Releasing brakes after setting TO power
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Releasing brakes after setting TO power
Hi everybody,
I would like to ask you: if you set take off power and hold the brakes, do they get hot if the airplane is not moving? If not, how does the energy, produced by the engines, dessipates?
I would like to ask you: if you set take off power and hold the brakes, do they get hot if the airplane is not moving? If not, how does the energy, produced by the engines, dessipates?
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Yeah and the energy? You will move our little planet against its atmosphere
If you are able to hold the aircraft with the brakes, the energy from the engines is dissipated by stirring up the atmosphere.
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If the brakes are on and the plane doesn't move it's like having thrusters bolted to the earth: you push some air in one direction and what the thrusters bolted to (the earth) must go in the other direction - there's the law of conservation of momentum and you don't get any momentum out of the fuel, just energy. If you look at the situation from a frame of reference where you spin with the earth (so the earth seems to be at rest at first) it's simple to see where the energy's going to: part of it makes the air go faster and the other part makes the earth start to spin. From a point of reference where the earth is already spinning at the start it's be a bit less obvious: when your plane points west you actually slow down the rotation of the earth, so it afterwards has less energy, thus that bit and the energy from the fuel is all gone into speeding up the air while, when you point the plane east, the rotational speed of the earth will increase while the rotation of the atmosphere will be slowed, i.e. part of the energy of the air plus the energy of the fuel will all go into the earth. And, finally, from a frame of reference where the rotation of the earth doesn't change (i.e. one that's gets accelerated with the earth, which is the one you're probably thinking in when you sit in the cockpit) it looks as if the energy from the fuel has all gone into just accelerating air. So the answer to where the energy goes depends on the frame of reference you use to look at it - but there are the proper transformations in physics for going from on frame to the others. But because of the conservation of momentum thing take good care to make as many starts going east as you do going west or you may stop the earth from turning - or speed it up that much that centrifugal forces outweight gravity and you will lose your job because everyone then can fly without a plane;-)
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jtt,
Pretty good analysis except it seems you have ignored the torques on the earth caused by the surface drag of the winds just stirred up by the engines.
If this doesn't balance out exactly, then we have just influenced the weather!
Pretty good analysis except it seems you have ignored the torques on the earth caused by the surface drag of the winds just stirred up by the engines.
If this doesn't balance out exactly, then we have just influenced the weather!
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Having done max power runups we usually set the opposing wing to about 60% thrust, the weight of the aircraft has allot to do with it. I have been in an observer seat on a short field (abnormal/ferry) T/O, empty MD-11, brakes set, thrust set to about 80% N1, I could feel the damn gear kick underneath us on release and we broke ground in short order. Just about blew our initial assigned altitude before the guys could clean her up. Great memory....
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We needed max performance takeoff at TGU in the 757 but we never held the brakes to max power. The procedure said to do it but spooled up and a simutaneous brake release with max power didn't scare the passengers to death wondering why the whole airplane was shaking. We might be talking about 50 ft difference in takeoff distance versus scaring the crap out of everybody for no reason. After a 4 ft fence at the end there was a 70 ft drop off into the valley so it was like taking off from an aircraft carrier.
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Work = force * displacement
If displacement = zero then
work = zero.
In the case of a jet it doesn't mean no fuel is consumed. Just that the engines are 0% efficient (if you define efficiency in the right terms). All the fuel burnt is wasted accelerating air, heating it up, making noise etc.
The equation also explains how fridge magnets manage to stay up without an energy source. They don't need one as they also do no work.
If displacement = zero then
work = zero.
In the case of a jet it doesn't mean no fuel is consumed. Just that the engines are 0% efficient (if you define efficiency in the right terms). All the fuel burnt is wasted accelerating air, heating it up, making noise etc.
The equation also explains how fridge magnets manage to stay up without an energy source. They don't need one as they also do no work.
Bubbers - 50 ft is about right. When the Nimrod was new the procedure was full power against brakes for all take offs. After about 5 years they were finding cracking around the main undercariage attachments, which were attributed to this procedure, and we switched to releasing the brakes as the engines spooled up. Field lengths were adjusted for take off calculations by 50feet.
If you were on a conveyor belt, but the brakes on the conveyor belt rollers were on, would the earth move?
If you were on a conveyor belt, but the brakes on the conveyor belt rollers were on, would the earth move?
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Ah but the engine is producing thrust and is, by definition, not 0% efficient. Hot gas at high speed is a good indication of that ie. displacing air.
However, if you take into account the engine AND airframe in one package then yes the aeroplane is not producing any power and is less than 0% efficient when it's not moving.
However, if you take into account the engine AND airframe in one package then yes the aeroplane is not producing any power and is less than 0% efficient when it's not moving.
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T/O pwr against the brakes.
But because of the conservation of momentum thing take good care to make as many starts going east as you do going west or you may stop the earth from turning - or speed it up that much that centrifugal forces outweight gravity and you will lose your job because everyone then can fly without a plane;-)
In such, it can be said, therefore, that if you repeat the process to often you eventually starve to death. Therefore, holding the power against the brakes produces malnutrition.
Notwithstanding the above experiment, at T/O thrust, as mentioned in the posts above, would drag the aircraft down the tarmac leaving a trail of burning rubber - even a 737 would produce the same results. And, witness thereof, once a 73 taxied with a slight problem with the park brake - which was unfortunately stuck on..They taxied a fair old way until a hi-viz coat waved at the pilot to stop.
Last edited by Natstrackalpha; 5th Feb 2012 at 15:14. Reason: can`t type
