Grumman Tubular Spar... Why?

USAF JD said:
The AA-5A have a thinner walled spar than the AA5-B. All the Grummans are faster than the same powered P & C, I was told this is due to the clean rivet free skin. Makes sense to me...
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It's because of the short wing.
 
asicer said:
"Hey Jim, we just got another airframe order. I'm gonna head over to Home Depot for some sewer pipe.... I mean wing spars."
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I've made many trips to Lowe's Aviation Supply myself. :)
 
PilotRPI said:
An I beam or box beam resists bending better than a hollow circle, for an equivalent cross sectional area,
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When it comes to aircraft structure "resists bending" often means " more prone to cracking". Why is tubing used in early aircraft design and construction ? Why are modern pressurized aircraft tube shaped? Why do hydraulic systems use tubing? Why do aircraft such as the 727 have tubular bolts holding the wings on? The hollow circle is proven historically, i.e. Roman archways.
 
GlennAB1 said:
When it comes to aircraft structure "resists bending" often means " more prone to cracking". Why is tubing used in early aircraft design and construction ? Why are modern pressurized aircraft tube shaped? Why do hydraulic systems use tubing? Why do aircraft such as the 727 have tubular bolts holding the wings on? The hollow circle is proven historically, i.e. Roman archways.
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None of those examples are relevant to spar construction.
 
Pretty simple, BeeDee used an irrigation pipe in his E-AB and it worked rather well. so American Yankee ran with it. off the shelf, at half the cost.
Remember the whole idea behind these was a cheap throw away trainer at first, It worked well enough they changed the idea.
 
Tom-D said:
BeeDee used an irrigation pipe in his E-AB and it worked rather well. so American Yankee ran with it. off the shelf, at half the cost.
Remember the whole idea behind these was a cheap throw away trainer at first, It worked well enough they changed the idea.
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Contrary to urban legend, the BD-1 (which morphed into the AA-1 Yankee) was never intended to be marketed as an E-AB or kit. It was always to be certificated and sold FAF ("fly-away factory") Springfield, Ohio. Later Bede projects (e.g., BD-4, BD-5, BD-10) were kits, but never the BD-1. See Post #6 above. In August 1963, Flying Magazine wrote,
NEW BD-1 LIGHTPLANE expected to be in production by year’s end at the Springfield, Ohio plant of Bede Aircraft Corp. Prototype airplane is now flying, with two additional being turned out (one of which will be utilized for static test for FAA certification). The small BD-1 will allow choice of four powerplants: three Continentals of either 65, 90 or 100 hp; one Lycoming of 108 hp. Basic price begins at $2,500 for the 65-hp model.
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BD-1_coverltr1.jpg BD-1_coverltr2.jpg
 
Tom-D said:
Pretty simple, BeeDee used an irrigation pipe in his E-AB and it worked rather well. so American Yankee ran with it. off the shelf, at half the cost.
Remember the whole idea behind these was a cheap throw away trainer at first, It worked well enough they changed the idea.
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The idea of the AA-1 was never to be a cheap trainer. The idea was to be a small aerobatic aircraft that would have removable wings, towed behind a car and could fit into a garage.

The tube spar was best for this concept in that it's design was simple, strong and could be removed easily. Same design concept Bede applied to the BD-5.
 
Pilawt said:
Contrary to urban legend, the BD-1 (which morphed into the AA-1 Yankee) was never intended to be marketed as an E-AB or kit.
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Where did I say it was? BeeDee's aircraft was better phrased as "proof of concept" I do not know if it was sold or not, wouldn't surprise me if it were in a collection some where.
 
Velocity173 said:
The idea of the AA-1 was never to be a cheap trainer.
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It sure as hell was, they were after the Cessna 150 market, and they put a big dent it it. Remember in that day every airport had a flight school and cheap aircraft were in big demand.
 
Tom-D said:
It sure as hell was, they were after the Cessna 150 market, and they put a big dent it it. Remember in that day every airport had a flight school and cheap aircraft were in big demand.
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Yes, the AA-1 evolved into a trainer but it's original prototype (BD-1) was to be an aerobatic aircraft with folding wings. The folding wing concept, along with aerobatic certification were abandoned after initial tests showed to be a poor performer. It was then redesigned into a trainer...keeping the original tubular spar.
 
Velocity173 said:
The idea of the AA-1 was never to be a cheap trainer. The idea was to be a small aerobatic aircraft that would have removable wings, towed behind a car and could fit into a garage.
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Change "AA-1" to "BD-1" and your statement is correct. Look at Bede's own brochure, reproduced below. Bede touted it primarily as a personal runabout and sport flyer, and only secondarily as a trainer. But while it was fundamentally a promising design, such details as aerobatic certification, removable wings, fantasy-world performance and bargain-basement price were all unfeasible. Bede was forced out of the project, and grown-ups took over the company and transformed it into American Aviation Corp. The reworked design, AA-1 Yankee, was a good performer (though well below Bede's wild claims) and fun to fly, and turned out to be a worthy head-on competitor to the C-150 as a fleet trainer. Not aerobatic, no removable wings; and price and operating costs were just about the same as a C-150. The FBO where I worked had several AA-1 Yankees, and I enjoyed instructing primary students in them.

BD-1_brochure2.jpg BD-1_brochure1.jpg BD-1_brochure4.jpg BD-1_brochure3.jpg
 
Dan Thomas said:
The BD-5 had the tubular spar. One of the guys that flew one of the BD-5J jets pulled too many Gs and bent the wings way up, and flew home.
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Hardly unique to the BD-5 or even tubular spars. People have been wings and continued to fly. TACA managed to bend a 737 and SWA is still flying it.
 
kyleb said:
None of those examples are relevant to spar construction.
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Tubes are used all over aircraft, obviously, by mistake... control mechanisms, drive mechanisms, landing gear struts...

If you'd ever worked around large aircraft you know that tubing is often used in hoisting equipment... by accident?
 
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GlennAB1 said:
Tubes are used all over aircraft, obviously, by mistake... control mechanisms, drive mechanisms, landing gear struts...

If you'd ever worked around large aircraft you know that tubing is often used in hoisting equipment... by accident?
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Tubes are totally effing fantastic for some applications. Great when loads can come from any direction. Best for resistance to buckling. Best when you have to deal with internal pressures. etc.

On the other hand, tubes are somewhere between fair and urine poor for other applications. For example, when the loading is known to be non-symmetrical (like a wing spar) tubes have too much material in all the wrong places and not enough in the right places. That's why you don't find tube spars in a 757. On the other hand, tubes are low cost COTS parts - that's you find them in hang gliders, Kitfox, etc. Like any engineering problem, you make compromises.


'splaining the loads in the spar in my ride (skip to the 4 minute mark)
 
SixPapaCharlie said:
I have been digging through the 330 NTSB reports on the AA5's and so far it seems like the predominate issue is fuel issues, and pilots veering off to the left of the runway. That really seems to have happened a lot in the reports. A lot of "Left tank was empty and fuel selector was positioned to the left tank" type stuff too. I haven't yet come across anything structural failing yet but I have only read about 40 of the reports.
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Tiger Flying made easy:

1. Have fuel and make sure selector is positioned to use it.:(
2. Don't try to land FAST ... if you do, look for the high speed taxiways to the left:confused:
3. At rotation, level in ground effect until you reach Vy. Trying to climb at initial rotation speed + a gust or shear = NTSB report:eek:
 
GlennAB1 said:
Tubes are used all over aircraft, obviously, by mistake... control mechanisms, drive mechanisms, landing gear struts...

If you'd ever worked around large aircraft you know that tubing is often used in hoisting equipment... by accident?
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The point the previous responder made was that none of your examples of use of "tubes" related to the loads and stresses applicable to a wing spar. For example, the fact an aircraft hull is a "tube" in order to efficiently contain the hoop stresses from internal pressure bears no relationship to the bending loads borne by a wing spar. Or the loads on a Roman arch - the benefit of which is primarily the ability to clear span a large dimension (the elliptical arch variation accomplishes this even better, and is no longer like a "tube"). :)

As Dr. Thorpe has posted, a tube of constant diameter is actually a poor choice for a spar...and that is why few airplanes have them.
 
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Tubes are great for tensile and compressive loads....bending not so much. Which is why most tube structures are trust like.

The biggest issue I'd have with a tube spar is hidden corrosion. ...from the inside out.
 
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GRG55 said:
As Dr. Thorpe has posted, a tube of constant diameter is actually a poor choice for a spar...and that is why few airplanes have them.
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And yet there has never been a spar failure in an AA5A or AA5B. Strange isn't it? As far as weight, the Tigers useful load isn't suffering, nor is it's performance. The obsession with the tubular spar is a waste of time. Overall, the Grumman line outperforms it's competition and does so economically.
 
Glen R said:
And yet there has never been a spar failure in an AA5A or AA5B. Strange isn't it?
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This is what I love about POA: a bunch of folks come and trash talk a design as being horrible and yet it hasn't actually proven to be a problem.
 
Checkout_my_Six said:
Tubes are great for tensile and compressive loads....bending not so much. Which is why most tube structures are trust like.

The biggest issue I'd have with a tube spar is hidden corrosion. ...from the inside out.
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As the Stinson and early pipers proved, with out linseed oil placed inside the tube during manufacturing, 15+ years later we see fractured tubes ADs on struts etc.
 
Glen R said:
And yet there has never been a spar failure in an AA5A or AA5B. Strange isn't it? As far as weight, the Tigers useful load isn't suffering, nor is it's performance. The obsession with the tubular spar is a waste of time. Overall, the Grumman line outperforms it's competition and does so economically.
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I don't find anything strange about it. It just means the engineers did their job. And some other part of that airframe will fail first when overstressed. Every airplane out there falls into this situation. Engineering involves compromises, and that is one reason airplanes ostensibly built for the same target criteria differ from one another.

Grummans are great airplanes, but to suggest the Grumman line "outperforms its competition" is a completely specious statement. Depends on how one wishes to measure "performance". Given they are no longer produced, while some of their competition survives, suggests they didn't outperform in enough respects, and by enough margin to matter.
 
Checkout_my_Six said:
The biggest issue I'd have with a tube spar is hidden corrosion. ...from the inside out.
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And that is why we have to x-ray the spar tubes on the Beech 18.

Fortunately, the linseed oil that Beech used to fill the tubes with during welding actually provides a pretty good protective coating inside.
 
Tom-D said:
As the Stinson and early pipers proved, with out linseed oil placed inside the tube during manufacturing, 15+ years later we see fractured tubes ADs on struts etc.
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When did they stop using linseed oil?
 
GRG55 said:
...the fact an aircraft hull is a "tube" in order to efficiently contain the hoop stresses from internal pressure bears no relationship to the bending loads borne by a wing spar.

As Dr. Thorpe has posted, a tube of constant diameter is actually a poor choice for a spar...and that is why few airplanes have them.
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You are sadly mistaken to believe there are no bending loads in an aircraft tube fuselage.

Tubing use presents challenges, i.e. attachment, and use of space, access to wing internal.

It would make no sense to have tube spar on aircraft with wing integral fuel tanks, which, most have.
 
GRG55 said:
Grummans are great airplanes, but to suggest the Grumman line "outperforms its competition" is a completely specious statement. Depends on how one wishes to measure "performance". Given they are no longer produced, while some of their competition survives, suggests they didn't outperform in enough respects, and by enough margin to matter.
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To say that airplane 'A' has more merit than airplane 'B' because 'A' is still in production and 'B' is not, or sold more units, ignores too many variables to be valid. If Grumman-American had Cessna's capital, management know-how, dealer and support network, marketing budget, and production facilities; and if 172s were built by a mom-'n'-pop shop that decided it wanted to ditch flying flivvers in favor of Gulfstream jets, then Tigers and Cheetahs would probably have outsold 172s since 1976.
 
GRG55 said:
Grummans are great airplanes, but to suggest the Grumman line "outperforms its competition" is a completely specious statement. Depends on how one wishes to measure "performance". Given they are no longer produced, while some of their competition survives, suggests they didn't outperform in enough respects, and by enough margin to matter.
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I sort of get your point, but aircraft manufacturers go out of business for all sorts of reasons*.

My favorite comparison is Tiger vs. Arrow.

About the same cruise speed, but the Tiger does it with fixed gear, fixed pitch prop and 20 hp less! I find that impressive.


*Ninja'd by pilawt.
 
Glen R said:
And yet there has never been a spar failure in an AA5A or AA5B. Strange isn't it?
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No. Not strange at all.

No one is saying that a tube doesn't work. Just look at your typical hang glider. Or a Grumman AA... Tubes can be cost effective, and can be made to work. But that does not make them the most efficient structural shape for this particular application. An engineer made a choice. That's what we do.
 
FastEddieB said:
I sort of get your point, but aircraft manufacturers go out of business for all sorts of reasons*.

My favorite comparison is Tiger vs. Arrow.

About the same cruise speed, but the Tiger does it with fixed gear, fixed pitch prop and 20 hp less! I find that impressive.


*Ninja'd by pilawt.
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I'd generally agree but most will tell you that Grumman was very "optimistic" with their book figures. I've got a friend who flew Tigers as a demo pilot and he said there's no chance of a stock Tiger getting anywhere near the 139 KTAS book number.
 
My Traveler punched a hole through the sky at 118 kts. Better than a comparable PA28 150 HP and more fun to fly too.
 
GRG55 said:
I don't find anything strange about it. It just means the engineers did their job. And some other part of that airframe will fail first when overstressed. Every airplane out there falls into this situation. Engineering involves compromises, and that is one reason airplanes ostensibly built for the same target criteria differ from one another.

Grummans are great airplanes, but to suggest the Grumman line "outperforms its competition" is a completely specious statement. Depends on how one wishes to measure "performance". Given they are no longer produced, while some of their competition survives, suggests they didn't outperform in enough respects, and by enough margin to matter.
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Fly an Archer, 172 and a Tiger. Then get back to me.
 
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