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Old 13th Nov 2018, 23:57
  #1139 (permalink)  
FCeng84
 
Join Date: Feb 2009
Location: Seattle
Posts: 379
A couple of thoughts on cruise deck angle and maximum performance (i.e., L/D):

1. At a given airplane configuration (read flap/gear position and payload/fuel distribution) the deck angle that generates lift = 1g will be a function of Mach number and airspeed. You can fly faster at a lower deck angle or slower at a higher deck angle. As fuel is burned during the flight, the lift required to balance weight will go down and the deck angle at a given speed will reduce. This is partially compensated during a flight by increasing altitude along a constant Mach line that yields lower impact pressure and thus less lift. All of this is factored into the airplane design.

2. While I am not an aerodynamicist and may be going out on a limb here, I believe that a configuration with the body at a slight positive deck angle will realize greater incremental benefit from the small amount of lift generated by the body vs the slight increase in body drag going from zero AOA to a small positive AOA. It is quite clear that negative AOA for the body would be a loser as it would result in increased drag and decreased lift.

3. Another variable in the mix is engine angle with respect to both the fuselage (left/right) and wing (up/down). Some of that is based on inlet flow distortion due to the forebody and considerations for engine out characteristics, but I believe that pitch/lift trim for cruise also factors in.

Standing by for someone with more aero background than me to comment.
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