Radio Technique

Radio Technique

George M. Myers

ABSTRACT: Failsafe Flight System (FFS)

SAFER RC FLYING may result from using T&D Flight Systems' Failsafe Flight System (FFS). The unit connects to your receiver in place of—or in parallel with—a servo and provides several safety functions.

I tested two models (list price $39.95):

• FFS-1: for PWM (Pulse-Width Modulation — ordinary AM and FM) receivers.
• FFS-2: for PCM (Pulse Code Modulation) receivers.

They share most functions. FFS-1 contains a pulse-omission detector for the Locator function; FFS-2 uses a timer instead, reflecting differences in PWM vs. PCM receiver behavior. Both models work with single- or dual-conversion receivers.

Functions

1. Nicad® Charger (CHG — both models)

• An onboard nine-volt transistor-radio battery automatically starts charging the airplane's Ni-Cd pack whenever the Ni-Cd pack terminal voltage drops below about 4.7 VDC. This helps prevent damaging deep discharge.
• Actual charging current varies; the 9 V battery supplies current as needed. In my tests the FFS units behaved like a stiff ~4.3 V regulator once the Ni-Cd pack fell to that level.

1. Backup Battery (BU — both models)

• A fresh nine-volt transistor-radio battery will operate the airplane for roughly 30 minutes in the event of total primary-pack failure (e.g., disconnected plugs, wiring failure, switch off, cell failure).

1. Low-Voltage Alarm / Expanded-Scale Voltmeter (ESV/LVA — both models)

• An internal beeper sounds whenever the Ni-Cd battery voltage drops below ~4.7 VDC. This serves as a continuous, load-tested voltmeter and warns before the primary battery is completely exhausted.

Model FFS-3 (PCM only, list $29.95) contains the ESV but not the LVA: it relies on the PCM receiver's own low-voltage/failsafe behavior. I am not enthusiastic about FFS-3 because some PCM receivers can mask a backup condition (failsafe signals can mimic valid transmitter signals), and receiver failsafe settings may be at voltages below the FFS alarm threshold.

Interference Monitor (INT)

• FFS-1: If you turn the airplane receiver on before the transmitter, the beeper should sound a steady tone. If the beeper is silent or stutters, that indicates a problem (dead batteries, another operator on your channel, or interference).
• FFS-2: Cannot reliably indicate interference because PCM receivers typically drive servos to a failsafe position when interference occurs; a PCM failsafe looks like a valid signal.

Lost Airplane Locator (LOC)

• FFS-1: If the plane goes down (e.g., in tall grass), switch the transmitter off and listen — the beeper may call you to the airplane.
• FFS-2: The beeper will sound about 18 minutes after the unit is switched on (or after the receiver is turned off, per the timer). This may help locate a downed airplane.

Switch-Position Indicator (SPI)

• FFS-1: If you turn only the transmitter off, the beeper sounds immediately. If both transmitter and receiver remain off, the beeper will sound when the batteries run down (LVA).
• FFS-2: The beeper will sound approximately 18 minutes after the receiver is left off or when something "twitches" again, alerting you that a switch has been left in the wrong position.

Both models provide leads for connecting an external noisemaker if the on-board high-pitched beeper is hard to hear. The large metal-can beeper was chosen after smaller units failed from vibration; it is epoxy-encapsulated for vibration resistance and uses the tin can as a resonator for louder sound.

SUMMARY OF TEST RESULTS

Each test entry shows modes S-1/S-2/S-3, measured receiver voltage (DCV), beeper level, current draw from the 9 V battery (mA), whether transmitter controlled servos (Ctl?), and notes.

1. Test 1 — Reference

• S-1: OFF, S-2: OFF, S-3: OFF
• DCV: 3.3 V
• Beep: No
• mA: 0
• Ctl?: —
• Notes: Reference

1. Test 2 — Failsafe

• S-1: ON, S-2: ON, S-3: ON
• DCV: 3.3 V
• Beep: Low
• mA: 14
• Ctl?: Yes
• Notes: Reference

1. Test 3 — Backup (B-U)

• S-1: ON, S-2: OFF, S-3: OFF
• DCV: 4.33 V
• Beep: Loud
• mA: 60
• Ctl?: —
• Notes: FFS + Rx + 4 servos

1. Test 4 — Locator/Interference/SPI

• S-1: ON, S-2: ON, S-3: OFF
• DCV: 3.3 V
• Beep: Low
• mA: 14
• Ctl?: —
• Notes: LOC/INT/SPI

1. Test 5 — Normal use

• S-1: ON, S-2: ON, S-3: ON
• DCV: 5.0 V
• Beep: No
• mA: 7
• Ctl?: Yes
• Notes: Normal use

1. Test 6 — Low-voltage alarm (LVA)

• S-1: ON, S-2: ON, S-3: ON
• DCV: 4.6 V
• Beep: Loud
• mA: 18
• Ctl?: Yes
• Notes: Low-voltage alarm

1. Test 7 — Charging Ni-Cd (CHG)

• S-1: ON, S-2: ON, S-3: ON
• DCV: 4.4 V
• Beep: Soft
• mA: 28
• Ctl?: Yes
• Notes: Charge Ni-Cd

1. Test 8 — Use Backup with servos moving

• S-1: ON, S-2: OFF, S-3: OFF
• DCV: 4.2 V
• Beep: Soft
• mA: 45
• Ctl?: —
• Notes: 2 servos moving

Abbreviations:

• S-1 = Locate / Interference / Switch-position indicator modes
• S-2 = Failsafe Flight System switch
• S-3 = Receiver switch
• DCV = Receiver voltage measured from a servo pigtail
• mA = Current out of the 9-volt transistor battery
• Ctl? = Control of servos by transmitter?

Test procedure and observations

• Installation: I prepared an aileron extender by cutting it in half, wiring the FFS pigtail, and plugging into the receiver and a servo. I added three more servos, a battery pack, and a switch harness.
• Systems tested: FFS-1 on a World Engines Expert (FM) system; FFS-2 on a Polk's Challenger 550 (PCM) system. Both worked well.
• Voltage regulation and current: In Nicad Charger mode the units tried to maintain ~4.33 V. Under continuous two-servo motion the 9 V battery current spiked as high as ~450 mA; receiver voltage (measured at a servo lead) stayed about 4.30 V with variations within a few hundredths to a few tenths of a volt. When servos stopped, current dropped to ~65 mA.
• Restart behavior: After discharging the Ni-Cd pack to about 3.3 V and switching the FFS back on, current climbed to ~120 mA while the pack recharged, then subsided to ~65 mA when the pack reached ~4.33 V.

Battery recommendations and capacity notes

• Use Duracell or Energizer nine-volt alkaline batteries—other brands may be less satisfactory. Do not use lithium nine-volt batteries; they have high shelf life but cannot supply the high current bursts the RC system may require.
• Do not substitute rechargeable nine-volt batteries blindly. Some rechargeables deliver higher surge current but lower usable capacity and will run down faster.
• The FFS board regulator will handle about 1 A continuously. In one bench test I saw ~1.5 A briefly with a heavy load, but voltage fell rapidly under that strain.
• Duracell MN1604 data (as provided by the manufacturer) indicate about 552 mAh capacity under light loads; typical quoted behavior was ~14 mA average into a 510-ohm load for ~40 hours. From test data, a fresh nine-volt should run the FFS ~40 hours in Lost Airplane Locator mode.
• Alex Kronfeld's claim of ~30 minutes emergency operation on a fresh battery is a conservative and reasonable estimate for powering servos in backup mode under typical loads.
• Battery replacement guidance: the unit will show backup servos slowing as nine-volt voltage approaches the seven-volt level—use this as a practical cue to replace the battery. If you want to preserve at least five minutes of backup time, retire the battery when it drops to ~7.2 V.

Options, variants, and scaling

• Units tested were configured to support up to four Ni-Cd packs. If you use five-cell packs or higher-capacity servos, T&D will configure cutoff voltages or a larger regulator accordingly.
• Planned variant: B-5-1 (boat failsafe) — list $39.95 — will include a two-amp regulator and a jumper for four/five Ni-Cd cells, but will omit locator and PCM-specific functions; usable with PWM or PCM.

Conclusions and final notes

The Failsafe Flight System is small (about 1.5 in. square), light (less than one ounce), inexpensive, and has low operating current (~7 mA). Used with fresh nine-volt alkaline batteries and with normal preflight checks (servo and rate checks, interference check), the unit performs as claimed and can provide useful backup charging, backup power, low-voltage warning, interference monitoring (PWM models), lost-airplane locating, and switch-position indication.

Failsafe Flight System — $39.95 list from T&D Flight Systems, P.O. Box 1782, Staten Island, NY 10314. For additional information call Alex Kronfeld at 1-718-894-9121.

Transcribed from original scans by AI. Minor OCR errors may remain.