S-21 OUTBOUND PROGRESS
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High Points of Dual Spar Design
In a two spar wing the load is shared between the spars. In the outbound 75% of the load is on the front spar and 25% on the rear. In the case of a single strut single spar wing the rear spar has to be there anyway, so why not share the load. This reduces the weight and size of the main spar. It also positions the front spar to the leading edge of the wing that allows more advantages…read on.
The front and rear spar locations are conveniently located where you would want the door posts. This allows for big doors for easy cabin entry and cargo loading.
Fuel On Center of Gravity
The wide spar spacing allows the wing tank to be located near the center of gravity. This minimizes CG movement between full and empty tanks. The CG of the wing tank is kept as close to the wing root as possible, keeping weight near the aircraft centerline.
Pre-formed leading edge
By including the leading edge in the front spar extrusion, the need to bend and fight a sheet metal leading edge is eliminated. The extra thickness of the leading edge provides a robust structure to resist bird strikes and hangar rash.
Ease of Assembly
Perhaps the hallmark of this design is the ease of assembly. In a matter of hours you will have a completed wing. This is due to a high level of pre-fabricated parts ready to assemble, and having an overall design concept that eliminates many of the critical and time consuming elements of sheet metal wing construction.
Conducted a brief .4 of an hour flight in, 104 degree F temps. DA was 5000, winds light from the south. It was a check flight, to make sure controls were responsive and symmetrical (they were) and make sure all things were a go for further testing. The most stressful part of flight testing new designs is getting the plane ready for data acquisition. A few crow hops can tell a lot. Controls were light and even. Being only 20 pounds heavier, it reached lift off speed in 5 to 6 seconds. The feel in the flare, excellent, very easy to land.
I was impressed with the effective ailerons. Very light but with an odd feel, more on that to come. The other concern was the force on the flaps. Flap extension force seemed less than the stock Raven wing flaps. This was the intent of the "bail" type flap, good to know it worked out. Back to the ailerons...I kept the flight short because that little voice said go back and investigate.
Back in the shop we measured the static break out forces up to 3.5 pounds. We then tweaked the hinges and got this to be at 2 pounds or less. An aileron system can have a lot of aerodynamic boost, as do these, but if there is system friction it will mask the feel, even though the break out pressure in flight is very light (2 pounds or less). That system friction masks the feedback and diminishes the self centering. The combination of light forces and little self centering would make most pilots uncomfortable. The stick release test was positive, going back to neutral, just a tad slower than what a RANS should. Tomorrow we test with the reduced system friction and take some readings. Ultimately we will have that nice feel you expect in our planes.
We are predicting at least 5 knots faster and no less stall speed. At first blush it seems that both of those goals are well within reach. Rate of climb was familiar and plenty.
I am very excited about further flight testing and will post more when possible. - Randy Schlitter
Outbound Progress Report 7-11-17
The new 20 plus gallon fuel tank is underway. In a matter of weeks we should be getting the first article and placing a first run order. The tank fits like a glove into the root of the wing to maximize volume. An important design feature: Drill out a few rivets and you can remove the tank if needed. This became a welcome feature, as our test wing tanks ended up needing replaced, due to potential leaks.
There is a neat trick we used to install the root rib. Since it needs to slip between the outer skin and all of the tank supports, we created a space between them using a piece of .125” nylon cord. Once the root rib is installed, simply pull the cord out, install the rivets, and the install is complete.
Aileron and Flap Assembly
Pre-formed skins make for fast assembly. We were knocking them out right at 2 hours. The parts come well formed for a cleco and rivet assembly. It starts with riveting the hinge and action plates to some of the ribs, then cleco all the ribs in place on the skin, slide in the spar, double check the fit up, rivet all open holes, set the surface square and transfer drill the bottom skin leading edge to the spar.
Aileron and Flap skins come formed to shape for smooth surfaces and fast assembly.
First step in aileron or flap assembly is to rivet hinges and action plates; these are where the push pull tube attaches to the ribs. The action ribs are .040 thick to withstand actuation forces, thus “action” ribs.
This is the prototype access hatch on the aileron, but this will give way to two smaller holes on either side of the action rib.
To help line up some holes a #30 punch works great.
A vice grip with a couple scraps of stringer material make an excellent squeezer tool, if needed, to fine tune the skin fit to minimize pillow effect.
A flat table and some ballast bags is all that is needed to assure a square and true control surface.
Balancing the ailerons requires them to be pinned to the hinges on the wing, then pulling down with a fish scale and to take a reading. Unpainted you want to go to at least 100% of what you pull on the scale. Details about this will be outlined in the tech manual, since painting adds weight.
Once the lead shot is in place we sealed it with more spray foam.
Adding a mixture of epoxy coated lead shot into the leading edge spar of the aileron provides a neat way to ballast. Prior to the install of the lead we injected spray foam at least 6” into the tube.
Ailerons are set vertical, to let the epoxy and foam cure.
The Joy of working ABS becomes apparent as you build up the wing tips. The material is light, strong, long-lasting and easy to work. Our prototype tips were pretty rough, but made useable by using scraps of ABS and ABS or PVC cement. The tips need the install of an inner rib that has 6 nut plates coming through the end rib. This method makes for a clean tip install and was pretty simple. We assembled the tips, taped them to the end of the wing, reached inside with a small 90-degree drill and transferred drilled the hole, off of the rib and into the tip. Remove the tip, install the nut plates, and bolt in place. You can make an ABS repair putty by mixing grindings with the PVC prep solvent. This sticky goo works great to repair or correct any flaws in the tip. Body putty and auto paints also stick really well to ABS.
Wing tip internal rib trimmed and ready to install. The metal strips are set to hold the rib stable for best match to the wing rib. Production rib will have webs to omit this step.
The glue sets fast, only a few strips of tape are needed to hold things in place.