Strange aerodynamics during winged reentry

[YL3GDY]

Hi all,

Some time ago I've tried to reentry on DGIV without autopilot turned on. Then I discovered a strange fact - in upper atmosphere aerodynamic forces attempt to increase pitch, but after some moment - to decrease pitch.

What is physical explanation of this fact?

 

[pete.dakota]

This is called shuttlecocking. It's the DG naturally trying to move into it's most efficient and aerodynamic attitude relative to the airflow. As the upper atmosphere is thin, this takes longer and seems slower than it does at lower altitudes.

 

[lalbanof]

The pitch is not "increasing"

Not because of aerodynamical forces. It is simply that the Earth is spheroidal and wherever you have almost no aerodynamical forces to speak of (say 100 Kms. and higher), the spacecraft will keep a constant "heading" (having quite some moments of inertia), while the horizon "falls" under it.

The craft will, in the presence of aerodynamical forces, if left to itself, will adopt 0 pitch (or oscillate about it) because it happens to be the minimum energy configuration for the craft, for a trajectory inside the atmosphere: in more technical words the one that minimizes the "action integral".