Tow Planes - General Set-up
I spent the bulk of the weekend at Visalia flying the ISSA Pegasus. Between tows I talked to a number of people who are interested in, or are in the process of building a tow plane. The majority are building the Pegasus II, (the second generation Pegasus), while others were building scale models. There were a lot of questions about setting-up tow planes (sorry, I can't call them "tugs"), with several questions specific to the Pegasus. This month's column will focus on items of general interest related to setting-up a tow plane, with specific comments about the Pegasus.

If the builder already has a background in giant scale power, the Pegasus or other tow plane will likely be an enjoyable project. If this is your first power plane, or if your expertise has been in smaller power planes, spend some time talking to guys that regularly fly big birds and when possible, look at their radio and engine installations. Power planes operate in an environment much different that sailplanes, and tow planes are that much more unique. A tow plane may spend more time in the air in one day than other planes would ever see in a month. As a result, the building and set-up should be well thought-out, including making the installation easily serviceable. For example, when possible, servos should be mounted externally on control surfaces, and all the electronics should be easily accessible. It should be easy to check the batteries, fuel level, and the engine needles should be within reach of a screwdriver. On big engines I prefer lock-nuts to T-nuts. Think utility and ease of service. The ISSA Pegasus originally had a front hatch held down by four screws - it looked good, but checking the fuel level was a pain. The screws were replaced by a hook and a rubber band, but what it lost in aesthetics, was more than made up in convenience.

Selecting an Engine
Tow planes need a reliable, low maintenance, "user- friendly" engine, and lots and lots of power. The DA-100 used in the ISSA Pegasus was originally designed and marketed to large-scale aerobatic models. It is well suited as a tow engine because the same power that allows aerobats to do eight-point rolls going straight up also allows the tow planes to haul the heavier and/or draggy sailplanes to altitude. It's a fair statement to say that engines that are popular in the giant sale aerobatic community are also good choices for a tow-plane. I know there are some people building 3W 150 and DA-150 powered tow planes, but before anyone runs out to buys 150cc motor, be aware that a 100cc powered Pegasus will pull even the largest 40% vintage and "glass slippers" with authority.

I'll avoid anything resembling a debate on which is "the best engine". The Pegasus will fly well with a wide variety of engines, but my choice is the Desert Aircraft 100cc twin, commonly known as the DA-100. Other good choices would be the 3W-100, 3W-78, 3W-85, an older 3W-80, Brison 6.4 twin, or BME 102 twin. And there are plenty of others. The ISSA Pegasus started off with a Brison 5.8 single on a Hyde mount, which in addition to not being nearly as strong as the DA, required constant maintenance from the vibration, both from being a big single, and from the soft mount. So whatever engine you use, it should be hard mounted. The interest in soft mounts has pretty much waned. If you are using a big single, build the airplane stout and plan on a thorough maintenance schedule after each session.

My only real caution with buying an engine for any airplane is not to try and start a trend. Stick with what other people have used and we know works. The engines listed above have been beaten to death by the aerobatic community and have proved to be reliable and strong. If you can find a good used 3W 120, that is also an excellent engine, a little heavier than the DA-100, but of comparable power. The "grunt" power will favor the 3W-120, the DA-100, or the 3W-100. Also, never overlook the performance benefit of a good prop…wood or carbon is your choice, but a prop does make a difference.

Pegasus Flaps
Tony Elliot's 3W-120 powered Pegasus does not have flaps, and flying it provides a good excuse not to build them. John Derstine and I agree that the Pegasus doesn't need flaps to land, as it has plenty of wing area and is stable at low speeds. It's a matter of personal choice - adding flaps is kind of sexy, but they will require at least one more channel, and a fair amount of building time. One of my "complaints" with the ISSA Pegasus is that we lose battery capacity by powering flaps servos that are rarely used. Also, my preference in flying the Pegasus is that the airplane remains more controllable with the flaps up. With the flaps down, because of the reduced airspeed both the ailerons and the elevator lose some effectiveness, and it becomes more important to fly the throttle. With the airplane "clean", I can come off tow, point the nose straight down, and make an aggressive landing approach. The airplane remains very predictable. And all that translates into less time to get up and down.

Hinges
There are two hinges commonly used for giant scale, Robart or Dubro. The Robarts are two opposed round and serrated "fingers", with a hinge pin the middle. Robarts have different sizes, use the giant scale or largest diameter. Dubro also has a "giant scale" hinge, wider and deeper than the smaller size. The Dubro hinges are flat on each side, also with a pin in the middle. Compared to the Robarts, the Dubro hinges are arguably easier to install, as they only require a slot, whereas the Robart's require a centered hole. I have never seen a reason to favor either one, so it's your choice. One trick that we noise makers use is to double up the hinges near the tip, and get the hinges as close to the tip of the aileron as possible. That minimizes the vibration at the tip of the aileron and greatly lessens the chances of the outboard hinge coming loose.

Radio, Receiver, and Servos
The ISSA Pegasus is controlled by a JR-388, an older, but still very good eight channel computer radio. The receiver is a JR 649, a nine-channel JR PCM receiver. The rear elevators are on a "Y", the tow release is on the Gear channel, the Flaps on a "Y" with a reverser, the ailerons are on individual channels, the throttle and rudder are normal, and we use an optic switch to kill the engine ignition. All eight channels are in use.

Why PCM
There are good arguments to be made in favor of PPM in a quiet environment such as a sailplane or a .40 size glow airplane. But with big gas engines, PCM should be the only choice. Why? Because the fail-safe function found on PCM systems is the only way to control the throttle in the event of frequency interference.

The ISSA Pegasus is equipped with a transmitter controlled optic kill switch (Electro Dynamics EDR-107) that is programmed using the PCM Fail-Safe function to kill the engine should the airplane experience sustained frequency interference. Other pilots may opt to simply have the engine go to idle, but in any event, the FS allows the user to program the condition. PPM can't do that, and what that means is that if a tow plane was idling on the grass getting ready to go, and someone accidentally turned on…well, it would get ugly in a hurry. With the ISSA Pegasus, the PCM is set to terminate the ignition and induce a few degrees of rudder - which I do to tell me that the airplane is experiencing channel interference. The bottom line is that PCM provides a much higher safety margin. Furthermore, because of the amount of time that the tow planes spend on the ground at idle, I encourage everyone setting up a tow plane to use PCM and to program a FS condition.

It's also worth noting that the ISSA Pegasus started off with a PPM receiver and radio, but on an early outing we experienced severe glitches from the ignition. The transmitter and receiver were replaced with the JR-388, and the 649 PCM receiver, and the system has worked flawlessly since. I do check the system every so often to make sure that the fail-safe is functioning properly.

PCM and Ignition RF Noise
I recently worked with a modeler who was experiencing glitches with his BME 5.8 (twin 100cc) powered airplane, which uses the widely marketed CH ignition. The receiver was a JR-549 PPM, and when replaced with the 649 PCM, the glitches ceased. Someone commented that the noise was still present, which is true, but the PCM technology "ignores" the momentary spikes that define ignition noise. Replacing the receiver essentially duplicated what we experienced with the ISSA Pegasus. As a result, PCM is often the "cure" for installations that are experiencing ignition interference. Also, the ignitions used by DA and 3W are very quiet and can be used with PPM receivers without interference, but the for reasons described above, my preference is PCM.

Electronic Kill Switch
Regardless of whether you opt for PCM, an electronic kill switch is recommended as a safety device. The beauty of the electronic kills switch is that it terminates the spark, rather than relying on the mechanics of the carburetor. There are a number of situations (linkage failing, dead servo, mounting bolt(s) coming loose, excessive vibration from prop strike…) where the mechanical linkage may be impaired, and the motor would not shut-off from simply moving the throttle trim. A friend of mine recently lost a 42% Giles G-202 aerobatic airplane when immediately after a mid-air he wasn't able to shut the engine off and the vibration from the broken prop literally tore the airplane apart.

Two switches are available, one from Electro-Dynamics, the other from Electronic Model Systems. Either one works well, and they both require a dedicated channel. In addition to the transmitter kill switch, the ISSA Pegasus has an externally mounted ignition switch, so both the transmitter and the airplane switch must be "on" for the ignition to fire. That may seem redundant, but it allows whoever is starting the engine not to rely on the person holding the transmitter that the ignition has been disabled. But the person flipping the prop must make sure that the transmitter ignition switch is "on" when he is starting the engine, or he'll have a sore shoulder in a hurry.

If you don't have an electronic kill switch, you can set-up a throttle kill by using an open mix - but it's nowhere near as good. It does work much better than relying on moving the trim, especially on a radio with an electronic throttle trim. On most radios a kill-switch can created by a same channel mix (throttle to throttle) and assigning a switch to activate the mix. When the switch is engaged, the servo moves to a fully closed position. The goal is not to rely on the throttle trim to kill the engine.

Set-up Wish List
A better alternative to Ys is a servo combining device like the JR Matchbox (4 servos to 1 port) or the Smart Fly "Equalizer" (2 to 1). For those who are unfamiliar with these new gadgets, they allow you to program the individual servos using the transmitter end-point, centering/sub trim, and servo direction function. A small selector selects the servo programming. In the case of the ISSA Pegasus, I would have preferred to use a multi-servo port one on the second elevator and the flap channels, rather than the Y's, although that would not have freed-up any channels. But it would have made setting the airplane up a little easier, as well as being able to make precise individual adjustments.

Servos
There is a good selection of high torque, ball bearing servos available from all the major radio companies that are well suited for tow planes. Metal geared servos are not necessary, nor are digitals, but neither one is a liability if that is your preference. The exception to metal gears is if you are running a big single, as the increased vibration tends to be hard on servo gears. And like engines, I don't encourage anyone to use servos that have not been well tested elsewhere.

Batteries
The ISSA Pegasus has two 2400Mah four-cell Nicad packs, with two Cermark heavy-duty switches. Each battery is plugged into separate ports on the receiver. (See the January tech talk for a discussion on battery redundancy). Whereas in other applications I favor five-cell packs for the increased speed and torque, the "price" of a five-cell pack is increased current drain (faster discharge), and not what we want for a tow plane. Plus, tow planes really don't benefit from increases in servo speed or torque. The ignition battery is also a 4 cell 2400Mah, and is controlled both by an external Cermark DSC Switch, and the transmitter controlled electronic switch.

Like servos, there are a lot of options related to batteries, but I do recommend redundancy, whether it be a "battery backer" or running two packs. The goal is to build in a lot of capacity without adding too much weight. The ISSA Pegasus started with two 1700Mah packs, but we upgraded to the 2400Mahs, which now allows the airplane to fly all morning without a re-charge.

Linkages
Among the giant-scale power community, the Rocket City hardware is used widely and their stuff works well, is relatively inexpensive, and easy to install. R/C stuff is available through most hobby retailers.

More Info
Contact information for a number of giant scale power companies is available through the IMAA (International Miniature Airplane Association, the AMA SIG for giant-scale power) web site, at www.fly-imaa.org. IMAA also publishes a quarterly magazine, called High Flight, that includes a lot of technical information on building and setting up large scale power planes. There are also a lot of experienced builders and pilots either active on the ISSA forum, or "lurking", so don't hesitate to ask if you have a tow-plane related question.

There's a whole lot more, but hopefully, this is a start…