Chordwise location of lift for wing structural design

[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 !

 

[CWatters]

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.

 

[CWatters]

Google found this...
http://www.propdesigner.co.uk/html/spar_location.html

 

[Seraphin2288]

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) ?

 

[Seraphin2288]

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

 

[CWatters]

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).

 

[Seraphin2288]

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 C_L (worst structural case) and add a constant pitching moment due to Cm0 :)