Two plane qunestions

Factual Questions
1

  1. So, I’m sitting in my window seat this morning, waiting for us to get out to the runway and carry me far away from New Jersey. The engines start up, and out of the one I can see (I’m sitting a row or two behind the wing), comes a big cloud of…something. It didn’t look particularly like smoke, more like steam or car exhaust. I’ve never seeen it on a plane, before, though, and was just curious. Do plane engines do the same type of exhaust-cloud thing as car engines? And also, why belch it all out at once?

  2. This is a more abstract one. How steeply, generally, can the average commercial airliner bank? As we took off from Minneapolis, it seemed that the moment the wheels left the ground, we were turning, hard - it felt like the wings were at maybe a 70-degree angle to the ground. I found it mildly unsettling, becuase from what I know of the physics of flight, that makes a crash a lot more likely. I know, probably not going to happen, but it could.

Both planes, IIRC, were A300s or something similar (large planes, underwing engines, 3-aisle-3 seating arrangement).

2

I’ll let a pilot ahand the second question, but as to your first, yes, jet engines produce hot gasses and water vapor just like any other combustion engine. You can see this exhaust on cold days when the engine first starts, just like you did, but the exhaust on a jet engine heats up very quickly, so that you no longer can see the condensed water vapor, because it’s too hot - just like a car that’s been running for several minutes.

3

For #2, an airliner can (theoretically) bank as much as any other airplane, even fly upside down. The airline industry frowns on this practice. Yes, they might be able to collect a lot of spare change when they clean up the plane afterwards, but most passengers would find it a little uncomfortable, and complain. :eek: :eek: :eek:

4

Jet engines do produce exhaust. I’ve never flown big planes, but it’s not surprising that going to takeoff power on the runway (more or less like flooring it in your car) would produce a bit more visible exhaust. But I’ll let a mechanic answer that in more detail.

On the other question the bank of the airplane is related to the Gs that are being felt by the passengers. If, in straight and level flight, a plane were to turn at a 60 degree bank (ie, the angle between the horizon and each wing is 60 degrees), and the plane remains level, the passengers would be feeling 2 Gs. IIRC, your plane would be certificated to withstand at least 4.5 Gs, but I can’t imagine that any pilot would ever subject his passengers to anything like a 45 degree bank. It would cause airplane-unfriendly people a bit of discomfort.

5

But there’s two other considerations here. First, an airliner could not maintain a bank angle that would produce 9 Gs (that’d be something like 88 degrees of bank) because its wings might soon depart the fuselage.

Second, wings have different efficiencies. As bank angle increases, stall speed also increases. (A stall is when a wing stops producing lift because the airflow over the wing is no longer sufficient.) I’m no engineer, but I’m pretty sure an airliner would stall very quickly if an extreme bank were sustained for any amount of time. There was once a large Boeing airliner that was rolled in flight testing, but I’m talking about making a turn with a high bank angle.

6

Most of the time, a large jet will bank at about 30 degrees, sometimes as much as 45. I know you thought you were in a 70 degree bank, but it was probably between 30 and 45. It just SEEMS like more. I suppose it could have been as high as 60 degrees, but trust me, that’s a very steep turn in an airplane.

7

This isn’t a large jet, and they have a good reason to turn quickly, but still they keep the angle well below 60°:

Landing in Baghdad Sept, 2003

8

Ah, interesting.

With regards to banking: say the plane was banking at a steeper-than-usual angle. It was immediately after takeoff - we were climbing steeply as well, which is possibly why it seemed so steep. Is there any sort of ‘situation’ in which sharper-than-usual turn is called for, or is it just sloppy flying on the part of the pilot? I mean, I recall a thread about aborted approaches/landings and the reasons for that (ie, obstacles on the runway), but I can’t think of anything that would be in the way once you’re out of the air.

Also (to hijack my own thread): I’ve noticed this every time I fly, but especially if my seat is towards the back of the plane. You pull up at the gate, everyone scrambles to get off the plane, and meanwhile, from down beneath the passenger compartment, there’s a series of thuds and a sort of vibration-like noise: it sort of is reminiscent sound-wise of a dental drill or something. Always wondered what it was - anyone?

(I could ask my sister who’s a pilot, but A: she laughs at me for my intense dislike of flying and the trouble I have when I do fly, and B: probably doesn’t know the answer to my most recent question.)

9

On this page there are a collection of cool aircraft videos - including the one of the Dash-80 (the Boeing 707 prototype) executing a barrel-roll.

10

The ground crew removing the baggage from the hold.

11

I am not a jet pilot, but I’ll take a stab at both questions:

Number 1, the exhaust question: yes, airplanes engines (of any sort) generate exhaust, which is more visible on cold mornings than warm. While I don’t know if it is the case in this situation, unburned jet fuel exiting an engine also looks like smoke. I’m presuming that jet engines do not start with 100% efficiency, so it is possible that it emitted a certain amount of unburned fuel on initial start, along with the usual exhaust products. Once the engine is up and running this would no longer be happening.

Number 2, the bank angle question: a little more complicated.

Actually, I don’t think jetliners are stressed for sustained inverted flight, or sustained extreme banks in level flight. There is a significant difference between momentary inversion and continued inversion, for instance.

Getting back to the (in)famous barrel-rolled 707… during a properly executed barrel roll the stress on the airplane does not exceed 1 positive g. The physics is complicated, but the effect is that even while the airplane is inverted in such a roll the force felt inside the airplane is towards the airplane’s floor - and when inverted, exactly opposite that of Earth’s gravity. You can experience the same effect on roller coasters that turn riders upside-down.

During a normal turn in level flight, the pilot uses the controls to bank the plane and keep the “down” force down from the perspective of the occupants of the aircraft. In level flight, the greater the bank the greater the “down” force required (which, when banked is not really “down” in the sense of “towards the center of the Earth”, but we’re talking about down relative to the airplane’s designated floor and ceiling). At 15 degrees of bank the increase is unnoticable to human perception. At 30 you start to feel it, if you’re paying attention. At 45 you will certainly notice it. I don’t remember the exact figures for anything but 60 degrees of bank, which in level flight gives you a force of 2 g’s, twice that of normal gravity.

But that’s level flight. A banked turn while climbing increases the force imposed on the airplane, so a climbing turn in a 60 degree bank is greater than 2 g’s. A descending turn decreases the force on the airplane, so an airplane that couldn’t maintain controlled flight in a level 80 bank could continue controlled flight in a descending 80 degree bank (although it might be quite a steep descent) because that decreases the stress involved. (That’s stress on the airplane - stress on the passengers is another matter entirely :D…)

So, a climbing turn might feel steeper because of the difference feel of the forces involved. Likewise, a descending turn might look pretty darn scary if you can see the ground, but may be perfectly safe. (When I have passengers and need to make a steeper than normal turn I tell them to look up at the sky - with fewer points of reference the view is much less alarming to the non-pilot)

A pilot would do that to avoid a collision. Probably for no other reason.

As mentioned, the climb might have made the turn feel more extreme than it actually was.

As for after take-off - it is possible that there was a “traffic issue”. The pilot might have needed to make a tighter than normal turn to avoid other traffic

There are some airports that have obstacles close to the field, which might require a steeper than normal turn to maintain an adequate buffer zone between the aircraft and the hard object. This is getting to be more and more of an issue, what with cell towers and tall buildings going up everywhere. Some developers seem hazy on the idea that it’s bad for business for airplanes to become entangled in towers and buildings.

Some localities have very stringent anti-noise ordinances. This may require an airplane to turn sharply, or climb very steeply, to avoid directly overflying a “noise-sensitive” area. The people who make these rules are concerned for the comfort of those on the ground, not those in the air.

“Sloppy flying” is unlikely in the airlines.

12

How about a complete loop? Not sure what the term is, but the nose continuously raises till going straight up, then the plane continues to loop in the same direction till upside down, then pointing straight down, then levels off. Can passenger jets do that?

13

Aircraft are full of hydraulic motors, which make the “dental drill” sound. Motors actuate the large hatches (as for cargo), wheels, flaps and so on.

And on the subject of turns and g-forces, the g-force created by a turn is dependent on the aircraft speed and the tightness of the turn, not the angle of bank. But banking, through textbook aerodynamics, normally produces a turning effect; the “lift” vector from the wings is offset from the vertical, naturally pulling the aircraft into a turn. Still, it is controllable; military pilots can make extended banking turns and not have to worry about the g-forces that a tight turn at the same angle and speed would produce.

The OP concern was that the aircraft would lose lift, and that can be a real concern in steep banks; the airfoil lift vector tilts with the aircraft, and at some point, the vertical component will be less than the weight of the airplane, and you will sideslip. That is one reason why such maneouvers are best done at altitude, where there is room for this to happen.

14

Hampster test.

15

Airline pilot here …

A lot of folks have come up with great answers to NinjaChick’s questions, so I’ll try to hit the high points & correct the couple of misstatements.

Good job Broomstick. Bravo.

Original question 1: Jet fuel doesn’t vaporize nearly as easily as gasoline. As a result, getting a jet engine started requires a VERY rich mixture, and a lot of unburned fuel is ejected from the tailpipe. That looks like white smoke. Different engine models are more or less prone to it, and all do it more on cold mornings than warm. The Rolls Royce engines on the L-1011 were famous for producing huge clouds of startup smoke. Some 757s are equipped with a variant of that engine which also smokes a bit more than most.

All that is only at startup. Once the fire is lit, all jet engines run quite lean, as that’s the way to get good fuel economy which is just about the most important design metric.
Original question #2: The max bank angle you would have experienced after takeoff is 30 degrees, and it was probably more like 20. It feels steep because you’re close to the ground, nervous, etc. How rapidly they roll into the bank has a lot to do with how it feels as well.

You didn’t say which airport you left NJ from , but if it was LGA or JFK, there are elaborate departure routings to try to avoid flying over houses (fat chance in NYC!). These require relatively strong manuevering within a few seconds of liftoff. Banks angles of 20 degrees are all that’s needed, but you do start manuevering much closer to the ground (say 400’) than is typical (1500’) of other airports.

It’s nothing to get excited about; the 100 airplanes that took off on that runway ahead of you, and the next 100 after you, will all do exactly the same thing, hour after hour, day after day, year after year.
Question 3: The thudding from below after parking is the ramp folks unlocking the cargo door, raising it, then crawlng around in there to retrieve all the baggage and cargo. The drill-motor like noise is the electric motor & transmission that raises the door. On large airplanes, like the A300 you mentioned, those doors are the size of household 1-car garage doors & quite heavy.
BobLibDem: Nope. A loop won’t work in a big jet. Others have explained the difference between bank angle and G-forces, so I won’t go into that. If a jetliner tried a loop they’d make a mess.

They typical way to do a loop is to start out in a slight descent at max power, then when you hit absolute max speed you level off & begin to pull up in the loop. You can only pull up as rapidly as the strenght of the aircraft will permit. Pull too hard & the wings or tail break off.

So essentially it becomes a race between how long it takes to turn through 180 degrees to make the first half of the loop and the rate at which speed is being lost during the steep climb, net of the engines’ best efforts to add more speed.

Airliners are 2.5G max airplanes. That means you need a gradual gentle pullup, not at all like an airshow airplane. And the engines are powerful, but not nearly as powerful per aircraft weight, as those on airshow planes or fighters.

Result: if you try a loop, you’ll run out of airspeed long before you get over the top. Somewhere around 70 or 80 degrees nose up you’ll run out of speed. You need to get to 180 degrees without running out of speed. Waay short.

Once you’re out of speed, the airplane will fall. If the ground is far enough away, you may be able to get it pointed the way it’s falling and regain control. If not, not.

You’ll also have the problem that the engines may hiccup & quit during all that low speed flailing, or break off the airplane. During the falling phase you may lose hydraulic power due to fluid sloshing in the pump reservoirs, which eliminates your ability to control the plane at all. Finally, the controls aren’t that big for the size of the aircraft, and if it really starts flailing around, the inertia of all that mass may be greater than the control power available.

Eventually it’ll settle down into more-or-less plummetting earthward nose first. Then you have an excellent chance to regain control if the machine is still intact.

But it might have to fall 3 or 4 miles before that happens. And depending on the speed at that point, it might take anoother 2 or 3 miles of room to recover from the dive to level flight. Last of all, durng that recovery you’re going to be accelerating like a banshee. At some point you may get going so fast that parts start to peel off.

All in all, a loop is NOT a good idea.

16

Wow, thanks Broomstick, LSLGuy, lots of info.

LSLGuy - the sharp-seeming turn wasn’t actually leaving NJ (I almost always fly out of Philly, actually, becuase it only take about half an hour to get to the airport from my house), but Minneapolis. And I believe that there’s a similar deal with pilots needing to turn and whatnot faster than usual out of Philly as well. I bet that could’ve been the case in Minneapolis.

17

As Broomstick says, many places have noise-abatement policies. For example, pilots departing SMO have to ‘dogleg’ over the golf course at the end of the runway so as not to fly over the houses.

IIRC, jets taking off eastbound from LAX (when the wind requires it – usually they take off over the ocean) have to reduce power to reduce the noise a little.

18

On the L-1011 the engine would be a variant of the RB211.
Apart from normal in- service start ups and the usual smoky emissions, we would be required to do test run after fitting a new engine. It was always best to inform the tower when this was going to happen!
The inhibiting oil protecting the engine could never all be removed so the first start of a new engine was always spectacular. If the tower weren’t aware of this, seeing an aircraft completely hidden by clouds of smoke used to make them call out the fire service. Our record was 14 fire vehicles arriving at the run pen.

19

Incorrect.

In a level turn G-force is dependent on the angle of bank. Speed is not the determining factor. I’ve done 60 degree banks at 45 mph and at 200 mph - they are ALL 2 g’s

Specifically, the g-force generated in a coordinated, level turn is related to the cosine of the angle of bank. Every time. Regardless of speed.

As far as “tightness” of turn - that usually varies with the speed, yes. A 60 degree bank at 100 mph will have a shorter radius than the same bank at 200 mph. You probably could use the elevator to artificially tighten the turn but it’s certainly not the normal procedure, and it will just put you closer to a stall, which you don’t want in a steep bank anyway. And it will increase the g-forces even further by adding more force the the turning vector.

Actually, the lift vector is usually split in two - one vector to hold you up against Earth’s gravity, the other vector being what actually turns you. In level flight, the two vectors are 1 (what holds you up) + 0 (because there is no turning force). In a turn, it’s always 1 + [the necessary vector to turn].

Well, yes, as long as the pilot is concious, the wing is not stalled, and you don’t have some sort of aerodynamic flutter going on the aircraft is controllable - where you get problems is when (most commonly) the pilot stalls the aircraft while in a steep bank (it’s can be an interesting ride), or the g forces cause the pilot to pass out (always a Bad Thing), or, as LSL Guy put it, you get up so much speed parts start to peel off the airplane (which is also a Bad Thing). However, I can tell you from experience that you do have to exert more physical effort to fly while in a 60 degree bank than while straight and level. Just for starters, your arms weigh twice as much but you still have to hold them out in front of you.

Civilian pilots can make extended, banking turns, too - but you have to respect the limits of both yourself and your airplane. As part of my primary training I was required to demonstrate my ability to enter and maintain steep banks with precision and grace - and exit them with similar skill. If the military guys beat us civilians out it’s because their airplanes have much more power and they’re wearing g-suits. In all cases, the physics are the same.

Mmm… not exactly. If you’re trying to describe a stall, it’s when the angle of attack of the wing exceeds a critical angle (which varies based on the airfoil) at which point the airflow over the wing ceases to be laminar and becomes turbulent, which pretty much kills your lift. You don’t sideslip, you fall. If one wing stalls before the other (quite possible, particularly in steep turns where it becomes more difficult to maintain proper coordination between dimished handling characteristics at high angles of attack and the g-forces you have to deal with) you can set up a very nice rotation, which is how you get a spin. You don’t sideslip, you roll - in an ucoordinated stall out of a steep bank you might roll inverted. Among other things.

Lest you non-pilots panic - even as a student pilot I was taught to keep all turns properly coordinated. If you did stall from a steep turn, as long as the turn was coordinated, it’s not a screaming carnival ride. You’ll notice something, but the pilot will very quickly (in a second or two) regain control. No rolling. I was making coordinated 60 degree banks, and “accelerated stalls” from 50 degree banks as a student pilot and maintaining control. The guys flying the airlines are MUCH more experienced and better trained than I am, I would expect them to do even better.

Mind you, that was expected manuvers in training - when things unexpectedly happen it’s a slightly different matter. Even so, as long as the airplane is intact the pilot should regain control in seconds even if something does happen. Since I fly very small, very lightweight airplanes I have certainly had my share of “upsets” and managing them is not a big deal. That’s what all the training is for. The biggest danger low to the ground is that there is a such a short interval between something going wrong and the ground - which is why standard take off procedure is to go up as rapidly as practical. Altitude is your friend, it gives you time in which to act.

20

It may be true that in actual piloting, the g-force is the same because the turn is flown in such a way as to make it the same (by coordinating the bank angle, speed and radius). But the g-force tending to slide you outward along your seat is (mv^2/r) times the cosine of the bank angle, so it actually depends on speed more strongly than on bank angle. The fact that piloting practice prevents you from independently varying the speed much doesn’t mean speed is irrelevant.

For your first statement to be true, your turning radius in the 200 mph turn would have to be about 20 times what it was for the 45 mph turn - does that sound about right?