Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Let's talk about Fineness Ratios ( pointy front ends )

aerohead · 03-04-2010, 07:59 PM

Hucho pushes 5-to-1 @ zero ground clearance,which in mirror-image ground-reflection yields 2.5:1.
Aircraft are all skin friction,they have no profile drag except at take-off and landing.
Cars are 7-9% skin friction.
The 55% form drag is what the big dogs say we should go after.
The blunt hemispherical nose allows very stable flow acceleration within a very favorable pressure regime.A sharp or conical nose can actually be worse as they provide very unstable acceleration.The flow doesn't actually follow the nose but rather a discontinuity within the onset flow.You ca see it in all wind tunnel smoke photos.
After reaching maximum cross-section,even with turbulent boundary layer,the flow cannot tolerate radical pressure gradients.
The aft-body has got to taper gently allowing flow deceleration for pressure recovery.Too steep,and it separates,and the base pressure will be the same as at the point of separation.

03-04-2010, 07:59 PM	#27 (permalink)
aerohead Master EcoModder Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 16,335 Thanks: 24,453 Thanked 7,394 Times in 4,789 Posts	5-to-1 Hucho pushes 5-to-1 @ zero ground clearance,which in mirror-image ground-reflection yields 2.5:1. Aircraft are all skin friction,they have no profile drag except at take-off and landing. Cars are 7-9% skin friction. The 55% form drag is what the big dogs say we should go after. The blunt hemispherical nose allows very stable flow acceleration within a very favorable pressure regime.A sharp or conical nose can actually be worse as they provide very unstable acceleration.The flow doesn't actually follow the nose but rather a discontinuity within the onset flow.You ca see it in all wind tunnel smoke photos. After reaching maximum cross-section,even with turbulent boundary layer,the flow cannot tolerate radical pressure gradients. The aft-body has got to taper gently allowing flow deceleration for pressure recovery.Too steep,and it separates,and the base pressure will be the same as at the point of separation.