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Measure Your Electric’s Power

E-power basics to get more from your models

How To Measure Your Electric's Power

Using the Astro Flight Meters really helps sort through the flexible power combinations electric flight affords.

Here are many variables that can be adjusted when configuring an electric power system, and the choices are growing given the avalanche of new motors and batteries coming to market. One tool that will help any modeler sort out these choices is the Astro Flight Whattmeter.

When used in conjunction with a tachometer, the Whattmeter can reveal the relative efficiencies of different motors when compared with the same propeller at the same rpm (if one motor consumes more power at a given rpm, it is not as efficient as another that consumes less power). You can also test different brands of propellers with the same nominal pitch and diameter to see if more power is consumed by one than another at a given rpm. And you can test larger or smaller props to see which produce the level of current your motor was designed to handle.

One of the most interesting statistics is “power loading,” defined here as the “watts per pound” consumed by the model in a full throttle, static bench test. The power loadings of models reviewed in this magazine have ranged from 22W/lb. to as high as 200W/lb. Gentle flying park flyers tend to be at the lower end of this scale, and aerobatic 3D aircraft tend to cluster toward the higher end.

A way to easily measure and compare power parameters of different aircraft and system components has long been very desirable. Electric flight pioneer Bob Boucher of Astro Flight faced these same issues decades ago, and, lucky for the rest of us, did something about it. Read on for tips on using the Astro Whattmeter and its sibling, the Micro Meter.

COOL TOOLS

Astro Flight offers two multi function digital meters to help us set up and monitor our electric models. These allow you to see the volts, watts and amps measured by the meter during a test of an electric motor. They also report the milliamp-hours that have flowed through the meter during any given test.


Left: With the Whattmeter hooked up, but throttle off, the meter shows the unloaded voltage of an 8-cell NiMH motor battery. Right: Once throttle is applied, the battery voltage will sag under load. 144 watts tells me that this less than two pound model will have great performance.

GETTING THE NUMBERS

The first step to measure the power is to make sure everything is plugged in and functioning (and if you are at the flying field, that you have the proper frequency pin). Turn on your transmitter, then the model. Make sure that the throttle responds to the transmitter, then return the throttle to the full off position. Now that you know the system runs, and that it is set to off for safety, you can hook up the meter.

Disconnect the motor battery and plug it into the source leads of the meter (the right side of the meter is labeled source). At this point the meter will display the voltage of your pack, but no current. Now make sure the model is secured and nothing is near the prop, then plug the load leads into the motors speed control.

Slowly advance the throttle stick to full. Notice first that the amps will climb from zero. Because of the load, the voltage will be slightly lower than with the throttle off. Observe the performance of the plane and write down the amps, volts and watts at 15 seconds and 30 seconds, then pull the throttle back to zero, turn off the airplane radio and then the transmitter. We recommend looking at the power loading 30 seconds into the static run to get past the initial power spike. The power loading at 30 seconds, for any battery type, is more representative of average input power levels than the performance when you first throttle up.

HOW WILL IT FLY?

You can get a good idea of in-flight performance from the numbers. While results can be influenced by other factors such as wing loading and general efficiency of the airframe, watts per pound (W/lb.) is a common and worthwhile number used for predicting and comparing performance. We also use watts per ounce here at Fly RC, since a pound is a coarse unit for models that might only weigh two or three ounces. Take the watts reading displayed at 30 seconds and divide it by the weight of the model in ounces. This gives you the power loading in watts per ounce. Multiply that by 16 to get watts per pound.

The generally accepted power level for basic sport flying with some mild aerobatics is around 50 W/lb. (3.1 W/oz.). More sedate flyers with low wing loadings can fly well with as little as 30 W/lb. (1.9 W/oz.), while aerobatic models or warbirds often have power loadings of 75 W/lb. (4.7 W/oz.) or more. Models set up for hovering and 3D maneuvering want all the power you can get. 100-150 W/lb. (6.3-9.4 W/oz.) are typical numbers for these models.

HOW TO PREDICT DURATION

To predict flight duration, start with the current draw in amps at full power and the battery capacity in amp hours (Ah). Lets assume a 10 amp draw and a 700mAh battery. A 700mAh battery has .7Ah of capacity.


 

 

The easiest way to juggle the numbers is to convert the amp-hours to amp-minutes. Multiply .7Ah X 60 to get 42 amp-minutes. A 10-amp draw (max power) will result in a duration of 4.2 minutes (42 amp-minutes/10 amps). A little over four minutes isnt that long, but most models arent flown at full throttle from takeoff to touchdown. If you know you can fly your model at half throttle, you can expect it will stay aloft for about eight minutes at that throttle setting. If this isnt going to cut it, youll want a battery with a larger capacity.

The Astro Model 100 Micro Meter and 101 Super Whattmeter (both retail for $59.95) look very similar; the main difference is their power measuring and reporting capability. The 100 Micro Meter is designed to monitor power systems with currents up to 15 amps, typical of smaller models like park flyers. The 101 Super Whattmeter will handle up to 75 amps for larger models. The Micro Meter measures in smaller increments (e.g., 1/100 volt, compared to the Whattmeter’s 1/10 volt).


News Flash!

Astro Meter

At this years Weak Signals show in Toledo, Astro Flight introduced an improved version of their compact servo tester that should prove as indispensable as their meters. In the context of this article, you can use the servo tester to operate an electronic speed control while setting up and testing power systems. Just plug your ESC into the servo tester and hook up the motor and battery. Speed controllers with a BEC circuit will power the tester. For larger ESCs without a BEC, connect a small receiver pack to the tester for power. The servo tester is much more convenient in use than a transmitter and receiver. At the field it is also much safer, since you dont need to worry about getting the pin to clear your frequency before turning on your transmitter. Naturally, you can use the tester for comparing, setting up and installing servos as well. The Astro Flight Servo Tester now features a 500mA test load to help ensure that your speed control or reciever batteries are functioning properly.
Price$19.95.

If you want to experiment, the sky is the limit. You can change the prop, add or remove cells in the battery pack, or even change motors. When I set up the Clancy Aviation Stagger Bee shown in the opening photo, I tried out a few dozen motor/prop/ battery combinations. By using my Astro Super Whattmeter, I was quickly able to get an idea of how the model would perform with various combinations. The same testing could have taken days if I had tried each combination by flying. Sure enough, the model responded as the numbers predicted. For reference, the Stagger Bee flew quite well at 55-60 W/lb., but it is a lot more fun at 74!

CONCLUSION

With an Astro Flight meter in your flight box, you will have a good idea of the performance and duration you can expect from a new model. You will also be able to compare your own numbers with those reported in electric reviews in Fly RC. These meters will better help you understand your models, giving you more quality flying time with airplanes that perform the way you want them to.

 

These numbers really start to become interesting as you change parameters and compare different systems. We have seen some 7.4V 2S Li-Poly batteries deliver higher wattage to the motor than some 7-cell 8.4V NiMH batteries, when, under power, both batteries exhibit comparable voltage. This may suggest lower internal resistance in the Li-Poly batteries. Weve seen brushless motors that increase climbing power compared to a brushed motor, and that also increase duration when flown at lower throttle levels.

You can also derive actual in-flight power consumption from observed performance. With the above example, lets say you actually average fifteen-minute flights before the BEC circuit on your speed control cuts motor power. If you divide 42 amp-minutes by 15 minutes, you get an average in-flight current draw of just 2.8 amps.

 

 

Links

 

Astro Flight Inc

 

www.astroflight.com, (310) 821-6242

 

Clancy Aviation, distributed by Global Hobby

 

http://clancyaviation.globalhobby.com (714) 963-0329