Re: unpowered glide ratios

From:         David Lednicer <dave@amiwest.com>
Date:         25 Oct 94 10:24:06 
References:   1 2 3
Next article
View raw article
  or MIME structure

	As to why a windmilling jet engine produces less drag than a 
stopped jet engine vs. a stopped prop produces less drag than a 
windmilling prop:

	Not much air gets through a stopped jet engine, resulting in very 
high spillage drag (kinda like holding a coffee mug open end first into 
the wind).  A windmilling jet engine passes more air, reducing the 
spillage and hence the drag.  On the other hand, a stopped prop is 
usually feathered, which is very low drag.  A windmilling prop has more 
blade surface area exposed and has higher drag.  The difference boils 
down to the fact that the jet engine is contained within a nacelle, while 
the prop is completely out in the freestream and that the prop has 
variable pitch, while the jet engine blades are not variable pitch 
(ignoring variable pitch stators).


-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299