Is it feasible to replace brakes on airplanes with electric motor/generator?

Question

Would it be feasible to replace the brakes on airplanes with electric motor/generator drive systems?

There are two problems I see with airplanes. When they land, the tires are not spinning to match the ground speed, as the craft touches the ground. So there is a sudden lurch as the tires touch the ground and suddenly begin spinning. The bigger the craft, the more tire mass that must be instantly accelerated.

This disrupts flight stability, and the jolt could be enough to cause a crash or mishap in poor landing conditions. It also causes unnecessary wear and stress to the tires due to the motionless rubber dragging across the runway pavement at first contact.

It would be best for the tires to be spun up to match the ground speed before the craft touches the ground, for the smoothest landing possible.

 

The second part is just simply the problem of dissipation of heat. It has been discussed elsewhere here that the heat from braking can cause fires and crashes.

As in train locomotives, with a braking motor/generator, the heat could potentially be moved elsewhere to resistive heat coils covering a large area, and cooled using forward thrust or fans.

 

With a motor/generator unit, the tires can be spun up to either near or matching ground speed, using them as motors powered by the APU, and then switched to be used as generators for braking after ground contact is made.

Has this ever been researched or used on airplanes?

(As a lower middle-class American that is not involved in the airplane industry, I have no hope of doing anything with this or profiting from it, even if it is a practical and potentially patentable new concept that has never been tried before.)

EDIT

1. I am not talking about regenerative braking or energy recovery into a storage battery. Someone has inferred that though I didn't say anything about it at all.

   I understand the storage battery would likely be too much extra weight for a plane to carry, vs the energy recovery of takeoff and landing. (Heh, have a 2000 ft breakaway extension cord on a reel, to drive it down the runway and disconnect right before it lifts off the ground.)

   I am only looking at resistive / dynamic braking, dumping the heat in huge resistor banks with a high speed blower, the same as is used on train locomotives. On a plane, air bleed from the engines can blow across the the resistor banks.

2. Probably should pluralize the title. Rather than a single huge motor/generator, each brake assembly would be replaced by a separate one of these.

   So a large plane with say 20 wheels would have 20 motor/generator units, these running at high voltage to keep the amps and wire diameter small. This reduces weight and engineering complexity, eliminating gears and shafts linking all wheels on one set of gear, to one huge motor/gen unit.

3. This is not two separate devices, but one device that does both functions. Most (but not all) generic electric motors without electronic drive systems can also be a generator.

   This could use a permanent magnet motor, or with a powered field coil. Neodymium is capable of extremely high flux density, but loses it when exposed to high heat, so there are density / temperature tradeoffs of permanent vs powered field coils.

4. Tire hub could potentially be integrated as part of the motor/gen rotor/casing for weight savings (which is very important in aircraft), using a fixed core and a hub that rotates around it in reverse of conventional motor design.

Answer 1

On a hybrid or all-electric car, the motors on the wheels supplement or replace the engine, and by regenerating electricity from the braking, it extends the range of the battery.

On an aircraft, the engines would still be needed for the major part of the working life of the aircraft -- the time while it is in flight. So unlike the case of a car, you don't get any advantage with the motors themselves by adding the motor/generators to the wheels.

Beyond this, on a large aircraft, electricity is in ample supply when you have jet engines turning, so what's gained by regenerating the electricity from the wheels, during the 60-90 seconds of heavy braking on landing, isn't all that significant when compared to the electricity used and produced over the course of a whole flight. Not nearly as significant as a drive in the city with many cycles of accellerating to speed then decelerating to a stop, over and over.

Beyond this, the major players then are the added weight and complexity of the motor/generators, vs the gains in terms of (a) tire life, from spinning them up prior to landing, and (b) fuel saved by maneuvering on the ground from the wheel motors instead of the jet engines. When you consider that the costs of jet fuel are on the order of $1000 to multi-thousands of dollars per flight hour, the costs savings of longer tire life will lose out if the extra weight of the motor generators over conventional brakes means that you're spending more money over the course of the flight to carry the extra weight. Likewise, the fuel burn during taxi in & taxi our on the motors isn't all that great compared to an hour or many of high-power cruise... power settings for taxi are close to or at idle, so fuel consumption isn't all that high. Plus, it's common to taxi without all engines running, further reducing fuel burn on the ground.

The big killer for this idea is the weight penalty. Motor generators are significantly heavier than conventional brakes, and when you consider the amount of energy that brakes have to be able to absorb during a high-speed stop, you'd need some seriously beefy generators to accomplish what aircraft brakes can do. Or else, you'd have conventional brakes installed to supplement the generators, and the weight goes up further.

As a ballpark, on an hour flight in a 737, an extra 1000 lbs of weight increases the fuel burn by about 10 lbs, or about 1.5 gallons. Doesn't sound like much, but when you multiply the added weight times every flight hour that the aircraft is operated over the course of a year, it really adds up. Beyond that, motor/generators are a lot more complex than simple brakes, so they'd be more expensive initially and require more maintenance throughout their service life. Plus, I suspect, they'd need to be supplemented by conventional brakes that would be used in the event of a high-speed max-effort stop (rejected takeoff, landing on a short runway).

The end result of all of this is, no manufacturer I'm aware of has found that the tradeoffs make this sort of project worthwhile.

The best case is probably one of really long taxi times, but in that case a pushback tug could tow the aircraft to a point close to the runway, saving the fuel burn on the aircraft engines until it was time to start them. I'm sure this has been considered as well, and again, I'm not aware of any carrier that uses this approach (although outside the U.S., somebody might be).

It's an interesting question, but the costs & the math behind the engineering haven't worked out in its favor, at least thus far.

Answer 2

Feasible? Maybe. Sensible? Clearly no.

Let's first look at the energies involved: Braking a Boeing 747 to a complete stop will change its kinetic energy by 276.4 kWh. Drag and friction will maybe swallow 20% of that, so we have 221 kWh or 795.6 MJ to convert to electricity within 30 seconds. This means the generators have to be sized for a load of 26.5 MW. If you can live with a longer deceleration distance, the generator has more time to apply its drag and can be smaller for the same overall stopping power, but it will be hard to make your prospective pilots accept that your design will now need a much longer runway.

I found it hard to find online sources for the mass of aviation generators, and the best so far was Jan's answer here and this EAA page. It seems a lightweight starter/generator could have 5kW of power per kg of mass, so the 26.5 MW generator should weigh 5.3 tons.

Next would be energy storage: In order to keep the recuperated energy for taxiing and the next take-off run, the battery will need to weigh 800 kg (assuming 1 kg of battery mass per MJ of stored energy). Of course, the efficiency of the generator and the DC converter will be below 100%, so maybe the battery can be a little lighter. A 70% overall efficiency will only require 560 kg for the battery.

How much brake mass and fuel would that save? Again, aircraft brake masses are hard to find online, and I did not want to go through a proper disc brake sizing. But I bet it is a small fraction of the generator mass - maybe 10%! With 43 MJ/kg and maybe a quarter of the efficiency of electric wheel motors, the fuel needed is just 75 kg. Here I assumed we are feeding 800 MJ to the wheel motors and try to achieve an equivalent acceleration with conventional jet engines. Now consider that this fuel is no longer burdening the aircraft for the remainder of its trip while the batteries and generators still need to be carried around.

So it looks like the conventional solution has 10% of the mass of a proper generator brake and needs just 75 kg of fuel even for the biggest airliners. Carrying the generator and battery around will use much more fuel or require our hypothetical 747 to carry 61 passengers less. No airline can accept that!

Regarding the wheel spin-up on touch down you say:

This disrupts flight stability, and the jolt could be enough to cause a crash or mishap in poor landing conditions. It also causes unnecessary wear and stress to the tires due to the motionless rubber

Yes, spin-up causes some of the rubber to burn away (just watch any airliner landing - spin-up is signalled by a cloud of smoke, and skid marks on the runway) but I would not worry about flight stability once the aircraft is down on the ground. Even locked wheel brakes are not a real danger to flight safety on modern tricycle gears.

Answer 3

Can't see mentioned in the thread another issue would be aquaplaning on a wet runway. It is better to land hard and let the skidding tyres break through the water film on the runway. The tyres would be delayed in making contact with the runway in wet weather if they were sped up before touchdown.