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View Full Version : 23.2A on the ground, ???? in the air


John D
09-08-2005, 08:07 AM
My eMeter:D tells me that at full throttle I have 9.82volts, 23.2A, 228 watts and 18000 rpm. What happens when this gets in the air?

I presume the revs might go up a bit, what about the amps??

The setup I use is 2x 1250mah, 11.1v, 3s1p etecs (rated 10c) in parallel, castle creations 25A esc, mega 16/15/4, graupner speed 6.6x3 prop.

It occured to me that if the revs do increase, the current might as well.

Am I getting too close to the limits for the battery and esc?

Azarr
09-08-2005, 01:21 PM
The current goes down as the prop unloads.

Azarr
www.ecubedrc.com (http://www.ecubedrc.com)

John D
09-08-2005, 02:09 PM
"The current goes down as the prop unloads."

This probably rates as a stupid question but here goes anyway. When the prop unloads, do the revs increase, or is it only a decrease in current?

falingtrea
09-08-2005, 05:20 PM
I would say that you get some rpm increase as the prop unloads.

BTW is there any kind of unit that measures current in flight? Would that be a useful item? Or maybe a speed controller that monitors current that you can read back later?

Azarr
09-08-2005, 08:24 PM
There's a few on the market, but you need to be able to carry the weight, about 1.5 oz.

Here's one: http://www.eagletreesystems.com/

Azarr
www.ecubedrc.com (http://www.ecubedrc.com)

falingtrea
09-08-2005, 10:56 PM
There's a few on the market, but you need to be able to carry the weight, about 1.5 oz.


So if the capability to measure and store current and rpm could be put into a ESC with little or no added weight would that be useful?

Twmaster
09-08-2005, 11:47 PM
Perhaps it would be useful if somebody explains the term 'unloads' as it pertains to the propeller both in the air and statically on the ground ie while testing.

falingtrea
09-09-2005, 06:40 AM
Well this is a guess. When a motor and prop are running statically, the prop is basically "pushing" air. So you use a certain amount of your work just moving air and the prop sees more resistance. When the motor and prop are in free motion you are "pulling" through the air and have a different airflow and less resistance so the prop should spin faster for the same power input.

GeraldRosebery
09-09-2005, 11:22 PM
I would say that you get some rpm increase as the prop unloads.

BTW is there any kind of unit that measures current in flight? Would that be a useful item? Or maybe a speed controller that monitors current that you can read back later?

Some of the lighter watt meters can be sent up in you model and will record the information you want. I am sure someone can tell you which one as I can't recall the ad.

Greg Covey
09-12-2005, 10:45 PM
The rule of thumb we have been using for years is 10%-15% unloading in the air. This seems to work when you correlate average static current on the ground to the same throttle setting in the air. It also helps us to plan for more burst power when using throttle management.

Matt Kirsch
09-13-2005, 05:07 PM
Perhaps it would be useful if somebody explains the term 'unloads' as it pertains to the propeller both in the air and statically on the ground ie while testing.

On the ground, the propeller is pulling against relatively still air. This takes more power because the air has to be accelerated from a standstill.

In the air, the plane is moving forward, so the air is already moving through the prop. It takes less power to accelerate the moving air with the prop than it does to accelerate it from a standstill.

John D
09-14-2005, 04:57 AM
Thanks for the replies so far.

I understand how the prop unloads now. What I would like to know is what effect that unloading has on rpm and amps.

At 60% throttle with a 5.2x5.2 prop the emeter tells me I've got 10.89volts, 11.1amps, 121 watts and 15570 rpm.

As the prop unloads, what could I expect these numbers to read?

Greg Coveys' post says 10 - 15% unloading could be expected. Does this mean the revs might go up 10 - 15% with no increase in amps etc or do all the numbers go up 10 - 15%, or maybe the revs stay constant and the amps go down 10 - 15%?

regards

John

ForestCam
09-14-2005, 05:20 AM
As a prop unloads the amp draw goes down.
Simple way to explain it is take a vacuum with a hose attachment and put your hand over the hose, motor revs up because there is no airflow (ie. no resistance) so the amp draw drops. Now to mimic a prop in flight blow air from the exhaust from another vac into the hose and the same thing happens, more airflow, less resistance, motor speeds up and the amp draw drops.

Same thing goes for the HVAC industry, you have to know the amount of airflow for a given system to make sure you don't underload the blower motors.

jonnyjetprop
09-15-2005, 04:18 PM
I would count on less unloading than most people think. I wish I could remember the article, but the author was using the Eagle Tree system and was discussing his results. His 3D plane actually pulled more amps while flying than on the ground while his scale plane showed some unloading.

John

Matt
09-16-2005, 02:00 AM
The 3D type plane was probably using a large diameter prop with a low pitch. You would expect less unloading on a low pitch prop. It is interesting that the current increased while flying. Maybe the servo current draw added to the reading? If he/she was flying in a 3D manner, it would be similar to a static test.
Matt

Matt Kirsch
09-16-2005, 05:09 PM
Definitely. 3D consists of primarily low- and zero-airspeed maneuvers, so it's like a static test on the ground.

I believe it's technically correct to say that the closer the plane flies to "pitch speed," which is the theoretical speed that the propeller would "thread" itself through the air like a screw if it were 100% efficient, the more it unloads.

rcairflyer
09-23-2005, 04:32 AM
I had a wattage cobalt, 8 cell 1950 Sanyo's, APC5x5. I tested the motor static, and tested it again while blowing air across the Zagi. The motor drew more current. Try it yourself. Try a fan or low powered leaf blower at various distances from the aircraft. You might be surprised at the resulting measurements. Word from rcgroups was that if the prop is stalled and churning air, it is doing less work than if it was actually biting into the air.

Even if the motor unloads by 1 one amp, do you really want to operate at 95% of meltdown? If you need to get as close as possible to 100%, you ought to measure current during flight.

Bill

redgiki
09-25-2005, 09:35 PM
Even if the motor unloads by 1 one amp, do you really want to operate at 95% of meltdown? If you need to get as close as possible to 100%, you ought to measure current during flight.
For a lot of smaller-plane sport flyers, the unloading is actually pretty useless as far as "pushing the envelope" goes. We're operating well below the maximum burst discharge rate of our LiPos. The real appeal of knowing how much the prop unloads is figuring out how much longer we can fly on a given pack.

For instance, on my almost-stock Stryker, my static amp draw with a 6.5x3 prop is 10.5 amps on a 3S1P 2100mAh -- exactly 5C. That gives me 12oz of thrust according to my trusty kitchen scale, which is enough to rescue me from most precarious positions (flying weight: 22oz with battery installed).

2100mAh total charge / 10,500mA draw = 0.2 hours, or twelve minutes. The prop 'unwinding' in the air is pretty consistent with my observations, as at twelve minutes the battery is still going strong. The ESC will cut it off at just shy of 15 minutes of wide-open throttle if all I'm doing is full-speed laps.

So the fact that the prop unwinds means that sport flyers can set a timer right at the max amp draw theoretical flying limit, and then fly however they want to, knowing that when the timer goes off, they have a little time to bring the bird down under power.

My two cents as to the point of knowing that a prop unwinds 10-15% in the air :) My experience is that the peak RPMs don't seem to increase at all in straight-and-level flight, but the reduced amp draw vs. a static test is obvious from the flight times. The effect seems to be more pronounced the steeper the pitch of the propeller, and less pronounced the lower the speed you fly at or the more drag you have.

--
Matt B.