Effect of Span-Loading on Aircraft Performance

[Jane-DoH]

As I understand it aspect ratio is the ratio of the wingspan squared divided by the area of the wing, and span-loading is the ratio of the weight divided by the span.

While I understand the effect of aspect ratio on aircraft performance, I'm not 100% sure I completely understand the effect of span-loading.


R.C.

[barit1]

For a given gross weight, increased aspect ratio implies lower span loading. They aren't really independent of each other.

[Intruder]

It may affect wake vortex generation... Shorter span and higher weight (higher span loading) generally imply stronger vortices.

[FCeng84]

As correctly stated above, aspect ratio is wing area divided by span. Stated another way, aspect ratio is wing span divided by the mean aerodynamic cord length. Gliders have very high aspect ratios as their wing spans are long and their cords are short. Fighter jets tend to have relatively low aspect ratios (short wing span and long cords).

Span loading is not a single, scalar parameter. Span loading refers to the distribution of lift along the span of the wing. In general, an eliptical distribution of lift along the span is close to the optimal with respect to maximizing L/D. Wing structural loads vary with span loading profile as well. The more the load is carried outboard, the higher the bending loads along the wing and particularly at the side of body where the wing joins the fuselage.

PPRUNE members with more of an aerodynamic background may choose to expand further on the general topic of optimal spanwise loading.

[kenparry]

In general, an elliptical distribution of lift along the span is close to the optimal with respect to maximizing L/D.

Correct. This minimises induced drag. It can be obtained by using an elliptical wing planform (e. g Spitfire) - but that is expensive to build. For a specific design point, such as a given angle of attack, it can also be achieved by a simpler wing shape, such as straight taper, combined with a wing section that changes along the span. Washout may also be incorporated to give friendly stall characteristics.

[Pugilistic Animus]

Just to add...prediction of span-wise load distribution is also important for determining flight characteristics wrt to stability, for example overall pitching moment 'Cm'

Generally though, as mentioned, anything that reduces induced drag...ie increased aspect ratio or favorable overall load distribution, reduces the thrust required to overcome the induced drag, hence its relation to performance, although increased AR has to be balanced with the increase in weight and profile drag

It's helpful to remember that for an equilibrium flight condition, every time drag is increased there has to be a corresponding increase in thrust...whether discussing climb, cruise or turning performance...:)

[Pugilistic Animus]

Just in case anyone is interested the coefficient of drag varies with aspect ratio as follows: Cd'= Cd + Cl^2/pi( 1/A' - 1/A)

For computation of span-wise load distribution for non-elliptical loading Munk's integrals can be used with the help of computers...:)

[Jane-DoH]

FCeng84

Span loading is not a single, scalar parameter. Span loading refers to the distribution of lift along the span of the wing. In general, an eliptical distribution of lift along the span is close to the optimal with respect to maximizing L/D.

So, the parameter is not all that important when it comes to dividing weight by span as in terms of the way lift is distributed across the wing?

[FCeng84]

Jane-DoH

I'm not sure what you are meaning. Dividing weight by span yields an average lift over the span, but does not address the question of how the lift (or load) is distributed over the span. To fully understand the spanwise loading, the loading at each point along the span must be known.