Electric RC Airplanes – Things to Know Before Buying Your First Electric RC Plane
a guest post by Tony Murano
Flying electric airplanes have been around for quite some time in one form or another! I saw my first in the late ’80s in Tokyo.
No doubt electric RC airplanes have been responsible for the explosive growth and have helped rejuvenate & recapture the popularity of radio control flying.
Let’s face it – electrics are quiet – so we can remain friends with the neighbors.
With the introduction of lithium batteries and brushless motors along with advances in microelectronics, battery-powered RC airplanes growth will continue to grow.
Obviously, electric RC airplanes use different components than gas-powered RC airplanes.
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The power of RC electric flight is amazing, not to mention much easier to own and operate. With glow engines, there is the problem of maintaining them and making sure they are up to snuff.
Over my years as an RC pilot, I have found that beginners and advanced pilots both seem to like electric RC airplanes, so it is a great idea to buy one as your first airplane.
Simple electric RC airplanes are great because they are very easy to fly, and you really only have to master two controls. One control being the speed and the other being the direction you want to move in.
One of the best things about airplanes that are simple is the fact that they can be ready to fly within 15 minutes of opening the box! Basically, it is a no-hassle approach to flying RC airplanes, which is what most beginners like and need.
Electric RC trainer airplanes are also available for people who want to actually train with electric airplanes because that is all they intend to fly.
Electric RC Airplanes Components
Obviously electric RC airplanes use a few components that are different than used in gas-powered RC airplanes.
The following are the basic components that are found in nearly every electric model.
Electric RC airplanes operate on an electric motor and it is powered by a battery pack. Electric motors are nothing fancy and are cheaper than glow engines. So far my experience with electric motors has been great. I don’t think you will have any problems either, as long as you take care of your electric motor and try not to crash.
There are two types of motors used in RC airplanes – “brushed” & “brushless”
Offers simple low-cost power but they are fairly inefficient & the speed/torque remains moderately flat. It requires an electronic speed controller. Simple 2 wire connection. Swap the leads to change the spin direction.
High efficiency, higher speed range and a higher power output per weight. It requires an electronic speed controller. Use’s a 3 wire connection, if it spins the wrong direction, just swap 2 of the 3 wires, simple.
NiMh (nickel-metal hydride) batteries are used quite often, especially with basic or trainer airplanes.
Lithium Polymer or Lipo batteries use a different chemistry to function, this provides lower weight plus a great deal more power than NiMh batteries. Lipo’s also required and MUST be charged with a charger specifically designed for lithium batteries. See Li-po’s for more.
ESC ~ Electronic Speed Control Brushed or Brushless
ESC (electric speed controller) – think of this as an electronic version of the throttle that opens and closes the carburetor in a gas engine, it varies the power by changing the amount of fuel/air getting into the engine.
The ESC simply alters the amount of current that passes to the motor, thus changing the power output of the motor.
BEC ~ Battery Eliminator Circuit
Has two functions – it controls the power to the receiver and servos, it also monitors the charge level of the battery and when the batteries are close to being discharged it reduces or stops the power to the motor while still maintaining the current to the receiver and servos so you can land while still under control.
On most electric RC airplanes the BEC is already incorporated within the ESC but a few may not be, these will require a separate battery to power the radio system.
Technical Stuff – The transceivers are devices that have both a transmitter and a receiver which are combined and share common circuitry. You only need to build the interface circuitry to have a complete rf remote control system with the Ming transmitter & receiver modules. A relay is an electrical switch that opens and closes under the control of another electrical circuit.
What else will you need?
I think every RC airplane pilot should have a field box. They’re great for fixing some problems you might have out in the flying field. If you want to have a longer flying time with electric planes, you might want to think about buying an extra battery.
For even longer flying times, you could buy a car charger for your RC airplane battery. Something every beginner should consider buying is a good book or subscribing to an RC airplane magazine.
All of the above combinations of components have been proven successful although you’ll find that a plane with a brushless motor and a lithium battery tends to be the most efficient. This combination typically provides more power and longer flight times!
Why Choose Electric RC Planes?
Flying electric RC airplanes – Or RC park flyers is a fun, easy to learn to fly, thrilling high-tech sport and a great way of experiencing the challenge of flight – safely!
It’s also a fabulous stepping stone for boys and girls of any age who dream of becoming a future pilot. They can have fun and build confidence while learning aerodynamics and what makes a plane fly while refining their own flying skills.
Anyway, there are many ups and downs to electric RC planes. Let’s look at them now.
Electric RC Plane Pro’s
- Safer – Electrics are always remote started, no need to get your hands or fingers close to flip the prop with your fingers.
- Less expensive to build – Less expensive to fly.
- Requires a great deal less equipment to fly – basically a battery charger that you can connect to your car. No cans of fuel, fuel pump, starter battery, power panel, glow starter, engine starter, cleaning spray, paper towels.
- Fly year-round comfortably – electrics can be small enough to fly in gyms, backyards or local parks. Some micro electric RC airplanes can even fly in your own living room.
- Rechargeable batteries can be recharged hundreds of times. No cans of flammable fuels around the house.
- Many can fly very slowly, making it much easier to learn to fly.
Little if any noise to disturb those around. Good for keeping peace with the neighbors.
- Can be small and simple enough to fit in the trunk or backseat for a quick lunchtime flight.
- Possibly less of a carbon footprint compared to gas or nitro-fueled planes.
- Cleaner improved scale appearance- No cylinder and exhaust muffler ruining the outline of the plane.
- Properly set up – an electric RC airplane will often have as much and sometimes much more power than a gas or nitro system – with no tweaking of needle valves and tuning.
- Less noise
- Environmentally friendly: no smell – no oil residue – no smoke
- Easy to fly
- Easy to fix and replace parts
Electric RC Plane Cons
Waiting for a battery to charge – simple solution – get an extra battery!
- With the advances in this technology newer batteries called li-polys or li-pos can store and deliver an incredible amount of charge for their small size. They do require and you MUST have special equipment to charge and maintain these batteries.
- It is very important that you NEVER attempt to charge a li-poly with any charger unless it was made specifically for lithium polymer batteries. The good news is that with the right type of charger and a little care, you should expect to safely use your li-poly batteries for years to come.
- slow (some exceptions)
- short flight times
- not realistic looking or sounding
Basically, if you are looking for just some quick little flights every once in a while, then electric r/c airplanes are perfect for you. If you want to get into faster more advanced airplanes, then I suggest you check out gas and jet RC airplanes.
Understanding Electric Flight – A Beginner’s Guide To Remote Control Airplanes Electric Flight
If you read through the “Basics of Electric Flight” it may well have left you wanting more information on how to size engines for speed/thrust etc when designing and building your model plane. So, here is some more great stuff that helps people in the hobby have taught me, or I have figured out myself.
First some basics:
Pitch speed is the speed of the engine/prop combination through the air ignoring drag and prop slippage.
It is simply based on the multiplying the pitch of the propeller, by some number of revolutions for a unit of time.
For example, let’s say we have a 7×5 prop that we know is being driven at 10,000 rpm then the pitch speed of this model is 50,000 inches per minute (5 from prop pitch multiplied by 10,000 rpm). Once you have this number, conversion to more useful units (such as km/hr or mph) is just a matter of multiplying by the right constant value.
- To get km/hr, multiply by 0.001524
- To get mph, multiply by 0.000947
So for the example above, with the motor spinning at 10,000rpm, the pitch speed would be 76km/hr or 47mph.
How do you find the RPM? Many motor manufacturers will publish this information with their engine. Some don’t. If you don’t have published information you can often take an educated guess by looking at the recommended prop ranges for a motor, and assuming that the smallest published size will spin the motor at approximately 85% of kV * volts (so a 2000kv motor with 11.1v, you would expect approximately 19,000rpm. With the mid-range prop you would probably expect around 70% of kv*volts (so 15,500rpm from the example above), and with the largest recommended prop maybe 50-55% (so 12,000rpm from the example).
The best way to find the RPM if it isn’t published is to actually measure it with a tachometer.
Finally, if you have an inrunner which is reduced through a gearbox before spinning the prop you, of course, need to factor that into your calculations. For example, a 3000kv motor reduced through a 3:1 gearbox has an effective prop kv of 1000.
Thrust is a measure of the force generated by an engine/prop combination. Typically it is expressed either in ounces or grams (and those of you that know your SI units, of course, realize that grams are a mass, not a force and that this should be more properly described as grams at earth gravity).
Typically a wider diameter propellor will generate more thrust, to a point. Perhaps you have already noticed the tradeoff here. You generally get higher pitch speeds with a smaller prop, but you generally get more thrust with a larger prop.
If your engine doesn’t have thrust info provided with it, then you have to figure it out for yourself by measuring it. You’ll need some sort of rig that allows you to measure the force the engine generates at full throttle.
All things that move through a gaseous medium (ie air) must overcome air resistance. Aircraft are no exception, and they are further affected in this regard because their wings create additional drag.
The important things you need to know about air resistance is that its relationships to velocity is a squared one. That means that if you double your speed, your wind resistance increases by a factor of four. For example, if your wind resistance at 30km/hr is 100grams, then your wind resistance at 60km/hr will be 400grams, and at 120km/hr will be 1,600grams.
So How Fast Can a Model Go?
As a model accelerates the amount of drag increases as square of the velocity until either drag equals engine thrust, at which stage the plane will stop accelerating, or the plane reaches the pitch speed of the prop, at which stage it stops accelerating.
It’s a bit like gearing in a motor car. In first gear, you are “pitch speed” limited. You have plenty of power to overcome wind resistance, but eventually, you reach a point where the engine is at the red line and can’t (safely anyway) spin any faster. However, it top gear you tend to be drag limited – you accelerate to a point where wind resistance equals the acceleration the engine is providing and you have reached your top speed.
So, the plane’s top speed is governed by the lower of the point where thrust and drag intersect, or pitch speed. The graphs at the right will hopefully illustrate this – click on them for a blown up version.
Choosing the Right Motor/Prop for Your Model
Of course, it does depend on what your application is. If all you want is thrust for 3d work you are better off getting a lower KV motor and using a larger diameter prop – you are getting the most thrust you can, but typically paying for it with pitch speed (a bit like the 9×3.8 graph). Also, if you have a slow flying airframe you often need to pay for this in terms of drag as you try to go faster.
If you are after speed you have to make a compromise. Clearly, you want pitch speed, but you also need enough thrust to overcome air resistance. Normally you have to trade pitch speed against thrust. Sometimes it is easier to lower the air resistance of your model than increase the power (more on that later) if you want it to go faster.
So something I have seen come up on forums from time to time is something along the lines of “this guy showed up with *some fast plane* and blew away my *I thought it was fast plane*. I need to upgrade my power so that I can blow him away.”
Okay – time for some hard numbers. As mentioned above, drag increases as a square of velocity. So, to go twice as fast you need four times as much thrust (ignoring pitch speed for the moment).
Let’s say for example that you use 700grams of thrust to go 90km/hr, and decide that you need to catch a plane that can fly in level flight at 110km/hr. Take a guess at how many grams of thrust you need (ignoring pitch speed as a limiter for the moment). Got your guess ready? 1050grams. That’s right, 50% again to increase your speed by 20km/hr.
Unfortunately, Mr. Newton has more unpleasantness ready for us. Power is a measure of work over time. If we want to fly twice as fast we need four times as much work done (because we need four times the thrust from above) being done twice as quickly. Yes – you need 8 times the power to go twice as fast. So let’s say you were using 200watts to get 90km/hr, what would your power requirements be at 110km/hr? Got your guess ready… were you anywhere near 365watts?
Now – don’t despair. A power system upgrade can be a fine thing at times like this, but in many ways, you will probably get a much better return by reducing the drag coefficient of your model.
For example, if you reduce your drag coefficient by 20% you can fly a linear 20% faster for the same amount of thrust. So, in the example above you would have almost gotten to 110km/hr without putting in a single watt more of power.
Now, reducing the drag coefficient of your model (by reducing wing area, reducing the size of the airfoil etc) is not a free ride by any means. You will end up with a higher stall speed amongst other things making landings and takeoffs more difficult. Your model may well become unstable, you could change the center of gravity etc. In so many ways you could ruin a great plane – but you could go a bit faster too. So, if you want to make your stryker go faster, as well as upgrading the power, think about how you can reduce the drag coefficient.
What about Weight – Doesn’t It Affect Speed?
As best I can tell, weight does not affect straight-line speed in an aircraft. It affects acceleration, but not top speed. Obviously, if you are fighting gravity then mass does make a difference.
How Do I Tell Whether My Speed Is Pitch of Thurst Limited?
If a model is severely pitch limited it is usually pretty obvious – you push the model to full throttle, it jumps very quickly to a speed but then just stays there. The model has plenty of thrust, but just won’t go any faster.
If the model takes a little time to wind out to top speed (not ages, just some time), and can go significantly faster in a steep dive it is probably thrust limited.
If in a dive the model doesn’t pick up significantly more speed it might be thrust or pitch limited, but it is so close to pitching speed that it almost doesn’t matter (the model doesn’t pick up much speed because once the model passes its pitch speed in a diver the propellor starts acting as a brake).
So Is It Better To Be Thrust of Pitch Speed Limited?
It really does depend on the application. 3d planes are often pitch speed limited – they usually just want buckets of thrust for unlimited vertical. Most scale models fly best with thrust limited setups in my opinion.
Important Electric RC Airplanes Reminders
There are many types of electric RC planes you can choose from. There is your regular electric RC airplane, then there are park flyers and slow flyers and some RC jets that are powered electrically. Park and Slow flyers are great for some fun flights in your backyard or any local park. Electric r/c jets are very fast and fun to fly too. Follow those links to learn more about them.
Before a beginner decides to purchase an electric RC plane, you should keep these two things in mind:
- Wings should be high up on the fuselage
- Should be an electric trainer that is RTF or ARF
Having the wing high up on the fuselage creates more stability, and RTFs/ARFs are easy to put together and start flying.
Below is a list of equipment that every beginner should have to make their electric RC plane work properly.
An RTF and ARF kit usually includes everything needed with your purchase. Just go through the list to make sure you have it all.
- Airplane kit
- Electric motor and speed controller (maybe propeller too)
- Battery pack and appropriate charger
- Radio transmitter (AA or AAA batteries) and receiver
Just a reminder, always look for “not included”, and be sure to purchase what is not.