jumbojim747
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Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 3:43 am

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.
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RushmoreAir
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 3:55 am

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.

http://en.wikipedia.org/wiki/Angle_of_attack
0° Angle Of Attack In The Cruise (by Faro Sep 6 2007 in Tech Ops)
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jumbojim747
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 5:53 am

RushmoreAir thanks for the reply but wouldn't a nose down angle lose altitude?
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vikkyvik
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 6:00 am

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.
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rwessel
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 6:34 am

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
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 11:58 am

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?
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tdscanuck
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 1:38 pm

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.

Tom.
 
KPWMSpotter
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 1:40 pm

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.
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rfields5421
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 2:00 pm

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
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 3:45 pm

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.

Tom.
 
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CALTECH
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 5:00 pm

UNITED We Stand
 
timz
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 5:21 pm

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
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 17, 2012 7:29 pm

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!

e38

[Edited 2012-10-17 12:33:18]
 
tdscanuck
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 18, 2012 12:42 am

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!

Tom.
 
PC12Fan
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 18, 2012 1:53 pm

Excellent post guys! I've often wondered about this and why it's not "engineered" out. A lot of questions were answered and then some.

  
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mrocktor
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 18, 2012 10:02 pm

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
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 18, 2012 10:33 pm

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.

Tom.
 
rwessel
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 18, 2012 11:29 pm

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.
 
vikkyvik
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Fri Oct 19, 2012 4:50 am

Quoting rwessel (Reply 17):
Might a slight nose-up attitude actually be the minimum drag position for the fuselage?

That's what I'd always heard - that a slight nose-up attitude produces less drag - but I've never seen any explanation as to why.
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tdscanuck
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Fri Oct 19, 2012 1:14 pm

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."

Tom.
 
georgiaame
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Tue Oct 23, 2012 3:12 pm

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?
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vikkyvik
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Tue Oct 23, 2012 5:56 pm

Quoting georgiaame (Reply 20):

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.
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jumbojim747
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Tue Oct 23, 2012 5:59 pm

Ive always noticed that more on rear engine mounted aircraft even noticeable from the outside
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GAIsweetGAI
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Tue Oct 23, 2012 9:25 pm

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...
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tdscanuck
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 24, 2012 12:47 am

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.
 
georgiaame
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 24, 2012 2:07 am

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 small fortune, not to mention a loss of my Delta SkyMiles, to have to fly BA in business, in one of the rear facing seats. If what you say is true, and it sounds completely reasonable, I should then see the TAIL of the aircraft rising during the roll out. Fascinating!
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Fabo
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 24, 2012 11:09 am

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.
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tdscanuck
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 24, 2012 7:43 pm

Quoting georgiaame (Reply 25):
BA in business, in one of the rear facing seats. If what you say is true, and it sounds completely reasonable, I should then see the TAIL of the aircraft rising during the roll out.

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. However, since you're facing backwards, you'll now be looking "downhill." Having flown facing backwards in a BA business class seat, I completely agree, it's weird. You feel like you're going to go rolling down the cabin.

Tom.
 
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Starlionblue
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Wed Oct 24, 2012 8:00 pm

Quoting georgiaame (Reply 25):

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 small fortune, not to mention a loss of my Delta SkyMiles, to have to fly BA in business, in one of the rear facing seats. If what you say is true, and it sounds completely reasonable, I should then see the TAIL of the aircraft rising during the roll out. Fascinating!

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 if you are in a steady state turn and level the wings it can feel as if you are now banked in the opposite direction.

Once you start instrument training one of the main messages is: "Your senses can deceive you. Don't trust them."
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KELPkid
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 25, 2012 12:26 am

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 with a nose-down attitude. It drives me nuts to fly one    Cessna fixed the problem with the Model 337 (the retractable geared model)...
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timz
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 25, 2012 1:14 am

What did Cessna have to do to change that?
 
rwessel
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 25, 2012 3:03 am

Quoting KELPkid (Reply 29):
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 with a nose-down attitude. It drives me nuts to fly one Cessna fixed the problem with the Model 337 (the retractable geared model)...

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.
 
mrocktor
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Thu Oct 25, 2012 10:02 pm

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.

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 lift. The wing comparatively has a much smaller planform area and is already developing significant lift. The spanwise (feels wrong using the term for a fuselage) pressure differential you generate by giving your fuselage a small AoA when compared to what it would take to generate the same lift from the wing is affected by both factors. You'd need more AoA from the wing (because it is smaller), and you start from a flow pattern that already has a significant spanwise component.

Aero is black magic though... I'm not going to pretend to understand it.

Quoting rwessel (Reply 17):
Might a slight nose-up attitude actually be the minimum drag position for the fuselage?

It is, at least for the "tube with wings" configuration aircraft I have seen data on.
 
Aircellist
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Fri Oct 26, 2012 4:54 am

Quoting tdscanuck (Reply 24):
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".

You've been consistently giving interesting explanations, but this one is particularly fascinating!
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KELPkid
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Fri Oct 26, 2012 9:05 pm

Quoting rwessel (Reply 31):
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.

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 couple of demo flights on one particular 336 with my boss many years ago who was an affecionado of the type (Skymaster in general, not necessarily the 336), and was considering it for his 135 overnight air cargo business, I can tell you it most definitely cruised nose down (at least with two people aboard!). When VFR, it is hard to resist the urge to apply some back pressure on the yoke when your view of the world makes you think the trajectory of the aircraft is downward. BTW he decided that the 336 was a piece of smelly stuff  
Quoting timz (Reply 30):
What did Cessna have to do to change that?

See rwessel's reply, the angle of incidence of the wings and tail booms was changed. IIRC, changing one changes the other, as the tail booms are attached to the wing spar, not the fuselage...and counterintuitively, increasing the wing's angle of incidence fixed the nose down cruise attitude problem since it it changed the angle of incidence of the elevator at the same time (in the right direction)!
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B777LRF
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RE: Why Do Aircraft Cruise With A Nose Up Angle?

Fri Oct 26, 2012 10:40 pm

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 clocking 330 knots she was perfectly level, which is quite an unusual experience in an airliner. On the next leg we were up at FL280/M.79 and "normal service" was resumed, that is with approx. 3 degree nose up attitude; I liked 13K feet and 330kts much better.

Oh, forgot to mention, a 16 minute flight in a wide body is pretty fun, though the fly boys were more than a bit busy. Was late getting a clearance to descend, so at 9000ft we slowed to 240 and dropped the gear while keeping the spoilers all the way out. Result: A respectable 5000ft/min descent and big smiles all around. Flying boxes is just so much more fun than hauling SLF  
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