Carbon Tubing Dynamic Response Test
Entry posted by SegelFlieger
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Summary:
A static deflection test was previously performed on several carbon tubes allowing them to be compared based solely on “stiffness”, or the “spring constant” for each tube.
A link to the discussion can be found here:
http://kitelife.com/forum/topic/7329-carbon-tubing-comparison/
Experienced Rev fliers have responded with comments regarding the “reflex”, or response of the carbon tube frame being more important than the “stiffness” or weight of the frame. Experienced fliers select a preferred frame based on wind conditions and the selection of sail type (vent options: none, mid, full). Additionally, modifications may have been made to the “stock” sail design to improve performance of the kite. The data presented here represents the dynamic properties of a carbon tube independent of a sail.
The Data:
Let's get right to the data!
Here is a link to the current Excel spreadsheet with both static and dynamic test results for the rods that I have tested. A copy of the table is included in this post but does not let you sort by manufacturer or characteristic.
http://www.donkush.com/CarbonTubeProperties.xls
here is a copy sorted by 1st Amplitude 5” Perturbation:
Here is a copy of the worksheet "Deflection and Reflex comp" which compares dynamic measurements with the commonly used Rev 3-wrap rod with silver label:
A positive % means that the test value for that tube was greater than the Rev 3-wrap test value. Likewise, a negative % means the test value was less than the Rev 3-wrap test value.
Discussion:
Specifications for carbon tubing that you purchase for the purpose of constructing or modifying a frame combination yourself only include tube ID and OD dimensions, and an overall weight. Some tubes and frame sets will only provide a model ID and no other information. The more experienced kite flier or builder will be interested in more information than this. Hopefully the tests that I have conducted will be useful to you although you will see from the pictures that I don’t have a NIST certified lab. I was able to conduct these tests with readily available household items (for me these were readily available 
) and a little ingenuity.
Description of the Carbon Tubing Response Test:
A 31” tube was secured at one end of a test table by a ferrule allowing a full 31” section of tubing to be deflected and oscillate. The 31” length was deflected by 5”, released, and allowed to oscillate until it settled. The amplitude of the first vibration response was measured in inches. The frequency of vibration was measured in HZ (cycles per second), and the settling time was measured in seconds. (settling time is defined as the time it took for the tube to stop vibrating after the initial disturbance).
Test Table Picture:
The measurements were taken using audio technology. A small wire (whisker) was fastened to the end of each tube that was tested and a small microphone was placed exactly in-line with the stationary tube.
As the tube oscillated past the microphone it recorded the impact of the whisker. Since the audio was sampled at 44.1 KHz the data is quite precise. Frequencies and settling times were analyzed using audio editing software.
Data Sheet Picture:
A data sheet was used for each rod tested. The data was recorded and then entered into the excel spreadsheet that has been posted.
Sonic Performance Pictures:
Multiple tests were performed on each rod and the results were averaged. Here is an example of a rev 3-wrap rod showing 3 response tests:
Here is a picture of one test with markers drawn at each impact of the microphone. Two impacts define the Period in seconds. 1/Period defines frequency in Cycles per Second known as Hz (see highlighted and circled value in the picture). In the case of a 3 wrap rev rod the Period is .062 seconds on average and the frequency is (1/.062) or 16.129 Hz:
The first amplitude deflection however was measured manually. A toothpick was held perpendicular to the testing table and at the end of the tube. The toothpick was adjusted until the tube barely touched it during the first amplitude deflection.
I don’t have a picture demonstrating this test but here is a picture of a sample test sheet showing where the first reflex of an example test sheet for the 3-wrap rev rod where the first amplitude was marked. It is at the end of the perpendicular line drawn from the Centerline (CL):
What does this all mean?
Hopefully this information is useful to you and can inspire further discussions regarding frame preferences and sail choice in the forum.
I will offer my initial interpretations of the data with hesitation because I know that many of you will have your own observations...
Again, Wind Speed, Sail Choice, and Sail Modifications are all factors in the kite’s response and performance.
Frequency test:
The frequency test results are very similar between each rod with the exception of the SS P400 and the Rev 3-wrap Green-stripe. Almost all the rods oscillate at a slightly lower frequency than the Rev 3-wrap. The frequency value could indicate how quickly the rod, when used in a sail, will try to return to a static position after a disturbance. Perhaps a point for discussion…
1st Amplitude Response from 5”:
This test possibly indicates how “springy” a tube is. A large 1st amplitude would indicate that the rod wants to flex easily. Perhaps this would give you a very “bouncy” experience when used in a sail. Silver Race rod test results would indicate that they would seem considerably less bouncy than the 3-wrap. The SS P-90 is the bounciest of all which I would agree from my experience.
Settling Time:
This test would also seem to indicate how quickly a rod will return to a static condition after a disturbance. It adds another dimension to the frequency characteristic that describes how quickly the amplitudes during oscillation diminish to zero. Race Rods and SS PX tubes have a shorter Settling Time than the 3-wrap standard; this has me curious now and perhaps my first question back in the forum: For those of you with SS P-3X frames, how do you compare this frame to the Rev 3-wrap? The SS P400 seems like it wants to vibrate forever!
Well, I will end this blog now and look forward to continued discussions in the forum.
S.F.
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