Spinout.....

Spinout.....

Post by Mark James D » Sat, 09 Apr 1994 17:24:03

Quote:

>Just a few questions on your last post:

>> However, when the angle of attack reaches a certain threshold the
>> airfoil will stall and there will be a drastic reduction in lift.

>Why is this?

When the fin is at a small angle of attack the flow will closely follow
its shape. At typical board speeds the flow will be laminar in
roughly the first 1cm of the fin, turbulent thereafter - just a point of
clarification from earlier posts. As the angle is increased, the lift
will also increase (linearly). However, a critical angle is eventually
reached were the flow can no longer follow the shape of the fin. In
simple terms, it cannot "turn the corner" at the top of the fin:

                      /---->---->---->-------
        --->----->----  \
FLOW    --->----->------  \  <-- Fin          WAKE REGION
        --->----->---->---  \
        --->----->---->----   \      
        --->----->---->-----_____--->--->---  

(Whoever said the net wasn't AUTOCAD?)

Anyways.... if the flow was still "attached" then it would follow down the
back side of the fin, having accelerated around the tip and therefore dropped
in pressure (thus generating lift across the fin area). But in the case
shown above, the "separated" flow hasn't significantly changed in velocity
so the pressure difference is much smaller across the fin - translation:
lift drops in a big way and you eat the mast.....

Try sticking your hand out a sun roof at speed (everyone wants to be a kid
again). Same situation: the flow can't follow the transition from the
windshield to the roof and you should be able to feel both the wake
region and where the separated portion of the flow is going. This isn't an
endor***t to speed (but it does work better that way).

Now you know why dogs stick their heads out the window - it works there as
well.

Note: a number of people have shown that cavitation can be present.
I agree, but cavitation is not responsible for the physics behind spinout.
Whether its a factor in the process is another question but let's not
bore everyone with the debate.    

Quote:
>> Finally, there was an earlier post that suggesting fairing out
>> around the fin base - this will help to clean up the flow around the
>> base area and will improve lift performance in this region (notice the
>> new finbox designs).

>What is "fairing".

Fairing just means to smooth out - usually in the context of using some
type of filler followed by sanding to create a smooth shape where before
there was a dent, depression, etc... Consider yourself lucky if you
haven't ever experienced faring a sailboat hull or you dings in your car.

I forget who suggested this in the first place but I have one question:
how do you get the fin out or change its position if it's faired into
the hull?

I hope this was of use to people; for party tricks after a day of sailing
if nothing else.

Mark