Basics of Speed to Fly for Paragliding Pilots
The
expression Speed to Fly represents
the adjustments to a Paraglider's speed in relation to wind,
lift and sink. Maximizing glide based on this relationship is
a constant process while flying. Speed to Fly is a key that
not only makes you a better pilot, but helps build a connection
between you and the
wing.
This
article was written to provide the basics of Speed to Fly and
techniques to adjust your speed based on these variables. There
are tools that you can use (speed rings on variometers or GPS
systems), but not all pilots have these included in their gear.
Besides, learning to fly without instruments is a large part of
growing as a pilot.
Learning
to adjust Speed to Fly continuously while in flight can be done
with some simple observations. If you want to learn the math part
of the equation, many articles teach about Speed to Fly by using
a polar. The math part of the equation works for some, but not
everyone is good with numbers or looking at graphs. It's important
to learn the theory, then practice making adjustments to hone
a full understanding. Having a solid understanding of Speed to
Fly will help you fly higher, stay up longer, and improve your
senses of wind and lift.
The
basics apply equally to ridge soaring or thermal flying. The main
factors that affect Speed to Fly are only two: relative wind direction
and lift or sink.
The generalized
adjustments involved in Speed to Fly are:
In
Lift / Sink
- Fly
slower in lift
- Fly
faster in sink
In
Headwind / Tailwind
- Fly
faster into a headwind
- Fly
slower with a tailwind
Next,
you need to learn why each of these statements is true. In each
of the below descriptions, eliminate the secondary set of variables
and imagine that there is only one. For the wind descriptions,
assume there is 0 on the lift / sink components and for the
lift / sink descriptions, there is no headwind or tailwind.
Fly faster into a headwind
This is perhaps the easiest of these to grasp. In still air
(no wind and no lift or sink), most Paragliders get their
best glide at or very near trim speed. For simplicity, let's
say that the sample Paraglider flies at 20 MPH at trim. In
general, I like to promote that for any headwind above 12
- 15 MPH without lift, your best Speed to Fly will be most
to full speed bar.
This
is very easy to see in a stronger wind situation. Picture
yourself at 100 feet above the ground, pointing straight into
a 20 MPH headwind. At trim, you will hover downward toward
the ground at the sink rate your glider gets at trim speed.
You will land on the exact spot beneath you. If you fly any
slower, you will fly backward. If you fly faster (pushing
on a speed bar), you will improve your glide by moving forward.
You will in fact descend faster because of a slight increase
your sink rate, but the factor of forward motion will be a
greater.
A
slight headwind is not quite as easy to picture. Imagine yourself
at 100 ft above the ground, pointing straight into a 5 MPH
headwind. Flying slower will shorten your glide path. Flying
full speed will also shorten your glide path. To maximize
your flight path, about 25 to 50 % of the speed bar would
be close. Note that in lighter headwinds, the difference in
glide is not nearly as significant into stronger headwinds,
but it is significant nonetheless. I recommend that up to
about a 5 MPH headwind, trim is close enough to be acceptable.
Keep
in mind that the speed ranges I am discussing are generalizations
and not correct for all gliders.
To
summarize:
.
Fly
slower with a tailwind This
is very similar to the above, but instead of accelerating
as the headwind increases, you will fly progressively slower
as the tailwind is stronger. Here, Min Sink is like full speed
bar in the above "headwind" scenario. Other than
in a flair, Min Sink is as slow as you should ever fly in
a Paraglider. In any tailwind above 12 or 15 MPH, you will
likely get your best glide by flying at Min Sink Speed. Because
of the extra 12 + MPH groundspeed you will have over your
airspeed, time spent in the air gains you more distance. The
combination of your slowest descent rate and the high groundspeed
(Windspeed + Airspeed) will help your glide the most. Since
minimum sink speed maximizes the time you are in the air,
flying slower helps you fly the furthest with a tailwind.
Most
gliders get their best glide in 0 wind at or very close to
trim speed. With a 12 + MPH tailwind, most gliders will get
their best glide at or very close to min sink speed. In a
very light tailwind up to about 5 MPH, there will not be a
very significant improvement in glide by adding a little brake.
But, in winds above 5MPH it will begin to become much more
significant. So between 5 and 15, you will fly progressively
slower between light braking at 5 to min sink around 12 or
15 MPH tailwind.
Fly
slower in lift
The
rule with lift is pretty clear as well because if you can maintain
or climb, obviously this is going to extend your glide. If you
are in fact ascending, to maximize the ascent rate, you will
fly minimum sink. Anytime the air is going up at least as fast
as your Paraglider’s Min. Sink rate, flying Min Sink will get
you the furthest. You might notice that a pattern is developing
here. In 0 lift you start off at Trim and progressively go to
Min. Sink as the lift increases to match your gliders Min Sink
speed. For any lift that exceeds your gliders Min Sink rate,
you will go up the fastest at Min Sink speed. The faster you
go up, the higher you will get and with height comes distance.
Fly
Faster in sink
This is true because of the shorter time you will be in the
air as a result of the descending air. During this time, flying
faster becomes more of a factor than how fast you are descending.
This one is among the hardest to perceive, so you have to
either use your vario to sense the amount of sink, or use
the visual references.
Starting
at 0 lift/sink you fly trim. If your glider descends at 260
ft / Min at Best L/D (trim) and the sink cycle you are in
is down at 240 feet / Min you will be descending at 500 ft
/ Min continuing to fly at trim. By flying faster you will
descend a smaller percentage than the speed you increase.
In all of these examples, it is simply a vector of whether
speed or sink rate is going to help you glide the furthest.
Application
in the "Real World"
Most of
the time there are blends of wind and lift/sink factors. You
could encounter sink and a headwind at the same time. This is
an easy one, as both tell you to speed up. The combination will
lead you to fly faster. Contrarily, you could just as well fly
into a thermal as you are penetrating a headwind. With this
type of situation, you need to judge which factor is the dominant
one. If the thermal in this situation is strong enough for you
to maintain or climb a little near minimum sink speed and you
are still moving forward, then the you would likely continue
to fly near minimum sink speed (choosing that the lift is the
stronger factor). But if the thermal is not so strong and the
headwind is strong (at min sink you are not penetrating), then
the wind is the stronger factor in this situation. It becomes
pretty easy to pick the larger factor in the real world when
you have ground references to check.
Imagine
that you are gliding toward an LZ that has trees around it including
one at the far end. The top of the tree is in front of a contrasting
field. As you glide toward your LZ, you can use the top of the
tree to help you adjust your Speed to Fly. By referencing the
relative motion of this tree top to the background, you can
determine whether your glide will take you past the tree or
not. If the top of the tree is moving up relative to the field,
you will land short of the tree. If the tree top is moving down
relative to the field, your glide will carry you past the tree.
If it is perfectly still against the background, your current
glide will take you right to the tree top.
How
do you use reference points to adjust your Speed to Fly? In
the simplest sense, you make adjustments to speed with the goal
of improving your glide via the visual reference. If the tree
is moving downward relative to the field, you will try to adjust
your speed to maximize the tree moving downward, or, if the
tree is moving upward relative to the field, you will make adjustments
to minimize how fast it is going up.
You
can look in many directions and have different objects to cross
reference against. In one direction it might be the top of a
tree, in another the top of a ridge or mountain, in another
a radio tower in the distance. You can even use other gliders
in front of you if they are heading in the same direction. Although
other gliders are not fixed, you can tell if you are improving
your glide by adjusting the same way against them as for fixed
objects. The point is that no matter what direction you are
going there is almost always something to use as visual tool
to help adjust your Speed to Fly (The main exception here is
when you are way high over flatland or a lone mountain). Keep
in mind that only references in front of you will work effectively,
references to the side don't work (because you are not gliding
toward them).
Using
a vario is another tool that can help you fine tune Speed to
Fly. As stated above, if you can maintain or climb in lift,
then min sink will be the correct Speed to Fly. On the contrary,
if at min sink speed, you are descending, you should speed up
a little. If you are sinking moderate to fast, then your best
Speed to Fly will likely include some speed bar.
Using
the visual references can help you take Speed to Fly to the
next level. After you make an initial adjustment, you can your
reference point in front of you to see if there has been improvement
(if it was moving up relative to its background, it would now
be moving up slower ... and visa versa)
All
of the adjustments are a continual process and are never fixed.
When the wind gusts, you need to adjust your speed (fly faster
in a headwind) while flying through such a gust. As soon as
the gust ends, you would readjust your speed. For each change
in lift or sink, you will use the above 4 rules as an initial
in seeking the right Speed to Fly. If a factor is prolonged
and time permits, you can use any horizon reference to fine
tune your adjustment. You can even use other gliders in front
of you. The only hard and fast rule is that the reference should
be in the direction you are heading.
The
first step in learning this is to begin combining your awareness
of the basic adjustments. Then, you can begin to improve your
relative glide by managing speed to fly with better understanding.
Jeff
Greenbaum (12-6-04)
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