take off rev hold
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BINGO! Boeing has said pretty much the same thing - there is no meaningful performance advantage to static takeoff, full power statically is really hard on the engine (actually banned on some engine types), and there is not an insignificant risk of an engine surge if there is any side or tailwind present (inlet separation).
Inlets don't work particularly well without some forward airspeed. As a result, when sitting still, it doesn't take much of a crosswind (or worse, tailwind) to cause the inlet to start to separate when at high power - and a separated inlet can easily lead to an engine stall or surge (which are really hard on the engine). You're also more apt to suck up FOD when sitting statically than with some forward speed.
Back pre-FADEC, it was necessary to run the engines to takeoff power to check for proper operation of the bleeds and vanes - often requiring 'trim' adjustments to the hydromechanical control. At one time, I was involved in the engine run functional test requirements and would sometimes sit in for the engine runs. They'd always try to position the aircraft into the wind before the run, but that wasn't always practical - the engine run guys had developed a finely tuned ear to listen for inlet separation during the runs when the winds were less than favorable. Originally I was skeptical that you could actually hear the inlet separate with all the noise and vibration of an engine at takeoff - but then I was sitting in on an engine run and I started to hear this unusually growling noise - I was just about to say something like 'what's that' when the engine run guy snapped the throttle to idle - I'd just heard inlet separation...
Back pre-FADEC, it was necessary to run the engines to takeoff power to check for proper operation of the bleeds and vanes - often requiring 'trim' adjustments to the hydromechanical control. At one time, I was involved in the engine run functional test requirements and would sometimes sit in for the engine runs. They'd always try to position the aircraft into the wind before the run, but that wasn't always practical - the engine run guys had developed a finely tuned ear to listen for inlet separation during the runs when the winds were less than favorable. Originally I was skeptical that you could actually hear the inlet separate with all the noise and vibration of an engine at takeoff - but then I was sitting in on an engine run and I started to hear this unusually growling noise - I was just about to say something like 'what's that' when the engine run guy snapped the throttle to idle - I'd just heard inlet separation...
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Your posts from the flight test world are an endless source of fascination. Thank you. When you talk about "separation", am I right in understanding that the airflow separates somehow? If so, I'm having trouble visualising how airflow could separate in front of a what' essentially a giant vacuum.
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Your posts from the flight test world are an endless source of fascination. Thank you. When you talk about "separation", am I right in understanding that the airflow separates somehow? If so, I'm having trouble visualising how airflow could separate in front of a what' essentially a giant vacuum.
When going fast, the airflow is coming from straight ahead, actually expanding outward kind of as if the engine pod is just a blunt front end of a cigar and the air has to go around it. This is the design condition, and the cowl is designed for that with a slightly inward-turned lip to match the air expanding into it.
The slower you go, the more it sucks air from all around rather than straight ahead, so air comes at the lip inward from the outside. The most extreme of this, a standstill, has air coming in from all directions and is why you see engines on test stands (and some helicopters) have these big tuba horns put on the front to allow the air to flow in smoothly. A standard cowl can't do that, so the air has to make a nasty sharp turn around the lip, and it doesn't take much upset to stall it.
http://edallcoin.fatcow.com/ecollege..._image0024.jpg
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Hi Check Airman. Check this out. Pretty much what Tdracer explained with one example of an incident and some graphs.
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That being said, I'm just an interested nerd, who's also waiting for Tdacer to expand on, and fix any inaccuracies. For one, I think it might be that the design condition has the air coming from straight ahead (the slug of air that enters the inlet has the same diameter as it does far ahead before any disturbance) while the expanding flow is at a speed faster than design. And the primary reason the inlet expands is to slow down the air and increase its pressure. And I'm wondering how this interacts with the "matching" of the airflow field at the lip I mentioned earlier.
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Engine Thrust Management – Thrust Setting at Takeoff | Safety First (airbus.com)
Hi Check Airman. Check this out. Pretty much what Tdracer explained with one example of an incident and some graphs.
Hi Check Airman. Check this out. Pretty much what Tdracer explained with one example of an incident and some graphs.
That being said, I'm just an interested nerd, who's also waiting for Tdacer to expand on, and fix any inaccuracies. For one, I think it might be that the design condition has the air coming from straight ahead (the slug of air that enters the inlet has the same diameter as it does far ahead before any disturbance) while the expanding flow is at a speed faster than design. And the primary reason the inlet expands is to slow down the air and increase its pressure. And I'm wondering how this interacts with the "matching" of the airflow field at the lip I mentioned earlier.
Inlets are designed to work very well and be very efficient at cruise - not takeoff. When sitting statically, as you explained the air is coming from all around - not just straight ahead - and the air to the side or rear has to turn a pretty sharp corner going around that inlet lip, that's where the inlet can separate (it can also happen if you over-rotate on takeoff - the bottom of the inlet lip can separate and cause an engine surge at a really bad time). Fans really dislike lots of distortion at the fan face - and in severe cases that distortion can even make it to the core compressor which likes it even less...
Now, this can all be made better by putting a bigger radius on the inlet lip (i.e. making the inlet thicker/fatter) - but - that makes the inlet less efficient and increases cruise drag.
One solution that has been used is called 'blow-in doors' - spring loaded doors in the inlet that open inward due to the suction at low airspeeds so the air doesn't have to turn that sharp corner around the inlet. These are used on some military aircraft (I believe the Harrier had them) and were on the early 747s. The 747 was originally designed to cruise at Mach 0.87 and higher - so it had a very thin inlet lip - but that was prone to separation at low speed so they added blow-in doors. But that created a new problem - blow-in doors are noisy. Not only do they remove some of the acoustic treatment in the inlet, they create vortices that enter the fan and that creates a lot of noise. After the 747 entered service, it was determined that it seldom cruised that fast, so the later 747s got a redesigned inlet with a thicker inlet lip that didn't need blow-in doors.
