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Is it right that basically an airplane just needs to accelerate to climb?

Greater velocity of an airplane leads to greater lift - and since its weight remains constant (or even decreases) - a greater lift leads to a gain in altitude without the need to pitch. But why do airplanes climb by "pointing the nose up"? Is it to climb as fast as possible and to need less horizontal space?

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  • $\begingroup$ Well, hopefully someone really smart is going to come along and give a really brilliant and detailed answer. But in summary: Tilting the wings up actually causes the wings to create more lift, which is what increases altitude. Leveling the wings makes the lift equal to the weight of the craft, so you get no altitude change. $\endgroup$ – Jay Carr Feb 16 '15 at 17:52
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    $\begingroup$ @JayCarr not quite, depending on power, an aircraft can both climb and descend with the nose on the horizon, just as it can fly level (or even descend, like in an approach to landing) with the nose high. You're right that increasing the AoA will increase lift though (to a certain extent), there's a sweet spot for climbing the fastest called Vy. $\endgroup$ – falstro Feb 16 '15 at 17:58
  • $\begingroup$ I assume from the question context he meant "full power". Besides that, power being equal, positive AoA always creates more lift. So I don't disagree with you, but I think you've stumbled outside of the intended scope... $\endgroup$ – Jay Carr Feb 16 '15 at 18:00
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    $\begingroup$ @jay-carr, a briliant anwer would be "because nose-down you'll hit the ground" :-). Is there a jokes area on this site? $\endgroup$ – Paul Ogilvie Feb 16 '15 at 18:46
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    $\begingroup$ Watch a BUFF (B-52) take off. A BUFF doesn't rotate. Because of the bicycle and outrigger landing gear, he has to take off level. Very shortly after he breaks ground, he will drop the nose, and climb with the nose actually below the tail. It definitely takes a little getting used to. $\endgroup$ – John R. Strohm Feb 18 '15 at 17:28

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A climbing aircraft needs less aerodynamic lift than in horizontal flight, not more.

Now I have your attention, I hope. The reason is quite simple:

Lift equals weight, and just because the pilot choses a different flight path angle, the weight of the aircraft does not change. The total of all lifting forces must still balance the weight, but in climb you get a small lifting contribution from the engine(s) because its (their) thrust will point upwards just like the rest of the airframe.

Forces and their angles acting on a climbing aircraft in side view

Don't let the many arrows and greek letters confuse you. To be in equilibrium, lift (L, dark blue), drag (D, red), thrust (T, green) and weight (m⋅g, black) must add up such that they can be combined into a closed run of vectors. I've done this with the lighter-colored vectors around the weight. Since the flight path points upwards, so does the thrust which now has a small vertical component. The lift vector can be a little shorter now.

Consider the extreme case of vertical climb: Now all thrust supports the weight, and aerodynamic lift is no longer needed.

There is a second, much more subtle effect: When you climb, air gets thinner and engine performance goes down proportionally. At the same indicated air speed, the aircraft will continually decrease its climb speed, and this deceleration frees up a tiny inertial force, which again adds to lift and counteracts weight.

Conversely, at the beginning of a climb phase the aircraft needs to create momentarily more lift to accelerate itself upward. Only then, when climb speed increases, lift must be bigger than weight to overcome the inertial effect which at this moment works downwards. For the supernerds: If you integrate the lift deficit over time of the aforementioned effect and the extra lift over time for climb acceleration, both cancel exactly.

To answer your question directly: To climb you need to increase excess energy, not speed. This is normally done by increasing engine power output, or by trimming the airplane at a lower speed where drag is less, so more power remains for climbing. This question contains more details on how to get an aircraft to climb. Note especially @SteveV.'s bucket analogy.

If you use the airplane's kinetic energy as its source of thrust, the same mechanism can be applied to instationary climbs, where speed is traded for altitude, like in gliders.

The nose-up attitude is simply the result of a different flight path. Since the required aerodynamic lift will be almost the same, the angle of attack will also be almost the same and the whole aircraft needs to fly nose-up. This is similar to a car which has the same attitude towards the road, but when you drive uphill, both car and road will be tilted upwards.

This analogy breaks down when you change speed - flying at lower speed needs more angle of attack to still create the same lift, and this nose-up change will be added to your attitude angle.

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    $\begingroup$ Hmm... excuse me for being a layman at these things, but doesn't this answer completely collapse on itself as soon as we're looking at unpowered aircraft? A glider can climb by pitching nose-up, and that certainly has nothing to do with thrust...?!? Or have I misunderstood completely? $\endgroup$ – DevSolar Feb 17 '15 at 8:56
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    $\begingroup$ @DevSolar: A glider can only climb by slowing down, or by flying in raising air. The climb will soon end if no external energy source can be exploited. This answer looks at a stationary climb, which a glider is completely incapable of without external help. $\endgroup$ – Peter Kämpf Feb 17 '15 at 9:32
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    $\begingroup$ @PeterKämpf: From an energy-conserving point of view, your answer is correct. It is impossible to gain potential energy without investing some other form (chemical, kinetic). But I do not believe in the 1. law of TD (another discussion). Our disagreement can be tested: Does an airplane climb by accelerating (ceteris paribus) when thrust is perpendicular to weight? If thats the case - and I think it is - then lift is what matters and not some form of excess energy. $\endgroup$ – Chris Feb 17 '15 at 16:22
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    $\begingroup$ @Chris: How would you accelerate, if not by adding thrust? Ceteris paribus means we keep our trim point, so the added thrust will be converted into altitude, not speed. If you retrim the aircraft for a higher speed while adding thrust, it will not climb, but accelerate - just as commanded. I also think that belief has to do with religion, not physics. If you reject the first law of thermodynamics, we two will never understand each other. $\endgroup$ – Peter Kämpf Feb 17 '15 at 19:43
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    $\begingroup$ @PeterKämpf this is such a great answer. It has actually helped me reframe the entire way I think about lift and aerodynamics into a more consistent mental model. In particular, while pulling up temporarily increases AoA, a sustained climb is not due to excess AoA compared to level flight, but due to an excess angle of airplane to earth compared to level flight (which in turn causes thrust to increase in angle compared to level flight). Suddenly lots of things make more sense. Thanks! $\endgroup$ – Peter Jul 20 '16 at 4:56
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Consider the relative airflow. When an airplane is not climbing, the relative airflow is horizontal, and so the angle at which the air meets the wings, ie the angle of attack, is measured from the horizon (Case A in the diagram). However, when an air-plane is climbing, the relative wind is tilted downward by the climbing component of the airplane's velocity. If the airplane did not tilt the nose up, the angle of attack would approach zero as the rate of climb increased, reducing lift and efficiency (Case B), so the air-plane must tilt up the nose to keep the angle of attack in an efficient range (Case C). !enter image description here

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    $\begingroup$ @QuadmasterXLII: You are saying that the reason to pitch is to get the angle of attack at which you get Max L/D which is what you want in a climb. $\endgroup$ – Chris Feb 17 '15 at 16:45
  • $\begingroup$ I'd second that. The F-8 Crusader was very similar to an A-7 Corsair II, but the F-8 had the main wing attached to a hinged mechanism. This way, they could raise the wing by about 7 degrees, and the AoA, without needing to raise the nose so much. Most aircraft were hard to land on a carrier because you had to raise the nose so much on approach. The F-8 typically used the raised wing on approach (not in combat) so they had better visibility over the nose. This implies that if your wing could change incidence, you wouldn't need to raise/lower the nose of the plane. $\endgroup$ – Meower68 May 11 '15 at 14:03
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While the answer from @Peter Kämpf is all true and sound, i think it misses a point and does not really answer the OP's primary question.

Is it right that basically an airplane just needs to accelerate to climb?

Yes this is basically right. Higher horizontal speed produces more lift so it'll make the aircraft climb. https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift_formula.html

But it's not the only way to make an aircraft climb. Increasing pitch (while also giving more thrust) is the other and has been explained by Peter.

Which is more efficient? An aircraft is designed for optimal efficiency at cruise speed and level flight. So you may want to keep your speed within a narrow range around that optimum. Raising airspeed will also raise drag (to the square of v) see https://www.grc.nasa.gov/www/k-12/airplane/drageq.html Drag is what you absolutely want to minimize because it is energy that is completely lost (transformed into heat).

This is why increasing pitch/AoA, while maintaining airspeed constant is the better way to do. This way drag stays about the same. Of course you still need to provide more thrust (thus energy), as now part of your thrust is directed downward (and part of your lift backward), but you are converting this energy more directly to altitude, eliminating the loss in drag.

So to answer your question, yes it is possible to climb in a strict horizontal attitude by increasing airspeed, but it is more energy-efficient to climb by increasing pitch. (Thrust being increased both ways)

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  • $\begingroup$ "Higher horizontal speed produces more lift so it'll make the aircraft climb". Yes but only temporarly: Starting to climb while keeping the nose attitude will increase your fpa and decrease aoa, resulting in lower lift. It's only a balloning efect, not a sustained climb $\endgroup$ – Radu094 Sep 25 '17 at 10:01
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You can answer this question empirically.

Trim your plane for straight and level flight and set the power to Vy. Look at the AI or visual horizon, and note the pitch attitude.

Now set the power to its Vy climb setting and configure the aircraft (ball, cowl flaps, mixture, prop, etc) to climb configuration, but don't retrim the elevator. Adjust ailerons to maintain straight flight.

The airplane will pitch up by itself to its Vy climb attitude.

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I do think that you must consider the type of aircraft here! If I'm a hotshot new F-22 pilot with an obnoxious thrust to weight ratio trying to intercept some baddies and I need to quickly reach altitude, you can bet I'm gonna put the nose up and go like a rocket.

enter image description here

But really though, it's all about velocity vectors. If you want to go up, then travel up! The engines propel in the direction of the nose. (Unless you are that hotshot F-22 pilot from before). Also consider aircraft have speed limits under certain altitudes, and also consider that ole' Bernoulli is not the only reason airplanes fly, Mr. Newton has something to say about this as well.

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Because most of the lift comes from the angle of attack (AoA) of the wings. Higher AoA means more lift (up to a point).

Also most aircraft will pitch up as they increase speed due to design.

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    $\begingroup$ Pitch and AoA are related by you could use some more distinction here. $\endgroup$ – fooot Feb 16 '15 at 18:42
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As a general rule of thumb and without a long drawn out technical explanation of how and why here is a simple answer that my 8 year old son could grasp. In straight and level flight, if you reduce power without altering the Aircraft's attitude the Aircraft will descend, conversely if you increase the Aircraft's power it will climb. Now the same Aircraft without altering its power settings if you alter its attitude by lifting the nose it will slow down hence with the same power settings in you lower the nose the Aircraft will increase its speed. So your rule of thumb is "power equals height" and "attitude equals speed". Go and take a flying lesson and try it, and you will see what I mean.

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  • $\begingroup$ Welcome. Does this add new elements to the already existing answers? $\endgroup$ – mins May 17 '15 at 15:43
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Normally in an aircraft you change your altitude using the power. If you increase power, your altitude increases. If you reduce power the aircraft descends. In both cases the aircraft is normally at a near-level pitch angle.

The reason for this behavior is that the wing is permanently tilted upward by a certain amount, called the "chord angle" or "angle of incidence". The angle is chosen so that in normal conditions, with medium power the aircraft will stay at the same altitude. If the wings were flat, the aircraft would tend to descend constantly.

The main exception to the above is when you are taking off and want to gain altitude rapidly for safety reasons. In this case, the stick or yoke is pulled back and the aircraft tilts upward and climbs rapidly. What causes this is the elevator (or horizontal stabilizer) which is located on the tail of the aircraft:

Elevator controls pitch

The elevator allows the pilot to change the pitch of the wings. The more wing surface that is exposed to the air, the greater the upward force. You can demonstrate this yourself by holding your hand outside the window of a fast moving car. If you hold your hand level and then tilt the leading edge up, your hand will be forced upward by the wind, and vice versa. If you tilt the leading edge of your hand down, then your hand will be forced down by the wind. The same thing happens to an aircraft.

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    $\begingroup$ No, Tyler, level flight is when you stay at the same flight level. When you climb or descent, it's not level anymore. BTW, the F-104 had quite flat wings. I do remember seeing Starfighters climbing just like any other airplane. $\endgroup$ – Peter Kämpf Feb 17 '15 at 20:03
  • $\begingroup$ @PeterKämpf I have edited my answer to change the phraseology. Nearly all aircraft have a designed positive angle of incidence. Delta wings and some specialized high-performance aircraft are the exception. $\endgroup$ – Tyler Durden Feb 17 '15 at 20:35
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    $\begingroup$ In both cases the aircraft is normally at a near-level pitch angle. nope, not even close. you need a certain aplha to generate lift and in climb your pitch is that alpha PLUS the climb slope (gamma): see diagram in Peter's answer. $\endgroup$ – Federico Feb 24 '15 at 19:11
  • $\begingroup$ @Frederico I am pilot, I know how an aircraft descends and climbs. $\endgroup$ – Tyler Durden Feb 24 '15 at 19:14
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    $\begingroup$ (it's Federico, without the "r" or I won't be notified of replies). and I could reply that I analyze flight data for a living, you don't want to go the "truth by autorithy" road. as per "controlling altitude with power" it was already treated here: aviation.stackexchange.com/questions/2980 You can argue that for large airliners, when a FL change is required, the pitch is hardly moving (but it still is), I might agree, but that's far from being a general rule. $\endgroup$ – Federico Feb 26 '15 at 20:59
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The pilot choses a different flight path. This new flight path is going higher in altitude and by that is changing the potential energy. mass*gravity *9.81*delta Hight. We need to fly slower with a lower drag and use the extra energy to climb or we need to increase the power to the propellor to overcome the change in potential energy. When the altitude is changing we need also to increase the speed because of the lower air density. The lower air density is effecting the lift and the trust the prop can deliver for a giving RPM

We can calculate the trust by looking into the force vectors Lift and Weight. When the plane change course the lift vector and weight vector who were in opposite direction are in a climb path working under a small angel, the climb rate. To counter the weight we need to increase the lift from lift r1 to a lift r2. But the result is also a vector drag r1. This drag vector is added to the drag in level flight. As a conclusion we can say that we need to increase the trust to overcome the added drag and we need to increase the lift to counter the weight.enter image description here

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According to Wikipedia and what I recall from my earliest days in training as a private pilot:

Relation between angle of attack and lift[edit] A typical lift coefficient curve. The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases.

As the angle of attack increases so does lift. Exceeding the Critical Angle of Attack further illustrates this point.

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    $\begingroup$ Welcome. I don't think this answer gives anything new over and above the existing answers... $\endgroup$ – Rory Alsop Feb 15 '17 at 17:38
  • $\begingroup$ Good grief, almost none of the answers add anything new... Best answer yet was the "stick your hand out the window of a car" comparison. (I realized this fundamental truth at a very tender age) I think sometimes our members provide greatly overwrought engineering explanations, when by the simplicity of the question one can discern that the asker will not be able to digest such information. Detailed questions deserve detailed answers, simple questions deserve simple answers. $\endgroup$ – Michael Hall Feb 15 at 19:04

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