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ELECTRIC POWER SYSTEMS FOR AIRCRAFT

ELECTRIC AIRCRAFT POWER SYSTEM

At first, I thought electric power for an aircraft was a novelty. Especially because I created a few Light Sport Aircraft (LSA) that liked to use a lot of power to go fast (SportCruiser & Parrot) or take off from the water (Mermaid). It was clear to me that the electric power technology had not yet evolved to be viable for LSA aircraft. And IMHO it still has not.

However along came the Zigolo which requires a great deal less power to fly and even zero power once self-launched to soar. And if you have read any of my blogs I am not too keen on 2-stroke engine power. So I tried an off-the-shelf electrical system and sourced Kokam battery cells and wired it all up and flew to the Sun n Fun airshow in 2014. And back. It worked. Not only did I prove (at least to myself) that the Zigolo works well on electric power I came away with a much different perspective and attitude.

Electric power on a very light aircraft or ultralight is not only viable it offer by far the best flight experience.

Maybe my attitude is tainted by all the forced landings I experienced flying ultralights with snowmobile engines in the early 1980s. Or maybe I just value comfort, safety, and quiet more now that I am older and turning gray. I just know that in the past and even still today the first thing I do when flying an aircraft powered with a 2-stroke engine is climb for altitude and look for a place to land. Old habits die hard I guess. But it is exactly what John Moody still does. And it gets me out of shotgun range. I don’t want to be mistaken for a UAV!

Aircraft make noise for a few seconds and then, pass by. Ultralights make noise but don’t go anywhere and can be mistaken for a big mosquito irritating someone’s BBQ.

Electric aircraft motors are very reliable. It’s comfortable to fly low and slow. And the motors are nearly silent. However most of the electric motors on the market are converted from some other application like motorcycles or scooters, or scaled up RC motors. All of these have high RPMs which translates to high prop noise. No point to having a quiet motor if the prop noise remains the same.

Electric Powered Flight: Zigolo

I tried electric powered flight with the Zigolo last year and discovered that:

  • It is really great to fly on smooth electric power. And I feel very comfortable flying low and slow.
  • Sure it is ‘clean power’ but to me that means no oil stains and gas to mix and clean up than it does saving the environment.
  • Electric power works very well but for very light and low-energy aircraft.
  • Motors offered on the market are not optimized for aircraft use. Primarily they spin too fast which results in just as much prop noise as with gas power. So one of the main benefits of quiet flight is not there.
  • The best motors like out-runners are too expensive. The cheaper motors like pancake or in-runner motors are not efficient enough.
  • The best batteries are sold in pouches. This means they need specialized containment and configuration and protection. This must be done correctly and it is not that simple. Ask Boeing.
  • All systems need complete integration. Everything required for a system must be designed and incorporated together. It is not a good idea to try to piece a system together. It is not so easy to pull over to the side of road and get out when the battery pack torches off. SAFETY IS A PRIMARY CONSIDERATION!

These conclusions resulted in our creating our own system. Design criteria was basically high torque with low RPM. Things like weight and efficiency were secondary considerations. We know that even a heavy electric motor is a fraction of the weight of a gas motor. And all are generally efficient depending on what is measured. In reality the prop efficiency is a bigger criteria.

The result is not an in-runner or an out-runner but an Axial Flux PMAC motor. What that means is very powerful rare-earth magnets are fixed in place and flat-wire rotors are energized with AC current. Brushless of course. One of the keys is the controller which first changes the current from DC to AC then sends the current in pulses timed perfectly to spin the rotors.

The bearings are mounted outside the ‘hot zone’ so there are basically no other moving parts inside the motor besides the rotors. And the rotors have small turbine blades milled into them with the front and back in opposite orientation. This means the motor does not rely on any outside airflow to cool. The rotors push air toward each other which then is forces up and around the coils resulting in excellent cooling regardless of the motor location. It can be tractor, pusher, and/or fully cowled. No cooling drag.

Plus the motor housing offers protection from rain.

Our motor weighs twice as much as the competition! Well, that is not something to brag about but it only weighs 25 lbs. And that weigh helps with the CG when replacing a gas motor. In fact it helps improve the looks of the Merlin PSA considerably as we can extend the cowl to enhance the lines and looks of the aircraft.

The extra weight is from bigger coils and more magnets. This means more power. Which translates into more torque at lower RPMs. And that is our objective. We should have a max RPM of under 2,000 and a cruise RPM of around 1,000. At these prop speeds the prop noise should meet our targets.

 

low noise is achievable
electric - noise graph

Let’s talk about noise

Above are some slides from the 2015 CAFÉ Foundation on Electric Aircraft:

This shows that theoretically quiet flight is possible. In fact so quiet that the pilot can speak to people on the ground and be heard and understood. That means no noise complaints! And a very comfortable environment for the pilot to truly enjoy nature and the experience of aviating.

Although prop speed is the main issue it is not the only issue. We need to absorb all that torque and turn it into thrust. The standard prop designed for 2,500 RPM will be too fine of pitch or stall out if the pitch is increased too far. The solution is to add more blades. And to increase the C/L of the prop blades themselves.

Our prop supplier has done a great deal of work on this subject. You can read about some of their research and technical data on this site:

http://www.e-props.fr/

ground-testE-Prop 4-blade offset carbon propeller mounted to our electric motor and mounted on the Zigolo. Note that the custom carbon spinner is now finished

An added benefit is these carbon blades are very light. And now we manufactured a custom spinner that matches the prop and hub perfectly:

For examples of the interest of major companies, universities, and scientists in electric aircraft technology see the links to these two electric aircraft symposiums I have been attending and presenting at:

cafefoundation.org  and museumofflight.org

The weak link in electric power for aircraft is the limits of the battery energy density. Yes, LiPo batteries have 3 times the energy per weight than the older NiCad batteries. You now find Lipo in nearly all portable electronics from cell phones to notebooks to tablets. And of course in all the electric RC model aircraft.

And you hear about serious battery failures occasionally as well. This is usually a non-event in a car or a phone or a RC aircraft. But must be avoided on an aircraft for rather obvious reasons. That is why we have gone away from using the pouch cell technology. Rather we have our cells made in lightweight cylinders and then encapsulate them in a weave of carbon and Kevlar with an epoxy matrix.

This protection virtually eliminates the chance for a thermal runaway due to gas expansion.

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Screen Shot 2015-07-09 at 12.03.53 PM | Jul 9This system provides a very strong protective structure that does not allow the internal gases to expand which is one of the symptoms of cell failure. By eliminating the oxygen and gas expansion we can minimize or eliminate the chances of any cell failure. We also have a unique mounting system that keeps the cells move than 5mm apart so any failure does not propagate between cells. This also allows for greater air circulation and cooling. Plus we can arrange the cells in infinite geometry which means we can fit the cell package into any space or spaces available on nearly any aircraft.

Our custom and proprietary battery cells are a composition of lithium, cobalt, nickle, and mangalese optimized for high energy, low weight, and reasonable cost. Our design goal is > 1 hour of endurance. This provides plenty of power for the average 42 minute personal flight plus a reserve.

For those who may want to fly cross country, well that is not so practical on electric power yet, my friend Bertrand Piccard excepted of course. However we are researching a light weight fast-charger that could be taken along on the flight for remote and reletively quick recharging.

And solar charging is possible of course. Just do not expect to gain much energy in one hour flight to make any difference regardless of what some of the competition is saying. The batteries like the slow charge from solar. It is perfect on the hanger roof for charging and maintenance of the cells between flights.

hybridComing next year: Hybrid power. This ‘range-extender’ will allow the pilot to depart quietly on electric power and fire up the gas motor during cruise which will provide a couple of hours of endurance plus a faster cruise speed so one can really get somewhere. And, when you land the battery pack is fully recharged. This means you can have the $100 hamburger for about $10 and still get home the same day.

This year we are simply too busy with the new aircraft, the new motors, the new electric power systems, and the new amphibious floats. Therefore, the hybrid will have to wait. However, we have sourced some key components such as the clutch.

Contact

Chip W. Erwin
South Lakeland Airport
Lakeland, FL
262-408-0124

info@aeromarine-lsa.com

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