My daily commute has me pass by a large airport, where I get a really good view of various planes making the final 1000-2000 feet of their descent.
I’ve noticed that large jets (747s in particular) seem to almost have their nose pitched very slightly upwards and seem to descend in a ‘controlled fall’. Small jets definitely seem angled downwards. Am I just seeing things, or is this accurate? If so, how does the plane descend with a positive or neutral pitch?
(If it matters, the airplanes usually have their landing gear out by the point I see them.)
I’m sure you’ll get a much better explanation from a pilot, but when planes land, they are often nose-up so that they can maintain slower speeds for landing. Pilots will adjust the engine power to adjust the rate of climb (or descent), and the pitch to adjust the aircraft speed. As a plane lands, the pilot slows down by increasing pitch and then keeps on the correct glide slope by adjusting the engine. Smaller planes can fly at lower speeds (generally speaking) so they are often not nose-up during descent, while the larger aircraft usually are.
So far as I know, it’s routine for all airplanes to land with positive pitch. The rear landing gears make contact first, followed by the one under the nose.
There’s a tradeoff between lift and drag for different wing angles of attack. A higher angle of attack will give more lift at any given speed, but will also give more drag. When you’re landing, you need your lift to still be equal to the weight of the plane (or very nearly so), but you also need to be going at a slower speed than when you’re cruising, so you need to increase the angle of attack, both by extending flaps and by tilting the whole plane. This produces drag, but that’s OK, too, since you also want to be slowing down while you do this.
I realize that, at landing and slightly before it, the pitch is positive for all planes. But I’m talking about 1000-2000 feet up. Mainly, I guess I’m confused as to how the plane can be maintaining a positive pitch and be descending at the same time.
The relation of the longitudinal axis of the plane (as opposed to the axis of the wing, or elevator, or sources of thrust like a propeller or jet) has surprisingly little to do with the aerodynamics of climbs and/or stalls.
That particular nose-up attitude is what works best with the combination of
Wing and flap settings
Speed
Amount of thrust/power
for that particular airplane to have a stable, controlled approach and descent into an airport in a desired landing configuration.
There are other planes that may not look the same way in for approaches, and may indeed have their noses down. But a lot of planes are on the ‘back side of the power curve’ in approach configurations and need the nose up to provide the proper angle of attack for such slow speeds.
Pitch determines lift, which is a force, and if there’s a net force on an object, it’ll accelerate. Even when the plane is descending, you still want to keep the forces balanced on it, or very nearly so. If your lift were, say, 90% of your weight, you’d accelerate towards the ground at a meter per second per second, which would be what we generally call a “crash”.
A plane flying at constant speed in straight level flight has no acceleration, and hence all forces balanced, but a plane in a steady descent also has no acceleration, and hence must also have all forces balanced.
I spent the summer watching planes land and learning about them, and from what I’ve seen small regional jets (Bombardier CRJs, Embraer ERJs) tend to approach nose-down, and only flare upwards a few feet above the ground to ensure that the main landing gear (MLG) takes the first load. Turboprops (Dash-8s) tend to approach nose down or level, although they also flare for the MLG touchdown. Larger jets approach nose up, and don’t need to adjust their attitude to get the main gear down first.
“Angle of attack” refers to the pitch of the aircraft relative to the line on which it’s actually moving.
When travelling at very low speeds - as, for example, on final approach with full flaps - an aircraft must necessarily maintain a high angle of attack to create enough lift to maintain a steady rate of descent.
So if the slope of its descent is 3 degrees down, and its angle of attack (relative to that 3-degrees-down descent line) is 10 degrees, the plane will apear to be pitched nose-high, 7 degrees above horizontal.
The basic idea is that the angle between the wing and the actual flight path, called the angle of attack, can change depending on a number of factor, and it may not be exactly zero a lot of time. When a plane slows down when it is coming in to land, the decreased airspeed causes the amount of lift generated by the wing to decrease, if everything else remains the same. To compensate for this loss, the pilot will have to increase the pitch of the aircraft, thus increasing the angle of attack, and lift.
Just want to add that a Cessna 182 (a light single prop) can approach at a disturbingly low nose-down attitude with full flaps deployed and power off, and requires a pretty aggressive flare to avoid planting the prop or nose gear into the ground. This can be a bit nerve-wrecking to watch if you’re not expecting it.
Some Googling suggests that a 747’s cruise angle of attack is around 2.5 degrees. We can expect that at this AOA, the fuselage is very nearly horizontal - as that would give best efficiency. The 747’s AOA during approach is probably at least 12.5 degrees (the stall AOA is around 17 degrees). Thus, flying straight and level at approach speeds, the fuselage will be pitched up 10 degrees above the horizontal.
A typical final approach path descends at an angle of 3 degrees, during which the fuselage will appear to be pitched up 7 degrees above horizontal.
For someone who doesn’t claim to be a pilot that is exactly correct. Altitude is adjusted with engine power and speed is adjusted with pitch. When power is reduced for decent the plane is trimmed for the appropriate landing speed. Once it is established then the glide slope is maintained with engine power.
Tricycle geared aircraft are general designed to land on the mains so structurally the goal is not to flat land it like a tail dragger (3 point landing).
I don’t think that’s right. A 2.5 degree AoA for the wing in cruise flight doesn’t mean much once the plane is in a landing configuration, because flaps will be deployed to give the wing a significantly greater AoA, regardless of the position of the fuselage.
In other words, deploying the flaps gives the aircraft a greater angle of attack, meaning that the fuselage could maintain the same AoA with the flaps and the nose further down than in a clean configuration, in which the nose would have to be pointed further up.
As for the OP, I flew single engine land airplanes for a while, and there were certain aircraft in which once the flaps were fully deployed and the airplane set up for a short field landing (low airspeed, steep approach), I certainly had the impression that the nose was slightly low to the horizon. I can’t say for sure that it wasn’t some kind of optical illusion, but it sure felt that way.
Keep in mind that a descending airplane is a bit like a car going down a hill. It will have a tendency to gain speed, unless you do something to prevent it. And when you’re coming in for a landing, extra speed is exactly what you don’t want.
In a car, you can lightly press the brakes (or shift into a lower gear) to keep the car from gaining too much speed. In an airplane, lowering the flaps reshapes the wing to generate more lift at low speed, AND it increases the drag.
I think he was just citing the cruise numbers to figure out the wing AOA relative to a level fuselage, and I think that would be a constant[sup]1, 2[/sup]. AOA is the airflow relative to the chord line of the airfoil, and that chord line is based on fixed points. Put out all the flaps and slats you want, the chord line relative to the fuselage stays the same.
I don’t know if it’s true for airliners, but some planes have a slight twist in the wing. The inboard section is at a higher AOA than the tip; during a stall, there will still be airflow over the ailerons to maintain roll control. But that twist is also constant, so it doesn’t apply here.
Some planes have a movable wing (the F-8 Crusader, and possibly others). But those aren’t airliners and so it really doesn’t apply here.
I don’t know about the old taildraggers, but the Cessnas I used to fly did not stall during the landing flare. The angle-of-attack was high, but not high enough to stall.
The approach path is almost universally a 3 degree descent versus horizontal. The typical nose attitude once on-speed for landing is 2 to 4 degrees nose up versus horizontal for most airliners at their common landing weights. The angle between the path of the aircraft and its longitudinal axis is therefore 5 to 7 degrees. The landing flare in the last 10-30 feet brings the nose up another 3-ish degrees to almost zero out the descent angle for touchdown.