Setting up a Helicopter
From HeliNet
Contents |
Helicopter Setup
Blades
Main Blade Balancing
Mechanical Setup
Remember, that you can only fine tune the mechanical setup with the electronics, so it is important to get the mechanics as good as possible before moving on.
3D Flying
Symmetry about the 0° point.
F.A.I. F3C Style
The emphasis is on hovering maneuvers, a setup point around hovering pitch (5°-6°) should be established.
Symmetrical Setup Overview
The symmetrical setup starts by identifying the 0° point. At the rotor head, this is the point where the blade holders and their mixing levers are level. On the mast, this is the point where the swash plate is in the middle of its travel and the washout arms are level. Servo mid points are those points that position the servo output arm perpendicular to the control rod when the transmitter sticks are centered.
The radio only needs to be turned on when you are ready to connect the mechanical linkages to the servo. There are some radio adjustments that go hand-in-hand with the mechanical adjustments, and these are covered as we get to that point.
Main Rotor Head
The first mechanical alignment is that from the blade holder to the Bell/Hiller mixing levers, which combine the swash plate input with that of the flybar. They mix the direct Bell input, from the swashplate, with the indirect Hiller, from the flybar.
Adjust the linkage from the blade holder to the mixing lever so that the blade holder is at 0° while the mixing lever is perpendicular to the main shaft. Once you have adjusted one linkage simply copy the length to the opposite blade holder. It may be easier to use a ball link duplicator.
The second adjustment resides on the other end of the Bell/Hiller mixing lever. This is the input from the swash plate so it is essential that the swash plate be held square to the main shaft. The length of these two rods are adjusted so that the swash plate is in the middle of its travel. This ensures that you get the full range of movement on both the positive and negative pitch ranges.
If you are unsure where the mid point of travel is, look at your plans or assembly instructions. Usually, there is enough information here to determine the approximate mid-point. If you do not get the accuracy immediately, it is a simple matter to adjust these linkages when fine-tuning at the end of the mechanical setup. While adjusting the length of these two rods, ensure that the flybar is also level so that the blade holders are held parallel to the workbench (0°). After the length of one rod is determined, simply duplicate it for the other.
The result is: the swashplate will be in mid travel with washout arms and mixing levers level. The blade holders will have 0° pitch.
The last linkage on the rotor head to be adjusted is the simplest. These are the two linkages from the washout unit to the flybar. The purpose of the washout unit is to remove the collective pitch input while still allowing the cyclic input to deflect the flybar’s paddles.
Swashplate to the washout unit.
Place the swashplate in mid travel, level the washout units arms and adjust the rod length to fit between here and the flybar.The flybar must be level.
At this point, take a look at what you have achieved:
- Blade holders level at 0°
- Bell-Hiller mixing levers level
- Flybar level, paddles at 0°
- Washout control arms level
- Swashplate in mid-travel and square to the main shaft
Cyclic Servo to Swashplate Setup
The servos must be connected to the swashplate, usually via L-levers or bell-cranks. It is not important whether you work from the swashplate to the servos or vice versa. What you are going to accomplish is to make all levers 90° to the control rod when the servo is at neutral or center.
It is at this point that some radio work must be performed, so power up the transmitter and receiver. The first thing to do is expand all the travel adjustments to their maximum. On most radios this is known as ATV.
Next, select the output arms and wheels needed and fix the ball links to the arm. Many people like the aluminum output arms, and certainly, these seem easier and more secure when screwing a ball link down. A personal preference of mine is to place 2mm nut under the ball to lift it off direct contact with the servo arm. This allows more room for the shoulder of the ball link when it moves over the arm and removes the potential for interference.
Turn the radio on and ensure:
* Sticks centered * Trims centered * Hovering pitch hover trim centered * Hovering throttle trim centered * Subtrims at zero
Place the output arms/servo wheels on the cyclic servos. For each servo try to place the arms on the spline that most closely places the arm at 90° to the control rod. If it appears that you cannot get the arm perpendicular to the control rod perform the following:
Subtrim
Access the subtrim function for that channel (aileron or elevator) and adjust the subtrim so that the servo wheels rotates to the 90° position.
If you use the subtrims for recentering your trim sliders, you will offset your mechanical setup. To trim the helicopter in the hover, you readjust your linkages to the swashplate, this keeps the servo output arms symmetrical about their center or neutral.
Adjust the mechanical advantage of the servo arm so that the swashplate is not driven over its articulating ball. Alternatively, you can reduce the cyclic ATVs , but if using this method do not allow the ATV to fall much below 100%. Falling below 100% indicates that too much advantage exists which increases the load to the servo. Check to see that it does not bind by giving full fore-aft and left-right cyclic (place the cyclic stick in a "corner"), and rotate the head. You can feel if any binding takes place.
Technically, the best mechanical linkage arrangement exists when the arm length is at a maximum at both the servo and the control surface. The constraint is determined by the adjustment available at each of the arms which therefore affects the mechanical travel volume. In summary, try to keep the ATV adjustment to a minimum.
Throttle
The throttle linkage requires a little more thought. Check that the ATV is expanded to 150%. Place the throttle stick (transmitter) to full power and position the carburetor control arm to fully open the throttle barrel. Place ball links on the throttle control rod and roughly adjust in length to fit between the fully opened throttle and servo arm. The ball links on both the throttle and servo arms should be equidistant. I use about 12mm as reference.
Now slowly close the throttle. The ATV for low throttle is set at 150%, so change this in the transmitter as you fully close the throttle. The throttle trim should be all the way down in the cutoff position.
You are trying to adjust the throttle for the following:
* Throttle and servo arms move parallel to each other throughout travel * Full throttle is 150% ATV * Cutoff ATV is adjusted so that servo is not stalled
Why adjust full throttle ATV to 150%? Would 100% work well? The answer is yes, 100% works well until you program a mix to add throttle. On JR radios, the master to slave relationship will add a certain percentage of mix all the way to 150%; therefore, if your mechanical limit is set at 100% throttle, the servo will be stalled as the master channel attempts to drive the throttle. By adjusting the mechanical linkage so that full throttle is reached at 150% ATV, no master channel can overdrive the servo. This has to be compensated by adjusting the lower ATV range.
The throttle servo generally does not require that subtrim be adjusted. Of most importance being the equal arm lengths, and that the arms follow each other in parallel fashion. The end result being that 50% barrel open on the carburetor corresponds to 50% servo arm arc.
Collective Pitch
Collective pitch mechanical adjustment to the servo is similar to the aileron and elevator servos; however, the transmitter setup is more complicated.
The method to use on the transmitter starts with placing the flight mode switch to flight mode 2. I am using flight mode 2 in this example, but it could easily be flight mode 1, or whatever flight mode you would like to use for 3D. As 3D essentially means that the helicopter will behave similarly whether inverted or upright, we use whatever flight mode prepares the helicopter for that style of flying.
This is extremely convenient because it provides the template with which to prepare any other flight mode. To continue, adjust the ATV out to maximum and call up the pitch-curve programming screen. Use the pitch curve display to place the pitch stick at 50% input (which equates to 50% output).
Next, attach the arm to the servo and reposition the arm on the servo output splines to get it as close as possible to center (perpendicular to the control rod or rods). Without touching the pitch stick on the transmitter, go to the subtrim screen and adjust until the servo and arm are precisely centered.
Connect the pitch control rods and adjust the length as necessary. In the previous steps, the head was set so that swashplate mid travel was 0°. All that is needed now is to adjust the collective pitch control rods so that the swashplate is again in mid travel.
When finished, attach a blade or an old blade root from a broken blade. The short blade makes it easy to adjust pitch curves on the bench without swinging a blade into the wall. Use a pitch gage and check to see that the blade indicates 0° with the collective stick centered on the radio. Lower the collective to the bottom (full negative) and measure the angle with the pitch gauge. Raise the collective to maximum and again take a measurement. Hopefully, you will have approximately ±10° . If you have something less than this then adjust the mechanical advantage either at the servo or control arms to obtain this figure. The final setup for the pitch curve occurs during the radio setup portion of this article.
Anti-Torque (Tail Rotor)
Torque compensation is provided by the tail rotor. The tail is also used to provide yaw control, and, with the use of a gyro to dampen motion in the yaw axis. As the tail is not readily understood by beginners, it is quite often the most poorly adjusted. If the tail is improperly adjusted and does not function to control torque or dampen motion, it seriously affects the confidence level of a novice pilot and increases the learning period.
The tail differs from the rest of the setups because there are several factors that need to be considered. Probably the single greatest affect on how the tail is set up is determined by the gyro type, whether it is a standard yaw damper or heading hold type. To keep it basic, I will discuss the standard yaw-damping gyro, then later in advanced topics, the heading hold gyro.
In order for the transmitter software to properly calculate the amount of torque compensation required, it must know where the stick is when in the hover. Normally, tail blade pitch is increased above this point and subtracted below it. The mechanical setup then needs to be adjusted so that when the collective is in the hover position the servo arm is centered and square to the control rod. Also, the tail blades will be at hovering pitch of around 5° .
Essentially, the tail rotor zero point centers on the amount of tail blade pitch needed to counter torque at hovering power. To accomplish this, turn the radio on and insure that tail compensation mixing is off. Now, place the servo arm on the splined output shaft of the servo adjusting its position to 90° to the control rod (this rod travels all the way to the tail). Use the subtrim function to square it exactly. Note that this is a completely different method of setting up the tail when compared with a heading hold gyro. The heading hold gyro will be covered later.
Connect the control rod to the servo output arm, the distance should be between 12mm and 18mm out from the center. Read the instructions that came with the gyro as several manufacturers have suggested lever lengths.
The length of the control arm at the tail is usually dictated by the manufacturer. There may be only one hole to screw in the ball or several. Choose the middle length for starters. Adjust the length of the control rod, by screwing the ball link in or out to provide approximately 5° of tail blade pitch. This 5° is close to what will be needed when the heli is being hovered. Any changes to hovering pitch will be adjusted mechanically by adjusting the length of the control rod.
Install the gyro according to the manufacturer’s instructions. If the gyro has upset the neutral point of the servo, either adjust it using the neutral adjust on the gyro or via the transmitter’s subtrim function. Be sure that the gyro polarity is correct.
There are several important areas to check at this point if your first takeoff is not to be an overly exhilarating one.
* Are the tail blades bolted on correctly? * Is blade pitch providing thrust to counter the torque of the main rotor? * Does right tail on the transmitter increase blade pitch (for clockwise rotation rotors), and vice versa? * Does the gyro alter tail blade pitch to oppose the direction of yaw?
Leave all tail rotor mixing and compensation off until after the first flight. It is essential that several parameters are adjusted during first flight, prior to using the radio’s built-in programming.
Radio Setup
Now that the manual labor part of the hard work is done, you can utilize all the bells and whistles of your radio without resorting to getting your hands dirty. Exponential
Nobody should be flying without using exponential on the cyclic controls. On a JR radio, I enter a value of +30% to both my left/right and forward/aft cyclic. The net effect is to permit more stick deflection for a given swashplate movement around center. It feels as though the sticks are "soft" or "indirect" around their neutral positions.
The result of adding exponential is easy to observe while hovering. Without exponential, you can almost see the helicopter fuselage move about the rotor disk with every cyclic input. The model appears jerk in response to control input, which advances the misconception that it is more maneuverable. Incidentally, there is a delay between input and response in model helicopters as there is in the full size. This delay is usually quite constant for a given head speed and blade weight. The large initial control input does not result in a faster response because of this delay; however, the result of a large control input is simply a large response. The pilot reacts with a correction and the pattern repeats. The model then "feels" less stable and becomes intimidating for the novice.
A model flown with exponential appears much more solid in the hover. Control inputs are washed out due to the exponential, thereby softening most pilots’ tendency to over control. The helicopter is then perceived as being docile and forgiving. Remember that exponential softens the command around neutral but does not affect the total swashplate deflection at extreme stick positions. There is no loss of authority (you have it, when you need it).
The amount of exponential to add to the tail command is usually dependent on the operation of the gyro. On piezo gyros that support yaw rate demand, it may be necessary to add some exponential. Consult the instructions packaged with the gyro and abide by their recommendations as a starting point.
Pitch Curves
To program the pitch curves we work backwards from the 3D or "V" Curve. This is simply because we set the mechanical portion of our helicopter to symmetric about the 0° point. If we start with the "V" Curve, all we need are the programmed numbers for 3 points and we can generate everything needed to program any other curve.
Flight Mode 2, V Curve
Turn everything on and place a pitch gauge on 1 blade (make sure you have one blade marked so that the other blade can be tracked to it during the first flight). Place the collective at center stick and move the flight mode switch to the position that will correspond to your "V" Curve.
For this 5 point setup, center stick collective corresponds to point 3, or 50%. Blade angle should read 0° . If not use the data keys on the transmitter to be sure it is and make a note of the percentage output.
Place the collective to the ¾ stick position, or point 4 or 75%. Point 4 is the upright hovering point and should read approximately 5.5° . Again, make any adjustment and make note of the percentage output.
Finally, move the collective to full positive pitch, point 5 or 75% and adjust for +9° . One important item here is compression. If points 4 and 5 are close to each other in percentage output, for example 78% and 86% respectively, then adjust either the travel adjustment or mechanical advantage to spread them apart. Personally, I adjust my point 5 so that it requires 100% of output. Extra pitch is not made available for autorotations because 11° of pitch has no benefit at low rpm. Make a note of the reading.
To program the negative portion of the "V" Curve on JR radios, simply subtract the value from 100 and use that for the opposite point. For example, 78% for point 4 becomes 22% for point 2 and 95% at point 5 becomes 5% for point 1.
Normal Curve
Normal Curve is easy to prepare now that the critical programming percentages are known. Flip the flight mode switch to "normal" and place the collective stick at ¼ stick, or point 2, or 25%. In Normal the ¼ stick position corresponds to the 0° point. Use the figures generated for point 3 in the "V" Curve setup and enter it.
Next, move the collective to ½ stick, or point 3, or 50%. This is our hovering point while in normal curve. It is also critical for setting automatic tail rotor compensation when this becomes enabled, but more on that later. Enter the figure generated for point 4 in "V" Curve setup.
Points 5 for Normal and "V" Curve are equivalent at +9° ; therefore, use the same percentage.
Point 1 or full negative should correspond to approximately -5° or -6° . This value is not critical at this point. In fact, you could set it to 0° if you are a novice to smooth out your hovering while practicing. At 0° , it is hard to really dump your heli onto the ground in a moment of panic.
On the other hand, you might set point 1 at -6° . That would allow an entrance to an auto without a jump when the throttle hold is switched in. Either way, point 1 for normal can be set based on your style of flying when in Normal. Note that data is not programmed for point 4.
Throttle Curves
Again, start in your V Curve flight mode. The data entered for the various points is purely arbitrary at this point since we have only a rough estimate on how much engine power is going to be needed in the hover. As a general rule of thumb, the throttle barrel on the carburetor will be open approximately 50% when hovering. It may be advantages to pop the linkage off the carburetor and mark the high, low, and middle points with a laundry marker. V Curve
In V Curve, we run collective pitch from +9° to -9° with maximum engine power at both these points. That leaves 3 points to cater for: 1) inverted hover, 2) zero pitch, 3) upright hover.
Access the throttle curve function on the transmitter and move to point 2 (inverted hover). Enter a percentage that opens the throttle barrel to 50%. Typically it works out to be around 70% on JR radios with 150% ATV (with expanded ATV, 50% on the transmitter output does not equal 50% servo movement).
Use the figure determined for point 2 as the data for point 4 (upright hover).
Finally, we must use some value for V Curve point 3. At this position, the blades are at zero degrees collective pitch. As we are going to be doing aerobatics while in V Curve, it is essential that some power is available. Ideally, we want the head speed to remain constant to minimize torque changes; therefore, we need enough power available to maintain our hovering head speed, but less than that required to hover the helicopter. As a starting point, use a figure 10% less than the hover throttle settings. The term V Curve comes from what we have just accomplished. The throttle curve now resembles a "V" when graphed: Normal Curve
The graph also shows the Normal Curve. Remember, that in normal flight mode the helicopter hovers at the mid-stick position, point 3. Its value should be the same as point 4 when in V Curve flight mode. This is a great point to remember when fine-tuning after the first flight. Fine tune the Normal Cuve first, then transfer the numbers to V Curve.
Point 4 is shown with an average value of. In reality, most radios will compute this value for you or offer to leave the value inhibited. On my JR radio, I don’t even have a point 4 in Normal Curve. If it is not possible for you to similarly inhibit this point, simply calculate the mathematical midpoint between points 3 and 5.
Tail Rotor Curve
No curves are programmed before the first flight! Earlier it was mentioned that even revolution mixing is turned off until all the necessary adjustments have been made. Details on how to handle the tail follow the section on first flight.
First Flight
What is first flight? It is simply the most critical flight that takes place after assembly, rebuild, or reprogram of the system. If you are a beginner and have just completed your first helicopter, this is absolutely the time to receive experienced help. Do not even think about trying it yourself, its not worth the anxiety, the panic, and the cost! My own first flight lasted around seven seconds but as I watched it occur it appeared to last a lot longer. Even worse was the fact that the helicopter was an American "Super Mantis." The reason being that as I crashed so did the company, leaving me with an expensive first lesson. So, no matter how tempted you are, seek help.
Follow this section to understand what the first flight is all about. After reading it, you should understand that the significance of the fine adjustments made to the helicopter and the format for accomplishing these adjustments.
Engine tuning
It is quite necessary to begin here because one of the most critical adjustments depends on a smooth running engine that is producing a compromise of smoothness and power. Yes, it is somewhat of a compromise in that ultimately the engine is capable of producing more power but at higher vibration levels. This is especially true if the engine is set overly lean. On the other hand, if the engine is set overly rich the smoothness disappears as the engine attempts to lean out and 2-cycle. This changes the torque output of the engine, which can in turn play havoc with the gyro.
The engine should be producing smoke in the hover with a crisp response to the collective as power is increased. This is important because as the throttle is opened the load is increased. This is simply because as the left stick is moved (For Mode 2 fliers) both throttle and collective pitch are changed...
Based on and updated from material by Chris Berardi.
References
http://www.clevelandheli.co.uk/Learning%20to%20Fly.htm
http://ourworld.compuserve.com/homepages/chris_berardi/Helisetup.htm
