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Tools of the trade
Bell, RickIn my last column, I discussed different size helis as well as various gyros and radios. Let's look some more at why heli radios are better than airplane radios for controlling our helicopters.
To fly most airplanes, you need four channels or functions-ailerons, elevator, rudder and throttle. For an average helicopter-assuming it has collective pitch-- you need five channels: roll cyclic (ailerons), pitch cyclic (elevator), tail rotor (rudder), collective (main-rotor pitch) and throttle. To make things easier, let's refer to all heli control functions using fixed-- wing terminology: aileron, elevator and rudder. Most gyros now have switchable dual rates; therefore, a heli radio requires a sixth channel for this function.
As far as controlling the model is concerned, heli and airplane radios are set up in the same way: aileron is for roll, elevator is for pitch, and rudder is for yaw. The difference between the two types of radio lies in throttle and collective-pitch control; heli radios mix these two functions electronically. As you increase or decrease collective pitch, the radio increases or decreases throttle for you.
Heli radios can also compensate for torque. With helicopters, torque is produced by the engine to turn the main rotor; depending on the direction of main-rotor rotation, the body of the heli will try to rotate in the opposite direction. This is why tail rotors exist: to counter the effects of main-rotor torque.
It works like this: the heli is set up to hover with the collective/throttle stick at its midpoint of travel on the transmitter. The heli will hover with, say, 5 degrees of main-rotor pitch. To counter the main-- rotor torque, the tail rotor will need a certain amount of pitch, and this is initially set mechanically. Now, if you increase the collective pitch/throttle to climb, torque forces increase, and the heli body rotates in the opposite direction to the main rotor. Increased tail-rotor pitch is now needed to prevent the body from moving.
The opposite happens when you decrease collective pitch/throttle. A heli radio can be set up to automatically increase or decrease tail-rotor pitch to compensate for the torque changes. This function is known as ATS, or revolution mixing.
Features such as servo-reversing, dual rates, exponential and ATV are usually found on both types of radio, and they all work in much the same way.
KNOBS AND SWITCHES
If you have ever looked at a heli radio, I'm sure you've noticed that it has more knobs and switches than an airplane radio. These additions are for idle-up, throttle-hold and various engine and hover pitch adjustments. The idle-up switch allows the heli pilot to do aerobatics by placing the throttle in a predetermined curve that works with collective pitch. Throttle hold is for practicing autorotations (more on this in a future column).
The knobs allow small changes to be made to the hovering pitch and hovering throttle without having to go into the radio's program menu. These adjustments are for normal day-to-day tweaks to compensate for air temperature, humidity and other factors that affect your engine's performance.
The idle-up switch allows you to select different pitch and throttle settings (curves) so you can do aerobatics. If you have ever wondered how inverted hovering is possible, idle-up is the answer. In a normal setup, as collective pitch is decreased, so is the throttle. With idle-up, whether pitch is increased or decreased, the throttle is increased to maintain main rotor rpm.
The throttle-hold switch electronically uncouples the throttle and collective pitch so autorotations can be performed. When throttle-hold is activated, the engine goes to a low idle setting, the clutch disengages, and you control only the collective pitch. Autorotations are great fun to do; they are also the last maneuvers contest pilots do. If you're able to do autorotations, you can also save your heli from crashing if the engine quits in flight.
This has been a simple explanation of heli radio systems; I'll continue the discussion during the coming months. For more heli-radio info, also refer to Don Edberg's "Effective Programming" column in the June '98 issue.
TOOL TIME
Someone once said that if you build a crooked foundation, the best you can hope for is a crooked house. The same applies to helicopters. You need good tools and the know-how to use them to build a helicopter correctly. While certain tools are absolutely necessary, others just make building your heli easier.
Helicopters have many rotating parts that should be balanced for smooth running. A Robart* High Point balancer is the most popular balancer used, but I'm sure other brands will work as well. The important thing is to get one and use it. "What gets balanced?" you ask; rotor heads, tail rotors, clutch bells, main gears and cooling fans are the usual parts. While it may sound like a lot of stuff to balance, it takes only a few minutes with each to achieve good results. A balancer of some sort is probably the most important tool of all.
A dial indicator is next on the list. It's used to check the "run-out" (out-of-true condition) of the clutch assembly. On some helis, the starter shaft also needs to be true. Because these parts are turning at engine rpm, you want them to run as true as possible. Run-out of more than 0.002 inch is unacceptable. The vibration caused by these out-of-true parts will beat bearings to death very quickly and also cause other parts to wear out prematurely. The heli will shake visibly to let you know something is not right. The radio equipment will also take a beating and might even fail.
Allen drivers, nut drivers and Phillips screwdrivers are the most often used tools, and all R/C shops sell them. Though you can use the Allen wrenches that come with some kits, they do tend to wear out quickly. I have two sets of drivers-one for my shop and one for my field box. Bondhus makes a neat set of metric ball drivers that can get into hard-to-reach places-very handy! By the way, we're talking about metric hardware here. Also very handy are 7mm and 5.Smm open-end wrenches. (I bought mine at Sears. )
To get the servo movement to the various bellcranks, most helis use a lot of ball links; again, one of the must-have tools is a ball-link pliers. It makes snapping the links onto and off the balls very easy, and it doesn't scratch the ball or damage the link. Your local R/C shop should offer various ball-link pliers.
How do you get the links onto the pushrods?-by using a tool designed to screw the links on. Hobbico* makes a neat little tool to screw ball links onto pushrods-sure does save wear and tear on the fingers. This is not a complete list of useful heli tools, but they are the ones that you'll use the most.
Next time out, I'll begin a build-along project featuring the .32-size Hirobo* Shuttle RG heli to show general building techniques that can be applied to almost any helicopter because the Shuttle RG is typical of today's helicopter designs.
If there is anything you would like to see discussed in this column, let me know. If you write to me c/o Model Airplane News and would like a reply, please enclose an SASE.
Until next time, fly safely and have fun!
Correction: in my first column, the email address given was incorrect; my address is rbellO2@snet.net.
Copyright Air Age Publishing Feb 1999
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