Centre of lift for this style of wing?

[Hogster]

Hi all,

I'm designing my own version of the VIP pylon racer (http://insel.heim.at/malediven/350052/Modelle/VIP01.JPG) out of balsa instead of glass-fibre as per the ARTF model itself.

This is the wing so far and I'm curious as to where the centre of lift for such a wing would lie?

http://hogster.rchomepage.com/pylon_racer/wing_plan_view.jpg

The wingspan is 24" and I'm using the MH-42 aerofoil. The wing is fully built up, using two 1/8" spruce spars, 1/32" shear webs, and 1/16" sheeting all over. The tips are 1/4" sandwiched to 1/8", sanded to shape.

Also, am I right in presuming that the centre of lift is the same as the centre of gravity for a plane?

Any comments/thoughts/input is graciously welcomed

Thanks,


David :)

[Tom Harper]

Hogster,

I've seen some heated arguments over this topic.

The point at which the lift is applied used to be called the Cp. But that's been replaced by the Cm which is not a point. The lift is thought of as being distributed evenly over the wing with a twisting moment pivoting around the CG. In any case it does not impact the position of the CG.

For a wing of modest camber and no reflex the lift is essentially applied at the CG.

[vintage1]

Hogster,

I've seen some heated arguments over this topic.

The point at which the lift is applied used to be called the Cp. But that's been replaced by the Cm which is not a point. The lift is thought of as being distributed evenly over the wing with a twisting moment pivoting around the CG. In any case it does not impact the position of the CG.

For a wing of modest camber and no reflex the lift is essentially applied at the CG.

Its more a question of wanting CG and CP to coincide so that there is no net force on the tailpane..so that drag is at its lowest.

CP moves with AofA and airspeed anyway, so you can obnly do this for one speed.

In practice, you shold be adjusting CG for max speed once the model is up and flying.

[space_case]

What are you guys talking about? The CG could wind up almost anywhere before balancing. How can you possibly pre-suppose that the CG position will coincide with that of the CP??

Are you trying to say that the CG *should* be placed at the CP?

Firstly you need to calculate the MAC.

[vintage1]

What are you guys talking about? The CG could wind up almost anywhere before balancing. How can you possibly pre-suppose that the CG position will coincide with that of the CP??

Are you trying to say that the CG *should* be placed at the CP?



For best speed, yes.
This may not be stable though..

Firstly you need to calculate the MAC.

and some other stuff :D

[Ollie]

Center of force is not just lift force. Center of force is not just drag force. The total force on the wing and it's position makes the moment (torque nose down). The combination of lift, drag and moment accounts for wing total force and it's position.

The modern system puts the lift and drag on the aerodynamic center which doesn't shift and the moment (torque) is defined as around the aerodynamic center. The wing aerodynamic center is very, very near 25% the mean aerodynamic chord of the wing. Assume it aerodynamic center as 25% the mean aerdynamic chord. The problem is to find the mean aerodynamic chord from the planform of the wing.

The question of the aircraft design position of CG depends on stability. For positive stability, the CG is ahead the neutral point (aerodynamic center) of the whole aircraft. The aerodynamic center of the whole aircraft depends of the aerodynamic center of the wing, the aerodynamic of the tail and, the length of arm of the tail behind the wing.

Luckly, the birds (and many R/C fliers) know zilch aerodynamic theory and they fly nicely. Perhaps R/C plane designers could use aerodynamic theory to their advatage.

The problem for me is not enough aerodynamic knowledge to perfect my designs. When the aerodynamic theory is complex, some poople put forth myth for truth. I try to not put forth myth.

[Tom Harper]

The balance point of the model is chosen to establish a stable configuration. It is static rigging.

Cp has entered the realm of mythology (thanks Ollie). It was a misinterpretation of the chordwise twisting force (Cm).

Like Cp, the lift force could be thought of as being applied at a point. That would become the Mean Aerodynamic Center (MAC). But I think you have a static MAC and a dynamic mac. Dynamic mac would take into account the local efficiency, AoA and reynolds number of each segment of the wing panel. This would have to be modified by the pressure distribution of the local section.

Such a calculation might be useful for stress analysis, but I don't see it as valuable for model design. The CG is static rigging. The wing has three engineering vectors (Cl, Cm, Cd) which act at the neutral point.

A model airplane is a perfect analogue computer. When you fly it and measure it's behavior you get the output of it's calculations. The only way to know the performance of a wing like hogster's is to build more than one model, make measurements and compare the results. Or, better, to make interchangeable wings to compare on the same airframe. Then you get to rationalize the data, make some changes and try it again.

Most reliable models are an accumulation of compromises. As vintage points out you can only optimize for one situation. Control line speed is an interesting study in optimization. There are control line speed models that can go 180MPH but can't take off. Fun to watch!

[Tom Harper]

hogster,

You do nice work! Do you fly CW over there?

[Ollie]

There's more.
The undisturbed air velocity vector (magnitude and direction) is 90 degrees to the Lift vector (magnitude and direction). The drag vector (magnitude and direction) opposite to the air velocity vector. The torque (moment) axis is in the spanwise direction and thru the aerodynamic center of the wing.

If you use the original wing area, moment coffecient, zero lift angle of attack and platform you are not changing the designed CG position or aircraft stability. Are you changing the airfoil? Why?

[Hogster]

Interesting replies guys ..... presumably I can enter measurements of this wing into a normal calculator like this to find the CofG point (or the point under which the plane should balance)?

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Many thanks,


David

PS. Thanks Tom! What does CW mean/stand for? :confused:

[Ollie]

A CG design position is within a range flyable CG of positions from too much stability to little stability. After test flying, tune up the CG position to your flying style not to your calculations.

[Hogster]

Hi Ollie,

Yes that's what I usually do. Is it worth starting with it say 30% back from the leading edge? I usually do test glides anyway (where I run along with the plane and continually throw and catch it to test the glide slope ...).


David

[Sparky Paul]

If you straighten out the swoopy tips and put in a straight leading to trailing edge line, the wing is just a tapered wing, with the usual method of finding the m.a.c.
I eyeball in a tip that splits the difference between where the leading edge begins its turn to the trailing edge, and the outer point of the trailing edge.
Close enough for gummint work... :)

[HELModels]

Along those lines is an alternative. Since you have your wing in hand, get some graph paper and trace the planform onto the paper. Just like Newton's method, you can approximate the area and mean chord. Tighter graph paper means better approximation. Dividing up the wing into triangles and rectangles, circles and then adding em all up.

[Tom Harper]

Hogster,

CW=Clockwise. Is that how you fly around pylons. If not I'd put the aileron on the left wing.

[Hogster]

Ohh I see .... I'm not into pylon racing at all ... I just wanted to build a tight, sleek, high speed model and as I was on the verge of buying a VIP two years ago I thought, well why not! :)

Cheers,


David

[Tom Harper]

Way to go - and it looks great so far.

[BMatthews]

If you want to confirm the Neutral Point and CG suggestion from the calculator then you could build a little 12 inch span all sheet model from 1/16 sheet for the wings and tails and a bit of 1/8 x 1 for the fuselage. Test glide this test model to find out where the CG ends up once the model is able to fly a flat glide path with very little tendency to lift the nose up in recovering from a stall but definetly does not show signs of wanting to curve down and lawn dart itself. As you trim the CG back you need to compensate with some warping in of elevator trim to restore a flat glide. Throw it hard and soft to see what the pitch response is. When it's VERY close to neutral in that it doesn't nose up from extra speed and does not recover from a stall then you have the CG on or very close to the Neutral Point. For a racer like this that's pretty much where you want to end up. For the test flights run it about 3 or 4% forward of that point and move back from there.