Plane doesn't get to speed and makes a backflip on liftoff.

[DonParpan]

It only goes up to about 64 m/s and is uncontrollable after liftoff. I've been trying many things and I have no clue what's wrong. Definitely not my first plane... but I'm out of ideas.

 

[Corona688]

Your center of lift is far out of whack, it should be close to the center of gravity.

We can't see your center of thrust, but I suspect it's too low, either that or you have drag-inducing elements on the underside of your plane.

[Stoney3K]

Center of lift needs to be closer to the center of mass, and your main gear is WAY too far back. It should be underneath the center of lift or slightly behind it.

Think of the gear as the center of a see-saw and the center of lift as a piece of string trying to pull the plane up. Try to keep a balance there.

The backflipping is usually caused by the center of lift being in front of the center of mass (which it is not) or by engines that are placed way too low and cause the plane to pitch up.

[DonParpan]

there's nothing on the underside and I have quite a few planes with the center of lift further back. Center of thrust is in the middle of the plane so it shouldn't get it out of control like that.

and the really weird thing is that it won't accelerate much past 64.

[Corona688]

33 minutes ago, DonParpan said:

there's nothing on the underside and I have quite a few planes with the center of lift further back.

So do I, I must compensate with lots of control surfaces.

Take a look at your control surfaces, and where they are relative to the center of gravity.  They are so close to the center of gravity, they have to work real hard to make it turn.  That long neck, if it catches any drag, can and will outfight them easily.  Pilot this craft with a velvet touch.

Also, those diagonal fins, whichever way you turn them, will always press down on the back end of your plane, because turning them generates drag which tries to push them backwards.  When your fins are sideways and flat, that drag doesn't matter, but when they're above your aircraft, that drag will force the end down / nose up.

I also notice your wheels are far back from the center of gravity, which may make lift-off difficult.  Imagine it as a lever, with the wheel as the fulcrum.  Pushing 'up' presses down on the back end of your plane, right down on your wheels.  Put the wheels a little further forward and it will help that push turn the nose of your airplane up.

The SR-71 didn't have as much of a problem because its center of lift wasn't so wonky -- its neck generated lift! -- but for all of that, while SR-71 may have been the fastest craft in the world, it certainly wasn't the easiest to fly.  About one in every three was lost to an accident.  I know it'll break up your SR-71 look a little bit, but a set of canards in the front may give you the control authority you need to fly.

As for the engines, check if the thrust limiter has been turned on by accident, that's happened to me a few times.

Edited June 16, 2016 by Corona688

[Stoney3K]

I'd start with some bigger control surfaces on the back (the Elevons 2 and 3) instead of the tiny ones you have on there now. And maybe, put a few reaction wheels in front of the NERV engines to help it turn and keep it flying in the same direction with SAS.

Move the gear forward, a lot, that will definitely help you on takeoff. And rotate (tilt) the wings slightly to move the CoL forward without breaking up the look, this will also curve the wing so you will have more lift when flying straight-and-level.

The way the CoL is positioned right now means the nose will drop hard when you let go of the stick, which will make you lose lift and stall. You need a small distance between the CoL and the CoM to avoid backflips and keep stability, but this is too much. The control surfaces may not even be able to turn it enough to take off.

Edited June 16, 2016 by Stoney3K

[Snark]

The problem is not the center of lift.  It's the center of mass.  Just turn off the CoL display here, it's a distraction.

Your center of mass is way, way too far in the back of the plane.  You have this huge draggy fuselage and what-not sticking out in front, which desperately wants to be behind the center of mass.  And the fact that all your control surfaces are way in the back means that they're extremely ineffective:  they're so close to the CoM that they only have tiny lever arms to work with, which makes them very ineffective.

To fix the problem, move your CoM way far forward, and/or add control surfaces that are far forward or far behind the center of mass.

[FullMetalMachinist]

28 minutes ago, Snark said:

Just turn off the CoL display here, it's a distraction.

Your center of mass is way, way too far in the back of the plane.  You have this huge draggy fuselage and what-not sticking out in front, which desperately wants to be behind the center of mass. 

^So much this. There are cases where the CoL marker is useful, but if you rely too heavily on it, it'll bite you, because it doesn't tell the whole story. 

The Center of Lift marker shows (you guessed it) where the sum of all lift forces will act on your plane at the current angle of attack. However, there's also this pesky thing called drag that the game gives you no way of visualizing, save for looking at where draggy parts are compared to the CoM. 

So while the CoM/CoL relationship might look good, there's the hidden drag forces that aren't shown, but you still need to account for them. 

Also, the things said above about short distance from the CoM that the control surfaces are is spot on as well. 

[bewing]

Stick a pair of canards on the front and it will fly just fine as is.

[DonParpan]

Thanks for all the tips, but you all address the things I already said aren't the problem. This thing flies as if there's a deployed parachute at the front. Heavier bigger planes with lots of more drag get faster on the runway than this one.

Engines are at 100% ans the plane sill is slow. I had canards at the front. The fins were vertical once and I made many changes. I know that this isn't the easiest to fly thing and definitely not the best one but many of my planes go into that direction. I like them.

Nevermind... I just rebuilt it exactly like in the screenshot... It was just KSP doing weird stuff.