Why Fly Nose Up ?
 

This is one thing that's been bugging me for some time:

When in the cruise, all passenger jets fly nose up rather than with the fuselage level. To me this is a disadvantage in that the canted attitude creates more parasite drag + higher fuel burn, surely the best thing would be to increase the incidence of the wing and fly fuselage level?

This also helps in the climb, descent and flare; i.e. there isn't such a nose high attitude and visibility over the nose is better.

Anybody got a clue ?

I apologise for being a computer nerd - I'm actually quite a nice normal person ! However, I did do some work on SWORD at BA for a few years. It was determined that it was more fuel efficient for an a/c to fly at an angle of 3 or 4 degrees up from the vertical. Not really answered the question but apparently it did make considerable fuel savings. Kept a few programmers in employment for a year or two.

And no, I did not design or write the SWORD software. I just had to support it day and night. My pilot friends tell me the acronym stands for Sure Way Of Running Dry. Old joke I suspect.

Life's never quite so simple as you'd think, although I don't think the nose-up attitude, if you're right, is large.

The most efficient cruise is achieved at the speed at which the specific air range (nm/tonne of fuel or whatever measure you care to use) is greatest, not at which actual fuel consumption is lowest.

So, by the time you've factored in engine efficiency, altitude, variation in L:D with attitude, wind, colour of the pilots socks, etc. you just occasionally get a slightly surprising result.

For example, I was recently flight testing a slowish light aircraft perfectly content to cruise at about 85 kn, and most comfortable cruising about 75 kn. When I dropped all the numbers out I discovered a best range speed of 51 kn (giving an SAR of about 5 nm/L), whereas cruising at 75 kn (which as a pilot felt right) I got an SAR of nearer 3 nm/L - a huge reduction in range or, for a private owner whose primary cost is fuel not hours, a large increase in cost from A to B.

So, although I don't have enough experience of airliners to know if BF's facts are truly correct, they don't as presented surprise me hugely.

G

Isn't automation wonderful.

I typed "L colon D" meaning "lift to drag ratio", and get a green man with big teeth :D

Thank goodness I'm only paid to understand aeroplanes, computers frighten me.

G

Thanks for the input Ghengis, I take your point about best range speed but I'm really trying to understand why an aircraft designer would accept this "extra" drag when it isn't really necessary?

I'm thinking mainly about long haul flights where you're looking for that extra fraction of a percentage point.

sorry you guys you got it all wrong !!!!!!!!!!!!!!!
its so the pax can laugh at the trolly dollys struggling up and down the incline

BF - Good query - Can't resist taking a shot at it, but am not a certifiable aerodynamicist, so what follows is speculative in part:

1) Real world aircraft designs must accomodate constraints across the envelope. Some of the biggies are a) reasonable rotate / stall speeds at an AOA angle somewhat consistent with placement of the landing gear b) adequate elevator control authority at all allowed weights at all allowed speeds, including manoeuvers in steep banks, stall recovery, crosswind, engines out, etc. c) various vibration damping, balance, and stability issues in level flight, climb, turns, etc.

2) In order to satisfy the many constraints of 1), some aerodynamic efficiency is sacrificed for stability and controlability. As a result, the most efficient performance at any given speed / weight / altitude combination is not going to be wholly intuitive, and the parameters move around a lot for different speeds.

3) As aircraft grow bigger, they grow wider. While a circular cross-section is likely most efficient from pov of drag, an elipse or even a rectangle are vastly more efficient from the manufacturing and operational perspecitves of simpler structures, more flexible load capacity, passenger comfort, turnaround convenience, etc. so aerodynamic compromises are made for operational reasons as well as whole-envelope aerodynamics. As a result, practical large capacity aircraft tend to have 'fat bottoms'.

4) All of the above is pretty evident. The parts I am a little less sure about - from memory - are these: a)control and balance requirements from 1) require a lot more horizontal tail than simple efficiency would dictate. In level flight in a streamlined configuration, that big tail is actually burning energy to push the nose down. The efficient cruise configuration lets the tail drop into a more neutral zone of operation where it just holds up its end of the aircraft, contributing useful net lift with less opposition to the wing and thus less overall drag. This combines with b) the angular cant of the cabin portion of the airframe into the relative wind. The fuselage is designed to surf the breeze somewhat in long range cruise - acting as a lifting body - so in the tail-low surfing approach, the fuse actually increases lift/drag at appropriate speeds, squeezing out more forward inches per pound of fuel, even while making it harder to manage a cart in the aisles.

Just to throw my 2d into the ring...

Remember the old formula L= Co-efficient of Lift x 1/2 p x Vsquared x S

As p (the Greek letter rho representing air density) decreases with increased altitude, lift decreases. To maintain the same amount of lift you have to increase the Co-efficient of Lift or V (IAS) or S (the surface area of the wing). Increased IAS means more power ie. fuel burn. Increased S means something akin to Flowler flaps that would be very complicated. That leaves Co-efficient of Lift. Co-efficient of Lift can be increased by increasing the AoA. Hence, at higher altitude an aircraft needs to maintain a higher nose attitude.

Incidently, 4 degrees is generally recognised as roughly the most efficient AoA.

Why they don't just increase the Angle of Incidence (the angle at which the wings are attached to the fusilage), I don't know. For that, you'll have to ask an expert.

Hope this helps and I welcome any comments to correct any erroneous statements.

Cheers, LP :)

Have to agree and disagree with arcniz on this.

As far as my feeble memory can recall the tailplane/stab is always acting upside down. IE it is always pulling the nose UP not Down, (in a conventional stable non-flybywire a/c).
Also large airliners have adjustable/variable incidence stabs so the issue of pointing the a/c in the right attitude just to get the stab in line with airflow is a bit of a nonsense, sorry.

However, an engineer instructer once told me that the reason airliners fly slightly nose up is because the fuselage actually adds to the total a/c lift. On that we can agree.

Hope this helps:)

Just for info........i thought Diesel Tens flew nose down????

Don't know about the Tens, but the Nines certainly did !!

Thanks for the replies everybody - there's some good thinking going on out there.

Having thought about it a bit more, there's one thing that occurs to me although it could be considered a bit of a cop-out:

I know from my own calcs that the best you can do in engineering mathematics is an accurate approximation - we do qualification/certification testing in order to prove that our theory is correct.

Having done the flight testing at the designed cruise speed; is it perhaps considered too expensive/time consuming to correct a "minor" innacuracy in incidence setting and still meet delivery requirements?

However, I'm not giving up yet!

Its simple. They do it so if they hit a bump they tend to jump over it

Why fly nose up?
 

On most aircraft the wing is set about 1 degree up relative to the fuselage. At the start of the cruise when weight is high this will leave the fuselage a bit nose up. As weight reduces it will come down a bit. The general idea is to keep it aligned with the airflow as far as possible over the whole flight, however flow over the fuselage generates a bit of lift which is in the right direction even if the fuselage isn't as good at the job as the wings. If it was aligned with the flow at the start of cruise it would be pointing down at the end, generating both negative lift and positive drag.