Transcript Speed To Fly
Speed To Fly
Jim Crowhurst
Definitions
• Average speed: – Speed around the course.
• Speed to fly: – Speed you should fly at between thermals.
Speed Vs Distance
• Speed to Fly is a Trade off: – Fly slowly: Go further but arrive later – Fly Fast: Get there faster but travel not as far.
– What speed should we fly at between climbs?
Cruise Speeds
MacCready
• World Gliding Champion 1956 • MacCready Theory: There is an optimum speed to fly between thermals that combined with the thermal climb will result in the fastest average cross country speed. (reichman)
4 Knots
MacCready theory
4 Knots 4 Knots 80 knots 80 knots = 40 knots av. speed
Cruise Speed vs. X-Country Speed • 2 • 4 4 2 • 20 40 40 knots 60 80
3 knots 70 60 90 3 knots
Cruise Speed vs. X-Country Speed • Flying Faster 4 38 knots @ 90 knots cruise 2 • 20 40 60 80 • 2 • 4 • 6
Cruise Speed vs. X-Country Speed • 2 • 4 • 6 4 2 • 20 40 40 knots @ 80 knots cruise - ideal 60 80 Fly slow go slow Fly fast go slow
Failure Of MacCready
• Don’t know next thermal’s strength • Not flying in still air between thermals • The ground gets in the way!
MacCready Theory For Dummies • Look ahead: If you determine that the next climb is stronger than that achieved, leave the climb.
• Cruise faster if the expected climb is very good.
• Cruise slower if the next predicted climb is weaker
MacCready flying
• Next Climb.
• Performance of your glider.
Better the performance faster you go.
• What air you are currently flying through.
Stronger the sink…faster you go!
Speed Ring
• Your glider may have a vario with a speed ring.
• Simply put the arrow on the expected (average) climb rate and fly the speed that the needle points to.
• This is the most efficient speed to fly to the next thermal.
• Right?
Wrong!!
• • • If you leave a 3 knot climb, expecting another 3 knot climb, set a 3 knot ring setting….you will hit the weeds……soon!
Why?
We need to find the best climbs of the day, therefore sample as many thermals as possible.
MacCready Continued
• • Flying 10 knots too fast means you will get low and struggle for a while.
Flying 10 knots too slow will mean you get there a minute later but 500ft/1000ft higher and centre a climb faster………or chose another!
• Conclusion: Flying slower is faster than flying faster…!?
How Fast then?
• Rule of thumb……take average climb rate and halve it. Set that as MacCready setting.
• Allows you to fly fast but also allows you to sample more thermals.
It’s all Theory
• Pete Masson: 2001 Club Class World Champion – Average speed Governed by 3 things: Climb Rate Climb Rate Climb Rate Fly a little slower than MacCready says.
Cover more sky, sample more climbs, pick the best climb.
It’s all Theory
• Jay Rebbeck: 2002 Junior World Champion: “To achieve fast average speeds, it is far more important to climb in strong thermals than to stick to the perfect speed”.
MacCready Myths
• “I can’t figure out why I went so much slower than you, maybe I set the wrong MacCready setting on my glide computer”.
• No, he was just using more thermals that were weaker and not centering them as effectively.
Conclusions
• Above 4 knots it is important to have high speed.
• Below 4 knots it is important to search for thermals and go slower.
• Never fly faster but you can fly slower.
• Move around to stay in the lift.
• Stay in the lift and out of the sink!
Water-Ballast
• • • • • • Will move polar curve forward Will increase speed to fly between thermals.
Same LD will be achieved at higher speed Get to the same place faster Will climb slower Trade off between flying faster but climbing slower.
Water-Ballast Considerations
• • • Late in the day, climbs are slower and may need to dump earlier.
Sometimes it can pay to be in the air longer to get the next climb (poor day).
Keep ballast for into-wind final glide, keep it till the last moment.
Dolphin Flying
• • Under a cloud street: Fly slowly in the lift and fast in the sink.
In Strong conditions, pull up harder and steeper.
• • Don’t pull up in every scrap of lift as you fly further.- With little difference in performance.
Changing the AOA frequently affects the airflow over the wing and can affect performance.
Thermaling
• • • • • • What speed?
Min Sink Best L/D ? Wing loading ? Angle of bank ? Handling at that Speed • Whatever feels right……..
Other things to consider Height Bands • George Lee: Fly Fast when high and things are good: be picky with climb rates.
Fly a little slower at medium altitudes: cover more air so as to find a good climb.
Fly at zero MacCready when low: Maximise time in the air and maximise glide angle to find a climb.
Final Glides
• • • • The only certain time of knowing exactly what speed to fly at.
Set computer to MacCready of the wings level to wings level Climb-rate of that Climb. Leave the climb when on glide, If Av. Is 3 knots, set it to 3 knots. Never set it higher than 3 knots!