S-21 OUTBOUND PROGRESS
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Showing S-21 mock up at Oshkosh allowed us to gauge the reaction of to the concept, construction, and pricing. The good news is we are right on target in just about every area. We hustled back home and have been working diligently to make this plane happen.
Work on the tailcone has been progressing, the first step is to mate it to the cage. Minor changes have been made to make it easier, such as eliminating the overlap of the skins onto the cage. This will allow complete assembly of the tailcone without the potential damage to overhanging sections. It will also allow independent assembly of the cockpit section. Imagine setting the cage on the gear and being able to install the everything without the tail cone and wings attached. This keeps the project compact during assembly of the most time critical elements such as, avionics, engine install, and interior.
We tested the wing assembly jigs. These will come in the kit. To assure accurate wing twist we provide machined out supports of composite wood. Attach the jigs to the saw horses with the bottom edge of the supports level with the world. The sawhorses should be on level ground and set apart the distance between the wing root and strut attach point. The wing is then clamped or taped into the jigs, ready for installation of the bottom panel. Ok, so you may ask, how are the wings assembled up to this point? It is done on the same two sawhorses, only minus the jigs. The goal is to provide a kit with all the tricks built in for extra fast and accurate assembly, and yet still be within the FAA’s 51% ruling for EAB.
Some are wondering how the skins attach to the cage. That is a great topic for the next report!
Thanks for stopping by, stay tuned- RJS
NOTE* THE S-21 HAS A CASTERING NOSE WHEEL
Which color choice won? Not too big of a surprise, #8, which was the red/white/gray/black scheme was a strong first, with #7 not to far behind. There was a lot of enthusiasm over selection of the color, and many insisted their choices was, of course, the best. So maybe we will just have to build one of each eventually.
Preparing for static testing of the wing: We use some monster equipment to test wings and the many other components of a plane to make sure they will be airworthy for many years. The HAL’s as we call them do this with ease, since we can exert many tons of force into a test piece. The set time is considerable, since most of the time the HAL’s are tucked up against the back corner. The floor in the location was super reinforced. In our old plant we had some issues with anchors not remaining intact through out the test. So far (knock on concrete) the test lugs have been secure for about anything we throw at them.
Updated Station 8 bulkhead: Station 8 has been revised to be lighter and stronger to take the tail wheel loads. We get such a nice clean cut that you won’t, in general, have to remove burrs or do any edge sanding. This really speeds assembly. We should be seeing a completed tailcone for load testing before long.
Updated wing spar layouts and increased rib count: We increased the rib count by two. This omitted the need for several span wise stringers, and the added drag from rivet heads.
Designed tooling for drilling out longerons: A big chore is to transfer drill stringers or longerons. At least in the case of our angle longerons on the tailcone we are pretty optimistic we can provide them ready to install. More on this as we test out the tooling.
Bottom wing panel final designed approved and to be installed next week on test wing: The test wing, shown on the mock up at OSH will be fitted with the bottom panel next week and loaded into the HAL. Once it passes static testing there remains a number test for the wing, fuselage and tail members. Fun stuff! Proving the planes strength prior to flying it makes the test pilot feel a whole lot better!
Thanks for stopping by, stay tuned...RJS
LOAD TESTING BEGINS
It has been a busy week for the Outbound. We worked on several areas, tail-cone, wing, horizontal tail, vertical fin. The wing testing began late Friday, and went to 3 G's with no issues. It is proving a tough wing and right in line with the FEA. The photos show the extensive set up required to wing test. There will be several conditions investigated. From there we go into control surfaces tests, then onto fuselage, tail cone, and tail group testing.
The test wing weighed in at 51 pounds minus struts and fuel system. A fully dressed S-7S wing comes in at 74 pounds. The completed Outbound wing with struts and fuel system should tip the scale at 85. For a gross weight of 1800, this is not a bad strength to weight. For comparison, the S-19 wing, which is inherently heavier due to being cantilever, weighs in at 101 each. Please note the S-19 was tested for a 1475 gross, and de-rated to 1320 for SLSA.
Stay tuned, more to follow next week, then look for a pause as we go to Mt. Vernon, followed by the Black Hills fly-in. RJS
Horizontal Stabilizer rib tooling built and tested: Test runs of ribs produced spot-on parts. The drive to make things fit with final hole size and no de-burring presents an extra touch. The pay-off is reduced build time and a more accurate airframe. The builder will have to take out some “banana” in the longer ribs, which is only a few tweaks with the fluting pliers. This is however, a good thing, since it scores a positive in the 51% column for the builder. If you look close at the ribs, we locate the holes in the flange close to the bend. This reduces the chance of the skin and rib flange not being tight against each other. This is especially important when attaching the last skin panel, when there is no way to push up the flange against the skin.
Parts are continuing to be produced for the tail cone assembly, final version will enter static testing once wing testing is concluded.
The panel between station 8 and 9 creates a torsion stable “notch” to allow the install of the one-piece horizontal stabilizer. A small aluminum fairing will most likely close out the top, but the stabilizer is fully skinned from tip to tip.
Wing Static Testing
Although the wing tested is not 100% representative of what will fly on the S-21, it proved the spars, fittings, and ribs are up for the task. Loaded to 6g’s, which is ultimate for normal category (we are going for 6+4-, which means ultimate will be 9+ 6-), there was some deformation and skin buckling. Nothing unusual or unexpected. The test specimen did not have span wise stringers, and other features that will be incorporated into the final design. The value of testing an under spec wing gives real world data on where to beef up structure without adding excessive weight.
It has also provided us enough data to confirm some concepts for all metal wing options for current models that could be weight competitive with existing wings. The potential of the DZ wing is very exciting. The dual spar design has some very nice advantages, entry door design, tank location, less or no trim change on fuel burn off.
The fly-on wing design will be loaded into the testing equipment by mid October, expect a full report to follow.
Rear strut attach showed slight deformation at 6 g’s, the single strut version will void this load path, and if a two strut version happens, a compression strut will fix the issue.
The bottom of wing remains smooth after many loadings to 5 and up to 6 g's. Buckling at strut fitting locations appears after limit load, and is no factor on final versions.
A stronger rear spar fitting is required for the higher gross. This CNC fitting has ample safety factor.
Twice we had to stop testing to due fixture deflection. Testing will resume with final version of the wing and a much beefed up fixture.
Check out the Video on Facebook
In this video you see a sample wing being load tested to 6 g’s, which it made in flying colors, with the exception of some minor easy to remedy issues. We kept getting large deflections, and the source was not the wing, but the test rig kept bending. We stopped testing twice to shore up the rig. In the video you hear reference to reading the deflections. Deflections are the early tell tale sign of a components ultimate success. The DZ wing spars are proving consistent with the FEA predictions. This wing is showing very minimal defection at 6 g's which is great, since we hope to move ultimate up to 9 positive.
Thanks for tuning it for the latest on the Outbound. Look for the next posting in 3 weeks. RJS
Wing Skins and Flush Rivets
The D spar has a joggle where the top and bottom of the one piece skins lap over the spar. This first row of span-wise rivets need to be flush, since this is in a critical flow zone. Solid flush rivets are preferred, but we are testing blind flush rivets. The bottom skin will be attached using blind flush rivets. The option is possible for the entire wing to be flush. To dimple the wing skins and ribs adds a few hours, but some feel the need for the potential speed, and we are happy to try and make a pathway for this to happen.
Aero Servo Ailerons
I am a big fan of light roll force. To insure the Outbound handles in a way pilots will truly enjoy, we have decided to swap out the Friese style ailerons for aero-servo. The benefits are many; lighter roll forces, less weight required to balance, less drag, (since it omits the outrigger hinges) and they have the same reduction in adverse yaw.
Wing Final Design
After the static load test, further optimization was possible. We increased the rib count, and added stringers, which will be special extrusions cut to length and drilled to match the skins with final holes size.
Inside the wing at the strut station will be a welded steel cro-moly truss. This ties the two spars into the load path to the single strut. We will show you this truss and how it integrates into the wing, along with static test data. Since this truss is a critical element it will arrive powder coated and ready to bolt into place. Inspection panels will be located so the truss can be inspected during condition inspection. There was some concern about dissimilar corrosion. Between the spar and truss is a bearing plate of aluminum, then a shim of .020” Lexan. The Lexan provides a non-conductive barrier between the two metals. We have used this with great success on every S-12, and S-12XL Airaile to protect the boom from the steel collar that wraps around the boom.
We hope to integrate into the strut truss the mounting for the aileron bell crank. This will speed assembly, reduce weight, and part count.
We are doing final fit-up of the fly-on fuselage. This aspect of the project is testing the skin fit and method used to cover the welded cage. The front section is directly borrowed from the S-20 project but, from the boot cowling aft is all new. To attach the side and belly skins aft of the front section we have a network of small tabs that key in the skins. The skins are made to shape with most all the holes, except where the tabs are welded to the cage. The builder will transfer drill into the skin once positioned on the welded cage. This method assures an accurate fit. Capping strips that wrap around the cage tubes at the bottom of the door opening finish off the assembly.
Thanks for stopping by, stay tuned, more to follow! RJS
The wing is the thing. Our efforts have been concentrated mostly on the wing these past few weeks, doing the static load testing of various wing truss designs, to final skin layouts and control system final design.
We used 4 different truss designs in static load tests, to arrive at the most optimized and load proven article. We started with a 3.9 pound truss, went up to 5.5 pounds, and optimized the final version down to 4.88 pounds. Not bad for a part that will be loaded to 9500 plus pounds if you pull over 9 Gs at 1800 pounds gross.
What material to use for truss was debated, from milling out an aluminum bar stock, a custom extrusion, to welded steel tubing. The welded steel tubing won out because of ease of manufacture, and great weight to strength. The concern about having a steel part tucked up inside the wing was overcome by providing it with internal oil treatment and powder coating. We also will use .020” thick Lexan barriers that sandwich between the truss and wing spars.
The truss is used to connect the front and rear spars to the single lift strut (Luscombe style). Originally, we were to test fly a dual strut wing, like the one displayed at Airventure, thinking it would be faster to get something flying. After doing the wing test, it made sense to skip over the dual strut wing for the single strut design.
Of course a single strut wing needs a bigger strut over a dual set up. A larger strut was designed and extruded featuring dual internal webs. This increases the negative load carry with a very small weight increase. The larger strut weighs in at .98 pounds per foot. The total weight of the dual struts system is 11.25 pounds. The total with truss and single strut and fittings is 12.25 pounds. This is a weight increase of 2 pounds for the single strut design. Well worth it for the lower part count, ease of assembly and drag reduction.
Wing and fuselage stringers will be pre-drilled and also made from a custom extrusion. It is a small angle featuring a bulb-flange design. Stringers prevent skin buckling by reducing the free span area between bulkhead and ribs. The extrusion is small but mighty, providing stiffness for very low weight. Having them pre-drilled to final hole size will greatly advance assembly.
Wing tip design will feature a flush mount system. This means there will not be a row of screws outlining the root edge of the tip. The plan is to use thermal formed ABS in a two part design. The outer tip will be a typical teardrop shape, the second part glues flush to the outer parts rim forming a mating flange allowing the assembly to bolt to the tip rib. Two access panels will be provided to allow install and removal. It should be one of the most drag free and builder friendly ways of mounting a wing-tip.
We plan on flying a set of S-21 wings on the prototype Raven. This may allow early shipment of wing kits. Details of this plan will be sent to depositors. Meanwhile, tooling development continues on the rest of the tailcone and tail surfaces.
Stay tuned, more to follow…RJS