JumboJim747 From Australia, joined Oct 2004, 2462 posts, RR: 50 Posted (7 months 6 days 3 hours ago) and read 5716 times:
i have noticed that aircraft look like they are constantly in a climb angle while they are in cruise.
Why is that
Does it have something to do with safety or loss of height.
Thanks in advance.
RushmoreAir From United States of America, joined Nov 2011, 19 posts, RR: 0 Reply 1, posted (7 months 6 days 3 hours ago) and read 5716 times:
The incidence angle of the aircraft at cruise altitude is based on the specific airfoil (wing profile), the fuselage profile, and the center of gravity of the aircraft in particular. It so happens that most airliners have a C.G./aerodynamics combination that require a positive fuselage incidence angle in order to stay level at cruise - resulting in a slightly nose up position at cruise. But this is not so with all aircraft - for instance, the CR7 usually cruises with the nose slightly down.
vikkyvik From United States of America, joined Jul 2003, 8204 posts, RR: 28 Reply 3, posted (7 months 6 days 1 hour ago) and read 5639 times:
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Quoting JumboJim747 (Reply 2): RushmoreAir thanks for the reply but wouldn't a nose down angle lose altitude?
No, not necessarily. It's like when you're on final approach - most airliners have a nose-up attitude at that stage, but they're still descending.
Fly fast enough, and you may be able to have a nose-down attitude while still maintaining level flight. You're generating the same amount of lift, but trading angle-of-attack for speed.
"Two and a Half Men" was filmed in front of a live ostrich.
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 4, posted (7 months 6 days 1 hour ago) and read 5618 times:
Quoting JumboJim747 (Reply 2): RushmoreAir thanks for the reply but wouldn't a nose down angle lose altitude?
To expand on what vikkyvik said...
On most aircraft the wings are mounted at a small positive angle to the fuselage, and the lift comes from the wings meeting the oncoming air at a positive angle. For example, if an aircraft hand its wings mounted with a five degree incidence, and it was flying straight and level with a one degree down deck angle, the wings would still have a four degree positive angle of attack. Whether that's enough to sustain level flight is (somewhat simplified) dependent on the weight of the aircraft, the size of the wing and indicated airspeed.
When an airplane is climbing or descending, the angle of its flight path subtracts or adds to the angle of attack of the wings. Consider our hypothetical aircraft with a five degree wing mounting incidence and a minus one degree deck angle in a two degree descent - the angle of attack would be six degrees.
Note that the fuselage generates little lift, and is usually of a grossly inefficient shape for doing so. But it will generate some, unless its angle of attack (separate from the wing's) is zero. But since it's very inefficient, that sort of thing is minimized.
So why not always fly with the fuselage at a zero degree alpha? You really can't unless you significantly alter your airspeed or altitude (which alters your indicated airspeed) based on the weight of the aircraft. In addition to playing heck with your schedules, that adds inefficiencies of its own. Especially for very long range aircraft, which can burn off 40% or more of their takeoff weight in fuel during a flight. So airspeed tends to remain approximately constant from beginning to end of cruise, but alpha decreases.
Sasha From Russia, joined May 1999, 858 posts, RR: 0 Reply 5, posted (7 months 5 days 19 hours ago) and read 5513 times:
Wouldn't this have something to do with Weight and Balance and loadsheet/trimsheet for this particular flight? The pilots would set the nose angle with stab trim depending on how the pax were seated and luggage loaded and how much of that.
Anyone can confirm or disprove this?
Thanks
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 6, posted (7 months 5 days 18 hours ago) and read 5427 times:
Quoting Sasha (Reply 5): Wouldn't this have something to do with Weight and Balance and loadsheet/trimsheet for this particular flight?
Yes. For a particular weight and balance, there may be no feasible altitude/speed combination that will have you perfectly level. If there is a feasible solution, it only works at one altitude/speed combination. Since your weight and balance are constantly changing as fuel burns, even if you do manage to get level you won't stay there long.
Quoting Sasha (Reply 5): The pilots would set the nose angle with stab trim depending on how the pax were seated and luggage loaded and how much of that.
Anyone can confirm or disprove this?
Stab trim does not directly set the nose angle. Stab trim sets the tail downforce, which has to counter the moment due to lift vs. weight vs. CG. Since lift is directly a function of AoA (and hence pitch), the only change in pitch from changing the stab is whatever is needed to alter wing lift just enough to counter the change in tail downforce (this is typically a very small angle).
The normal cycle is to set stab based on neutralizing the elevators and then set nose angle to whatever is required to fly level.
KPWMSpotter From United States of America, joined Dec 2006, 325 posts, RR: 2 Reply 7, posted (7 months 5 days 18 hours ago) and read 5423 times:
The simple answer to the OP's question is "because people like it that way."
Ideally an aircraft will cruise with its fuselage oriented directly into the relative wind; this way there will be minimum drag and minimum pitching moment created by the fuselage. Realistically, passengers don't like to be straight and level. As a rule of thumb, passenger transport aircraft will be designed to cruise with a one or two degree nose-up deck angle at cruise flight, strictly for passenger comfort. When I go flying in a light Cessna 152 it always feels like the nose is trying to bury itself in the ground in level flight. Trust me, that's an unpleasant sensation.
When designing an aircraft, engineers will have a pretty good idea of the angle of attack required to produce the required lift at cruise. For example, a given airfoil may require an angle of attack of 6 degrees to produce the expected cruise lift coefficient. If the engineer wants to design a flat deck angle at cruise, they simply mount the wing at an angle of incidence of 6 degrees to the fuselage. Adjust the angle of incidence and you will adjust the cruise deck angle.
The loading of an aircraft will affect the required angle of attack (more weight = more lift = more alpha), but these changes in AOA are generally very small compared to the design incidence of the wing.
rfields5421 From United States of America, joined Jul 2007, 6143 posts, RR: 25 Reply 8, posted (7 months 5 days 17 hours ago) and read 5405 times:
My understanding on this is that one reason the wing is built into a position to give a slightly nose up angle for the fuselage when the wing is in it's best flight position - is so that when the aircraft achieves sufficient lift to begin to 'fly' while on the takeoff runway - it will begin to lift the nose of the aircraft.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 9, posted (7 months 5 days 15 hours ago) and read 5348 times:
Quoting rfields5421 (Reply 8): is so that when the aircraft achieves sufficient lift to begin to 'fly' while on the takeoff runway - it will begin to lift the nose of the aircraft.
Nope. There isn't a large airliner out there that will takeoff (at normal V2 speeds and properly trimmed) without rotating. It's the horizontal stabilizer trim angle and elevator position that gives you rotation, not the wing incidence angle. If you mistrim the stabilizer the aircraft may autorotate (the nose will come up on its own) but this is a very bad thing and a flight operations error. The aircraft should not want to rotate until the pilot makes a positive nose-up command.
timz From United States of America, joined Sep 1999, 6466 posts, RR: 8 Reply 11, posted (7 months 5 days 14 hours ago) and read 5270 times:
Quoting KPWMSpotter (Reply 7): passenger transport aircraft will be designed to cruise with a one or two degree nose-up deck angle at cruise flight, strictly for passenger comfort.
I'm guessing passengers don't mind a level cabin-- but you're right, no one likes a nose-down descent. To avoid that, designers set the wing incidence for slightly nose-up cruise.
e38 From United States of America, joined May 2008, 231 posts, RR: 0 Reply 12, posted (7 months 5 days 12 hours ago) and read 5155 times:
Quoting tdscanuck (Reply 9), "There isn't a large airliner out there that will takeoff (at normal V2 speeds and properly trimmed) without rotating."
Tom, I realize you said "airliner" so this is not to contradict your reply, but the B-52--obviously not an airliner--would takeoff without rotating, as the photos would suggest.
Interestingly, the B-52 only has one flap position (well, I guess you could call it two)--either all the way up or all the way down, and the same flap setting--down--is used for both takeoff and approach/landing. It takes 60 seconds for the flaps to go from flaps up to flaps down, and, of course, 60 seconds to retract them. Amazing aircraft!
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 13, posted (7 months 5 days 7 hours ago) and read 4991 times:
Quoting e38 (Reply 12): Tom, I realize you said "airliner" so this is not to contradict your reply, but the B-52--obviously not an airliner--would takeoff without rotating, as the photos would suggest.
Yeah, I'd originally written "large jet" then thought of the B-52 and changed it to "airliner" just to cover that issue. The B-52 is an amazing bird in a lot of ways, including screwing up almost all generalization about large jets!
mrocktor From United States of America, joined Jan 2005, 1645 posts, RR: 51 Reply 15, posted (7 months 4 days 9 hours ago) and read 4556 times:
Quoting KPWMSpotter (Reply 7): Ideally an aircraft will cruise with its fuselage oriented directly into the relative wind; this way there will be minimum drag and minimum pitching moment created by the fuselage.
This is not strictly true. For most aircraft you get a lift contribution from the fuselage such that it is "worth" keeping a very slight positive attitude for minimum overall drag (i.e. if you flew with the fuselage completely flat, you'd need more lift from the wings thus increasing your induced drag).
Of course, as said in other replies, your optimal drag condition is just one point in the envelope - and you are generally near but not at that particular point.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 16, posted (7 months 4 days 9 hours ago) and read 4544 times:
Quoting mrocktor (Reply 15): For most aircraft you get a lift contribution from the fuselage such that it is "worth" keeping a very slight positive attitude for minimum overall drag (i.e. if you flew with the fuselage completely flat, you'd need more lift from the wings thus increasing your induced drag).
I agree you do get some lift contribution from the fuselage, but I don't think that's desirable from a drag point-of-view...the aspect ratio of the fuselage is terrible, so it's induced drag on the fuselage is extremely high. Moving "fuselage lift" to the wings is far more efficient from a drag standpoing. There are other reasons to have a slightly nose-up fuselage attitude in cruise but I don't think drag is one of them.
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 17, posted (7 months 4 days 8 hours ago) and read 4521 times:
Quoting tdscanuck (Reply 16): There are other reasons to have a slightly nose-up fuselage attitude in cruise but I don't think drag is one of them.
Might a slight nose-up attitude actually be the minimum drag position for the fuselage? After all the overall flow field around the aircraft is down (front-to-back, although obviously most strongly around the wings), and since we're talking subsonic aircraft, that effect will extend forward of the wings.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 19, posted (7 months 3 days 18 hours ago) and read 4317 times:
Quoting rwessel (Reply 17): Might a slight nose-up attitude actually be the minimum drag position for the fuselage? After all the overall flow field around the aircraft is down (front-to-back, although obviously most strongly around the wings), and since we're talking subsonic aircraft, that effect will extend forward of the wings.
I think so. You'd want the fuselage at the lowest-drag position and I agree that, for subsonic normal operation, that's going to be slightly nose-up to align with the local flowfield (same reason the engines on a lot of T-tails are slightly nose-up).
I was mostly disgreeing with this part:
Quoting mrocktor (Reply 15): (i.e. if you flew with the fuselage completely flat, you'd need more lift from the wings thus increasing your induced drag)
You don't want the fuselage generating much lift because the induced drag savings on the wing will be more than offset by the induced drag increase on the fuselage.
This sentence was terrible:
Quoting tdscanuck (Reply 16): There are other reasons to have a slightly nose-up fuselage attitude in cruise but I don't think drag is one of them.
That should have said "I don't think *fuselage lift* is one of them."
georgiaame From United States of America, joined Aug 2005, 867 posts, RR: 6 Reply 20, posted (6 months 4 weeks 1 day 16 hours ago) and read 3461 times:
Speaking of level or non level flight, perhaps it's my imagination, but when seated inside a jetliner I can swear that the aircraft assume a modest nose up position, almost immediately into the takeoff roll and well before the wheels leave the ground. Watching takeoffs rolls on the ground, I don't see anything but a level run until - obviously - rotation. Does the nose of a commercial jet indeed rise, early into the roll? Or is it one too many gin and tonics in the lounge?
"Trust, but verify!" An old Russian proverb, quoted often by a modern American hero
vikkyvik From United States of America, joined Jul 2003, 8204 posts, RR: 28 Reply 21, posted (6 months 4 weeks 1 day 13 hours ago) and read 3397 times:
Well, there will be a slight nose-up pitch tendency when power is applied to the engines, as (for wing-mounted engines) they are generally below the CG.
On landing, there will be a bit more of a nose-down tendency, since the landing gear and brakes are typically a bit farther below the CG.
"Two and a Half Men" was filmed in front of a live ostrich.
GAIsweetGAI From Norway, joined Jul 2006, 883 posts, RR: 7 Reply 23, posted (6 months 4 weeks 1 day 10 hours ago) and read 3329 times:
Quoting tdscanuck (Reply 19): Quoting rwessel (Reply 17):
Might a slight nose-up attitude actually be the minimum drag position for the fuselage? After all the overall flow field around the aircraft is down (front-to-back, although obviously most strongly around the wings), and since we're talking subsonic aircraft, that effect will extend forward of the wings.
I think so. You'd want the fuselage at the lowest-drag position and I agree that, for subsonic normal operation, that's going to be slightly nose-up to align with the local flowfield (same reason the engines on a lot of T-tails are slightly nose-up).
From what I think I remember/understand, about 2 degrees nose-up is about the best L/D for a cylindrical fuselage...
"There is an art, or rather a knack to flying. The knack lies in learning how to throw yourself at the ground and miss."
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 24, posted (6 months 4 weeks 1 day 6 hours ago) and read 3283 times:
Quoting georgiaame (Reply 20): I can swear that the aircraft assume a modest nose up position, almost immediately into the takeoff roll and well before the wheels leave the ground.
The aircraft isn't tilting at all. And it's not too many gin-and-tonics...it's your inner ear playing tricks on you.
As soon as the takeoff roll starts, the aircraft (and you) begin accelerating forwards. The fluid in your inner ear sloshes to the back due to the acceleration...to your inner ear, it looks like gravity is now pointing somewhere behind you (human beings don't accelerate fast enough for long enough on their own for your brain to have evolved a model for externally imposed acceleration). Since your visual processors are party coupled into your inner ear (why the world spins when you're dizzy), your brain interprets the situation as "the airplane must be tilting nose up, that's the only way gravity could be pointing backwards from vertical".
Tom.
25 georgiaame: WOW! Now this is a topic that the FAA has NEVER brought up with us during our ENT (Ears, Nose, Throat) training sessions! It's going to cost me a smal
26 Fabo: Well, not really, since when you will be seated backwards, your inner ear would sense gravity pointing forwards, i.e again, tail must be going down.
27 tdscanuck: Like Fabo said, since you've reversed the direction of your inner ear along with your body, it will still look like the airplane is going nose-up. Ho
28 Starlionblue: This illusion is just one of many in aviation. Another classic is if you level off abruptly from a climb it can feel like are tumbling backwards. Or
29 KELPkid: A most annoying aircraft to fly is the original Cessna Model 336 Skymaster (the one with the fixed gear). Due to a design error by Cessna, it cruises
30 timz: What did Cessna have to do to change that?
31 rwessel: I thought it was the other way around - the 336 cruised nose-high, so they increased the incidence of the wings on the 337 to *lower* the nose.
32 mrocktor: Intuitively you'd think so, but I've seen CFD that says otherwise. Keep in mind that the fuselage has a huge planform area, and is starting from zero
33 Aircellist: You've been consistently giving interesting explanations, but this one is particularly fascinating!
34 KELPkid: They changed the angle of incidence all right...at least that's what everyone who seems to know anything about the Skymaster says. Having had flown a
35 B777LRF: Was out flying as a cargo attendant the other day, occupying a cockpit jumpseat for most of the flight. We were in an A300-600F, and at 13000ft clocki
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