[Bdegan]

I was taking some readings today and am getting rather large differences between on the ground static, and in the air flying.

As an example, I maidened a new plane today. But before the first flight I hooked up a watt meter to see what it was doing.

On the ground with a watt meter I got
78 amps peak
1611 watts peak

The in air numbers recorded with an Eagletree Datalogger.
The maiden was 5:56 in duration. According to the datalogger......
Peak amps was 52.36
Peak Watts 1149
Low PakV 21.55

Second flight :
Duration 11:58
Peak amps 60.77
Peak watts 1285
Low PakV 20.99

Is it normal for there to be such a difference. Does an electric system unload like a fueled system?



Never pay again for live sex! | Hot girls doing naughty stuff for free! | Chat for free!

[RAMJET]

sounds normal the greater the load the higher the draw ,can't draw more than being held on the ground.the advantage to the info is you can prop for actual draw in the air .this may over load the motor if held on the ground but you won't need to do that anymore anyway.

[GGrant]

The explanation could be that the air is thinner at higher altitudes, so it is easier to spin the prop. But I doubt that you were high enough to make a difference. LOL.

The real explanation is the movement of the plane. The faster the plane is moving, the less thrust is being produced by the prop (at a given RPM), and thus the drop in power. On the ground, the plane isn't moving when you are winding out the motor, so the battery is providing all the energy to move the air.

Yes, an electrical system does unload - except unlike a fuel system, the weight doesn't go down! As the battery discharges, its voltage gradually goes down. Simple formula: V=IR. Volts = Amps x Resistance. The circuit resistance remains (more or less) constant for a given throttle position. So as the voltage goes down, so does the amperage. To continue, Watts = Volts x Amps. So as the voltage drops, and the amperage drops, the wattage drops even faster. AT 90% of voltage, you'd also have 90% of the amperage, so you'd be pulling 81% of the wattage!

My guess is that you used about 1/2 the battery pack playing on the ground before the first flight. By the time you took off, the battery voltage was down a bit, that is why your peak values were lower on the first flight.

If you want to test the plane movement theory, try your ground test while holding the plane up in a 30 kph head wind. Peak amps should be noticibly lower.

[Max]

Brad- every amperage reading I have made static testing on my dataloger on the ground has had a reduction in the air. My planes arent as big as yours, but I get around te same reduction in current draw in the air- about 10%.

I even notice this on my heli's when I compare current load of a flight in hover, vs a flight where I fly forward in translational lift. Fwd flight has less load then hovering.

[Cougar429]

You also get into efficiency factors. If the motor is lugged down on the ground, (and we all seem to agree that is the case-with the correct prop) then you will find the motor outside the "Sweet Spot" in RPM and efficiency.

What would have been extra was to have a motor/prop RPM reading in flight.

With brushless motors that reduction is rather drastic, so you don't have to overprop it by much for it to happen. The easiest way to see if this is the case is to check for component heating at the end of a flight. If the motor, ESC and/or battery are excessively warm to the touch then you are pushing it close to or at the limits, which does reliablility no good.

p.s. There are a couple of other reasons, one you already factored out yourself. You will also get that with inadequate cooling airflow, so you need to look at that if something is excessive temp. The one you eliminated was asking more from the pack than it was able to provide, but in that case your pack voltage would have sagged.

[Guest]

Originally Posted by Bdegan

Is it normal for there to be such a difference. Does an electric system unload like a fueled system?

Yes, and yes.

[Bdegan]

Originally Posted by GGrant

My guess is that you used about 1/2 the battery pack playing on the ground before the first flight. By the time you took off, the battery voltage was down a bit, that is why your peak values were lower on the first flight.

Thanks for the info.
But I didn't use the run up battery for the first flight.
Ground run up - Battery 1
Maiden flight - Battery 2
second flight - Battery 3

The maiden flight was just flying around easy, trimming, etc.

The second flight was more of what I usually do. I threw in some hovers and some nice long up lines to work the motor and see what results I would get.

The battery i used for the second flight ( the longest at 12 minutes ) only used 3540 mah's.
So i have room to fly a bit more aggresively and stay in the battery safe zone.



Never pay again for live sex! | Hot girls doing naughty stuff for free! | Chat for free!

[Bdegan]

Originally Posted by Cougar429

p.s. There are a couple of other reasons, one you already factored out yourself. You will also get that with inadequate cooling airflow, so you need to look at that if something is excessive temp. The one you eliminated was asking more from the pack than it was able to provide, but in that case your pack voltage would have sagged.

Everything was quite cool, motor and esc. i am getting good airflow through the cowl.
The battery was warm, but not hot by any means.



Never pay again for live sex! | Hot girls doing naughty stuff for free! | Chat for free!

[Cougar429]

Don't forget, in a hover or any other high power/high alpha maneuver you're back to identical conditions as your initial ground run.

[Guest]

Originally Posted by Cougar429

Don't forget, in a hover or any other high power/high alpha maneuver you're back to identical conditions as your initial ground run.

Not really, static would be full power, hovers in high performance electrics are usually no where near full power, so still lower than static.

Brad, don't sweat it, looks like a good setup to me!