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Best Practice

‘Best Practice’ reflects accumulated knowledge and experience about what works and doesn’t work in achieving safe, reliable and enjoyable electric flight.  Rather than thinking about rules and regulations, think of these as the benchmarks.

Material is arranged in the sequence, from creation (Construction) through to disposal (Post Flying).

 

 

CONSTRUCTION

 

Power System Component Location

Position the ESC and motor battery so that the connecting wires are as short as possible. This reduces power loss and radiated radio interference.

Before deciding on the location for the motor battery pack, try moving the pack until CoG is correct.  This avoids adding extra weight to obtain balance.

Retain the motor battery securely.  A solid former epoxied in front of motor pack will help.  Prevent movement during flight, by packing the battery with foam or securing it with velcro

The Rx (and any Rx battery pack) should be located behind the motor battery pack, again to avoid damage in the event of an accident.

Keep the Rx as far as practical away from the ESC and its wires. This reduces the possibility of an ESC’s radiated ‘noise’ interfering with the Rx.

Heat Dissipation

Electrical power system components (ESC, motor and battery) can get hot during operation.  A good practice is to ensure that there is some airflow over the motor and ESC. 

For enclosed components, good airflow results when air exit holes are about three times the size of the entry holes.

Where components are used at well below their rated limits the need for cooling is greatly reduced and providing some space around the components may be sufficient to help any heat dissipate.

Cooling can be easily assessed during initial flight testing by keeping flights short and checking motor and ESC temperatures as soon as possible after landing.  Component should not be approaching too hot to hold.

Wire Size

For applications at less than 10A, use 18 gauge wire.  For currents up to 20A, 16 gauge is the minimum, and 14 is preferred.  For currents in the 40 to 50A range, 12 gauge wire is desirable.

BEC Applicability

If weight is at a premium for small aircraft, take advantage of the BEC function.  Examples are small slow-fly and park-fly type models.

Aircraft with high servo count may exceed a BEC’s current rating.  Check the ESC’s specifications.

When using batteries of more than 3S or more than four servos, and ESC with a linear BEC is likely to be inadequate and may overheat and fail.  In those cases, use either an ESC with a switching regulator BEC (SBEC), a separate UBEC, or a separate receiver battery.

If a UBEC or separate receiver battery are used to power the radio gear, the ESC’s BEC must be disabled.

Many ESCs designed for higher cell counts don’t include a BEC.

Don’t confuse BEC with low voltage cut-off.

Switches or fuses

Use care in incorporating switches.

Switches or fuses for the motor battery circuit that are rated to handle the currents used in electric models are usually large, bulky and are potential sources of power loss.  There are currently no known components that are suitable.

Rx switches are needed when no BEC function is used.  In these cases, a separate battery is used to supply Rx power and there needs to be a way to turn off that supply.

If a BEC function is included then a Rx switch is not recommended.  In some cases a powered up ESC may start a motor without a throttle command being present.  Therefore it is unsafe to allow the possibility for the motor battery pack and ESC being connected without the Rx (and Tx) being turned on.

Electrical Noise Reduction

Ferrite cores on ESC leads can help reduce the effects of interference from the ESC on the Rx’s power supply.  If not supplied as part of the ESC they can be purchased at electronic parts shops like Jaycar.

Battery Connections

Use connectors that eliminate or reduce the possibility of misconnection.  Deans Ultra plugs are a preferred choice in most applications.  In cases where high currents are expected bullet connectors are commonly used.

Battery Connection Convention

Red for positive, black for negative

With Deans plugs, install female (socket) connectors on power supplies and batteries.  The positive lead is always connected to the top of the ‘T’.

With bullet connectors, there is a trend to mix gender on the supply (battery) side with female positive and female negative.  These connectors should be protected by a sheath made from plastic or fuel tube. Phrases like ‘Red Hot Female’ and ‘Blackmail’ help in remembering the protocol.

Disabling BEC

Disabling a BEC is not usually a programmable function and therefore must be achieved by physical means.  The aim is to get the servo signal to the ESC without getting power from it.  This can be accomplished by open circuiting the red wire in the ESC servo lead.

One approach is to use a servo extension lead and sever the red wire, preserving the integrity of the ESC for other possible uses.

Another method is to pull out the red wire’s pin on the ESC servo connector.

Motor/ESC connection

Use bullet connectors to join an ESC and motor unless the lowest possible power system loss is required when soldering is an alternative.

Connections must be insulated so that they can’t inadvertently make electrical contact with each other or the motor.  Heatshrink tubing is the normal choice..

Twin engine aircraft

Use a single motor battery pack to supply both motors.

For brushless motors, use a separate ESC for each motor.

Consider how to deal with low voltage cut-off.  Each ESC will trip at a slightly different voltage because of small manufacturing tolerances.  If the aircraft is flown until the battery is nearly exhausted, one motor can slow or stop before the other, making control very difficult.  A timer may be the best solution to avoid getting close to the low cut off threshold.

 

PREPARATION FOR A FLYING SESSION

Know The Rules

Australian Government rules determine where and how you fly.  Information is on the MAAA website at Know the rules

LiPo Charging

Only charge LiPo batteries on a charger specifically design for LiPo chemistry.  They must not be charged with a peak detector charger.

On multi-chemistry chargers ensure that ‘LiPo’ is selected.

On chargers with a manual set up, ensure the number of cells (or voltage) and capacity is set correctly.

When using chargers that automatically determine the number of cells, confirm that the correct number is detected.  This is particularly important when charging higher cell count batteries as it is possible, for example, that an almost fully charged 5-cell battery could be detected as a discharged 6-cell battery. If charging is allowed to continue, the battery will be charged to well beyond its safe maximum voltage. An undetected error could lead to a fire.

Note the battery manufacturer’s specification for maximum charge rate.  For example, if it’s a 2C 2200mAh pack then maximum current is 4.4A.

Never leave charging LiPo cells unattended (at any charge rate).

Charge LiPo cells in an open space on a non-flammable surface (such as a brick or quarry tile) and away from flammable materials.

Have a dry powder fire extinguisher or a bucket of dry sand within reach.

Cell balancing

Manufacturing techniques result in some differences between almost otherwise identical cells.  When charging LiPos, the effect can be that one cell reaches its safe fully charged voltage (4.2 volts) before the others.  If charging is continued, this cell can be damaged (or worse).

Many modern LiPo chargers are balancing chargers and automatically limit the charging of each cell as it approaches its safe maximum.  Alternatively, an external balancer can be connected to the battery. 

Balancing will optimise the performance of the cells in a pack and eliminate any risks associated with overcharging (fire is one).

Not every charge needs to be a ‘balanced’ charge but consider the risks if overcharging results.

 

PRE FLIGHT

Handling

The only time an electric motor can be considered safe is when it is not connected to a battery pack.  Always treat an electric model as if it were about to burst into full power.  Keep the propeller clear at all times.

Arming Sequence

The preferred start-up sequence is Tx on, Rx on (if switched), then connect motor battery.

Make sure that the throttle is set to its minimum (off) position before turning on the Tx.

Some 2.4GHz systems require the Rx be turned on before the Tx.  Proceed with extreme caution and restrain the aircraft during start up sequence.

Tx Precautions

Be aware of servo reversing switches on the front of some Txs.  Don’t accidentally reverse the throttle direction.

Hand Launch

Throwing Strength

Throwing strength depends on the model.  Light, high power/thrust to weight models will fly out of the hand.  Models with higher wing loadings, if thrown too softly, won’t get enough airspeed to fly.  Heavy, high wing loaded aircraft may be impossible to hand-launch, striking the ground before they can accelerate to flying speed. If thrown too hard the model may not come out of the hand on a straight path and may be very hard to control.  The aim is for a smooth, straight launch and to match the model’s flying speed. There is no formula, just judgment.

Throwing Technique

Launching a plane is a bit more like throwing a dart or a javelin than a ball.  If launching your own aircraft, use the hand that is on the throttle side of the Tx.  Throttle is not a self centering function so it doesn’t need to be held in place during the launch.  Holding the plane overhead provides a little more altitude.  Give the plane a good solid flat push.

Holding the Plane

Grip the plane close to, but behind, the C of G.

Holding the Tx

If launching your own aircraft, angle the Tx so that its weight is supported by the palm of the non-throwing hand, not just the fingers.  Keep the thumb on the Tx stick.

Throttle

On lower powered models, full throttle launches may be appropriate.  On high powered models, full throttle could induce a torque roll and one half to two-thirds throttle may be more appropriate. If launching your own aircraft, bring throttle to ‘off’, hold the Tx in one hand, aircraft in the other and nudge the throttle ‘on’ with your chin.

Launch Angle

Usually the best angle is flat and level, or just slightly nose up.  If the nose is too high there is risk of stalling.

Climbing Out

Depending on the aircraft, don't try to climb out straight away.  Similar to an ROG take-off, in the first few seconds after launch let the model fly straight and level while it builds airspeed.

Direction

Launch into the wind.

 

IN FLIGHT

Flight Times

Plan on only using 70% of the motor battery’s capacity.  LiPo cells have a minimum safe discharge voltage and a good, properly programmed ESC should ensure that the motor is throttled back or stopped before the battery is too discharged.  However, battery service life can be greatly extended by ensuring at the end of each flight there is still 20-30% of the available charge in the pack.

Time initial test flights and keep them fairly short.  Determine how much charge is put back into the battery when it is re-charged, then calculate how long you can fly until the battery is about 70% discharged.

Restrict flight times to that duration by using a talking timer or Tx function to warn when the time has elapsed.  Remember that the amount of throttle used during the flight will greatly affect the amount of battery charge that is consumed.

 

AFTER FLIGHT

Aircraft Handling

After a flight, helpers or spectators may need to be reminded that the only time an electric motor can be considered safe is when it is not connected to a battery pack.  Make the aircraft safe as soon as possible.

Motor Battery Removal

Batteries such as A123 cells may be left in aircraft but LiPos should be removed and stored separately.

 

POST FLYING SESSION

LiPo storage

Do not allow the batteries to experience extreme temperatures.

Never allow a battery to stay directly in the sun or near another heat source.

LiPo batteries do self-discharge, although they do so at very slow rates; eventually they do lose their charge.  Packs that fall under 2.5 volts per cell can become unusable. Avoid storing discharged batteries for a long period of time. as they will eventually discharge to below a safe voltage.

Batteries age.  For long-term storage, battery deterioration can be minimized by storing them about half charged, in a cool (not cold) place.

LiPo Damage

If a pack is involved in a crash or is damaged, remove the pack from the model.  Inspect the pack for damage to the cells, wiring or connections. A charged, damaged pack could catch fire at any time. Safely dispose of damaged batteries.

LiPo Disposal

Put the pack in a safe open area

Connect a moderate resistance across the cell terminals until the cell is completely discharged. CAUTION: The pack may get extremely hot during the discharge.

Puncture the plastic envelope and immerse in salt water for several hours.

The pack can be discarded in household (non-recyclable) rubbish.

(Used with BEFA permission)

LiPo Transport

See http://auspost.com.au/media/documents/LithiumBatteries_CustGuidelines.pdf

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