Why is an aircraft aisle not level when cruising?

[Eboy]

I searched online for an answer to this but just saw arguing. I am not an expert and am sorry if I am asking this question incorrectly or using the wrong terminology.

An aisle in an aircraft during cruise is almost imperceptibly sloped down from front to back (if a can is dropped it rolls back). Why not just make the aisle level? What I see online is that the wings have to have a certain angle of attack. It seems to me, though, that aircraft designers could tweak the wing incidence angle so the wings have the right angle of attack with zero pitch. Thank you for considering my question.

[DaveReidUK]

I searched online for an answer to this but just saw arguing. I am not an expert and am sorry if I am asking this question incorrectly or using the wrong terminology.

An aisle in an aircraft during cruise is almost imperceptibly sloped down from front to back (if a can is dropped it rolls back). Why not just make the aisle level? What I see online is that the wings have to have a certain angle of attack. It seems to me, though, that aircraft designers could tweak the wing incidence angle so the wings have the right angle of attack with zero pitch. Thank you for considering my question.

The "right angle of attack" will vary during the course of a flight as the aircraft burns fuel and therefore gets lighter, requiring less lift (i.e. a lower angle of attack) from the wings.

[GordonR_Cape]

Briefly touched on in a recent discussion elsewhere: https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-57.html#post10310305

As has been mentioned, transport aircraft that are optimized to maximize L/D are designed to fly with a small, positive pitch attitude. To design an airplane to fly with deck angle of exactly zero in cruise would be to leave unrealized performance on the table! Separately, optimization of takeoff, landing, and ground operations usually results in a slightly negative deck angle when taxiing. I strongly doubt that any commercial transport airplane has ever been designed to have exactly zero deck angle either during ground taxi or cruise.

[TURIN]

You are correct. The wing does have an incidence angle. However, the c of g and aircraft mass will change throughout the flight as fuel is used (swept wing aircraft) the 'deck angle' will therefore change as the angle of attack alters to compensate for altitude, airspeed, and lift requirement. There is also the fact that the fuselage adds to the total lift of the aircraft and there will be a sweet spot angle for the entire aircraft to meet the airflow.

Someone clever will be along in a bit with some maths to explain.

[RevMan2]

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1644x1006/04560bbb_0568_4abf_bc4c_f05633a938cf_d072e6d288ccc98db59398d d147188da01768d32.gif

[vctenderness]

RevMan2. I was going to do that but just did t want to show off!

[TURIN]

See I told you so.

[S.o.S.]

Thank You RevMan2 just please don't ask us any questions ...

[krismiler]

On the DC3, the Pilots could tell when the flight attendant walked to the rear of the aircraft because the autopilot applied forward trim. This is an adjustment the pilot makes to enable the aircraft to fly level without having to apply forward or backward pressure on the controls.

At the beginning of a flight when the aircraft is heavy with fuel it will fly slow and nose up, as fuel burns off it gets lighter and flys faster, the nose then needs to be moved down or it will climb.

[cooperplace]

On the DC3, the Pilots could tell when the flight attendant walked to the rear of the aircraft because the autopilot applied forward trim.
this explains everything. Clearly in the big jets, if the floor is sloping front to back, it's because all the FA's are in the back gossiping.

[krismiler]

No smart arse, it's to illustrate how changing weight distribution affects an aircraft and how it is dealt with, either by the pilots if being flown manually or by the automatics if engaged. The person asking the question is obviously not a pilot so I've kept the explanation as simple as possible. I've avoided using complicated technical terms such as centre of pressure, angle of attack,formulae for lift and why a rearward CofG is beneficial in certain situations but detrimental in others.

[RevMan2]

Clearly in the big jets, if the floor is sloping front to back, it's because all the FA's are in the back gossiping.

And if the floor’s sloping back to front, it’s because the cockpit just got paid and haven’t had time to get the cash to the bank....

[PJD1]

And if the floor’s sloping back to front, it’s because the cockpit just got paid and haven’t had time to get the cash to the bank....

I was wondering what the "pw" stood for in your equation above - now I understand, it's pilot's wallet!

[cooperplace]

No smart arse,.

OK, I apologize, it's because the FA's are all in the back not gossiping but hard at work in the galley.

[jimjim1]

There is also the fact that the fuselage adds to the total lift of the aircraft and there will be a sweet spot angle for the entire aircraft to meet the airflow.

I think this is the key point.

I suspect I have read somewhere that for best efficiency the fuselage is operated at positive angle of attack and it generates some lift. The floor of course could be arranged to be horizontal even with a +ve pitch on the hull, however that is apparently not done:-)

[DaveReidUK]

Gosh, explaining all of this has been a bit of an uphill struggle.

[PAXboy]

Ouch DRUK. After that observation - it's downhill all the way ...

[Hokulea]

RevMan2 - you don't happen to be a mapmaker, do you? ;)

[RevMan2]

Hokulea - Sshhhhh!

[eckhard]

The floor of course could be arranged to be horizontal even with a +ve pitch on the hull, however that is apparently not done:-)

I suppose as most aircraft fuselages have a circular cross-section, the installation of a sloping floor would involve a tapering planform, which would add to complexity and cost.

The crew bunks on the 747-400 were slightly inclined, down towards the nose, which made them flat. Very comfy they were, too!

[Davef68]

Can you define your variables please?

[WHBM]

Separately, optimization of takeoff, landing, and ground operations usually results in a slightly negative deck angle when taxiing.Now I do notice, of two common types at London City, that the Embraer 170 has a quite noticeable nose-upward angle when on the ground, whereas the stretched Embraer 190 has a quite noticeable nose-downward tilt. Always intrigued me why this difference would be on two otherwise comparable types.

The Handley Page Hermes of 1950 had apparently such a nose-upward angle in the cruise that passing aircraft were known to enquire, presumably genuinely, if everything was alright. It was always accepted that it should have had a forward fuselage plug to balance it. Quite how the designers at Radlett got it so wrong (it was calculated to have notably impacted the fuel consumption) is not recorded. BOAC disposed of them prematurely after just a few years of service.

[TLDNMCL]

Can you define your variables please?

Now there's a euphamism (or two) for you. Apologies, my mind has been operating a little differently since reading about the ex-Cranwell Flt. Sgt's after hours activities...

[PDR1]

It was always accepted that it should have had a forward fuselage plug to balance it.

That may or may not be true, but CG position has no effect on flying attitude. The fuselage attitude is set almost entirely by justthe wing incidence - the angle at which the wings are nailed to the fuselage. The CG position would affect how hard the tailplane has to work to hold the wing in that attitude, but not the attitude itself.

FWIW I understand that on transport aircraft (and indeed any aircraft designed for long periods of straight&level cruising) the wing incidence is chosen such that the rear fuselage is aligned to the airflow to reduce drag, and the airflow has significant downwash aft of the wings as a result of generating all that lift. Thus the aeroplane "sits" nose-up. This does mean that the forward fuselage has a significant angle of attack, but the drag produced by that is less than the drag that would be produced by the rear fuselage if flown with the fuselage in a level attitude. Obviously as the fuel is burned off the required wing AoA reduces, and that's why for VERY long ranges the optimum approach is to cruise-climb so that the required AoA increases at the same rate as the fuel-burn decreases it (if you see what I mean).

Someone mentioned using fuselage lift as a contribution to the overall lift - actually you don't want to do this. The reason is simply that the fuselage is very bad at developing lift because it has such a tiny aspect ratio. So each pound of lift (on old money) that you get from the fuselage produces about 30 times the induced drag that you'd get by developing the same amount of lift from the wings. So you really, really want the fuselage to be as close as possible to zero lift coefficient, and that could also be why the wing incidence is chosen to get zero AoA on as much of the fuselage as possible in the cruise.

PDR

[Ancient Observer]

If there were designers in Radlett, it is a shame that they did not turn their attention to Radlett itself.

[boisbrule]

PDR1, I think what the designer wants is not zero lift from the fuselage, but that the lift from the fuselage is what would be delivered by the adjacent bits of wing if they were projected across the fuselage span and there were no fuselage. That is, what you want is a smooth spanwise lift distribution including the fuselage, part of the ideal elliptical lift distribution. I'm not an aerodynamicist but I suspect that this is a lot easier said than done for different angles of attack, and that designers seek a compromise between low drag in cruise and perhaps low drag, say, at liftoff with an engine out.

Also, I think that for long range one would cruise climb to maintain a constant angle of attack, for minimum drag, or in practice a slightly lower angle and higher speed, for speed stability, staying on the right side of the drag curve, I recall reading a graph of Lindberg's IAS against time in the Spirit of St. Louis, showing a continuously decreasing speed at roughly constant altitude, corresponding to a constant angle of attack and a decreasing fuel load.

*****

That may or may not be true, but CG position has no effect on flying attitude. The fuselage attitude is set almost entirely by justthe wing incidence - the angle at which the wings are nailed to the fuselage. The CG position would affect how hard the tailplane has to work to hold the wing in that attitude, but not the attitude itself.

FWIW I understand that on transport aircraft (and indeed any aircraft designed for long periods of straight&level cruising) the wing incidence is chosen such that the rear fuselage is aligned to the airflow to reduce drag, and the airflow has significant downwash aft of the wings as a result of generating all that lift. Thus the aeroplane "sits" nose-up. This does mean that the forward fuselage has a significant angle of attack, but the drag produced by that is less than the drag that would be produced by the rear fuselage if flown with the fuselage in a level attitude. Obviously as the fuel is burned off the required wing AoA reduces, and that's why for VERY long ranges the optimum approach is to cruise-climb so that the required AoA increases at the same rate as the fuel-burn decreases it (if you see what I mean).

Someone mentioned using fuselage lift as a contribution to the overall lift - actually you don't want to do this. The reason is simply that the fuselage is very bad at developing lift because it has such a tiny aspect ratio. So each pound of lift (on old money) that you get from the fuselage produces about 30 times the induced drag that you'd get by developing the same amount of lift from the wings. So you really, really want the fuselage to be as close as possible to zero lift coefficient, and that could also be why the wing incidence is chosen to get zero AoA on as much of the fuselage as possible in the cruise.

PDR

[old,not bold]

I think that those of us who are fairly simple-minded, like me, only need to know that airliners tend to be cruised at the optimum speed for fuel economy, which is usually rather slower than maximum continuous cruise speed. To maintain its assigned flight level at this slower speed calls for a larger angle of attack, achieved by pointing the nose up a bit. So the cabin floor slopes upwards.

(IIRC the VC10 was particularly noted for this, with an optimum economic cruise speed only a few knots higher than the clean stalling speed at cruise altitudes. Cabin crew developed good leg muscles pushing the carts uphill.)

[DaveReidUK]

I think that those of us who are fairly simple-minded, like me, only need to know that airliners tend to be cruised at the optimum speed for fuel economy, which is usually rather slower than maximum continuous cruise speed.

I think you may have overdone the simplification slightly. :O

When you introduce Cost Index into the mix, the optimum cruising speed may differ from LRC, and may also vary depending on wind conditions.

[ZeBedie]

Whilst the fuselage is inefficient at producing lift, it would be really bad for efficiency if the fuselage produced a down-force (negative lift). So the designer aims to make the fuselage slightly lifting in all possible cruise conditions, to avoid the costly negative lift situation. Thus the fuselage flies at a lift producing AoA because that's much better than striving for the unattainable ideal of a lift neutral fuselage.