Tweet
Disclaimer: The information provided here is for information only, as this is a deviation from Bearhawk plans. The information contained here has been collected from several different engineering sources, and have been interpreted to the best of my ability. However, there may be errors in what you see here, and it's up to me to find out whether or not they meet my interpretations during static load testing. If you see that I've miscalculated or misinterpreted something, feel free to let me know!
The spars for the Expedition wing are about one inch taller than the Patrol spars, and two inches taller than the Bearhawk 4-place spars. While doing the beam calculations for the main spar, it turns out that the cap strips could remain at 1-1/4" wide and (theoretically) support the weight of a 3500-pound airplane at +4.4g with a 1.5 factor of safety (+6.6g). But the stresses were just a little close to the yield strength of 2024 (the point where the metal will deform and not have enough elasticity to return), so I chose to increase the spar caps from 1-1/4" to 1-1/2". That will also give me some buffer for additional weight applied by floats in the future. These wings are being designed (on paper) to support 4000 pounds and stay within yield.
A couple of CAD drawings of the root-end attach fitting and the spar splice are attached.
Because the Expedition wing is two feet longer than the Patrol wing, when coupled with the extra gross weight, the attachments needed to be improved upon a bit. Starting with the root attachment plate, the forces applied at that fitting can potentially reach about 18,500 pounds. That exceeds the capacity of the stock 3/8" bolt by about 2,000 pounds. Moving up to a 7/16" bolt will meet the strength requirements with nearly 4000 pounds to spare. The -5 rivets in the top and bottom of the original attach plates are good for about 16,500 pounds of force collectively. The 4-place Bearhawk wing places about 10,000 pounds at +6.6g on the root attach fitting. The Expedition, however, is expected to nearly double that. So the attach plate was created as the 4-place fitting is shaped, and extended two inches. This allows 14 of the -5 rivets to be used, which is good to 29,000 pounds. More than I need, for sure. But I actually cut my spar webs 2-1/4" inches too short, so they don't go out to the tip as they should. After a call to Bob, he said they'd be strong enough with a .032" filler in between the attach plates, but I opted to error on the side of caution here.
The splice plate, of course, joins the two spar webs together, but also provides a place for the lift strut to attach. After reviewing both the Bearhawk 4-place and the Patrol plans, I noticed that there were some differences in the number of rivets used to splice the plates together. The 4-place has 53 rivets in its splice plate, where the Patrol has only 40. I found that interesting given that both are more-or-less 2500-pound airplanes. So I looked up web splicing and repairs in AC-43.13, Chapter 4, paragraph 4-58.g, Splicing of Sheets. Armed with that information, with normally accepted rivet spacing, the splice on my spar requires only 31 rivets. But to provide proper support to the entire area, similar to the way Bob assembled the 4-place and Patrol wings, I used 58.
The wing strut attach fittings are still .190" 4130. Where the lift strut attaches, however, was resized to accommodate the additional wing loads. The 4-place can see forces up to 11,500 pounds at this point. The Expedition, however, with its longer wing and greater weight, can apply as much as 21,600 pounds at this point. The stock 3/8" bolt at this location is good for about 16,500 pounds (8280 pounds in double shear). The Expedition will need a 7/16" bolt here, for about 22,500 pounds capacity. The radius and associated offset were also increased to tearout capacity of the steel at this point. The five 1/4" bolts that hold the strut attach fittings onto the wing spar should be good for either plane, with nearly 37,000 pounds capacity between them. But I created the fitting such that it would be able to support a 5/16" bolt if needed.
That's it for the moment! I hope to start cutting out spar parts this week and get moving on them. I'll also be putting the aft spar drawings together as well. I expect the format to be similar to the main spar. But I wanted to share what was upcoming and the theory behind it all!
~Chris
The spars for the Expedition wing are about one inch taller than the Patrol spars, and two inches taller than the Bearhawk 4-place spars. While doing the beam calculations for the main spar, it turns out that the cap strips could remain at 1-1/4" wide and (theoretically) support the weight of a 3500-pound airplane at +4.4g with a 1.5 factor of safety (+6.6g). But the stresses were just a little close to the yield strength of 2024 (the point where the metal will deform and not have enough elasticity to return), so I chose to increase the spar caps from 1-1/4" to 1-1/2". That will also give me some buffer for additional weight applied by floats in the future. These wings are being designed (on paper) to support 4000 pounds and stay within yield.
A couple of CAD drawings of the root-end attach fitting and the spar splice are attached.
Because the Expedition wing is two feet longer than the Patrol wing, when coupled with the extra gross weight, the attachments needed to be improved upon a bit. Starting with the root attachment plate, the forces applied at that fitting can potentially reach about 18,500 pounds. That exceeds the capacity of the stock 3/8" bolt by about 2,000 pounds. Moving up to a 7/16" bolt will meet the strength requirements with nearly 4000 pounds to spare. The -5 rivets in the top and bottom of the original attach plates are good for about 16,500 pounds of force collectively. The 4-place Bearhawk wing places about 10,000 pounds at +6.6g on the root attach fitting. The Expedition, however, is expected to nearly double that. So the attach plate was created as the 4-place fitting is shaped, and extended two inches. This allows 14 of the -5 rivets to be used, which is good to 29,000 pounds. More than I need, for sure. But I actually cut my spar webs 2-1/4" inches too short, so they don't go out to the tip as they should. After a call to Bob, he said they'd be strong enough with a .032" filler in between the attach plates, but I opted to error on the side of caution here.
The splice plate, of course, joins the two spar webs together, but also provides a place for the lift strut to attach. After reviewing both the Bearhawk 4-place and the Patrol plans, I noticed that there were some differences in the number of rivets used to splice the plates together. The 4-place has 53 rivets in its splice plate, where the Patrol has only 40. I found that interesting given that both are more-or-less 2500-pound airplanes. So I looked up web splicing and repairs in AC-43.13, Chapter 4, paragraph 4-58.g, Splicing of Sheets. Armed with that information, with normally accepted rivet spacing, the splice on my spar requires only 31 rivets. But to provide proper support to the entire area, similar to the way Bob assembled the 4-place and Patrol wings, I used 58.
The wing strut attach fittings are still .190" 4130. Where the lift strut attaches, however, was resized to accommodate the additional wing loads. The 4-place can see forces up to 11,500 pounds at this point. The Expedition, however, with its longer wing and greater weight, can apply as much as 21,600 pounds at this point. The stock 3/8" bolt at this location is good for about 16,500 pounds (8280 pounds in double shear). The Expedition will need a 7/16" bolt here, for about 22,500 pounds capacity. The radius and associated offset were also increased to tearout capacity of the steel at this point. The five 1/4" bolts that hold the strut attach fittings onto the wing spar should be good for either plane, with nearly 37,000 pounds capacity between them. But I created the fitting such that it would be able to support a 5/16" bolt if needed.
That's it for the moment! I hope to start cutting out spar parts this week and get moving on them. I'll also be putting the aft spar drawings together as well. I expect the format to be similar to the main spar. But I wanted to share what was upcoming and the theory behind it all!
~Chris
Comment