I managed to get quite high in a helicopter today. I haven't seen anyone else get to this altitude so I thought I would share how to do it. Basically:

- Use a wing instead of one of the stock propellers.
- Don't use rigid attachment, let the wings stretch out with centrifugal force.
- Place the wings as far from the rotor as possible without losing control
- Place struts between the wings to prevent a catastrophic failure

Currently, the limiting factor preventing me from going any higher is that the craft becomes unstable and starts vibrating rapidly. If I can overcome this, then I suspect I can go higher still.

The craft above uses two standard rotors, in a counter rotating configuration. High altitude flight is probably most useful on Eve as part of an ascent stage.

In more detail:

**Stock propellers.**

The stock propellers have variable pitch. In theory this makes them ideal for any scenario: at high speeds, use a high angle of attack to get the most velocity, and at low speeds, use a low angle of attack to get the most acceleration.

Unfortunately, stock propellers seem to have a kind of hard ceiling where they stop being effective. At their "max" altitude, these propellers rapidly stop producing lift and fail. Adding blades does not seem to help much. The ceiling for every propeller is below 20km ASL on Kerbin. If you don't want to exceed this altitude however, then stock propellers will give the best performance.

**Rigid attachment**

This leaves us with wings. Wings still produce effective lift well above 20km ASL. Unfortunately, we lose out on variable pitch. This means that for performance in high altitude flight, we will have to sacrifice some performance at low altitudes. Luckily for us, once the propellers are spinning, their attachment will spin further out, giving us a pseudo variable pitch propeller.

In all of my experiments, enabling rigid attachment for propeller blades causes worse performance. Not only does it prevent the blades from stretching out, but there also appear to be other limitations. For example: if the above craft is in a vacuum, then at full throttle, I only achieve 74 RPM. This doesn't make any sense since her should be no friction. In atmosphere, this is even less. Without rigid attachment, I can get to the maximum rotor speed of 460 RPM easily. There appears to be no combination of blades where turning on rigid attachment is beneficial. Correct me if I'm wrong.

**Place the wings as far from the rotor as possible without losing control**

The maximum power output from the rotor (and therefore maximum vertical velocity) will be achieved when the rotor is spinning at max RPM at max torque. At low altitudes air resistance will slow down the propellers and so the rotor will output it's max torque, but low RPM. As the craft ascends, it will become easier and easier to spin the blades so the rotors will be at max RPM but won't need a lot of torque. There is a sweet spot in the middle where the craft performs at it's best.

You can increase the altitude of this sweet spot (and therefore it's maximum altitude) by: adding more blades, increasing the area of the blades, increasing the angle of attack and placing the blades further away from the rotor. Of all the these changes, moving the blades away from the center appears to have the greatest effect.

**Place struts between the wings to prevent a catastrophic failure**

Unfortunately, if you offset the parts too much at high RPM one or more of the blades may stretch too much making the vessel unbalanced. Sometimes the craft just explodes. To remedy this, you can place struts from one blade to the next, which ensures they stay centered around the rotors. This way, the blades stretch evenly and remain functional.

At the moment, the craft above suffers from fairly severe vibrations at high RPM, which prevents me from moving the blades away any further from the center, but if I can overcome this, I think higher altitudes are possible.


How high can helicopters go? (at least 39km Kerbin, 50km Eve)