Why do airliners pitch up during cruise?

Question

In my experience as a passenger, when the plane stands at the airport and you enter it, the aisle is pretty much horizontal. (Obviously, I've never flown on a DC-3). After takeoff we pitch sharply upwards (duh!), but even after the captain comes on the PA with "we have now reached our cruise altitude", the aisle continues to point a few degrees upward. Usually it's only at "we're starting our descent into XYZ" that the body returns to a more or less horizontal pitch.

Why is this? I have some hypotheses, but don't know which, if any, of them hold truth.

1. It's just how the aerodynamics happen to work out. (Unlikely -- it would seem to be a simple engineering matter to attach the wings to the body at an angle such that the right angle of attack for cruise flight corresponded to the body being horizontal).

2. Pitching the body upwards slightly allows it to generate some lift, free of a proportional increase in drag relative to pushing the cylindrical body straight through the air.

3. Passenger jets are deliberately designed such that they never need to pitch lower than horizontal in a routine descent, lest passengers might panic and think they're nosediving to their death. (Also unlikely, unless there is really no hidden costs in terms of drag, etc.)

4. Landings might be more difficult if the plane had to point its nose downwards in order to get closer to the ground -- the nosewheel would hit the runway first. (On the other hand, simply leveling out would count as a flare, so it's not clear to me that's actually a disadvantage).

Answer 1

Mainly the options 2 and 1 from your list.

What nobody mentioned so far, but is rather important here is that the angle of attack depends on speed¹ and weight. While the "speed" does not vary much for most airliners, the difference between empty and fully loaded airliner is rather large.

Now if the fuselage was tilted down, it would generate lift directed down and associated drag with the net result of simply a lot of extra drag. So the aircraft is designed so that it does not fly (in level cruise) pitched down even when light. But that means that when it is fully loaded, it flies slightly pitched up, because it needs more lift and higher angle of attack to get it.

¹ _{It depends on the "indicated speed", which is not speed at all but rather dynamic pressure expressed as speed at which it would occur when moving through air of standard sea level pressure (1013.25 hPa). The pressure at high altitudes where airliners normally fly is significantly lower, so the indicated speed is usually does not exceed 250 knots when true speed is well over 400 knots.}

Answer 2

Ultimately the answer to your question is number (1) from your options above - That's just how the aerodynamics happen to work out.

To generate a certain amount of lift requires that the wing have a specific angle of attack at a given speed. As the wings are more-or-less permanently attached to the fuselage setting the angle of attack requires us to pitch the entire aircraft, changing the deck angle.

There is a very nice web page illustrating this in (greatly exaggerated) detail, so I won't reproduce the full explanation & drawings here.

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As to why the aerodynamics work out that way - that probably has something to do with numbers (3) and (4) - passenger comfort, and ease of executing maneuvers (like landings).

Most airliners I've been on have a slight nose-up deck angle (maybe 2 degrees) in cruise, which isn't really "noticeable" to most people unless you're carrying a bubble-level or staring very intently at your drink, but the exact angle for climb, cruise, and descent will vary depending on the aircraft and how it's been loaded (you can pretty much descend with any deck angle you like).
Most people would probably object to flying around with a 10 or 15 degree deck angle (whether nose-up or nose-down), so as a concession to passenger comfort the wings are attached to the fuselage in a way that is aerodynamically favorable (not trying to drag the fat bottom-side of the plane through the sky) and comfortable for the passengers.

Conversely most GA aircraft I've been in seem to cruise with a flat or slightly nose-down deck angle, though that could just as easily be an optical illusion from having the big window in front where you can see it. (I'm estimating the deck angle based on the wet compass tilt since I generally don't serve drinks.)

Answer 3

You actually have some pretty good hypothesis here!

To start with, it truly is "a simple engineering matter to attach the wings to the body at an angle such that the right angle of attack for cruise flight corresponded to the body being horizontal", but you missed the design goals slightly. This angle is called the angle of incidence, or the rigging angle of the wing and is the difference between the fuselage angle and the wing angle. This is a very specific angle that is calculated as part of the design of the aircraft.

The wing incidence must satisfy the following design requirements:

1. The wing must be able to generate the desired lift coefficient during cruising flight.
2. The wing must produce minimum drag during cruising flight.
3. The wing setting angle must be such that the wing angle of attack could be safely varied (in fact increased) during take-off operation.
4. The wing setting angle must be such that the fuselage generates minimum drag during cruising flight (i.e. the fuselage angle of attack must be zero in cruise).

These design requirements naturally match with the wing airfoil angle of attack corresponding to the airfoil ideal lift coefficient (see figure 5.26).

The typical number for wing incidence for majority of aircraft is between 0 to 4 degrees. As a general guidance, the wing setting angle in supersonic fighters, is between 0 to 1 degrees; in GA aircraft, between 2 to 4 degrees; and in jet transport aircraft is between 3 to 5 degrees.

Notice that the proper angle of incidence will have the wing at its most efficient during cruise while the fuselage also generates the minimum amount of drag. Both of these are calculated independently and the angle of incidence is set so that both conditions are satisfied at the same time.

Aircraft design decisions are usually a compromise between different, many times conflicting goals. He goes on to say that it may be adjusted from the optimum in some cases:

The wing setting angle may be modified as the design process progresses. For instance, a fuselage with large unsweep over the rear portion to accept aft cargo doors may have their minimum drag at a small positive angle of attack. In such cases, the wing incidence will be reduced accordingly. Another, les fundamental, consideration is that stopping performance during landing operation to get as much weight on the braked wheels as possible. Thus, there is a benefit to reduce the wing incidence slightly to the extent that the change is not felt significantly in the cabin. Reducing the nose gear length will do the same thing. This technique is limited in passenger aircraft because a level cabin floor is desirable on the ground. But, for fighter aircraft, the level floor is not a design consideration.

Another possible reason to adjust the angle of incidence to a non-optimum number is to ensure that when landing the aircraft the nose is not pitched down, in order to avoid a higher likelihood of hitting the nosewheel first on landing.

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Most of the material for this answer came from this document written by Mohammad Sadraey at Daniel Webster College

Answer 4

You need a slight pitch-up attitude to fly: you need a certain angle of attack in order to produce the required lift to remain airborne.

As you have imagined, usually the wings are tilted with respect to the fuselage so that in cruise the wings will have the required AoA while the fuselage will be horizontal to minimize drag (and thus fuel consumption).

Passenger comfort is much less important than this (and passengers can always lean back their seats).

My hypothesis for your observation is that the "pitch-up" you observe is either a visual/sensory illusion due to the lack of external point of reference (no, the clouds are not a valid reference and the ground is too far away) or you have flown in aircrafts where the floor is not parallel to the aircraft x-y plane. Personally, I would lean towards the first hypothesis, as having a non-horizontal floor in flight creates all kind of nuisances to cabin service (again, something you want to avoid).

Answer 5

I believe the answer may be option 5:

5. The aircraft changes during flight.

At the beginning of a flight an aircraft is loaded with fuel. This increases the weight of the aircraft and thus requires an increase in the AoA to maintain level flight. As the aircraft burns fuel the mass of the aircraft decreases and consequently the AoA must decrease also.

Depending on the aircraft type and the length of the flight, the weight of the fuel can be very significant. For example, a Boeing 777-200 has a maximum fuel capacity of 117,340 lts. This ammount of fuel weighs aprox. 95,000 kg. The maximum take off weight for this type of aircraft is slightly over 247,000 kg. This means that a 777 fully loaded with fuel and with maximum take off weight will lose a bit less than 40% of its mass during flight.

Answer 6

Passenger comfort is a large consideration! It's simply more comfortable for passengers to be reclined by a degree or two than to sit exactly level.

I have hopped off a 777 onto a small a/c (if I recall correctly, a Baron of some sort) to island hop and the fuselage was pretty much level. It certainly felt less comfortable and somehow, even as if it was fighting not to descend. I would prefer to be a little inclined.

I believe that some aircraft cruise slightly nose down.

[EDIT]

CAVEAT. I am not an expert in physiology or aircraft design.

Comfort is a big factor and the question is not "do airliners fly pitch up" but "why do they?" That they do is a fact. I suspect that there are many factors, e.g. the fuselage itself, when pitched up, will generate some lift but ask any experienced flyer about sitting in a rearwards facing seat, or walking towards the front when in a descent which results in a level pitch or pitch down attitude. It feels less comfortable than when nose up.

If the aircraft is level, and you are level, you experience +1g vertically and 0g horizontally. If you recline the seat, you still experience +1 and 0. If the aircraft is pitch up, you experience a small -ve g component in the horizontal. Quite what effect this has psychologically, I don't know but it does make a difference. I've heard, anecdotally, that facing backwards, e.g. in a British Airways biz class seat is less comfortable than in a forward facing one for exactly this reason.

Answer 7

Don't know how but I stumbled upon this interesting question which has generated some quite elaborated answers.

For a conventional jetliner, the angle at which the wing is attached to the fuselage is simply chosen so that the total drag of the airplane is minimised during cruise condition. And the total drag at cruise is minimised when the fuselage pitches up a couple of degrees. That's all.

Obviously also the confort of the flight crew pushing food carts in the aisle plays an important role during design phase.