Can't take off

[cowboycomando54]

So for what ever reason I keep encountering a rotational torque being applied to my starting tech jet planes on take off, despite the fact they are perfectly symmetrical in design and are a based on a generic leer jet profile. Large swept wings in front, smaller elevators in on the tail, vertical tail fin, and two small engines located above the elevators, yet some how I am getting some torque being generated, causing the plane to yaw hard to one side or another. My COM is in front of the center of lift and my center of thrust is behind that. Regardless if SAS is on or off and the steering is locked or not, I can't correct the random yaw. What am doing wrong?

[KerbolExplorer]

can you post a picture of one of your planes?

[cowboycomando54]

[bewing]

Are the planes still on the ground? Leftward veering during takeoff runs is a common problem, but has many fundamental reasons and fixes. Some fixes are subject to more debate than others.

Directional instability almost always comes from having more drag at the front of the vehicle than at the back. But there are questions about where the drag is coming or missing from. The wheels? The nose? The tail?

 

[OHara]

I guess you are having trouble staying straight on the takeoff roll.   The way the wheels are placed the plane will keep weight on the wheels a long time on the takeoff roll, and rear-wheel steering is unstable because if you veer a little left, the side-force of the front wheels pushes your nose further left.  You depend on the traction of the tail-wheel and aerodynamics on the tail-fin to overcome that over-steering tendency.

Why does the veer even start? It is easy to put wheels on at a slight angle -- editor tools 'C' for angle-snap, and 'F' to snap relative to the whole craft, help to avoid this -- but your wheels look straight.  Even perfectly symmetrically built craft do tend to veer one way or the other, which seems to bother some people, but for craft designed to point stably it does not bother me.

Standard suggestions are
 (1) move the front wheels very near the center of mass,
 (2) turn the friction to zero on the front wheels, 
 (3) set controls nose-up during the takeoff and landing roll to get relatively less weight on the front wheels, more on the back.   
There has been philosophical debate in this forum about which solves the 'fundamental' problem, which you can find or ignore as you like, but it is fun to try all the methods.

Also notice that tail-wheel aircraft often sit on the ground with the nose pointing up (link with some pictures) at an angle such that the wings lift the weight off the wheels at a speed not much faster than minimum controllable airspeed.  That gets the plane flying at a lower speed, which is practical in real aircraft, and also helps in KSP.

[Vanamonde]

Firstly, those landing gear might be too weak for a plane of that size, and if weight causes them to flex, they can become misaligned, resulting in wobbling. Secondly, as the nose rises as you start to become airborne, the plane pictured will become a unicycle, trying to balance on the single rear wheel. Generally, tricycle landing gear works better for KSP planes (with the gear placed so as to leave the plane sitting slightly nose-up, because KSP wings generate no lift at zero angle of attack). 

[Spricigo]

12 hours ago, Vanamonde said:

Generally, tricycle landing gear works better for KSP planes

Not really,  taildraggers are equally effective. Is just a matter of getting a few detail right:

• using gear that can support the weight of the craft without flexing
• put the main gear next to the CoM
• positioning the rear wheel so it don't need to do more effort than the main gear (it's called 'main' for a reason)

Like many other things in KSP, once you get the hang of it, you wonder why you had so much trouble before.

[GrouchyDevotee]

Looks like it could use elevators. Using the flaperons and ailerons for pitch control can work, but are not necessarily very responsive or stable.

Tail dragger designs (reverse trike) are notoriously difficult to control on the ground.