Fitting electric nosewheels to aircraft could reduce weight and improve efficiency – so where are they?
Aircraft ground manoeuvring wastes energy. With gas turbines running inefficiently at minimum power while taxiing, or while just waiting for a slot, a typical A320/737 will consume 600 kgs of kerosene every day of a short haul duty cycle. On top of that, conventional aircraft friction brakes impose large loads on gear and tyres when they hit the runway for every landing and instantly spin up from stationary to 1,000 RPM.
In 2007 an American engineer – Steven Sullivan – was awarded a US patent for an electric nosewheel which would cut both of these costs. His idea was for the wheel to be given an electric motor, to provide ground propulsion, pre-spin at landing, and electric braking after landing. Power would come from a fuel cell, (not from the gas turbine APUs currently fitted to all commercial aircraft). Mr Sullivan’s company, Delos Aerospace, was also named on the patent.
Nothing happened for several years, though in 2012 Germany’s aerospace testing centre did some work with fuel cell APUs. Then, in 2014, Airbus teamed up with South Africa’s National Aerospace Centre to work on fuel cell APUs and electric nosewheels.
Again, nothing much happened. Enquiries by Electrans suggest that a research student has only just been selected by the NAC and South Africa’s HydrogenSA to carry out research work on APU fuel cells stacks.
Meanwhile Delos Aerospace, and presumably Mr Sullivan, are currently based in a suburban condo in Reston, Virginia, Electric aircraft wheels are firmly not part of the aerospace scene.
Is this for technical reasons? A number of technical challenges exist. First, you need a large amount of torque to move a stationary loaded airliner – 4,000 NM for a 737, for example. In-hub electric motors can deliver that, given enough current, but where does one get the current?
A hard cell
All commercial aircraft are fitted with auxiliary power units – APUs. These are small gas turbines, optimised for the load imposed by aircon, lighting, avionics, and a margin for compressed air for main engine startup. The current needed for a high torque nose wheel motor is probably beyond the APU. Beef up the APU and you end up hauling a dead weight to 35,000 feet every sector, just to save a few kilos of fuel on the ground. So, the APU is not the answer.
A better source would be a high capacity fuel cell, running from a small Hydrogen tank. Hydrogen’s energy density is three times higher than kerosene, a good fuel cell will work at 65% efficiency, and will consume nothing at idle. So, instead of burning 600kgs of kerosene a day (at 20% efficiency) an A320 would burn roughly one tenth of that, for roughly one quarter of the time, using 10kgs of Hydrogen per day. Tankage is not a problem – a Toyota Mirai carries 10kg of Hydrogen in two small gas tanks, so just fit those. Perhaps those wins explain why Airbus has begun its research into the fuel cell stack.
But there’s another challenge to overcome. As it lands an aircraft needs enormous braking power very quickly for a very short period – about 30 seconds to destroy the energy of 50 tonnes moving at 150 knots. At present this is provided by friction brakes, which produce intense heat in the process. A normal landing will heat brakes up to 400 degrees C, a short sharp landing to 600 degrees. Apart from the thermal stress caused to pads, tyres and gear, brake temperatures affect duty cycles – you cant take off again until your brakes have cooled, in case you need full braking power to abort the takeoff (hot brakes cause brake fade).
An electric nosewheel would replace friction braking with electromagnetic braking. Weight, heat and space limitations would prevent the use of eddy current braking (the flywheel would be too big and heavy) which means dealing instead with a surge of current produced by the braking wheel. The only place that this can go is a very fast, very light, very high capacity capacitor. Electrans cant find any data in the public domain, but it is a fair guess that the technology to do that is neither generic nor cheap. It may also not be light, neutralising the weight gain from carrying fuel for taxiing on each sector. Graphene sponge capacitors might serve, but they are years from commercialisation.
If the tech issues were solved then roll-out into the aviation sector would be glacially slow. Certification would take several years. After certification operators would have to be persuaded that the win (fuel cost savings) was worth the risk and inconvenience of the unknown and untested. Airports would have to be persuaded to handle high pressure Hydrogen on the apron. Then, if electric braking were adopted, penetration would come slowly as a generation of existing plant was slowly retired. The cost wins are not large enough to justify retrofit.
In sum, electric nose wheels are an idea whose time should come, but not for a decade or three.