Plane design

[tomf]

I've been playing KSP for a few years now, and the controlability of my planes are still entirely random.

My latest, ultr tight budget attempt flies ok, except that it it is only stable when pointing about 30 degrees above or below it's current velocity.

With SAS on if I take my fingers of the controls it will happily sit there with the nose up gaining altitude, or the nose down losing it but I have to fight contantly to get it to stay somewhere in the middle.

What am I doing wrong?

[allmhuran]

Pictures in the SPH showing the craft, with the centre of lift, mass and thrust markers turned on would help a lot!

[O-Doc]

The easiest way to refine these pitch moment is to angle the control surfaces on the craft until it flies level without SAS. Also, you can trim roll, pitch and yaw(Mod + W/S/A/D/Q/E) to get level flight, this is useful as you fuel drains.

http://wiki.kerbalspaceprogram.com/wiki/Key_bindings

[tomf]

[Hodo]

Your CoL is to close to the center of the CoM, odds are your fuel is draining and causing the crafts CoM to shift forward.

[GoSlash27]

tomf,

your tailplanes are fooling the lift marker and misleading you. Your wings should attach at or slightly behind your CoM, never ahead of it. The lift of the wings happens at the attachment point in KSP. You also shouldn't use a tailplane as a horizontal tail, but rather a control surface such as a delta deluxe. You want it to generate correcting forces (usually downward) rather than lift.

As hodo pointed out, try to make your fuel tank the center of mass so that it won't move as fuel drains.

Also, you should add a little dihedral (rotate your wingtips up and engines down) in order to add roll stability.

Best,

-Slashy

Edited August 2, 2014 by GoSlash27

[Voculus]

I'm not sure how much fuel is in the center tank, but adding some more, if possible, will bring the CoM forward a bit, which should help with stability issues. Also, you have too many intakes! One standard intake on the front of each engine nacelle will service each engine for its entire practical operating envelope. That, or even just two radial intakes in the rear, similar to what you have now.

Another alternative, if it fits your budget, is to add another fuselage section in the center, in front of the tailboom, thus extending the overall length of your ship, and moving the CoL a bit further back.

[allmhuran]

Pretty cool looking thing, but yes, yoru centre of lift needs to be further behind your centre of mass. Either move the wings back, or add some more mass to the front. Command pods obviously have a lot of mass, a bunch of RTG's will work too.

And as Hodo mentioned, be careful about where the fuel is. As the fuel drains out of the tanks they will get lighter, and the centre of mass will shift as a result. To see how it shifts you can right click on the fuel tanks in the hanger and drag the fuel level bar all the way to the left. You can then see position of the centre of mass when the tanks are empty.

One other thing: It looks like you might have asymmetric drag at the back. Air intakes create a lot more drag than most parts, so you want to be sure you have good symmetry. From the picture it looks like you have two intakes on the top of the craft that are not matched by intakes on the bottom (because they'd scrape on takeoff I suppose). This means you will have additional drag high on the fueselage, creating a torque that will tend to pitch the plane up. But you're only using basic jets, not turbojets, so there's no need to spam intakes. You can probably get away with just one, or you could stick circular intakes on the front of your engine tanks, or stick an engine nacelle in the middle where you have the fuel tank.

Ah, just thought of something else. I'm not sure if this still applies, but as I recall, in past versions of KSP the game would decide which way control surfaces should move based on where they were mounted to the fuselage. This includes (I think) the vectoring of engines. Your engine nozzles are behind the CoM, so to pitch up you'd want them to point upwards, but because they are attached to a wing, and the wing is mounted to the fuselage in *front* of the CoM, they will actually do the opposite. I haven't checked this recently so I'm not sure if this behaviour still applies in more recent versions, but if it does you can just right click on the engine to disable the thrust vectoring.

Edited August 2, 2014 by allmhuran

[Wanderfound]

Your CoL is to close to the center of the CoM, odds are your fuel is draining and causing the crafts CoM to shift forward.

Yes. All of the fuel tanks appear to be ahead of CoM (generally a bad idea), and there isn't enough slack in the CoM/CoL arrangement to get away with it. Once half the fuel is gone, that thing is going to try to fly backwards.

You'd also probably be better off with nacelles in place of the fuselage pieces that you've stuck the engines on. Less weight and a bonus intake.

[tomf]

Thank you all that is much more stable, so the page on the wiki that says that The centr of lift should be in front of mass is just wrong?

http://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Basic_Plane_Design

[allmhuran]

Thank you all that is much more stable, so the page on the wiki that says that The centr of lift should be in front of mass is just wrong?

http://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Basic_Plane_Design

It's either flat out wrong, or just written in a confusing way.

The author is describing the process of building an aircraft, and is putting the wings on before putting the empennage (tail assembly) on. Adding the tail assembly will shift the centre of lift backwards. So if you follow the tutorial, with the centre of lift *just* ahead of the centre of mass when you put the wings on, and then you put the tail assembly (including elevators, etc) on, the CoL will in most cases shift back behind the CoM into a correct alignment. But it's not clearly stated that this is the end result to shoot for, so it's not a good tutorial.

A far, far better tutorial can be found here.

[GoSlash27]

Aye. Typo on their part. And as I said upstream, you should not use your horizontal tail to generate lift, so even if they were shooting for what allmhuran is figuring, it would still fly funny.

Step #1 is to balance your aircraft longitudinally without the wings. Ideally, the center of mass should be in the middle of the fuel tanks. Then you balance everything else around it like a balance beam. In fact... I *actually* balance beam my aircraft on the runway to ensure that this is happening. A light mass far away can counterbalance a heavier mass in- close.

Step #2, add the wings. Preferably slightly low with dihedral. You don't want the wings touching or you'll have no roll stability, as wings produce lift and drag where they're attached, *not* in their center. They should attach slightly- behind the CoM.

Step #3, add control surfaces. It's ok to have a vertical tail (move it back to give it leverage), but you don't want horizontal tailplanes adding lift, so use ctrl surfaces for that and ignore how they move your center of lift (it's misleading).

Step #4, balance- beam it again at the center of the fuel tank (both empty and full) juggling your dead weight until it's perfect.

Voila! A perfect flying airplane!

Best,

-Slashy

Edited August 2, 2014 by GoSlash27