Speed to fly

Speed to fly is a principle used by soaring pilots when flying between sources of lift, usually thermals, 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. ["Modern Elementary Gliding", British Gliding Association, n.d. Appendix D: "Making the Most of it".] The idea is usually attributed to Paul MacCready though an early version of the theory was first described by Wolfgang Späte in 1938. [cite journal
last = Pettersson
first = Åke
authorlink =
coauthors =
title = Letters
journal = Sailplane & Gliding
volume = 57
issue = 5
pages = 6
publisher = British Gliding Association
date = Oct-Nov 2006
url =
doi =
id =
accessdate = ] 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. [cite journal
last = Edwards
first = Anthony
authorlink =
coauthors =
title = Letters
journal = Sailplane & Gliding
volume = 57
issue = 6
pages = 7
publisher = British Gliding Association
date = Dec 2006-Jan 2007
url =
doi =
id =
accessdate = ] 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 airspeed 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. [1 meter per second = approximately 2 knots (more precisely 1.944 knots).] 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. [cite book
title = Glider Flying Handbook
year = 2003
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-8083-15
page = p. 4-8
url=http://www.faa.gov/library/manuals/aircraft/glider_handbook/ ]

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.

References

External links

* [http://home.att.net/~jdburch/polar.htm Speed to fly]
* [http://faculty.chicagogsb.edu/john.cochrane/research/Papers/newmcred.pdf MacCready Theory with Uncertain Lift and Limited Altitude] Paper from Technical Soaring 23 (3) (July 1999) 88-96, by John H. Cochrane
* [http://faculty.chicagogsb.edu/john.cochrane/research/Papers/Flyingfaster_new.doc Just a little faster, please] paper by John H. Cochrane
* [http://www.betsybyars.com/guy/soaring_symposia/72price.html The Price You Pay for McCready Speeds] by Wil Schuemann, from the Proceedings of the 1972 Soaring Symposium
* [http://www.betsybyars.com/guy/soaring_symposia/72-dj.html Competition Philosophy] by Dick Johnson, from the Proceedings of the 1972 Soaring Symposium