Streamlined Strut Fairing Material

[old man emu]

The plans of the plane I want to build call for the interplane struts to be constructed from 3/4" round tube, then streamlined with a balsa fairing. Since balsa is not cheap, I thought I would use streamlined 4130 tubing, but that stuff doesn't have the strength of round tube.

 

As I was looking around for a way out, I came across a mob which sells plastic streamlined fairing. Only problem is that the mob is in Canada. www.ultralightnews.ca

 

Has anyone heard of this stuff in Australia?

 

Old Man Emu

 

 

[Bubbleboy]

Hi Mark....I have been looking into the aluminium stuff and you will need to sell your first born to pay for it! There is a crowd in the USA the Pietenpol guys use which is supposed to be very good. If you can get a few orders together, it would be worth it with the freight I guess. Im not up to that stage yet. Have a look in the Matronics site under the Pietenpol category, im sure you will find it.

 

Scotty

 

 

[old man emu]

That Canadian site can supply at $5.32 per foot plus freight.

 

OME

 

 

[Gnarly Gnu]

Couldn't you knock some up out of high density foam? Either CNC cut or just a simple hot wire cut, trim to length, sand, coat....

 

 

[fly_tornado]

guy on the tornado list did that trick. pretty easy to build and a lot cheaper

 

 

[old man emu]

Couldn't you knock some up out of high density foam? Either CNC cut or just a simple hot wire cut, trim to length, sand, coat....

Great idea! I'll just have to draw up a template to cut them with hot wire.

 

OME

 

 

[sain]

Hi OME,

 

This method might be simpler and lighter:

 

http://www.homebuiltairplanes.com/forums/light-stuff-area/4669-lexan-strut-fairings.html

 

 

[old man emu]

I had a long think about using foam.

 

Firstly I would have to design and cut metal (aluminium) cutting templates. The biggest problem here is determining the size of the major axis to get a good looking shape, and where the metal tube would sit along the major axis.

 

 

Then I thought about how I would go about cutting the shape from foam. The limiting factor here is the length of the hot-wire bow. The longest piece of tubing I'd need is 40 inches long, so that is a manageable length with a 12V DC power supply.

 

The next thing is to work out how to produce the lengthwise hole that the metal tubing would fit into. The length of the foam piece is too much to drill out, unless I could lay my hands on a fence post auger, and could set up some way to hold everything rigid while I line bored.

 

The alternative is to produce one length of cut-to- shape foam about 42" long and use it as a mold to lay fibreglass over. There is also the possibility of using the 42" length as a former for the heated Lexan sheet method described here http://www.homebuiltairplanes.com/forums/light-stuff-area/4669-lexan-strut-fairings.html

 

Alternatively, I could just ditch the whole foam former idea and simply heat Lexan. However, I don't know how good a shape this would produce.

 

Old Man Emu

 

 

[Gnarly Gnu]

The lexan idea is neat. I guess I was picturing two solid halves of foam that would nest either side of the round strut & glued on, that avoids adding to the overall thickness a bit like this: co> where the 'o' is the strut & red bits the foam.

 

Cutting the shape could be a hassle though, I have a couple of large CNC machines sitting about so strange shapes are a daily occurrence. Some foam suppliers will economically cut long lengths using their CNC hot wire machines if you supply a simple CAD drawing. But yeah, the lexan is probably less hassle and more robust.

 

 

[Litespeed]

Another alternative is to make some fairings out of alloy sheet.

 

You can buy rolled sheet which is sold as alloy flashing at bunnings in 6 and 12 inch widths. It is nice and soft, easy to form will slowly work harden. It comes in a really thin 0.4mm and 0.7mm.

 

With a bit of practice, a perfectly suitable set of light fairings can be made to any shape you need and cheap.

 

I have a lot of fun playing with this rolled sheet and with practice beautiful pieces are possible.

 

And it polishes like a metal airplane should.

 

All you really need is the strut tubes and a smaller tube or rod of anything thats straight- even dowel. Combined with a straight edge, some clamps and a bench.

 

This plus a drill and rivets is all you need to make a nice set.

 

But there a many ways to any destination.

 

cheers

 

Phil

 

 

[pudestcon]

The next thing is to work out how to produce the lengthwise hole that the metal tubing would fit into. The length of the foam piece is too much to drill out, unless I could lay my hands on a fence post auger, and could set up some way to hold everything rigid while I line bored.Old Man Emu

Just support the foam vertically, heat up a ball bearing the correct size and release it on top of the foam. a few seconds later you will find it at the bottom of the foam piece. Voila! one neat hole - or so I'm told!!!!

 

Pud

 

 

[old man emu]

Wow! What a lot of ideas!

 

Gnarly:

 

Cutting a 42" block of foam with a hot wire is easy as. It's done all the time by blokes who make radio controlled planes, and they are doing it with much thinner aerofoil shapes than would be needed for tube streamlining.

 

Litespeed:

 

Using aluminium flashing sounds good, too. I might be forced to pop down to Bunnings today to check it out. Could you post a sketch of how you would set uo the metal tubing and dowel piece to get the shape? I was wondering if it would be possible to use the foam core to shape the flashing over.

 

Pud:

 

We all know about your hot balls

 

OME

 

 

[Bryon]

OME, check out this supplier http://streamline.8k.com/FAIRINGS.htm

 

Cheers

 

Bryon

 

 

[old man emu]

OK. Just back from Bunnings (well I had to leave ... the cleaners wanted to lock up and go home!)

 

Aluminium Weather Flash: 10M x 150mm x 0.7mm $33.90

 

SUNPAL PVC roll 0.8mm thick , 1 metre wide, $27.30 per metre.

 

I haven't priced Lexan. I don't think the flashing would be stiff enough to bear up to hangar rash.

 

I'll have a go at drawing a shape to see how wide a strip I would have to cut to form the piece.

 

OME

 

 

[Litespeed]

Pretty crap at making a drawing for the computer but will have a go.

 

Once the alloy is formed into a relatively tight curve- as in a fairing- it is actually very stiff and should cop any hanger rash.

 

 

[sfGnome]

I wonder... What if you used the Al flashing to make the shape, and then fixed it to the strut by filling the space between with expanding foam? Block off both ends while you're filling it so that the foam expands against itself and fills the whole area evenly. Not only will it hold the fairing in place, but it will protect it pretty well from dents too. Not sure about the weight, but I can't imagine it being too heavy.

 

 

[old man emu]

Here's a drawing of a high aspect ration streamlining with the tube show. The tube is supposed to be 3/4". Using the length and width, the aspect ratio is 4.3.

 

 

I noticed when reading one source that a cylinder has a coefficient of drag of 1, and a streamlined tube has a coefficient of drag of about 0.45.

 

OME

 

 

[old man emu]

OK. I took the dog out for a walk this morning and used the time to work on the ins and outs of making these fairings.

 

I'll test some of my theories today and if all goes well, "Making Streamlined Tube Fairings" will be the topic of the next "What has Old Man Emu Scratched up?"

 

OME

 

 

[facthunter]

Most of the benefit is by altering the trailing profile of a round tube. Some of the foams dissolve with almost any solvent . If you cover the load carrying tube you lose the ability to inspect that section easily. Attaching the extra pieces can cause surface fretting of the Al tube . Nev

 

 

[facthunter]

Why don't you make the struts of wood to an airfoil section fabric covered. Taper the ends down and fit them into a conical sheet steel piece that threads onto a high tensile rod that passes through the strut end to end attaching to a similar fitting the other end. This gives a high strength in tension and a good compressive strength too and would look very authentic. Nev

 

 

[djpacro]

The plans of the plane I want to build call for the interplane struts to be constructed from 3/4" round tube ........... I thought I would use streamlined 4130 tubing, but that stuff doesn't have the strength of round tube.

Easy enough to work out what size & thickness of streamline tube you need for the same strength as the round tube (of course it would be heavier). The Pitts, for example, has streamline tube for the cabane struts and, for some models, the aileron slave tubes.

..... then streamlined with a balsa fairing. Since balsa is not cheap....

Streamline tube is a lot more expensive than round tube so I'd be surprised that the balsa design would be more expensive than it. I wonder if the balsa design is any more expensive than other solutions discussed here (even if nowhere near as elegant).

[skeptic36]

Wouldn't the foam idea make the struts much thicker thereby increasing drag?

 

Maybe you could ask airborne where they source the fairings they use on their late model fast wings

 

http://www.airborne.com.au/images/galleries/XT912-SST/pages/IMG_8377.htm

 

Regards Bill

 

 

[old man emu]

Wouldn't the foam idea make the struts much thicker thereby increasing drag? Bill

The idea of the foam formers is to have something to shape the PVC film around, and to provide a means of attaching the fairing to the round tube without having to weaken it by drilling holes for screws.

 

I've done the drawings so that the widest part of the fairing is only 1/4" thicker than the tube (1/8" on each side). That's not too much considering the benefits of streamling. Also, the streamlining is designed as a symmetrical aerofoil so the flow of air around the fairing will not break up and cause the eddy on the downwind side thaat you et with a cylindrical tube.

 

DJP

 

Yes, there are tables which give the corresponding streamlined tubing size for a given round tube size, but as you say, the streamlined tube is heavier and more expensive than round tube. The idea is to reduce drag without a substantial increase in weight.

 

Cost is a big factor in the completion of a building project. If I had boundless amounts of money to throw at a plane, I'd go out and buy a factory built one. But since that is not the case, I'm looking at every way I can reduce costs and still be able to produce a quality plane at the end.

 

OME

 

 

[Guest DJH]

Just wondering if you've priced the balsa, there is a company in S.A. that supplies planks of balsa for surf board builders. Might be an less expensive way to buy the stuff. I am in no way associated with the company, here's the link.

 

http://www.balsacentral.com/epages/auszacdb.sf/?ObjectPath=/Shops/balsacentral

 

 

[sfGnome]

Wouldn't the foam idea make the struts much thicker thereby increasing drag?

I recall seeing a video demonstration (might have been on this site - I really don't remember) showing that a streamlined strut could be many times the diameter of a round one before the drag was equivalent. However, I also can't remember how many times more it was, so just call me Forgetful Jones...