downward lift when pitched down

[VR_Dev]

Could someone try to explain to me why the lift switches to downward when its pitched down. It causes my zoid to nosedive as soon as it pitches below 0 degrees. The inner wings are backwards/upside down but I have the same problem with them the correct direction.

any advice would be greatly appreciated

Edited February 14, 2014 by clown_baby

[Claw]

When you pitch things in the SPH, it shows you the resulting lift as if it were Angle of Attack. So if you pitch it down by 5 degrees, then the lift line acts as if you have put your aircraft in -5 degrees of AoA. At negative AoA, wings in KSP produce lift downward instead of upward (because they are symmetric).

[VR_Dev]

i figured pitching it in sph was different then flying. I just can't figure out why it nosedives past -90 as soon as it drops below 0 degrees.

[Claw]

Are you talking about it nosediving in flight? Or just asking why it does it in the SPH?

In the SPH, it just shows the direction. So once you go into negative AoA it points down.

If you level your ship and put on wings that are pitched down, it will do the same thing. It basically assumes the relative wind will be going down the axis of the body and shows you the resulting lift direction.

Edited February 14, 2014 by Claw

[VR_Dev]

No i mean when i fly. It flies ok as long as the pitch is above 0 degrees. After about 15 seconds of flight it nosedives uncontrollably every time. I think the COM is too low and causes the top engine to push it downwards. Its hard to balance the COM and lift in the SPH for legs up and legs down. I wish there were a way to move the hinges in the SPH

Edited February 14, 2014 by clown_baby

[VR_Dev]

Ill just have to keep playing with it. I really wanna release the craft file but it needs to fly reliably first. thanks for the help

[Claw]

Ahh yes, this is a different problem. Based on your pictures, I thought you were about the Center of Lift in the SPH. If you can, post a picture of your ship with the Center of Mass, Center of Thrust, and the Center of Lift shown. What you are saying is probably true.

Although I am less familiar with some of the mods you are using.

[VR_Dev]

its just the b9 part mod and the robotics mod. nothing fancy. heres the pics with how i have it set up now. it launches and flies for about 15 seconds before nosediving every time. the only thing i can think is the lift is too high

[ComradeGoat]

Is that an engine way up there on top? I see your COT is about right, but I wonder if the RAPIER on the bottom is giving a misleading impression. Does the pitching stop if you cut thrust to zero? If so, it's your engine placement.

Edited February 14, 2014 by ComradeGoat

[Claw]

Check what Comrade said. You can set the thrust limit for a given engine to zero using the right-click tweakable menu and the SPH will update the thrust vector. Just remember to put it back when you're done or you'll get an in-flight surprise.

Also, what is your takeoff profile? If you are launching VTOL and then moving forward (while moving up) then you probably are causing downward lift. That might make an offset CoT problem even worse.

Also, that picture angle makes it hard to tell if your CoL is forward of your CoM. It seems pretty high, so if it is directly above your CoM that could cause some instability too.

Edited February 14, 2014 by Claw

[VR_Dev]

Thanks for the info on the tweakables. That helps a ton. I created a new rapier engine with no thrust so I could try to figure out the forward thrust, but thats way easier. That engine will be placed inside the ship with just the bottom of it exposed so it looks flush with the ship. You can kind of see the set up in the top two pics. Ideally that engine will be directly under the COM to accomplish a verticle takeoff/landing.

http://imgur.com/a/2QBMK

This is the album of my first version of the ship. It isn't as pretty but as you can see it takes off verticaly with the feet down, then lifts the legs up to fly. The dificult part is having it balanced for a hover then balanced for flight when the legs lift. The COM moves backwards and upwards as the legs lift up.

I know this is off the topic but thats the game plan for this thing, it's just going to take a lot of tweaking to get the hover>legs up>flight>legs down>hover>land sequence to work. Thanks again for the 0 thrust tweakable, thats seriously huge.

edit: yeah the Lift is behind the COM in that pic. I can post a better one when I get home. To me it looks there like the thrust is pushing below the COM, which should make it pitch upwards. But still the thing just nosedives after about 20 seconds, even with the downward rapier engine on

Edited February 14, 2014 by clown_baby