CubCrafters Carbon Cub FX-3 Electrical Questions

[Meconiates]

Had some more electrical questions:

1) Does anyone know what the electrical draw is for a full IFR panel on a FX3 (G3X, G5, GNC355, two Comms, etc).

2) What does a heated pitot draw? I assume there is an on / off switch for the heated pitot?

3) In a scenario where you are drawing more than 40amp, can you damage the alternator? Or will it just supply 40amp and then the battery will be discharging?

4) Anyone running a EarthX 1200 battery? I wonder if that’s prudent upgrade to give yourself more margin of safety for a situation where you are temporarily drawing more than alternator can handle ?

I was at the Yakima factory today and was trying to finalize my build sheet. And a concern was raised that I am going to exceed the electrical capacity if I did all of the following:

- Full IFR with pitot heat
- 100w Landing light with 10amp
Breaker
- 100w taxi light with 10am breaker
- TA245 USBC dual (45w each plug) so there is another 90w / 10amp breaker

 

[Neal]

I moved this post out of the landing lights MOD discussion, probably best for its own thread in configuring a new FX-3 to be built by the factory.

Probably some questions to send to [email protected] and get Pete's help.

It was stated by @earthxbatteries not to use the 1200, the 900 is the correct battery for us.

At some point increasing electrical demands is going to go beyond the design and circuit, yeah, not stating anything we all know. So there certainly has to be limits in what is used IMHO. I don't think you can throw an "extreme" upsize to the alternator without needing wiring, fuses, on and on to support the loads.

If no one can help then I'd say you really need to dig into the documentation to pull the electrical specs for each and every LRU, component, etc. The ole AI / chatGPT on and on may help but needs to be verified.

 

[Cactus Charlie]

2) What does a heated pitot draw? I assume there is an on / off switch for the heated pitot?

There are two verions of the heated pitot. CubCrafters usually fits the GAP 26-10 which is the uncontrolled version. That means it is either on or off as set by the switch.

Like all resistive heaters the current depends on the probe temperature. When the probe is turned on there will be a large current peak but current will quickly reduce to a steady state value that is influenced by ambient temperature.

I cannot tell you the peak current because G3X logged data is heavily lagged. When I turned on my GAP 26-10 the data log shows a peak current of 6.5 A and stabilized current of 4.9 A with ambient 36 deg F. Actual peak current at switch-on would have been greater than 6.5 A but less than 20 A CB rating.

 

[Cactus Charlie]

3) In a scenario where you are drawing more than 40amp, can you damage the alternator? Or will it just supply 40amp and then the battery will be discharging?

Don't know for sure. I expect the alternator will supply over 40 A but will overheat and may fail. I would expect the voltage to reduce as current draw is increased and that would eventually limit the current it can supply.

Since alternator is critical for sustained flight I would be inclined not to overload it.

I have an amber alert for alternator current greater than 30 A (corrected from 35 A). It tripped once very briefly after a difficult engine start.

 

[Cactus Charlie]

1) Does anyone know what the electrical draw is for a full IFR panel on a FX3 (G3X, G5, GNC355, two Comms, etc).

I don't have that configuration but remember that peak avionics load will be when transmitting with AP engaged and servos at peak torque.

I did some battery current measurements on my system (standard G3X VFR plus GPS 175 and G5) a while ago, Here are my notes -

With engine running the ignition load will be about 2.5 A, not the zero increase shown in the table. Transmitting will add about 3 A.

It's easy to go over the top with added equipment. Think about what you really need for the way you plan to use the aircraft. Don't add weight, cost, and complexity for equipment you don't need for your mission.

 

[Cactus Charlie]

Here is a plot of alternator and battery current for a typical flight in my FX-3. Strobes on, lights wig/wag, no autopilot, no pitot heat. Peaks were correlated to radio TX and fuel pump use.

 

[ve6yeq]

In my journey to understand my EX-3's electrical system, I have discovered that it is not as simple as just adding up current draws. I then switched to adding up average and peak draws but that still does not match reality. Very few things behave on face value. Some examples.

• My EarthX battery has a specification of no more than 80 Amps of charge current and yet it works fine with a 40 A fuse. The battery will draw significantly more than 80 A if fully discharged and connected to a power source capable of more than 80 Amps so for example connecting it directly to a 120 A alternator will cause bad things to happen. However, the battery is almost never fully discharged (when you need to use an external current limited charger to charge the battery before use) and the 40 A alternator does not produce full power at 1000 RPM after start so in practice, everything “just works”.
• Some power draws are constant power (e.g. XeVision LED lights) such that the current draw changes with voltage but not time.
• Some power draws are very time dependent (e.g. WAT LED lights drop greatly in the first few seconds and does not level out for 15 min).
• Some power draws are very conditional (VHF AM radio transmit)
• Some power draws are inversely proportional to outside temperature (e.g. pitot heat)

So overall, just adding numbers does not really work.

 

[ve6yeq]

As far as

Had some more electrical questions:
1) Does anyone know what the electrical draw is for a full IFR panel on a FX3 (G3X, G5, GNC355, two Comms, etc).

My measurements indicate that the full "IFR" panel (including AP servos) draws approximately 5 A at 13.2 Vdc, with a peak current of 8.5 A. This differs from simply summing the average and peak current draws; without the AP servos, the average should be 8 A and the peak 12+ A.

2) What does a heated pitot draw? I assume there is an on / off switch for the heated pitot?

The heated pitot draw is inversely proportional to OAT and airspeed; at lower OAT and higher airspeed, more power is drawn. I do not have the temperature-regulated version, as I plan to use pitot heat only when required, not as a precaution (I have GPS speed and known power/performance while I wait for it to melt). The temperature-regulated version reduces average power consumption while on and prevents people from being burned on the ground.

3) In a scenario where you are drawing more than 40amp, can you damage the alternator? Or will it just supply 40amp and then the battery will be discharging?

When the alternator is overloaded (a normal condition, especially just after engine start), its voltage output drops, reducing the power available for tasks such as main battery charging. Remember that you only get 40 A when the RPM is above 2500 RPM, and the temperature is cold.

4) Anyone running a EarthX 1200 battery? I wonder if that’s prudent upgrade to give yourself more margin of safety for a situation where you are temporarily drawing more than alternator can handle ?

The ETX-1200 is for more starting current not for more capacity. The ETX-1200 might supply too much current to the starter motor causing damage. The ETX-680 is correctly sized and the ETX-900 is oversized.

I was at the Yakima factory today and was trying to finalize my build sheet. And a concern was raised that I am going to exceed the electrical capacity if I did all of the following:
- Full IFR with pitot heat
- 100w Landing light with 10amp
Breaker
- 100w taxi light with 10am breaker
- TA245 USBC dual (45w each plug) so there is another 90w / 10amp breaker

You will not damage anything, but you may not have the IFR reserves you expect if the alternator or main battery fails. Remember that on landing, the alternator is not producing full power (the engine is at idle), so the battery will support some of the power draw on final and during taxi, and you might need to idle for a couple of minutes to recharge the main battery after powering off the lights, pitot heat, etc., before shutting down the engine.

 

[Cactus Charlie]

Remember that on landing, the alternator is not producing full power (the engine is at idle), so the battery will support some of the power draw on final and during taxi, and you might need to idle for a couple of minutes to recharge the main battery after powering off the lights, pitot heat, etc., before shutting down the engine.

I have never seen negative battery current with engine running. I alert for that condition. My alternator will power all my loads at idle and is capable of at least 33 A battery recovery at 1,200 rpm

I think you are overstating the low rpm case but, if not, perhaps you could share your data.

Here is a sample of my data that shows alternator output and rpm after a difficult start.