Loaded Up
On Memorial Day we had everything in place to do the load test on our aft fuselage and tail structure. Weather was also finally a go. Cameras were checked and their batteries charged. The airframe structure was attached to the blue testing fixture and setup in the parking area near our building. We used an old forklift for a counterweight. We had made stands for some rulers to take measurements of deflection. Then, after lunch, the testing crew was ready to go. We had two teenagers (extended family) help us with loading/unloading 46 lbs. gravel bags. Many hands make light work!
We preloaded the structure to 900 pounds of load - just over 4 g’s as a warm-up and to stretch what is going to stretch. This helps calibrate the deflection measurements. All new structures are going to settle and stretch under first loads. The result after unloading the structure is the baseline “Zero”. We noticed a couple areas of skin compression (slightest bulge between rivets) on the bottom box skin.
Test #1 was 1,196 pounds or 5 g’s at full gross weight at dive speed - Vne +10% (8 g’s at acro gross and maneuvering speed Va). We held that for 30 seconds or so. Measured deflection of 1.5” at the aft fuselage. In the long bay 2/3 the way back, we have two lightening holes in that panel and saw that there was a significant distortion of material between them. We had discussed adding panel stiffeners but were going for light structure. After removing the test weight, we decided to add the stiffeners to help with that distortion. There was no permanent deformation, so the structure was still good. The skins looked like they had taken a load, but still fine.
Test #2 was to 6 g’s at full gross weight, dive speed (9 g’s at acro weight and maneuvering). Deflections were about 1.75 inches at the fuselage and we held it for a good 30 seconds. The new stiffeners really helped with distortion of the panels and will be added to the kits. We then supported the fuselage and loaded more bags on for another test. Test #3 was 1,380 pounds and held as just as well. We didn’t unload but loaded more bags on for Test #4 at 1,518 pounds. Held that for almost a minute. Test #5 - At this point we unloaded three bags from one side of the horizontal stabilizer for asymmetric testing. This twisted the fuselage about 1/4 of an inch. Held for 30 seconds and then we unloaded the entire weight. That was equivalent to 6.5 g’s at full gross weight and dive speed (10.2 g’s at acro weight and maneuvering speed). Deflection was 2 inches and we had .125 inches of permanent deflection in the fuselage after all loads were removed. There is less than a .06 inches in the horizontal of permanent deflection. The horizontal stabilizer and mounting structure is rugged as hell and could take a lot more load. The side and bottom fuselage skins had definite wrinkles at a couple spots. The main longerons and bulkheads are perfect.
The deflections were more than estimated by the CAD software and that was a concern. Then, when reviewing the video, we found that the forklift was lifting and getting light on the main wheels. This caused a much larger measurement than it actually was. We should have turned the forklift around and had the counterweighted end facing our test rig instead of the forks. So we actually had about an inch of deformation over the length of the fuselage frame under max load, not the 2.0 inches indicated. We are happy with that result and it’s everything we designed the structure to do.
Adding the two panel stiffeners was already in the design space and we can also adjust the size of the lightening holes to help with distortions. Thickening the upper and lower box skins will help with compression bulges and only will add about 2 pounds. We had thinned those two skins to save the 2 pounds. We can save some weight elsewhere and make this structure more robust. The CAD engineering simulation software was good with deflection and loads, but not so good at buckling and distortion. We knew this going in and were worried about it buckling. It did not. The main longerons and mounting blocks held perfectly. The skins show some permanent distortions, but that is to be expected when loaded above yeild.
Results show the aft fuselage and tail structure to be capable of 5g at full gross (1350 pounds) up to yellow line cruise Vno (4g at Vne - but never a good idea to push it that hard!). This has a .5x safety factor before breaking structure. At acro weight (1235 pounds) she is a +6/-4 at Va maneuvering speed or low cruise.
So its capable of x g’s means the structure will deform under load, then return to original dimensions when released from the load. The max it can do this is called the design load. Above that load it may deform and not return all the way to original. This is called the yield region. Aluminum has a plastic yeild region that stretches and distorts. Then above the safety factor yield region - it may structurally fail and break. We call that point the ultimate load.
So for this plane, 6 g is the design load for aerobatic maneuvers. Above that is the safety factor yield region with an ultimate of at least 9 g’s before breaking. It can be bent, but will bring you home for repairs. Above 9g, all bets are off of bringing the plane home.
This is just what we want for the structural design. Mild, gentleman’s aerobatics (less than 4.5g) with a very good safety factor - up to 6 g’s for if you get in trouble in a maneuver. More than that and you might bend something.
If you are looking for serious acro - this is not the plane for that. This IS the plane for a lot of fun maneuvering and mock dogfights. Keep it Fun!