You bring the power to idle BEFORE you touchdown. On very windy days (especially with strong crosswinds) you might carry some power all the way to touchdown, but as soon as the main wheels are on the ground the throttles come back to idle.
The reversers do NOT come out airborne. In fact, the throttles must be in idle for the reversers to be deployed - there is mechanical lockout that prevents reverser actuation at any throttle position above idle. Also, the previously mentioned squat switch (AKA Weight On Wheels) must sense that the aircraft is on the ground before the reversers can deploy.
You may now continue with the Suck Squeeze Bang Blow discussion…
Thank you. Now this is making sense. As I stated in the my OP the ‘roar’ occurs after the plane has touched down.
I can also sleep more soundly tonight knowing that the reversers cannot be deployed unless the throttle position is in idle. All day yesterday I had visions of cartwheeling at 37,000 feet above the Grand Canyon on my next flight.
Yes but then the turbine would not be needed at all. A turbofan is essentially a bunch of rotors–which are themselves wings–rotating around an axle. The compression stages are a bunch of wings essentially pulling the engine forward and the exhaust stages are essentially pulling the engine backwards, but since there are usually many more compression stages than exhaust, the net effect is that the engine pulls itself forward. The compression stages must be the source of the populsion, because the compression stages must be able to keep pushing air through the engine all the time. The engine can’t function any other way.
Generally, you can tell what type of engine a vehicle has by its exhaust nozzle. Atmospheric engines almost always have convergent nozzles, and reactive-thrust engines (rockets) almost always have convergent+divergent nozzles (think of the space-shuttle big rocket nozzles). Another example would be that if a convergent-divergent nozzle is more efficient for a rocket, then why don’t all modern jet aircraft use them too? The reason is that aircraft engines are not producing thrust by reactive propulsion.
~
Because not all jet aircraft are supersonic. Generally, subsonic aircraft use convergent nozzles, while supersonic aircraft use convergent-divergent nozzles. It mainly depends on whether they have supersonic flow in the exhaust or not.
Yes, they are. According to your theory, how does a J-33 jet engine (which draws air in from the sides) produce thrust, if not from reactive propulsion?
Stuff flows out back, vehicle moves forward. For rockets or jet airliners.
I thought as much. (thank you sir – and Rick and ElvisL1ves)
One unsubstantiated tale I once read, that pegged my areyouseriousmeter, was about the early (turbojet) DC-8s, some of which had an odd looking add-on-behind-the-engine-assembly reverser set up, to the effect that in the early days you could partly deploy the reversers on the two outboard engines, as some sort of aerobrake-on-steroids
You can tell the powerplant engineer community is mostly male. Otherwise there would be a “Talk” cycle at both ends and the middle of the sequence…
The engine itself uses a double-sided compressor, but installed in aircraft it had intake ductwork that faced forward. It can produce thrust even just mounted on a test stand, but the intakes are closer to the forward end. It’s also not a very efficient design overall, it dates from the WWII-era.
The problem with thinking that a jet engine is just like a rocket because “air squirts out the back” is that all the air that comes out the back of a jet engine had to be pulled in the front. The compressor is a bunch of wings that can only pull the air in front of it, and if the compressor can’t keep pulling air into the engine, the whole thing would stop. If you cut off the whole compressor stage and spun it at the usual speed, it would still create thrust. If you did the same with the turbine stage, it would create little if any air movement at all.
…If a rocket and a jet engine both create reactive propulsion, then why don’t aircraft all use rocket engines? Rocket engines are simpler to build, aren’t they?
~
The problem with looking at any single part of the engine in isolation is that it will do nothing as the engine works as a unit. No turbine stage = nothing driving the compressor stage to make it spin.
Because a rocket requires that all of its fuel, including oxygen, is carried on board. A jet engine takes half of it’s fuel for the combustion process from the outside air and so doesn’t need to carry as much stuff on board.
Here’s a question to ponder, if a jet engine is pulling the aircraft with its compressor stage then why does it even have a turbine stage? Why don’t piston engine aircraft have propellors that look like compressors?
The reason you’ve never quite understood thrust reversers is that you don’t understand jet engines. The fact that they do work is evidence against your idea of how a jet engine works.
The turbine isn’t needed. Turbine jets are not the only style of jet engine; there are ramjets and scramjets with virtually no moving parts and pulsejets with only a bare minimum of moving parts. They all work on the same compress-combust-exhaust principle, though. The turbine is just the best way anyone’s figured out for achieving good performance and fuel economy at typical high-subsonic speeds.
Rocket engines have to carry all of their fuel and reaction mass; jets use the atmosphere around them for the overwhelming majority of their reaction mass, so they can carry a heavier load, or carry a load farther, using any given amount of fuel. [On preview, I see the Death Ray has beaten me to it on that score.]
Most rocket engines are designed to be fired exactly once. Building a rocket engine that could withstand the hours-long day-in, day-out cruising required of airliners might be a lot harder than you’d expect. Even the Space Shuttle’s liquid-fueled engines - by far the most advanced rocket motors ever made, and an engineering miracle on a par with the more-famous heat tiles - have an severely constrained lifetime. I think it’s only a few dozen uses (couldn’t find the cite I had in mind, sorry).
I´d like to point out another contributing factor to the landing noise are the spoilers, they deploy as soon as the wheels touch the runway and make quite a roar by themselves. Imagine installing a vertical metal plate on the roof of your car and driving at 150 Km/h, that´s the kind of noise you´d get.
The turbine stage is there to spin the compressor stage, but the turbine stage does not contribute to the accelleration of the exhaust stream, in fact, it impedes it. The compressor stages are larger and more-numerous to overcome the resistance of the turbine stages and keep air moving through the engine. The only reason air moves through the engine is because the compressor can push it through.
The truth is they can, but the reason they usually don’t is because a ducted fan usually needs a higher RPM for efficient operation than a piston-engine can turn at, so turbofans are turbine-driven. The propeller of a prop-aircraft and the compressor of a turbine engine really are doing the same thing. They are wings mounted on axles and flying in a fluid, that can only work by pulling on the air in front of them. And on an aircraft’s wings, the spoilers are on the top surface, because disrupting that surface flow is what keeps the wing from creating lift.
~
Thrust reversers were first being developed at about the same time as the DC8 & 707. So several designs were tried out before they settled on the typical modern design. And like any new technology, it was misused / over-used a bit at first.
Yes, the inboard thrust reversers could be used in flight as additional speedbrakes / spoliers to steepen the descent. Only idle thrust was authorized IIRC, but it could be (and routinely was) done. It was noisy in the cabin and produced a bunch of vibration, so it wasn’t popular with the airline’s Marketing department or some of the passengers.
Thrust reversers were notoriously unreliable mechanisms in the early days. By virtue of a lot of safeguards, they tended to fail by refusing to open, rather than by opening at awkward times or refusing to close again. But there were enough glitches on the DC8 where one engine stayed in reverse idle that most operators eventually abandoned the use of inflight reverse.
Contrary to the scare-mongering in some posts above, a reverser deployment in flight is not necessarily instant doom for all aboard. In some cases, yes, but in most cases we simply diagnose the problem, shut down the engine and carry on without it. With no thrust coming out of the shut-down engine, there’s not much effect from a deployed reverser.
One looking more like the DC8 example above or the L1011 example here http://home.swipnet.se/~w-26408/211rev1.jpg (thanks again Jurph) have almost no adverse effect on handling once the engine is shut down. Some extra vibration and drag, yes, but nothing to sweat about.
Wings create lift mostly by pushing air downwards. The Bernoulli effect is only a small part of it. Spoilers would work equally well on the top OR the bottom of the wing–they’re just a drag surface, after all. They’re on top so as not create ground turbulence which would cause instability on landing.
The compressor is there to take air and compress it prior to going into the combustion chamber, it is not there to drag the aeroplane through the air. Older jet engines had no bypass airflow, so all the compressor stage was doing was supplying air to the combustion chamber. Do you not think that if the compressor is there to act as a propellor, that high bypass engines would have been the initial design aim?
How do you think that jet engines work at supersonic speeds? Aircraft flying near or faster than the speed of sound actually have to have the airflow to the compressor slowed to subsonic speeds for the engine to function properly. How does that tally with the compressor pulling the aircraft?
Military aircraft get a huge thrust boost by using afterburners. How do these work? By supplying extra fuel to the exhaust (ie after the turbine stage) and burning it with left over oxygen in the exhaust gasses. They can get upto a 50% increase in thrust yet the turbine and compressor are turning no faster.
Indeed, I have seen gliders that have spoilers on both surfaces of the wing. However, I think top surface airflow is far more important than bottom surface airflow and so top surface spoilers work better. They do more than just create drag, they disrupt the smooth airflow over the wing which spoils its lift regardless of the process creating the lift (bernoulli/newton etc).
Spoilers, also called speed brakes, have two functions.
One is simple drag increase, and they could be mounted almost anywhere and be in almost any shape to do that.
The second function is lift disruption. At touchdown, the wing is still going fast enough to provide a bunch of lift, thereby carying a bunch on the aircraft’s weight and reducing traction for the tires for steering and braking. That’s not desirable.
By raising the panels on the top surface of the wing, the lifting effect is “spoiled”, hence the name, and a lot more weight is transferred onto the tires.
The half-flying, half rolling condition is not a stable one and as pilots we want to get through that transition as rapidly as reasonably possible.
Most airliners built after the 1970s have auto-spoliers which activate on touchdown, precisely to hasten that transition. It also tends to prevent really bad bounces from lofting the airplane more than a couple of feet in the air. A large airliner doing a 10’ bounce after a really lousy landing can easily turn into an accident. Keeping the plane close to the ground means a bad landing is just an embarassment.
Likewise, during an aborted takeoff, we need maximum wheel braking to get stopped in the remaining runway. Killing the wing’s lift ASAP is a kay part of that. So wing-mounted lift spoilers are essential, whereas the simple aerodynamic drag they also provide is only an incidental benfit worth a few percent at most.
And by mounting the spoilers on top of the wing, the airflow is deflected upwards, producing a downward force. Pivoting spoilers on the bottom of the wing would produce an upwards vector that would not be desirable. Yes, some sailplanes have bottom-mounted spoilers, but they’re not found on big airplanes.
The times I’ve seen non-flap drag devices on the bottom of sailplanes’ wings, they have been either the Schempp-Hirth bar style brake that extends from inside the wing in the vertical direction only, or door-type spoilers that open from the front (i.e. are hinged from the rear of the spoiler - suicide door style). This is what you see on the ubiquitous Schweizer 2-33. Neither of these configurations really add lift, but in all the cases I’ve seen of bottom-mounted spoilers they have been in conjunction with spoilers on the top of the wings too. I suspect that a front hinged underwing spoiler would indeed not provide enough of the desired braking/spoiling effect, which explains why we don’t see them much.
The other reason to use top- & bottom- mounted spoilers on small aircraft is to minimize the pitching moment. When we deploy top-mounted spoilers on a larger aircraft, there’s usually a pretty good pitch moment to counteract at the same time.
When we’re using them inflight, we can anticipate and counteract this. We also use them gingerly. Simply yanking the control handle from fully-stowed to fully-deployed would produce a real jolt in the cabin. We’ll typically take more like 5 seconds to go from fully stowed to fully deployed to keep everything nice and smooth.
Conversely, in a sailplane (which category I’ve flown only a handful of times), the spoilers are used as an almost-throttle during approach and landing. So there’s a pretty continuous jockeying of the spoiler control to steepen or flatten the descent and control the energy level. Having a big nose-up / nose-down pitch excursion with every tweak of the “throttle” would suck. Better to have very nearly offsetting moments so the control is almost pure drag.
