Electric-powered Remote-Controlled Model Airplanes

The Hangar Photos and discussions of all the airplanes, including how some of them were constructed, setup, and tuned.

Batteries Battery experiments I've done and general info on batteries used in electric airplanes

Simulators Links and discussion of remote control flight simulators

CDROM motors Experiments with CDROM motors

Introduction

I built and flew small gas-powered control-line airplanes when I was young. I was somewhat interested in some sort of flying models, particularly to teach the grandkids about models, but was put off by safety issues and the mess of the fuel. I happened to drop into a hobby shop and was surprised to discover the existence of electirc-powered (small motors and batteries) model airplanes. That was intriguing!

It turns out that electric-powered model airplanes are becoming quite popular and that newer batteries and motors are available that yield flying times of anywhere from 7 to 20 minutes, depending on the plane, motor, and other factors. There is a challenge, however; batteries weigh a lot, relative to the weight of the plane, so building a plane that will fly easily and for a long time is more difficult than when flying with gas-powered engines. Another advantage of small electric planes is that they can often be flown at a local ball park instead of having to go to a flying club field.

I'm now learning about many of the factors that influence electric-powered model airplanes. Factors like the weight of materials, gluing techniques, aerodynamics, balancing airplanes for flight, why a plane won't fly, etc. There are several critical factors that make an electric airplane fly well:

• Weight - keep it light by using light-weight materials. Air weighs less than balsa and balsa weighs less than plastic and glue. Use a light-weight covering if you're building with balsa. RAMicrolight covering is like Monokote, but weighs a lot less (.035 oz/sq ft for Microlight vs .25 oz/sq ft for Monokote). It is also not as tough. There are intermediate weight coverings such as Econokote or Oracover or Coverite (about .22 oz / sq ft). Use the correct type of balsa in various parts of the model to botain the best weight-to-strength ratio. A tutorial on balsa and how to identify the three grain types can be found at RCModelers.com, courtesy of SIG. You'll need a good set of scales to measure the weight of materials you use (Scales I use).
• Motor - select the proper motor. The less expensive 'can' type motors are inexpensive, but inefficient. A coreless or brushless or cobalt motor will be more efficient and deliver more battery power to the propeller. The price difference is impressive. A 'can' motor often costs less than $20 for 65% efficiency while the coreless, brushless or cobalt motors are often $100-$300 for 80-90% efficiency, depending on the size of the motor. Another option is a coreless motor. It has no metal core and therefore weighs much less than other motors. A coreless motor may have efficiencies of 80-90%.
• Batteries - select the proper type and capacity battery for the motor. I've enjoyed experimenting with batteries for my electric airplanes (see battery experiment results). NiCd batteries are best for high-current draw (big motors). NiMh batteries are lighter, but have lower current draw capacity. Lithium Ion batteries store more power, but have the lowest current draw capacity. If we define 'C' as the capacity of the battery, then NiCd batteries can discharge at 10 - 20 times C. NiMh batteries work best at about 5 times C and LiIon batteries work up to about 2 times C. For a 700mAh battery, this translates into discharge rates of 7-14 amps for NiCd, 3.5 amps for NiMh, and 1.4 amps for LiIon. I've also developed basic battery info on several types of batteries as they pertain to the electric flight hobby. I've included with links to the Panasonic web site where you can find details right from the manufacturer. Today (late 2007), battery vendors are now producing Lithium Polymer batteries that replace the metal can container with light plastic. The current delivery capability has also been increased from about 2C to 20C or more. Competitive airplanes are now being powered by electric motors and are winning major competitions.
• Propeller - higher RPM direct-drive motors drive smaller propellers efficiently while geared motors drive larger diameter propellers better. A larger diameter propeller at lower RPM is more efficient than a smaller propeller at higher RPM. However, the application determines the propeller diameter. The propeller pitch also needs to be correct. One can trade pitch for diameter, over a small range. A 10x8 propeller will be nearly equivalent to an 11x4.7 propeller. The 11x4.7 will be more efficient and provide more power while the 10x8 will provide a higher top speed. The propeller also needs to match the motor's performance characteristics. Too large a propeller (diameter or pitch or both) will load the motor more and draw more current with not much performance in return. Too small a propeller will not provide optimum performance.
• Drag - eliminate obvious sources of drag. Round the leading and trailing edges of the wings and tail surfaces. Eliminate rough spots and lumps, particularly on the lifting surfaces.
  

As an example of the tradeoffs, the Tiger Moth uses an inexpensive 'can' type motor geared at 5.9:1 that draws about 1.5 amps at full speed when driven by a 7-cell (8.4-volt) battery. The propeller provided with the Tiger Moth is a 9x7, but I've found that a 10x4.7 provides better performance. I haven't noticed that the speed is slower with the 10x4.7 prop, but I have noticed that there seems to be more power for takeoffs and loops. The 650mAh and 720mAh battery packs I use provide me about 20 minutes of flying time each, as expected (figure 720mAh / 1.5A = .48 hours = 28 minutes). The actual flying time will be less due to losses in the speed controller and inefficiencies in the discharge of the battery pack. I've flown for 25 minutes on this plane/motor/prop/battery combination by running at about 1/2 throttle. I've tried an 11x4.7 and found the performance to be less. The thrust measurements I've made show the same difference in thrust with this motor and various propeller combinations - the 10x4.7 provides greater thrust.

Safety Note: one shouldn't fly R/C planes within 3 miles of an existing field due to transmitter interference. Also be aware of other people and pets around a ball park - even small planes can injure someone, though much less seriously than a heavy gas-powered plane. Join the Academy of Model Aeronautics to obtain liability insurance coverage and to learn the rules of flying.

Another Safety Note: Spinning propellers and flying models are dangerous, even on small models. Propellers have sharp edges and are spinning at speeds of 1,000 to 13,000 RPM. Even on electric planes, a spinning propeller will cut fingers. Being hit by a small model that weighs the same as a football will feel much like being hit by a football travelling at that same speed. If you fly models at a local recreational park, make sure any spectators are kept away from your flying area. Do not fly over spectators, even though you and they often enjoy it. And watch out for dogs that like to chase flying objects. I find that the fenced ball fields are the second best place to fly if I don't go to the flying club's field.

Helicopter Humor

The flight of helicopters is sometimes humorously described as "beating the air into submission". A quote from Harry Reasoner (a well-known TV news reporter in the 1970s) pretty much says it all:

Helicopter Pilots are Different

"The thing is, helicopters are different from planes. An airplane by it's nature wants to fly, and if not interfered with too strongly by unusual events or by a deliberately incompetent pilot, it will fly. A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other, and if there is any disturbance in this delicate balance the helicopter stops flying; immediately and disastrously. There is no such thing as a gliding helicopter.

This is why being a helicopter pilot is so different from being an airplane pilot, and why in generality, airplane pilots are open, clear-eyed, buoyant extroverts and helicopter pilots are brooding introspective anticipators of trouble. They know if something bad has not happened it is about to."

Other humorous helicopter stories and comics, including Harry's quote, can be found at www.jollygreen.org.

Here is a video of a helicopter that is flown through a forest with the same style that you might see an RC flyer. The only difference is that the heli flyer here isn't likely to walk away from a crash or blade strike.
Heli in forest <

How they inspect high voltage lines from helicopters. Just like birds sitting on high voltage lines, these guys equalize voltage between the heli and the lines, then climb onto the lines to do their inspection and/or repairs. Just don't touch anything else!
Cable Inspector

Other flying links

Links to interesting videos, pictures, and other oddities of airplanes and flying.

• RC airplanes are pretty incredible in the right hands. Here are a few examples.
  Scale models doing aerobatics This is a site with a number of aerobatic airplanes showing in flight.
  Indoor F3P-AM qualification video In this one the pilot is flying a routing for a competition, coreographed to music. It is flown in a basketball gym. It is my favorite to show what a great pilot can do.
  Extra330    Vertigo XL These two are also an indoor flying routine, but the space is much larger than the gym in the previous video.
  Indoor combat RC combat is wehre the planes typically trail a streamer. The winner collects the most streamers from the other planes.
• Videos of 'human gliders' where the individuals wear suits that emulate a flying squirrel. They land by deploying parachutes.
  Biertijd
  Helicopter jumping & gliding
• JetMan - mounting small turbine engines on smallwings that are worn like a suit. This fellow can gain altitude. Landing is just like the jumpers above - by parachute.
  JetMan
• This lo-res 8.5MB (6min 45sec) video looks through the HUD (Heads Up Display) on the flight deck. You hear the flight deck audio of the astronauts interacting with ground control.
  The video starts at about FL830 (83,000 ft),
  260 Kts, Mach 2.5 velocity and 1.1 Gs.
  You'll hear the term HAC (Heading Alignment Cylinder) used a lot.
  It is the shuttle's version of a traffic pattern to line up with the runway centerline. It is a circular instead of rectangular pattern.
  Atlantis Landing
• Head-mounted viewer that controls a gymballed camera on an RC electric airplane. This fellow has a number of videos on google and YouTube.
  Gymballed Camera & Head mounted display
• DIY Drones (also known as Aerial Unmanned Vehicles or AUVs).
  diydrones.com