englandair
Posts: 2193
Joined: Tue Oct 10, 2000 4:34 am

Why Is It........?

Sun Oct 29, 2000 3:33 am

Why is it that even when the aircraft's cruising, the cabin crew seem to pull/push the trollies uphill?
 
Guest

RE: Why Is It........?

Sun Oct 29, 2000 4:35 am

Pilots always needs to do minor and sometime major adjustements on the throttles while in flight due due a lot of things, winds, CG always moving ...ect
Once on a flight on an A310 the pilot told me that managing the throtlle is a real "full-time job".

Nicolas - Montreal
 
Guest

RE: Why Is It........?

Sun Oct 29, 2000 4:57 am

Also if you ever have a chance to visit the flight deck, take a look at the attitude indicator and you will notice that it's showing the aircraft to be cruising with the nose slightly up.
 
calpilot
Posts: 881
Joined: Sun Oct 17, 1999 5:16 am

Level Flt., Not Level Pitch.

Sun Oct 29, 2000 5:05 am

Sorry Nicolas, that is not correct.

The reason is because large transport aircraft do not have 0 degrees of pitch in level flight. For instance my B757 uses about 4 degrees of nose up pitch for level flight.

I don't know what that A310 pilot was talking about with the throtlles, unless the A/T's were inop. Even then it's not that bad! Even the B727, and DC9, take only small adjustments throughout cruise...
 
Guest

RE: Why Is It........?

Sun Oct 29, 2000 5:58 am

CALpilot: I was not talking only during cruise, but also all flight mode, climb, cruise, descent.
 
DL_Mech
Posts: 2061
Joined: Fri Feb 18, 2000 7:48 am

RE: Why Is It........?

Sun Oct 29, 2000 10:02 am

I was told by a Lockheed engineer that the floor on the L-1011 was designed to be level at cruise. However, when the 70's fuel crunch appeared, airlines started flying the airplane at a lower Mach number to save fuel. The airplane started flying with a slight nose up pitch and f/a's have been pushing carts uphill ever since. You can see that the streaks on the fuselage (cabin door leaks,etc.) all streak at an upward angle.
This plane is built to withstand anything... except a bad pilot.
 
Greeneyes53787
Posts: 817
Joined: Tue Aug 08, 2000 10:34 am

RE: Why Is It........?

Tue Oct 31, 2000 3:39 am

I think the first responder misread the question and thought it pertained to throttles instead of the cart of food inside the passenger cabin.

As to the nose high attitude discussion, I think this idea of moving the crew and passengers around to suit the wings' needs is primitive. With wing boxes fixed (like they are) wings can still be designed to pivot slightly. I don't advocate doing this for take-off or landing. But I think a modifyable wing pitch is a good idea for comfortable flight.

It is an expensive proposition, but probably not as much as people think.

Greeneyes
 
prebennorholm
Posts: 6409
Joined: Tue Mar 21, 2000 6:25 am

RE: Why Is It........?

Tue Oct 31, 2000 8:43 am

What goes up, must come down....
When an airliner cruises at level flight - and we are talking about a heavier-than-air vehicle, not a blimp or such. Then it only does so because its wing accellerates huge amounts of air masses downwards. The energy of accellerating thousands of tonnes of air downwards every minute is simply what keeps the plane in the air.
Most airliners are designed so the fuselage is approximately lined up with the average airflow around it. It means that it must be pointing a little upwards to create minimum drag.
So when the F/Aa are excercising their muscles pulling the buze cart upwards, then they must send a thought to the airline accountant who smiles at the lower fuel bill.
I read somewhere many years ago that the A300 was designed to cruise with a more or less horizontal fuselage attitude in order to maximise comfort mostly for the F/As, but at a small fuel burn disadvantage. Which didn't mean too much since the early A300s were rather short legged. I really don't know if Airbus has discontinued that trend or what. It seems to me that an A320 is more flat at cruise than for instance a 737. But there are so many variables in flight that I as a passenger have no chance to judge that with any accurracy.
In any case I would assume that also Airbus longhoulers - A330/340s - have a fuselage attitude optimised for fuel economy and nothing else.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
 
CP744
Posts: 192
Joined: Wed Jul 26, 2000 3:08 am

RE: Why Is It........?

Tue Oct 31, 2000 9:52 am

I don't want to sound out to lunch.... but...... Am I wrong by thinking the aircraft stays in the air through the pressure difference between the top and bottom of the wing.... I don't think it has anything to do with pushing air downwards?????? Hence Bernoulli’s (sp) principle. Please correct me if I'm wrong.......

Bill
 
timz
Posts: 6084
Joined: Fri Sep 17, 1999 7:43 am

Passenger Psychology

Tue Oct 31, 2000 10:11 am

Presumably the designers could set the wing incidence so the fuselage was level in the cruise, but there may be another reason they don't do that.

I once read that someone (don't recall whether it was Boeing or Douglas or whoever) had found (decades ago) that passengers don't like being pointed down. We don't care how far back the seat tilts during the climb, but we are uncomfortable being tilted forward during the descent.
 
Airgypsy
Posts: 130
Joined: Tue Nov 23, 1999 11:02 am

RE: Why Is It........?

Tue Oct 31, 2000 4:59 pm

Critical flight phases require the Angle of Incidence (angle of the wing to the fuselage) to be within a certain range depending on the ammount of lift required.
Not enought AOI and the tail would drag on landing and takeoff.
Too Much AOI and the aircraft fuselage would begin to contribute to "down force"
Perfect AOI is the one that lifts the aircraft without banging the tail and takeoff or landing and keeps the lift of the fuselage neutral to positive at the highest speed.
Angle of Attack (AOA) reduces with speed increase. Ever watch a B-52 depart empty? It appears to be at a nosedown angle because of the huge lift the wing generates. Looks normal when fully loaded.
And this brings in the other factor. The F/A will get a lot more exercise on a fully booked flight.
Airgypsy
 
prebennorholm
Posts: 6409
Joined: Tue Mar 21, 2000 6:25 am

RE: Why Is It........?

Wed Nov 01, 2000 4:10 am

CP744 wrote:
-------------------------------
I don't want to sound out to lunch.... but...... Am I wrong by thinking the aircraft stays in the air through the pressure difference between the top and bottom of the wing.... I don't think it has anything to do with pushing air downwards?????? Hence Bernoulli’s (sp) principle. Please correct me if I'm wrong.......

Bill
-------------------------------
Bill, you are certainly right. But just as the aircraft reacts on that pressure difference - by flying in the air instead of sitting on the ground - then the air masses reacts the same way (or more precisely the opposite way).
There is no way that the air masses can stay unaffected by for instance a 700,000 lbs 747, which tells the air to carry it.
In layman's terms: The air masses are pressed downwards.
To every action there is a corresponding reaction.
It is not so that there is a constant pressure difference which keeps the plane in the air. Instead thousand of tonnes of air per minute is accellerated downwards by the pressure difference. The force needed for that accelleration is the "reaction".
Fortunately air is not weightless - it would make flight impossible.
If you take a cube of air at sea level with sides of the same length as the wing span of a 747-400, then the weight of the air inside that cube is some 300 or 350 tonnes, or roughly the same as a fully loaded 747-400. It takes a lot of power to "push" such air masses downwards in only a fraction of a second. It takes exactly the same power as pushing a 300-350 tonnes boulder the same distance in the same time. That's - again in layman's terms - what makes flight possible.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
 
CP744
Posts: 192
Joined: Wed Jul 26, 2000 3:08 am

RE: Why Is It........?

Wed Nov 01, 2000 9:36 am

Thanks..Preben.....

Bill
 
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fanoftristars
Posts: 1544
Joined: Mon Jul 17, 2000 9:03 am

RE: Why Is It........?

Wed Nov 01, 2000 11:57 am

My friends mom, who was a flight attendent for United years ago, said she hated working the DC-10 because it had an unusually high angle to push the carts up and down. Is this true or just an incorrect perception?
"FLY DELTA JETS"
 
fr8tdog
Posts: 120
Joined: Wed Feb 16, 2000 4:25 pm

RE: Why Is It........?

Thu Nov 02, 2000 4:01 am

UhOh not the Airpump theory again!.....
It is a combination of Bernoulli's principle and Newtons third law that allows the wing to produce lift.
(in simple terms.)

The airpump theory does not describe, how a rotating drum can develop lift......







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