Speed to fly
Encyclopedia
Speed to fly is a principle used by soaring
pilots when flying between sources of lift, usually thermal
s, ridge lift
and wave
. The aim is to maximize the average cross-country
speed by optimizing the airspeed
in both rising and sinking air. The optimal airspeed is independent of the wind speed, because the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed. The idea is usually attributed to Paul MacCready
, although an early version of the theory was first described by Wolfgang Späte
in 1938.
However Späte may not have considered sinking air between thermals, and there is no mention of this until 1947 when Ernest Dewing and George Pirie independently included this aspect. Paul MacCready, however, certainly invented the "ring", which allowed an easy indication of the optimal speed to fly.
indicator and a variometer
. The pilot will use the polar curve
information for the particular glider
to derive the exact speeds to fly depending on the lift and sink conditions in which the glider is flying. This is commonly done using a speed to fly ring (known as a 'MacCready Ring') which is fitted around the aircraft's variometer. The ring is usually calibrated in either knots or meters per second and its markings are based on the aircraft's polar curve. During the glide between thermals, the index arrow is set at the rate of climb expected in the next thermal. On the speed ring, the variometer needle points to the optimum speed to fly between thermals.
Electronic versions of the MacCready Ring are built into glide computers that will give audible warnings to the pilot to speed up or slow down. Similar facilities can also be built into a PDA
. The computer is connected to sensors that detect the aircraft's airspeed
and rate of sink. If linked to a GPS
, and using a computed or manual estimate of the windspeed, the glide computer can also calculate the speed and altitude necessary to glide to a particular destination. This glide is known as the final glide because no further lift should be necessary to reach the goal. During this glide, speed to fly information is needed to ensure that the remaining height is used efficiently.
Gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.Gliding as a sport began in the 1920s...
pilots when flying between sources of lift, usually thermal
Thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...
s, ridge lift
Ridge lift
Ridge lift is created when a wind strikes an obstacle, usually a mountain ridge or cliff, that is large and steep enough to deflect the wind upward....
and wave
Lee waves
In meteorology, lee waves are atmospheric standing waves. The most common form is mountain waves, which are atmospheric internal gravity waves...
. The aim is to maximize the average cross-country
Cross-country flying
Cross-Country flying is a type of distance flying which is performed in a powered aircraft on legs over a given distance and in operations between two points using navigational techniques; and an unpowered aircraft by using upcurrents to gain altitude for extended flying time...
speed by optimizing the airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
in both rising and sinking air. The optimal airspeed is independent of the wind speed, because the fastest average speed achievable through the airmass corresponds to the fastest achievable average groundspeed. The idea is usually attributed to Paul MacCready
Paul MacCready
Paul B. MacCready, Jr. was an American aeronautical engineer. He was the founder of AeroVironment and the designer of the human-powered aircraft that won the Kremer prize...
, although an early version of the theory was first described by Wolfgang Späte
Wolfgang Späte
Major Wolfgang Späte was a German World War II Luftwaffe flying ace. He was also a recipient of the Knight's Cross of the Iron Cross with Oak Leaves...
in 1938.
However Späte may not have considered sinking air between thermals, and there is no mention of this until 1947 when Ernest Dewing and George Pirie independently included this aspect. Paul MacCready, however, certainly invented the "ring", which allowed an easy indication of the optimal speed to fly.
Instrumentation
The minimal instrumentation required is an airspeedAirspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
indicator and a variometer
Variometer
The term variometer also refers to a type of variable transformer or an instrument for measuring the magnitude and direction of a Magnetic field....
. The pilot will use the polar curve
Polar curve (aviation)
A polar curve is a graph which contrasts the sink rate of an aircraft with its horizontal speed.-Measuring a glider's performance:...
information for the particular glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
to derive the exact speeds to fly depending on the lift and sink conditions in which the glider is flying. This is commonly done using a speed to fly ring (known as a 'MacCready Ring') which is fitted around the aircraft's variometer. The ring is usually calibrated in either knots or meters per second and its markings are based on the aircraft's polar curve. During the glide between thermals, the index arrow is set at the rate of climb expected in the next thermal. On the speed ring, the variometer needle points to the optimum speed to fly between thermals.
Electronic versions of the MacCready Ring are built into glide computers that will give audible warnings to the pilot to speed up or slow down. Similar facilities can also be built into a PDA
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
. The computer is connected to sensors that detect the aircraft's airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
and rate of sink. If linked to a GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
, and using a computed or manual estimate of the windspeed, the glide computer can also calculate the speed and altitude necessary to glide to a particular destination. This glide is known as the final glide because no further lift should be necessary to reach the goal. During this glide, speed to fly information is needed to ensure that the remaining height is used efficiently.
External links
- Performance Airspeeds for the Soaring Challenged by Jim D. Burch mirror
- MacCready Theory with Uncertain Lift and Limited Altitude Paper from Technical Soaring 23 (3) (July 1999) 88-96, by John H. Cochrane
- Just a little faster, please paper by John H. Cochrane
- The Price You Pay for McCready Speeds by Wil Schuemann, from the Proceedings of the 1972 Soaring Symposium
- Competition Philosophy by Dick Johnson, from the Proceedings of the 1972 Soaring Symposium
- Introduction to Cross Country Soaring by Kai Gersten, 1999 (Revised 2006)
- Canadian Advanced Soaring - Techniques