Aerodynamic loads on blade & conversion into test loads.

Dear Everyone,

This topic is for discussion of various issues and considerations while predicting the aerodynamic loads and challenges for converting these loads into structural test loads [during static and fatigue testing of the blade].

During static test: The blade bending moment is simulated by applying the concentrated loads at three or four location [radial positions]. Of course after performing buckling analysis of the blade, the number of loading points is decided. These loads can’t generate the exact bending moment derived by the software code.

Here we can discuss updating in the assumption which will approach towards real operating condition of the turbine. Is there anyone who is involved in conversion of aerodynamic loads into test loads?

It is correct that in testing we cannot EXACTLY match the target bending moments; however, we can get reasonably close. Here is the simple explanation on how the loads are derived.

During static testing, multiple load stations along the blade span are used to apply the test loads calculated from the sum of the linear moments reacted at the root of the blade. By positioning these stations strategically, and adjusting the pull load at each one, it is possible to achieve an applied bending moment similar in curvature to target bending moment. The attached picture shows a theoretical 40-m blade using five load stations (labeled winch); notice how the applied load is linear between each of the load stations, yet the combined bending moment curve matches closely with the target. This is an acceptable and widely used approach.

During fatigue testing, we can employ a similar method by adjusting the saddle or ballast weights, with the assumption that we are performing at test at resonance. In this case though the dynamic bending moment is directly related to the mode shape / deflection shape and therefore saddle locations and weights can “kink” the applied loads to match the target loads.

Refer to the IEC 61400-23 standard for more detail. Here you will find the theory behind structural testing and the partial safety factors needed to obtain the target test loads from the aerodynamic loads.

Though not on wind turbine blades per se, I’ve carried out structural tests on aircraft external structures. Perhaps my experience is relevant to you. A lot is published about aerodynamic load test practices on aircraft structures, especially on helicopter rotor blades for example, which you might also find useful. Use sources not only from “universities” but from the “regulators”, who concentrate much more on the means than the ends.

Generally the analysis of loads leads to several critical cases which you must pass by test. Either you’re seeking certification from an external body, or just looking for verification. No matter who’s watching, you won’t have a valid test unless the loads are applied and distributed in ways that realistically represent the loading scenarios you want.

Usually they include:
1- shear load and distribution
2- bending moment distribution
3- torsional distribution

The loads have to be applied in some way. They can be weights hanging below the WT blade, or weights stacked on the surface (eg bags of sand or lead pellets). The use of gravity to apply the load is often chosen because it’s invariable and the fall to the floor if something goes disastrously wrong won’t likely cause injury. Using cables or ropes under tension can be more dangerous because releasing the load in a failure - things will “spring” all over the place. The chance for injury is higher.

If there’s a “catch” in using gravity, it’s that large deflections affect the distribution of load that you’re trying to achieve. Either you have to anticipate deflections or be prepared to put on more load to compensate. Safety factors would be specified by IEC guidelines, I suppose but I shouldn’t guess, and I won’t go into what they are for aircraft/rotorcraft.

Test load combinations the simulate any critical case will have to get the distribution right, as was pointed out before. In addition, the application of the load could be too concentrated - excessive pressure at a single point can cause local deformation or damage that invalidates the entire test. Some times an attachment for a weight was used that caused a false “stiffening” of the structure, also invalidating the test results.

Proceding carefully and safely are the key ingredients. Cost and the amount of caution required go up exponentially with the size of the blade being tested (it goes without saying). Good luck.

Dear Steven,

Thanks for the detail information.

Do you have any document related to this procedure? If yes, Is it possible for you to share the same?

Hi Shankar,

I doubt I could find much that would be directly useful to you, if that’s what you mean. My involvement in certification tests for wing extensions and external equipment on aircraft was the basis of my earlier comments. The aerodynamics and behaviour of wings and rotor blades are similar, but only in general terms. Any specific configuration will have its own challenges.

I looked at your company’s website and apparently you already have the facility you need. I can’t imagine what I can add that isn’t already at your disposal. As a test facility you should be given the loads by your customer. If all they are offering you is the distribution of aerodynamic load from CFD analysis, and you are converting it to a test system with 3 applied point loads, then you should submit your test plan to them for approval first.

You may find the Boeing videos of their wing load tests informative.