From: drela@athena.mit.edu (Mark Drela)
Newsgroups: rec.bicycles.tech
Subject: Re: Effects of wind
Date: 24 May 1996 23:44:29 GMT

In article <4o2h0t$rce@rap.SanDiegoCA.NCR.COM>, jsr@sparc.SanDiegoCA.NCR.COM (Joseph Riel) writes:
|> Jobst Brandt (jbrandt@hpl.hp.com) wrote:
|> 
|> : First you'll have to explain how a wheel can extract wind from a
|> : crosswind.  I have seen this claim in promotional articles always
|> : without a thread of proof or credible measurements.
|> 
|> If it can generate lift then it can certainly extract power.
|> Consider a direct crosswind.  The total air drag on the rider is
|>
|>  [snip]
|> 
|> The force opposing the rider is
|> 
|>   Fb = drag*(Vb/V) - lift*(Vw/V)
|>
|>      = V*(Kd*Vb - Kl*Vw)
|> 
|> Note that as Vw -> 0 [not a requirement, but makes the result simple],
|> 
|>   dFb/dVw = - Kl*Vb
|> 
|> so that an increase in the cross wind will reduce the effective drag
|> on the bike.

You differentiated incorrectly.  I get

  dFb                     Vw
  ---  =  (Kd Vb - Kl Vw) --
  dVw                     V

which is negative only if  Kl/Kd  >  Vb/Vw  , i.e. only if the 
lift/drag ratio exceeds the wind component ratio.  This is only
true for airfoil-type bodies, and only at sufficiently large
crosswind angles (more than a few degrees, typically).  It
is never true for bluff bodies like a bike rider, but may be 
true for some airfoil-like bike components like disk wheels.
When you add everything up, though, any crosswind always means
more drag for the bike rider.

                   Mark Drela
             _______________________________
    o/LO  .' 
     O  .'  Gravity-Powered Technologies Lab 
      .'  MIT Aero-Astro Department  37-475
     '

From: drela@athena.mit.edu (Mark Drela)
Newsgroups: rec.bicycles.tech
Subject: Re: Effects of wind
Date: 24 May 1996 23:59:32 GMT

In article <DrxEKq.7FG@hpl.hp.com>, jbrandt@hpl.hp.com (Jobst Brandt) writes:
|> Joseph Riel writes:
|> 
|> >> Who said there was "lift".  That is the main contention.  I have seen
|> >> no evidence that there is lift or a positive rotational force from a
|> >> side wind if the wheel has no net pitch to its blades... and they don't
|> >> have it.
|> 
|> > It appears to me that the wheel does have pitch.  Remember, the
|> > effective wind direction is not perpendicular to the wheel.  Or are
|> > you saying that a symmetric member cannot generate lift with an
|> > appropriate angle of attack?
|> 
|> An airfoil must have an angle with respect to the direction of motion,
|> otherwise differences in pressure on its two faces can have no in-line
|> effect on the vehicle.  That leaves only transverse forces other than
|> its surface friction that retards forward motion.

No.  An airfoil must have an angle with respect to the direction of the WIND,
not with respect to the direction of motion.  As I stated in other posts, the
net force on an airfoil of even modest lift/drag ratio is towards its leading 
edge.  The reason is that the pressure at the leading edge tip becomes strongly
negative at any appreciable angle of attack.  The result is a forward force 
(called "leading edge suction" in aero jargon) which easily overcomes the 
aft friction forces.  This helps to propel sailboats and specially-bred 
airfoil-shaped bike riders.  Alas, it can't propel airplanes, which don't 
have the luxury of being able to resist sideways motion by pushing against 
water or earth, and are doomed to fly straight into their apparent wind.

                   Mark Drela
             _______________________________
    o/LO  .' 
     O  .'  Gravity-Powered Technologies Lab 
      .'  MIT Aero-Astro Department  37-475
     '

From: drela@athena.mit.edu (Mark Drela)
Newsgroups: rec.bicycles.tech
Subject: Re: Effects of wind
Date: 24 May 1996 23:28:36 GMT

In article <Drx91u.Knp@hpl.hp.com>, jbrandt@hpl.hp.com (Jobst Brandt) writes:
|>  [snip]
|>
|> >  Greenwell, et al.
|> >  Aerodynamic Characteristics of Low-Drag Bicycle Wheels.
|> >  Aeronautical Journal. March 1995.
|> 
|> I recall reading this report and the measurements, but these were made
|> not in a large cross section wind tunnel, but rather in what can be
|> called a ducted passage that was not much larger than the wheel and
|> its angle of attack to the wind.  This was, as I saw it, an invalid
|> test, and it was sponsored by the manufacturer.

The test was done in a 2m x 2m wind tunnel, equipped with a standard
multiple-axis tunnel force balance.  The wheels were loaned or donated 
by a local bike store in Britain, where the tests were done.  I think 
you're confusing it with the tests done by Chet Kyle.

|> It was this test that led me to mount one of these wheels on a fork
|> and extend it, on the end of a broom stick, from a moving car as I
|> described.  No rotation could be initiated or sustained in any
|> orientation with respect to the wind.

Your test tried to measure a driving TORQUE on the wheel, which would
be due to airloads on the spokes.  The tunnel test I mentioned measured
a driving FORCE on the wheel, which is due do airloads on the rim.
The two effects are mostly decoupled on a spoked wheel.

|> I still see no explanation of what causes this effect and that it can
|> be demonstrated by a simple test as I performed it.  I am no convinced
|> there is anything other than promotional hype and wishful thinking
|> going on.

It is the same effect that makes a sailboat keel to have a net force
towards the bow when the boat "skids" sideways at a slight angle when
close-hauled on either side.  Lift acts perpendicular to the relative 
wind, not perpendicular to the surface.  Likewise, drag acts parallel
to the wind, not parallel to the surface.  If the inequality

    Lift       V_parallel
    ----  >  ---------------
    Drag     V_perpendicular

holds, as it does for any half-decent airfoil, the net force is
towards the front of the airfoil (or wheel).  There is a mountain 
of data acquired since the Wright brothers that supports this.

                   Mark Drela
             _______________________________
    o/LO  .' 
     O  .'  Gravity-Powered Technologies Lab 
      .'  MIT Aero-Astro Department  37-475
     '