Air Data Boom Vibrations

[Sarge14]

ello all,
I have recently been assigned the task of fixing part of an experiment that was conducted last summer and will be repeated this summer. Part of the experiment involves taking air data measurements from a flying helicopter (Bell 206). This was done using an air data boom (

http://www.spaceagecontrol.com/100510.pdf)

that was attached to the end of a tapered boom extending the boom past the front of the helicopter rotor.
The boom was attached to the step of the landing skids of the helicopter at two points. This was done somewhat haphazardly and during flight tests the end of the boom vibrated substantially and caused the alpha vane at the end to oscillate a considerable amount. This created an assortment of problems in terms of correlation of the data obtained.
I have been asked to minimize the vibration of the tip of this data boom so as to improve the air data measurments. The only real restrictions are that the data boom (and also an instrument pod) are located out near the front of the rotor and that it be attached in some way to the landing skid step. The size, material and attachment types are all fair game though I am trying to do this as simply and cost effectively as possible ( ie active control via piezo etc not really feasible). I also have to have this ready to go by mid June.
My general Idea of how to solve the problem is to calculate the eigenvectors and modes of a beam and to find the nodes associated with the 1/rev and 2/rev excitations of the rotor. I then intend to clamp the boom at these nodes. I am by no means a vibration expert but have substantial engineering education.
My question is if any of you vibration experts out there have any pointers on what else I might do to solve this problem and reduce the tip deflection of the boom. Any and all pointers would be greatly appreciated. Sorry for the length of this post but I figured that without a proper (or at least substantial) explanation of the problem, people would continue to ask more.

Thanks in advance for any pointers.

 

[kopterdoctor]

having worked on 206 series helicopters for quite a number of years, I would cosider the landing gear a less than ideal
location to mount a vibration sensitive device.I would try the belly of the fuselage, and also have the vibration level checked, and corrected if necessary to minimize the 2 per characteristic of that helicopter type.

 

[Sarge14]

I agree that the landing gear is a less than ideal (if not one of the worst) places to mount the device. Though I did not see the actual oscillation of the boom last summer, from what I understand it was really quite large in amplitude. Small vibrations are ok. I am just looking for any tricks of the trade people may have encounted when faced with a problem of this nature. Because the boom must be built off site and without access to the helicopter I am really trying to stick to the current skid mount location. I plan on checking vibration levels when I have access to the 206 in a few months.

 

[Sparweb]

The Bell 206 has lots of locations available to which you can mount things underneath. Don't put it on the shakiest part of the airframe just because "it was done that way before".
Since you don't have access to the helicopter (or any others, I assume), then you at least need a station diagram to figure out where you are.
I also assume you want the probe extended forward of the helicopter, on a boom of some sort. Make the boom bigger, stiffer, and then mount it on the softest attachment you can get away with.
Are there things in the way, like a sling mirror, wire-strike blade, camera, etc. that preclude some locations? If not, then you could look around at how they're mounted for ideas for mounting, or bracing, your boom.
The 206B is one of the more commonly modified civil aircraft of any class out there. Perhaps the simplest source of inspiration is

http://www.airliners.net.

My 2 cents.

Steven Fahey, CET

 

[MikeHalloran]

You might be able to add a dynamic vibration absorber near the tip, e.g. a short stiff wire rope with weights on the ends, as used to damp vibration in long span overhead electric cables.

It would be simpler, and maybe involve slightly less drag, to add cable stays to the distal and proximal ends of the associated skid strut and a third wire to somewhere on the opposite skid, but that probably would bend the probe, since it would be outside the centroid of the anchor points. But you'd have to check the airframe for the odd loading, and you probably don't have time or data for that.

My next thought was a cable stayed lance, sort of like the cable stayed pipes you see around grain elevators and other places where long slender pipes are used as compression members in what amount to trusses to support themselves.

Considering the drag from all the cables, if you need to go fast, you're probably better off with a big 'ol tube, like a refueling probe.



Mike Halloran
Pembroke Pines, FL, USA

 

[HANDCALC]

About 17 years ago, on our Bell 206B, we installed the same air data probe (Space Age Control P/N # 100510-1) onto a boom that was attached to a centreline mount under the nose. Although this design has been adequate for our research purposes, a stiffer boom is desirable for a number of reasons which are beyond the scope of this post.

If you have the time and money, consider the following:

Step 1 - Identify the range of the 1/Rev and 2/Rev main rotor forcing frequencies, in both the "power on" and "power off" conditions. See Type Certificate Datasheet No. H92 (Canada) or H2SW (United States) for details. This information is needed to choose a boom length and stiffness that avoids resonance of the air-data boom with the rotor forcing frequencies.

Step 2 - Look in a book like "Theory of Vibrations with Applications" by William T. Thomson for the equation that describes the boom natural frequency for your boundary conditions. Use Dunkerley's formula to combine cases, if required. These formulas will result in something like Wn^2=Constant*[(E*I)/(M*L^3)]

Step 3 - Increase the outer and inner diameters of the boom to increase the Moment of Inertia I, and reduce the length L; especially if you want to increase the stiffness and natural frequency of vibration for the air-data boom.

Step 4 - Meet with the pilot and AME or Director of Maintenance to ensure they are happy with the proposed nose-boom design

Step 5 - Ground test the air-data boom (rotors not turning) using an accelerometer, in both the vertical and lateral directions. Ensure you are well away from the frequencies determined in Step 1. Also check damping - to see how quickly the vibrations damp out.

Step 6 - Do a stress analysis or proof load test of the mounting structure, to ensure the air-data boom can withstand the inertial and aerodynamic loads expected in flight.

Step 7 - Comply with the FAA or Transport Canada's requirements associated with aircraft modifications, to ensure that airworthiness certification is maintained. You'll probably require a US (DER) or CDN (DAR) to assist with certification via an LSTC.

Finally - Perform ground tests on the nose-boom (with rotors turning) and if everything looks good proceeed to flight-tests that clear the nose-boom over the required flight envelope. Mounting a nose-boom to a helicopter is serious business. Ask others for help if you are not sure that what is proposed is safe for flight.

Hope this was helpful?