# Are tube spars an insult to the engineering community?

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#### TFF

##### Well-Known Member
For Grumman it would not just be skilled labor. You need a shop station, which means more building overhead, tooling, which has to be production certified, and a larger cash of material sizes to keep in inventory. It’s not just saving $140 per hour, probably closer to$1400 an hour.

The biggest issue with an AA1 is its certified. Having put a 160 hp on one that now has duel G5s, GNS430, and some digital engine instruments, RV wheel pants, it’s not as good as an RV6 only because the STC limits prop pitch, and that limits speed. Fuel tank capacity is not good either. Stuck at 130 kts instead of closer to 150. The owner believes in certified, that’s why he doesn’t have an RV6 for about the same price. It’s probably the most expensive AA1 in existence.

#### BrianW

##### Well-Known Member
This thread concerning round tube spars reminded me of a WW2 expedient.
A round tube was shown deformed to a shallow figure 8 in a spar.
I doodled the shape and found that the depth was reduced. This would seem to be weakening?
I notice too, that deforming the circular section to a rectangle provides an increased depth - which is disproportionately stiffer.

#### cluttonfred

##### Well-Known Member
HBA Supporter
FWIW, I think the entire premise of this this thread is misleading. Structural efficiency is only one of many factors to consider when looking at what makes good design or engineering. Personally, I have always admired the simplicity and ease of construction and repair of the simple, bolt-together type of tube and fabric ultralight like the Best Off Sky Ranger and similar designs. Different strokes....

#### TLAR

##### Well-Known Member
The Skyranger is a well designed aircraft, the company has really worked hard to sex it up, see website for updates

#### stanislavz

##### Well-Known Member
HBA Supporter
Personally, I have always admired the simplicity and ease of construction and repair of the simple, bolt-together type of tube and fabric ultralight like the Best Off Sky Ranger and similar designs. Different strokes....
I have it in my shed. It is really simple to assemble/repair. You need only some wrenches and flat table. And it looks ok, especially Nynja or Cheetah derivative.

But ! Cover for wings is more than 2.5k euro. X-lam is even more.. And it is expendable.

And if fuselage is nice and sound, as i look into wing tubes, i really do fell, where is a better soluyion.

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#### TLAR

##### Well-Known Member
Do you want to sell it?

HBA Supporter

#### Gregory Perkins

##### Well-Known Member
This thread concerning round tube spars reminded me of a WW2 expedient.
A round tube was shown deformed to a shallow figure 8 in a spar.
I doodled the shape and found that the depth was reduced. This would seem to be weakening?
I notice too, that deforming the circular section to a rectangle provides an increased depth - which is disproportionately stiffer.
Square vs Round tube of same depth and weight .... So it is assumed the square is stronger even though the wall thickness would have to be less because proportionally more material is at the extreme top and bottom. I can see that but what if the failure on testing was in the shear web area because it was now thinner in the square example. I would think that the failure mode would be in compression and the thickness of the wall tubing exponentially affects the stiffness so if failure is in compression by buckling, perhaps the stiffer walls of the round tube could be stronger overall in bending compared to square tube of same weight but with thinner wall sections..... There are other considerations.... would be hard to use the square tubes in the leading and trailing edges of ladder type UL planes.

#### stanislavz

##### Well-Known Member
HBA Supporter
There are other considerations.... would be hard to use the square tubes in the leading and trailing edges of ladder type UL planes.
It is not good solution anyway. Kitfox for its later variants, uses leading edge former, to make proper airfoil.

Or even full nose covering:

In my conclusion - if tube to be formed as D edge accurate to airfoil shape - it is ok. But if it is only a tube - it do not have its place in ladder styled wing.

#### stanislavz

##### Well-Known Member
HBA Supporter
Could not resist to add this photo from kitfox too (Skyranger also do have sleeved tubes near struts mount) :

The question is - how far they will go with current ladder type of wing ? Next step - is full composite/metal skin ?

#### TLAR

##### Well-Known Member
Aluminum and steel tube are wonderful materials. Proven and easy to produce a fuselage. The cost is driving folks to Carbon

#### pictsidhe

##### Banned
Rigid skin is hard to do at low wing loading. The need for hangar rash resistance drives the thickness and weight higher than aero loads dictate.

#### stanislavz

##### Well-Known Member
HBA Supporter
Rigid skin is hard to do at low wing loading.
It is. But at weight of ladder tubes, you are able to build rigid skin D tube for 1/3 of chord at least. And if you subtract 1/4 of aileron/flaps from total chord - you are left with 45% of chord to cover.. Fabric cowering opt to cover full wing, rigid skin - only rear part. Regardless of aluminum (0.4mm) or composite (200gsm/foam/200gsm or 450 gsm + more ribs), it will be at 1-1.2 kg per m2 area. Which is for 11.5 m^2 wing (minus fuselage area etc.. ) area 9-10.8 kg - minus weight of fabric cowering..

#### wsimpso1

##### Super Moderator
Staff member
Log Member

To go back to the OP's question, this engineer:
• Is not offended by tube spars;
• Recognizes the trade of simplicity for weight gains can be useful in externally braced wings;
• Wants everyone to know how big the weight penalty is of straight spars in cantilever wings;
• And of course, wants everyone to keep firmly in mind that WEIGHT IS THE ENEMY.
Billski

#### pictsidhe

##### Banned
Are you still going on about weight, Billski?

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Are you still going on about weight, Billski?

If Billski ever STOPS talking about weight, it will be a great loss to the aviation community. One of the things that has been shown many times is that it is pretty easy to make an airplane out of commercially available "Home Depot" materials, concrete, EMT, wafer board, rebar, MDF... on and on.

Making it actually light enough to actually fly using these materials is not so easy

But to make it light enough to actually fly well is starting to be quite a challenge, even for engineers.

It's actually hard enough to make a legal ultralight out of certified aircraft materials.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Are you still going on about weight, Billski?
There are new folks coming on here all the time that treat weight like it is no big deal. So of course I am going to remind us of it frequently. Besides, I like saying it.

Billski

#### stanislavz

##### Well-Known Member
HBA Supporter
There are new folks coming on here all the time that treat weight like it is no big deal.
There is lak-16 glider, which officially have 3 different types of wing - plywood D nose, fabric cowered, rib less foam cored, foam cored D nose, fabric rear part. Will make bigger article on it later.

Its max speed is in two numbers area. And cored wing was smaller one one from all three 9 square meters, best surface, but bad handling on rainy/dust period. Bigger on was first one 12. Latest one was in between.

Weight and hangar/school rush - second one was good, third was ok.

Weight was in similar region..

#### pictsidhe

##### Banned
I'm just jesting. Yeah, we have way too many noobs who think they can TLAR a plane, making it 'plenty' strong enough without any maths, and have it work.

Nope. That doesn't work. Not the first time, or the second or third. If you have designed a bunch, you will get an idea of your starting point. You are also adept at the maths by then, so you tweak it anyway.

You need to optimise the everything or the plane will be a turkey.

Maybe we should make a sticky? It should include basic structural design, too. I know I'm not the only one wincing when someone announces they're going to use cheapo PVC pipe spars and 'they should be plenty strong enough'

You can't eyeball strong enough AND light enough.

#### stanislavz

##### Well-Known Member
HBA Supporter
You need to optimise the everything or the plane will be a turkey.
You cant do that on all parts - you have finite number of materials gauge to use on you project.