Can someone explain to me why my shuttles are flipping over in atmo at subsonic speeds?

[TeeGee]

I've been building shuttles ever since the mk3 parts came out and have never had this problem before. 

I'm testing the aerodynamics of my newest shuttle, and have become so flustered that I need to ask for help.

WHY is this happening? I can't find any rational explanation for this issue... especially at subsonic speeds.  

 

[bewing]

When CoM is above CoL, you are balancing a rock on top of a fulcrum. The craft naturally wants to stabilize in the other orientation, with the CoM hanging down from the point where the lifting force is being applied. All you have to do is raise your wings up one click to get them aligned, and it'll be neutrally balanced. As it is, your craft wants to fly upside-down.

[AeroGav]

CoL underneath CoM shouldn't affect pitch or yaw stability much, if at all, but it does make the plane less stable in roll - unless there's any dihedral to counteract this effect, the plane will have negative stability in roll, but I got the impression the OP was more concerned about pitch. 

@TeeGee I recommend downloading CorrectCoL  mod  , it updates the stock CoL indicator so it takes into account fuselage parts and not just wing parts.    You have a lot more fuselage in front of CoM than behind it, and fuselage does generate some lift, so it is possible CoL is not as far aft as you think it is.

Also I'd like to shout out to RCS build aid since it gives a nice red dot that shows where your CoM will be with empty fuel tanks.

BTW are you actually test flying it like this, since presumably it can't make orbit alone, and normally rides up with a load of solid boosters?

edit -  those Vector engines at the back weigh 12 tons.  You only have  a 4 ton cockpit at the front to balance them.   What happens when you empty the cargo bay, where does your CoM move to ?

Edited February 4, 2017 by AeroGav

[The_Rocketeer]

I would suggest moving the CoL further backward from the CoM - about twice as far. Although this will make your shuttle nose-heavy, the greater leverage of your elevator surfaces should be amply able to correct this as long as you don't approach stall speed, and if flying under power it shouldn't present a problem at all.

I see where @bewing is coming from, and I certainly won't contradict him, but I have built and flown plenty of ships with CoM above CoL and never had any problem. There could be a lot of reasons why that should be the case - dynamic correction, intuitive design choices etc - so I certainly don't say he's wrong, but you might find it better suits your aesthetic to work around the problem than utilise absolute symmetry.

Edited February 4, 2017 by The_Rocketeer

[TeeGee]

1 hour ago, bewing said:

When CoM is above CoL, you are balancing a rock on top of a fulcrum. The craft naturally wants to stabilize in the other orientation, with the CoM hanging down from the point where the lifting force is being applied. All you have to do is raise your wings up one click to get them aligned, and it'll be neutrally balanced. As it is, your craft wants to fly upside-down.

 

I've built every derviation and permutation of aircrafts in KSP ever since 2013... I've never had such an issue happen with previous aircraft before that had CoL balls on the ventral part of the CoM.

[AeroGav]

Is it just me, or are those strakes angled down?

That will move the CoL indicator aft in the SPH, because you effectively have nose down trim.

But, when you pitch up, the main wings will loose lift quicker because they are starting off from a larger aoa,  so it will try to pitch up at the stall.

All lifting surfaces should be at the same angle of attack, or if not,  surfaces at the front of a plane should be at a larger built in AoA (incidence) than ones at the back, that way the front end looses lift first and pushes the nose down.  You have the opposite - a plane that like to deep stall.

[bewing]

1 hour ago, The_Rocketeer said:

I certainly won't contradict him

Go ahead and contradict me, it's fine!

40 minutes ago, AeroGav said:

Is it just me, or are those strakes angled down?

I think the wings have a little bit of incidence, and the strakes are actually flat.

 

But anyway, the generic answer to the question of flipping is:

If your Center of Drag moves far enough in front of your CoM that your SAS system does not have enough control authority (at some AoA) to compensate, then your craft will flip.

The problem is that your CoD moves around depending on your attitude, so it cannot be displayed, and you have to guess where it is.

And if you want to move the CoD backwards, you also have to guess about that, too. Decreasing the drag at the nose helps. Increasing the drag at the back end helps (for stability, but is obviously bad for fuel efficiency).

 

Edited February 4, 2017 by bewing

[GoSlash27]

@bewing is correct; it's because the pressure center is ahead of the CoM. In addition to the CoM being slightly ahead of the CoL, both should be near the longitudinal center of the aircraft.

Best,
-Slashy

 

[AeroGav]

6 hours ago, bewing said:

 

I think the wings have a little bit of incidence, and the strakes are actually flat.

 

That would also be a problem.

If your craft has any angle of incidence on any of its lifting surfaces, then you must ensure the lifting surfaces towards the front have an equal or greater amount of incidence than those behind.    

At the moment, the centre of lift will move forward as he pitches up, because lift production gets into diminishing returns with increasing angle of attack well before the actual stall, and the rear wings are starting out from a higher angle of attack, so hit diminishing returns first.  Eventually CoL moves in front of CoM and the shuttle becomes uncontrollable.

What OP should do

1 - angle the stakes up by the same amount as the wings, or slightly more

2 - this will cause the CoL indicator to move forwards in SPH - but really that was it's true position all along.

3 - he'll then have to move the wings aft to get CoL behind CoM.

 

[Sharpy]

By the way, what's in that cargo bay?

If you carry a satellite in there and dump it in orbit, your CoM will move way back.

[TeeGee]

On 2017-02-07 at 11:11 AM, Sharpy said:

By the way, what's in that cargo bay?

If you carry a satellite in there and dump it in orbit, your CoM will move way back.

 

Nothing. The plane is empty.

 

I've actually solved the problem. I installed the proper CoM and CoL mod as well as RCS build aid to see what was really going on. 

Turns out the CoL was much closer to the CoM than the in game mechanics was telling me. So I moved back the wings in order to establish more stability, especially during supersonic flight.

Regarding the comment about the CoL being closer to the incidence, I didn't touch any of that on my shuttle to fix it. As a matter of fact, I've built shuttles that have their wings on the BELLY of the craft, even lower than what I've posted above. I don't think that is a real issue.