Mark Drela wrote:
> FWIW, Zinn's explanation of how this fork works is incredibly convoluted.
> The concept is simple:
>
> Conventional fork blades form a venturi between them,
> so the spokes see a brief 1.2x larger headwind when
> they pass through the fork (Figures A, B).
> This implies a brief 1.2^3 = 1.7x larger power loss,
> giving a _very_ significant average power loss increase
> (most of the spoke power loss occurs between 2 and 10 o'clock
> on the wheel).
>
> By making the fork blades into airfoils which lift outward,
> the centerline velocity is _decreased_ by 0.9x (Figure D),
> which makes the local spoke drag power loss 0.9^3 = 0.7x smaller
> as they pass between the fork blades.
>
>
> That's the basic idea. The reason for the 2-element
> airfoil fork is that it's the only way to make the
> fork blades have sufficient outward lift, without
> an excessive fork drag increase. This increase
> would counteract the spoke drag benefit. Zinn's
> explanation of how this works ("slot sucks the air out", etc)
> makes no sense, but then again a correct explanation
> wouldn't make much sense to a non-aero audience either.
>
>
> BTW, Zinn seems to be relying on 3rd-hand information.
> I had nothing to do with the tunnel tests. One of the
> italian teams and Cervelo was in our tunnel doing tests.
> While I was rubbernecking I hit on the idea of the
> outward-lifting fork. I quickly cooked up the
> 2-element fork with some CFD calculations,
> and handed them the plots you see in the article.
> It's kind of amusing to see my penciled doodles
> and annotations on some of the plots.
>
> So I really have no idea if it actually works or not.
> If the fork blades lift as the CFD predicts, the
> momentary spoke power between the forks is reduced
> by a factor of 0.7/1.7 = 0.4x, which is pretty huge.
> The net effective wheel drag reduction will of course
> be more modest.
>
>
>
>
From my understanding of air flow, if the air moving through the forks
is moving faster then it would suck air INTO the slots rather than out,
the same as a carburettor sucks fuel in, and didn't Bernoulli
demonstrate that a moving liquid creates a low pressure?
Secondly, any non symmetrical airfoil shape will have more drag than a
symmetrical airfoil, so even if you reduce airflow through the fork
you're going to increase the overall drag from the fork legs.
I don't have access to a wind tunnel so I have to assume things.
Dorfus