Where to have CoL

[Axilourous]

So, people are saying to have the CoL behind the CoM, but then when I do that they are like "NO!!! UR DOING IT WRONG!!!!" And I'm just sitting there like, what. So, basically, where do I put the Col and CoM

[Gaarst]

If you want a stable craft, then CoL should be behind CoM. For a plane it shouldn't be too far though or you'll dive straight to the ground.

Some issues may arise due to the fact that CoL != CoP, but if you are not building a Space Shuttle replica you shouldn't have this problem.

Anyway, if you have a problem with a craft in particular, screenshots are very helpful.

[Leafbaron]

I have found that having putting CoL really far behind CoM doesn't matter that much as long as you have the ample control surfaces to provide enough control authority. In fact, i have found doing this makes planes fly much smoother, they don't turn 45 degrees into the air stream with a tap of the control key. 

[Rocket In My Pocket]

I typically place it just below and behind the COM for most standard plane designs. The markers should be close enough to be overlapping, like this:

I find this gives a good trade off between stability and maneuverability.

Edited December 7, 2016 by Rocket In My Pocket

[Axilourous]

Thanks you peoples. I will try it later

[Starman4308]

Best advice: put CoL way in front of CoM.

Then fly backwards.

For actually-serious advice: A, put CoL a bit behind CoM (further back means more stability and more margin for fuel depletion; closer to CoM gives more maneuverability), and B, check to make sure that your aerodynamically stable aircraft stays aerodynamically stable when the fuel is gone.

Way back in 0.25, I lost an SSTO that way; very stable at the start of the flight, but after delivering a full load of fuel to orbit, the CoM had shifted significantly backwards, and it span out of control after reentry.

Edited December 7, 2016 by Starman4308

[Sharpy]

My rule of thumb:

Balls concentric or blue ahead - flippy.

Blue ball touching back of orange from inside - twitchy, likely unstable

Blue ball's center on orange ball's edge - responsive

Balls touching sides - stable

Balls roughly orange ball size apart - might start dipping.

More - lawn arrow.

[wumpus]

Don't forget to check where it moves when you use up that fuel.  There is a reason that planes tend to store fuel in the wings (that and it makes all kinds of structural sense).  Stability matters when landing.

[Randox]

You may want to take a look at a guide, like this one.

It goes through CoL vs CoM placement, engine placement and angle (for asymmetric thrust), wing placement and angle, angle of incidence, and landing gear type and placement.

 

The problem with planes that naturally want to nose up is that they aren't self stabilising, at least not in the vertical plane, and will actually want to nose up and over so that they are flying backwards . The Centre of Lift always wants to be behind the Centre of Mass; like a Wind Vane (same principle really).

The further the separation between the CoM and CoL, the more stable the plane will become, since you are effectively creating a longer lever on which the forces that keep the plane flying forwards are acting. At the same time, you will need more control authority to counter the effects of gravity, which increases drag (and if you don't have enough control authority, can make it difficult to pull up out  of a dive).

You can also play around with the settings on the control surfaces themselves, including putting them on at an angle.

[rawghi]

On 12/7/2016 at 7:03 PM, Starman4308 said:

Best advice: put CoL way in front of CoM.

Then fly backwards.

I had a rough day. I saw this, 30 mins laughing

[DoctorDavinci]

On Wednesday, December 07, 2016 at 0:51 PM, Rocket In My Pocket said:

I typically place it just below and behind the COM for most standard plane designs. The markers should be close enough to be overlapping, like this:

I find this gives a good trade off between stability and maneuverability.

I put the CoL behind the CoM, however the CoL 'ball' is always in contact with the CoM ... the amount of 'contact' depends on the flight characteristics of the craft in question

I have craft where the offset between the two is close to 1/3rd the radius of the CoM 'ball' (judged from the center of said ball) but this is due to the general operational speed of said craft. Then I have other craft which, due to their flight characteristics (as well as weight, thrust, lift, drag, dimensions, major and minor lifting surface positions etc....) the CoL is offset by 1/2 the CoM radius (or more)

A good general rule of thumb to follow is to always be sure your CoL 'ball' is in contact with the CoM 'ball' and that the horizontal lines running through the 'balls' are close to intersection

Another thing is to pay attention to where your CoM is when full of fuel and when empty ... best practice is to do your best to make sure that there is as little variance as possible between empty and full as these states being different will change the flight characteristics of your craft in flight as it uses fuel

Edited December 8, 2016 by DoctorDavinci

[Leafbaron]

Stable craft: CoL ball Above and behind CoM ball

Manueverable craft: CoL ball below and right on CoM.

When I make space planes I tend to make them as stable as possible with still respectable control authority, because the last thing I want is for my plane to flip around at re-entry speeds possibly breaking and killing crew. Additionally, at higher altitude and speeds, even if the CoL is slightly behind the CoM you can and most likely will still lost control. 

[Sharpy]

One thing not to be forgotten - center of drag.

Your plane WILL be flip-happy if your center of drag is too ar ahead.

Too bad the game doesn't show it. But I had a plane that was simultaneously flip-happy and lawn arrow!  Rolling off the end of runway, heading down for the sea, then suddenly certain speed exceeded and... heading for the sea tail first.

I had managed to make the front extremely draggy (surface-attaching several MK3 parts where normally node-attach would go; I think I was improvising a tiltable cockpit / front ramp in stock). The plane got flip-happy, so I kept moving CoL backwards, and as result I got a flip-happy lawn arrow.

In normal case all you need is the proper tail, with a rudder. But if you mess up your drag profile, that won't suffice, and the only real solution is fixing your drag profile.

[Axilourous]

What do you guys mean by lawn arrow?

[foamyesque]

4 minutes ago, Axilourous said:

What do you guys mean by lawn arrow?

 

Gravity will tend to pull your prograde vector down (particularly if you don't have counteracting forces, such as angled wings). A stable plane will tend to have its nose point prograde, so your plane will tilt downwards. If it's too stable, your control surfaces won't be able to pull the nose up again, and so it will stay locked into an ever-steeper dive, eventually ploughing into the ground nose first.

Like a dart or an arrow would.

 

One thing I have not seen mentioned here is that you can actually get away with a dynamically unstable plane if you have a long wing (e.g. look at the Concorde's). The long wing impedes pitch rotation rate so that despite being unstable and therefore not needing much in the way of control surfaces to steer, the actual changes to the angle of attack occur slowly enough to be compensated for by manual piloting or SAS control.

[Snark]

Moving to Gameplay Questions.

[DrLicor]

It's kinda base rule to put the center of lift behind the CoM. The further behind, the more stable your draft will be, however, closer to the CoM makes your craft in general more maneuverable. Try to make your crafts as aerodynamic as possible to prevent the drag problems. 
When placing your CoT in front of your CoM, you can put your CoL in front of the CoM if I'm right. The effect is that your engine is pulling the craft instead of pushing. So when things get off center, your engine will stabilize it, disadvantage is that you can glide anymore, because the plan will to flip. 

So with the CoT in front of the CoM:
The center of lift can be placed in front of the CoM, but your engine needs to maintain thrust. I don't really know why you would want your CoL in front of the CoM, but maybe you have some designs where it looks better.

With the CoT behind the CoM:
You always have to place your CoL behind the CoM, further away is more stable, less maneuverable.
 

Also different wings have their influence on your plane:
Straight wings are better for low-speed planes, swept wings are better for high-speed planes.
 

About the position of the wings:
High position is more stable.
Mid position is balanced.
Low wing position is more maneuverable. 

Dihedral low wing and anhedral high position wings are balanced, that's because you keep the CoL in-line with the CoM.

If you place a dihedral wing high, your CoL will be above the CoM, which gives you a very stable plane.
If you place a anhedral winglow, your CoL will be under the CoM, which gives you a very maneuverable plane.

 

The amount of lift your plane creates, isn't a constant number.
Real life aircraft designers design their planes that the force of lift [L] = force [F] that pushes them down on cruise speed. (aircraft mass * gravity = [F] in Newtons)

https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift_formula.html

Edited December 9, 2016 by DrLicor

[Val]

14 hours ago, foamyesque said:

One thing I have not seen mentioned here is that you can actually get away with a dynamically unstable plane if you have a long wing (e.g. look at the Concorde's). The long wing impedes pitch rotation rate so that despite being unstable and therefore not needing much in the way of control surfaces to steer, the actual changes to the angle of attack occur slowly enough to be compensated for by manual piloting or SAS control.

Do you have a source for this? I've never heard of it.

[Sharpy]

let's clarify: overly 'stable' airplanes have a very annoying tendency to 'dip', pitch down all the time. It's not a show-stopper, but it's annoying as you have to use trim or other means to keep your plane flying straight.

'lawn arrow' is a situation when the airplane's downwards tendency is too strong for your aerodynamic control, and you end up in the sea past the end of the runway.

A proper rocket's gravity turn is based on this - if you dip it too much too early you won't be able to get it back up, but the dipping tendency is what drives your turn.

[foamyesque]

5 hours ago, Val said:

Do you have a source for this? I've never heard of it.

Trying it in KSP

I suspect it falls out of moment of inertia. With the same CoL and lift amount, a long wing has mass further out that it needs to slew around, and so even though it produces the same torque (the larger resistance at the back being countered by the larger force imparted by the front) the rotation rate is not as large. This is particularly true since a long wing generally means a long fuselage, and thus also a much higher moment of inertia for the plane in that regard. It's triply true if the heaviest parts of the fuselage (e.g. engines or payloads in the KSP context) are at the tips. That can also have disadvantages; it's called inertial coupling and it can overwhelm control surfaces.