:

• Leave the gyro in heading hold mode and move the rudder stick around while carrying the helicopter and keep moving it until just after you put it down.

• Switch the gyro to rate mode, move the helicopter, then return the gyro to heading hold mode.

The flight characteristics of the two modes of operation are different. With a rate gyro, the tail of the helicopter tends to follow the helicopter through a turn. A helicopter with a heading hold gyro, on the other hand, must be steered through a turn.

Happy flying ...