RC Soaring

Author: Mike Garton

Edition: Model Aviation - 2000/11
Page Numbers: 82, 83, 84

RADIO CONTROL SOARING

Mike Garton, 506 NE 6th St., Ankeny IA 50021; E-mail: [email protected]

THE THEME of this month's column is Hand-Launched Glider (HLG) technology.

I'll provide some details about Denny Maize's X-Terminator, then I'll discuss the flight-pack components for hand launch. It will let you know what the top competitors are using and why.

Denny Maize originally designed the Terminator several years ago. It is a nice-flying, competitive HLG. The plans, templates, and detailed instructions are freely downloadable from the Web site (address at the end of the column). In a quest to make the airplane stronger, lighter, and cleaner, Denny has created a composite fuselage. With this new fuselage, he calls his model the X-Terminator.

Fuselage and grip

The X-Terminator fuselage is pod-and-boom style. The pod is molded fiberglass, with carbon reinforcements. The tailboom is a carbon kite spar. There is a molded-in two-finger grip on the bottom of the pod; this grip is durable and comfortable.

A large hatch opens to expose almost the entire nose section of the pod. Relatively large carbon rods on the lip of the hatch opening maintain strength through the hatch area. With the top of the pod exposed, radio installation is easy. Servos are installed on their sides with double-stick tape. No servo tray is needed.

Wing saddle and attachment

The wing saddle is flat. Wing-root chords ranging from six to eight inches fit well, but root chords outside this range can be accommodated. Denny often tries different airfoils on the model; the thinned SD7080 is one of his current favorites.

There is a former at the front of the wing saddle. A dowel sticking out of the wing's leading edge slides into a hole in the former. The wing is held down with a nylon bolt at the back of the saddle.

Dimensions and weight

The fuselage is more than 44 inches long if the tailboom is left untrimmed. Denny usually trims approximately six inches, to give a long—but not extreme—tailboom. The pod with boom, formers, and hatch typically weighs 1.1–1.2 ounces. Ready-to-fly weight is roughly eight ounces.

Denny has been selling "short kits" for the X-Terminator's fuselage and tail. The kits include a pod, contest balsa tail surfaces, ultralight laminating Mylar™ to cover the tail surfaces, carbon pushrods with plastic housings, and Spectra sleeving for the control-arm hinges. The pod has the tailboom, formers, and hatch hold-down preinstalled. The short kits do not include wings.

Denny sells these kits for $55, including shipping—a great price for a high-performance HLG fuselage kit. Denny prefers to be contacted by mail or by E-mail (addresses are at the end of the column).

Importance of weight

Weight is very important in HLGs. Typical contest airplanes weigh seven to nine ounces. Many designs need only one ounce of ballast to fly in high winds. This sensitivity to weight is great incentive to save fractions of an ounce.

Flight-pack components for hand launch

The hot flight pack right now is a Hitec 555 receiver with the case removed, two Hitec HS-50 servos in the fuselage, two Cirrus CS-10 servos in the wings, and a three- or four-cell 110 mAh battery. This four-servo flight pack weighs less than 2.5 ounces.

Most pilots use a four-cell 110 mAh Ni-Cd pack, which nominally weighs one ounce. Some pilots use a three-cell (3.6-volt) pack, although radio manufacturers do not endorse this practice. Those who use a three-cell pack time it in simulated use on the bench before attempting flight. The three-cell pack is incompatible with many receivers and some servos, but pilots have reported success with the Berg receiver from RC-Direct and the Hitec 555.

Besides providing slight weight savings, the lower voltage decreases the current draw of the servos. That means a longer possible flight time. On the downside, the servos move slower and range may be affected. Try the three-cell pack at your own risk.

Moderate- to high-current peak chargers can damage small Ni-Cads. Sirius Electronics makes a nice pack charger, which has one output at a 100 mAh rate. It can safely charge four to eight cells of 50–200 mAh capacity. With the Sirius charger, hooking the battery up backward will not hurt it. The charger takes an hour to energize a fully discharged 110 mAh pack, and less if the pack was not fully discharged. The charger is a little slow, but it's very gentle on the batteries; they do not get warm during charge. Charging a three-cell pack takes a more-flexible charger.

A few pilots are experimenting with lithium (LiM) battery technology. Many larger-capacity cells are available, but there is not much available in the smaller capacities (and weights). Two Tadiran LiM cells can be used to create a six-volt, 800 mAh, 34-gram pack. LiM and Nickel Metal Hydride (NiMH) batteries require chargers made specifically for them. Tadiran LiM cells and LiM chargers are available from Magellan Technologies (address at the end of the column).

Servos

Hitec HS-50 servos are the most popular ones for elevator and rudder in competition HLGs. The servos weigh 5.8 grams, and they have 8.4 oz.-in. of torque at 4.8 volts. This is plenty for an eight-ounce low-speed airplane. The HS-50s have less gear slop than some of the other microservos.

The Cirrus CS-10 servos are popular for use in wings. They weigh 5.4 grams and have 7 oz.-in. of torque at 4.8 volts. The CS-10s are the thinnest (0.37 inch) of the microservos. A typical HLG has a seven-inch root chord and a 7%-thick airfoil. This makes it 0.49 inch at the thickest root.

Often, the servos cannot be located at the thickest portion because of the spar. Subtract for skin thickness, and you can see why servos are squeezed. At 0.44-inch thick, the HS-50s do not fit into most HLG wings. In addition to weight and size, consider average current draw and "start-up currents" when choosing servos for competition HLGs. The HS-50s have very low current draw, which means that the pilot can use a smaller pack for contest rounds.

When some servos start to move, the current draw is very high for a very few milliseconds. In motor terminology, this is called "start-up current." These spikes become very important if you are running four servos from a small Ni-Cd pack.

When the battery is forced to produce a high current, its voltage instantaneously sags. A crash will result if the voltage drops below the operating voltage of your receiver. I have experienced that scenario.

I was using a Hitec 535 receiver, and four FMA S-90 servos with a 110 mAh Ni-Cd. I had chosen the S-90 servos for their relatively high torque (19 oz.-in. from a microservo). I expected a high current draw from the servos because of the high torque. I figured I could safely fly my flaperon model with coupled ailerons and rudder for at least a 15-minute contest round.

When I moved the right transmitter stick, all four servos were affected. The airplane locked up in flight and crashed after 10 minutes. In the lab, we learned that each of my S-90 servos pulls one amp of current for a few milliseconds when it starts moving. The amazing amount of torque these servos produce causes an amazing amount of start-up current! The four servos starting up at the same time probably made the battery voltage sag below what the 535 receiver would handle.

This is only an issue with small-capacity packs and some servos.

A friend bench-tested some receivers with a current-limited power supply, to see what would happen when the voltage got very low. Under lab conditions, the voltage to each receiver was slowly decreased. Voltage, current draw, and servo speed were watched carefully.

Some of the receivers locked up suddenly before the servos slowed down. In the absence of a timer, a pilot would have no warning before this happened.

The Hitec 535 receiver dropped out (locked up) at 3.1 volts, and it would not restart until voltage was increased to 4.0 volts. The 535 has an excellent reputation for normal uses, but it is not the best choice with the components I chose. The Hitec 555 receiver would be better.

A "low-voltage friendly" receiver gives a warning, by slowing its servos way down before it quits and locks up. Wiggling your transmitter sticks before each throw is a good habit. Hopefully the pilot would notice that the servos are sluggish.

The Hitec 555 receiver dropped out at 2.4 volts in tests, and would restart when the voltage was increased to 2.5 volts. The servos slowed way down before the receiver stopped receiving.

The difference between the "dropout voltage" and "restart voltage" is important. If a current spike sags the battery below the dropout voltage for a few milliseconds, the current will drop, the voltage will pop back up, and the low restart voltage will allow the battery to proceed with no noticeable interruption.

A large difference between the dropout voltage and the restart voltage means that it is much more likely to lock up and stay there.

If you use the receiver as the manufacturer intended, you should never see these dropout conditions (i.e., use a larger-capacity four-cell pack, and stop flying when the pack reaches 4.6 volts). It is standard practice to quit flying before your battery runs down. It is a luxury to have a receiver that drops out after the servos slow down. This has become more of an issue, as people push the envelope with competition HLGs.