ElectroFlightRC - Basics of Electric Flight

There are several things you can measure to evaluate the performance of a power configuration and its suitability to your application. On the ElectroFlightRC.com website, we track the following metrics:

Watts:

This is the peak watts generated at full throttle.

Amps:

This is the peak Amps pulled at full throttle.

mAh drawn:

Total mAh drawn from the battery during the flight. This can be measured as the total mAh put back into the battery by the charger or it can be measured by a device on the plane as the mAh are being drawn.

Minimum Voltage (during load):

This is the minimum voltage that was seen on the battery pack during the flight or testing.

All Up Weight (AUW):

This is the total weight of the plane including the batteries.

RPM:

This is the RPM of the propeller at full throttle.

Thrust:

This is the thrust generated by the power setup.

Flight Time:

This is the amount of time the plane can be flow (in various flight styles) before the battery is depleted to a safe level (i.e. 80% rule).

Top Speed:

This is the fastest speed that the plane can go at level flight and at full throttle.

Battery Configuration:

This is the battery configuration used in the power system. For example 12s2p @ 10,000 mAh. We also specify the individual batteries. So for the previous example, it may be 4 x 6s1p 5000mAh or it could be 6 x 3s1p 5000mAh.

Not all these metrics are necessary to evaluate the suitability of the power system, but some are more important than others. Most important are Watts, Amps and Minimum Voltage. AUW is also important once you have the plane all together and with the battery(ies) installed. Those values will tell you if your motor, ESC and batteries will run within their capabilities.

Combine the Watts with the AUW and you will find out if the plane will have enough power to perform as desired. For example, if you want to fly 3D, you need 150 to 200 watts per lb. Simply divide the peak watts by the AUW of the plane and if the result is between 150 to 200, then you know you are on target. If it isn’t in the range, all you may have to do is try different props. See Lawrie Henrickson’s article Specification of an Electric Power System Rev 2.pdf for more information on the subject.

Minimum Voltage will tell you if your batteries are being stressed too much.

Some things to remember before starting your testing

It is important to evaluate the suitability of a finalized power setup before you maiden a plane or any time after you change any component in the power configuration.

You will be doing this on the ground, but on the ElectroFlightRC website, we have also made an allowance to enter separate in-flight figures (where it makes sense - like for Watts).

Remember that you will not be going full throttle during the full flight. When performing your ground tests, you should test full throttle, but for limited time - no more than a few seconds or you may risk overloading or overheating the components. Also, when performing ground tests, remember that the static nature of the ground test will cause the readings to be higher for watts and Amps, and to be lower for RPM - compared to in-flight readings. Consequently the mAh will be drawn a bit faster as well.

Another difference with doing a static test is that there is less air circulating over the components and maybe even no air over some of the components - like the battery or ESC. So be mindful of the lack of cooling when doing your testing.

Safety concerns

Be prepared. Wear safety glasses. Make sure that everything is connected properly and that the motor / propeller are safely mounted to a sturdy surface. Do not try to run the motor while it is in your hands. Do not stand in front of the propeller or in the line of rotation while running the RPM’s up. If the prop is not tightened properly, it could come off. Or the prop could break if struck by something and the debris could go flying along the axis of rotation - or behind in the prop wash. Have everyone stand back and in a safe place. Do not rush. That’s when you make mistakes. Trust me. Take the time to think everything through.

These “toys” must be taken seriously. They can be dangerous. I don’t want to scare you with the following links, but hopefully they will instill a sense of respect for these machines. Not for those with a weak stomach:

Quadrocopter accident

EDF Testing Accident

I told you it wasn’t for the weak stomach. Anyway, you get the point. Be careful and be safe!!!

Measuring performance

So how do we measure all these different things? Lets go though it one at a time:

Watts and Amps:

Ok this is not one at a time, but these two can be measured with one device.

To measure watts and amps, you need an amp meter. I use one called Watts Up - pictured below.

The device pictured above does not have connectors, but you will install the connectors that you use. This will make it easy to connect the device when doing your testing.

All you have to do is to simply connect the device between your battery and your ESC. The screen will start displaying results. Make sure your plane is well secured so it won't move, then start applying some throttle. As you increase throttle, look at the Amp and Watt reading. Make sure you don't pull more Amps than what your ESC and motor can handle. Keep increasing the throttle until you reach full throttle. Make a note of the Watts and Amps at half throttle and at full throttle. That's it.

mAh

As you recall, the mAh (or milli Amp Hours) represent the fuel gauge - so to speak. A battery contains so many mAh's and when you fly your plane, you consume a portion of the mAh's from the battery.

To measure the mAh's drawn from your batteries, you can use the Watts Up device but it is only useful for ground testing because it does not have any logging features. Plus, it's a little on the heavy/bulky side for smaller planes. It wouldn't make any sense really to use that device for this purpose during a ground test unless you intend to test your system for the full length of a flight. This could pose some problems as discussed before because of poor cooling of components. Plus, the Amp draw is going to be higher during a ground/static test (versus in-flight) which would yield inacurate data anyway.

So, the most practical value to measure is the mAh draw during flight. How do we do this? Well, I use a live telemetry device from Quanum as described in the Useful Tools Section. This device will also display current Amp draw, Watts and battery voltage. As it does not have any logging capabilities, you will have to have someone working with you while you fly to record those figures. The mAh measurement is cumulative so you can get that at the end of the flight.

There are other devices that can also be used to measure mAh during flight. Do some research and you will find a fairly wide range of devices. One that comes to mind is from Eagle Tree.

Another method worth mentioning is ESC's with logging capabilities. The Castle Creations Phoenix ICE series is a good example.

Minimum Voltage

As with mAh, it makes more sense to record Minimum Voltage while in flight. See the sub-section on measuring mAh drawn. You will use the same device to measure Minimum Voltage.

All Up Weight

This is a fairly simple one to measure. You can use your regular scale if your plane fits on it and/or you can balance it without affecting the weight too much. Some scales have a minimum weight before they register a measurement so your plane would have to meet that minimum requirement if that's the case with your scale.

I bought a hand-held scale used for weighing baggage. They are inexpensive and have a range of a few ounces to 50+ lbs. I bought mine on e-bay, but many luggage stores carry them. I believe that HobbyKing carries one as well as other online hobby stores.

Here is a picture of a model similar to the one I have:

xScale

To weigh a tail-dragger plane, lift the tail up and balance the plane on its nose and hook the scale to the tail wheel. Lift and hold until you get a steady reading.

To weigh a plane with tricycle landing gear, you can do the same thing but hook the scale to the nose gear.

If the plane is too big or is of an odd configuration where hooking the scale to the landing gear is not practicle, you can use straps around the fuselage and/or wings. You can weigh the straps separately and substract that from the measured weight for a more accurate measurement. If your scale has a "Tare" function, you can zero the scale with the straps hooked on and then you can weigh the plane.

RPM

To measure RPM of the propeller during a ground test is pretty simple. There are hand-held infra-red tachometers that are pretty inexpensive. Here is an example:

Turnigy Multi-blade Tachometer

You don't even have to read the instructions for this one. There is a button that changes the setting between 2, 3 or 4 propeller readings. Set it to match your prop and then just press the other button and hold the device a few centimeters behind the propeller. That's it.

To measure the RPMs while in flight, you will need a data logging device or an ESC with data logging capabilities. The Castle Creations Phoenix ICE models have this feature. You can download the data after the flight.

Eagle Tree has two product lines that can log data from various sensors. For RPM, they have two or three different sensors. One is infra-red like the example above, and has to be mounted on your plane to get a clear reading during flight. The second has to be connected to the ESC wires to get a reading.

Thrust

The only way to measure thrust is on the ground. I simply use the same scale I use to measure the AUW. I attache it to the tail of my plane with straps or just hook it to the tail wheel and let the plane pull on the scale while I push the throttle forward. All you have to do is make sure the plane can roll freely without too much resistance. Exercise caution when doing this test. Make sure your scale is securely attached to the plane. You wouldn't want it to get loose. In any event, be ready to pull the throttle back if anything happens. Keep everyone a safe distance away from the prop.

Flight Time

Simple. Just start a timer when you take off and stop it when you land.

Top Speed

There are three ways to measure the top speed of your airplane but first let me just make a point of describing ground speed versus air speed.

Ground speed is the speed at which the plane moves relative to the ground. If a plane is moving with the wind, it will move faster in reference to the ground than if it was moving against the wind (at a constant airspeed).

Air speed is the speed at which the plane moves relative to the air mass it is in. This is the true speed of the plane.

Now, here is a description of the three methods for measuring the top speed of your plane:

Radar Gun

You can use a Radar Gun to measure the speed of an RC plane. This method measures the ground speed of the plane. In order to get as accurate a reading as possible, you need to do two passes. The first pass with the wind and the second against the wind. Add the two speeds together and divide by 2. That will give you the average speed of the two runs. Try to make the two passes as close together as possible so that the wind conditions are as similar as possible for each run.

There are many radar guns out there that will do the job. The price can range from less than $100 to ... well the sky is the limit. Some work better than others at picking up smaller planes, especially if it's at a longer distance. Bushnell has an inexpensive model that seems to work well - according to what I've read. Do your research and see what you can find.

Pitot Tube

I've used a stand-alone device made by EagleTree that works very well. Here is a picture of the product and a second picture of the included pitot tube:

This device measures the actual air speed and is capable of measuring speeds from 2MPH to 350MPH with a resolution of about 1MPH. It is very simple to install and operate. Another nice feature of this device is that it can work completely stand alone or it can work with the EagleTree telemetry data logger.

GPS

There are data logging devices out there that use a GPS chip to measure speed. I have used one of these devices and it seems to work pretty well. It is completely stand-alone and is very simple to install and operate. I purchased the device from velocity-rc.com. Here is a picture of the device:

This device measures ground speed but unlike with a radar gun, you can't measure a separate upwind and downwind pass unless you land your plane and reset the device. This is because, the device only records the top speed since it was turned on. So it's still possible to do an average of two passes, but you do have to land in between. You may not care about doing an average, and that is up to your personal preference anyway.

Actually, another nice feature of this device is that it also displays the highest altitude and the total distance travelled.

Doppler

I haven't tried this yet, but there exists software that uses the doppler effect to measure the speed of RC airplanes. The one I've been seeing more and more of is from RCSpeedo. They have a version for iPhone, Android and Windows. This app measures the ground speed of the airplane, but because you can get instant readings by using the iPhone or Android app, you can do two passes (with and against the wind) and calculate the average. Take a look at the website, it describes exactly how the app works and how to get correct measurements.

Power Setup Data Collection Sheet

For your convenience, we have prepared a data collection sheet which you can print and bring with you to the field. You can use this sheet to conveniently write down all the performance measurements for later entry into the ElectroFlightRC website.

Click here to download the PDF.