what is old is new again ... 1

[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

 

[WKTaylor]

In the fuel crisis of the mid 1970s airlines were scrambling for max fuel efficiency from the old legacy jets... such as 707s, 727s, 737-200s, etc... so the call went out.. find ways to conserve fuel!

Example: Taxi with [1, twin] or [2 of four] engine where possible. Spool-up/check/rev-up the other engine(s) only when absolutely necessary for take-off, etc. shut-down engines as soon as possible after landing. ETC...

A small group of 707-320 pilots tried in-flight tactics too... and soon learned that extending trailing edge flaps ~5-Degs provided a 7-to-9% fuel saving on long flights. They rationalized that flap stresses in cruise flight were acceptable. The ONLY critical aspect they HAD to observe was to 'pull/collar' the flap circuit breaker to prevent inadvertent extension at cruise... which could be catastrophic for many reasons... over-stress, pitch-over-etc. Yep this worked fine... UNTIL... a co-pilot bumped the flap handle 'down' before collaring the breaker... over-stressing all flaps and starting a pitch-over. Luckily they caught the runaway flaps... in-time... but flap and track and rear-spar damage was done and the secret exposed. Then no-one was amused.

About 15-years ago I got an urgent call of a possible inflight emergency [IFE]... a 'similar-to Boeing MIL aircraft' had a 'bumped-flap handle incident' which started TE flaps extending... but was 'stopped' at ~7-deg max extension... after the urgent call 'flaps moving' from an eyes-open-crew-chief alerted the crew which stopped/retracted them. IFE CALL: 'Was Damage done?'... 'do they have to land ASAP?'... etc. I consulted my memory of the system... and explained 707 history with cruise flap extensions... and that they had stopped the flaps in range where damage was highly unlikely... and likelihood of damage was slight [in their circumstance] and recommended 'continue to destination and then perform a precautionary flap-overspeed inspection'. Last I heard there was 'no damage noted' and aircraft was returned to service'... and just needed an info-NOTE in the Acft records

 

[rb1957]

you'd've thought (well, I'd've thought) that modern control systems would prevent unsafe flap extension, or that we'd be building airplane controls for long enough to know to design them so they can't be bumped into an unsafe position.

As for your inflight incident ... if the plane's flying ok, keep flying; I might have suggested a flaps up (or partial flaps) landing, depending on the risk of a high speed landing. Maybe loose as much weight as possible before landing. Want to bet that that plane has an incident later and this event (and maybe your role in it !??) comes to light in a "Mayday" episode ?