VTOL

[RizzoTheRat]

Are there any tricks to getting a horizontally oriented VTOL craft to work besides very careful balancing of the CoM and CoL?  I had a play with a quad jet VTOL the other day, and despite having some vernors on a long tail boom it was very unstable in pitch.  My only successful VTOL craft have been skycranes with the CoM well below the CoT which makes them a lot more stable.  Are there mods which will vary the power to individual engines to give better roll/pitch control, or do I just need better balance and more powerful tip thrusters?

[Val]

With a single VTOL thruster, CoM below the point of thrust is really the only way to increase stability, other than adding other types of stability like reaction wheels and RCS.

But if you have multiple VTOL thrusters, angling them outwards from CoM will improve stability, but cost you a little in cosine losses.

And that is why it works. Let's say you have 4 VTOL thrusters, front, back, left and right, and they're angled 5° out.

If your craft pitches forward, then the forward thruster will start pointing more straight down, reducing its cosine loss, while the rear thruster gets even more angle, increasing its cosine loss. The net effect is that the craft will try to pitch back until the thrusters balance again.

Edited February 22, 2016 by Val
Grammar

[RizzoTheRat]

Gah, why the hell didn't I think of that.  Exactly the same principal as a tailplane and pitch stability.  Thanks.

[genericeventhandler]

If the CoM is way away from the CoT tweak the engine thrust limit to bring the CoT towards the CoM. Reduce the thrust in any rear thrusting engines.

i've made successful STOL craft on kerbin that are perfectly capable of VTOL on mun 

[WhiteKnuckle]

1 hour ago, RizzoTheRat said:

Are there any tricks to getting a horizontally oriented VTOL craft to work besides very careful balancing of the CoM and CoL?  I had a play with a quad jet VTOL the other day, and despite having some vernors on a long tail boom it was very unstable in pitch.  My only successful VTOL craft have been skycranes with the CoM well below the CoT which makes them a lot more stable.  Are there mods which will vary the power to individual engines to give better roll/pitch control, or do I just need better balance and more powerful tip thrusters?

When I'm building a VTOL, my goal is always to be able to turn on the vertical engines and have the craft lift off the runway without SAS being on or me having to make any control inputs. Vernors and reaction control wheels are fine for aiding maneuverability, but the craft shouldn't need them just to balance.

If you've got any pics of the plane I'll be happy to give specific advice, or you can check the link in my signature to see some of my VTOLs including a few quad engine ones that might give you some inspiration.

 

[FullMetalMachinist]

Sorry to be the "there's a mod for that" person, but there's a mod for that. 

[Rushligh]

Gotta agree with full metal machinist. there are a few very simple mods for that.
 

is the more complicated of the two major ones; this will however allow you to make far more complicated, stable, and faster VTOL craft. you'll have to control all the throttle's yourself however, and set all the values manually.
 

TCA is the simpler and more 'beginner friendly' version. it doesn't require as much design and it mostly automated. it also allows you to use engines as thrusters. very useful if you want to mount smaller LFO engines in place of RCS thrusters. its more for landing/take off vertically, not as much for inflight.

in theory you could  use both of them together; they don't conflict. and neither add's parts to the game (DTCS has one part which acts as a controller like a MekJeb cpu)

there are other mods out there, but these are the most common/supported/multipurpose. even if you run stock parts i would suggest these plugins.

[Hodo]

The trick with a good VTOL, is to make sure your CoT is in line with the CoM for the vertical mode.  I like to build everything then set the rear engine or the pusher engines to 0 thrust on the limiter, and see where the CoT ends up.  I will then adjust the fuel load and vertical engine placement until I get them perfectly or as close as I can get them to centered with each other.  If it all works out well I can usually fly the craft without RCS or SAS support or minimum of either.  I like to adjust the fuel loads as it changes.  I suggest RCS Build aid to help with the build of any craft and should be stock with the game.  I also suggest TAC Fuel balancer but it isn't required to make a good VTOL.

 

Here is a few of my more recent VTOLs that are in use as of 1.05 for me.

Unmanned SSTO science craft and VTOL.

Atmospheric rescue and science craft VTOL.