@Titaninfernal1 oh ok. thanks

There is one more thing, I have found that if there is any fuselage where there is nothing connected to the front of you, your plane will be impressively unstable, if you were to create a new manual connection... Extremely simple and fast using the translator, sometimes I forget to use it.

@Selcukk não... Mas uso tradutor as vezes

@Titaninfernal1 can you write English?

Existe mais uma coisa, eu descobri que se existir alguma fuselagem onde não existe nada conectado com a sua frente, seu avião ficará impressionantemente instável, se fosse criar uma nova coneção manual com a sua frente conectando-o com uma parte aleatória, seu avião ficaram muito mais estável, fiz teste com aviões da WW2 e funcionou, pode ser isso, espero ajudar em algum projeto

@Formula350 it helped a lot thank you

To clarify further, the Center of Mass (CoM - Red sphere/lines), Center of Thrust (CoT - Yellow), and Center of Lift (CoL - Blue) are dependant on your PROPULSION as well -- in other words, the type of plane you're building.
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WWII Jets had their engines on the wings, so CoT was more mid-point.
WWII Props had their engines either on the wings for multi-engine, or one at the absolute front, so CoT was either similar to those Jets, or right at the front.
"Modern Military Jets" (roughly Korean-war and newer) generally have their Jet engine at the absolute rear, so CoT is opposite to that of a single-engine Prop.
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I'm not an expert yet but I've worked out this much so far, on how to place your CoL and CoM in respect to CoT:
REAR Engine: CoT - - - CoL - - CoM
WING Engine: CoL - CoT - - CoM
FRONT Engine: CoL - CoM - - - - CoT
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Lastly, ideally only use "Symmetrical" airfoils (Wing part setting) on the Tail (Rudder and Elevators), with the Primary wings set to "Semi-Symmetric" or "Flat Bottom". If your main wings are set to "Symmetrical", it has a chance of being really unstable. (Example: the new "Twin Prop" included in the game; unless they included my fixed version I sent them, which if it has interior lighting now then they have)
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It's a shame these forums only display posts for a few days, as I'm sure there's a wealth of this sort of knowledge that could be found by just clicking through old pages. (I know we can search but that doesn't allow for 'stumbling across gems' T_T)

@ZeroWithSlashedO thx

@CrimsonOnigiri thanks :))

@Bellcat oh np thanks :)

it is advised to have the com in front of the col just a bit.
Although, having the com just in front of the col will make your plane harder to control, but it gives more maneuverability. If too close, your aircraft will perform PSM easily. if it's pretty close, it would either make the plane able to fly straight, or pitch up slowly over time. This I think is based of physics quality. This comment was based on my experience in low physics.

Moving the COM forward increases stability, but also decreases turn rate

@CrimsonOnigiri, oh yeah sorry, my bad. @Selcukk, my apologies for your inconvenience but please reread post I have edited due to an error CrimsonOnigiri has found. Thank you.

Also, for stability. You need to monitor the drag points. Usually those are the cause for instability. It's best you keep all parts in 0 dragScale in xml. And leave a few exposed parts in the middle to be 1 in dragScale. It may also have to do with what kind of stability you want. Do you want it to be able to fly straight? Or stable as in you want it to smoothly turn or etc?

@Bellcat wait what? The CoM is behind the CoL? Isn't it the other way around??

@Bellcat hahah thanks

@LarryTad Thanks I will try

Creating a stable aircraft in simple planes is a very complex process. You need to make sure that the center of lift is at the back of the center of mass. You also need to make sure that you have the wings correctly used accurately. You may need a lot of trial and error to make a plane that flies decently. You are lucky you are a gamer that plays simpleplanes because if you are a designer at a big aircraft company, there is not much any room for trial and error as their prototypes are expensive.

Try making parts invisible or weightless @Selcukk it works for me all the time.