Fuel saving on taxi... feasible?
 

Croydon's trams apparently use a system whereby they can store the energy absorbed by the brakes during deceleration and use this to assist in them getting underway again. This results in less power being used from the (unsightly!) overhead supply.

Could this be developed for use in aircraft for a fuelless (excluding APU) taxi out/in or would the extra weight of the gear just burn off all that 'saved' fuel in the cruise?

While on the subject of saving money, how about using the system mentioned above to spin the wheels up before touchdown and save on tire wear? Not an engineer or anything, just have strange ideas sometimes!

Got a call back from Dragon's Den about another idea but when they realised it was just an idea they weren't interested!

Hi All,

Does this mean that the A380 engines 2 and 3 will succumb to more wear and tear as the reverse thrust is only available for those two engines?Sorry to veer off a lil with regards to the a/c type.

Warmest Regards,
Vibes

Probably because it costs more to install and service the motor and control switches that spin the wheels.

I use to get these ideas a lot from my young engineers. After I sent them back to their calculators to do a cost estimate comparison, they never came back again.

It's easy to be immaginative, it's tough to be practical.

The additional machinery for reverse on those engines will certainly take a lot of maintenance and be prone to breakdown. I believe Airbus were all for leaving reverse out altogether, but it was customer demand that led to reverse being installed in 2 engines only.

Reverse thrust economics
 

Reverse thrust on any operation should be conservative and idle reverse is the general recommendation for gas turbine engines be they turbo props/jets/fans.

Economics becomes redundant where safety issues of a particularly marginally safe to unsafe landing conditions or pilot excursions(correlated with experience and recency levels on the type) are factored.

If the weights calculated correlate to those loaded(i.e no fraudulent weights) the performance data should guide the pilot quite accurately(even with airframe and engine degradation-as performance engineers look into such factors).Now its up to the pilot to fly the bugs as referenced for the scheduled performance computed.Anything excessive will eat into both saftey and economics but we are dealing with infinitismaly micro economics.

Even for a stabilized and on the bugs reference speed landing It could throw all the perfection out of perspective if the airline has an urgency to meet the gate arrival time due to schedule tardiness or ill passenger and pilot tries to short cut a runway exit taxiway...hence instinctively gulping more fuel with more reverse thrust.

Reverse thrust for most turbojet/fans is more effective at high engine RPMs
(ref DP Davies-Handling the Big Jets) and so if landing weights, runway length and surface conditions,pilot experience level do not bias toward safety demands...then Reverse thrust is just a luxury(aerodynamic braking and good old wheel brakes are sufficient to do the job...that is WHY Reverse Thrust IS NOT FACTORED in scheduled landing performance for category A airplanes.

Taxi fuel allowance for a heavy jet is in the vicinity of 200kg.Fuel burn on landing is roughly only10% of that.

You can try to save pennies if you like(and they do count with a massive scale of operation no doubt!)but ask yourself the fundamental questions:

Is it at the expense of safety?...at the expense of macro economics,scheduling,maintenance cost,noise penalty fees,etc)?...at the expense of passenger comfort levels(especially an early morning landing)?...at the expense of noise rules(environmental sensitivity)...and so on and so forth....:rolleyes: