Streamlined Tube

[rb1957]

I'm trying to get some streamlined tube to mount an antenna on top of; sigh, the gory details are I have to position a TCAS antenna roughly 2' above the airplane skin (to clear a radome that we're adding; yes, it has to go here).

Don't care about material (Al, Steel)
Not too fussy on gage (in my mind, not a pleasant place usually, i'm thinking of 0.063" 2024)
Minor diameter something like 3"
Major diameter something like 6"
only need 2' - 3' length
a paper trail would be nice (but not essential)
this project is not civilian certified

you're my last chance, before we fabricate something ...
purchasing says it unavailable,
some micro-light leds lead to nothing ...

any help, gratefully appreciated

 

[amorrison]

Check out heli/gyro blades at

http://www.vortechinternational.com/

0012 = symmetrical blade profile.

If you need something thicker Google for "0018" rotor blades - unusual but maybe they exist.

 

[CESSNA1]

RB1957: How about a strut fairing off of a runout Cessna 152/172/180/182/etc.

Regards
Dave

 

[aviat]

Float struts and spreader bars use large streamline tubing, but 6" is quite large. You could try the following sites as they manufacture floats, and could provide leads for tubing.

http://www.wipaire.com/

http://www.kenmoreair.com/index.php?flash_loaded=1

 

[GregLocock]

Tooling up for your own aluminium extrusions is amazingly cheap.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[berkshire]

rb1957
You might look at mast extrusions for sailboats, a Kenyan 3650 is 5x3.5 They are usually 6063 t6 or 6061 t6
They also have an airfoil spreader section SP 6 which is 6.5x1.75 with a .125 wall. Check

http://www.RigRite.com/spars

Hope this helps,
Brian Evans.

 

[Sparweb]

You could use a 3" round tube and flush-rivet a triangular trailing edge onto it...

I'd fight like hell to do anything other than that. Put it on the vertical stab, the nose radome, or maybe dual antennas with a splitter so one makes up for the other when it's shaded.




Steven Fahey, CET

 

[rb1957]

thx everyone for the ideas. i'd thought of struts, and of fabricating a fairing on a round tube, hadn't thought of boat masts/booms (should've). i've also thought to ask Aloca and the like (i mean, boat suppliers and such are getting their extrusions from somewhere !)

 

[MikeHalloran]

At my last job, I used a home-made tube in marine exhaust systems that's relatively simple to fabricate, and looks 'steamlined'.

To get such a tube of 3" x 6" diameter, you would first make up a tube 7.5" diameter by whatever length you need, then scribe parallel axial lines 6.96" apart on its surface, and slice it on those lines, producing two partial cylinders and some scrap. In cross section each partial cylinder would resemble an arc of 3.75" radius by 106 degrees.

When we first made them, my tube mill, Francis, actually made up full tubes and sliced them. Later he found a way to roll the partial tubes directly. Work with your tin shop; it's probably easier for them to make a similar tube with a short straight tangent (length determined by the size of their slip roll) than to make them fully curved.

In 12 gage stainless, it's simple to butt the edges at the inner surface and bond the pieces with small fillet welds, producing a tube with a shape like this: ()

In the foil that airplanes are made out of, it would be a little more difficult, but a decent TIG welder could make it look real nice in .063" steel.



Mike Halloran
Pembroke Pines, FL, USA

 

[copter]

We just recently fabricated a similar unit it was only about 4" high. You could use the same line of thought for making your mount. It was fabricated from sheet and tig welded..

 

[WKTaylor]

rb'57

You never gave us a clue as to the operational environment this antenna mast will see, such as:

V(TAS,IAS)max

G(X,Y,Z)max

H(altitude)max

T(temperature)min/max

Evironment [desert, arctic, oceanic, rain-size/rate, icing, lightning-strike(?), etc]

Install life: Test program X-hours-service and 6-months: or remaining service life of acft such as 25,000+-hrs and 30-years???

Vibration environment?

Regards, Wil Taylor

 

[rb1957]

g'day wil,

no, i didn't ... the plane isn't black, but it is dark grey; that 's why i didn't think the operational environment was particularly relevant. we tend to err of the heavy side of things, and can afford special inspctions ... lucky us, eh !

this plane will probably ('cause i don't really know !) operate similar to a commercial transport, in terms of mission profile; limited flying time is expected (hangar queen).

right now i'm tending towards a fabricated solution, welded or riveted (depending on the shop's feeling on the day). probably a 4" - 6" diameter tube (6061 if we're welding), 0.06" - 0.08" wall, and similar thickness sheet for the fairing.

cheers

 

[WKTaylor]

rb'57...

First. A "no-kidding" story...

I worked with a bunch of Army guys on a limited mod (such as Your'es), for putting Rotor blade sections into a real-world icing situation. The Test-set-up hung the blades off the side of Beech King-Aire with an "aerodynamic" frame-work around them. My mechanical/structures review before flight indicated it was structurally questionable and potentially too flexible. I recommended a critical review by the "vendor" who designed and built it [I will NOT say which company it was]. The review resulted in minor beef-ups here/there on the frame work and strut. After just a short winter-flying season (about 75-hrs) the Army guys removed it from the acft. I was asked to inspect and evaluate the structure: since a few "small cracks" were noted. A FEW SMALL CRACKS MY ASS... every part [skins, ribs, spars, fittings, boom-arms, actuator attachments, etc] showed signs of cracking, severe fretting, distortion, or loosened fasteners. I recommended scraping ever piece of the equipment... and suggested they NOT use the same design-fabrication shop, EVER again [industry reputation NOT withstanding... the company did a lousy job on that equipment... probably put a junior engineer on the task].

NOW...

The 24-inch antenna stand-off You need (adjacent to another external body) in a "generalized transport flight environment" would make me nervous to say the least. Buffeting/vibration/flutter/fatigue, rain/ice, thermal cycles, bird-strike, lightning-strike, bonding/groundin for static dissipation, etc... make this a dicey issue to evaluate at best.

RB'57... a question (food-for-thought): if the mast fails, or is damaged, in a least opportune time in-flight [or a during a critical mission-task, etc]... what is the greatest harm it COULD it do (to the acft or the mission)????

Sorry if I come-off a bit "hard" sounding... my military aero-experiences have led me to [absolutely] believe that "Murphy was an optimist".


Regards, Wil Taylor

 

[Sparweb]

I could add that I've seen these kinds of failures too...

http://www.eng-tips.com/viewthread.cfm?qid=67807

The lesson I learned is to use horribly large margins built on extremely conservative assumptions, and then back it up with boilerplate to make absolutely sure. It seems people just have to have these kinds of antennas.


Steven Fahey, CET