Based on previous threads, I understand that jet aircraft engines consume a lot of fuel at idle. Do they also produce an appreciable amount of thrust?
For reference, the engines on a 747-400 each produce a maximum of about 60,000 pounds of thrust. How much thrust do they produce in an idle state?
I don’t know much about jets, but the word “idle” suggests that they put out zero thrust. Just like when my car is idling, no?
No. Most cars with auto transmissions will go forward on flat pavement when the brakes are released with it in ‘gear’ (drive).
Jet engines are air pumps. If it’s running, it’s pumping air. I worked on 707’s years ago and you COULD NOT walk behind one when it was ‘idling’. Even many feet away, it would blow you over (yes, I’ve seen it).
Dunno how much ‘thrust’ though…
Jet engine in ‘ground idle’ will produce something like 10% of rated thrust. I’ll see if I can find a reference.
How much thrust do jet engines put out at idle?
Not much, in general. However, some unscrupulous airline mechanics have found that mixing a bit of ethanol in the fuel tends to increase the amount that they do put out.
The smaller Cessna Citations come equipped with thrust attenuators, a paddle that is inserted into the jet stream to deflect thrust sideways when the aircraft is on the ground. Even at idle, their engines produce enough thrust for the aircraft to build-up speed, so the attenuators keep the pilot from having to ride the brakes down the taxiway. They are used on landing to reduce thrust to the minimum and shorten the rollout, but they are not considered thrust reversers.
As I understand it WRT larger jets, they have enough thrust at idle to taxi, but not enough to overcome being stationary. So throttle up to get moving, reduce to idle to keep moving.
A couple of thousand pounds on a 767 if I’m reading correctly. See the notes to figure 6.1.1 on page 154 of this pdf from boeing
There’s some guys with an RB211 in their back garden who made the grass move with their ground run. Video link
10% sounds like a reasonable figure, hopefully AdamF can source a cite. My text books say that thrust is proportional to fuel flow but I think that is a bit simplistic. Unfortunately I know what the idle fuel flow is but not the fuel flow for rated thrust on the ground.
The BAe146 has enough to keep moving (you have to touch brakes occasionally to keep the speed under control) but not enough to get moving.
I found a good reference. Boeing 777F has GE90-110 engines, with rated thrust of 110,000 lbf. As per Boeing datasheet, these engines produce 4013 lbf at ground idle, or about 3.6%. Since four-jet airliners have less excess power, their ground idle will be somewhat higher.
Enough to knock a person down if they are foolish enough to walk behind the engine. I’ve been foolish.
I remember reading that one reason why Concorde was given priority takeoff clearance was that even at idle the engines produced enough thrust to require braking all the way to the runway. Too many delays would risk overheating the brakes.
Of course they weren’t your typical aircraft engines.
I’ve taxied behind idling jets (while they were sitting at gates) in Piper and Cessna 4-seaters and felt quite a bit of buffeting.
I’m not quite following that. If you were to say put 4 x GE90-110 engines on an aeroplane the idle thrust would still be 3.6% of maximum. I thought 4 engine aeroplanes have less excess thrust because they are only required to perform with one engine out, i.e. a loss of 25% thrust as opposed to 50% for a twin jet, but that doesn’t relate to the difference between idle and max thrust for an engine.
:dubious:
A delay means they’re just sitting there, with the wheels not moving. No heat is being dumped into the brakes under those conditions. They could be delayed an hour, and the brakes wouldn’t get warm at all.
OTOH, this site claims that the Concorde burns a lot of fuel at idle, so a takeoff delay costs more dollars per minute (in fuel) than other planes. If true, that’s pretty strong incentive to get it in the air ASAP after starting the engines.
If a plane has excessive idle thrust, you do have to ride the brakes when taxiing.
Some airlines taxi with one or more engines shut down, to save fuel.
If you had an hour-long delay and you spent it doing laps on the tarmac, then yes, I could see how this might cook the brakes. But sitting still won’t, regardless of how many engines are/aren’t running.
I think Elvis was just addressing the fact that there is enough idle thrust to require riding the brakes when moving. It can be an issue at airports with a long taxi to the runway after a short turn around.
Idle thrust is set based on how much electricity / compressed air / hydraulic power the aircraft needs on the ground. So if the same plane had 4 engines, it would still have about the same idle thrust, but lower rated thrust.
It a pure turbojet thrust is almost directly proportional to fuel flow. Which would imply idle thrust is around 5-8% of max thrust for most engines. For a turbofan, the low end thrust is proportionally less, so the cite above of 3-4% would be a good ballpark for any modern airline-sized jet engine.
As explained by others, idle thrust on all engines is usually not enough to get moving unless we’re real light weight, have them all running, and are starting off downwind with a bit of slope.
But once taxing, idle thrust on all engines is usually enough to slowly accelerate to too high a speed for safe taxi. So we periodically apply brakes back down to a slow speed and then let the speed build up again. Riding the brakes leads to shortened brake life and at the limit case, overheated brakes.
We want cool brakes at the start of takeoff since our ability to abort the takeoff successfully depends on their total energy-absorbing capability. If we start a takeoff with well-heated brakes and have to abort at max abort speed, the brakes may become “full” of energy before we get down to zero speed. At that point they fade to ineffectiveness and we roll off the end of the runway unable to influence our speed. At 20 mph that’d be embarrassing. At 75mph that might be real hard on the aircraft and the roads and cars and buildings just past the end of the airport.
We often taxi out (or in) on fewer than all engines. It saves fuel and it also saves brake wear. The countervailing point is that to get moving on fewer than all engines takes more thrust from the one(s) you do have running. Exhaust velocity, and hence the blast damage radius behind the aircraft, is more than proportional to thrust. You’re much more likely to blow over equipment or blow in building windows when pulling away with less than all engines running.
Finally, as to Concorde… They were very fuel limited. And they had engines that were very thirsty on the ground. So unlike more typical airliners, they simply could not load an extra hour’s taxi fuel on board to allow for takeoff queue congestion at JFK at peak departure times.
As well their abort scenarios were even more of a knife-edge energy balance than more typical airliners. Brake energy required goes up at the square of the speed whereat you abort. And they had abort speeds 150% of those of more typical airliners. So they really needed cool brakes & tires to start.