not too many jet planes can do this in flight and for those planes that can it’s a rough ride. the only civilian jets I can think of were the DC8 and Hawker Trident and they’re both pretty much gone unless there are any freighters versions left flying.
I’ll bite: what practical reason is there for a civilian jet to be built with the capability of mid-air reverse thrust?
[[very tiny stupid hijack]]
Do commercial jet pilots use a foot pedal, like a gas pedal, to increase thrust? I never thought about until now. Or if they use a key to turn start the engine.]]
Thrust is controlled by thrust levers. They’re normally placed on the centre console between the pilot seats and you use them with your hands. Pedals are used to control the rudder which is mainly to keep the aeroplane “in balance” which just mean that it is flying straight through the sky without slipping sideways.
Light aircraft will often, but not always, have an ignition key much like a car. Large aircraft don’t have that. (I guess it was assumed that if you are in the cockpit and know how to start the aeroplane then you are meant to be there.)
to slow the plane down and descend faster. When you’re hauling a couple hundred thousand lbs of stuff it’s like anything else subjected to the laws of physics.
The last time I was in a commercial plane that was higher than the captain wanted to be (reference the arrival point) she deployed what looked like barn doors on top of the wings as speed brakes. That was in a plane that was many decades newer than a DC-8.
From a mechanical standpoint, the thrust reverser already exists as a mechanical method of slowing a plane once it’s landed. Using it in the air is one less device to add.
From a practical standpoint it’s a damn dangerous thing if both sides don’t deploy. It’s brought at least one plane down inflight and seriously damaged another taking off.
You have to have that device no matter what because you need them as ground spoilers. Your brakes aren’t going to do much good without them.
Well that was my point for the thought behind using them as an in-flight spoiler device. it’s hard on the air-frame and fatal if it deploys in split fashion. I saw pictures of a DC8 that had a split thrust reverser deployment on the ground and it dragged the aircraft diagonally off the runway.
Wait, what? Are you calling the spoilers - the large panels that rise up on top of a wing - a type of thrust reverser, or do I just have no reading comprehension today? I’ve never heard them referred to as such: spoilers are on the wings, thrust reversers are on engines.
I thought Magiver was saying we could ditch speed brakes, one less thing to go wrong, for aircraft where thrust reversers could be used in flight. To which I replied… can’t do it because you need the “speed brakes” as spoilers on the ground. To say nothing of spoilerons.
If that was an incorrect reading I apologize. I must admit I’m somewhat confused myself. It’s late.
no I’m saying the use of an engine thrust reverser in flight serves the same function as a spoiler in that it slows the plane down. It wasn’t a particularly good idea and no modern commercial jet uses it to my knowledge. the last one I can think of was the Concorde. Now turboprops are a little different and changing prop pitch allows for some braking capacity.
Why don’t engines crack up during thrust reversal? To my mind the words imply going from Drive to Reverse without passing Neutral; shouldn’t Neutral include enough time for fan blades to spin down?
Also, Richard, thanks for your answer (of course I remember about the the thrust control now now; I guess I was more intrigued with the pedals). But, OK, you’ve left me hanging on a cliff: if not by a key, how do you start up a commercial airplane? I’m guessing button.
nothing in the engine reverses. The backward flow of air is partially diverted forward by mechanical vanes of one sort or another
hereis a clam-shell design on the back of a jet engine.
A few things:
As stated above, spoilers are secondary flight controls that reside on the top of the wing. They serve a few purposes. First, in flight you may notice the spoilers being actuated slightly as the airplane rolls into and out of a turn. These are the flight spoilers, and they assist the ailerons with roll control and keep the airplane flying true while it is rolling (by preventing adverse yaw). The second thing they do is act as speedbrakes while the airplane is airborne. The pilot can deploy them incrementally to assist in expediting a descent or slowing down. Finally, when the airplane touches down, the flight and ground spoilers deploy (usually automatically) to add aerodynamic drag and - wait for it - spoil the lift of the wing. They disrupt the airflow over the top of the wing, preventing the airplane from lifting off again in the event of a hard landing or strong gust of wind.
Thrust reversers are mounted to the engines, and they work by redirecting the airflow from the engines. There are a few different types of thrust reversers, but the idea is the same. Turn on your faucet and stick a spoon into the flow of water. You have youself a poorly designed thrust reverser. As Magiver explained, the engines don’t reverse their direction of rotation, they are still generating thrust, it is simply redirected forward(ish).
An in-flight thrust reverser deployment is a very serious emergency for most airplanes. Either the reversers are way too big and place huge stresses on the airframe if deployed at high speed, or the asymmetric thrust is dangerously destabilizing to the airplane. I only know of a few airplanes that can use thrust reversers in flight. The Concorde, DC-8, and C-5 could all pull numbers 2 & 3 into reverse in flight as a way of assisting slowing down / going down. I’ve heard of one or two old Russian airplanes that have the ability as well. NASA also has (had?) their Orbiter landing trainer (a modified GII) that could deploy TR’s in flight, but that was a bit of an extreme example.
I’ve had TR’s deployed on me in the simulator on a few different types of airplanes. If it happens at a critical time it’s a handful, to say the least.
Thanks very much. Ignorance fought.
I understand perfectly in concept, and generally enough in mechanical design, but I’m having trouble visually identifying features from the stowed to deployed configurations in the pictures.
I see a back-plate extended from the engine body, but I must have trouble understanding space (I always did, which is why I always bombed on IQ tests). Where is the fish-lip component on the deployed photo?
Airflow would be right to left in these images of a clamshell reverser.
Stowed: Office of Budget and Planning
Deployed: Office of Budget and Planning
And a cascade reverser: http://www.aerospaceweb.org/question/propulsion/thrust-reverser/thrust-reverser2.gif
Spoilers aren’t necessarily critical and reverse thrust is not taken into account when calculating landing performance for airliners. On the BAe146, the landing distance increases by 33% if the spoilers aren’t working, so they are helpful but not critical.
Well, assuming you have a power source, normally the APU or a ground power cart, you select the engine you want to start, then you push a button to engage the starter, then you move the thrust lever to the fuel on position. After that the start takes care of itself, though you have to be prepared to abort the start in the even that it goes wrong.
The problem with some of the suggestions and videos is that they are from the sky. I think this sound can only be heard from the ground.
I don’t hear it when I fly. Only every night when I’m in bed. It’s not doppler, but it may be a combination of the engines revving down, the configuration change, AND being on the ground.
If I’m hearing the same sound I think the OP is speaking of… Let me see if I can describe it like this: Imagine a tie-fighter shooting a laser blast. Now super slo-mo slow it down. Pitch the note waaay down several octaves. It’s like a blast, but reallly low-pitched.