plane takeoff help

[jab136]

so I am trying to build a space plane and for some reason when I try to take off it pulls to one side or the other (can't find rhyme or reason to which side it pulls towards), I have dihedral wings with control surfaces as well as vertical stabilizers, and my wheels are balanced cm is below and in front of cl and rear wheels are lower than front wheels so I shouldn't be having issues, but I am. Using multiple mods but they shouldn't be affecting anything (well maybe ferram aerospace but still...) only other mod that applies is B9 because some of the parts on the plane are from that mod.

I rebuilt my plane but am still having the same issue, the pics are posted on page 2 please help.

Edited April 15, 2014 by jab136

[Grim187]

I've had this problem a few times, its usually turned out to be the wheels or SAS being weird.

try a low throttle taxi down the runway to help you diagnose.

edit: i think CM is should be above CL...slightly.

Edited April 13, 2014 by Grim187

[White Owl]

A screenshot of the plane might help.

Also, I've been reading that lots of people have issues with excessive drag when using B9 landing gear and FAR. A fix is being worked on. Think that could be the problem?

[NQMT]

Checklist:

Are your wheels angled inward/outward?

Are your wheels cambered?

Are they placed unevenly?

If any or all of these are 'yes', fix. Try again.

[vetrox]

I always find it to be weak joints, even with the new joints. Strut wings together and strut wings to main fuselage. The effect is doubled if the landing gear is directly attached to wings

[Claw]

Yeah, a picture would help a ton. I can throw a list of things it "might" be up here, but it's easier to give specific advice if we can see it.

[jab136]

I also forgot to mention that I also have kerbal joint reinforcement installed

[XnuffX]

Get rid of one of the front wheels!

[jab136]

I had that previously, the two wheels were because I wasn't sure the single wheel was straight. neither configuration worked

[Tex_NL]

Center of lift is way too far behind the center of mass. In this configuration the tail will lift and the plane will start to fall over, steering either left or right.

Easiest solution: add some canard wings to move the CoL forward (to just behind CoM) and it will take off just fine.

[Kasuha]

Your centrer of thrust seems to be quite high, maybe it's pushing nose down? Unless you are only using the two bottom engines during liftoff.

I would also consider putting rear wheels more forwards, just behind center of mass. It helps with pitching up.

[ThorBeorn]

I can see two problems with your design.

1: Your wheels are cambered (pointing outwards). This is great for when you want a for instance a wheelchair to be easy to manuever/turn. But this is also causing aircraft in KSP to turn randomly and crashing to the sides.

2: What Tex_NL says. Your center of lift is quite a bit far behind your center of mass. This can make your plane impossible to pitch up and if it manages to take off, it will have to fly at a large AoA to stay in the air. This increases drag and hurts overall performance.

[Random Tank]

Also, those intakes on the rear engines are pointing.... backwards.

Novel, I'll give you that; but air comes from the front my friend!

[jab136]

Also, those intakes on the rear engines are pointing.... backwards.

Novel, I'll give you that; but air comes from the front my friend!

well ****, I didn't even notice that. I think I may have to redesign the plane, any suggestions on how to attach the rockets so I don't screw up my center of thrust and torque the rocket when I fire them?

[jab136]

I completely re-did my design (pretty much a completely new plane) and it is still pulling to a random side. please help

[Tex_NL]

Can you please so some pictures from the side. Current pictures make it near impossible to judge the CoL, CoM, CoT, landing gear relations.

[jab136]

[Tex_NL]

Thanks, that helps a lot.

First of all your CoL is still WAY too far behind the CoM!

Secondly (still a bit hard to see) your rear landing gear is also too far behind the CoM.

The faster you go on the runway the more lift you'll generate. Since your CoL is far to the rear all that lift is trying to lift the tail. (Imagine you've attached a piece of string at the CoL and lift the plane from there. You're plane will hang nearly vertical nose down instead of near horizontal.) When the tail is lifted all that carries the front is your single nose gear and your plane starts to fall, and steer, either left or right. Commonly known as wheelbarrowing.

To make your plane stable you'll need to bring the CoL forward. Either by moving your wings forward or as I've already said in my earlier post that can easiest be achieved by adding some canard wings near the nose.

About the rear gear:

Let's assume you did have your CoL just behind the CoM that you still would have difficulties getting off the runway. To raise the nose you would have to lower the tail of your plane. But you can not lower the tail because that's where your landing gear is. The closer your landing gear is to the CoM the easier it is to 'rotate' your plane. Unfortunately, if you go too close it becomes instable again.

Edit: After giving you plane a closer inspection it looks like you could use bigger wings as well.

70+ tons with those small wings? I doubt it to be airworthy even IF it was properly balanced.

Edited April 15, 2014 by Tex_NL

[Taki117]

With design number 2 you might want to consider putting your rear landing gear on your lower fuel tanks to reduce the risk of tail strikes. Also, you have B9 and FAR, so design it to look real, you're not going to ever get it perfect, there will always be some drift. However, if you are pulling one way or the other rapidly (Resulting in unplanned dis-assembly) then it's an issue with your design. Also, you could probably drop those pre-coolers. Yes Interstellar makes them useful, but unless you plan on going Mach 2.5 deep in the atmosphere you're not going to overheat anything. (My current spaceplane doesn't even use them)

You could also stand a set of canards on the front. I believe what's happening is you are Wheelbarrowing. With your tail lifting off the ground first, resulting in more pressure being put on your nose gear, one point of contact and any small amount of thrust that isn't exactly perfect will cause a loss of control. Also, do you really need that many rockets? You could put the same setup along the edge of your aircraft and then attach wings to them, saves weight and looks much better than your current design. I'm also curious how you have your jet engines attached, that could also be a problem.

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[jab136]

the rockets are for space flight, also it was the only way I could think of to balance the center of thrust and still fit the fuel tanks, I removed the pre-coolers, moved the cp closer to the cm and moved the rear landing gear forward and wider, it didn't help and for some reason the landing gear seems wobbly.

[Voculus]

In addition to the other design fundamentals posted, you can also edit the B9 wheel CFG files, as they are far too wobbly to be useful a lot of the time. You want to change the "sidewaysstiffness" value to something very low, like 0.2 for example.

[jab136]

the current design doesn't use B9 and I don't know how to edit the cfg files (on mac)

[Hodo]

Ok looking at a few of your designs, you have issues with these problems.

Your landing gear are not straight, they have to much camber, which in KSP is bad. It causes random steering issues.

Next your CoL is to far back, it should be right behind your CoM. This keeps your craft pretty stable at super sonic speeds and gives you room for the CoM to shift while fuel drains.

Here is one of my OLD SSTOs.

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[jab136]

yah but wouldn't the fuel drain cause the Cm to move back behind the cl creating an unstable plane?

[GoatRider]

yah but wouldn't the fuel drain cause the Cm to move back behind the cl creating an unstable plane?

You can empty the tanks in the SPH or VAB by clicking or right-clicking on them, and the moving the slider. That will show you the empty CoM. Don't forget to fill them up again before flying!