Helicopter Height Ceiling and (lack of) Autorotation

[AHHans]

When playing around with the new helicopter blades I noticed that they seem to have a rather sharp height ceiling. When my Eve-ascent quadcopter gets above ca. 5500m on Kerbin or 17000m on Eve then it dramatically loses vertical speed and then starts dropping. My old version from before 1.7.3 that used elevons as the lifting surfaces showed a more gradual behavior: it didn't ascent as fast in the lower atmosphere and lost vertical speed much more slowly when it got higher. So that it could de-facto hover at its service ceiling. With the new helicopter blades my craft ascends much faster, but then rather abruptly goes from ascending at 20 m/s or so to falling within a couple of seconds.

Also there is no autorotation, but rather the opposite effect: once it started dropping the rotors slow down quickly. To get them to rotate again I need a lot of torque, much more than to "just" start ascending from a standing start. Which means that if I don't have rather overpowered motors (i.e. otherwise unnecessary weight) the craft is unrecoverable once it starts dropping.

Has anyone else noticed this?

[OHara]

Auto-rotation seems to be working for me.  It does require very slight negative pitch on the blades, and to use it for practical purposes you must not let the rotor slow down very much before you make that negative pitch.  The blade is a windmill through the descent, then you go to positive pitch to convert the energy in the blades into a burst of downwash to slow the craft before touchdown.

KSP 1.7.3 cheats a bit with the forces on propeller/helicopter blades, including a strong reduction in the torque on the rotor due to drag of the blades.  That cheat might be a bit much, I was hovering for several minutes  with no power other than that being slowly bled from the energy  in the rotor+blades.

Blade-pitch is extremely important in the new KSP helicopters, because that pitch affects whether you have excess torque to increase rotor speed, and there is a lot of momentum in the typical rotor+blades, so its current speed limits what you can do for the next minute or so.

[mystifeid]

As @OHara observed, blade pitch is important - are you adjusting the pitch as you ascend?

Also, might it not be better to try to get up as quickly as possible (big motors, max torque) and release while you still have a reasonable vertical speed?

This may not help you getting into orbit around Eve but...

I played with getting a single stage Terrier into orbit around Kerbin. The first attempts were hitting 9000m-10000m before stalling and then they didn't have enough fuel to reach orbit. I was launching with reduced torque, rpm and blade pitch (props not blades) because the craft became unstable if it accelerated/ascended too fast. Safety first, so 20m/s up was it with me trying to increase the aforementioned torque, rpm and pitch as I went up. It was tricky. I kept adding more fuel so as to make orbit with each attempt getting closer.

In the process I found that it was much easier and a heck of a lot faster to launch at maximum rpm and maximum torque, then just use blade pitch to smoothly accelerate to 140m/s up (much faster and bad things happened). I try to hold that speed for as long as possible by adjusting the prop pitch but because I am moving relatively fast, the cutoff point where the props start to become useless also comes much faster. This is the time to release and fire the rocket.

The first time I made orbit I released with 0m/s vertical speed - ie I maxed out the height first - and made it into orbit with about 50m/s dV  to spare.

Now I am releasing with 100m/s vertical speed - although at a lower altitude - and am reaching orbit with more than 1000m/s dV left. Big difference.

(Edit - with planes it doesn't matter so much but in this case I found it really, really helped to turn on caps-lock and use the fine adjustment for changing the prop pitch. One tap of the axis group keys gave about 0.5% adjustment)

Edited July 14, 2019 by mystifeid

[AHHans]

4 hours ago, OHara said:

It does require very slight negative pitch on the blades, and to use it for practical purposes you must not let the rotor slow down very much before you make that negative pitch.

what? Why does it need negative pitch for autorotation? That doesn't make sense... Well, I guess that's Kerbal physics.

I use a servo between the engine and the blades to set the collective pitch. And of course I set the limits for the servo to not allow negative pitch. So I never tried that.

3 hours ago, mystifeid said:

blade pitch is important - are you adjusting the pitch as you ascend?

Yes. I'm constantly monitoring blade pitch, engine RPM, and ascent speed.  With the old (elevon-based) blades I needed to gradually lower the blade pitch the higher I got to get the best ascent speed.  (Also because the torque wasn't enough to keep the engine at max RPM for most of the ascent.) With the new blades I can use a fairly high pitch and still get max RPM and a pretty high ascent rate, that increases with elevation -- until it starts decreasing and things go pear shaped. Lowering the pitch at that point (from ca. 16 deg to 5 deg or whatever) doesn't really help much if I don't have the spare engine power.

I guess my current strategy is at least related to your "high speed" approach: I ascent at best speed (maxing out at around 20 m/s) until the ascent speed starts to drop, then I fire up the rockets and drop the rotors. For me that's because If I don't do that then I start dropping fast, and with the low TWR that I have in Eve's atmosphere I would spend a long time just reversing the drop-speed if I'm not quick...

Well, Eve is a poodle! In my folding-wing Eve plane the parachutes rip my craft into pieces during the descent. Not because of high stress during unfolding, but because the static forces are too high. Guess I have to attach them to more than one spot...

[mystifeid]

31 minutes ago, AHHans said:

Lowering the pitch at that point (from ca. 16 deg to 5 deg or whatever) doesn't really help much if I don't have the spare engine power.

Are you using the biggest motors? In the test above I was launching with blade authority limit at 100% then increasing it to 101% until my speed got to about 50m/s. Decreased it to about 82% until speed got close to 130m/s then increased again, slowly, to maintain speed, sometimes to as much as 125%.

36 minutes ago, AHHans said:

For me that's because If I don't do that then I start dropping fast, and with the low TWR that I have in Eve's atmosphere I would spend a long time just reversing the drop-speed if I'm not quick...

Even on Kerbin it takes a long time to reverse a descent.

Anyway, good luck with your endeavors!

[AHHans]

@mystifeid "16 deg" on a servo between engine and blade is probably not the same as "15" units of blade authority.

No, I'm not using the biggest motors. On the contrary, I'll try to use the smallest motor that I can get away with. Extra weight is just extra cost getting it to the surface of Eve.

And thanks my plane now not only survives aerobraking on Eve (having folded its wings into the shadow of a 10m heat-shield), but can also go from parachuting to flying, and can get it's 3-Kerbal capsule back into Eve Orbit. Even if the latter is tricky and requires  to get the ascend trajectory just right.

Btw.:

2 hours ago, AHHans said:

Well, Eve is a poodle!

Well, te forums seem to have a "nasty"-word filter. I wouldn't compare Eve to one of my favorite vacuum-stage engines. "Poodle, when a Terrier just doesn't have enough bite."