Tapered Aluminum Tube

DOes anyone know if it's possible to get aluminum tubes that have two different diameters on either end? Like a 755mm long aluminum tube that is 25mm outer diameter for most of it's length, but has a taper near one end so that there is about 5~10cm (or so, it's not too critical) of 20mm diameter tubing there.

Check with the places that sell camping equipment. My friends 8 person tent has aluminum poles that have one end smaller than the other so they fit together to make the center frame.

Hi dknguyen,

going from a wider diameter to a smaller one will be a problem.

It'll mean to shrink the material, which is possible, but hard to achieve.

The other way round it (widening) is easier to do and you can even do it yourself.

Use a 20mm (bore) aluminum pipe and drive an iron cone with the desired diameter into it. Use a shallow angle for easy shaping and make one end fitting snug into the aluminum pipe. When the desired length is achieved heat the aluminum slightly for easy pull-out of the iron cone.

Plumbers use that method to make "trumpets" at one end of a copper pipe for easy and tight connection.

Shrinking the pipe from 25 to 20mm can be done using a lathe and pressing the aluminum using a roller and a calibrated 20mm iron rod in the support.

Hans

Hmm. This was for a helicopter tailboom that is 25mm to maintain rigidity at it's extra long length. BUt the tail case I wanted to use only fits on a 20mm tailboom. I think I'll just have to live with the shorter tailboom that is 20mm.

I wasn't sure how rigid was enough, but I found some examples. I knew 600mm x 20mm is rigid enough, but that was too short. But I found an example of 690mm x 20mm exists and works, but then it goes to 755mm x 25mm.

Hi dknguyen,

I imagined it was for a helicopter model. You might want to try something different and easier, not less elegant.

This is how i'd do it without lots of tools and machinery: Get two different dia aluminum pipes (20 and 25mm) which you cut to the desired length. Use a piece of pinewood shaped as cone to connect both pieces and use two component glue (Epoxy).

If the drive shaft for the tail rotor has to pass through drill a hole leaving enough space for it.

Supposedly you're going to paint the tail boom and nobody will see the piece of wood between the two sections if you do the job accurately what I assume.

If necessary smoothen the joints using putty.

Depending on the thickness of the aluminum pipes I'd consider glueing stabilizing ribs into it (a lot of work, I know) for maximum rigidity.

Please consider the length of the drive shaft. If it is unbalanced (not 100% straight) it will rip off the tail boom in no time. Plan for intermediate (maintenance free, sealed on both sides) ball bearings, using one at each end and one in the middle of the boom.

The bearings don't have to be pressed into the aluminum pipe. Rather press them onto the shaft and have them fit snug into the pipes.

When finished, don't forget to post a photo of your helicopter.

Regards

Hans

dknguyen said:

Hmm. This was for a helicopter tailboom that is 25mm to maintain rigidity at it's extra long length. BUt the tail case I wanted to use only fits on a 20mm tailboom. I think I'll just have to live with the shorter tailboom that is 20mm.

I wasn't sure how rigid was enough, but I found some examples. I knew 600mm x 20mm is rigid enough, but that was too short. But I found an example of 690mm x 20mm exists and works, but then it goes to 755mm x 25mm.

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I'm not sure what "gauge" your talking about however, my friend is an aerospace machinist I recall he went to a Boeing facility to learn how to buff an aluminum skin after repair? I didn't understand process at all but, it makes me wonder if a good Tinsmith could design and build what you want ?

kv

Boncuk You think that wood adapter is gonna hold up? Heli's are pretty nasty on the vibration side. You could just set the two pipes up on a jig so they're straight butted up against one another and use epoxy and carbon fiber tape, not gonna get much stronger than that.

THis is a belt drive so it's gonna need more room than for a shaft drive so there's no room for anything inside the tube. But anyways, 690mm x 20mm seems to work so that's what I'm gonna use. Narrow enough and long enough. I suppose I don't need a 755mm long tailboom. It's just nice to have extra tail authority and to have more clearance between main and tail rotors. But you do lose some efficiency since the tailrotor doesn't make use of the main rotor's blade tip vortex.

BUt I would have though that wood would help with vibration. It lacks tensile strength of aluminum or metal though.

Taking this route would allow me to not have to frankenstein the tailboom with the rest of the airframe. I would just have to just rob parts from slightly different versions of the same helicopter from the same company. But I'm probably just going to wait until the company does it themselves. They seem to be working on the exact same thing: Piecing the "smaller" helicopter airframe with the longer tailboom of the "larger" helicopter, but not the extra long tailboom of the larger helicopter which has the larger diameter. The airframe is actually the same airframe. It just comes with a slightly smaller main rotor assembly and a smaller canopy (so the canopy is going to look a bit undersized relative to the tailboom, but who cares...it's lighter).

The result is a helicopter that can run 550-600mm blades, whereas the smaller heli could run 500-550mm blades while the larger heli could run 600-690mm blades (which meant extra weight to accomodate the larger blades that would have gone unused for the most part, but for me the issue was the larger tailboom diameter to support the extra tailboom length was incompatible with my desired tail rotor assembly).

Sceadwian said:

Boncuk You think that wood adapter is gonna hold up? Heli's are pretty nasty on the vibration side. You could just set the two pipes up on a jig so they're straight butted up against one another and use epoxy and carbon fiber tape, not gonna get much stronger than that.

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Vibrations are caused in any aircraft. Engineers normally design airframes with some flexibility to compensate for vibrations.

You might have observed the wings of an airliner "oscillating" by 3 feet and more in turbulent air, e.g. when entering the jet stream or in a thunderstorm.

The wooden adapter is really strong enough to hold up, provided each part is made with the necessary precision (not to fill the gap between two parts with excessive epoxy).

My concern is more the stability of the aluminum pipe.

Here is a good one to think about the problem of rigidity:

An aircraft manufacturer designed a new aircraft. During test flight the aircraft had to recover from a steep dive which caused the wings to break off. The design was altered and the same thing happened again. A cleaning woman (responsible for the restrooms) observed the air show and suggested to drill holes at the wing roots equally spaced. The engineers gave it a try (because they didn't have a better idea ) and the follow on test flight was successful. Asked about the idea of drilling the holes she answered: "Have you ever experienced toilet paper tearing apart where it's perforated?"

Boncuk

dknguyen said:

BUt I would have though that wood would help with vibration. It lacks tensile strength of aluminum or metal though.

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Did I say to use balsa wood?

Use pine or oak wood. It has more strength than aluminum.

I don't understand the statement about toilet paper tearing at the perforations.

I think it's a joke on the structure of a material relative to the material it's actually made out of. Wood is fibrous and pretty well intertwined, it's natures version of carbon fiber basically. That's my take on it at least. I didn't find it THAT funny, but I'm not a mechanical engineer =>

dknguyen said:

DOes anyone know if it's possible to get aluminum tubes that have two different diameters on either end? Like a 755mm long aluminum tube that is 25mm outer diameter for most of it's length, but has a taper near one end so that there is about 5~10cm (or so, it's not too critical) of MM diameter tubing there.

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Seems like it would be much easier to make it out of fiberglass.

dknguyen said:

I don't understand the statement about toilet paper tearing at the perforations.

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That's the point. It normally does not tear at the perforations.

Rolf said:

Seems like it would be much easier to make it out of fiberglass.

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Sounds good to me. How much time is it going to take for a mould?

Probably use the hard paper core of a toilet paper roll.

Well now lets be careful to not paper over this problem.

sorry, Lefty

Boncuk said:

That's the point. It normally does not tear at the perforations.

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By perforations, do you mean the dotted lines made so you can tear the paper?

25 mm seems huge. This must be one big heli. What scale it it?

Maybe you can get a carbon shaft adapter made at a machine shop, may cost a few bucks though. I think a aluminum adapter would have too much weight.

THe 25mm thick tailboom is for a heli that can normally accomodate 500-550mm blades whose tailboom has been stretched to accomodate 600mm blades, whose tailboom has been stretched again for more tail authority.

It sounds like aluminum is fine...better than carbon fiber in some cases since it's easier and cheaper to make an aluminum tube with proper straightness and tolerance than it is to do the same with a carbon fiber tube. THese things seem to be able to carry an amazing amount of weight (you just up the headspeed and work the batteries harder for more power but less flight time)- it depends on how well you want to do aerobatics and autorotations with it.

dknguyen said:

DOes anyone know if it's possible to get aluminum tubes that have two different diameters on either end? Like a 755mm long aluminum tube that is 25mm outer diameter for most of it's length, but has a taper near one end so that there is about 5~10cm (or so, it's not too critical) of 20mm diameter tubing there.

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Easton produces such items for use in bicycles, large antennas, baseball bats, arrows, tent poles, etc. I'm not sure if they sell privately or if you need to go through a dealer. Below are some links to assist in what you may be looking for:

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