Speed to Fly theory, or S2F, is a strategy used when racing
gliding aircraft, such as
hang gliders and
sailplanes. The main focus of the system is to find the
optimal mixture of time spent
climbing in lift vs. time spent gliding between climbs. Glider races are usually based on the time it takes one to reach a
goal, and in any interesting race that goal is far enough away from the start that one can't simply climb to a certain
altitude and glide straight in to goal - one has to find and exploit
lift (usually
thermal lift) a number of times between the start area and the goal line. Typical hang glider race distances are 30 to 70 miles, either one-way,
dogleg with a specified turn point,
triangle, or out-and-return and take 2 to 5 hours to complete. The major contests (U.S. Nationals, Forbes Flatlands (Australia), and others) usually include at least one 100+ mile
task as well.
Gliding aircraft descend more quickly as one speeds up, so flying at top speed tends to put one close to the surface, where lift is smaller and weaker, fairly quickly, so one has to spend extra time climbing. This is where speed to fly theory comes in.
All serious pilots take their gliders for flights in still air where they measure
sink rate at various speeds and plot them on a
graph in what is known as a polar curve or
L/D Polar, with
airspeed on the horizontal axis and rate of climb or sink on the vertical axis. The shape of the curve is constant for a given aircraft and
wing loading, but its position with respect to the
origin moves to account for rising or sinking air, headwinds or tailwinds, whatever the motion of the
airmass is. A very general rule of thumb is "slow down in lift, speed up in sink", but that is too subjective and imprecise for racing.
As stated a glider sinks faster the faster it goes, and if two identical gliders leave the same point in the same direction at different speeds, the one going faster will get directly above point B sooner than the other, but lower than the other arrives above the same point. Assuming equal skills, they'll climb in the lift at the same rate. Say they'll climb at 300 feet per minute (
FPM). If the first pilot lost 500 feet
relative to the second and arrived 1 minute sooner, he'd have gained 300' back before the second glider arrived, but he'd never make up the remaining 200' (barring
pilot error). Likewise, if he intially lost 500' but arrived 2 minutes earlier, he'd be 100' ABOVE the second pilot when he or she finally arrived. This is what speed to fly theory is all about - finding that magic mix of speed and climb.
To find the
best glide speed in a moving airmass, one plots one's polar shifted down or up for sink or lift, left or right for winds, and then draws a line from the origin
tangent to the curve. The point on the x (airspeed) axis directly above or below the intersection is the speed to fly in those conditions. For racing, one has to
estimate the climb rate in the next thermal one will find and plot one's polar curve accordingly. Graphing in flight
is
cumbersome, especially out in the airflow when flying a
hang glider, so Dr.
Paul MacCready came up with a device known as the
MacCready Speed Ring to simplify application of speed to fly.
Aircraft use an instrument called a
variometer which displays rate of climb or sink, and these, like most aviation instruments, usually employ round
dial indicators, with an arrow which is horizontal at
zero sink and moves up for lift and down for sink. The speed ring has marks for various airspeeds
spread along an arc and the ring mounts on the rim of the variometer; the marks are spaced according to one's glide polar. One rotates the ring and sets the "pointer" for the expected climb rate in the next thermal, then, as one glides toward goal or the next turnpoint one watches the vario and flies at whatever airspeed on the ring the pointer on the vario is pointing to as it indicates sink (or climb) rate. If one encounters stronger sink, a higher airspeed is indicated, and so on.
This simple technology was a great boon to racing pilots through the mid 1980's, but was complicated by the arrival of digital variometers which used non-circular bar graphs or digital displays. Within a few years advances in microelectronics led to instruments which incorporated
altimeter and airspeed functions, along with user programmable memory, so pilots could enter their polar data and the instrument would determine speed to fly automatically, though the pilot is still required to estimate the next rate of climb. The units typically emit tones to indicate whether to speed up or slow down, so the pilot doesn't even have to look at the instrument. Many of these instruments also contain or can be linked to
GPS units to make even more refined speed suggestions by calculating current wind vectors. A further application of speed to fly is for knowing when to forget about finding lift and go on "final glide" to goal - one enters the goal
coordinates and altitude and the instrument monitors current altitude, distance from goal, and motion of the airmass and beeps when the mix is just right.
This system reduces some variables, and helps pilots avoid flying too fast or slow because of tension or other emotional influences, but it depends on flawless flying and
accurate prediction of conditions, which are very difficult
things.