lr27
Well-Known Member
PROBABLY A FALSE ALARM!
Stan Hall had an article in the July-August 1999 Sailplane Builder called How to Size the Caps in a Wooden Box Spar-WITHOUT MATH. It also shows up on page 457 in the second volume of his collected works. Just for fun, I tried to evaluate the Sky Pup. However, when I got to drawing line 4, I noticed that the deeper the spar height was, the thicker the cap it recommended, which seems backwards. It also recommended a cap far thicker than the Sky Pup actually uses. Anyway, I gave up on it, but perhaps someone can explain what I did wrong. Or maybe some of you out there have run into the same problem. Any clarification appreciated.
Thanks!
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Warning, the following may be bogus:
Incidentally, I estimated the bending moments using another one of his articles which seem correct, or at least very close. For 6g's, I got about 95,000 inch pounds at the side of the fuselage where the wing attaches, and about 25,000 for just outboard of the last doubler for the wing joint, on the outer panel. I guessed these were the most likely to be critical.
Since I didn't trust the whiz-bang nomograph any more, I decided that the top cap would be in compression, and the bottom in an equal amount of tension. A quick look seemed to confirm that the top cap was the critical one. For an approximation, I figured the forces would be applied at the respective centroids of the 1/4" X 4" top cap and the 3/16" X 4" bottom cap. I imagine the errors are small, because the caps are thin relative to the depth of the spar. The crushing strength of douglas fir is not all THAT much higher than compression at the elastic limit, so I figured the ultimate load was the determining factor. The spar is 11.156" deep on the outside, and the centroids are 10.94 inches apart. At the 7,000 lb. crushing strength, I got an ultimate of about 5.5 g's. The limit load would be 3.65 g's. Of course, there's a bit of plywood, as I recall, on top of the spar cap, and maybe a little spacer of random wood, but I ignored those. That's using the figure in the 1944 edition of ANC-18. If we go by the 1951 edition, we get 4.4 and 2.93 g's. I suppose if we go inverted, we'd still have 4.1 ultimate and 2.7 g's, going by the 1944 edition. Compared to other sources, the 1951 edition seems quite pessimistic, but not all the source agree exactly.
I'll probably be looking at this in other ways soon.
BTW, I figured the bending moment between the cabin uprights was probably constant, and about the same as just outboard of the cabin. Close enough, I hope. Unless I missed some piece of plywood stuck on the side of the spar between the uprights.
Anyone who goes by my calculations is nuts. I'm probably doing something wrong.
(not mine)
Stan Hall had an article in the July-August 1999 Sailplane Builder called How to Size the Caps in a Wooden Box Spar-WITHOUT MATH. It also shows up on page 457 in the second volume of his collected works. Just for fun, I tried to evaluate the Sky Pup. However, when I got to drawing line 4, I noticed that the deeper the spar height was, the thicker the cap it recommended, which seems backwards. It also recommended a cap far thicker than the Sky Pup actually uses. Anyway, I gave up on it, but perhaps someone can explain what I did wrong. Or maybe some of you out there have run into the same problem. Any clarification appreciated.
Thanks!
---------------------
Warning, the following may be bogus:
Incidentally, I estimated the bending moments using another one of his articles which seem correct, or at least very close. For 6g's, I got about 95,000 inch pounds at the side of the fuselage where the wing attaches, and about 25,000 for just outboard of the last doubler for the wing joint, on the outer panel. I guessed these were the most likely to be critical.
Since I didn't trust the whiz-bang nomograph any more, I decided that the top cap would be in compression, and the bottom in an equal amount of tension. A quick look seemed to confirm that the top cap was the critical one. For an approximation, I figured the forces would be applied at the respective centroids of the 1/4" X 4" top cap and the 3/16" X 4" bottom cap. I imagine the errors are small, because the caps are thin relative to the depth of the spar. The crushing strength of douglas fir is not all THAT much higher than compression at the elastic limit, so I figured the ultimate load was the determining factor. The spar is 11.156" deep on the outside, and the centroids are 10.94 inches apart. At the 7,000 lb. crushing strength, I got an ultimate of about 5.5 g's. The limit load would be 3.65 g's. Of course, there's a bit of plywood, as I recall, on top of the spar cap, and maybe a little spacer of random wood, but I ignored those. That's using the figure in the 1944 edition of ANC-18. If we go by the 1951 edition, we get 4.4 and 2.93 g's. I suppose if we go inverted, we'd still have 4.1 ultimate and 2.7 g's, going by the 1944 edition. Compared to other sources, the 1951 edition seems quite pessimistic, but not all the source agree exactly.
I'll probably be looking at this in other ways soon.
BTW, I figured the bending moment between the cabin uprights was probably constant, and about the same as just outboard of the cabin. Close enough, I hope. Unless I missed some piece of plywood stuck on the side of the spar between the uprights.
Anyone who goes by my calculations is nuts. I'm probably doing something wrong.
(not mine)
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