Aircraft nose-diving

[JTMilne]

Hi, this question has probably been answered before, but I can't seem to find a specific answer.

Most of my planes are nose diving, even though I try putting the engine really low e.g.

FORWARD DIRECTION---------->

......................^

......................|

......................|

....................COL COM

<----COT

COT = Centre of thrust

COM = Centre of mass

COL = Centre of lift

And it still tips forward! help! thanks.

Edited January 25, 2015 by JTMilne
it deleted the spaces lol

[Foxster]

So, you have COM in front of COL, right? Make sure its only just so, have it too far and the craft will nose dive.

Where is your fuel? If its at the back then the back will get lighter as the aircraft uses fuel.

Are your using SAS (and/or RCS) to help keeps things level?

Is your probe or capsule pointed in the right direction i.e. is the nav ball the right way around with ground at the bottom and sky at the top?

[Kryxal]

Pictures would help a great deal more ... we can't tell just how far behind the CoM your CoL is, nor how your pitch authority is.

[Janos1986]

Asymmetric thrust helps sometimes.

Try to tilt the engine nozzles slightly upwards until you manage to balance your pitch.

Note that this will give you some trouble in vacuum if you plan to leave atmosphere so have some reaction wheels to balance that out, if you're staying on kerbin you should be fine.

Also, more pictures would help, since position and angle of wings and engines are fundamental.

[JTMilne]

Ok sorry for the late reply, here is a screenshot of one of my planes. I set the thrust limiter off on all but the main engine.

[covername]

Drop the center of lift down to were the COM is. With lift that far above and behind the COM you are putting an upward torque on your center of mass. The further away your COM is the more torque is created. Being above the COM COL is further away

[MarvinKitFox]

Yep.

For such a short vehicle, your CoL is too far from your CoM, and your primary pitch control is too close.

(CoL above CoM is actually a good idea, for an atmospheric flyer. It tends to make your plane lazy but stable)

This plane will want to be stable while aiming down at about 40* degrees, and the elevators will have to work overtime to compensate.

Using off-center thrust is one way to combat this, but it is quite a dicey technique as the relative effect of engine vs. drag vs. lift will dynamically change with speed, alt and throttle.

When it causes your drag to also go below your CoM (your air inlets contribute a lot to that), ... well....

Quick fix:

Move the wings (with engines et al) a bit forward, and your tail surfaces as far aft as you can.

This will better align CoM and CoL, and give ailerons more grip.

[JTMilne]

Awesome thanks heaps guys!!!! Really really helpful hopefully I can make some better vehicles now

[RexKramer]

Getting your pitch control surfaces farther away from your COM will also give your more pitch authority. In other words, moving your elevator control farther back (aft), or putting a canard pitch control on the ship as far froward as you can make it.

You have the right idea placing the COL behind the COM. However, for such a short craft, your COM is too far back. Yes, moving the COL closer to the COM will make the craft less stable, but this is unavoidable in a very short ship.

In general, you can place the COL above the COM will no ill effects, I do this often. However, again because your ship is so short, having the COL so far above the COM is causing pitch moments which your elevators can't compensate for.

I think if you move the COL closer to the COM, that plane will fly well, and be extremely maneuverable. One suggestion to make the plane more stable: Use some structural fuselage sections to make the plane longer. They don't add much weight or drag, and having a longer plane makes it easier to have a stable plane.

- - - Updated - - -

I see you have 4 sepatrons powering the command pod- I bet that's an exciting ride!

[Frank327]

I think making your plane so short is a bad idea in general. It decreases the margin for error between it being flip-happy and being uncontrollable. With longer planes you can have more distance between your pitch control and COM while still putting your COL and COM relatively close together. It makes them more stable. The air intake body is a really useful part for early airplanes behind the fuel tank.

I'm not an expert on this stuff though. It's just what I noticed building some planes myself.

[Commander Jebidiah]

Hes trying to make a replica of a harrier and moving the winga will make it look strange.

My suggestion is to add the small control surfaces inside the plane and ahead as well as below the COM and doing this wont make the plane look different but will make the plane peform better and if this makes the plane to sensitive just disable some of the control surfaces.