In message <199401161613.LAA04051@cray-ymp.acm.stuorg.vt.edu>, Allen Briggs wri
tes:

>> - Why does buttered toast always land butter-side down?
>
>To understand this, one must look at a more obvious case: jelly or
>marmalade on toast.  This is much more messy, but leads us to the
>obvious cause of the problem.  When there is jelly on the toast, it
>forms an airfoil.  As toast is never dropped from a point where it's
>parallel to the floor, there is air moving over both sides--faster, of
>course, over the jellied side.  The faster air movement causes that side
>to have a lower air pressure which the toast moves to fill, pushed from
>the higher air pressure on the other side.

I disagree.  In my opinion the non-jellied side (using your example)
has regions of micro-turbulence (so called "swirls") caused my the
pores of the bread.  Because these regions are well below the boundary
layer between laminar flow and turbulent flow (except around the well
studied crust-bread interface region) they actually *contribute* to
the flow (by reducing drag) and hence cause layers of lower pressure
air on the *non-jellied* side.

In the case of butter the regions of micro-turbulence do not occur
(obviously) and hence do not contribute to the laminar flow of air
over the toast.

I guess it goes without saying that the toast becomes an asymetric
airfoil at this point and given sufficient altitude will begin to
produce lift with regions of lower air pressure on top. (and, why the
oft noted "inside toast loop" is much easier with the butter on the
bottom, or outside.

I'd reference the obvious NACA airfoil studies but you no doubt have
already read these.

-brad

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