The definition "Geräte mit einer maximalen Bewegungsenergie unter oder gleich 79 Joule" is ambiguous to the point that a person can not know if/when they are breaking the law. In addition, the lawmaker made a gross oversimplification of energy to damage conversion, which I believe was the goal of having this category.
The common interpretation is that this means "device must have a mass < 250g". But that is clearly not the case - an airplane model weighting 250g and flying at 100km/h (look up Zohd Drift) has kinetic energy of 96J.
That clearly means that the mass is not the deciding factor, the energy is. So I could fly a 300g model or even a 1kg model, provided that they fly slowly, right? Examples:
- Zohd Drift - a sailplane with mass of about 300g and Vne 50km/h = 28J
- Easystar - trainer, 800g, can fly very slowly, below 10km/h; at 50km/h its energy is 77J
Now here it gets interesting. If I limit my Easystar so it can not fly faster than 50km/h, can I fly it up to 30m, because it qualifies as a toy?
The general line of thought is "no, because at 30m it has a potential energy of 240J". That seems reasonable at first, but the law limits the kinetic energy, not the potential energy.
The argument then goes that in a free fall, the potential energy is converted to kinetic energy which would exceed 79J. There are two major problems with this argument:
1. potential energy does not convert to kinetic energy without a loss
2. potential energy does not equal maximum kinetic energy a device can achieve
The second point is more interesting so let's focus on it:
Q: If a quadcopter weighting 250g is flying at 30m, what is the maximum kinetic energy it can achieve?
A: Its potential energy + the energy of the rotors + wind energy!
In this case, the worst possible scenario is that a drone suddenly finds itself at 30m AGL in a downburst, with its rotors at full down thrust. That is in essence the "maximum kinetic energy" of such flying drone.
(Or, for a model airplane, if it flies level at 10m/s 20m AGL, what is the maximum kinetic energy it can develop? In this case a part of forward velocity can translate to vertical velocity, and if it has a prop it can accelerate a lot in 20m).
How does one know what qualifies as a toy to stay within the law!? Even if I know the maximum descend speed, how do I take into consideration a possible (but unlikely) downburst?
But clearly this is what the lawmakers had in mind, otherwise they could simply have written "with maximum potential energy of 79J", or "with maximum kinetic energy - developed by own propulsion in still air - of 79J".
But no, the law clearly states "with maximum kinetic energy of 79J". To play the devil's advocate, to stay strictly within the law, one should not fly anything with mass grater than 10E-12g. Why? Because, to our current knowledge, the maximum speed an object could achieve is 299792458m/s, and only such object would have its kinetic energy truly limited to 79J.
Which is clearly nonsense, so that is probably not what the law means. But how can we know what it does mean?
Does it mean:
1. The model must be unable to exceed kinetic energy of 79J
Example: 10E-12g maximum mass
2. Fly within the limits of potential energy < 79J
- Anything with mass<250g is OK
- Flying a 250g model at 100 km/h is ok
3. Fly with a device which can not develop kinetic energy over 79J with its own propulsion in still air
Example:
- flying a light model airplane limited to 50km/h max (easystar, light electric glider)
- flying a 500g drone which can fly no faster than 60 km/h
4. Fly with a toy which can not develop kinetic energy over 79J with its own propulsion AND stay within the limits of potential energy <79J
- flying a light model airplane at about 50km/h max (easystar, light electric glider) up to 10m AGL
- flying a 500g drone which can not fly faster than 60km/h up to 15m
5. Fly with a toy which can not develop kinetic energy over 79J with its own propulsion AND stay within the limits of potential energy <79J at 30m AGL
- Fly anything that weights below 250g and can fly no faster than 90km/h with its own propulsion
None of this makes much sense. Personally, I would rather be hit by an easystar falling from the sky than a drone or an airplane flying at 90km/h!
Which brings me to the second point - the fact that the lawmakers have totally disregarded absorption energy and pressure! The limit of 79J is considered "safe", but is it?
A dart has a mass of about 20g, and an experienced darts player throws it at about 5.5m/s, giving it kinetic energy of just 0.3J, but I would really not like to stand in its way due to it's sharp point, which is going to release that energy over a very small surface, likely penetrating skin or (rarely) even skull!
https://europepmc.org/article/med/18990959 On the other hand, a foam airplane which flies at 50hm/h has kinetic energy of 77J. But if you've seen those - they have pusher prop and a long, soft, foamy nose, which compresses and absorbs a lot of energy. I mean, just look at this - the plane bounces off the grass on impact. That will not leave much damage on a person or property.
https://youtu.be/bdZiwYNaOvA?t=111 A drone is completely different. It's hard, its mass is concentrated, lots of sharp edges, at 79J it can cause some damage. And the same goes for DLG - small, very light (sub 250g) gliders made out of gfk:
https://www.youtube.com/watch?v=PdoiMff7IDk Taking that wing or nose to the head is no joke, but those are (probably) toys!
For all these reasons the definition as stated in the law is really poor. Until there are some court cases I'm afraid there is not going to be much clarity on what the law actually allows. The key issue is that it tries to generalise all devices: qadcopters, model airplanes, different build and materials and different damage potentials into a single definition, which just doesn't work.