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AF330

At low speed....

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Hi,

 

I had some questions about basics of flight.

What happens when we turn off the engines? Do we stall or not? Does our AoA increase? So we slow down but what else?

 

Thanks

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It depends on what you do with the Flight Controls.

Although the rest of this topic should become quite "fun".

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Yeah....

But do you stall? No, isn't it? You will lose speed till a point where the plane will pitch down to take back speed....

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Again: It depends on what you're doing with the flight controls.

Are you pitching for speed, altitude, or AOA? Depending on what you're doing, the airplane will do different things.

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If nothing changes in the actual position of the flight controls, assuming an underwing-engined commercial airliner, the aircraft will slow down and pitch down.Then it will gain speed, and pitch up again, possibly climbing somewhat, but not as high as where it started. Due to the pitch up, it will of course lose speed. Then it will pitch down and gain speed again. This oscillation will probably continue until it meets terra firma.

It *may* however, end up in a stable dive, where speed and pitch remain more or less constant. It will of course still descend.

This would depend on the type of aircraft.

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The same exact thing happens. As long as the position of the flight controls doesn't change (trimmed for a certain speed), there won't be any difference to the scenario above.
The only thing that will change is the rate at which changes happen, since there still is some, though not much, residual thrust when an engine is brought to idle, but not shut down.

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Airplanes don't need engines to fly, they need engines to remain in the air rather than glide down to the ground.

 

When an airplane doesn't have an engine to produce thrust to generate airspeed to generate lift, then it must use gravity to get the necessary airspeed.

 

The only alternative is to ride air currents to climb higher and extend your time in the air, but that is only a realistic option for purpose built gliders.

 

The oscillations described by KriVa are known as phugoids.

 

For a famous example of a 767 losing both engines and gliding to a safe landing, look for the story of the "Gimli Glider".

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Thanks,

 

But I tried in a small sim, the plane pitched down, kept the same speed it had before ditching. The flight path during descent was the same all the

way down. The plane was not pitching up: only the Vertical Speed (negative) was sometimes changing but was always negative till the ground.

 

Is that normal? Could you please do the test?

Take a Cessna 208 (or whatever) to 3000ft - 105/110 kts. Reduce thrust and don't touch the F-CTL's. See what happens! ;)

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Like I said, it depends on how the airplane is designed. Some will describe the phugoid, some will be in a stable descent.

 

 

...are known as phugoids.

 

Thanks for that! I couldn't, for the life of me, remember the name.

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Thanks,

 

And how does a Boeing (777/747) or an Airbus (330/340) react? Lile a stable descent or a phugoid?

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Thanks,

 

And how does a Boeing (777/747) or an Airbus (330/340) react? Lile a stable descent or a phugoid?

 

As has been described, it will follow a phugoidal path which will result overall in a net loss of altitude. This is all due to a basics law of phsyics, namely Netwon's law that states (and I paraphrase for the sake of clarity) "A body will remain at rest or at a constant speed unless a force acting upon it causes it to change".

 

When an aircraft is in level flight at a constant speed then all the forces acting on the aircraft are equal and opposite: the downward force acting on the aircraft (its weight) is counteracted by the equal yet opposite force of lift which is generated by the aircraft's wings. It is crucial to note that the amount of lift the aircraft's wings generates is proportional to the speed the wing moves through the air (airspeed). This speed through the air is maintained by the engines producing an equal and opposite force to the drag caused on the aircraft by the air which tries to slow the aircraft down.  

 

Now assuming the aircraft is trimmed and stable and maintaining a constant speed and altitude when you remove the power, the aircraft will begin to slow. This is because the force of drag is now greater than the forward force of the aircraft (which is now purely momentum as the forward force of the thrust has been removed). This reduction in airspeed causes a reduction in lift. The reduction in lift causes the aircraft to begin descending as the downwards force (weight) is now greater than the upwards force (lift). However, as the aircraft descends the aircraft is accelerated due to the force of gravity. This acceleration causes the speed to increase. As the speed increases, lift once again increases until it is greater than the downwards force of the weight, at which point the aircraft will start to level off and gradually climb. As the aircraft climbs, the drag caused by the air along with gravity (now slowing the aircraft) will cause the speed to reduce until once again the resulting lift is reduced to the point it is less than the weight, adn the aircraft begins to gradually level off and descend again. Rinse and repeat

 

As a result of this, although the aircraft does regain some altitude when it climbs again it will never reach the same height as it started due to energy being lost to friction with the air, and thus the aircraft will actually be effectively descending gradually until it hits the ground. This is similar to dropping a bouncy ball on the floor - you will notice that unless you throw it, it will never return to the height it was released from as energy is lost as the ball deforms at the bottom of the bounce.

 

NB: It is also important to note that the aircraft will not enter a phugoidal state if it doesn't reach or exceed the speed it was trimmed to fly level at before power was removed. This could be the case if for some reason the aircraft was operating at a high power setting and thus higher speed than normal. In such a case accelerating force due to gravity in the descent would be overcome by the drag caused by the air before it reaches the trimmed speed. In this case, the aircraft may enter a gradually steeper descent which will then reduce as the aircraft's speed increases, but it will never actually level off or climb.

 

NB2: This is a very simplified model and does not account for thermals and winds which occur in the real wolrd and would/could significantly alter the path of the aircraft depending on their strength.

 

NB3: Your experience may differ depending on the specific aircraft but roughly speaking from analysis of the forces involved this is how the aircraft would react. Terms and conditions apply. No refunds. See website for details.

 

Edit:

 

pic5.gif

Hopefully this image (courtesy of NASA) will clarify things a bit more.

 

Imagine the aircraft is in flight, and imagine the size of the arrow shows the size of the force. The Lift and Weight arrows act in opposite directions but are the same size so they counteract each other meaning overall the aircraft neither climbs nor falls. Same goes for Thrust and Drag: it is neither speeding up nor slowing down. It is in level flight at a constant speed. Now remove thrust. Drag is now greater than thrust, reducing airspeed. Lower airspeed means less lift. As a result Weight is now greater than Lift. Aircraft falls. etc.

 

Hope this helps.

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Puffmac: Your explanation was just great!!!!!

 

One more question: I got a book for christmas. It said that the AoA increases (before ditching) when we reduce thrust....is it normal?

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Puffmac: Your explanation was just great!!!!!

 

One more question: I got a book for christmas. It said that the AoA increases (before ditching) when we reduce thrust....is it normal?

 

Glad my answer was of use!

 

As for the AoA question, I can't think off the top of my head as to why a reduction in throttle would cause a pitch up reaction. What is the name of the book and what type of aircraft is it referring to?

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It's in french: " Les bases du pilotage - Jean Zilio".

You can also find it in English! ;)

 

By the way why is weight = lift during climb????

 

If you reduce speed, you reduce lift so you increase AoA, no?

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It's in french: " Les bases du pilotage - Jean Zilio".

You can also find it in English! ;)

 

By the way why is weight = lift during climb????

 

If you reduce speed, you reduce lift so you increase AoA, no?

Lift > Weight during the climb, if Lift = Weight the aircraft will neither descend nor climb.

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Lift > Weight during the climb, if Lift = Weight the aircraft will neither descend nor climb.

 

Nope, that's wrong. Lift = weight in a steady (constant VS) climb or descent as well as in level flight.  Lift > weight means that the aircraft will accelerate upwards (increasing VS).

 

Actually, and to further confuse people, Lift < Weight when a powered aircraft climbs.  That's because the aircraft is pitched up, so the engine thrust carries some of the weight.

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If you reduce speed, you reduce lift so you increase AoA, no?

 

Yes that's correct - in some cases at least.   If you reduce speed and keep the pitch attitude of the aircraft constant (ie. hold the nose up), lift is reduced so the aircraft accelerates vertically downwards. This means that the airflow hits the wing more from below, ie. at a higher AOA, so that the lift increases again.  Eventually  a steady descent is achieved, at a slower forward speed and higher AOA so that lift=weight again.

 

This is a potentially dangerous situation because the AOA cannot be increased indefinitely, eventually the wing will stall and lose most of its lift (and the aircraft then accelerates downwards FAST!)  Most aircraft are designed so that the nose drops when the speed is reduced (by making the wing lose more lift than the tailplane), thus keeping the speed high and the AOA low.  So on most aircraft this will only happen if the nose is deliberately kept high by the pilot or automatic systems (including Airbus FBW, which is why it needs all those protections).

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Nope, that's wrong. Lift = weight in a steady (constant VS) climb or descent as well as in level flight.  Lift > weight means that the aircraft will accelerate upwards (increasing VS).

 

Actually, and to further confuse people, Lift < Weight when a powered aircraft climbs.  That's because the aircraft is pitched up, so the engine thrust carries some of the weight.

 

I stand corrected.  :P

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Thanks a lot,

 

So our AoA increases. Why? Because we slow down and we lose lift. But why don't we stall? Because at a certain AoA/speed - before stall - the plane will pitch down. Am I right?

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But why don't we stall?

 

Well, at cruise speeds AOA is usually about 5 degrees and most wings stall at about 12 degrees (ball park figures here - there is a lot of variation between different wing profiles).  So if you slow down a bit and hold the nose up you won't stall.  Slow down a bit more while holding the nose up and you still won't stall.  Slow down even more and insist on holding the nose up - BAM! 

 

If you let the plane pitch down (or push the nose down yourself) you'll generally be OK, assuming the ground isn't too near.

 

One can't be specific because the pitch-down response to slowing down- longitudinal stability in tech-speak - varies from aircraft to aircraft and depends critically on the position of the CG.  In some designs and under certain conditions the response to nearing the stall may be to pitch UP, which is generally fatal.  The early jet fighters such as the F-100 and the F-104 were (in)famous for this, and led to the invention of the stick-pushers that are installed in modern transport aircraft.

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