Thoughts about ... the speedbar

Gas pedal: "Mechanical device allowing the driver to increase the vehicle's speed."

In paragliding, a speed system has exactly the same function: to increase the airspeed of the aircraft. This accessory, which may frighten the novice pilot, can also prove to be a formidable ally in many cases. As with any tool, it's important to be familiar with it, to learn how to use it wisely and not to fear it unnecessarily.
We'll try to decipher how it works, the consequences on the wing's behavior, and when and how to use it.

I.- A few notions of aerodynamics ... :

To fully understand how the gas pedal works and the consequences for its use, it's important to have assimilated certain aerodynamic concepts, in particular those relating to the speed polar, attitude and angle of incidence.

I.1- Speed polar :

The airspeed polar is a graph describing the vertical airspeed as a function of the horizontal airspeed of a stabilized straight flight aircraft (paraglider, hang glider, glider, etc.). This curve can be used to illustrate the different flight regimes of a free-flying wing, and to determine its overall performance within each regime: speed, sink rate, glide ratio. Even if the shape of the curve remains generally the same, there are certain peculiarities depending on the type of wing (leisure, performance, mini, etc.) and the flight characteristics expected at the design stage.
We consider this curve to be the graphic representation of the glider's performance. A good understanding of this curve will help you to better understand the effects of using the gas pedal, as described below. We'll concentrate on this one, but without going into too much detail.

The different flight regimes are each defined by a zone on the graph:

Notable features include

- Speeds above the minimum sink rate, known as the "1st regime" (downward part of the curve).

- Speeds below the minimum sink rate, known as the "2nd regime" (rising part of the curve).

- The stall speed (Point A):
Located at the extreme left of the curve, the stall point corresponds to the minimum speed required to remain in the flight envelope. Below this speed, the wing stalls and stops flying!

- The speed corresponding to the minimum sink rate (Point B):
This is the highest point on the curve, where the curve intersects its horizontal tangent. We're in the flight regime that allows us to descend (vertically) as quickly as possible.

- The maximum glide speed range (Zone C):
Located at the junction of the straight line passing through "0" and tangent to the curve, this zone corresponds to the best possible glide for the designated wing. We'll see later that this zone of max glide can be more or less important depending on the wing, as the polar curve can be likened to a straight line over a certain portion.

- Max speed (Point D):
This point, at the extreme right of the curve, corresponds to the maximum speed that can be reached by the glider, after using all possible acceleration systems. There is no exit from the flight envelope just after this point, but we are approaching the acceptable limit for safety.

This polar diagram shows that there is a direct link between speed, glide ratio and sink rate (in other words: performance). When using the gas pedal, we must therefore be fully aware of the interaction between these 3 parameters.

I.2- Basis and incidence :

To allow aerodynamic forces to come into play and make flight possible, it is necessary for the air streams coming from the relative wind to attack the glider at a given angle: the angle of incidence. This is formed by the plane of the sail and the direction of the relative wind (which also corresponds roughly to the paraglider's air path). This angle is directly related to the glider's trim, which corresponds to the angle between the wing plane and the horizontal.

At the design stage, and depending on the purpose of each model (competition, hang glider, school, cross-country, etc.), the designers calculate a pitch (and therefore an angle of trim) which will define the glider's nominal flight characteristics, i.e. without pilot action. There is no standard for this setting: it is specific to each glider model. You can find this "neutral" point on the polar chart, and determine the nominal parameters: cruising speed, sink rate and glide ratio. On modern paragliders, this point is generally located at the beginning of the maximum glide zone (Zone C), whether on recreational or performance wings. On mountain or speed-riding/flying mini-sails, this point may lie a little further down the curve (between C and D), in which case you'll need to apply a good dose of brake to find the maximum glide ratio.

II.- Accelerate ...

Now that we've (briefly) reviewed the aerodynamic principles associated with paraglider speed, let's take a closer look at the "acceleration" aspect.
If we want to increase a wing's speed, we're going to have to increase its trim angle to make it steeper. Today, there are 2 systems for doing this: foot throttles and trimmers (also called displays). Both use the same principle: deforming the risers to vary their relative lengths in order to modify the nominal pitch.

The foot gas pedal is the most common device found on today's paragliders. Made up of one or more bars, it is connected to the risers by a system of cords running through pulleys on the sides of the harness. The principle remains the same for all harnesses, but it's up to you to find out about the specific features of your harness and the correct way to install your gas pedal. Don't hesitate to consult a professional if you have any doubts.

By pushing on the bars with his feet, the pilot acts on the cords, which then activate the system installed on the risers. This system then accentuates the angle of pitch and generates an increase in the wing's airspeed. The A / B / C risers can be shortened linearly or not. Some systems even allowed the rear risers to be released when the front risers were lowered. But this system was soon abandoned, as the phenomenon was amplified in the event of a frontal collapse! The 2-pulley pulley system (in blue on the diagram) reduces the effort required of the foot.

Trim tabs work in exactly the same way, but are manually operated to lock the risers in a certain position. They are generally found on wings where the use of a gas pedal is complicated or simply impossible, such as in tandem or speed-riding (because of the skis!). They can also be found on paramotor wings. Trim tabs can be used to trim the glider more nose-up or nose-down. They are positioned on the rear or the front, and sometimes on both. Their position and action are totally dependent on the purpose of the glider.
In a tandem glider, for example, trimmers can be used to adapt flight speed to the weight of the passenger. Or to make a glider more lively at take-off if there's wind.

WARNING!

The trimmers can therefore offer a relatively precise adjustment, but as they have to be operated by hand, they can present a major disadvantage in the event of bulk, as the wing will have to be managed with the brakes: your hands will already be quite busy! You'll then have to manage a wing in a more nose-up or nose-down configuration, and therefore potentially less easy to recover in stabilized flight. With a more nose-down setting, the glider will be much livelier, and collapses more violent.

III - Action - Reaction!

All you have to do is activate the throttle or trimmers to modify trim and pitch, and allow the glider to pick up speed. There are many complex interactions between trim and incidence angles. In our case, where acceleration and increased speed are concerned, we'll simplify matters and assume that the greater the pitch, the lower the angle of incidence.

When we accelerate, we move a point along the polar curve to the right. Accelerating will therefore have the following direct physical consequences (see polar curve of speeds):
1.- Increase airspeed
2.- Change glide ratio
3.- Increase sink rate
4.- Reduce angle of incidence

More generally, flying a wing at higher speeds will accentuate all these effects, including reactions to collapses. Approvals require our paragliders to undergo all kinds of flight incidents at cruising speed (nominal stall), but also at various accelerated speeds (modified stall). The effects of these flight incidents are all the more violent the higher the speed. Managing these accelerated incidents will therefore be more complicated than at cruise speed. It's worth taking a close look at the certification reports to determine the cases which have finally classified the wing in a particular category (A / B / C / D). A single case may suffice, such as a large asymmetric collapse with the gas pedal pushed to its maximum. This is not necessarily a common occurrence, but it's something to be aware of.

As you reduce the angle of incidence, you'll find that the air streams will attack the profile more and more along the axis of the sail (lower incidence), and the risk of them passing over it (negative incidence) and causing a collapse increases. The sails are set up so that this doesn't happen in calm air, but more "tonic" aerological conditions are quite conducive to this kind of inconvenience. This is one of the only risks when using the gas pedal.

IV - The gas pedal: a management tool :

The main tool on a paraglider for controlling the various movements are the brakes. They are used to control the 3 axes (roll, yaw, pitch). They are also used to control speed, but only to a very limited extent, and mainly to slow down the glider's speed. When we want to pick up speed, we have very little room for manoeuvre using only the brake handles.
But there are many situations where it can be interesting to be able to pick up speed according to the aerology, for example in the case of a poor analysis of the current conditions...
The 4 consequences of using the gas pedal (Paragraph III) can then become precious allies, together or independently.

There are 2 ways to use the gas pedal:
- As a safety tool
- As a performance tool.

IV.1- The gas pedal as a safety tool :

First of all, consider the speed system as an additional safety feature to your paragliding equipment. Theoretically, you shouldn't find yourself in aerologically problematic situations, but who knows? It could well come in handy if you need to accelerate and enjoy some of the induced consequences (see above). It's not necessary to carry out this experiment in a secure environment (SIV type), since the glider is not supposed to leave the flight envelope, but to get off to a good start it's essential to carry out your first experiments in calm conditions, with sufficient height and away from any relief or obstacles.

As we saw with the speed polar, accelerating can help to degrade overall wing performance, especially towards its maximum. So "getting away" remains one of the main reasons for using the gas pedal as a safety tool. To get away more quickly from an "unpleasant" area, to avoid getting sucked into the cloud, or simply to fight the wind (or the breeze) and reach your intended landing. The gas pedal can also be combined with certain descent techniques, such as big ears, to make them even more effective. To learn more about these descent techniques and how to use them, it's essential to consult a professional.

IV.2- The gas pedal as a performance tool :

Less interesting on "leisure" wings, as they generally have their best upper-arm performance (notably glide ratio), the gas pedal becomes indispensable on competition wings to optimize the different flight regimes according to the conditions encountered. Performance wings have a "flatter" polar curve, enabling them to use the gas pedal while retaining their best glide over a greater or lesser range.
In competition or cross-country flying, or more generally in performance flying, you have to constantly juggle your objective, your timing and the aerological conditions encountered. Sometimes it's better to lose glide so as to arrive more quickly in a generous updraft, or sometimes it's better to optimize sink rate as much as possible so as to gain height in a thermal before moving on to the next terrain. The cases are many and varied, and could be the subject of an entire course, but that's not the point here. For those who would like to delve deeper into the subject, I recommend Baptiste LAMBERT's excellent video on the Mc Cready. In simple terms, you can see that when flying into the wind, the gas pedal improves glide ratio and ground speed. In accelerated transition, successful pilots even learn to control their wing's pitching movements by playing with the gas pedal, pushing or releasing at the right moments. When this is not possible, they use the rear risers, as this generates less drag than using the brakes, and allows them to maintain maximum performance. As we'll see later, the use of the brakes should be avoided when the gas pedal is activated.

V.- Precautions :

As we've seen, using the speed system isn't all that complicated, but it does have an influence on flying. It is therefore essential to follow the instructions below if you are to use it with confidence:

- The throttle must be correctly adjusted for safe and optimum use. The first thing to check is that the adjustment length is not too short, and will not activate the system without the pilot's voluntary action. The risk is to find yourself flying with a permanently accelerated wing. To be able to use it over the full range of travel, the 2 pulleys on the risers must be in contact when the legs are extended. If this isn't the case, and there's still some travel left in the pulleys, it's not a big deal - just adjust the next time you fly. In general, the adjustment is not easily accessible in the air, and you'll need to spend some time on the gantry to rough out the adjustment and refine it after a few flights to measure the adjustments to be made in real conditions (lengthen / shorten / symmetrize). It's important to use the risers of the glider in question to make the adjustment, and not the pulley system that may be present on the gantry straps because of its potentially different dimensions. In "performance" use, this adjustment is particularly important, as it will enable you to adjust each flight regime according to the bar used, in relation to the polar of the wing concerned.

- In the event of bulk, it's important to release the gas pedal immediately so as not to amplify the induced effects. The same collapse (type and amplitude) will be more violent and complicated to manage when the gas pedal is maintained. In addition, the reduction in angle of attack during acceleration makes the glider more fragile and sensitive to collapse. Particular care must therefore be taken when flying close to the ground and terrain. If you use trimmers, it will be very difficult, if not impossible, to neutralize them quickly in the event of a collapse, and you'll have to manage the situation more carefully.

- When using the gas pedal, it's imperative not to touch the brakes, otherwise you risk triggering a shutdown! It may seem paradoxical, but it's true. Without going into too much detail, the action of the brakes will generate a depression towards the trailing edge, which will then cause the center of thrust to move backwards. To realign this new center of thrust with the pilot's center of gravity (pendulum system), the glider will have to move forward and dive: risk of accelerated frontal! This explains why brake length adjustment is so important. If set too short, the trailing edge may be deformed when the gas pedal is used, leading to an accelerated frontal collapse without the pilot even having taken any voluntary action on the brakes.
And if you feel the need to brake, why did you accelerate? Well, to hold the glider, of course, if conditions are a little turbulent. But what if you're not allowed to touch the brakes? It's possible to hold the sail by using the rear risers, often equipped with small handles (at least on recent models). By acting on them, you can hold the sail without risking collapse. To steer the wing in accelerated flight (without using the brakes), we recommend adding harness steering to the use of the rear risers. For more experienced pilots, it's also possible to prevent collapses by controlling pitch with the gas pedal.

Conclusion:

You shouldn't think of the gas pedal as something frightening and complicated. It's a tool that's relatively easy to use, and deserves a little time to get to grips with it so that you can use it calmly when you need it. You need to take the time to set it up and adjust it correctly: its effectiveness will depend on it in case of need (a bit like the emergency parachute...).

Now you've read enough!
Go fly, and don't hesitate to test your gas pedal to make it your best ally!