Re: Nose high during cruise?

From:         "Paul Vijgen" <Paul_Vijgen.LFCPO@qmgate.larc.nasa.gov>
Date:         10 Mar 94 13:07:22 PST
References:   1
Next article
View raw article
  or MIME structure

In article <airliners.1994.980@orchard.Chicago.COM>, sdd@larc.nasa.gov (Steve 
Derry) writes:
> 
> 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 
> 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.vijgen@larc.nasa.gov