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Jun 28, 2020, 06:59 AM
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Relation of vg to airfoil


I'm working on CADing up a foam plane to build and am working out the cg and have been wondering something. Is there a relationship between the cg and the curve of the airfoil? Should the cg be at the highest point of the airfoil?
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Jun 28, 2020, 08:26 AM
treefinder
springer's Avatar
I dont think it is a "laws of physics " requirement, but frequently I find the airfoils I use have their max camber near where the plane's CG locates in plan view. Usually CG is fwd like 4-40 airfoil, but CG at 30%.
Jun 28, 2020, 08:51 AM
An itch?. Scratch build.
eflightray's Avatar
Try this calculator to find out properly --

https://rcplanes.online/cg_calc.htm

It's not as difficult as it may first appear, just a few dimension required.

.
Jun 28, 2020, 11:56 AM
treefinder
springer's Avatar
To add to my and Ray's comment, CG is essentially based on arrangement of plan view areas, so not specifically associated with wing camber. You will see that in the calculator he posted.
Jun 28, 2020, 02:52 PM
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@eflightray - yup! used that calculator already. But thanks for sharing.
@springer -
Quote:
I dont think it is a "laws of physics " requirement, but frequently I find the airfoils I use have their max camber near where the plane's CG locates in plan view. Usually CG is fwd like 4-40 airfoil, but CG at 30%.
This is what I am finding too and is why I was wondering if there is an association between the two.

The train of thought the two of you are sharing is whether or not the camber affects the location of CG. which is one train of though.

but what if the CG tells us where the camber is? Just another way to think of it. if that makes sense.

I suppose having the camber roughly in the area of the CG is better but for RC probably not a critical number.
Jun 29, 2020, 01:04 AM
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Quote:
Originally Posted by riptyde
I'm working on CADing up a foam plane to build and am working out the cg and have been wondering something. Is there a relationship between the cg and the curve of the airfoil? Should the cg be at the highest point of the airfoil?
Not really. Center of gravity position is all about stability, and stability is all about the change in lift with a change in angle of attack. Nearly all airfoils have approximately the same lift curve slope, so the stability is not strongly affected by airfoil shape. At least when operating in the regime where the flow is fully attached.

Where the curve of the airfoil becomes important is with regard to trim. Airfoils produce not only lift and drag, but also a moment - a pure couple. The moment is not just due to the effective location of the lift, but exists even at zero lift. The more curved the airfoil, the stronger this moment is. It needs to be countered by either the incidence of the tail, or a constant elevator deflection.

The way you're talking about the curve of the airfoil suggests you are considering a flat-bottomed airfoil, where the only curvature is on top. The pitching moment is controlled by the mean line of the airfoil, which is half way between the bottom and the top. So with a flat bottom, the thickness and the mean line are related, with the mean line being half the thickness. There are other airfoils where this is not true. Symmetrical airfoils, for example, have a straight line as their mean line and all the curvature is due to thickness. Many foamies have bent sheets of foam, where the thickness is constant and all the curvature comes from the mean line.

The effects of thickness area all due to the boundary layer. The air that actually touches the surface sticks to it, and there's a thin zone between the surface itself and the general flow where the velocity of the air increases relative to the surface as you move outward from the surface. This zone of shearing is called the boundary layer. The flow in the boundary layer can be smooth (called laminar flow) or it can be full of eddies (called turbulent flow). Designing airfoil shapes is all about controlling the development of the boundary layer as it flows from where the air first meets the wing to the trailing edge. All of the profile drag, and phenomena like flow separation and stall, plus the transition from laminar to tubulent flow, are all aspects of the boundary layer development. The effects of thickness and camber (the mean line) are complicated, but can be calculated with programs like Xfoil or Javafoil.

It's really only coincidence that the center of gravity location tends to end up near the maximum thickness of the wing.
Jun 29, 2020, 08:05 AM
Registered User
There are many airfoils that have max thickness and/or max camber well behind where the CG would be.
An example: http://airfoiltools.com/airfoil/deta...l=naca66206-il

It's just that the majority of subsonic airfoil cases are such where the max thickness will turn out to be quite near CG on a real airplane. Sometimes fore, sometimes aft, depending on a mix of various other airplane design decisions (tail implementation, degree of maneuverability desired, etc.).

Also, consider non-conventional designs (canard aircraft like the Rutan Long EZ and others, tandem wing airplanes, etc.), where the aircraft CG is way forward of the max camber of the main wing (sometimes even forward of main wing leading edge!).
Last edited by nuteman; Jun 30, 2020 at 12:56 AM.


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