Let's talk about Fineness Ratios ( pointy front ends )

[3-Wheeler]

Originally Posted by Old Tele man

...exactly, because high-speed jet wings have almost razor-sharp leading edges, not the smoothly rounded edges seen on a Piper Cub, etc.

Jets have sharp edges on the leading edge of the wings for completely different reasons:

1) They fly above the speed of sound
2) The entire surface of wing moves, not just the flaps
a) this is for control when flying super sonic; a flap does not affect the air flow when flying that fast
3) A sharp leading edge provides less drag at super sonic speeds
4) A strange as it sounds, the transfer ports in two cycle engines should NOT be sharp for exactly the same reasons as above; i.e. the air velocity is LOWER than the speed of sound.

Jim.

[Frank Lee]

Bottom breathing has nothing to do with fineness ratio.

[moonmonkey]

one benifit of extending the front would be ,closer to ideal length vs height,,, i know my echo is way short and too tall. also i think forcing more of the air up and over the car is ,in many,many cases better than letting it flow where it wants,,.

[Cd]

Originally Posted by Frank Lee

Bottom breathing has nothing to do with fineness ratio.

Whoops.
I meant stagnation point Thanks for catching that Frank !( fixed ).

Moonmonkey : I have yet to do any kid of testing on the fully enclosed grille block versus just having a small hole ( which I had prior )
The car is semi retired now.

[MetroMPG]

Originally Posted by Cd

I'm starting a new thread because that one is a mess. This one is dedicated solely to fineness ratios

Good luck with the new thread.

[gone-ot]

...instead of "fineness-ratio" will it be "dullness-ratio"?

[aerohead]

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.

[Otto]

Originally Posted by aerohead

... 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.

NASA guy told me ~8 degrees was max slope on aft body, to avoid undue separation and turbulence. German sailplane research showed essentially that, as seen in oil smear tests in wind tunnel.

[bgd73]

most cars have this already. I called it fathead. the front ends...

Upon 140 hours of welding even an old subaru, I spotted it from above, in a second story window looking down...the front of the car is wider than the back...by 1.5 inches. by the windshield, top of roof to the back is not a square. The roof rack with straight rails can even be used as a measurement..

A man plows snow...head on. the stuff that follows must be this "finess" or "fineness" cleaning up the mess created, in relation to aero.
The pointy front end stuff or sharp dome must be for extremely high speed. but then again, not all that necessary, The front of an old 707 is like a goonie bird but capable of 500mph...on engines with 25% of the thrust of today.

[aerohead]

Originally Posted by Otto

NASA guy told me ~8 degrees was max slope on aft body, to avoid undue separation and turbulence. German sailplane research showed essentially that, as seen in oil smear tests in wind tunnel.

Remember,they're talking about sections or bodies of revolution with very low thickness and high fineness,and they're operating in turbulent free air,trimmed for max efficiency,at altitude.
Also,during landing and take-off the wings can survive angles of attack close to stall,and with turbulators ( vortex generators ) they can push that angle another 6-degrees.
With suction slots,wings can maintain attached flow up to around 40-degrees.
Mairs research ascertained that for automobiles,22-degrees would be the limit.
You'll see this angle with Klemperer's Cd 0.16 'minivan',Walter E. Lay's Cd 0.12 'pumpkin seed'.It's also the terminal angle for the L/D 2.5 streamline body which Hucho references as the drag-minimum in ground-reflection,developed by Ludwig Prandtl under inspiration from Edmund Rumpler.
And before you say it,some of the exotic fancy laminar sections with remarkable low drag can suffer up to 300% drag increases when used as 'cars'.