New wings for the Culver Cadet

[Neal LaFrance]

Hi Culver Pilots, This is Neal again, with some thoughts you might want to investigate. Are there Culver owners that would like to strap a new set of wings on their airplanes?? Those 70 year old wings are holding up a lot of precious cargo. I have flown those old wings through some heavy turbulence and I cringe every time. The wings were designed for 21/2 to 3 G's under old part 23 using casein glue. The wood is still in pretty good condition but the holes through the wood to support al the metal pieces are of unknown condition. So much for the negative outlook, lets look on the bright side. I love to fly this little airplane and I believe that many of you out there would like to know it is safe. The only way to be sure is to start with new wings. I have the drawings and would like do my part by giving them to the forum. There are requests that go with this donation. Some one will make a parts list of material and do a cost analysis on what the cost will be to make new wings. (If we use all the metal parts on your present wings costs could be decreased.) Should a professional wood working shop make all the wood parts?  Your feedback would be greatly appreciated.

[JoeB]

Hi Neal,
Thank you for sharing your knowledge with us all and for contributing so freely to this small Culver community!
My personal experience is in metal work and welding, the only wood wings I have ever built was when I spent hours gluing my fingers together with balsa models as a kid (which was probably good practice). 
I need to be honest I have some big projects going on right now, but I would love to take a look at the drawings and do my part to help keep these airplanes flying.  I think being able to offer a set of plans and a material list to current/future Culver owners would be a huge step in the right direction.
Creating a required materials/parts list and cost analysis is something I think I could complete in the near future.  But the next step after that, to properly prepare and start the construction of wings, is honestly 12+ months down the road for me.
If you think I could help in any way, I am willing!
Let me know your thoughts on this
Best regards,
Joe

[Eddie Z]

Hello Neal,
I may  be interested in the project as I have been playing with Culvers for awhile. I have talked to you about cowlings at one point as I live in San Diego. My thoughts on this would be Kit form all the way similar to as in a Pitts special where you can buy kits in wood stock with patterns for the ribs, etc. As a kit the costs dont hit you as high immediateley.  I also have two sets of wings at home to look at.
Die-cut or Laser cut parts would be a dream but would probably be cost prohibative. We should get ahold and talk as I have a bunch of interesting Culver Documentation, other stuff that would be noce to scan and put on this Forum.

ED

[Brett Lovett]

I find the construction of the wings for the Cadet very interesting.  They certainly are not typical of the strut braced wood wings of the day.  Ribs are cut-out plywood which I believe is stronger than the "conventional" truss construction.  Instead of the typical wires for drag/anti-drag bracing the Culver uses a drag/anti-drag structure of welded steel tubing. 

The Cadet was certified under CAR 4a.  I believe the sequence of certification rules for light aircraft was Aero. Bulletin 7A (Prior to Nov 1, 1937), CAR 4a (Nov 1, 1937 to Nov 13, 1945), CAR 3 (Nov 13, 1945 to Feb 1, 1965), FAR 23 (Feb 1, 1965 to present).  I've seen the load factor requirements of CAR 4a before, but I can't locate them now.  As I recall they were somewhere between the current "utility" and "acrobatic" category requirements, and definitely higher than the "Normal" category requirements, of the current FAR 23.

Pertinent Service Memos for the wings are:
http://www.culvercadet.com/servicememos/servicememo09.pdf,
http://www.culvercadet.com/servicememos/servicememo19.pdf,
http://www.culvercadet.com/servicememos/servicememo21.pdf.

Fatal accidents due to the failure of the main spar at the landing gear hinge spar channel (as addressed in service memo 9) is what ultimately resulted in the acrobatic/instrument limitation currently in place on our aircraft.  In all of the subject accidents the aircraft were likely above 145 mph at the time of the failure.  At least one was suspected of a hard landing prior to the accident.  As I recall there were 3 accidents total, 2 prior to C.A.A. Airworthiness Maintenance No. 56/Culver Service Memo 9, and 1 prior to C.A.A. Airworthiness Maintenance Bulletin No. 68/Culver Service Memo 21.  As far as I can find there have been no accidents due to structural failure in the NTSB Aviation Accident Database which goes back to 1962.

[Paul Rule]

Good post Brett,  I agree with what you have said.  I believe I recall that the CAR 4a required a loading of 4.2 G but will have to find it again to be sure.  The inboard lower aluminum bushing of the landing gear attachment cuts into the lower spar cap and creates "THE" high stress point.  If the gear attach fitting is not removed this area can not be inspected.  Because of the work to remove the fitting it is difficult to inspect. 

It is reasuring to remember that the fatigue life of wood is so high that it has never been precicely established!  These wings should last longer then all of us if they only had to stand up to normal fiying!  Its moisture and over stressing that gets them.

In two years when I move to NM full time (where my hanger is)  I plan to build some spars.  I have several as patterns and I have a jig (for routing out) that I think was used by one of the factories for forming the top & bottom faces.  I plan to try a "D" shaped aluminum bushing (withe flat of the "D" down) to reduce or eliminate the cutting into the lower cap as much as possible.  A "D" shape will carry the same vertical load but the side loading (into the end grain) will need to be investigated.   

It is interesting to note that in ANC-18 (Design of Wood Aircraft Structures, 1944) on page 235 &236 the Cadet spar was the subject of at least 5 test loadings to failure with modifications between each.  The production spar, it appears, was loaded to failure at a moment of 174138 in. lbs. on the fuselage (outboard) attach bolt.  I calculate that the 1/2 spanwise center of lift is very close to the inboard aileron hinge.  Using that (48" out from the attach bolt) and estimating the weight of one wing to be 100 lbs...(sombody put one on a scale and report back...!)  gives a design load of 4.37 Gs and an ultimate (fail) load of 6.56 Gs which is VERY close to the current utility catigory.

The indication from these same tests is that without the stress point at the gear attach bushing the design load would increase to 5.7 Gs.

[Brett Lovett]

Great information Paul. 

[Eddie Z]

If I get a chance I will scan the original Engineering report on the Wing certification, it will make for some fun reading for people interested in the wing design !

[Dan Rhinehart]

Does anyone know what the difference is between the 1940 "hollow spar" (hollow spot between the wing attach points and landing gear attach points, and the later spars(1941-1942) that this area is solid? Was there a problem with the older hollow spar, hence said problem was corrected.

[Brett Lovett]

Does anyone know what the difference is between the 1940 "hollow spar" (hollow spot between the wing attach points and landing gear attach points, and the later spars(1941-1942) that this area is solid? Was there a problem with the older hollow spar, hence said problem was corrected.

I wasn't aware there was a "hollow spar", or a change to the spar design.  If I recall correctly the testing for Service Memo 9 was conducted on an August 1941 build aircraft (just a few serial numbers away from mine).   That Service Memo deals heavily with the point of landing gear attachment to the wing spar, and gives no indication of any difference in wing spar construction between serial numbers 101 and 363.  There certainly is not a service bulletin/memo or AD I'm aware of that modifies the spar on a portion of the serial numbers, indicating that if there was a design change it was more likely for simplifying production or a general improvement rather than an airworthiness concern or any serious problem.  Is anyone else aware of a "hollow spar" design, or a change to the spar design made during production?

[Bill Poynter]

I've attached a photo of a hollow-spar Cadet with a hole punched throught the spar webbing.

[Brett Lovett]

Thanks Bill.  Any idea when they switched to a solid spar?

[Bill Poynter]

One possibility is that they reinforced the spars on the PQ8 drones and just used the same spar for the Cadets.

[Brett Lovett]

It is interesting to note that in ANC-18 (Design of Wood Aircraft Structures, 1944) on page 235 &236 the Cadet spar was the subject of at least 5 test loadings to failure with modifications between each.  The production spar, it appears, was loaded to failure at a moment of 174138 in. lbs. on the fuselage (outboard) attach bolt.

Paul,

I may have spoken prematurely to modifications of the Culver Cadet spar design.  Any chance you could scan copies of the pages relevant to the Cadet?  The only downloadable versions of ANC-18 I've been able to find online end at page 234.

[Paul Rule]

I will get a scan of those pages but not until probably mid July. 

The hollow spar is true.  I do not know when the change occured but I have both kinds of spars.  The hollow was between the outboard fuselage attach and the inboard gear attach.  The hollow was front and back of the center 45% shear ply and the transition to solid blocking was cut on a double curve.  There were also vertical blocks at all rib attach points. 

I doubt that there is any important strength difference between solid & hollow because the failures of the test spars were eithere at the inboard attach bolt or at the landing gear bushings as far as I know.

[Woody]

The spars on N37823, serial #373 is solid out past the landing gear then transitions to a hollow, ply covered wing to the end.