3 Point Bending Test with 2lb force to the centre of a 26 inch span
Then an e-mail from Rick Walba:
Rick Walba - 16th March SALGlider
Allegro boom test - Tailboom stiffness thread.
I just finished testing the bending stiffness of a standard Allegro boom from my stock, along the lines
laid out by Martin Kopplow. First, 2" (51mm) of the boom was securely clamped, with a 2", snug fitting and
properly tapered metal plug inserted to prevent
distortion. With a 1 kilo weight suspended 500mm out, deflection was .590" (14.98mm). You'll have to take my
word that my eyes are that good reading a machinist's
scale, as I didn't think to set up a dial gauge even though I had one.
I'll add that, gauging from my experience with the SuperGee, with its longer tail to rear wing attachment
and moment arm, I think the standard full pod set-up for DLG could probably suffice with a boom that was
2/3 the stiffness of these booms. So I'm going to say that a boom showing the same deflection with 2/3 kilo
I would think adequate for the hardest launches.
Caused questions from Martin Kopplow:
Martin Kopplow - 16th March 2004 SALGlider
Subject : Tailboom stiffness
Hello Rick,
To me that reads as if the allegro boom is pretty stiff. Did you build some "machine" for the test? If so: How did you approach the torsion testing: I
tried to fix an arm to a boom, but found that the (removable) joint between arm and boom was somewhat soft, so the readout was influenced. Next idea I
have is to separate the "hand" (indicator) from the arm (load), so there are only negligible forces at work.
And the reply:
Rick Walba - 16 March 2004 SALGlider
Subject : Tailboom stiffness
The Allegro boom was designed for the Allegro 2M RES so yes, it is pretty stiff. Big end od is .62", the
wall is about .012, the CF fibres run very straight in the CF prepreg' and the resin content is only 38%.
I hardly designed a machine, just clamped some items to a heavy wooden workbench. I used a "hefty"
adjustable bar-clamp to hold the plugged boom in a steel v-block that sat on an aluminium
plate. The other end of the boom where the weight was attached by a string hung over the bench. I had a heavy piece of
metal clamped down and sticking out from the bench parallel to the boom to have a platform to measure
from. Next time I will just mount a dial gauge to this platform.
I haven't done the torsion test yet. I am going to machine an arm, with a collar clamp that I will
probably epoxy some abrasive grit to. You could probably drill a snug fitting hole in a
0.25" metal bar and CA it to the boom. Heating the bar should release it without damage to the boom. You don't want any soft
or springy materials in the clamping arrangement,
because these will definitely yield some and throw your reading off. For instance, the bar in the
bar-clamp I used was 1.25x.188", because the 1x.125" bar-clamp would flex under load.
Rick
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