Why Does A Plane Seem to Slow...

Factual Questions
1

Shortly after takeoff, a commercial jet will seem to slow down. Why is this? I have several WAGS, but none convince me. First, the law of inertia. The plane does not need to produce the same amount of thrust to keep the object in motion. Also, I read how cooler air, being more dense, does not require a plane to go as fast to yield the same amount of lift as in hot (less dense) air. So, it can reduce speed and maintain the same lift (assuming air at 10,000-35,000 ft IS significantly cooler).

To the contrary, I read a commercial jet must reach about 120mph to take off. But, it will cruise at 550-600mph once above 10,000 ft before which there is a safety “speed limit” (perhaps to reduce wake in congested skies?) If this is true, a plane would need to speed up, not slow down!

Last, could it be an illusion? At those high elevations, it is hard to judge your speed. Familiar landmarks are but on the scale of toys, and the horizon expands outward significantly. Thus, you might not realize how fast you’re going!

So, what’s the SD on this phenomenon?

2

I’ve watched a lot of planes take off. They definitely don’t seem to slow down after TO, so whatever it is you’re experiencing as a passenger, I have to vote “illusion.”

3

[QUOTE=Q.E.D.]
I’ve watched a lot of planes take off. They definitely don’t seem to slow down after TO, so whatever it is you’re experiencing as a passenger, I have to vote “illusion.”
[/QUOTE]

Actually, for the record, I should add: A book sold in a “Boeing Store” in Arlington, VA claims it will answer this question within its ~30 pages, but never did. In the least, I am not the only one to experience this feeling. (The cover’s designer was out of synch with the author, I guess.) - Jinx

4

Yeah. Take-off in a large airliner is pure sex, just like a motorcycle. There’s the push in the small of the back, the deep roar of the engines, the vibrations up through the landing gear. The moment of take off, however, has always been anticlimactic for me. The vibrations stop, and you get a sense of “this is the beginning of the next few hours of The Great Boringness”. And, of course, you no longer have grass and bitumen whizzing by your window in a blur.

5

[QUOTE=Jinx]
Thus, I am not the only one to experience this feeling.
[/QUOTE]

Pretty sure I didn’t say you were.

6

At many airports the aircraft actually do slow down after take-off for noise abatement.

7

It’s probably the plane leveling off. You misinterpret the resultant vector of the airplane pitch and gravity as being due to acceleration. When the nose comes down you feel like the plane has slowed.

8

I don’t think they’re slowing down, but they are throttling back. I’m sure one of our resident airline transport pilots will be here shortly, but my understanding of flying is high thrust to get off the ground, but slowing the engine speed down a bit after the initial climb to keep the aircraft under the speed limits (<250 kts below 10,000’, <280 kts below 18,000’). If the higher takeoff thrust was maintained, the aircraft would plow through the speed limits on its way up.

In my defense if that’s inaccurate, I don’t fly 'em, I only put 'em out when they crash.

9

[QUOTE=Jinx]
Shortly after takeoff, a commercial jet will seem to slow down. Why is this?
[/quote]

During take-off, you’re accelerating at a pretty fast rate. Even in a car, when you go from 0 to 60 in a few seconds it seems like you’re really moving. Cruise along at a steady 60 mph and it seems like you’re not going fast at all.

[QUOTE=Jinx]
Familiar landmarks are but on the scale of toys, and the horizon expands outward significantly. Thus, you might not realize how fast you’re going!

[/QUOTE]

Sure. Fly past a house a few feet in the air and it’s there one second and gone the next. Fly over a house hundreds of feet in the air and it’s in your view for a while. The former is going to seem like you’re going a lot faster.

10

Jet airplanes absolutely hate low altitudes from an efficiency standpoint. They need to get to the higher altitudes and thinner air as fast as possible to make them efficient. Anything below that is just $$$$ coming out of the exhaust and that is both terrible for the airline company as well as the speed to the destination. Pilots definitely don’t want to putter around at lower altitudes any second longer than they have to.

11

I am not a physicist or a pilot, but: As the climb gets steeper, the forward speed component is reduced. And vertical acceleration is more resisted by gravity than horizontal acceleration is reduced by drag, so vertical speed may not grow as much as horizontal speed drops off.

12

[QUOTE=KCB615]
I don’t think they’re slowing down, but they are throttling back. I’m sure one of our resident airline transport pilots will be here shortly
[/QUOTE]

I guess that would be me.

As far as slowing goes, the answer is no- airliners do not slow after takeoff but it may seem that way for any number of reasons. At my company there is a profile that is flown on most takeoffs that may look similar to this dependent on aircraft weight.

0-130kts- Runway acceleration to liftoff
145-150kts- Initial Climb
@1000 feet above ground acceleration to 200 kts
@3000 feet above ground acceleration to 250 kts
(In USA 250kts is the speed limit below 10,000 feet)
above 10,000 feet speed increases again to 290 knots or faster

Many times on a departure there are particular restrictions on altitude that has to do with the local airspace requirements. Once the aircraft reaches its initial assigned altitude of say 5000 feet, you feel a level off and the engines will decrease power and you may feel whats like deceleration as you are feeling a few different forces at once. The engines need to produce but a fraction of the original takeoff power to stay level at 250kts.

Using the profile above: the initial assigned altitude at takeoff was 3000 feet which coincidentally is the acceleration altitude to 250kts. In this example you would certainly feel a deceleration because you are leaving a state of acceleration. Once the aircraft is level and not accelerating it is said to be balanced, very similar to being in a car on the highway.

[QUOTE=Jinx]
Also, I read how cooler air, being more dense, does not require a plane to go as fast to yield the same amount of lift as in hot (less dense) air. So, it can reduce speed and maintain the same lift (assuming air at 10,000-35,000 ft IS significantly cooler).
[/QUOTE]

I believe you are referring to what is known as density altitude, this in itself doesn’t have a whole lot to do with your original question. When it comes to higher altitude there are less particles of air per the same area at 30000 feet as opposed to sea level which is why an aircraft will fly at 480kts (true airspeed) where in the cockpit we will be indicating 300kts (indicated airspeed.) Higher still then Mach becomes a factor- that is based on temperature.

With all that said temperature can have a big effect on aircraft performance but it does not have an appreciable affect on what it feels like in the back as a passenger.

In the end an airliner is a high performance machine with a lot of power, and when we pilots in front add power or remove it it’s hard to not notice especially in the takeoff and landing phases due to the large speed changes.

13

You need to think of this in terms acceleration. You only feel acceleration. So I don’t think that it’s slowing down. That would be ridiculous. What it probably is, is reduced acceleration. When you get past takeoff but yet still not yet to cruising altitude you have to stop accelerating. It’s like when you go from 0 to 60. Once you hit 60 and you stop accelerating you are thrown forward. So no, they don’t slow down, they stop accelerating until they get past the required 10,000 feet.

14

[QUOTE=TheLoadedDog]
The vibrations stop, and you get a sense of “this is the beginning of the next few hours of The Great Boringness”. And, of course, you no longer have grass and bitumen whizzing by your window in a blur.
[/QUOTE]

I’ve only flown in a commercial airliner twice and that was a there and return journey from Belfast to Liverpool, so I’m a newcomer to plane travel still. It means the buzz goes on for a little longer, when you’re just off the ground and the plane is moving slightly from what feels like side to side and you know your off the ground and fighting gravity with aerodynamics :smiley:

It helps that check in in Belfast is so straightforward, few queues, relatively empty departure lounge, I finished off most of the novel Lolita waiting for my flight.

15

Nice intro post, ricec4ke!

16

Even in a fast car, raw acceleration is awesome. Going from 0-120 MPH in a quarter mile in 11 seconds feels like riding a beast. You won’t forget it!

Cruising at 120 MPH on a huge open stretch of road in a competent car will feel completely…well…boring. Wow…dull even.

On a plane, the whole effect described above is magnified. The accel to speed is much much more dramatic, and the cruising is even more boring because of the lack of reference points and the overall smoothness of steady flight.

Heck, in a car, even 0-60 can seem like a wild ride. Cruising at 75? ZZZZZZZZZZZZZZZZZ

17

Of course, the closer you are to the ground, the faster speed feels. Cruising at 120 MPH in a car might be boring, but if you’re in a street luge…

18

Technically, speed doesn’t “feel” fast at all. You feel acceleration, but you don’t feel anything if you’re doing a fixed speed, even if that fixed speed is 500 mph.

19

[QUOTE=Merkwurdigliebe]
You need to think of this in terms acceleration. You only feel acceleration. So I don’t think that it’s slowing down. That would be ridiculous. What it probably is, is reduced acceleration. When you get past takeoff but yet still not yet to cruising altitude you have to stop accelerating. It’s like when you go from 0 to 60. Once you hit 60 and you stop accelerating you are thrown forward. So no, they don’t slow down, they stop accelerating until they get past the required 10,000 feet.
[/QUOTE]

Or perhaps more accurately, there is a reduction in accelleration. Failure to accellerate in any way would probably result in falling. :wink:

20

[QUOTE=DSYoungEsq]
Or perhaps more accurately, there is a reduction in accelleration. Failure to accellerate in any way would probably result in falling. :wink:
[/QUOTE]

Eh?

If you mean accelerating in that they’re cancelling the downward acceleration of gravity, then sure. But the plane doesn’t necessarily need to accelerate past speed X - just maintain its airspeed.