A REQUESTED OPINION ON THE EFFECT OF WIND ON HPVs.
by Matt Weaver (Peter Ross was reasonably concerned with the influence of wind on HPV performance, and inquired my opinion on several points. The following is a slightly revised version of the e-mail response I sent Peter. Peter presented my response to Human Power for publication. Matt Weaver.) BODY SHAPES AND INFLUENCE OF THE WIND
"Stubby Wings"
Fully streamlined bicycle bodies, especially those with fairly long or tall
trailing edges and low ground clearance, act much like half of a stubby wing
turned on edge (the other "imaginary" half extends into the ground). As the
ground clearance increases, the body's sensitivity to the wind decreases. But
even with good ground clearance, if there is still an appreciable trailing
edge
the body will produce definite lift in a side wind. Typically it is not too
efficient, but by virtue of high vehicle speeds and low body drag, the forward
thrust is seldom negligible.
Estimated sailing characteristics
These lift characteristics can be well approximated for certain body shapes,
and others can ultimately be plugged into a computer if coordinates are
available (assuming the overall body flow is "well attached" - which can be
checked by the computer, and which is valid for the top streamlined bikes, but
not for many automobiles). I am familiar with both methods and have tools and
know the math, though I've looked at side-wind effects primarily to compute
the
lift and "moment" forces on the body for stability and steering-geometry
considerations (regarding the "moment" - most bodies like to torque away from
the side wind, as if the lifting force is acting on an imaginary point beyond
the front of the nose).
Some lift and drag relations
Some bodies are designed with low-drag "natural laminar flow" airfoil
sections. These airfoils have low drag at zero flow incidence (i.e. no side
wind). As the side wind increases beyond a certain point, the pressure
distribution over the section changes such that the boundary layer on the
downwind side of the airfoil becomes largely turbulent instead of extensively
laminar. At that stage the drag of say a 50-60% natural-laminar-flow airfoil
section will roughly double. The lower-drag part of the characteristic is
sometimes referred to as the "low-drag bucket" on lift/drag plots of various
airfoils. You can shape an airfoil to have a "wider" low-drag bucket (that
is,
it would cover a wider range of side winds) but the bucket becomes less deep
(i.e., the low-drag portion is increased).
"Turbulent" as used here refers to the state of the thin blanket of
"boundary-layer" (BL) air near the body surface. It is not the same thing as
"separation" - which is what happens at the rear of blunt bodies such as most
automobiles. A "laminar" BL has much lower shear or "skin friction" rubbing
against the surface of a body than a comparable turbulent BL. A BL starts
in a
laminar state, and may irreversibly "trip" (a bit like a wave breaking on the
beach) into a "turbulent" BL. The skin friction increases locally as much as
ten-fold as the BL transitions from laminar to turbulent, and drag of
identical-looking sections can vary as much as three-fold, depending on the
overall state of the BL.
Lift of symmetric sections is independent of drag
Interestingly, regardless of whether the boundary layer is laminar or not, or
what the airfoil section profile is (large nose-pointy tail, or pointy
nose-shorter tail...), the lift coefficient of a symmetric airfoil section is
largely a function of the incidence of the flow alone.
Lift, drag, and forward thrust
As the side wind increases with low-drag "natural laminar flow" sections, the
lift keeps rising, but the forward thrust will reduce temporarily when the
body
drag jumps up, but typically any wind produces a net forward thrust for the
body at all times regardless of whether or not the body drag has gone up (i.e.
the body drag has increased, but the forward or "thrust" component of the lift
has increased more than the drag and thus gives a net propulsion).
Not considering changes in drag coefficient, the potential forward "thrust"
from the wind goes roughly with the square of the side-wind speed.
Estimates of sailing characteristics of the "White Hawk"
I'll spare the summary of math details here because they go on for a page or
so, and just give you numbers for a single case. The "White Hawk" as well as
the "Tomahawk" look like fairly good vehicles for side winds with their low
ground clearance and fully developed height and trailing edges. Other fast
streamlined bicycles exhibit similar estimates: slightly greater sensitivity
for the Gold Rush and Cheetah, and slightly less for the Varna Mephisto and
Cutting Edge.
Estimates for a 5-MPH (2.24 m/s) side wind at 50-MPH (22.35 m/s) vehicle speed
are as follows based on a model from images of the "White Hawk",
Course geometry and side winds
A linear "out and back" closed course is sensitive to wind direction; a
circular course is not. Closed courses of varying eccentricity naturally fit
somewhere in between. Giving respective weights, with 1.0 = perfect
perpendicular side wind, the following holds:
Considering the previous case listed above with an average 5-MPH (2.24-m/s)
wind (regardless of direction) and a circular course, wind power would amount
to about 0.045 horsepower (34 W), or about 1.5-MPH (0.67-m/s) increase in
vehicle speed near 50 mph (22.35 m/s).
Wind and hour records
I hadn't considered the matter closely until you asked, nor did I even know
there was an issue with Lar's record!
"5 to 10 MPH winds reasonably significant"
I would consider the above estimations reasonable - of about 1.5-MPH speed
increase in a closed-loop hour run near 50 MPH with a 5-MPH side wind. For
winds much higher than 10 mph, the vehicle might start to have a hard time,
but
I would consider increases in average speed as much as 3 MPH or a little more
very likely before handling begins to deteriorate.
"High winds"
Higher winds - such as the 15-to-25-MPH (6.7-11.2-m/s) range as experienced in
Yreka - are adverse for most streamlined bicycles. Furthermore, the lifting
characteristics may be nearly maxed out or stall separating - or in other
words
- lift is no longer increasing much, but drag is. The bicycle gets buffeted
around and likely ends up running slower.
"Displacement of HPV records."
Considering the history of the hour record for the last ten years, the average
increment between records has been less than 1.5 MPH (0.67 m/s), and thus a
modest wind in the 5-to-10-MPH range could result in a displacement of an
otherwise true "human powered vehicle" record. I think such displacements
generally stifle efforts to set records, and ultimately slow the
improvement of
both the vehicles and the records. Such displacements are bound to happen,
and
have, and the effects are evident too.
Considering that, I'd say the wind is relevant if you are seeking to know what
is the most efficient "human-powered vehicle" rather than say the most
effective "local-natural-power vehicle" (e.g. local muscle, wind, gravity,
solar). The question then is it worth the trouble to measure the wind, or
how might you measure it?
Measuring the wind
An "instant-wind-speed" anemometer is next to useless (unless it feeds into a
data-logger). What you want to know is how much total wind (flow distance)
has
occurred within the hour period and ideally sub-periods within it. The
average
wind (total distance/ time) is a good approximation. A more exact calculation
would be to have additionally the total distance every 2 to 5 minutes or so or
even arbitrary weighted sub-periods, and do a "velocity-squared" average.
Interestingly, the power assist of the wind for a streamlined bike goes with
the velocity squared - the reason in part being that the vehicle velocity is
largely independent of the wind speed, unlike say a windmill. For a windmill,
the "velocity-cubed" power average is the most accurate (as is commonly
done in
assessing windmill sites using wind speed distribution (Weibull curve, etc...)
characteristics of the site.
Large tracks.
Interestingly, for a large site - such as two-mile (three-km) or larger track,
the instantaneous wind speed at one end of the track tends to correlate poorly
with the opposite end, but the average wind speeds are remarkably consistent
(assuming one end is not grossly "shadowed" by some structure). This makes
average-wind-speed measurements for a long event like the hour far more
meaningful than wind-speed measurements for say, a top-speed sprint.
Sprints.
The top-speed sprint covers a large distance (often two miles or more of
acceleration) and the wind speed for the brief instant the vehicle passes
through the time trap has little to do with the wind speed over the
entirety of
the course. In other words, the wind speed at the end of the course may or
may
not correlate well with the wind the vehicle experiences over the acceleration
distance, but at least it gives a probable idea, especially if multiple runs
and readings are performed. It would make more sense in the case of a sprint
to measure the wind speed for the entire duration of the run-up and sprint and
optionally take a velocity-squared average. This can be done by simply
reading
the time and feet of wind when the bicycle starts rolling and when it finishes
and then divide total distance/total time. Good runs are then actually less
likely to get ruled out because of a little gust and vice-versa (i.e. if it is
generally calm, everyone will have good runs, rather than some getting
ruled-out).
Probability of getting "Legal" winds
A quick analysis of available weather-station data for a number of locations
suggests that the probability is high to get "legal" (sub 1.66 m/s) winds as
previous hour records required - in the early morning hours. Typically a
window of three hours or so after sunrise exists. After that, winds are
likely
to persist until midway through the following night. Later in the day, such
low winds are far less certain and each site must be considered individually.
So, if you want to set an hour record with low winds, plan on running in the
early morning.
Key "power assist" areas
For streamlined bikes, there are three major areas of "power assist" that are
most likely to occur.
Wind as discussed - looks as if it gives modest yet significant assist by
default to leading streamlined bicycles before it grows too large and hinders
handling.
Pressure gradient, the unrecognized aerodynamic advantage, becomes probable if
a "pursuit" automobile is following behind a streamlined bicycle. The effect
is far more dramatic than most HPV enthusiasts would suspect. An automobile
can approach a streamlined bicycle from behind and gently accelerate, and long
after the cyclist stops pedaling the streamlined bicycle will continue to
"ride" the pressure-gradient "wave" extending significantly several car
lengths
in front of the automobile. The streamlined bicycle essentially
accelerates on
its own. (For instance, a truck approaching a streamlined bike from behind
may
never be able quite to catch up no matter how fast it can go!) This effect
holds true up to Mach numbers well over 0.5, and eventually diminishes to
zero
approaching the speed of sound.
There are some neat research references dating back to the 1930s well
documenting this effect. Dolphins know it well, often racing for hours
directly in front of ships. It also applies to a crouched racing cyclist on a
standard bicycle, but it is far less dramatic than with a fully streamlined
bicycle.. I have carefully tested this after witnessing some unusual
performances in several time trials in the Tour de France. Even with a
compact
car the effect is very repeatable and correlates well with some
"unexplainable" margins.
At least one of the exceptional performances in a trans-continental race may
have utilized the pressure-gradient advantage. Without going into specific
incidents, the general application has been to follow the cyclist closely with
a large support vehicle, and to go so far as to substantially increase the
frontal area (and extent of the pressure field) of the vehicle by mounting a
large "billboard" banner extending several feet above the top front edge of
the vehicle.
I should add one interesting quotation that may relate to this question, and
that appeared in the San Francisco Chronicle (Monday, February 8th, 1993): "I
felt like I was dying," "At [x] miles per hour I'd gone anaerobic, but I forced
myself to keep turning the cranks and our speed kept rising. It felt like the
[n] was accelerating on its own, a real testimony to aerodynamics."
The "felt like I was dying" anaerobic state is strongly associated with a
decay
in power output, which is associated with a fundamental reduction in speed
with
few exceptions, not "accelerating on its own." A rather interesting quote!
The "HPV".
Anyway, I like to call an "HPV" an HPV if it really is what its title claims -
a "human-powered vehicle." Then you can look at the very fundamental contest
that combines solely the power of a human's muscles and a human's mind to pure
movement, speed and efficiency going from A to B - the very stuff so
central to
our existence and so pervasive in our daily lives. Such a contest reveals
vehicle efficiencies far exceeding all other vehicles I know of - for a given
speed times occupancy, nothing does as much with as little.
To clarify, in my opinion the Du Pont prize for top speed was brilliantly won
fair and square by Freddy Markham in Gardner Martin's Gold Rush at 65.48 MPH
(29.27 m/s), but the fastest official "human-powered-vehicle" record now
stands
at 62.51 MPH (27.94 m/s) set in 1999 by Sam Whittingham in Georgi Georgiev's
Varna Mephisto in dead-flat near-zero-wind conditions.
To increase this "HPV"
record to, say, 70 MPH (31 m/s) will require a nearly 40% increase in rider
output or vehicle efficiency.
There is lots of energy all around us in the form of wind, and surely we
should
better utilize it. But I consider racing a "hybrid" sail-HPV, from which
phenomenal performance is possible, a different contest.
Milwaukee hour
Just a note on the Milwaukee hour you mentioned - I crashed for the first time
at about 48 mph (21 m/s) earlier that day, and I was a bit uneasy to race at
all. I decided I'd run if I rode on the outside of the track, and we measured
and estimated I traveled over 3 miles (5 km) extra over the course of the
run.
Not to mention I started about 1/2 lap late as my windshield came loose just
before the start of the race! I honestly didn't notice the wind. My
hydration
system failed, my head was overheating with the ventilation messed up due to
the last-minute windshield fix, and so I was more or less persisting to the
finish!
To my surprise, I actually passed the Gold Rush on the last lap. I believe he
was ahead of me at the 60-minute mark, but the rules required us to complete
our last lap. Gardner ran down the track and signaled to Freddy to ride
through the last lap. If you have video of the race, you might see this all
happen. The officials said I was a lap behind Fred, and it took Freddy to
tell
them that I was in fact on the same lap as he before they'd admit their
error.
I was happy with Freddy being the winner since he led most of the race, so no
matter what the rules said there. Interestingly they somehow managed to get
our marks recorded to the nearest 0.001 mile for the record! (I wonder what
measurement they used?!)
The next day was faster - I wish they recorded the lap times. Other than the
caution lap, I averaged just shy of 48 MPH (21 m/s), and would have been happy
to keep going at that rate except there were more trees off to the side of the
course than I cared for so I limited my speed except for the last lap which I
sped up and averaged just under 52 MPH (23 m/s). I wanted to go fast
enough to
"wear down" Freddy so that he wouldn't take me at the finish with some mean
sprint, but not so fast that I might crash and get hurt. Freddy and Paul
Swift
and Bobby Livingston were all real excited after that event - they all said
they wanted to ride the Cutting Edge and see who could go the fastest! I'd
love
to see what they could do! The only problem was that the bike was too long
for
all but Paul Swift's legs!
Conclusions
Matt Weaver,
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The above Technical Note was originally published in Human Power, the IHPVA technical Journal, issue Number 49, Winter 1999-2000. It is posted here with the permission of Matt Weaver, Copyright © 1999, 2000 |
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Copyright © 2000,
C.C. Broome
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