Question about Aerodynamics. Lift to Center of mass issue.

[Arugela]

I have planes designed with Rapier engines to get into orbit and hopefully beyone. My main issue is a weird problem I haven't discerened about lifet to center of mass. I always end up lining up the life to the center of mass so that they are basicallly perfectly aligned. but the mass is always a little above or below. This causes problems with getting proper flight controls. I always do a loop de loop test and it usually shows in that right after take off. The reason this matters is that I am doing high orbit long jet runs and it and reentry need very good aerodynamics to help control in weird situation. I've noticed, I think, that the best way to deal with anomolies is to getperfect alignment and then try to control the plane. Could be wrong but this is my current line of thought. My question is how do I control teh center of mass or lift up and down. I use those forward facing wings but I can't place enough to get it down or up enough for my needs usually. What are the most efficient ways to adjust this up or down to alling completely. I'm using many wings on a plane(usually 12-25) each with 2 elevators in groups of 3(1 bottom 1 middle 1 top) all aligned. I need to find out how to fine tune this.

This is an example pic of a prototype for the planes I'm designing.

[GoSlash27]

Arugela,

The center of lift doesn't have to be perfectly aligned vertically with center of mass to get high- speed stability, but it helps. Getting that is just a matter of adjusting the wing attachment height and how much the wings slope up from the fuselage.

From what I'm reading here, it seems to me that your problem isn't one of static balance, but rather drag distribution. Some parts in stock KSP have higher drag than normal. Especially intakes. Other parts can have lower drag than normal, such as wings. Control surfaces can have anywhere from negative drag (infinigliding) to high drag, depending on how they're aligned at the moment.

A lot of people don't realize this, and it leads to a lot of frustration with planes that seem to handle fine... and then become completely unflyable at high speeds. So here's what you do:

*Intakes go in the back of the plane. Use radial intakes because they're convenient for this.

*If you use control surfaces in the back of the plane, also use them in an equal amount in the front.

* Don't try to use control surfaces to crutch an unbalanced design. For example, don't build a nose-heavy plane and then slap a canard on it. Your plane will *look* balanced, but it'll try to fly backwards at high speed.

*Build the plane without control surfaces and get it balanced, then add control surfaces in such a way that it remains balanced.

If you do this, you will have a plane that doesn't flip out at high speed and during reentry.

Best,

-Slashy

Edited December 27, 2014 by GoSlash27

[numerobis]

Another thing with control surfaces is that their lift increases with angle of attack much faster than wings' lift does. That makes canards extra dangerous: as you get further away from a flat angle of attack, the lift up front increases more than the lift in back, which pushes you to a steeper angle of attack, etc and then you die.

In the SPH, turn on the center of mass and center of lift markers. Then click on the root node and use shift-wasd to rotate the spacecraft. You'll see how the lift marker will be changing as you fly.

[Fearless Son]

Another thing with control surfaces is that their lift increases with angle of attack much faster than wings' lift does. That makes canards extra dangerous: as you get further away from a flat angle of attack, the lift up front increases more than the lift in back, which pushes you to a steeper angle of attack, etc and then you die.

In the SPH, turn on the center of mass and center of lift markers. Then click on the root node and use shift-wasd to rotate the spacecraft. You'll see how the lift marker will be changing as you fly.

What I like to do (and someone please correct me if I am wrong) is to look at where my center of lift is relative to my center of mass, then make very slight adjustments to the canards to make sure my center of lift is properly aligned just behind my center of mass. I do not do this wholely, but to a small adjustment in the canard leads to a large change in the balance, so it is great for putting the finishing touches on an otherwise nearly stable design.

[Arugela]

I've actually found I have a slight twist to the left(very slight. Not like a wing is missing on one side.). Maybe placeing the structural air vents without geometry considerations has them slightly off. I found out I had bottoms wings with a differnt lift factor than the middle and top ones. I corrected this with elevators. now I have to find the source of the twisting.

I don't neccesarily see it when flying but I can see it in the model in the hanger. the one side of the lift is not identical to the other side on the mass ball when lined up.

Edited December 27, 2014 by Arugela

[numerobis]

I almost always use canards, but I do so knowing the risks. The com / col relationship depends on your angle of attack, it's not fixed.

[Arugela]

Out of curiosity, does putting the wings backworks do anything in stock? If not will it later?

[hypermetric]

I've actually found I have a slight twist to the left(very slight. Not like a wing is missing on one side.). Maybe placeing the structural air vents without geometry considerations has them slightly off. I found out I had bottoms wings with a differnt lift factor than the middle and top ones. I corrected this with elevators. now I have to find the source of the twisting.

I don't neccesarily see it when flying but I can see it in the model in the hanger. the one side of the lift is not identical to the other side on the mass ball when lined up.

If I even think I have a balance problem I redo all my symmetry. Detach anything you have two of and if there is still two when its detached throw one of them out. Now check out your fuselage on the off chance there is still a problem there. Reattach everything making sure symmetry is on. I find subassemblies helpful here, as well as saving and version control. In previous versions of the game I found it less glitchy to work without symmetry until have attached everything I want to attach. Once I am ready I grab the wing or whatever part connects to the central fuselage and duplicate the whole thing with symmetry.

[Fearless Son]

I've actually found I have a slight twist to the left(very slight. Not like a wing is missing on one side.). Maybe placeing the structural air vents without geometry considerations has them slightly off. I found out I had bottoms wings with a differnt lift factor than the middle and top ones. I corrected this with elevators. now I have to find the source of the twisting.

Is this twist only on the runway, or is it during flight? If on the runway, that usually indicates that the gear are too close together or the weight is too great on one side. If in the air, I would think it is also the weight distribution. One of the big "invisible" causes of this is the fuel distribution, if fuel is getting used up from a tank on one side of the plane but not on the other, hence one side of the plane gets slightly lighter than the other side and this becomes more pronounced the longer the flight goes on.

In short, I recommend checking the fuel line flow. Maybe something is connected backwards?

[GoSlash27]

Out of curiosity, does putting the wings backworks do anything in stock? If not will it later?

No, wings work the same forward or backward. I can't say what will happen after the next update.

Cheers,

-Slashy