DaviddesJ

Agreed. Several good points there. I was just responding to a specific claim: What takes longer is for the increased thrust to increase the speed of the plane since there's a lot of inertia to overcome. So it may feel like several seconds to increase thrust. I still don't think that makes sense.

BoeingBoy

Since I'm the one that said that, what doesn't make sense? That overcoming inertia is necessary to accelerate the plane or that feeling the acceleration can take longer than relatively small changes in thrust, like from reduced T/O to full T/O thrust?

While I don't know what turbojets you've operated but I'll admit that my experience is limited to JT8D's and CFM56's - the first a low bypass turbojet and the second what's now a medium bypass turbojet. But on those engines I can assure you that the time from the engines producing reduced T/O thrust to full T/O thrust (or more) is the shortest segment of the "recognize a problem, decide to increase thrust, move thrust levers, get increased thrust" sequence.

There is inertia to overcome by the engine when thrust is increased - the rotational speed of the rotating masses in the engine has to be increased. But that's a small mass compared to the mass of the plane and consequently it's inertia. As one or two of the passengers posted here, it "felt like the plane shifted gears" when the feeling of acceleration increased. The feeling of acceleration increased.

Jim

DaviddesJ

This doesn't make sense. It's inconsistent with Newton's second law: F=m*a. The mass of the plane doesn't change, so the acceleration experienced by the plane from the engines is exactly equal to the force generated by the engines, divided by the mass of the plane. There's no "inertia to overcome". You don't need to know anything about planes to know this, just physics.

PHLGovFlyer pointed out that other things can change the forces being applied to the plane, and therefore the acceleration. For example, changes in wing configuration will change the drag and therefore the acceleration. However, these changes also take effect right away (i.e., the current acceleration is always determined by the current speed and wing configuration, not by how long it's been in that configuration).

This inertia is why it takes the engines some time to increase thrust. But it doesn't affect the acceleration of the plane based on that thrust. If the engines are producing full thrust, that means that the turbine is already spinning at its full speed. The inertia of the rotating mass inside the engine is (most of) the reason that it takes the plane a while to increase thrust after the command is given to increase thrust. But it doesn't have anything to do with how much the plane accelerates as a function of the thrust.

BoeingBoy

Good ole Isaac - an object in motion tends to remain in motion unless acted on by an outside force. That's inertia - the resistance to a change, whether speed, spin rate, whatever. You can't just ignore the inertia of a mass like an airplane with passengers, cargo/bags, fuel, etc.

At a given instant, the speed of the plane is fixed. Increasing/decreasing the speed requires a force acting on the plane - pure Isaac. With a plane taking off, you can plot the speed as it increases due to engine thrust being greater than inertia and drag - a simple line graph. Changing the angle of that line graph - increasing speed faster (aka, increasing acceleration) - requires a increase in the force acting on the plane (aka, thrust). Because of the plane's inertia, you don't get a break in the speed graph - the line doesn't jump to a new slope - you get an increase to a new line that takes a finite amount of time. But the rate of change in speed happens as the increased thrust acts on the plane's inertia.

Your problem is simple - you consider the inertia of the rotating masses of the engine but ignore the much greater inertia of the plane. Changing the rate of acceleration is, from a pure physics point of view, no different than changing the trajectory - going from level steady-state flight to a climb for instance. You first increase the angle of attack, which makes the wing produce more lift, which then results in a change in trajectory - a climb. It's a process, not an instant change. Why? Because the plane has inertia.

Jim

DaviddesJ

It doesn't matter to the passengers what speed the plane is going. You don't feel the speed, you feel the acceleration. When the acceleration changes from X to Y, the passengers feel that change immediately.

BoeingBoy

Which is what I said originally - that there is a slight delay in feeling the change in acceleration because the rate of change in speed (i.e; acceleration) is not instantaneous - and you said didn't make sense...

When I asked why it didn't make sense, you said that the plane has no inertia and only the rotating masses of the engine have inertia.

Of course, this all started when you said that it takes 2-3 seconds to change the thrust produced by the engine. From idle to meaningful thrust maybe, from reduced T/O thrust (the lowest conceivable when the accident commenced) to full rated thrust or more - no way.

Jim

DaviddesJ

I didn't say that. I said that the inertia of the airplane is irrelevant in this case; it does not take any amount of time for the engine thrust to "overcome the inertia" in order to accelerate the plane. The change in thrust of the engines is instantly felt by the passengers, as a change in acceleration.

I will find out from my engineering contacts how long it takes for the engines to increase thrust from takeoff to full throttle. I think it's longer than you think it is, but I think I can get some actual data. Does anyone here know which engine was on this plane?

BoeingBoy

Either IAE V25xx or CFM56-5Bx. All the CFM's are on East airplanes and all of the IAE's are on West airplanes except for maybe 1 or 2 - I know the Airbuses started all coming with the IAE's regardless of which side got them but I'm not sure whether any of the newest A320's came to the East.

Inertia isn't revelent? So as thrust changes when the thrust levers are advanced (which you say takes some slight time) the acceleration changes from one rate to the new rate instantly? Now we're not talking theory - forget Isaac. To the passengers I'll grant you that the time needed to accelerate to a new rate of speed change is so short - but definitely not the 2 or 3 seconds you think - that they don't sense the rate of change in acceleration. But if it took your 2-3 seconds to change thrust and some interval for the changing thrust to change the rate of acceleration (the inertia of the plane that you want to ignore despite Isaac) they might feel the changing acceleration until it stabilized at a new value.

We haven't discussed the perception of acceleration because of factors besides changing thrust, although it's been discussed in this thread. In that real world, there's changing thrust, changing pitch, and a small change in engine sound. Ole Isaac would insist on including all those. But you've said that it takes 2-3 seconds for the thrust to change but the passengers "instantly" feel the new acceleration rate.

Jim

DaviddesJ

There is no interval for the change in thrust to change the acceleration.
When the thrust changes, the acceleration changes instantly. There's some flexing of the frame as the forces are carried through the wings and body of the aircraft, but that would take only a small number of milliseconds, way less than anyone can notice.

I don't know as much as you do about aircraft, but I did study physics at MIT.

BoeingBoy

Maybe that's the problem. You're thinking theory and I'm giving you reality.

1st, the thrust doesn't change instantly. It takes a small amount of time to move the thrust levers from one position to another - you don't just slam the thrust levers forward since that would overboost the engine.

Then there's the plane's inertia - changing from one acceleration to a greater acceleration requires changing the amount of inertia from one level to another - and when you're dealing with 75 tons or more of mass it does take some finite amount of time. Even you said that changing thrust isn't instantaneous so how can the change in acceleration rates be instantaneous? Even MIT professors should know that unless they're teaching pure mass-less theory. Isaac would know better.

Finally there's passenger perception. How many passengers recognize the change in acceleration between when T/O thrust is applied to when the plane takes off for a normal T/O? I'd bet not many if any although the acceleration does change slightly. How many recognize the changes in acceleration as the nose is raised and the plane becomes airborne? Between becoming airborne and the gear being up? Again, not many if any although the acceleration does change. As I said earlier, the passengers almost certainly would only perceive a change in acceleration, not the increase as it happened between one T/O thrust setting and another, between one pitch angle and another, between gear down and gear up. But perception is not reality. If you want to delve into perception, that's a whole different discussion.

However, it appears that neither of us will convince the other. You're convinced that your MIT education gave you all the answers while I'm convinced that my 27 years of experience and college applied physics classes gave me some of the answers - and no, I didn't go to MIT but to a state college in backwards NC.

Jim

PHLGovFlyer

Gentlemen, you appear to be talking past each other.

Jim, you're correct that the engines take some time to increase thrust, commonly referred to as "spool up time". This is where the inertia of the moving parts of the engines comes into play as you mention. Simply dumping more gas into the combustion section of the turbine does not instantly increase thrust output. I'm not expert on the engines used on the A320 but I'd guess that spool up from idle to max thrust is at least 4 to 5 seconds and that spool up from nominal takeoff thrust to max thrust is in the ballpark of 1 second as you mentioned.

David, you're correct that as thrust changes it affects the acceleration of the aircraft immediately. However, you also have to consider drag.

Jamming the throttles to the max does not always result in a particular net acceleration of the aircraft, even after max thrust is reached. Acceleration will be lower if drag forces are high. Acceleration is also lower if the mass of the aircraft is high. The real issue is that net longitudinal forces on an aircraft can vary considerably (as I describe in my post above), even at max thrust. An aircraft can even decelerate after the engines reach max thrust output if the drag forces are high enough (though it would be unusual, e.g. an accident or edge of the flight envelope scenario...). This depends largely on configuration (flaps, slats, gear, etc.) and angle of attack.

The inertia of the plane (the plane's mass really) is pretty much constant for a short duration event like the A320 accident. However, the NET forces on the plane likely changed very rapidly, much more rapidly than engine thrust could be changed. The change in those net forces was also likely extraordinarily large and rapid compared to typical takeoff operations so it's natural that the passengers would be disoriented.

As a side note, I often enjoy closing my eyes about midway through a takeoff roll and trying to perceive the aircraft's attitude (pitch and roll primarily) about 30 seconds after liftoff. It's amazing how having your eyes closed for just a short period during a normal takeoff can completely mislead your sense of balance.

P.S.: My degree is in Aerospace Engineering so I guess I got the best of both worlds

phlwookie

FWIW, this incident was mentioned on another board as having some similarities: http://www.ntsb.gov/doclib/recletter.../A02_06_07.pdf

Also, on http://www.avherald.com/h?article=471583da&opt=0 there is this update (bolding mine): "The French BEA reported in their weekly bulletin released on Mar 18th 2014, that the aircraft was at about 20 feet AGL when the takeoff was rejected. During the rejected takeoff the nose gear collapsed and the aircraft slightly veered off the side of the runway. The passengers were evacuated. Initial examination showed foreign object ingestion into engine #1." See that link for a bit more, but we might finally have a reason for rejecting the takeoff: perhaps tire debris got into the engine in a noticeable manner. I supposed FOD ingestion could have happened later, though, so this is still speculative.

BoeingBoy

That is the case with the 727-100 on an approach with only one engine operating. Even max thrust will not produce acceleration, but rather deceleration, in level flight. That's because the airplane is what's generally called "behind the power curve" - a term you may have heard in your Aerospace Engineering training or since. It's another of those student pilot exercises used to demonstrate that the plane can be in low speed/high power situations where the only way out is to trade altitude for speed - descend to get in front of the power curve then climb.

On the 727-100 with only one engine operating and gear/partial flaps extended (normal landing flaps aren't used) the procedure is to apply max rated power, continue the descent to accelerate while cleaning up the gear/flaps and once single engine climb speed is reached transition to the climb. Obviously you need enough altitude to trade altitude for speed before reaching the ground.

The 727-200 was different - the nose gear was heavier and retracted forward. On that plane putting the gear down was the point where you were committed to landing.

One thing I still stand by, although from a passenger perception angle it doesn't really make any difference. Since thrust doesn't increase instantly, the acceleration rate doesn't change from the value reduced thrust gives to the value rated thrust gives instantly either. You can't include the moment of inertia of the rotating parts of the engine but omit the inertia due to the planes weight. It does change in a short period of time, so I've stated that the passengers may perceive an instant jump in acceleration from one value to the other. But perceived is not fact in this case.

While I don't know anything about the IAE engines, the can't be that different than the CFM's and based on that the change from full reduced thrust to rated thrust is probably more like half a second. Reduced thrust is something like 90% or more of rated thrust and it's distinctly different than initial spool up from idle to 20% rated thrust.

Jim

DaviddesJ

This statement just doesn't make any sense. I think you should know what you don't know, but I'm not going to convince you that you have some serious misunderstandings about basic physics, so I'm done responding to this. It's not like it's that important anyway. If I find out the answer to the time the engines take to increase thrust, I'll post it.

DaviddesJ

Yeah, you pointed that out already, and I agree entirely.