What's wrong with physics?

[jfmoyen]

Dear kerbonauts,

I tried to build a simple VTOL craft, for learning purpose (I'm rather green...):

https://www.dropbox.com/s/inoec82zwism5y1/Capture-1.jpg?dl=0

However it behaves in a strange way:

- engage SAS (so you don't have to worry about keeping it upright)

- throttle up; about 2/3 should take you to the sky

Now I tried to land, and the fun starts here; depending where I am etc, even with cutting the power fully (with X), the craft will not go down ! In fact, with idle engine and nose up, it will go down a bit, maybe 100 or 200 m; vertical speed will decrease; and then the plane will go back up to the sky, without touching power or anything of course !

Where comes? I can imagine it has to do with engines picking up efficiency as speed increases (as airflow becomes larger), but still with idle engines the plane should not be able to climb ?

Thanks for your advice !

Craft file

Edited October 20, 2014 by jfmoyen

[Red Iron Crown]

First, welcome to the forums!

Your craft is infinigliding. You have a lot of control surface on a very light craft, every time they move they add some forward thrust (this is definitely a glitch with the physics that will likely be addressed when aerodynamics get reworked in a later version). The SAS is actuating them to try to keep the nose pointed upward as the ship falls and tries to turn nose down. If you turn off SAS your craft will start to fall and point nose down.

You can minimize the effect by right clicking the control surfaces and disabling pitch, yaw, and roll control. They will just work as fixed fins then, but your ship has enough reaction wheels that it should still be controllable.

As an aside, the basic jet engine used in the craft has its maximum thrust at zero airspeed, the turbojet is the one that will increase in thrust up until 1000m/s, then it will start to drop off again.

[jfmoyen]

Hello, Red Iron Crown,

Thanks for the hint. Unfortunately there seems to be more than that. I did disable the control surface, but the same effect is still there (perhaps a wee little bit smaller ?). Some quantitative tests:

I've been stabilizing my craft at ca. 1000 m, pitch 90°. The V-speed still cycles, very slightly; the airflow is also cycling, going from 0.40 gently down to 0.05, and then suddently up to 0.40, and progressively down again.

My craft has a max thrust of 150 kN, close enough to 15 T in low atmosphere; at the time of the experiment it had a mass of 3.8 T. With a thrust on the first notch of the dial, so somewhere near 7% if the scale is accurate (which should have given me about 1T of thrust, way below the weight of the craft at any rate !) the vertical speed was fluctuating, roughly around - 1 m/s (between -0.1 and 1.9 or something like that). With no thrust at all (X) it had a V-speed fluctuating around -2 m/s (-4 to -0.5). It was certainly not accelerating downwards, as I would have expected.

Now at the same time, the craft was drifting laterally, at some 15-20 m/s : surely, this is not going to yield 3-4 T of lift, with an angle of attack of 90° ?

So, what makes this craft immune to gravity ? ;-) Can the craft infiniglide with no active control surfaces ?

Thanks

[Red Iron Crown]

Control surfaces can still apply force when disabled, maybe try the craft again with the surfaces removed for testing? Was SAS still on when you were testing? If so, what happens when you turn it off?

Regarding the airflow meter, you will get some odd readings when changing between upward and downward flight as angle of attack is part of the intake air calculation, but that shouldn't have any effect on your craft's performance at 1000m.

You might also find it useful to right click your engine and limit its thrust, this will give you finer control for testing.

[jfmoyen]

Control surfaces can still apply force when disabled, maybe try the craft again with the surfaces removed for testing?

Well, yes, that seems to be it. Without control surfaces, the craft behaves pretty much as I would expect -- falling like a stone at low thrust. Which does not make it easier to fly, incidentaly :-) But you're right, a thrust limitation will help in that respect.

[MarvinKitFox]

Unfortunately there seems to be more than that. ...

not accelerating downwards, as I would have expected.

See what this guy manages, with just control surfaces.

A flying Lead Zeppelin

Control surfaces, even locked, and stock physics are NOT very compatible!

[S1gmoid]

The stock aerodynamics is terrible - in a recent post, Squad said the only good thing about it is that it exists and runs.

Some people seem to derive a sick pleasure from abusing it for the lulz , but if you are more interested in a real flight simulation sort of experience, I suggest you install Ferram Aerospace Research.

[jfmoyen]

The stock aerodynamics is terrible - in a recent post, Squad said the only good thing about it is that it exists and runs.

Some people seem to derive a sick pleasure from abusing it for the lulz , but if you are more interested in a real flight simulation sort of experience, I suggest you install Ferram Aerospace Research.

I'm more interested in a fun game, frankly. But of course, that sort of physics bug is not fun :-)

(the Zeppelin is awesome !)

[Axelord FTW]

I'm more interested in a fun game

If you meant to imply that FAR is not fun, well let me tell how wrong I think you are.

Switching over to FAR is the best decision I've made regarding KSP so far. Whole new learning experience (or unlearning if you consider what Stock teaches you) that makes heavyweight launches more than MORE FUEL MORE ENGINES. Airplane are really fun to fly too.

[jfmoyen]

If you meant to imply that FAR is not fun, well let me tell how wrong I think you are.

No, I just meant that realism is not my chief concern. I'm not playing KSP for a job :-)

That said, installing FAR is rather high on my to-do list (has been so for a while and even more now) !

[S1gmoid]

No, I just meant that realism is not my chief concern. I'm not playing KSP for a job :-)

That said, installing FAR is rather high on my to-do list (has been so for a while and even more now) !

None of us are. But for me, one of the main fun factors of the game is its educational aspect.

[rosenkranz]

Yeah, between FAR and Fine Print, I've never flown so much is KSP. I'm usually a die-hard rocketier.

Note, I still can't land for anything. Even with nav utilis and the hud thingy I still end up using just flying low over KSC and hitting the chutes to low the whole plane to the ground.