Helicopter Vibration Exposure

MIL-STD-180G, Table 514.6C-X, Vibration induced by the main rotor and blade passing frequencies…

Here is a script for calculating the sinusoidal frequencies and amplitudes: helicopter.m

– Tom Irvine

Propeller Aircraft Vibration Exposure PSD

Here is Matlab script for plotting the MIL-STD-810G, Figure 514.6D-2, Category 13, Propeller Aircraft Vibration Exposure PSD, including the overall GRMS level.

propeller_psd_grms.m

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See also:

Bombardier Q-400 Turboprop Acoustics

Aircraft Shock & Vibration

– Tom Irvine

Time Waveform Replication

Time Waveform Replication (TWR) for shock testing on shaker systems has been receiving some attention in MIL-STD-810G and elsewhere.  The goal is to test components to measured field time histories for realism.

 I did some related work with NASA’s Robin Ferebee a few years ago, published as:

An Alternative Method Of Specifying Shock Test Criteria, NASA/TM-2008-215253 & PowerPoint slide overview

The concern was the water impact shock of the Shuttle Solid Rocket Booster (SRB).  A large IEA avionics box was mounted in the fore and aft of each booster.  Each IEA was recovered and flown on a future mission.

So there was a need to perform a shaker table shock test on the IEA using realistic time histories from measured impact data, to verify that the IEA could survive this event

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Here is the software for the method in C/C++ :  TWR.zip

I will include equivalent versions in Matlab, Python, Fortran and Perl in a future update revision.

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– Tom Irvine

Rail Impact Shock

Question:

MIL-STD-810G, Method 526 Rail Impact, outlines the physical test of an actual system at 4, 6, and 8 mph. Is there an equivalent laboratory shock test for equipment which is to be transported via railroad car?

Answer 1: from Tom Irvine

U.S. Army TB 55-100, Transportability Criteria for Shock & Vibration 1964, has two time histories which are roughly half-sine pulses as shown in the above figure.

Answer 2: from Bill Shust

As far as I know there is no lab-based test that the American Association of Railroads (AAR) recommends to simulate the full-size shock. For example, any new cargo restraint methods are only approved after the actual 2,4,6,8 mph car-to-car impacts (years back, I operated several of those tests in Pueblo, CO). TTCI in Pueblo, (and to a lesser extent, I think) NATC in Nevada, and NRC in Canada are the most common providers of these tests.

I work regularly with the AAR rule makers, and I’ve talked with Howie Gaberson about this topic in detail over the years. (If you are familiar with his pseudo velocity shock response spectrum, that approach creates three-axes graphs — that indicate max. defl., max. velocity change, and max. acceleration of a shock pulse. It has been a while since I looked at some PVSRS data, but I seem to remember that the higher railroad shocks have the peak delta-V line around 80-150 ips, deflections of 36-48”, and 150-200g (this is on the railcar structure itself).) No lab-based tests that I know of have the low frequency content that the real railcars have. Also, it is common for the cargo to receive secondary shocks as their box/pallet runs out of travel and then contacts some kind of bulkhead.

Here are some references that would be good background information, however there are almost as many approaches to interpreting railcar impacts as there are testing organizations in the business:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr22.pdf

IEC 61373 (which is a euro. spec. for electromechanical equip. intended to reside and operate in a rail environment)

The selection of AAR Damage & Lading research reposts found on p. 54 of this catalog:

https://www.aarpublications.com/~/media/AARPublications/FreePubs/Catalog.ashx

On pages 50-53 are pages of AAR approved ways to restrain various kinds of loads so that they don’t get too much damage—this doesn’t totally eliminate certain railroad customers from cutting corners by using too little internal void-filling materials, such as air bags, or correct blocking/bracing, though.)

Tom’s older mil-spec test is still a valid set of data. Some of the newer railcar designs either (a) have better impact absorption hardware at their couplers, and/or (b) are restricted from being humped (i.e. sorted in the way that allows cars to roll down a gentle hill into each other). This means that not all of today’s traffic will see the impacts published 40 years ago. But the vast majority of cargo is still on cars that are not operated much differently (with respect to lengthwise shock) from the 1950s or 1960s).

For expediency, engineers who are tackling a subset of this issue often eliminate some part of the shock that they are concerned with (e.g. band-limiting a piece of longitudinal field data with cutoff frequencies tailored to what they already know about their product), and then perform a lab-based proxy with that modified understanding. So, the railcar builders look at the same shock differently than each of the various cargo makers.

Sincerely,
Bill Shust, P.E.
http://www.objective-engineers.com