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Re: Arfon's Aerodynamics

To: land-speed@autox.team.net
Subject: Re: Arfon's Aerodynamics
From: Ralph Forsythe <rf-list@centerone.com>
Date: Wed, 15 Jan 2003 22:12:32 -0700
>maybe DrMayf could photograph a drop falling very close to a long vertical
>board?

*VERY* close.  Probably would have to be within tolerances that are beyond 
the ability of his wife standing on a ladder, I'm afraid.

For no other reason than the scale involved, this is entering the area 
where a wind tunnel with smoke lines might provide more useful information, 
with a somewhat larger model(s) and boards you can place.

Also, the shape a non-static object takes in free fall might not be the 
best aerodynamic shape - it's just what the force of the air impacting it 
made it into (see raindrop shape - that's far from aerodynamic).  And 
ground effect is a major issue to take into consideration; look at an 
aircraft for example - ground effect happens usually within a wing's length 
of the ground, and can cause significantly increased lifting force.  For 
example, your average Cessna 172 rotates at 55 kts and climbs out at 65 
kts.  However, while within ground effect it's possible to climb out at 
10-15 kts below normal climb speed, which of course poses an issue when 
you're pointed up, not gaining speed, and leave ground effect.  See 
'takeoff stalls' for more info on that.  Gravity's a bitch.

IMO (and this is just THEORY, of course I've not built this and tried it - 
YET) the best compromise for a salt-faring vehicle would be one that could 
sit as close to the ground as possible (I mean, if you can get it within an 
inch of the salt, go for it) and have just enough negative lift to overcome 
the ground effect.  You want to avoid low pressure under the vehicle, 
however if you could somehow achieve neutral pressure between the vehicle 
and the salt while minimizing skin friction, and ever so slight down 
pressure from the top of the vehicle, that would work I think.  The sides 
would probably assume the teardrop shape however the top/bottom profile 
would need to be altered.  Of course the transition should be smooth and 
gradual.

Oh - someone else posted about the Stealth planes.  Yes, they are pointy 
shaped, all over the aircraft.  However AFAIK they are not terribly 
quick.  They are made for invisibility, not speed, and the angles you see 
are designed specifically to make radar ignore it.  I think speed was 
secondary because it's hard to shoot at what you can't find 
anyway.  Fighter jets are shaped like they are for a reason - radar sees 
'em, but they move *quick*.  However of course supersonic velocities change 
things a bit so those shouldn't necessarily be used as models either 
(unless that's your goal).  Comparing things designed to fly, to our 
vehicles which generally shouldn't, could be trouble too... =)

-rf

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