Nose high during cruise?
Steve Derry
aircraft appears to maintain a nose-up pitch angle. This is not only
based on personal perception, but also the stewardesses say that the push
carts tend to move downhill (i.e. toward the rear of the aircraft).
Now during straight-and-level steady-state flight, the only noticeable
body acceleration should be gravity, which is balanced by upward lift.
All other forces should be balanced so as to produce no acceleration.
I thought that for efficiency, airliners were designed with the
appropriate angle of incidence between wings and fuselage so that at
cruise angle of attack, the fuselage would be "level" with the oncoming
airstream to minimize drag.
Why then do I notice this "uphill" effect? Is the fuselage actually
pitched up slightly? I have noticed this on many flights on jet
airliners, but have never noticed a "level / nose on the horizon"
attitude during a lengthy cruise.
--
Steve Derry
<s.d....@larc.nasa.gov>
David Stocker
> On many flights on jet airliners, I have noticed that during cruise the
> aircraft appears to maintain a nose-up pitch angle. This is not only
> based on personal perception, but also the stewardesses say that the push
> carts tend to move downhill (i.e. toward the rear of the aircraft).
>
[snip snip]
>
> Why then do I notice this "uphill" effect? Is the fuselage actually
> pitched up slightly? I have noticed this on many flights on jet
> airliners, but have never noticed a "level / nose on the horizon"
> attitude during a lengthy cruise.
>
I've noticed this too - I always assumed that the plane was
trimmed out to that attitude. The lift produced by the wings
is a function of the airspeed and angle of the relative wind
over them, this must balance with the weight of the plane,
which is of course not all located at the CG but distributed
about. While I suppose it is possible for everything to
balance in a perfectly level flight attitude, I would think
this is rarely the case.
When I am flying (not jetliners, mind you!), and I sense
that I am flying in a nose high attitude, I adjust the trim
(nose down) and power (up) accordingly. But this makes the
plane less stable in pitch, and it becomes harder to hold
a fixed altitude. Perhaps there is a stibility margin
involved with such a nose high attitude. Of course this
is not as efficient (i.e. most any airplane that doesn't
have afterburners can get "behind the power curve" - it can
be in such a nose-high attitude that it will take full power
just to hold altitude, and climb is not possible without
bringing the nose DOWN) - but the added stability is desired.
David Stocker
stock...@salem.ge.com (PP-ASEL, CAP, EAA, AOPA)
GE Drive Systems, 1501 Roanoke Blvd, Salem, Virginia, USA
703-387-7844 GE Dial Comm: 278-7844 Fax: 703-387-7651
Paul Vijgen
>
> On many flights on jet airliners, I have noticed that during cruise the
> aircraft appears to maintain a nose-up pitch angle.
>
> appropriate angle of incidence between wings and fuselage so that at
> cruise angle of attack, the fuselage would be "level" with the oncoming
> airstream to minimize drag.
>
> Why then do I notice this "uphill" effect? Is the fuselage actually
> pitched up slightly?
(i) Most currently used jet aircraft were originally designed in the 1960's
(747, DC10, 737, 727), i.e., before the '73 oil "crisis". To reduce fuel
burn, cruise Mach number in modern operation is likely lower than the
original design Mach number to reduce wave drag. For fixed weight and
altitude, a higher CL, and a higher angle of attack is needed.To test this
hypothesis, check the cabin attitude during cruise of recent designs, e.g.
A330/340 or 777.
(ii) Similarly, many current airliners are stretched and heavier versions of
original designs: increased weight requires again more lift and higher angle
of attack while other parameters are constant.
(iii) A relatively large fraction of weight of long-range aircraft is burnt-
off during cruising flight. Initially (i.e., first hours on transpacific
flight), a high aircraft incidence angle is required.
Paul Vijgen
p.m.v...@larc.nasa.gov
David Lednicer
There has been speculation in the "Nose high during cruise"
discussion as to the reason for this phenomona. Its really rather
simple. At the beginning of a flight, especially a long range one, the
airliner is rather heavy, particularly with fuel. To generate the lift
necessary to maintain altitude, it most fly at a certain angle of
attack. As the fuel burns off, the lift necessary decreases and the
angle of attack decreases. The angle that the wings are at (incidence)
relative to the fuselage is determined by defining a "typical" flight
condition, but as one might imagine, aircraft only fly at this "typical"
condition for short periods, hence you can sense a nose up or nose down
floor angle.
Incidentally, changing the trim tab setting on an aircraft does
not affect the stability - it only changes the elevator hinge moment and
stick force.
The Boeing 727 is notorious for cruising rather nose up.
This led to the famous "bootleg" manuver that those TWA pilots got caught
using over Michigan. In cruise, 727 pilots would throw the breakers on the
slats and then put the flaps out 5 deg or so to decrease the angle of
attack necessary to generate the lift required and thereby decrease the
floor angle. Legend has it that on the TWA flight, the flight engineer
didn't know what was going on and flipped the breakers for the slats back
on, leading to slat deployment in cruise. The pilots then tried to
retract the slats, but one stuck from the high airloads, resulting in a
wild ride before it ripped off. The airplane landed safely at Detroit
Metro and the cockpit voice recorder was found to be erased! Some of the
debris landed in one of my friend's front yard.
When we helped Valsan add winglets to the 727, we ran into a
problem with the loads, so we fell back on the same trick. We found that
by rigging the flaps several degrees down, we could move the loads back
in where they needed to be and at the same time we gained a slight fuel
burn decrease. Currently there are two wingletted 727s in Delta's fleet
and several privately owned ones (I think one is the Limited's - not only
do they have winglets, but they also have the Valsan reengining on a a
727-100 - giving them one hot ship!).
ps - I haven't been getting airliners for a while - it seems that some
weeny deleted my subscription!
-------------------------------------------------------------------
David Lednicer | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc. | email: da...@amiwest.com
2133 152nd Ave NE | tel: (206) 643-9090
Redmond, WA 98052 | fax: (206) 746-1299
Paul Raveling
: On many flights on jet airliners, I have noticed that during cruise the
: aircraft appears to maintain a nose-up pitch angle. ...
...
: I thought that for efficiency, airliners were designed with the
: cruise angle of attack, the fuselage would be "level" with the oncoming
: airstream to minimize drag.
Actually the nose-high attitude is deliberate a deliberate measure
to improve fuel economy. If you look at the fuselage alone it adds
a bit of drag, but that's far overshadowed by the improvement it makes
in the wing/fuselage's span loading (spanwise pressure
distribution). An additional contribution is that a positive
deck angle reduces the aircraft's nose-down pitching moment;
that allows using a slightly smaller horizontal stabilizer and
slightly lighter structure.
If you look at an aircraft nose-on, the optimal spanwise lift
distribution is elliptical. That means it's a nice smooth curve
even where it crosses the fuselage. Typically there's a huge
area of low pressure over the wings at their roots. If the
fuselage doesn't have an area of low pressure over it, the
airframe will pump energy into generating a vortex of sorts --
well, at least some screwy airflow -- at the wing/fuselage
junction. To minimize this mess airliner designers deliberately
gave the planes a significant positive deck angle in order to
minimize this pressure discontinuity.
The effect is quite significant on airliner economy. When
Eastern Airlines complained about the deck angle on their new
L1011's in 1974, Lockeed responded with a letter whose second
paragraph is this quote:
"Had Lockheed designed the L-1011 to fly with a level floor
in cruise, EAL's operating costs for their present fleet
of these airplanes would have increased by three million
dollars per year."
That quote was based on a fleet of 25 L1011's and a fuel cost
of 22 cents per gallon.
Lockheed cited these penalties for leveling the fuselage:
"In summary, the total drag increase associated with the
effect of fuselage rotation is equal to 2.2% of total
aircraft drag for each 1 degree that the floor angle
is decreased. ... on the L-1011 the structural penalties
associated with each 1 degree reduction in floor attitude
is about 900 pounds..."
-------------------
Paul Raveling
Rave...@netcom.com
Philip M. Chuang
> aircraft appears to maintain a nose-up pitch angle.
This is especially true for ultra-long-range cruise.
I read in FLIGHT International that Cathay Pacific
took this into account and adjusted their engine nacelles
of their long-range 747's 2 degrees downward, and
manageed to save fuel burn and increase the payload-
range of these planes. They call it POD-NOD.
Terrell D. Drinkard
> aircraft appears to maintain a nose-up pitch angle.
>
> appropriate angle of incidence between wings and fuselage so that at
> cruise angle of attack, the fuselage would be "level" with the oncoming
> airstream to minimize drag.
>
> Why then do I notice this "uphill" effect? Is the fuselage actually
> pitched up slightly?
I love the questions that come off this group. I always end up learning
quite a bit myself. Keep them coming!
I cornered one of the senior aero guys and sure enough, the aero fraternity
is responsible for the deck angle you've noticed.
The body, as a surface of revolution, develops a moment when flown through
the air at a positive angle of attack (nose up). Inviscid effect. If too
much angle of attack in used, the body begins to develop lift along with
the moment. Viscous effect. The body is incredibly inefficient at making
lift, so the angle of attack must be kept quite small. Also, the cabin
crew has a heck of a time moving those serving carts uphill if the deck
angle is too large. What does this moment buy us?
Reduction in trim drag through reduction in the amount of moment (lift
times tail arm) generated by the horizontal tail. Less lift/moment
generated by the tail, less induced drag generated by the tail.
Terry
--
Terry
drin...@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."