I remember seeing on TV a while ago that there was someone trying to beat a land speed record. I really don't remember very many details of the show, but one thing I do remember is that they needed some way of stopping the vehicle very well, without locking up the wheels.
The solution involved an aluminum "disc" on the axle that caused drag the closer it was moved to a coppper(???) "disc" on th wheel. As long as the vehicle was moving, the two discs could not be pressed into making contact. The more force used to press the discs together, the stronger the braking power was.
This sounds like the ultimate permanent ABS brake system. No wearing out of friction materials, the very simplest of ABS implementation, etc.
I can't find anything online about this kind of braking system, or the physics behind it. I don't have enough details to do a decent Google search. I'm hoping someone here recognizes what I'm talking about and ca give more information.
My bet is that one or the other of the disks wasn't either copper or aluminum, but a bunch of strong magnets that *AREN'T* spinning - Braking by eddy-currents induced in the spinning disk as it gets closer to the magnet disk.
For a practical demonstration, find yourself a chunk of aluminum flashing, bend it into a "V"-ish shape, set it at an angle, and drop a round magnet on it to roll down it. You'll see some rather odd twitches and jumps as it does. What's the mechanism? Eddy-currents - The moving magnet acts like the stator of a alternator, while the chunk of aluminum acts like a turn of wire from the rotor, generating electricity in the aluminum, which, under well-known electro-magnetic laws, the magnet tries to grab, with rather surprising strength. (Depending on the strength and field orientation of the magnet, and the angle you've got your "V" set at, the magnet may actually come to a complete stop for a short moment before the electrical field dies, releasing it to start rolling down the slope again, only to stop dead again, and repeat over and over until it gets to the bottom.)
Be aware, though, that there's no such thing as a free lunch... Just like a normal rotor-and-pads system, the spinning disk is going to get hot - potentially "meltdown" hot...
True. However, eddy current brakes need another auxillary brake like a friction brake, or say the eddy current brake is the aux brake. The problem with eddy current brakes is that they only work with considerable motion. The braking falls to very low values at low speeds so you still need a brake to "park" it where you want to stop it.
Yes, but without a magnet in there, aint gonna be no eddy currents. I don't think the OP is really remembering enough details.
The other problem with all electromagnetic braking systems is that their effectiveness decreases as speed approaches zero, so its hard to use them to come to a complete and total stop. For example, diesel-electric locomotives use dynamic braking (operating the traction motors as generators and dumping the electricity to a resistor grid cooled by a big fan) to slow down or regulate speed on long downgrades, but to come to a complete stop they have to use the friction brakes at some point.
Considering the application the OP originally saw it connected with, I'd say that's one of those "no big deal" things. I'd be betting on the system being intended to slow the beast down to "conventional speeds" without locking (or even grabbing real hard for an instant) a wheel at some ridiculous speed, thus causing the whole thing to go every which way but loose. Once the thing is down to a reasonable speed, conventional brakes can take over without anywhere near as much concern for whether they lock or grab.
As in, a brake locking or grabbing at 500+ MPH is likely to result in a TOTAL disaster, but locking/grabbing at lower speeds (for grins, let's say 30 MPH or below) is likely going to be mildly annoying, perhaps cause some (relatively) insignificant loss of control, or maybe even do a little damage, but in comparison to the potential consequences of the same lock/grab happening at 500+ MPH, it will effectively be a non-event.
Not sure they're all the way there on the science end of things (with the description, anyway)
"Testing using this braking system has shown that the 7.5 ton NAE can be successfully stopped from speed of over 200mph within one mile at a resulting thermal energy generation of only 300 degrees F."
Kinetic energy gets turned to heat, there's no way around it. How much material getting heated to 300F I wonder? And how much heat do they manage to dump with cooling before they get stopped?
So do other braking systems like jet aircraft. Jets are diverted to slow the plane down and then friction brakes assist the end stages of stopping and steering.
However, any system which uses less energy than friction is a step or stride in the right direction.
But thrust reversers would bring the plane to a complete stop, and even cause it to back up. The reverse thrust does not drop significantly as the plane slows down.
But remember, braking is to destroy kinetic energy. Unless that energy is recovered in some way, you do not increase efficiency. Take the difference between dynamic braking and regenerative braking in locomotives. Dynamic braking uses the electric motors as generators, dumping energy into a giant bank of electrical resistors. This heats the air. The waste heat is then lost. So even though friction is not being used, there is NO savings in energy.
With regenerative braking, the locomotive is equipped with a large battery bank. Here the electrical energy from the motors acting as generators charges the batteries. This DOES affect fuel consumption.
Eliminating friction braking is not the issue. Storing braking energy IS.
I believe jets cease use of the thrust-reversers once they hit a certain speed to either a) reduce the chance of kicking up debris into the intake b) heating the airframe etc. ahead of the engine.
In one case, an idiot pilot used the thrust reversers to help back the jet out of a snowy gate, the snow was injested into the engines, and caused engine failure on takeoff. And it crashed.
Sorry, Dave, but I'm going to have to call "bullshit" on this one.
The engine(s) may indeed have swallowed some snow if the thrust reversers were used as described, but in point of fact, if they were going to fail catastrophically (or even just shut down without any noticeable/significant damage) they would have done so almost instantly at the time the snow was sucked in. Not several minutes later, after taxiing to takeoff position, waiting through whatever hold time there may have been, any de-icing procedures that may have needed doing (and if the terminal was snowed in that bad, there damn-well would have been de-icing needed), going through the final checklist before takeoff, including throttling up the engines to make sure they could/would throttle up, and about a bazillion other little items that make it impossible to go directly from "at the gate" to "rolling for takeoff" in less than 5-10 minutes. (By which time, every trace of the snow they sucked up would have long since vanished out the tailpipes as steam)
Never mind the fact that unless you're talking "hundreds of gallons per second" quantities (which wouldn't be the case in the scenario you describe - unless the plane was completely buried, and in that case, I'd expect the airport to be shut down due to weather conditions...) jets fairly routinely fly through precipitation with next-best-thing-to-zero difficulty.
Either give me a link to the FAA's crash report for the incident, or give me the exact date, airport, and flight number so I can look it up for myself in the FAA crash database.
(I have no doubt you're going to be unable to do either)
Uhhhh.... you CAN'T "use less energy than friction!" Friction turns the kinetic energy of the vehicle into heat. Other things like induced eddy currents and reversed jet engines also do that, but they ADDITIONALLY use more energy to create the reverse thrust, or to induce the eddy currents.
Maybe you're thinking of some system that RECOVERS the kinetic energy for re-use.
Its "a". And the thrust reversers are purely optional- they are NOT required to be functioning for the airplane to fly. I've been on several commercial flights where the reversers were never used, and I asked the flight crew if they were "tagged out" (labelled inoperative and disabled) and they said "they sure are."
Urban legend, most likely.
Wing-mounted engines are right near the ground and kick up so much crud that they can't safely be used to back up the airplane, except in the case of the air force C-17, which has an elaborate thrust reverser system to kick all the blast air upward.
Tail-mounted engines are another matter. It is often standard procedure to "power back" MD-80s and Boeing 727s out of gates. The 727 was particularly nifty in that it could be backed with only the center engine, which diverted all its blast to the sides AND had its air intake atop the fuselage. Virtually no chance of ingesting dirt kicked up by thrust reversal.
" Due to the deep accumulations of snow, however, the TUG was unsuccesful in its attempts to push the aircraft, and contrary to company policy, Wheaton elected to use the reverse thrust to back the airplane out of the gate. The reversers were engaged for a minute and a half, but only succeeded in sucking large amounts of storm debris into the engines. Finally, cooler heads prevailed, and a TUG with chains attached was called in. The aircraft was successfully pushed back. "
" The investigation of the crash concluded that the combination of the crew's use of thrust reverse on the ground, and their failure to active the engine anti-ice system, caused the crash. By failing to activate the engine anti-ice, the large amouts of snow and ice that were sucked into the engines during reverse thrust use was allowed to remain there, unchallenged. The ice buildup on the compressor inlet pressure probe, the probe which measures engine power, can cause false readings, as was the case here. The indications in the cockpit showed an Engine Pressure Ratio of 2.04, while the power plants were in reality only producing 1.70 EPR, or about 70% of available power. The combination of the ice covered wings and low power caused an immediate stall on takeoff that resulted in 74 lives lost. "
Aviation Accident Report number AAR-82/08
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NTSB Identification: DCA82AA011 Scheduled 14 CFR Part 121: Air Carrier operation of AIR FLORIDA, INC Accident occurred Wednesday, January 13, 1982 in WASHINGTON, DC Probable Cause Approval Date: 1/13/1983 Aircraft: BOEING 737-222, registration: N62AF Injuries: 78 Fatal, 6 Serious, 3 Minor.
The Safety Board's full report on this investigation is provided as Aviation Accident Report number AAR-82/08. To obtain a copy of this report, or to view the executive summary online, please see the Web site at
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SCHED FLT FROM WASHINGTON, DC TO FT LAUDERDALE, FL. THE RWY WAS CLSD OVR 1 HR DRG MOD TO HVY SNWFALL FOR SNW RMVL, THUS DELAYING DEPARTURES. THE ACFT WAS DEICED WITH A HEATED ETHYLENE GLYCOL & WTR SOLUTION WITHOUT ENG INLET PLUGS OR PITOT STATIC CVRS INSTLD. CONTRARY TO PROCEDURES, REVERSE THRUST WAS USED TO HELP A TUG DRG PUSHBACK FROM THE GATE & BLEW SNW.AFT PUSHBACK, THE FLT WAS DELAYED 49 MIN WHILE SNW CONTD IN SUBFRZG CONDS. WHILE WAITING, THE ACFT WAS PSND NEAR THE EXHAUST OF THE ACFT AHEAD. DRG TKOF, THE EPR'S WERE SET FOR 2.04, BUT AN ANOMALY WAS NOTED IN ENG INST READING. THE CAPTELECTED TO CONT TKOF. THE ACFT TKOF APRX 2000 FT & 15 SEC PAST THE NORMAL TKOF PT. AFT LEFT-OFF, IT INITIALLY CLIMBED, BUT FAILED TO ACLT. THE STALL WARNING STICKSHAKER ACTIVATED SHRTLY AFT TKOF & CONTD TIL THE ACFT SETTLED, HIT A BRIDGE &SVRL VEHECLES, THEN PLUNGED INTO A FRZN RVR. INVESTIGATION RRVEALED ENG INLET PRES PROBES BCM BLKD WITH ICE, RESULTED INHI EPR INDCN, PSBL PITCHUP W SNW/ICE FRZ ON WNGS, NO RWY DSTC MRKRS WERE AVAILABLE, CREW HAD LMTD CLD WX OPNL EXPERIENCE
The National Transportation Safety Board determines the probable cause(s) of this accident as follows: WING..ICE PLANNING/DECISION..IMPROPER..PILOT IN COMMAND MISCELLANEOUS..ICE ANTI-ICE/DEICE SYSTEM..NOT USED..PILOT IN COMMAND ABORTED TAKEOFF..NOT PERFORMED..PILOT IN COMMAND
REVERSERS..IMPROPER USE OF..PILOT IN COMMAND * (my emphasis)
ATC CLEARANCE..DELAYED LACK OF EXPERIENCE..PILOT IN COMMAND AIRCRAFT PERFORMANCE..OTHER AIRCRAFT HANDLING..REDUCED AIRSPEED..INADEQUATE THROTTLE/POWER CONTROL..DELAYED OBJECT..VEHICLE
My recollection was 75% accurate, I'll say (it was 26 years ago, after all). But I think I still get to say "Oh snap, you got told!" (smiley crap applies- it was a bit improbable)
Thrust reversers spit a crapload of snow into the engines, they didn't turn on the engine de-icers (and screwed some other procedures up) and they crashed on takeoff.
Apparently thrust reversers are sometimes used to help back up on the ground, according to some other things I saw.
Drop a powerful magnet down a length of copper pipe. Even if the magnet clears the ID by a few mm, it can take several seconds for it to drop a couple of feet.
With the comment that use of the reversers and/or engine failure (The engines didn't fail - they just didnt' work as well as the pilot thought they were working) seems to be a *VERY* minor part of this one, IMO. Were it me making the conclusions, I'd call it a pretty clear case of "pilot screwed the pooch". I also note that the De-ice (or lack thereof...) that should have happened might well have prevented the crash, even with the plugged sensor. I wonder how many tons overweight that bird was due to ice buildup when it left the ground...
(memory kicks in... Wasn't this the plane that clipped a truck/bus/both, and a bridge, then fell into the Potomac? Timeframe and situation sounds just about right, but 26 years later, as you say, memory fogs...)
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