Static vs Dynamic load limits

[TheSerb]

Hi,

The limitloads (per definition in e.g. FAR part 23) relate to dynamic or static loads?

I have seen airliners being tested to ultimate design limit, but it looks like static testing.

So, hypothetically, if you experience turbulence (gust load) that exerts +3.8g and then -1.5 followed by another +3.8g, are the limits still safe?

I assume that within +3.8 and -1.5g structure is exposed only to elastic deformations. But I wonder how many such dynamic cycles (ie.g. how long can it fly in strong turbulence) can a structure withstand or did they design it so that n-number of cycles is approximatelly unlimited within these limits.

I am aware of fatigue problem etc due to other factors from environment but I wish to clear this stuff in my head :}

Thanks for replies!

[Grunf]

Hello.

Limit loads always relate only to static loads, as per FAR 25, Subpart C, 25.305 with a full definition on the required timing of the loads (3 secs of ultimate loads without deformation if there is no dynamic test).

To clarify things that means that if you do not perform dynamic testing your structure wll have to withstand ultimate loads (ultimate = 1.5 x limit) for duration of 3 sec which has to be shown on static tests.

In general you perform static test as a certification requirement followed by flight tests (more or less they overlap).

A rule of thumb is that your structure has to pass the important limit loads tests prior to the first flight of first FTA (Flight Test Article). As I said this is just a rule of thumb not a specification. It depends on how the OEM agrees with local regulatory body on what is the condition for the first flight.

As for the second part of your question it is covered by 25.341, mostly taking care of defining gust loads. You can also use the Subpart G defining continuous gust criteria (dynamic response).

Someone might contradict me but I have rarely seen too much consideration for dynamic response of the structure (actually I have seen pretty scary stuff in terms of neglect of dynamic loads on structure).

Since regulatory agencies usually don't have neither the knowledge nor the competence to comment on these issues, in general they don't question these issues too much.

If you want to know more please check FAR 25 online (http://www.access.gpo.gov/nara/cfr/waisidx_04/14cfr25_04.html).

Interpretation and application of FAR 25 covering loads is a big business so don't think that everything you see there is applied straight forward .

Everything is OPEN FOR INTERPRETATION.

Cheers