COG or COM understanding? Flight dynamics

[Julio Jerez]

Let us do this.
You are stuck in some part of the code, I beleive the problem is thetcalcualtion of angular drag on the fuselage.
The emperical equations of lift and Drag using the fusekage body as a rigid body braeks down when becaus ethey center of presure
does not represent a good aproximation o fthe airfoi.

This is because the plane body will have a center of preasure too close to the plane origin. the force the change of velocity of the center of preasure of the fuselage will not model the angular drag of the plane, or will do a poor aproximation of it.
you can brake the fuselae into two large pieces: front and back, the calculate separate lift and drag for each of them.
then the drag do to rolling will come as an emarging behaviour.
for that you need to model corss section area of teh foil along the direction of is instantenioes velocity.
basically if you remember the Drag equation is

Fd = k * A * r * v ^ 2

A is the area of the cross section of the foild alon the velocity.
what this does is that creates a variable Drag depanding on teh rotion speed

For a plane fuselage when the plane faces the direction of motion A is very small, hoewver when the plane pitch up or down
because now you have a Center of preasure for the front Fuselage, that part will have a non zero velocity the will in one direction and a non zero velocity on the oposite direction on teh back fuselage.
the two part will see a much larger Cross section area and therfore a larg rollig resistance drag from pitch.
The same will go for the wings, ou only need to model the geometry of the cross section of the fiol area along the wing velocity.
did you get my explanation?

Ok i have done some extensive testing today and here is my conclusion: If i have the drag coeficients normal then the plane flies great, but at 0 thrust the plane stop in the air and falls to ground. If i lower the drag coeficients then the plane glide, but when i turn the plane is uncontrollable (like flying in space).

when you do that, you are basically doing hand and intuition what the proccess that I expleined above does buy the geometry of the vehicle.
bascailly the coeficeent of Drag is calculated by the Area of the cross section of the air foil facing the velocity of the same foil.

[andygib]

Ok yes I understand. I am currently calculating drag for fuselage but that is only acting as linear drag and not angular drag due to fuselage local position being so close to the com. As I am currently calculating direction and velocity for each airfoil It should be easy to split the airfoil up in two as you said and give them separate drag coefficients.

I'll try this next and see what happens.

Thanks again for this on going help. Much appreciated.

I'll get a new video showing result if it seems better.

Andy

[andygib]

Another quick question, should I be usgin individicual weight or mass's for each arifoil? At the moment i am just setting the body mass (2000), should I have a mass for each airfoil and use them somwhere in the calculations?

Thanks
Andy

[Julio Jerez]

the plane is consided a single body, the mass o fteh airfoil should be tha mass of the complete body, you are just calculating a force as a point on that body

[andygib]

Ok, thought so thats fine.

Well, im sure you ready to give up, but i have split the body into to but i hasnt made any difference to the "glide" issue. And thinking about it, it wouldnt, this will make "angular" drag work better but the problem i have is linear drag, the linear drag is "too much", when i put thrust down the plane stops in the air very fast and falls tothe ground, it doesnt "keep on going" due to momentum and then it should glide, the forward force should come with gravity?

But if i lower the drag coeficient, the plane flies like its in space?

Andy

[Julio Jerez]

No I am no ready to give up, I am curios to see what you get.

what I can do is that I can add demo to the sanbox demos that simulate a flight vehicles with airfilds. a Sesna seems a good sample for that.

however I do not knwo if thsi will help yuo because you use Newton tright a third party wraper, an most likely it is some version of newton 2.xx
Ther are many change from core 200 to 300, so the script may be different.

[andygib]

Thats good to hear, as so far its great, just the glide issue is the problem.

Writing some code may till help, if i was able to just look over the code for understanding it, plus i could always not use the newton in leadwers and use newton as standalone and integrate it with leadwerks.

Thanks
Andy

[andygib]

Hi Julio, think im getting somewhere now. I was using the incorrect drag equation, since using the correct one things seem much better.

Ill tidy it up and make a new video soon.

There is still a few things im unsure if i am doing right.

do i calculate AOA for EACH airfoil?

If so, what about rudder, i assume the AOA plane is different to other airfoils.

Also, if i dropped a plane high in the air, the plane would immidiatly dive because the weight is at the front? IF i do this in my simulator, the plane falls exactly flat, therefore never AOA stays above stall angle and i can never gain control? What am i doing wrong here?

Thanks
Andy

[Julio Jerez]

do i calculate AOA for EACH airfoil?

yes

If so, what about rudder, i assume the AOA plane is different to other airfoils.

yes ruder is simply a vertical foil with symetry, the angle of attack is what makes the plane turn
when the ruder is center the AOA is zero so side lifg, but when AOA is poistive or negatibe they you get positive or negative side lift force.

Also, if i dropped a plane high in the air, the plane would immidiatly dive because the weight is at the front? IF i do this in my simulator, the plane falls exactly flat, therefore never AOA stays above stall angle and i can never gain control? What am i doing wrong here?

when you do that the plane will be under the inflency of a very strong drag and litle lift, since grad force apply at the center of preasure o fteh foil, all you need to do is to plane the center of presure of teh main wings behind the center of mass of the plane.
this gnerates the aligning torque because the drag force ar large even the distance for the Cente of preasure to the COM is small, it is all geometry.

[andygib]

Hi, ok. Well im still have "some issues" that i dont understand. Ill try my bestto explain.

Ok, so if we are calculating AOA for each arifoil, how does this work with elevator as we have a "max aoa" value, and AOA is assumed to be between 0 -18deg (approc) but so when elevator I control by alter AOA, but if AOA for elevator is at 0 (stable flight) and i want to push nose down, if i push up to move elevator up the AOA is already 0, if i push up AOA will go negative?

What is the best way of "controlling" plane, i am alter AOA, so when i press a key it alters the airfoil AOA, but as described above this doesnt work so well?

Thanks
Andy

[Julio Jerez]

I will make a sketch to clarify all the confusion.

you will see that AOA doe sno really exist, AOA is the prokection of teh cross section of teh surface are of a airforl over the airfoil velocity velocity.
in any book aboput aerodynamics of Flight you will be reading teh turn Control surface, this is what airplai do to control teh vehicle,
they change teh are of the airfild with by gemerticlly chnagin with ruders, or chngin teh porjection by chnge the orintion of teh plvehicle relative to teh diretion of flight

wait for the image, it will clarify most confusions.

[andygib]

Hi Julio, any news on the image/diagram you have been doing?

Thanks
Andy

[Julio Jerez]

no I did not have the time, have to do Christmas chores.

I see if I can do today.

[andygib]

Any luck Julio? I know your busy, just really think your diagram will help clear up some issues.

Thanks
Andy

[andygib]

Hi. I'm sure you have been very busy with Christmas and all and so although I have a few questions this is a simple reminder about the diagram. I feel I've come far from when we started to give up now.

Ok so a few questions, one that I have assumed ind now wondering if is right and that is the mass value? What should I using for mass? Upto now I have set the mass to the weight of the aircraft which I find off the Internet for a Cessna. I have sometimes used lbs and other times kg, this is because I'm unsure on which units I should be using. Or even if mass should be the weight of the airplane?

I'm not using weight anywhere else in any equations, simply setting mass to weight, is this correct?

Thanks
Andy