To have or not to have 7 kg?

In the GPS Triangle category, achieving maximum permissible wing loading is essential. The maximum wing area is not specified as a fixed value; rather, it is indirectly limited by a combination of key technical constraints:

• Maximum wingspan: 5 meters

• Maximum takeoff weight: 7 kg

• Maximum wing loading: 75 g/dm²

To meet the 7 kg weight limit without exceeding the maximum permissible wing loading, the model's wing area must be at least 93.33 dm². With this wing area and weight, you are essentially locked into a high wing-loading configuration, and the model will still fly like a brick. The only advantage of this setup is speed.

Summary of what we already know (efficiency zone):

1. Weight does not change the polar profile:

• Cl(α), Cd(α), Cl/Cd are the properties of the profile and size Re

• weight only shifts the operating point along the polar curve

2. With increasing weight:

• the required Cl is increasing

• optimal speed increases

• declining time in ascent

• glide ratio (L/D max) remains almost the same
  

3. Ideal glide ratio (L/D max):

• always occurs at the same time Cl ≈ 0,9–1,1

• higher weight is achieved at higher speed

Either the weight of the ballast maintains your glide at high speeds on the course, or it sends you to the ground before you can even reach the next thermal. This formula expresses the terminal velocity of an object moving through a fluid-such as free-falling through the atmosphere.

In simple terms, it tells us how fast an object can fall before air resistance (drag) balances the force of gravity, causing it to stop accelerating.

For a given CL​, the following applies:

• V (Velocity): The airspeed attained by the body.
• ∼ (Proportionality): This symbol indicates that the velocity is not exactly equal to this fraction, but is directly proportional to it. (The complete formula would include additional constants such as air density and the lift coefficient).

• m (Weight): The mass of the falling (or in our case, flying) object.

• S (Area): The area of the object's cross-section (perpendicular to the direction of motion)—essentially the surface area "hitting" the air.
  

Imagine a model weighing 1 kg with a minimum speed of 30 km/h. If you add ballast (such as a camera) and increase its weight to 2 kg—effectively doubling it—by how much will the stall speed increase?

• The answer is not 2x more (60 km/h).

• It will be approximately 1.41 × 30 = 42 km/h.

Why exactly 1.41? For a model to stay airborne, lift must balance the aircraft's weight. Lift increases with the square of the velocity. This means that to carry a higher mass, you don’t need to increase speed linearly, but only in proportion to the square root of the weight increase factor.

• The mass has increased by a factor of 2

• The stall speed formula works with the square root

• The fall speed will therefore increase by 41%, not 100%

The formula is a simplification, but it's enough to give you a clear idea of how mass impacts flight performance.

But beware! Not all kilograms are created equal. By reducing the weight of a real-world SkyTouch GPS model from 7 kg to 6 kg, the stall speed (and with it, the optimal glide speed) decreases by approximately 8%. While 8% might seem like a nice figure (especially when we talk about speed increases), in the GPS Triangle Sport Class, it represents a fundamental difference.

The price for higher speed is a higher sink rate.

If you are flying at 7 kg and hit "dead air" without any lift, you will be back on the ground exactly 8% sooner than with a 6 kg model. By loading the model to 7 kg, you are shifting the entire polar curve to the right. In strong conditions (climbs exceeding 2.5 m/s), 7 kg is the clear choice, as the speed gained on the course far outweighs the disadvantage in thermalling.

To understand the battle between mass and thermals, we need to look at what actually happens:

1. What does a thermal "want"?

Thermals do not reward speed; instead, they demand:

• A low stall speed
• The ability to maintain a high CL​
• A tight turning radius
• Stable and "smooth" flight characteristics

All of these factors work against weight.

2. What does higher weight actually do?
When you increase the weight:

• you have to fly faster to maintain the required lift
• you fly at a lower CL​ (lift coefficient)
• and the airfoil moves away from its ideal operating range (the 'sweet spot')
• resulting in an increase in stall speed
• and the time spent climbing is reduced
• in weak or narrow thermals, a heavy model will simply 'fall out'

And what is the difference in climb performance between a 6 kg and a 7 kg model in a 1 m/s thermal? It is fundamental. In weak thermals, speed is no longer the deciding factor; staying airborne is. This is where feelings end and hard physics takes over—the exact moment that decides the outcome of the early morning rounds."

The net climb rate for the SkyTouch GPS in a weak 1.0 m/s thermal:

• 6 kg model: 0.55 m/s (100 m gain in 3:02 minutes)
• 7 kg 0,45 m/s ( 100/0,45= 3:42 
  A difference of 0.10 m/s might seem negligible at first glance, but the difference in time spent climbing to gain 100 meters is a staggering 40 seconds!

How to identify the right weight in weak thermals?

Monitor the time required to gain 50 meters of altitude:

• Under 1:40 – Your weight is Spot on / OK
• 1:40 to 2:00 – You are on the Limit / Marginal
• Over 2:00 – You are Too heavy