MacCready for Paraglider Pilots
Speed to fly answers the question 'what does the air I am in right now demand?'. MacCready theory adds a second, forward-looking question: 'how strong will my next climb be?'. Together they tell you how fast to glide between thermals on a cross-country flight.
Paul MacCready's idea, borrowed from sailplane racing, is that altitude has a price, and the price is set by your next thermal. If climbs are strong, height is cheap to replace — so you can afford to burn it for speed. If climbs are weak, height is precious — glide carefully.
What the MacCready value actually means
The MacCready setting — 'MC value' or ring setting — is one number: the average climb rate, in m/s, you expect in the next thermal you will stop for. Set MC 2 and you are asserting: the next climb I take will average 2 m/s from entry to exit, including the messy centring at the bottom.
Mathematically, the optimum glide speed comes from the same tangent construction as speed to fly: shift the origin upward by the MC value, as if you were gliding through air rising at that rate that you refuse to stop in. A higher MC pushes the tangent point right — glide faster.
Set MC to zero and the theory hands back plain best glide: maximise distance, ignore time. That is your setting when getting there at all is in doubt — scraping toward a landing field or stretching a final glide over unlandable terrain.
Why expecting a strong climb makes you fly faster
Think of it as time accounting. Gliding faster means arriving at the next thermal lower — you traded height for time on the glide. The height you sacrificed must be climbed back, and the cost of climbing depends entirely on how strong the thermal is.
If the next climb averages 3 m/s, 100 metres of height costs 33 seconds to replace — cheap, so race. If it averages 0.5 m/s, the same 100 metres costs over three minutes — expensive, so glide near best glide and hoard your altitude. Flying MC correctly minimises total time: glide plus climb.
This is why the same transition is flown differently at 2 pm under a booming sky than at 6 pm in dying thermals. Nothing about the air between the thermals changed — only the price of the altitude you will need to buy back at the far end.
Honest caveats for paragliding
MacCready theory was built for sailplanes with glide ratios of 40 or better and strong, reliable climbs. Paragliders climb at 1–2 m/s on a good day, and our glide degrades noticeably on bar — the polar steepens where a sailplane's stays flat. The optimal MC speeds are therefore closer to trim than the theory's sailplane pedigree suggests.
The theory also assumes you will find that next climb. On a paraglider, arriving 200 metres lower can mean arriving below the working band and bombing out — an asymmetric risk no tangent line captures. Most experienced pilots deliberately fly a 'degraded' MacCready: set roughly half the climb rate you honestly expect.
In practice: fly closer to MC on strong days with high cloudbases, on final glide, and downwind of a reliable house thermal. Fly conservatively — MC near zero — when low, in weak conditions, or over bad terrain. MacCready is a ceiling on aggression, not a floor.