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Are tube spars an insult to the engineering community?

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BJC

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The T/S-18 stabilator uses an aluminum tube for the spar.

The Evans VP’s use an aluminum tube for the spar of the flying rudder.


BJC
 

Gregory Perkins

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It looks [to me] like the spars are rectangular bar?!
Here is the detailed picture I meant to send. The built up "beam" has round tubes top and bottom with round tube shear web bent and smashed so to allow riveting to the top and bottom tubes. This was a very early 80s design and the problem at SunFun was that it could not fly all with the other ULs during the UL Show Case where none of the ULs were supposed to pass each other. In 80 and 81 etc lots of the ULs were still flying 15 and 20 hp motors... This was before Rotax. The empty weight was 188 and max speed was 85 with a 25hp motor. Take another look at that spar and MARVEL. I once talked to the designer and at the show
he and his partner would take turns doing pull-ups on the wing on each side of the plane at the same time.... or so he said.....
 

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rv7charlie

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I've got your proper engineered tube spar, right here. ;-)
shop counterweight (Medium).jpg
That's the counterweight arm for a 10' X 30' one piece door. The 6" irrigation tubing had a bit more flex than my redneck TLAR engineering felt would be safe. So there's a 10' run of 5 quarter X 6" treated deck board on edge inside, centered at the top hinge point. If memory serves, there's about 90 lbs of lead & steel up at the top. The door is classic tube (irrigation) and gusset (road signs) construction. I could write a book about the things that are wrong with it. An actual 2nd year engineering student could probably fill a semester. But it's survived about 15 years, so far. The bigger door in the background actually had some calculations (back of envelope) go into it.

Works fine to put a plane in; not recommended to put in a plane.
Charlie

doors (Medium).jpg
 

mcrae0104

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I expect there are several unspecified assumptions among those making the negative statements about tube spars. I have the following questions.

What are those assumptions?
1) The ratio of moment of inertia to weight per unit length should be optimized.

2) Moment of inertia should be tailored relative to bending moment at each station.

It is recognized that other factors such as ease of assembly and availability come into play. Because design requirements and the differing value each designer places on weight, ease of assembly, availability, and willingness to learn basic engineering principles (Fb=My/I) varies, different solutions may be more or less appropriate. YMMV.
 

BJC

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I've got your proper engineered tube spar, right here. ;-)
View attachment 102359
That's the counterweight arm for a 10' X 30' one piece door. The 6" irrigation tubing had a bit more flex than my redneck TLAR engineering felt would be safe. So there's a 10' run of 5 quarter X 6" treated deck board on edge inside, centered at the top hinge point. If memory serves, there's about 90 lbs of lead & steel up at the top. The door is classic tube (irrigation) and gusset (road signs) construction. I could write a book about the things that are wrong with it. An actual 2nd year engineering student could probably fill a semester. But it's survived about 15 years, so far. The bigger door in the background actually had some calculations (back of envelope) go into it.

Works fine to put a plane in; not recommended to put in a plane.
Charlie

View attachment 102358
Well that certainly discourages low fly-bys.


BJC
 

rv7charlie

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Nah; it's stealth. At least compared to the trees that are 20' taller and 50' closer to the runway threshold.
 

Victor Bravo

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The tube spar is indeed an insult to the engineering community.

The engineering community is an insult to the sales and marketing department. The sales and marketing department is an insult to the management C-suite. Management is an insult to the regulatory body. And so on...

But the tube spar is sometimes a very clever solution to a whole bunch of different needs by many different stakeholders.

But IMHO the engineering community (of which I am not a member unfortunately) should not be insulted by cleverness, they should be inspired by it.

If the boffins can come up with a better solution for the same set of cost/complexity problems, then they can pee on extruded tubes all they want :)
 

Tiger Tim

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Contrary to popular opinion, the Gossamer Condor used an aluminum main spar, chemically etched in a pipe to thin the outboard sections.
IIRC they even managed to etch a taper in the wall thickness by standing the spar on end and either slowly filling or slowly draining the etching acid. I forget which.
 

TLAR

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Raceair Skylite requires a 12ft, 21/4” .083 wall 6061-T6 main spar, $390 ACS.
I believe you could make one with carbon fiber at the same wall thickness cheaper
 

GeeZee

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I have the lil Bitts plans and its true that spruce sure isn’t giving away that alum tubing. Over 800$ for bitts spars (175$ of it for shipping!). I noticed that .083 wall is 32$/ft but If you go up to .125 wall it’s only 7.96$/ft. that change will cost you about 2 1/2 lbs but save considerable $$. Personally, I’d build a Skylite as light sport anyway so the small weight penalty would be ok.
 

TFF

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That’s another sliding scale. Performance vs Cheap. Ed Fisher is into performance. The cost to him was not as important to making the best he could make. Also consider that what may be a popular size 30 years ago when he designed it may be out of vogue now. It might have been cheaper even. My guess is he went for weight. He was a pylon racer in the day and is a perfectionist.
 

GeeZee

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So true, but probably the biggest factor in his design choices is the Skylight was designed to be a true legal ultralight so every ounce mattered. For me personally if I build one I’m gonna slap an N number on it even if it weighed 258#.
 

trimtab

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Any strutted wing has large compression loads. It's a simple exercise to calculate the comp loads on a Cessna, for example. You won't forget about what is happening just above your head in a tight turn ever again. Hang gliders have tubular members for several good reasons. They have to sustain loads in a lot of directions from setup and ground handling, for example. My gliders were all at least +/- 10g ships, and it was literally impractical to get anywhere close to that physically: the L:D max was 11:1 at 1g, and flex at a load of 2 g's hopelessly pushed that down to about 6:1 or so when thermaling at 60ish degrees. The tubes were simple to inspect and replace from landing dings, etc. And the wires meant significant comp loads while the tubes are flexed rather significantly to form the leading edge shape for example, which also cause large torsion loads on the leading edge tubes andarge compression loads on the cross tubes.

From an airplane standpoint, if cost is the issue, labor is a key driver for commercial production. If 50 lbs is thrown away to save 30 hours of $140 skilled labor, and burdened with a margin and turnover costs, then the choice might become interesting. It was for the Grummans somehow.

My interest in the tubular spar has been for a design I completed for a model. The model changes angle of incidence from 0 to 45 degrees. The two wing mounted motors change angle by 15 degrees. The model takes off vertically at somewhere between 35 and 40 degrees of incidence (it depends on a number of things). The model only controls the incidence at the moment and not the motor angles. As such, it can take off vertically in one configuration and fly at roughly 30-35 mph, or take off conventionally in another config and fly at just over 100mph (measured via radar gun). The idea to use tube spars in a full size aircraft is to allow the incidence change and load vector changes easily. An I -beam or other webbed design required a design that became less attractive fast, and a circular tube was a way to solve structural and motion issues at once. The model has a mechanism that fixes motor angles for each flight while the wing is moved with another axis (using 6 axes). Moving the motors requires another model and mechanism, and it is my fall project.

The change in force vectors in the design for the wing makes the extra weight in a tubular spar less wasteful.
 

Riggerrob

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The T/S-18 stabilator uses an aluminum tube for the spar.

The Evans VP’s use an aluminum tube for the spar of the flying rudder.


BJC
That is because Volksplane rudder tubular spars serve two functions: structure and hinge. Using one component to serve two functions often simplifies construction and weight.
For the same reason, many kit-planes - with all-flying horizontal stabilators - use tubular tail spars (e.g. Zenith 601). Even the fanciest CF-18 and F-22 horizontal stabilator spars taper to tubular where they join the fuselage wall. They slide into round bearings that translate loads to the fuselage walls.
 
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