Chordwise location of lift for wing structural design

• Seraphin2288
In summary, the speaker is designing a new aeromodel and needs information on the location of the lift to calculate the bending moment and torque on the wing box. They have found conflicting information on the location, with some sources saying it should be at the quarter chord while others suggest 45% of the local chord. They are seeking clarification on which value is used in aeronautical engineering and if they should include the pitching moment coefficient of the airfoil in their calculations. The conversation also touches on the use of multiple spars and the tendency of wings to twist under aerodynamic load. The speaker has found a potential solution by plotting a range of locations against angle of attack and considering the center of torsion for maximum angle of attack. They
Seraphin2288
Hi everybody,

I'm designing the wing of a new aeromodel and I need a few informations for the structural design of the wing box that I couldn't find in the few litterature I have by now. So, here comes the question...

For the sizing of the box beam that will be the main resistant part of the wing, I need to know the location of the lift to calculate the bending moment and torque applied on the beam. The spanwise lift equation is (quite) easy to obtain for the calculation of bending moment.
My problem in then to find the chordwise location of the lift to calculate the torque applied on the wing box.

The most obvious would be to locate it at the quarter chord (aerodynamic center of the airfoil section), yet in some books they locate it at about 45% of the local chord (which would be the center of pressure, if I'm not mistaking).

Do you know which value is used in aeronautical engineering ? I thought that maybe it is equivalent to account the lift at 0.25c and add the pitching moment using the pitching moment coefficient of the airfoil or to apply a single vertical force (equal to the lift, of course) at 0.45c, but I'm really not sure...

Who could help me ?? :D

Thanks everybody !

The centre of lift will move depending on the angle of attack so I don't think it's an easy question to answer. Most models I have built have had the main spar at about the thickest point, say around 1/3 chord. They frequently have a secondary spar at perhaps 2/3 chord.

Wings frequently have a tendency to twist under aerodynamic load. It's common to make the area between leading edge an main spar into what is called a D section box. I have a glider that uses Kevlar sheet with the weave at 45 degrees in this area. This forms a D section box that is resistant to twisting.

berkeman
Hi CWatters,

Yes, generally putting the spar at 1/3 chord is a good compromise, yet my design uses a wing box, with 3 spars (one at 0.1c, one at 0.38.c and the third at 0.78c). So, in any case, the lift application point will by "inside" the wing box. Yet, I have to know its exact location to size the box in torsion.
The link you sent me is very interesting, maybe I could plot a range of locations VS angle of attack and then find the critical case for torsion and/or bending combined.
And, if I well understand the document, it could be interesting to locate the center of torsion of the box at the center of pressure for maximum angle of attack, so the distance (and then torsion) would be zero when the lift is maximum, and then the distorsion of the wing would not be significative. What do you think ?
And in this case, shall I add the pitching moment (Cm0) of the airfoil integrated over the wing span ? Or will it be included in the torsion moment Lift*(distance with the torsion center) ?

Thinking it twice, I guess the total torsion moment would be Torsion=Lift*(distance to torsion center) + (torsion due to Cm0). It looks more logical to me, yet I'd like to have your opinion about it :p

The pitching moment is due to the centre of lift being behind the aerodynamic centre so I think you are double counting it. I think you either need "Lift*(distance to torsion center)" _or_ the pitching force (modified to take into account that the aerodynamic center and your box aren't in the same place).

FactChecker
Only a part of the pitching moment is due to that distance, for any unsymetrical airfoil there is an associated pure torque that, when considered at the AC, remains constant with incidence. So even if the torsion center of the box was at the application point of lift (whether it is the AC or the CP), there would be a torsion moment.

Just found that from "aerodynamics for engineering students" (actually it's just a plot of the equation given in your link). As I interprate it, the most logical would be to apply the lift at quarter chord, as it gets almost at the AC for high CL (worst structural case) and add a constant pitching moment due to Cm0 :)

What is the chordwise location of lift for wing structural design?

The chordwise location of lift refers to the point along the chord of a wing where the majority of the lift force is generated. It is an important factor in wing design as it affects the overall stability and performance of an aircraft.

How is the chordwise location of lift determined?

The chordwise location of lift is determined by the wing's airfoil shape and angle of attack. The airfoil shape determines the distribution of lift along the chord, while the angle of attack affects the magnitude of lift at different points along the chord.

Why is the chordwise location of lift important in wing structural design?

The chordwise location of lift is important because it affects the bending and torsional forces experienced by the wing. A poorly designed wing with an incorrect chordwise location of lift may be subject to excessive stress and could fail under certain flight conditions.

What are some methods for determining the chordwise location of lift?

There are several methods for determining the chordwise location of lift, including wind tunnel testing, computational fluid dynamics (CFD) simulations, and theoretical calculations using airfoil data. Each method has its own advantages and limitations, and a combination of methods is often used in wing design.

How does the chordwise location of lift affect the stability and performance of an aircraft?

The chordwise location of lift has a significant impact on the stability and performance of an aircraft. An incorrect location can result in poor handling characteristics, decreased efficiency, and increased drag. It is crucial for the wing to be designed with the correct chordwise location of lift to ensure safe and efficient flight.

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