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WHO NEEDS A WATT METER?

Understanding the value of this tool

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Ed Anderson29/03/2010 16:58:26
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234 forum posts
WHO NEEDS A WATT METER?
A personal experience reveals their value.

I enjoy electric planes.  They are quiet, convenient, can be fast or
slow and are fairly inexpensive to fly.

A few months back I picked up a Watts-up wattmeter.


I thought it would be a good investment as I was doing more in the area of
mixing and matching motors, props, and the like.  It is small and simple to
use so I put it in my field box.  It wasn't long before it started to show
its value.

We were flying one afternoon when one of the club members felt he was
 not getting good performance from a new plane he had built.  I put he wattmeter
 on the plane and determined he was pulling about 9 amps.  Turned out the pack he
was using really was not up to the load and the voltage was dropping off
excessively.  As a result he was not getting the RPM out of the prop that he
expected.  Problem discovered and cause identified in a few seconds.  He
needed stronger battery packs.

A few weeks later we did the same thing with another plane.  There was a
concern that the LiPo being used might be getting over worked.  However the
Wattmeter showed that it was working well within its rated capacity.  Flying
went on with confidence.

I recently purchased an Easy Glider Electric from another club member.  He
had upgraded the motor from the stock speed 400 to a brushless, a 27 amp ESC
and was using 2 cell 2100 MAh LIPOs.  I bought the whole package.

The plane flies very nicely on the 2 cell packs, but I had a 3 cell pack
that I thought I might add to the rotation and REALLY boost the power.  The
ESC could handle 3 cell LiPo so I did not see a problem.  I assumed the
system was probably running at about 18 amps which was within the rating of
this pack.  Should be a good fit.

Fortunately before I tried it in the plane I put the watt meter on the
system.  I was surprised to see that the system was running at 26 amps on
the 2 cell lipo packs.  That was much higher than I had expected.  It turned
out that the 2 cell packs were an excellent match for the motor and speed
control.  The amp load was well within the specs of the 2 cell packs being
used and the plane flew very nicely on this combo.

If I had blindly put a 3 cell pack in there I would have pushed well past
the ESC's 27 amp rating and probably burned out the speed controller.  Or,
in the case of my 3 cell pack, it would probably have pushed over 30 amps
into the system due to the higher voltage, but it was not rated for that
high of an amperage and would probably have had a short life working at that
level.  I would have thought it was just a crummy battery pack but in fact I
would have been over working it.

Operating in the blind I would have ruined the ESC, or the pack, or both.  A
very expensive mistake.  Certainly more than the cost of the watt meter.  It
had just paid for itself.

A few days ago I pulled out my old Electrajet to prepare to sell it.  I had
purchased it almost 3 years ago, but had never really been happy with the
plane and my interests have turned more toward gliders and slow flyers
rather than a pusher jet.  When I purchased it I also bought some cells and
made up some 8 cell packs.  However it really didn't seem to have the zip I
thought it should.  I just attributed it to the speed 400 motor and the
plane being too heavy.

I put the watt meter on the motor/battery combo.  The motor sounded about as
I had recalled.  When I checked the meter, low and behold, those 8 cell
packs were duds!   They were 9.6V 8 cell 1000 MAh packs rated for 10C. At
rest, fresh off the charger they were reading 11 volts, but when I hooked
them up they were both dropping to 7 volts while delivering 9 amps.  That is
way too much drop!  The problem was not the plane or the weight of the plane
but the quality of the cells I had used.

I tried one of my 15C Lipo packs and that held voltage well, delivering 13
amps.  The motor screamed!  Now that was more like what I had expected.
Hummm, maybe I won't sell it after all.  I just need to put better battery
packs in it.

I also tried a 1000 MAh 2 cell lithium pack that is rated at 10 C.  The
voltage sagged to 6.6 volts almost immediately.  The motor ran but I was
clearly over stressing the pack.  This pack would have been ruined in very
few flights if I had used it to fly the plane regularly.

I share this story only to help you understand that, without a watt meter,
or the use of a multi meter with knowledge and skill, we are working in the
blind.  We really don't know what is happening in our power systems.


WHO NEEDS A WATT METER?

While the watt meter is a nice to have, some people don't need one.  If you
are buying RTF planes, or ARF or kit planes and are using the manufacturer's
supplied motor and battery packs, I would say you can be pretty confident
that all is well.

However, if you start mixing and matching motors, gear boxes, props,
controllers, battery packs and the like, you are really working in the blind
if you are not measuring the energy flow in the system.  In my case, I
started making my own battery packs but
Steve Hargreaves - Moderator29/03/2010 17:17:00
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Amen to all that  !!!!
Tim Kearsley30/03/2010 07:58:11
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505 forum posts
The only thing I'd add is that something like the Eagle Tree data logger is even better than a Wattmeter.  I've had one for a while now, and I always make the first few flights with a new model with it installed, so that I can get a profile of how the power train is behaving for the duration of a flight, at different throttle settings etc.  It's a bit like the difference between a still camera and a video camera - the Wattmeter is great for snapshots of what's going on, but a data logger gives you a picture over time.
 
Tim.

Ed Anderson30/03/2010 11:25:06
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234 forum posts
Tim,
 
I have been told that the motor draw is less in the air than on the bench due to the unloading of the prop.  Have you ever compared bench data with air data?  If so, what can you tell us?
 
 
David Ashby - RCME30/03/2010 12:55:35
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I use the Jeti Duplex 2.4GHz telemetry feature to meaure real-time flying current draw.  For instance the Multiplex Mentor drew about 30 amps on the ground and 26 amps in the air - although it was interesting to see how that did vary depending on how the model was flying - it draws more current in the climb.
 
My foamy EDF jets seem to draw about the same in the air as on the ground. We were talking about it recently here.

Edited By Bruce Richards - Moderator on 30/03/2010 13:39:11

Tim Mackey30/03/2010 14:49:43
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The various prop driven models I have tested show a difference of anywhere between 15 -30% lower current in the air. It varies due to prop size, model size and power, and flying conditions.
These days, if I need to do a rough and ready guesstimate, I allow 20% drop from static.
Ed Anderson30/03/2010 14:52:27
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234 forum posts
Interesting that the EDFs vary less than the prop planes.
 
 
If you were going to set-up a model for maximum performance in the air, would you set-up for 20% over wattage on the bench with the expectation that you will be OK in the air based on the difference you have seen from bench to air?

Edited By Ed Anderson on 30/03/2010 14:57:32

David Ashby - RCME30/03/2010 15:07:05
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Posted by Ed Anderson on 30/03/2010 14:52:27:
Interesting that the EDFs vary less than the prop planes.
 
 
 

 Many speculated that EDFs drew more in the air than on the ground but I haven't seen that yet. Can only speak for the models I've recently been playing with - FMS F-16 with 70mm fan and the Top Gun A-10.

Tim Mackey30/03/2010 15:44:17
20523 forum posts
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Posted by Ed Anderson on 30/03/2010 14:52:27:
Interesting that the EDFs vary less than the prop planes.
 
 
If you were going to set-up a model for maximum performance in the air, would you set-up for 20% over wattage on the bench with the expectation that you will be OK in the air based on the difference you have seen from bench to air?

Edited By Ed Anderson on 30/03/2010 14:57:32

 
Probably not, as the only ones where I am likely to really want to wring out the very last drop of juice are EDfs, and as stated, they vary little.  With "normal" prop models, I prefer to stay fairly well within specs, so like the added "safety" of knowing that things unload in flight.....as long as its not ordnance!

Edited By Timbo - Administrator on 30/03/2010 15:44:46

Tim Mackey30/03/2010 15:48:37
20523 forum posts
270 photos
15 articles
Posted by David Ashby - RCME Administrator on 30/03/2010 15:07:05:
Posted by Ed Anderson on 30/03/2010 14:52:27:
Interesting that the EDFs vary less than the prop planes.
 
 
 

 Many speculated that EDFs drew more in the air than on the ground but I haven't seen that yet. Can only speak for the models I've recently been playing with - FMS F-16 with 70mm fan and the Top Gun A-10.

 I have only "eagle treed" three edfs, and in two cases the difference was minimal ( very slight unloading ) and the third did show a very slight increse....due I suspect to the rather difficult conditions on the day.
I guess a lot of it has to do with eflux tube design, FSA, and intake area, and so on. Altering the length and bore of the eflux affects static thrust versus speed ( bit like prop diameter versus pitch ) and in flight of course, air is being forcibly fed to the fan, rather than static where its merely sucked in .
Myron Beaumont30/03/2010 21:00:55
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Timbo
Just thinking about your last remark ! The speed of the A/C is due to the thrust of the motor (EDF) to achieve it Now then the talk of an unloading in flight is surely due to "throttling back"so to speak for a while 'til you need to increase speed again after drag has taken over for maybe 10 seconds or so & slowed you down a bit ! This effect with an EDF is probably less noticeable than with a prop because a prop will almost certainly cause a greater drag factor if slowed slightly (greater frontal area effect ?).In fact it is (as you know) used to slow an A/C in a dive  As I say --Just a thought
Dusty30/03/2010 21:16:52
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A EDF rotor can become a SLUG when stopped and cause a lot of drag!
Chris Channon20/06/2010 17:23:54
478 forum posts
Hi Ed, just found this thread, what a brilliant opening post. As i am somewhat a bit slow on the uptake as regards anything electric, i do like playing with motors etc, i will be buying a watt-meter tomorrow.
 
Brilliant post,
Regards
 
Chris.

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