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Ho do you get the CG on a delta wing?

#1
I am wondering the best way to get the CG on a delta wing or any swept back flying wing? I know how to get the CG on a straight wing, trainer etc. On a straight wing I usually go one third of the cord from the leading edge and that seems to work fine.

Would I do the same with a delta wing, measuring one third from the leading edge of the ROOT of the wing?

Thanks in advance.
 

earthsciteach

Moderator
Moderator
#3
I am thinking about making Ho 229 based on Marek Pacynski's 1:50 paper model, and I don't know where should it's CG be. I want to make it as scale as possible and it's slightly different from most flying rc wings, and I'd use some advice ;) I don't know if it's important but I think I'll be using elevons with it. Thanks for info in advance.
You can use this cg calculator to determine the cg. You will just have to approximate the wing as a series of straight lines. It will work just fine. http://rcwingcog.a0001.net/?i=1
 

Mid7night

Jetman
Mentor
#4
I like the simplified one, because it's a bit quicker and cleaner when you're just looking at the wing: http://fwcg.3dzone.dk/

Basically, if it's just a delta wing or flying wing, you only need to calculate the MAC (Mean Aerodynamic Chord), and then choose what percent-chord you want to place your CG on. 15-25% MAC is usually a good target for new designs. I usually favor more nose-heavy (~20% MAC), and then play with the CG after that depending on how the maiden goes.

As for measuring where the CG goes, the calculator will tell you the distance as referenced from the leading-edge tip of the ROOT of the wing.
 
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Konrad

Posting Elsewhere
#7
The cg on a delta will be further back than 30%, most likely. Use this cg calculator to determine its location: http://fwcg.3dzone.dk/
I'm a bit lost. The greater the sweep the further aft the CofG will appear to be as measured at the root of the wing. But as the last 20% or so of the airfoil (wing area) in a flying wing (delta or plank) is used for stability. The center of gravity is actually placed a lot further forward when looking at the MAC (Mean Aerodynamic Chord), something like 25% to 20% of MAC.
 

earthsciteach

Moderator
Moderator
#8
It makes sense to me that the cg would be further back on a delta. The centroid of a triangle, which can be considered the center of lift, is close to around 60% of root chord. The cg would be a bit ahead of the cl, so that puts it way past 30%.
 

Konrad

Posting Elsewhere
#9
It makes sense to me that the cg would be further back on a delta. The centroid of a triangle, which can be considered the center of lift, is close to around 60% of root chord. The cg would be a bit ahead of the cl, so that puts it way past 30%.
On a delta wing and as measured at the root (centerline).

As I read the OP's question he is also asking about swept back flying wings. When I think of CofG gravity I'm looking at the MAC, not wing root. But as the OP is wanting to reference the wing root, I now see your point for the delta wing.

To the OP I would think it best to try to look at the CofG placement as a function of the MAC of the wing. Since there are so many variable at play, making any generalizations relative to the wing root rather meaningless
 
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Mid7night

Jetman
Mentor
#10
CG location is always calculated as a percentage of wing MAC, however it is most commonly LOCATED in reference to the wing root leading edge, because the root-LE is a physical point of reference, whereas the wing MAC is an imaginary line somewhere in space.. Both calculator links given above will tell you both numbers; %MAC and CG distance from root-LE.
 

Konrad

Posting Elsewhere
#11
And the "physical reference point" is usually measured in a unit of linear length (mm, or inches), not as a percentage of the chord (Root or MAC).

So to my thinking the "flying wing" has the center of gravity place a lot farther forward on the wing than that of the conventional layout. 20% to 25% MAC for the flying wing vs 30% to 33% MAC for conventional aircraft in general terms. (I like aft CofG for control as apposed to stability)

Wing sweep (fore or aft) has the same effect on CofG wether it is tailless aircraft or one with a separate stabilizer. The same is true for the delta wing, weather it lacks a stabilizer like the Mirage or has one like the Mig 21.

My concern is with how does one figure out what percentage of the MAC one wants? This deals with static margin of stability. I did not see in the calculators how one determines this number. What I saw was how to find this number on ones wing platform. I did see some general numbers given here by myself, others and in the calculators. The point I wanted to bring to light was that with the flying wing the CofG much further forward (MAC) that what one would see with conventional aircraft layouts. I thought that saying the the CofG was further aft might be misleading (true for the delta as measure at the root but not true if using MAC even for a delta).

Since the OP was asking about both the delta and swept plank layout I wanted to be sure I (and hopefully the OP) understood the answer.
 

Flite Risk

Well-known member
#16
:::slightly off topic:::
:::slightly related :::


first plane i ever built was a chuck glider hack (swept wing) long before J.Bix and J.Scott ever went to walmart and made that video.

So I was trying to balance this swept wing powered Chuck glider and find the literal balance point, that is, when it had no airflow over the wings and was on my bench.

I was trying to find out, where did it balance because I thought I was looking for the literal "center of gravity ".

; - p

What I should have been trying to find is the point on the wings at which when enough air is moving over them is enough pressure built up to cause lift.

Some people call it the center of pressure but even that is misleading because the word "center" indicates middle of a body (to me anyway (as in get out a measuring device mark the middle and BadaBing you are good to go)).

A more accurate word would, to me be, point. As in point of pressure or even
point of lift.<---- Thats very literal I like that. Used in a sentence it might sound like this; Balance the aircraft on its point of lift. If I did that I might have had a successful first RC plane.

I realize its me vs. a whole industry and study of science but semantics matter until you know.

Because;
you know what you know,
You know what you don't know,
But,
You don't know what you don't know.

(I didnt really have a mentor to help me through this build just J.Bix and a few videos about receivers and servos etc.)

My first ever attempted rcplane,Chuck glider with a motor and servos failed in many pieces after many times being fixed.

In hind sight I never did successfully balance the aircraft on its point of lift . It was stupid tail heavy.

maybe I will try that project again one day.
 
#17
:::slightly off topic:::
:::slightly related :::


first plane i ever built was a chuck glider hack (swept wing) long before J.Bix and J.Scott ever went to walmart and made that video.

So I was trying to balance this swept wing powered Chuck glider and find the literal balance point, that is, when it had no airflow over the wings and was on my bench.

I was trying to find out, where did it balance because I thought I was looking for the literal "center of gravity ".

; - p

What I should have been trying to find is the point on the wings at which when enough air is moving over them is enough pressure built up to cause lift.

Some people call it the center of pressure but even that is misleading because the word "center" indicates middle of a body (to me anyway (as in get out a measuring device mark the middle and BadaBing you are good to go)).

A more accurate word would, to me be, point. As in point of pressure or even
point of lift.<---- Thats very literal I like that. Used in a sentence it might sound like this; Balance the aircraft on its point of lift. If I did that I might have had a successful first RC plane.

I realize its me vs. a whole industry and study of science but semantics matter until you know.

Because;
you know what you know,
You know what you don't know,
But,
You don't know what you don't know.

(I didnt really have a mentor to help me through this build just J.Bix and a few videos about receivers and servos etc.)

My first ever attempted rcplane,Chuck glider with a motor and servos failed in many pieces after many times being fixed.

In hind sight I never did successfully balance the aircraft on its point of lift . It was stupid tail heavy.

maybe I will try that project again one day.
The center of gravity has to do with the wing area of both the main wings and elevators combined and the space between them both. If you don't have elevators than your just calculating the wing area of the Delta wing. The C of G is the balance point according to the wing area you calculate not the center of a model by any means. Here is a chart I use.
https://www.ecalc.ch/cgcalc.php
 
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Flite Risk

Well-known member
#18
I did try the Weight, Arm, Moment calculation .
That didnt pan out. I didnt really know what I was doing then, arguably I still don't now. Ha!
 
#19
I did try the Weight, Arm, Moment calculation .
That didnt pan out. I didnt really know what I was doing then, arguably I still don't now. Ha!
Ok, try this one. It's much easier to follow for a Delta wing.
https://fwcg.3dzone.dk/ when you click in the space to put a measure, the diagram shows you where to measure. It's easy...
Use the other one when you have more wings to work with. Or this one is maybe more useful than the first one I gave you...
https://rcplanes.online/cg_calc.htm
 
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