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Are tube spars an insult to the engineering community?

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Jay Kempf

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I never said it would be lighter. And it isn't the same price by a long shot. If you have ever sourced thin wall accurate aluminum extrusion, single cell, double cell you would know. Bede had a non-standard aluminum extrusion, somehow talked Grumman into a contract and ended up getting a large amount of stock of 2024 thin wall, drawn over a mandrel, DOM, for his homebuilt projects for cheap. So that is how it was made accurate. The spar was ID sized and the spar stub was OD sized. Doing a custom D shape wouldn't even be stable in bending and it would have to be multicell. How you would size it for a wing joiner is another animal altogether. Apples and oranges. You comparison doesn't exist. Comparing complexity to lightness has other assumptions.
 

Jay Kempf

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I work with composites almost daily. 24 hour bond cycles, clamps, tables to work on that are straight and air tight, vacuum processes, yadda... and consumables, how can I forget filling dumpsters full of consumables. I yay yay...

Gluing two sticks onto a thin web is easy but it isn't the point. The Bede wing was like 12 parts per side and two hours of work, virtually no tools other than a caulk gun for the adhesive, some pop rivets, a 2x4 through the tube and two saw horses, oh and some string. Not 60 pinch clamps per bond and wait over night, and the parts came in the kit that way. No prep. Hose clamps held the rib sections to the spar for curing. He actually specified Proseal. MMA's would have been a better choice, but wait, it was 1970. I am not sure MMA's existed back then or even modern heat cured resins. The ribs sections were Polyester. My head goes all swimmy just thinking about molding with that stuff.

Like I said before, if it had held fuel better it would have been genius. Want more fuel, cut slosh holes and drain back holes in another foot of wing.

I cannot and will not defend Bede for the record of his career. I have worked for dreamers. One notable one recently. But this overall concept was a really sound concept and it has stood the test. Still flying, lots of them, carrying good loads to real speeds, for decades. Lots of Grumman examples out there doing the same. So let's not say the spar of the design was optimized for weight or structural perfection. I am not saying that.

But to hate on a properly design load carrying tube is just narrow sighted. Too many examples out there to discount the whole category. I am using readily available carbon tube in tube wing joiners on my large UAV project cause they are cheap and work perfectly. Round is easy to size fits. Easy to manufacture, simple, easy to drill and mold sockets to bond them into. So it is the right compromise. Could I transfer mold a funky shape cause it would be lighter, yup, would it be a better design, not likely. And I would do it on a full scale design if it worked and the materials and processes made sense and were available and simpler than the alternative.
 

wsimpso1

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The title of this thread (Are tube spars an insult to the engineering community?) is taken from a post in the "Prandtl lift distribution for conventional configurations" thread?

This inference has been mentioned several times in various areas. I think the subject needs more discussion, in particular in relation to a D spar.

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?

Straight constant wall tube versus tapered or stepped spar or variable wall thickness.

Assuming a cantilever Hershey bar wing what is the % weight penalty for a tube versus I beam?

How is this modified by a strutted wing?

Is the D spar designed as an I beam with a leading edge D for additional torsional stiffness?

If the D spar is constrained to the same material for the entire D is it still superior to the tube spar?

Does the tube versus I beam spar assume or require both a forward and aft I beam spar versus a single tube spar or Ultralight like leading edge and trailing edge tube spars?

How does a non Hershey Bar plan form change the results of the comparison? Taper, elliptical, sweep, etc. ?

How does airfoil pitching moment and / or twist affect the trade study?

All comments welcome.
Asking for an awful lot of analysis. Here is why most of us who could won't even bother:

Straight spars of any sort in a cantilever wing are about 2/3 excess weight;

Round tubes are inefficient beams, that is their weight to strength and weight to stiffness ratios are higher than other suitable beam designs:

Straight tubes are straight spars of inefficient design. Usually that is not a good idea because Weight Is The Enemy.

Now if you have enough weight budget to afford the extra spar weight, knock yourself out. But that could have all been extra payload or a lighter bird instead.

Billski
 

BJC

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Its much easier to critique someone else's work than to make all the decisions and trade-offs that go into a design. The tubular spars in Bede aircraft may not be optional from a weight perspective, but they met the objectives for those airplanes. And there were quite a few aircraft with tubular spars that were built as TC’ed aircraft and as E-AB aircraft. Probably many more than all the airplanes built from all the designs of those of us who participate in HBA.COM.

My answer to the OP question is “no”.


BJC
 

Hot Wings

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but they met the objectives for those airplanes.
Bean counter override formula ... (mu^x)/n which should be as low as possible.
x>1*
n= Need
mu = Money Unit
n to be sourced from the marketing staff or, if your own project determined by your personal bank account due to fixed mu value.

* good scroungers can use a value for n <1
 

Vigilant1

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Not directly related: I recall reading about the British effort to develop radar in the days immediately preceding WWII. It is quite a story. One decision the team had to make was the frequency to be used. They had the technology to build relatively long wavelength transmitters and receivers, but they already had some promising laboratory tests indicating that they'd be able to build higher-frequency radar units that would offer better resolution and smaller antenna sizes--they'd be a lot better, and many members of the team wanted to put their efforts there. One of the lead scientists recalled that, given the events on the continent, they decided to freeze their design and begin rapidly producing fieldable low frequency equipment. This made all the difference in the Battle of Britain. It is difficult to overstate how differently things would have played out if the RAF hadn't had radar information.

As I remember his sentiment: "We went with what we had, because in these matters often the better hardware comes too late, and the perfect device never comes at all."

The tradeoffs may be in time, money, weight, complexity, or even safety. But they always exist.
 
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Mad MAC

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All too often engineering decisions can be best surmised as none of the good ides will work, which of the bad ideas do we go with.

I am surprised we don't see more tube spars in composite aircraft as a good quality source of filament wound tapered tubes are yacht masts (very popular for rotorcraft spray booms).
 

Victor Bravo

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Bean counter override formula ... (mu^x)/n which should be as low as possible.
x>1*
n= Need
mu = Money Unit
n to be sourced from the marketing staff or, if your own project determined by your personal bank account due to fixed mu value.
I believe that the plain language translation for this formula is" Shoot the engineer and ship the product."
 

wsimpso1

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Let's get into details. First off I have built highly tailored composite spars by both moldless and molded methods. Simple to tool/fixture, light, sturdy, straight, easily bonded to skins and the few ribs they need. I have also built a number of straight round tubes for use as push-pull rods for my control system. In each case they were in support of weight optimized systems. So, I know what I am talking about in construction and use of these gadgets...

Normally I preach min weight of the airplane as being critical. WEIGHT IS THE ENEMY in airplanes... It increases takeoff and landing speeds, increases runway needs, decreases climb rate and service ceilings, and adds to kinetic energy to be absorbed if you ever have a forced landing. Enough said about why?

Tube spar (which in all of the airplane experience I know of is an aluminum tube of constant diameter and wall thickness) in cantilever wings is a big part of wing weight. There are aluminum ribs to be aligned and attached, and alignment is critical to how well it will fly. A lot of ribs and adhesives. Besides the rib weight is all that adhesive weight, and then the little issue of aluminum bonding in a home shop being a life limited item. This is usually way less than optimal weight when compared to a tailored spar and riveted ribs.

Get to composites and well, I see only disadvantages. Even a tailored round tube spar built on a mandrel (even a foam core is a mandrel) is going to be both heavier and harder to fixture than a more traditional channel spar, so offers only disadvantages. No matter how you build the spar, a tube spar will be substantially heavier than the same material set in a channel or box spar. The beauty of even a simple mold built channel spar is the outside shape that has to fit the wing skins can be close to perfect, minimizing glue line thickness (this can be very large part of the assembly weight if the spar fit is loose). Try figuring all that out and getting it put together with a mandrel with a thin tube everywhere, then progressively more and more cap material and web material laid on as you go to the root end. Then in composite wings, we assemble primarily by adhesive connection between spar and skins - there are few ribs - so adhesive weights can be kept low. Think about how much adhesive would be needed to get a composite wing skin securely bonded to a round tube...

You need some really compelling reasons to take a bunch of performance out of your airplane. And in composites, round tube makes it no easier, no quicker all while adding to fuss and weight. Why?

Billski
 
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wsimpso1

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"Better is the enemy of Good Enough"
In metal, you take a measurable performance hit forever to save a few hours in the shop. Fixturing will have to be very carefully done. It might be a trade worth making.

In composites, you take a measurable performance hit forever that costs more and saves no time or effort in the shop. Just don't...

Billski
 

stanislavz

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Ultralight fabric skin?
I think it is only one viable solution - fabric skin and V strut braced wing. Kind of standard wing for ultralights in this part of Atlantic.

Good example is ikarus c42 and c52. First one was all bolted aluminium tubes - from fuselage to wings. It was ok looking plane and cheap. Still in production today.

When ikarus wanted to make it more competitive with composite airplanes - make c52 - but it was in a range of cheapest fully composite aircraft. But performance wise it was just a little better c42.. And as far as i know c52 silently disappear from market.
 

rv7charlie

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EDIT: corrections to my original post; I used Glastar instead of Sportsman specs in the original post.

Oooh, can I play?

Having owned a BD4 for about a year almost 20 years ago, I claim the right to have an opinion, even if it's wrong. ;-)

No argument about the structural inelegance. But...any RV-x, with a hershey bar wing? (I've owned two RV4s, I currently own a -6, and I'm building a -7.) Fixturing, especially for the -6 and earlier non-prepunched planes, certainly had to be carefully done. Building even the pre-punched wing is incredibly time consuming.
Constant chord wing is both structurally and aerodynamically 'inefficient'. Yet....the RV completions counter is approaching 11,000. While they did a lot 'wrong', they must be doing something right.

So, how big a compromise is the tube spar?
Specs of the original model, from the build book (with the acknowledgement that Jim Bede was often fraudulently optimistic, but see real world numbers, later):
original BD4
1200 lbs empty is a realistic number for one with a 4 cyl engine, built properly (light). Gross is 2000 lbs. The original is a '2+2'; not really a 4 seat plane.

A Sportsman is a much more recent and highly respected design, widely considered to be a 2+2, using what's supposed to be the most efficient materials choices for each part of the airframe:
Glasair Sportsman 2+2 - Wikipedia
Hmmm....1350 lbs empty weight. 2350 lbs gross. (standard category numbers)

800 lbs vs 1000 lbs useful load, but the Sportsman uses struts instead of a cantilever wing. Sportsman stalls about 10-15 mph slower (depending on individual builds & loading), and cruises about 20-25 mph slower (again, depending on build quality/cleanliness/loading).
One more edit: the current BD4C has a 1200 lb useful load and a *big* cockpit.

So, how much real world penalty for the whole a/c is there in the tube spar? Of course, you don't get to walk around, dodge and otherwise deal with that strut every time you try to move around under the wing. So I guess you lose all that flexibility training you get with the Sportsman. Oh yeah, you get to take up a lot more hangar space with the Sportsman, due to the extra span. My BD would easily taxi into the 30' width of my hangar 'lean-to' addition, while even the older Glastar belonging to a friend took up a big part of my 48' wide main hangar.

Here are extensive systematic test reports on an original BD4 from someone qualified to evaluate:
Ken Kopp's Calc Sheet
Ken Kopp's Flight Test Reports

My 180 HP, carb/mags, rather heavy, rather aero-dirty, trigear copy did a laid back 135 kts (155 mph) @ 10 gph. Empty weight roughly the same as a typical 2 seat RV, with gross weight is listed as 2000 lbs; quite a bit more than a 2 seat RV, and a *lot* more space for baggage, fuel capacity, and payload, if flown as a 2 seat a/c. Properly cleaned up copies are at least 30-40 mph faster than mine was. My one long cross country was from MS to OSH with a neighbor flying his 235HP C182. He carried a bit more weight. Both of us with wives; additionally, he had one 100 lb daughter and requisite baggage. Climb about equal. I could outrun him down low; above 8000 ft he could slowly pull ahead of me (longer wing).

Now, having said all that, I wouldn't have handed coffee money to Jim Bede and expected to see a cup of coffee afterward. A lot of people, including a friend of mine, lost significant money over a lot of years to his schemes. I hope his son is running the business in a more ethical manner.

Charlie
 
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WonderousMountain

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Bipandum plane !
I would like to get it down to 12ft tube spars
after the first larger surface proves concept.
Went with rectolinear forms to fight intersection turbulance.
octoplane - Assembly 1 Drawing 1.pngIMG_20200921_142628.jpg
 

TFF

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My mom could probably put an original BD4 together. It was designed for the person who’s tool box is in the kitchen drawer, a claw hammer, a Phillips, and a lost straight slot, in there with the pruning shears and a bunch of old mail. Simplicity is hard.

It was never meant to be end all be all airplane. My main library had the build book, and as a 12 year old building models, I knew I could build that. It was my first understanding of a homebuilt airplane.
 

rv7charlie

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Oh, I did forget one thing in comparing the Sportsman to the BD4C. You have to figure out what to do with that pesky $70,000 you have left after buying the kit.

And what to do with all the time you didn't spend building the wings.
 
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