Grumman Tubular Spar... Why?

SixPapaCharlie

SixPapaCharlie

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I joined up on the Grumman Gang which led to GPA forum which is super quiet.

Are there any benefits to this style of wing spar?
Any drawbacks?

Just curious about why they did it differently.
I have started my research and this is an item often mentioned during inspections which this is probably true on all prebuys. The spar is not trivial.

My A&P went over a Tiger in annual and he showed me the ins and outs of the plane and said you have to pay close attention to this spar because if it has even a small dent in it, it's game over.

Not sure how one would get dented but back to the point... Pros and Cons to this tube?
 
A tube is much easier (think less expensive) to construct than a traditional spar (I-beam or box beam). The Kitfox I believe uses a pair of them, and they might be different sizes. An I beam or box beam resists bending better than a hollow circle, for an equivalent cross sectional area, but they are all reliable in terms of how to calculate strength and bending.

Also, I believe in at least some of the Grummans they use the hollow space in the tube as a fuel tank. Neat idea, as long as the fuel doesn't get stuck to the outside length of the spar in a spin and prevent recovery.
 
SixPapaCharlie said:
I joined up on the Grumman Gang which led to GPA forum which is super quiet.

Are there any benefits to this style of wing spar?
Any drawbacks?

Just curious about why they did it differently.
I have started my research and this is an item often mentioned during inspections which this is probably true on all prebuys. The spar is not trivial.

My A&P went over a Tiger in annual and he showed me the ins and outs of the plane and said you have to pay close attention to this spar because if it has even a small dent in it, it's game over.

Not sure how one would get dented but back to the point... Pros and Cons to this tube?
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Cheap
 
Great place to put fuel in that it's easy to see on the sight gauges.

Another thing I heard was if you spin the plane the fuel can end up moving to the wing tips, which isn't good.

You also can't stick the tanks like you can with cessnas and pipers, which isn't that big of a deal with how fail safe the fuel sight tubes are
 
Advantage: Cost. Easy to attach a curved wing skin.

Disadvantage: Strength to Weight.
 
The AA-5 series is derived from the two-seat AA-1 (Yankee, Trainer, Tr-2, Lynx), which in turn was based on Jim Bede's "Bede BD-1" which first flew in July 1963.

bd-1_2.jpg


Jim had -- um -- issues as a businessman and salesman, but he was an innovative designer. The BD-1 was originally intended to have folding wings that could be detached and folded back so the airplane could be trailered home, and the spar also did double duty as the fuel tank. With engines of between 65 and 108 hp, that would have been enough fuel capacity.

bd-1_6402b.jpg
bd-1_6402c.jpg


It took the departure of Bede from the project, and some tough decisions by the new management (American Aviation Corp., later bought by Grumman to become Grumman-American) to get the airplane certified and brought to market in 1968 as the American AA-1 Yankee. The tubular fuel tank/spar remained, but the wings were no longer removable, nor was the airplane certified for aerobatics as Bede had promised it would be. The AA-1 performed very well for its 108 hp, but nowhere near Bede's fanciful claims for the BD-1.

A longer version of the AA-1A wing was used for the four-seat AA-5 series, but the spar did not provide enough fuel capacity for the larger engines, so a conventional metal tank was fitted in the wing.

I'm told that the only structural difference between the 180 hp AA-5B Tiger and the 150 hp AA-5A Cheetah is that the Tiger's spar is made of slightly thicker metal. When production was winding down in 1979, about 30 Cheetahs were built with Tiger spars. Only problem is, nobody kept track of which ones they were. You'd have to check with a micrometer to know for sure.

I've never heard of any problems with the tubular spars; and certainly never heard of a dented one in an airplane that had not otherwise been significantly damaged.
 
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Why did they glue the seams rather than rivet? maybe a little thinking out side of the box, is there any know cases of the spar failing?
 
tecprotb said:
Why did they glue the seams rather than rivet?
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The intent was to reduce labor cost and eliminate surface irregularities and drag from rivets. Said a 1964 article,

"A wing panel is made by slipping five identical aluminum ribs over the cylindrical spar, attaching a trailing‑edge spar and wrapping around two sheets of aluminum. Ribs, skin and trailing edge spar are secured by bonding. With a fiberglass wing tip added, a smooth and aerodynamically clean wing (completely free of rivet and screw‑bead protrusions) is obtained."​

maybe a little thinking out side of the box, is there any know cases of the spar failing?
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Never heard of one.
 
Not only cheap to build but strong. Never heard of one failing. Fuel is only in the AA-1s. AA-5s have separate tanks.

Drawback? Not the spars fault but some have a brass landing gear bracket that attaches to the spar. If it's been in a corrosive environment, you better pull the fiberglass fairing and check that area. Fletch-air has pics of bad corrosion in that area and requires a complete spar retro fit.

Great little planes with light controls, superb visibility, decent speed and you can fly with the canopy open.
 
tecprotb said:
Why did they glue the seams rather than rivet? maybe a little thinking out side of the box, is there any know cases of the spar failing?
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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.
 
There's really no issue with the glue. Only ones that had dissbonding problems were the few made with the "purple passion" stuff. You'll know if is has it because you can see the purple glue in some of the crevices. Even then, most if not all of the purple glue planes have been repaired with rivets. If it's white, you're good.
 
SixPapaCharlie said:
A lot of "Left tank was empty and fuel selector was positioned to the left tank" type stuff too.
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Which is especially dumb because the tip of the fuel selector handle actually touches the gauge of the selected tank.

upload_2016-4-7_10-10-30.png

Create an idiot-proof system, and nature quickly responds with a more advanced idiot. o_O
 
That spar is very strong. There was an AG-5B I think it was that hit some severe wake turbulence and bent the spar but they landed OK. I think it's in the NTSB's.
 
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...
 
jkaduk said:
That spar is very strong. There was an AG-5B I think it was that hit some severe wake turbulence and bent the spar but they landed OK. I think it's in the NTSB's.
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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.
 
jkaduk said:
That spar is very strong. There was an AG-5B I think it was that hit some severe wake turbulence and bent the spar but they landed OK. I think it's in the NTSB's.
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NTSB Identification: LAX00LA035
Accident occurred Friday, November 12, 1999 in VAN NUYS, CA
Probable Cause Approval Date: 08/16/2001
Aircraft: American General Aircraft AG5B, registration: N1195L
Injuries: 3 Uninjured.

The outboard 1/3 of the aircraft's left wing was deformed upward about 5 degrees with respect to the inboard section during an encounter with wake turbulence while on downwind leg for landing. The upper wing skins of both wings exhibited compression buckling, and the mass balance weights on both ailerons were separated and fell from the aircraft. The pilot reported that the flight was unremarkable until, while downwind for landing, there was an instantaneous jolt of sudden severe turbulence that ended before he could take any action. Data obtained from the airport noise abatement office showed that 2 minutes 23 seconds prior to the encounter, a heavy transport aircraft passed over the location on an ILS approach to another airport 6 miles away; the transport airplane was about 500 feet higher than the accident airplane as it crossed the location. The air traffic control tower at the airport where the accident occurred made a wake turbulence cautionary broadcast to all aircraft 30 seconds prior to when the accident aircraft first contacted the tower for landing. The cautionary broadcast was not repeated.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:​
  • The pilot's encounter with unknown and unanticipated wake turbulence from a heavy transport aircraft, which was landing at another airport. A factor in the accident was the tower controller's failure to repeat a previously broadcast wake turbulence cautionary advisory when the aircraft later checked in on tower frequency.

And the bonded honeycomb cabin structure is very strong, too. I was training for my CFI ticket in 1970 at an FBO at Santa Barbara, CA. They had a couple of AA-1 Yankees on their rental line. A 17-y/o private pilot decided to try flying with the canopy full open (you're only supposed to open it 8 or 9 inches or so in flight). The canopy came off and lodged against the tail. The airplane came down into a stand of old oak trees near Solvang, CA. The only injury was a broken leg when he tried to get out of the airplane and fell out of the tree.
 
Pilawt said:
Which is especially dumb because the tip of the fuel selector handle actually touches the gauge of the selected tank.

View attachment 44936

Create an idiot-proof system, and nature quickly responds with a more advanced idiot. o_O
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A "gotcha" is the one-half mark on the fuel gauge is about 2/3rds of the way up the gauge. A quick glance may leave you thinking you have more than half a tank when you don't. I learned that lesson the hard way during my first year of flying.
 
Capt. Geoffrey Thorpe said:
Advantage: Cost. Easy to attach a curved wing skin.

Disadvantage: Strength to Weight.
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Actually don't think that is the case - my reading consistently shows tubular structures have more favorable strength-to-weight characteristics vs box structure. Google can be our friend.
 
bullwinkle said:
I was under the impression that the Grumman spars are extremely strong. A circle is one of the best shapes for resisting deformation in all directions.
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The loads in a wing spar do not come from all directions. And, I didn't say it wasn't strong, it is just heavy for the strength it provides for the bending moment / sheer load caused by lift.
 
wrbix said:
Actually don't think that is the case - my reading consistently shows tubular structures have more favorable strength-to-weight characteristics vs box structure. Google can be our friend.
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Tubes are best when you are dealing with torsion or compression. Wing spars are loaded primarily in bending in one direction - I or box shapes can be the lightest for this application.
 
I looked at the one in for annual. They have the interior out, That spar (running through the mid section is much bigger than imagined. It looks pretty sturdy.

This isn't it but one from google:
grummanstripped.jpg
 
wrbix said:
Actually don't think that is the case - my reading consistently shows tubular structures have more favorable strength-to-weight characteristics vs box structure. Google can be our friend.
Click to expand...

Google can be your friend but in this case your friend is speaking of generalities and the discussion is about a wing spar.
 
Capt. Geoffrey Thorpe said:
Tubes are best when you are dealing with torsion or compression. Wing spars are loaded primarily in bending in one direction - I or box shapes can be the lightest for this application.
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The tubular spar seems an elegant and economical solution, typical of Jim Bede's creative thinking.

An "I" or box section bending strength is partly a function of the depth of the web. A main spar with a deeper web is easier to fit into a "fat", high camber wing like those on a Piper Cherokee or Vans RV. The Grumman GA wing seems rather thinner - is it laminar flow?
 
GRG55 said:
The Grumman GA wing seems rather thinner - is it laminar flow?
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As originally designed and implemented in the BD-1 and AA-1 Yankee, yes (I can't remember the actual airfoil designation offhand). But a characteristic of that airfoil is greatly increased drag at high angles of attack, not a good thing for an underpowered trainer. So the Model AA-1A "Trainer" of 1971 had a modified, more rounded leading edge with flat undersurface that was much better behaved at high AOA, and had a tamer stall and much lower stall speed, with only a small cruise speed penalty. All subsequent AA-1x and AA-5x models use that same modified "laminar" airfoil. That's why the original AA-1 is sometimes referred to as the "slick-wing" or "hot-wing" Yankee.

The leading edge modification was pretty much an "eyeball" job by Grumman-American engineers, and didn't conform to any particular standardized airfoil designation; so it's just referred to as "modified". It is similar to what Cessna did to the leading edge of the Cardinal wing in 1970 and most of the rest of the single-engine fleet in 1972-73; and the outboard sections of the PA-28 and PA-32 tapered wings.

Edit: Just found this:
AA-1: uses a NACA 64-415 airfoil, with a modified leading edge.
AA-1A (B,C, AA-5A and AA5B): is documented as using the same 64-415
. . . . . . but again noted as having a modified leading edge conic section.​
 
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GRG55 said:
The tubular spar seems an elegant and economical solution, typical of Jim Bede's creative thinking.
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Indeed. The cost / complexity of a round extrusion (c.o.t.s?) is MUCH less than an aircraft wing spar riveted up from flat stock and angles. And, for some applications is a very good choice.
Just like a rectangular wing planform vs an elliptical planform - one is inexpensive and gets the job done, the other is much more expensive to fabricate but is a bit more efficient.

Ya pays yer money and ya makes yer choice.
 
Pilawt said:
As originally designed and implemented in the BD-1 and AA-1 Yankee, yes (I can't remember the actual airfoil designation offhand). But a characteristic of that airfoil is greatly increased drag at high angles of attack, not a good thing for an underpowered trainer. So the Model AA-1A "Trainer" of 1971 had a modified, more rounded leading edge with flat undersurface that was much better behaved at high AOA, and had a tamer stall and much lower stall speed, with only a small cruise speed penalty. All subsequent AA-1x and AA-5x models use that same modified "laminar" airfoil. That's why the original AA-1 is sometimes referred to as the "slick-wing" or "hot-wing" Yankee.

The leading edge modification was pretty much an "eyeball" job by Grumman-American engineers, and didn't conform to any particular standardized airfoil designation; so it's just referred to as "modified". It is similar to what Cessna did to the leading esdge of the Cardinal wing in 1970 and most of the rest of the single-engine fleet in 1972-73; and the outboard sections of the PA-28 and PA-32 tapered wings.

Edit: Just found this:
AA-1: uses a NACA 64-415 airfoil, with a modified leading edge.
AA-1A (B,C, AA-5A and AA5B): is documented as using the same 64-415
. . . . . . but again noted as having a modified leading edge conic section.​
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Most of what you said is correct and is what is reported on most sites. But to be clear, there is more of a speed difference in the real world than books suggest. I am usually a good 10 knots faster than an A, B, or C model Yankee with my AA1. Honestly, I cant tell a difference in stall behavior between the two. They are both pretty mellow and are no worse than a Cherokee. The only real advantage to the later wing is their better ability to handle high weights on hot days. On cool days with similar weights I can out climb the later models. Add in some humidity and the later models can out climb me by about the same amount i can them in the cold. I probably wouldn't consider any AA1 series other than the original "hot wing" as the later ones aren't much faster than a 150 and you give up a lot of short field performance.
 
Grum.Man said:
Most of what you said is correct and is what is reported on most sites. But to be clear, there is more of a speed difference in the real world than books suggest. I am usually a good 10 knots faster than an A, B, or C model Yankee with my AA1. Honestly, I cant tell a difference in stall behavior between the two. They are both pretty mellow and are no worse than a Cherokee.
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And I think you are correct about the speed. I gave primary instruction in several original AA-1s (including s/n 7, the first Yankee on the West Coast) before the AA-1A even came out. Other than being noisy (this was before headsets were common) I thought they were good trainers, and would teach students things that a Cherokee would gloss over -- like don't try to yank the airplane off the ground and get behind the power curve! They were especially good trainers for students intending to transition to higher-performance, high-wing-loading airplanes. I do recall the Yankee having a sharper stall break than the Cherokees on our line. When I bought an AA-5A Cheetah decades later it seemed considerably more docile than what I recalled from the AA-1.
 
Capt. Geoffrey Thorpe said:
Indeed. The cost / complexity of a round extrusion (c.o.t.s?) is MUCH less than an aircraft wing spar riveted up from flat stock and angles.
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"Hey Jim, we just got another airframe order. I'm gonna head over to Home Depot for some sewer pipe.... I mean wing spars."
 
They are economical to build although I remember a wing bolt AD costing me 400 or so.
 
Pilawt said:
And I think you are correct about the speed. I gave primary instruction in several original AA-1s (including s/n 7, the first Yankee on the West Coast) before the AA-1A even came out. Other than being noisy (this was before headsets were common) I thought they were good trainers, and would teach students things that a Cherokee would gloss over -- like don't try to yank the airplane off the ground and get behind the power curve! They were especially good trainers for students intending to transition to higher-performance, high-wing-loading airplanes. I do recall the Yankee having a sharper stall break than the Cherokees on our line. When I bought an AA-5A Cheetah decades later it seemed considerably more docile than what I recalled from the AA-1.
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You definitely do not want to entice the Yankee's to leave mother earth too early, they will just sit there and plow air until they get to the speed they wanted to be at to start with. Power off I can barely tell a stall from a mush in my AA1. Power on, yea it has a pronounced drop and buffet but the deck angle is so great that I could not imagine anyone ever having a power on stall.
 
Grum.Man said:
You definitely do not want to entice the Yankee's to leave mother earth too early, they will just sit there and plow air until they get to the speed they wanted to be at to start with. Power off I can barely tell a stall from a mush in my AA1. Power on, yea it has a pronounced drop and buffet but the deck angle is so great that I could not imagine anyone ever having a power on stall.
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It is more common at high DA where "full power" is not so much. But some still manage to do it a sea level.
 
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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Actually BD's was an irrigation pipe.
 
Grum.Man said:
Power on, yea it has a pronounced drop and buffet but the deck angle is so great that I could not imagine anyone ever having a power on stall.
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The only time I came close to a departure stall was when it was completely dark outside. As I made it past the runway, I lost sight of any lights. I'm not instrument rated but had to fly in the pattern on instruments. Tower gave me right pattern, which prevented me from seeing lights until I got on the downwind. The airport was located next to a high terrain and I was afraid of hitting it. Combination of these factors made me lose track of airspeed. Deck angle was unavailable due to darkness (the AI obviously showed it, but...).
 
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