DIGITAL FUEL FLOW AND TOTALIZER CIRCUIT DESIGN AND INSTALLATION Mircraft certified 20 to 30 or more years ago still use analog instrumentation to display engine, avionic and system functions. With the introduction of low cost digital circuitry, many of these functions on newly certified and retrofitted aircraft are now displayed with greater accuracy. Avionics using 7 segment light emitting diodes or gas dis-

charge displays are easier to read, less operator error prone and less likely to fail or require service with time compared to their analog equivalents. Digital instrumentation also reduces pilot workload. One area of engine instrumentation becoming very popular is the digital fuel flow meter and fuel totalizer. In most older aircraft, fuel flow is displayed by pressure gages calibrated as fuel flow in gallons or pounds per hour. This calibration is nonlinear, and is most difficult to interpret at cruise conditions when flows are relatively low. A digital fuel flow and totalizer circuit has been developed, using FAA approved flow transducers, and is proposed to supplement existing instrumentation. In the critical take-off condition the analog gages are preferred since they offer positive proof, without regard for accuracy, that engine function and fuel flow is normal. I have "lusted" for a digital fuel flow meter for my Twin Comanche for sometime, but the high cost of STC'd units has been a restraint. While looking in some old notes for bolt torque values on lubricated and unlubricated threads, I came across an article from Popular Electronics describing a homemade digital speedometer. It seemed reasona-

ble to me that a circuit that counts pulses and displays the results as MPH could also be used to count pulses and display GPH. With a great deal of help from my friend Dennis Kubeldis at Measurement Analysis Corporation in Torrance, an improved circuit using the latest generation IC's and displays has evolved. The simplicity of the circuit is based on using the FloScan transducers and the integrated displays with logic.

by Hans D. Neubert EAA 63118 6051 Prado St. Anaheim, CA 92807

Fuel flow is related to a digital value by use of a transducer that emits a signal pulse for a given flow rate, and the appropriate timing circuitry to drive the displays. A block diagram of the current system is shown in Figure 1. Circuit timing and calibration depends on the transducer model. For the circuit herein, FloScan Model 201A is used, which has a flow range of .3 to 30 gallons per hour, and has a K-factor of 100,000 pulses per gallon. This transducer is appropriate for fuel injected engines. For carbureted engines, FloScan recommends their model 264PB transducer which incorporates a pulsation isolation/response equaliza-

tion preamplifier to minimize the perturbations caused by the diaphragm fuel pump. The complete circuit for measurement of fuel flow and total gallons consumed requires five TTL integrated circuits, the displays, and two voltage regulators. The complete circuit for a multiengined aircraft is shown in Figures 2 and 3. For single engine aircraft, the circuit must be modified due to the different transducer K-factor and elimination of the transducer combiner circuit. For single engine applications, the complete circuit can be packaged into the popular 3-1/8 inch diameter instrument format (Figure 4). Power is taken from the aircraft accessory circuit buss through a 5 amp circuit breaker. The entire circuit draws 1 amp at 14 volts. The circuit operates using internally regulated 5 and 12 volts. The FloScan Model 201A flow transducer has a K-factor of 100,000 pulses

PLANE PWR. SUPPLY + 14.5 VDC

GROUND

\/__\/ SENSOR SIGNAL

RIGHT SENSOR

REGULATED ,+ 12 VDC

FUEL FLOW METER SENSOR SIGNAL

LEFT SENSOR

Figure 1.1-1 - Fuel flow and totalizer block diagram. 26 JANUARY 1990

Figure 2 - Front view of circuit including transducers.

Figure 3 - Rear view of circuit including transducers.

per gallon, uniform within 2.5% over the full flow range of .3 to 30 gallons per hour. To display 1.0 gallons per hour, the circuit must count 10 pulses and reset. This process is repeated as long as the circuit is active. The digital displays have their decimal point fixed between the two rightmost displays. Therefore, the observed "1.0" is actually "10". The timing required is determined as follows: time = 10 / [100,000 P/G * 1 Hr/3600 Sec * 1 G/Hr] = .36 seconds = 360 milliseconds. Circuit frequency is 1/.36 = 2.778 Hz. If the fuel flow is 1.0 gallons per hour, then 10 pulses will result over a .36 second timespan, and will be displayed as "1.0". The combined logic displays have

been on the market for only a few years, and are used since they not only contain the 7 segment LED display, but also the TTL logic, counter, latch, decoder and driver. What previously required 4 discrete devices per display digit is all contained in one device. This saves circuit board space and the additional connections. The numeric display chips count pulses, and when strobe, display the value in the counter. Three devices are ganged together to yield a XX.X display. Timing for the circuit is accomplished via a precision programmable waveform generator (clock). External resistors and capacitors determine the clock frequency. Values are chosen to adjust the clock at 2.778 Hz. The output of the clock is used as input

for a dual one-shot multivibrator. The output of the on-shot provides the strobe signal for the displays, while the inverted form is the clear signal. Pulses from the flow transducers are counted in the displays, displayed by the LED's when strobe, and the counters reset when cleared. This process repeats every 360 milliseconds. To measure total fuel used, pulses are counted directly and displayed without regard for time. In order to display, "1.0" gallons used, the 100,000 pulses are divided by 10,000. This results in 10, again displayed as "1.0", with the decimal point fixed between the two rightmost displays. The divide by 10,000 is accomplished by using two dual 4-bit decade counters. For circuits using a flow transducer having a different K-factor, as in the case of carbureted engines, this portion of the circuit also requires revision. For multiengines, as in the case of the current circuit, pulses from each of the two transducers are summed using one section of an exclusive OR gate. Provided the two inputs received are not closer than 12 nanoseconds apart, the circuit will combine the inputs from both transducers. The displays as well as the divide by 10,000 circuit is initialized by the use of a RC leg and an unused section of the exclusive-OR gate. The circuit does not employ battery backup, or some of the frills found in commercial units. Total gallons used will remain displayed unless power to the unit is interrupted. Installation of the unit is accomplished in a manner similar to existing avionics. Power is taken from the aircraft buss through a 5 amp accessory circuit breaker, and connected to the + V wire. Ground is made locally near the unit, and connected to the black wire. When power is supplied to the unit, the rightmost display of each three display segment will indicate ".0". The transducers are mounted at a convenient location on the firewall using 1.75 x 1.75 x 1.5 x . 125 aluminum extrusion. The extrusion is mounted to the firewall with 10-32 aircraft screws and locknuts. The transducer is mounted to the extrusion with 1/4-20 aircraft screws and locknuts. A 3/8 inch diameter hole is drilled into the firewall for a wire feedthrough grommet. The mounting detail of the transducers is shown in Figure 5. Regulated 12 volts to the transducers is provided by the circuit. The square wave output of the transducer is referenced to 5 volts. Connectivity between the circuit chassis and the transducer is done via a No. 22 twisted pair with shielded ground. The red wire of the transducer is connected to the 12 volt line from the circuit, the white wire is connected to the signal return input to the circuit, while the black wire is connected to ground using one of the SPORT AVIATION 27

thiness of the aircraft due to the following reasons: 1. Circuit operating frequency of

Figure 4 - Packaging for single engine aircraft.

mounting screws. The shielded ground of the cable is not connected at the transducer, only at the circuit. The purpose of the shielded ground is to minimize signal noise from the engine compartment. Quick disconnect electrical connectors are used at both ends of the signal/power/shielded ground cable. Routing of the cable follows the aircraft existing pathways. The fuel line between the fuel/air servo regulator and the fuel injection divider block is disconnected at the servo regulator and connected to the "OUT" side of the transducer using a standard AN fitting. A new fuel hose is fabricated and connected from the fuel/air servo regulator to the "IN" side of the transducer. All fuel must pass through the transducer. Sharp bends and elbow fittings should be avoided in the fuel hose installation to minimize flow turbulence and signal errors. Verification of a leak free installation can be accomplished by temporarily disconnecting the fuel hose at the injection divider block located at the top of the engine and capping with a plug, and

operating the boost pumps. Verification of correct fuel flow readings and total fuel used is done by flight experience, with accurate measurements of the supply quantity and the tank levels. Unless the circuit is suspect of providing incorrect readings, the unit is maintenance free. In the unlikely event of difficulty with the circuit, disconnect the power/ground connector and placard the unit as "Out of Service" per the appropriate FAR. The transducer has a rated life of 10,000 hours, and is not expected to cause operational difficulty. The transducer is also maintenance free. In the unlikely event of difficulty with the transducer, disconnect the unit from the mounting angle and install a new transducer. Flight operations may be continued by temporarily installing a close nipple between the two ends of the hoses connected to the transducer. Compatibility The addition of the circuit or transducers is not expected to affect the airwor-

AN 4-14 BOLT

AN 910-4 WSHR + 12V SIGNAL GND

1.75X1.75X.125 ALUM. EXTR.

LOCKNUT TYP 2 PLCS

AN 3-4 BOLT AN910-4L WSHR

LOCKNUT TYP 3 PLCS

Figure 5 - Transducer mounting details. 28 JANUARY 1990

2.778 Hz (or any of its harmonics) will not interfere with existing navaid equipment due to the low frequency. 2. The circuit is fully enclosed in a chassis box, minimizing energy emittance. 3. The fuel flow measurement is independent of the existing engine measurement instrumentation. 4. Fuel pressure drop at maximum takeoff power will be approximately .6 psi due to the transducer, which is 2.3% of the 26 psi operating pressure. Pressure drop will be less at cruise conditions. 5. FloScan is sole source supplier for identical (and similar) transducers for STC'd fuel flow units, as well as supplier to Piper, Cessna and Beechcraft. 6. The FloScan transducer design uses a helical flow path design to vent any entrained vapor bubbles.

Summary A relatively simple circuit has been developed to operate as a digital fuel flow meter and totalizer. The prototype circuit was developed for a multi-engine aircraft using hand wiring and soldering, a tedious process. If there is sufficient interest by both single and multi-engine aircraft owners (a post card will do), professional quality circuit boards and complete kits will be developed and marketed. Single engine aircraft complete kits will probably run about $400, while multi-engine aircraft kits will run about $650. The kits would include all parts, hardware, electronics, transducer^) except fuel hoses and fittings. Production aircraft will require a Form 337 to be filed with the local FSDO, with an appropriate log book entry.

ABOUT THE AUTHOR

Hans Neubert is an expert in composite materials and analysis, holding BS and MS degrees in Mechanical and Aerospace Engineering, respectively. With 23 years of composite experience, he has been an independent consultant to the aerospace industry for the last 10 years. He is an FAA Designated Engineering Representative in structural analysis. Previous articles in SPORT AVIATION are found in July, September, December 1976, April 1977 and January 1978. He owns a Twin Comanche and a perennial 90% finished homebuilt. Digital electronics have always fascinated him, and provide a new learning experience.