Grumman Tubular Spar... Why?

Velocity173 said:
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.
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Baloney. I've got 500 hours in one. It's within reason. Just like Piper and Cessna "book numbers" are a knot or two off. LOL
 
Capt. Geoffrey Thorpe said:
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.
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Maybe the laminar flow wing design played a factor in the decision. Not much room for a box.
 
Pilawt said:
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.
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Yes. Some people feel the same way about the Betamax ;)
 
Glen R said:
Maybe the laminar flow wing design played a factor in the decision. Not much room for a box.
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The overall height of the spar and the thickness of the spar caps are the critical items in sizing for bending moments. An I beam can be shorter and lighter than a tube spar to meet the same bending load requirements.
 
FastEddieB said:
I flight planned for 132 kts @ 10 gph.

That's close to Arrow cruise speed, right?
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But an Arrow looks/feels more like a real airplane, with the retracts and the blue lever. This is a big thing for some people.
 
One of the purposes of the tubular spar was to accommodate a deicing modification involving two hairdryers and inverters providing hot air to the leading edge through ducts off the spar.
(anyone remember that thread, well before xenforo)
 
Let'sgoflying! said:
One of the purposes of the tubular spar was to accommodate a deicing modification involving two hairdryers and inverters providing hot air to the leading edge through ducts off the spar.
(anyone remember that thread, well before xenforo)
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Redneck bleed air?
 
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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A tube of constant diameter is a great choice for a spar... if your intention is to fill it full of avgas and use it as a fuel tank, which is what Jim Bede did for the BD-1, which became the AA-1, which became the Tiger.
 
I think the constant diameter had more to do with making the wings interchangeable than any other reason.
 
An I-beam is more efficient than a tube in bending loads, but a tubular spar may offer benefit with wing twist. I-beam designs often requires a front and rear spar. However, the decision to use the tube spar was probably driven by cost and simplicity rather than wing twist considerations.
 
asicer said:
A tube of constant diameter is a great choice for a spar... if your intention is to fill it full of avgas and use it as a fuel tank, which is what Jim Bede did for the BD-1, which became the AA-1, which became the Tiger.
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The AA-1 did not become the Tiger. The Cheetah and Tiger derived from the Traveler which was designed from the beginning as a four place airplane. The two seat Grummans used the spar for fuel. The AA5 series used wet wings, not the spar.
 
If you've ever looked in the leading edge cavity of aircraft that have wing ice protection, lift augmentation, etc., imagine how a tube spar would complicate the installation.
 
SoCalPilot88 said:
An I-beam is more efficient than a tube in bending loads, but a tubular spar may offer benefit with wing twist. I-beam designs often requires a front and rear spar. However, the decision to use the tube spar was probably driven by cost and simplicity rather than wing twist considerations.
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That tube spar takes all the lift, drag and torsion loads. All of them. There are no secondary attach points as with many other single-spar designs. Nothing inside the root fairing but flap and aileron torque tubes and fuel lines and wiring and the one spar.

There's an AD on the spar attach system: http://www.airweb.faa.gov/Regulator...3464c2386257065005d5abd/$FILE/95-19-15 R1.pdf
 
Glen R said:
The AA-1 did not become the Tiger. The Cheetah and Tiger derived from the Traveler which was designed from the beginning as a four place airplane. The two seat Grummans used the spar for fuel. The AA5 series used wet wings, not the spar.
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True. But many of the design principles carried forward from the AA1 to the AA5 including the tubular spar, albeit without doubling as a fuel tank.

edit: I'd also remind you that the AA2 Patriot used a laminated channel spar instead of the AA1 and AA5 tubular spar. Presumably the AA3 retract and AA4 twin would have also used the same spar arrangement as the AA2.

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Glen R said:
Baloney. I've got 500 hours in one. It's within reason. Just like Piper and Cessna "book numbers" are a knot or two off. LOL
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I have 1,000 hours in a Tiger, and I flight planned for 135 KTAS, and that was flying less than 75% typically.
 
asicer said:
edit: I'd also remind you that the AA2 Patriot used a laminated channel spar instead of the AA1 and AA5 tubular spar. Presumably the AA3 retract and AA4 twin would have also used the same spar arrangement as the AA2.
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True. The AA2 was designed from the git-go with retractable and twin-engine derivatives in mind. They doubtless anticipated that while the tubular spar was a simple, effective solution for a light, fixed-gear single (especially when it could double as a fuel tank, as in the AA-1), it would not be feasible to attach a retractable landing gear or wing-mounted engines to it. That's also why the AA2 had a conventional, steerable, oleo-shock-absorber nose gear, instead of the AA-1/AA-5-style castering nose gear.

It certainly wasn't a very pretty airplane.

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Pilawt said:
True. The AA2 was designed from the git-go with retractable and twin-engine derivatives in mind. They doubtless anticipated that while the tubular spar was a simple, effective solution for a light, fixed-gear single (especially when it could double as a fuel tank, as in the AA-1), it would not be feasible to attach a retractable landing gear or wing-mounted engines to it. That's also why the AA2 had a conventional, steerable, oleo-shock-absorber nose gear, instead of the AA-1/AA-5-style castering nose gear.

It certainly wasn't a very pretty airplane.

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Probably looked better with its skirt(s) hiked up.
 
Slightly off topic. Saw a gorgeous Grumman Cougar out at the airport yesterday. Got my ME in one. Sad there aren't more of them.
 
Simple question, If the market for this aircraft was as a quick little aircraft, why didn't the manufacturer put a 180 horse engine, constant speed prop and retracting gear on it ?
I believe we would have had a real pace setter and best seller.
But no. :(
 
Never have seen the marketing materials for the Tiger, but I would think the approach was: "...get the performance of a complex (i.e. retractable) airplane in an aerodynamically clean SIMPLE airplane with an 180HP 0360".
At least that's what the appeal was/is to me..
 
wrbix said:
Never have seen the marketing materials for the Tiger, but I would think the approach was: "...get the performance of a complex (i.e. retractable) airplane in an aerodynamically clean SIMPLE airplane with an 180HP 0360".
At least that's what the appeal was/is to me..
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High performance, sportiness, but with simplicity, was the key concept in Tiger marketing. With the tubular spar, fuselage-mounted MLG and castering nose gear, it would have been tough to make a retractable out of the AA-1 or AA-5 series.

It was rumored that they were testing an aerobatic Tiger, and there was talk of a larger engine and C/S prop, but nothing ever came of either. The airplane did fine as it was. Plus, it was a small company without a lot of capital for R&D. That's ultimately why the all-new AA-2 was dropped, and instead they developed a four-place line out of the existing AA-1. Eventually Grumman-American was bought out by Allan Paulson's American Jet Industries -- but he wanted it for the "Grumman" (Gulfstream), not the "American" (the light singles), and promptly terminated lightplane production in 1979.

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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.
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Yup. For a given mass of aluminum, forming it into an I-beam will carry more bending load than forming it into a tube. This is engineering 101, for greatest moment of inertia, distribute your material as far from the centroid as possible, for the given loading direction.

However, forming it into a tube spar instead of a more typical built-up I-beam spar requires fewer detail parts, fewer fasteners, fewer manufacturing operations, and less labor. A tradeoff of weight for cost.

The biggest issue I'd have with a tube spar is hidden corrosion. ...from the inside out.
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When you pull the wingtip off the Grumman, the inside of the spar is easily inspectable. I have heard of some minor corrosion on the ID, but never anything that could cause the spar to be scrapped. It is also really easy to fog with your preferred anti-corrosion treatment.

The really important place to inspect the Grumman spar for corrosion is directly below the wing walk, on the outer top surface of the spar. 40 years of fatass pilots climbing in, and the fiberglass wing-walk fairing sagging from solar heating, can cause the wing walk to remain in contact wih he top of the spar. Water can get trapped between the surfaces and start corrosion. Easy to inspect for if you know to do it, there is a large inpection panel right below the problem area.

I have heard of a few center spars being scrapped for this reason, and there are currently no new ones available. The factory/ type certificate owner has talked about a production run, but expects they would need to sell for $12,000 to be profitable in the expected sales quantity... as of August 2014.
 
Glen R said:
Fly an Archer, 172 and a Tiger. Then get back to me.
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That depends entirely on how you define "Performance". Between these three, the Tiger will be fastest, the Archer will carry the greatest load, and the Skyhawk will take off and land from the shortest runway. What is the mission of the day?
 
The useful load on the Tiger and Archer are very close. I carried 4 in the Tiger on a regular basis. The 172 can use shorter runways but 4 people? Good luck. Apples and oranges. Flying qualities, Tiger hands down.
 
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