Helicopter Landing - Sudden Drop

[afoolishmoon]

Hello, apologies if this is redundant (I couldn't find anything relating to this when I searched). I've been trying to wrap my head around helicopter physics and built a pretty stable test design using a contra-rotating rotor to stop spin (thank you Brikoleur!). I can fly around pretty easy, but one thing keep going wrong: Landing. Basically can't do it. Here's what happens:

I fly around a bit to test aerodynamics, then decide to land. I lower my blade RPM and descend slowly, but as I approach 0 m/s (descent) the rate of slowing increases very suddenly and I begin a free-fall like drop. I quickly jam the rotors back to max RPM, but there's a lag before the change starts reducing my descent and I'm typically around 20-40 m/s before it starts (slowly) reducing. Often this either ends in hitting so hard I explode, or just hard enough that I bounce hard upward and rocket back to high altitude (even if I drop RPM right before I hit). If I get it semi-stable I can ping-pong up and down, but can't maintain a slow descent. It feels as if the sweet zone for descending slowly is so VERY narrow that I can't stay in it, like balancing two spheres on one another. I can capture a video of this if it is unclear... Just hoping it's a known cause.

To be clear, when the sudden drop happens, I am NOT touching the controls. SAS is on, but my hands are off the RPM.

Does anyone know what I'm doing wrong? What's causing this sudden drop?

Edited July 15, 2019 by afoolishmoon

[OHara]

What you describe, sounds like a proper helicopter.  See the similar description here (link).

The helicopter blades need to approach the air at a reasonable angle of incidence in order to be effective.  Moving edge-on through the air does nothing, and presenting too much of the bottom surface to the air causes mostly drag.  The blades are moving forward around 100m/s,  so if the vertical speed of the craft is 20m/s up or down, the up/down component of air motion through the disk of the blades changes the angle at which they approach the blades.  So there is a sweet-spot of vertical velocity where the blades are effective.  Helicopters generally do not climb or descend very quickly, not without also moving forward.

And then, as you say, changing RPM takes a long time because the rotor+blades has so much momentum.  Doing so when the craft is moving down and the blades has a lot of drag, is very difficult.

It works better to keep roughly constant RPM and adjust the pitch of the blades, called the collective in helicopters, to change from climbing to descending.  That requires assigning the 'deploy limit' of the blades to an axis group.  When descending, you might even want the blades to pitch slightly down (a.k.a. 'pulling beta'), so that as you descend through the air, the air keeps the blade moving in the sense of a windmill (a.k.a 'autorotation').

Edited July 16, 2019 by OHara

[afoolishmoon]

41 minutes ago, OHara said:

It works better to keep roughly constant RPM and adjust the pitch of the blades, called the collective in helicopters, to change from climbing to descending.  That requires assigning the 'deploy limit' of the blades to an axis group.  When descending, you might even want the blades to pitch slightly down (a.k.a. 'pulling beta'), so that as you descend through the air, the air keeps the blade moving in the sense of a windmill (a.k.a 'autorotation').

100% right, thanks for the advice! I found by changing the angle 15-20 degrees I could force the craft to "auto-land" very gently.