14
$\begingroup$

I have seen many cockpit videos of airplanes landing, and nearly none of them have their nose down for losing altitude. How does this happen? and how does an airplane, such as A320, descend without its nose pointing down? And how does this happen in a controlled manner?

$\endgroup$
  • 6
    $\begingroup$ Of note, Air France Flight 447 on a Boeing A330 did exactly this for 38,000 ft and crashed. $\endgroup$ – Nelson Oct 21 at 7:21
  • 58
    $\begingroup$ Which way is the nose pointing on a brick? Throw a brick in the air, and it descends just fine without caring which way is is its nose! $\endgroup$ – Graham Oct 21 at 9:00
  • 4
    $\begingroup$ "The paradox of the glide. By pointing the nose down less steeply, you descend more steeply. By pointing the nose down more steeply, you can glide further." books.google.com/… $\endgroup$ – Jeff Y Oct 21 at 13:36
  • 13
    $\begingroup$ @Nelson "Boeing A330"? Now that would be a major merger! $\endgroup$ – Glen Yates Oct 21 at 17:33
  • 4
    $\begingroup$ Imagine the plane reducing speed to 0 in air. With the nose pointing forward, would it then hover? $\endgroup$ – Viktor Mellgren Oct 22 at 11:54
33
$\begingroup$

A plane descends when it does not have enough thrust to maintain its altitude. A plane can descend with its nose pointed up or down so long as there is not enough thrust to maintain altitude. Altering the pitch of an aircraft is used to control airspeed: pitching up slows an aircraft down (and may cause a climb if done from a level attitude) and pitching down generally speeds up an aircraft (and causes a descent if done from a level attitude). So if you want to descend at a slow speed you reduce power and pitch the nose up.

Since you want to land at a slow speed you will need to pitch up and slow the aircraft down while reducing power to descend nicely. This article covers it nicely in terms of flying an approach. It's also covered in this question.

$\endgroup$
  • 46
    $\begingroup$ Shouldn't it read "when it does not have enough lift to maintain its altitude? $\endgroup$ – Tashus Oct 20 at 21:35
  • 11
    $\begingroup$ I'd say this answer, while "good enough", glosses over/is imprecise regarding some details. E.g., from level flight, pitching up will generally cause a momentary climb, but if the airplane's moving slowly enough, will then enter a descent unless power increases. Similarly, a descent at slow speed may often be accomplished simply by reducing power and maintaining, rather than increasing, pitch angle. It's true that airspeed may be controlled with pitch angle, but the implication that a given pitch angle will always produce the same given airspeed is incorrect (and almost certainly unintended). $\endgroup$ – Peter Duniho Oct 20 at 21:55
  • 13
    $\begingroup$ Also, gliders ("thrust"?). $\endgroup$ – DevSolar Oct 21 at 8:38
  • 9
    $\begingroup$ @PeterDuniho that isn't the aerodynamic sense at all - what you've described is the usual parameters of a fixed-wing flight, but excludes without explanation many other things that are aerodynamically possible. An aircraft with surplus thrust can enter a dive and thus lose altitude; a glider with no thrust can, in the right updraft, gain altitude. In all these cases, in the aerodynamic sense it's the lift that determines the aircraft's altitude. Airspeed just happens to be one of the most straightforward ways to change lift, and thrust is a good way to affect that. $\endgroup$ – Will Oct 21 at 9:25
  • 8
    $\begingroup$ As someone who has done lots of low-level passes and a fair amount of aerobatics on gliders, I will need to be explained where the "thrust" came from in the parts where the glider was going up. $\endgroup$ – Martin Argerami Oct 21 at 13:31
13
$\begingroup$

When an aircraft ascends or descends, its inertial velocity vector (let's simplify by saying the ground is inertial) will be pointed upward or downward. The angle with respect to the horizon, or ground, is called the flight path angle ($\gamma$).

There are two more angles relevant to this story:

  • The pitch angle ($\theta$) is the aircraft attitude with respect to the horizon.
  • The angle of attack (AOA) ($\alpha$) is the air flow incidence against the aircraft.

When aircraft is not turning and there is no wind with respect to the ground, the relationship between the three angles are:

$$\theta=\alpha+\gamma$$

So as you can see, the plane can descend with a positive pitch angle. Or it can do level flight with a non-zero pitch angle. Pitch angle has no direct relation to the flight path.

enter image description here

Image: https://www.basicairdata.eu/knowledge-center/measurement/in-flight-angle-of-attack-usage/

$\endgroup$
10
$\begingroup$

The extremely coarse "explain it like I'm 5" answer:

You know how they talk about in school, how the curved top part of the wing has faster air across it at lower pressure, and that creates lift? They make a big deal about the Bernoulli Principle and how it's not just a wedge pushing through the air. Yeah, they didn't tell you the whole story. Airplanes also do the "wedge" thing. They make lift by angling the wing upward. In fact if you've built a paper or stamped balsa wood airplane, you may notice that it flies even though its wings are obviously flat. You could build a 737 with flat wings and it would fly. It would not get very good fuel economy.

The faster a plane goes, the better the wings use the Bernoulli Principle. The slower, the worse; so at slower speeds they must tilt the wings upward and using them in wedge fashion.

The nose is connected to the wings, so they must lift the nose to tilt the wings.

$\endgroup$
  • 6
    $\begingroup$ nose is connected to the wings citation needed. ; ) $\endgroup$ – Grimm The Opiner Oct 22 at 7:35
2
$\begingroup$

Really short version:

"How does an aircraft descend without its nose pointing down?"

By descending with its nose pointing level or up.

Thrust is not required to do this.

Here's an example:


Now for a tiny bit more detail:


How does an aircraft descend without its nose pointing down?

By flying along a shallow glide path at a moderate to high angle-of-attack.

Angle of glide path (relative to airmass, and expressed as a negative number) plus angle-of-attack minus angle-of-incidence equals pitch attitude of fuselage.

The resulting pitch attitude may be positive (nose up), negative (nose down), or flat, depending on the values of the other three angles involved.

If we ignore the angle-of-incidence, which is rather small in most airliners, then we can say that to a first approximation, the fuselage will be in a nose-up pitch attitude whenever the angle-of-attack is larger than the angle of the glide path measured with respect to the surrounding airmass, which in still air, is the same as the angle of the glide path measured with respect to the ground.

Draw a sketch that shows the wing's angle-of-attack in relation to the glide path through the airmass, and you'll see. For normal climb or glide angles a heavily-loaded jet needs to fly at a high angle-of-attack to generate enough lift when flying slowly. Also, airliner-style approaches to landing typically involve maintaining enough power to create a rather shallow glide path. The result is a nose-high pitch attitude.

In a descent, gravity is supplying power to the aircraft, which decreases the thrust requirement associated with any given airspeed, i.e. which decreases the thrust required to offset drag and keep the airspeed constant, i.e. which increases the airspeed associated with any given thrust setting, all compared to level (horizontal) flight. (The opposite happens in a powered climb.) But this "boost" from gravity doesn't depend on whether the aircraft's pitch attitude is nose-up or nose-down. It only depends on how steeply the flight path through the airmass is inclined downward.

One way to more comprehensively answer this question would be to create a 3 x 3 or 4 x 4 grid of 9 or 16 panels. Each column would show a fixed angle-of-attack (and therefore also a fixed lift coefficient and drag coefficient), progressively decreasing from left to right. Each row would have a fixed thrust setting, progressively increasing from top to bottom. Each panel would have a sketch showing angle-of-attack, direction of flight path through airmass (i.e. direction of airspeed velocity vector), pitch attitude, and airspeed. Some of the aircraft would have a nose-up pitch attitude, and others a nose-down pitch attitude.

For more, these related answers to related questions:

"Are we changing the angle of attack by changing the pitch of an aircraft?"

"Where is the zone of reversed commands?"

"Does lift equal weight in a climb?"

"What produces Thrust along the line of flight in a glider?"

"'Gravitational' power vs. engine power"

"Descending on a given glide slope (e.g. ILS) at a given airspeed— is the size of the lift vector different in headwind versus tailwind?"

"Is excess lift or excess power needed for a climb?"

$\endgroup$
  • $\begingroup$ Supplemental info that I might add to answer -A constant pitch "input" will, to a first approximation, maintain a constant angle-of-attack, not a constant pitch attitude. Even at a constant angle-of-attack there is some variation in airspeed as power varies, but in the opposite direction as you might expect. For example holding the same high angle-attack you'd normally use for an efficient glide, and adding enough thrust to sustain a 60-degree climb path, means that the airspeed must ultimately decrease, or else the excess lift will tend to make the climb path to curve closer to vertical. $\endgroup$ – quiet flyer Oct 20 at 22:13
  • $\begingroup$ Ctd - During the time this deceleration is happening, you'll need to temporarily decrease the angle-of-attack a bit if you don't want to "overshoot" your target 60-degree climb angle.Recalling of course that less lift is required for a climb than for level flight. Exactly the same logic holds true in a 60- degree dive, which would be only possible if you deployed some sort of large drogue chute or other similar device to increase the drag coefficient, as long as we're still talking about flying at a high angle-of-attack. $\endgroup$ – quiet flyer Oct 20 at 22:13
  • $\begingroup$ Ctd - see aviation.stackexchange.com/questions/40921/… $\endgroup$ – quiet flyer Oct 20 at 22:15
  • 1
    $\begingroup$ Winter project: create the table described above. $\endgroup$ – quiet flyer Oct 21 at 2:08
0
$\begingroup$

The angle at which the nose of the aircraft points has less to do with descending or climbing than your intuition would lead you to think. Airspeed affects the rate of climb, not attitude. The faster the air flows over the wings, the more rapidly the aircraft will climb. Slower air flow causes the aircraft to descend.

$\endgroup$
-1
$\begingroup$

The airplane flies because lift created by the wings counteracts its weight created by gravity. To create lift the surface of the wing must be in relative forward motion through the mass of air. Thrust created by the engine generates the forward motion which is opposed by drag. As long as thrust is greater than drag, the wing can move forward and generate lift. Gravity is universal and operates in all objects. Lift, on the other hand applies only to whatever is attached to a wing. To generate lift the wing must have the air moving around it following certain rules, otherwise it is incapable of generating lift. These rules are conceptualized as the angle of attack or the angle between the cord of the wing and the relative motion of the aircraft. At high angles of attack the wing is incapable of generating lift. Gravity, experienced by the airplane as weight does not follow any particular rules, all objects fall from the sky in the absence of lift. An airplane can have its nose up or down and gravity will still apply and if the lift is less than the weight, the airplane will go down. In fact, during landing most airplanes will maintain a nose-high attitude while going down into the runway.

$\endgroup$
  • $\begingroup$ The wing moves forward only when thrust is greater than drag? Go directly to jail, do not pass go, do not collect 200$. Also be aware that in a climb, lift is less than weight; your answer implies otherwise. Basically you are presenting an Aristotelian world view here rather than a Newtonian one. $\endgroup$ – quiet flyer Oct 23 at 0:02
  • $\begingroup$ Yes. My simplified view is Aristotelian and and entirely ignores inertia, which also explains why in a climb lift is greater than climb as long as all other forces stay the same. This explanation is much easier to grasp for the uninitiated and is a good intuitive first approximation. $\endgroup$ – LDBerriz Oct 23 at 15:11
  • $\begingroup$ But you are just making things up that aren't the least bit true! Ignoring inertia (e.g. not being able analyze situations involving acceleration) has nothing to do with it; your "explanation" is completely wrong even for steady-state conditions. $\endgroup$ – quiet flyer Oct 23 at 15:20
  • $\begingroup$ I assume you meant to say in your comment that in a climb, lift is greater than weight. Not true. $\endgroup$ – quiet flyer Oct 23 at 15:28
  • $\begingroup$ How does a rocket climb vertically? $\endgroup$ – LDBerriz Oct 23 at 15:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.