what is old is new again ...

[rb1957]

From Flight (a couple days ago) ...

German researchers claim aerodynamic and fuel-burn benefits from predictive load-alleviation​

This was tried some time ago, in like the 90s. Yes, it may save fuel costs (back then I think it was intended to smooth out gust loads for passenger comfort) ...
but this also impacts (negatively) the fatigue spectrum of the wings (particularly) by reducing the overload cycles and this increases the crack growth rate, and reduces the maintenance intervals.

 

[3DDave]

Always interesting to see estimates with 2 significant digits. Good on them for not rounding up to 7.5%.

 

[rb1957]

well, they are german !? (precision is a virtue, but too much of it isn't)

 

[GregLocock]

Found this, seems related.

https://elib.dlr.de/209223/1/Krengel_2024_ECCOMAS_FuelBurnEfficiency_of_LoadAlleviation_inOAD.pdf

Summary. This study shows the integration of the overall aircraft design with a concurrentwing planform optimization for long-range aircraft. The focus is on wing design and active loadcontrol with trailing edge flaps. A physics-based framework for structural wing design withsimplified aeroelastic load cases is the computational backbone of the research. The goal is toexplore the potential of load alleviation while optimizing a conventional cantilever aircraft configuration with a surrogate model based approach. It is shown that load alleviation significantlyreduces the wing mass, and has an effect on the empennage. In comparing optimized aircraftdesigns with and without load alleviation, the fuel burn benefit of load alleviation is between1.6% and 19.5%, with 11.6% as the most realistic estimate. This includes a 4% improvementfrom active load alleviation and 7.6% from the wing planform optimization. Simplified constraints like maximum wingspan and minimum roll control authority are investigated. Implicitconditions such as constant wing loading and static margin are included to ensure comparabilityacross the designs.

 

[SWComposites]

i thought the 787 had some sort of gust load alleviation system

 

[Lou Scannon]

...like, if a gust load is too high, the wings come off?

 

[SWComposites]

no, that is only in hang gliders
the aircraft uses the control surfaces to control the wing loading

 

[rb1957]

well yes, if the gust load is too high the wings will come off, like a plane landing at 24 fps will also crash. but as in all things aviation (or engineering ?) all things positive come with some cost. Now they're "hyping" fuel efficiency (more than usual) as a carbon reduction initiative, and so what if maintenance costs go up

 

[dcascap]

why reducing overload cycles increase crack growth? Is it due to reduction of crack tip plasticity or am I missing something?

 

[rb1957]

in Aluminium, reducing the overload reduce the retardation. Overloads overstress the crack tip which sets up a compression field ahead of the crack tip which the crack tip has to fight through under the routine (smaller) gust loads. This significantly reduced the crack growth rate of these smaller cycles. Smaller overloads leads to smaller retardation and faster crack growth.

For composite structures, like modern wings, it is probably different (being more sensitive to high loads and more tolerant to low ones). So previous lessons may not apply !

 

[dcascap]

Yes true, I was also thinking about ductile metals, but yeah composites can be quite different