Why is an aircraft aisle not level when cruising?

[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]

Originally Posted by Davef68

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]

Originally Posted by WHBM

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.

*****

Originally Posted by PDR1

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]

Originally Posted by 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.

I think you may have overdone the simplification slightly.

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.