Definition of terms:
- Angle of Attack – The angle at which the airfoil’s chord line meets the relative airflow from the glide slope.
- Best Glide or Best L/D Speed – The angle of attack at which the glider flies the furthest relative to the air (for example: if you flew a wing from a mountain in completely still air, this angle of attack would go the furthest). The expression L/D (Ell over Dee) means Lift / Drag. Best L/D is the scientific way of specifying the most efficient angle of attack for distance relative to the air.
- Descent Rate – The vertical speed of an airfoil relative to the ground. In the USA, we use feet/minute, in metric countries they use meters/second to specify descent rates.
- Drag – The resistance force to the flow of the air. There are several forms of resistance and these include form drag, induced drag and parasitic drag. Wikipedia has a good section on the cause of the physics of drag (but this is much more than can be covered in this article). For our purpose, it is just important to picture that when you pull the brakes down, the trailing edge of the airfoil is pulled down and this increases resistance to the relative airflow under the wing. Also, as the wing flies, there is drag from the wing itself, the lines and the pilots profile.
- Full Stall – The loss of lift from an airfoil when its angle of attack becomes momentarily so high that the airflow on the top surface separates from the wing. When the airflow is no longer connected to the wing, the lift forces fail and the airfoil stops flying.
- Minimum Sink – The angle of attack at which a wing descends at the slowest vertical speed through the air. Relative to the ground, this is true as well as you are relating descent as downward, which the ground is.
- For Memory —- >Minimum = Slowest, Sink= Descent rate
- Trim Speed – The angle of attack that paragliders fly at without any brake pull or speed bar use. In other words, the natural flying attitude of the paraglider without any input to change the angle of attack. Most paragliders achieve Best L/D speed at or very close to their Trim Speed.
- Spin – When one side of the wing is stalled, the wing rotates around the stalled wing and this is called a spin. So, a spin is an asymmetrical stall of the wing
Airfoils create lift when they move forward through the air. When the correct combination of airspeed and angle of attack are reached to support the airfoil and its payload, the airfoil begins to fly. As the airfoil flies through the air it is always descending relative to the air it is flying through. As airfoils moves through the air, the air flowing over the top of the wing speeds up. Because of this, there is a relative difference in pressure on the top of the wing versus the bottom. The air above the wing is exerting less pressure on the wing than the air below. This results in a pressure differential that combines to become the lifting force. Below the wing, the air is deflecting slightly from the bottom of the wing.
The airfoil is moving in a forward direction where the relative airflow veers slightly from the glide slope into the flatter, bottom of the wing. The air moves faster and exerts less pressure as it passes over the top of the wing up where the tall and curved part of the airfoil is. A flat piece of wood or a door will have a lower pressure above at some angle of attack, but the curve of the airfoil allows a much greater manipulation of the angles and enhances the lower relative pressure above the wing.
Bernoulli’s Principal
Bernoulli was a hydrodynamicist. He studied the airfoil shape and how the water behaved as this shape moved forward relative to the water. Air behaves almost the same as the water in relation to the airfoil shape. Going forward, I will refer to air as our discussion is about airfoils. Bernoulli found that the relatively faster moving air above the wing exerts less pressure on the wing than the slower moving air below the wing (this has been called the “Bernoulli Effect”). This is why we talk about higher pressure below and lower pressure above. Technically, it is the differences in these pressures that are the keys to the formation of lift. He documented that if you marked two particles adjacent at the front of the airfoil the one on top would move faster than the one moving under the wing. He showed that there is a lower pressure dome above the front section of wing as well as some increased pressure below.
The speed at which an airfoil moves relative to the air is called “airspeed”. In non-powered, gliding craft (Hang Gliders, Paragliders and Sailplanes) the airspeed is controlled through manipulation of the wing’s angle relative to its glide slope through the air and is called the wing’s angle of attack. In a paraglider, we can manage this angle primarily with pulling or releasing the brake handles. The natural angle of attack that a paraglider flies at without any brake or speed bar usage (Trim Speed), will be very close to the speed at which the airfoil will go the furthest through the air. In technical terms such an angle of attack is referred to as Best L/D speed (stated as “best ell over dee speed”). When a paraglider airfoil is gliding without any pull of brakes, this would be referred to as flying at “Trim Speed” . Trim Speed on most class A or B wings is very close to the glider’s Best L/D speed. The relation between trim and Best L/D speed is not the same for non-standard paragliding wings (speed wings, acro or specialty wings) and high performance wings (some do fall into this, but there is more variation, so it is always prudent to learn your specific wing characteristics) .
Airfoils glide downward though the air all of the time. The angle of the glide slope compared to the chord line is the definition of angle of attack. If you see a paraglider climbing, it is really descending through the air it is flying in. The paraglider is going up because the air is rising faster than the airfoil is descending.
Managing Speed and Descent Rate
When the brake toggles are pulled, the angle of attack increases. Pulling the brakes creates some drag at the back of the wing and this pulls the wing slightly back. When the wing is pulled back like this, the chord line raises and this causes the wing to fly slower. It is important to note that the drag from the brakes is the indirect reason the wing slows, the primary is the increase in the angle of attack.
When the wing flies slower than trim, the glide through the air is not as efficient (distance wise, through the air) as it was at trim speed. This resultant increase in angle of attack results in slower airspeed and slower descent rate (exception is slowing more than minimum sink – then the wing descent increases as the airfoil flies slower).
Paraglider wings are set to be very close to Best L/D speed at trim. Regardless of winds or lift and sink, you will always fly the most efficiently through the air at this speed. The caveat here is that in the real world there are winds and lift/sink variables. One of the most important facets of flying a gliding craft is to understand the dynamics of “speed to fly” theory (this is a full article in itself). This theory stipulates what speeds will help you fly further in relation to the ground with respect to these variables. The controls that you use to manage the speed and descent rate of the paraglider are mainly the brakes and the speed bar. Prior to getting into speed to fly, it is best to learn how the speed and descent rate work with respect to airspeed and angle of attack. After getting a solid grasp on this, then add in the speed to fly adjustments.
Below, the angle of attack is increased. First, the chord line in raised because the airfoil has increased drag (this pulls the airfoil back a little, raising the front of the airfoil). Secondly, with this increased angle of attack, the wing does not fly as efficiently forward as Best L/D speed (trim is very close to this). The increase in angle of attack causes the airfoil to fly slower. From trim speed, your descent rate will decrease as you pull the brakes downward until you reach the minimum sink angle of attack. If you pull further than minimum sink speed, the glider will continue to slow but will begin descending faster.
Minimum Sink
As the brakes are pulled, the sink rate of the glider decreases with the speed. When the brakes are pulled to a certain point, the slowest rate of vertical speed through the air can be reached. This speed is called Minimum Sink. If you pull the brakes further than minimum sink, the glider will continue to fly slower, but it will begin to move downward faster. Flying slower than minimum sink speed is not beneficial and the glider is much closer to a stalled angle of attack, so it can be dangerous. Minimum sink is a very useful speed, but making sure not to fly slower than this should be in your awareness all of the time while flying. Stalls happen more often from a sudden or dramatic change of angle of attack.
For example, when a glider enters a thermal, the lifting air of the thermal raises the angle of attack. During events like this, pilots sometimes pull the brakes quickly (before the glider can react to being slowed and having the pilot’s weight swing forward from this event). If a pilot pulls the brakes just as the wing enters a thermal, the nose will rise and the wing will slow as the pilot swings forward. It is better to let the brakes up when the glider first enters lift or even hits a gust that lifts the front of the glider. After this, wait for the wing to come back overhead before slowing smoothly to no more than minimum sink.
Minimum Sink is a fixed angle of attack. It does not change as it is related to how fast the airfoil descends through the air. On entry level to intermediate paragliders it is usually pulling the brakes down below shoulders to around mid or high chest area, but for every glider, you need to investigate and find out a close proximity to how much brake will achieve this angle of attack. In addition, pilots and paragliding manufacturers set the brake line lengths differently. So, work with your instructor or paragliding manufacturer to ascertain how much brake pull will achieve minimum sink.
Stalls
Airfoils stall when the airflow over the top of the wing separates as a result of an angle of attack that is too high. Yanking the brakes down hard and fast while flying a paraglider could lead to a full stall. A common misunderstanding is that stalls are speed related. In fact, stalls can occur at higher speeds when the angle of attack increases dramatically in a short period of time (a “pitching moment”). Jets can stall when a pilot pulls out of a high speed dive too fast. The point here is to understand the direct relation of angle of attack to stalls and to not be afraid of using your full range of speeds while flying a paraglider. Most paragliders will continue to fly at speeds much slower than minimum sink speed, but doing so has no benefit and if you did this in turbulent air, the variation in the airflow could induce a stall to occur.
The more common way that stalls can happen is when a pilot reacts wrong when entering a gust or thermal. Pilots are taught to slow the glider in lift and thermals, but when a paraglider first enters a thermal or gust, the wing slows as a result of either or both and as a result of the airfoil slowing down above, the pilot swings forward. The worst thing to do when this happens is to slow the glider down by pulling the brakes. Doing so will enhance the already increasing angle of attack and could lead to a full stall or, while turning, perhaps a spin (spins are the result of an asymmetrical stall). Instead, when the paraglider first enters lift or a gust, if the brakes are pulled slightly, ease them up to help minimize the angle of attack increase from this. It is fine to slow the glider smoothly after the wing has returned to being overhead.
Stalls occur when the angle of attack gets too high. In the below image, picture that when an airfoil without power has just been flying fast and then the brakes are pulled hard or fast (even a short distance), the wing will then change its angle of attack quickly (a pitching moment). The energy diminishes quickly as the airspeed drops. After climbing for a short time, the glider runs out of speed and the relative airflow becomes steeper as the wing no longer has energy to go forward and up. When this all comes together, the result is the flow separation “STALL” depicted in the below graphic.
View the above animation to see the true result of the angle of attach increasing rapidly. Note, the separation of air from the top surface of the wing.
Below is a link to a Video that is from a Zoom Ground School about the Paraglider airfoil and the related aerodynamics.
This video is about how Airfoils work.
