How wind affects groundspeed is critical to understanding your flying speeds and direction. That airspeed is unaffected by wind speed is one of the most critical points any aviation instructor needs to pass to their students. Wind only affects and changes groundspeed and vectored directions, called “track”, when there is wind.
When flying in still air (i.e., no wind), airspeed will match groundspeed. With winds directly blowing toward you, your groundspeed will be reduced by the wind speed. With the wind blowing directly behind you, your groundspeed will be increased by the wind speed. In either case, airspeed will be dictated only by the angle of attack of the paraglider’s airfoil. This is controlled via the brakes and/or speed bar.
If you point your paraglider 90 degrees to the wind, you will fly in the direction you are pointing at the airspeed you are flying at. But, at the same time, you will have sideways drift at the exact speed of the wind.
Terms to understand:
Heading: The direction an aircraft is pointing. On an airplane, you would use the fuselage. On a paraglider, picture a line in the exact center pointing forward. This could be a compass bearing.
Drift: A sideways motion when an aircraft is flying through any crosswind. The sideways component, drift, is at the exact speed of the wind.
Track/Tracking: A flying craft’s true line of motion relative to the ground. When a wing is flying perpendicular to the wind, it will drift at the speed of the wind to the side at the same time as pointing to its heading. The combination is a vector downwind of the heading; this is called a “ground track” or just “track”.
In the above picture, if the wind is stronger, the drift will be greater. In zero wind, the glider will go directly towards its heading and there will be no drift. If it helps, you can use the very common example of a boat crossing a river. The wind is like a river. If you point directly across a river flowing at 3 MPH, you will drift in the same direction and at the same speed as the river as you cross it. For this example, you would drift at 3 MPH. Because of the drift, the boat would not travel where it was pointing. Instead, it would track slightly downstream, just as a paraglider tracks downwind as it crosses a wind.
EXAMPLES / Discussion:
You are flying at trim (I am using 20 MPH as the airspeed of your glider at trim) straight into a 10 MPH due west, headwind. If the brakes stay at trim after the turn, the groundspeed will be just 10 MPH. Your airspeed will remain constant at 20 MPH. When you are flying directly into a headwind or directly downwind, there is no drift or track effect, only groundspeed changes.
If you turn exactly 90 degrees to the right in heading to the north, you will be flying perpendicular to the wind and pointing due north. Following the turn, you continue to fly at trim speed. Your average airspeed will be 20 MPH. You will now be drifting east at a wind speed of10 MPH. Relative to due north, where the airfoil is pointed, “heading”, you will be vectored slightly to the right and track a bit downwind (moving sideways at the speed of the wind).
If you want to track due north, you will have to point your wing slightly to the west. This is called crabbing. If the brakes are at trim while crabbing, the airspeed will still be 20 MPH. Catching on?
You then turn 90 degrees, again to the right. You are now headed east and going directly downwind. Your trim airspeed will remain at 20 MPH. Your groundspeed will now be 30 MPH. If you are going exactly downwind, your track and heading will be the same.
In all of the examples above, you could substitute any other flying speed for all of the examples, and the result would be the same. We could use, instead of trim, pulling the brakes to a few inches above the shoulder (we can call this 17 MPH). For upwind or downwind, we just adjust the math (10 MPH headwind would get 7 MPH groundspeed; 10 MPH tailwind would get 27 MPH groundspeed). For sideways to the wind, downwind, upwind or any possible direction, the amount of brakes pulled (or speedbar pushed) will dictate the airspeed the wing flies at. Airspeed is not affected by wind! Wind changes nothing but groundspeed, drift, and tracking.
A common dilemma that new pilots face is mistaking a higher groundspeed for airspeed. It is imperative to recognize a tailwind when low (below 100 ft). Instead of slowing the glider down with the brakes, the pilot should turn back into the wind while high enough to safely complete the turn.
The equation for Groundspeed calculations is:
Groundspeed = Airspeed + or – wind speed
+ when going directly downwind
or
– when going direct upwind
For tracking, vectors need some math for exact ground speeds. The fastest groundspeed is always directly downwind and the slowest is always directly into the wind.
Math can be tricky for vectors. At 45% to the wind, if you are tracking perpendicular to a ridge, you might guess the wind speed at 10 MPH if you were flying at trim/20 MPH. The math actually says that the wind would be just over 14 MPH for this vector (71% of your airspeed). If you turn straight into the wind after realizing the wind is increasing, you will only be going about 6 MPH at trim. I would recommend being a bit more cautious with this. If you notice that the wind is picking up, watch for a 30 degree crab angle. At 30 degrees, you can estimate half of your trim speed as being the wind velocity.
Eventually you will have, or, perhaps you are already using a GPS device. It will display your groundspeed. If you are flying at trim speed and GPS reports a groundspeed of 12 MPH, your airspeed will still be 20 (trim on your glider with your all up weight = 20 MPH). This means that if you are flying directly into a headwind, then it is an 8 MPH wind. The headwind is subtracted from your air speed.
Air foils fly through the air. If a parcel of air is moving, we call it wind. The wing does not know if the parcel is moving or not, it just flies through the air within the parcel at the speed that the angle of attack dictates. So, the brake position and speed bar control airspeed and wind have no effect. Wind only affects ground speed and, when flying across it, tracking direction. Airspeed is only dictated by the brake position and or use of a speedbar.
Low ground speed mainly shows the wind subtracting from the equation:
+ when tailwind or TW vector
– when headwind or HW vector
The BIG Picture:
Airfoils fly through the air. If a parcel of air is moving, we call it wind. The wing does not know if the parcel is moving or not; it just flies through the air within the parcel at the speed dictated dictated by the angle of attack. The brake position and speed bar are the tools that change airspeed, the wind has no effect. Wind does affects ground speed and causes drift when you are not flying directly into the wind or directly downwind.
If you are moving very slowly over the ground, unless you are pulling the brakes way down or yanking them hard, stalling should not be a concern. “Stall” is not a speed! There is a minimum flying speed for airfoils, but stalls can happen at speeds faster than this minimum. Stalls are most common when the angle of attack is “suddenly” increased. If you fly no slower than near minimum sink (hands near top of shoulder), and do not ever pull brakes hard and fast, stalls are very unlikely to come into play.
Never pull the brakes after entering a gust or thermal when you have swung forward as a result. Stalls and Spins (Asymmetric Stalls) mostly happen from this. Pulling the brakes as a wing enters lift or strong lift can compound the angle of attack to cause a stall or spin. Although stalls and spins are very rare, this is a recipe better not tried. Instead, after entering lift or a thermal, keep your hands up until you feel yourself swing back under the wing.
