Not a new subject I know, but I thought I would start a new post that deals with rework of an existing stock Skybolt to fit a tall pilot (I'm 6 ft 3 in), as opposed to customizing for the taller pilot during the build phase. I bought a Skybolt that was already in license, but I bought with the knowledge I would need to modify the pilot seating before starting to fly it. With the stock seating, the top of my head extends too high above the turtledeck and would likely be immersed in the slipstream. I know the designer intended the pilot to sit bolt upright with the windshield and instrument panel close to his face and legs bowed out like a cowboy on his horse, that part I get. However, looking at old photos of 1920's airmail pilots sitting in their open cockpit biplanes, they all seem to sit low and hunkered way down in the cockpit, barely peering over the edge of the coaming. I figure those guys knew what they were doing, so why not try to make my plane more like theirs?
Extending the rudder cables to gain sufficient leg room was easy, gaining head room is more challenging. Gaining space to accommodate a parachute is more challenging still.
The Stits cover on my fuselage is over thirty years old, but still serviceable, so my preference at this point is to preserve the existing fabric cover. Preserving the fabric rules out gaining headroom by cutting out fuselage truss tubes and re-welding to recline or re-position the seat back as other forum members have reported doing during construction. A check of the current Skybolt plans indicates my fuselage truss has standard geometry with the seatback upper crosstube cluster measuring 85 inches aft of the firewall.
The Skybolt seat bottom design seems to be typical of the sport biplane, just a chair frame with a sling or basket weave strapping between the chair rails to hold a cushion or a seatpack chute. You can gain some headroom by elongating the sling or basket weave strapping, but this approach was not sufficient for me. I found the seat side rails, which are 16.25 inch apart, are too narrow to let me lower myself all the way to the bottom truss tubes without pinching the sides of my hip bones on the side rails and having the front cross rail dig into the bottom of my thighs.
My solution was to remove the old chair frame tubes and install a new bolt-on seat pan as shown on the photos. I fabricated a new seat pan with a recurve shape that lowers my rear as much as is practical while still clearing the bottom truss tubes. The forward part has extensions for added thigh support. The side rails toe out at an angle that parallels the fuselage sides, a shape that better matches the proper bowlegged thigh angle. The new seat frame tubes are .75 dia X .035W 4130 steel tube, same as the old seat frame.
Front attachments are by bolted sleeves that engage the upright tubes of the old seat frame. One front attachment sleeve is welded to the new seat pan, its opposite is a separate sleeve that bolts both to the old upright tube and to a stub tube on the new seat pan. This separate sleeve is needed to accommodate assembly because the existing seat frame upright tubes are not parallel, you install by first sliding the free sleeve over one stub, then positioning the seat pan in place with the end having the fixed sleeve onto its stub, and finally sliding the free sleeve up to engage the opposite side, then fasten with two AN4 bolts on the fixed sleeve side and four AN4s on the sliding sleeve side.
The aft attachments use saddle clamps that fit around the lower truss cross tube and bolt up to flat plates that are welded to the aft ends of the new seat frame side rails, using four AN4 bolts per side. There are lugs with redundant attach points for the seat harness and lap belt that are welded to the new seat frame tubes, made of .050 4130 sheet. A similar lug on the front cross tube retains the harness crotch strap. There are separate reinforcement straps that wrap around the seat back frame upright tubes and pick up the seat harness lug clevis bolt on each side. These reinforcement straps are there to react forward crash loads. Naturally, if preserving fabric was not an issue, this new seat frame could be just be installed by welding to the fuselage truss tubes without using all these bolts and fittings.
The seat pan has a skin made of .040 2024-T3 sheet hand formed to match the frame contour. Under the skin are .063 thick .75 X.75 extruded 2024 aluminum stiffening angles that rivet to the skin and pick up attachment angles that are welded to the seat pan side rails. I figure that with my weight, and pulling 4.5 gee, my tusche will be pushing down with a force over 1000 lbs on this seat pan, so I wanted something plenty stiff and strong. There is an extruded aluminum MS piano hinge riveted to the aft edge to accommodate a seat back.
The seat back is made of 7 mm, 7 ply Baltic birch plywood, screwed and glued. There is a 2 inch pan-down in the seatback that can accommodate a parachute or a seat cushion without robbing any leg room. I have not checked the fit with a chute yet, however. The lower part of the pan-down area must be carefully configured to maintain clearance from the elevator reverser bellcrank located just behind the pilot. The upper corners of the seat back secure to the seat frame using quarter turn camloc fasteners. You can disengage the camlocs and pivot the seatback forward for easy access to the aft fuselage spaces.
With the seat installed, there is 38 inches between the surface of the seat pan and the top of the turtleback. And the sight line to the instrument panel and through the bubble windshield is much better now.
Weight came out to 6.15 lb added for the new seat pan, minus 1.60 lb removed for the old seat frame, 3.8 lb added for the new seat back minus 1.3 lb removed for the old .040 sheet aluminum flat seat back, resulting in a total weight gain of about 7.1 pounds.
Now it’s on to a new baggage compartment, baggage compartment door and shoulder harness attachments. Maybe I can gain back some weight savings there.
Mark
Extending the rudder cables to gain sufficient leg room was easy, gaining head room is more challenging. Gaining space to accommodate a parachute is more challenging still.
The Stits cover on my fuselage is over thirty years old, but still serviceable, so my preference at this point is to preserve the existing fabric cover. Preserving the fabric rules out gaining headroom by cutting out fuselage truss tubes and re-welding to recline or re-position the seat back as other forum members have reported doing during construction. A check of the current Skybolt plans indicates my fuselage truss has standard geometry with the seatback upper crosstube cluster measuring 85 inches aft of the firewall.
The Skybolt seat bottom design seems to be typical of the sport biplane, just a chair frame with a sling or basket weave strapping between the chair rails to hold a cushion or a seatpack chute. You can gain some headroom by elongating the sling or basket weave strapping, but this approach was not sufficient for me. I found the seat side rails, which are 16.25 inch apart, are too narrow to let me lower myself all the way to the bottom truss tubes without pinching the sides of my hip bones on the side rails and having the front cross rail dig into the bottom of my thighs.
My solution was to remove the old chair frame tubes and install a new bolt-on seat pan as shown on the photos. I fabricated a new seat pan with a recurve shape that lowers my rear as much as is practical while still clearing the bottom truss tubes. The forward part has extensions for added thigh support. The side rails toe out at an angle that parallels the fuselage sides, a shape that better matches the proper bowlegged thigh angle. The new seat frame tubes are .75 dia X .035W 4130 steel tube, same as the old seat frame.
Front attachments are by bolted sleeves that engage the upright tubes of the old seat frame. One front attachment sleeve is welded to the new seat pan, its opposite is a separate sleeve that bolts both to the old upright tube and to a stub tube on the new seat pan. This separate sleeve is needed to accommodate assembly because the existing seat frame upright tubes are not parallel, you install by first sliding the free sleeve over one stub, then positioning the seat pan in place with the end having the fixed sleeve onto its stub, and finally sliding the free sleeve up to engage the opposite side, then fasten with two AN4 bolts on the fixed sleeve side and four AN4s on the sliding sleeve side.
The aft attachments use saddle clamps that fit around the lower truss cross tube and bolt up to flat plates that are welded to the aft ends of the new seat frame side rails, using four AN4 bolts per side. There are lugs with redundant attach points for the seat harness and lap belt that are welded to the new seat frame tubes, made of .050 4130 sheet. A similar lug on the front cross tube retains the harness crotch strap. There are separate reinforcement straps that wrap around the seat back frame upright tubes and pick up the seat harness lug clevis bolt on each side. These reinforcement straps are there to react forward crash loads. Naturally, if preserving fabric was not an issue, this new seat frame could be just be installed by welding to the fuselage truss tubes without using all these bolts and fittings.
The seat pan has a skin made of .040 2024-T3 sheet hand formed to match the frame contour. Under the skin are .063 thick .75 X.75 extruded 2024 aluminum stiffening angles that rivet to the skin and pick up attachment angles that are welded to the seat pan side rails. I figure that with my weight, and pulling 4.5 gee, my tusche will be pushing down with a force over 1000 lbs on this seat pan, so I wanted something plenty stiff and strong. There is an extruded aluminum MS piano hinge riveted to the aft edge to accommodate a seat back.
The seat back is made of 7 mm, 7 ply Baltic birch plywood, screwed and glued. There is a 2 inch pan-down in the seatback that can accommodate a parachute or a seat cushion without robbing any leg room. I have not checked the fit with a chute yet, however. The lower part of the pan-down area must be carefully configured to maintain clearance from the elevator reverser bellcrank located just behind the pilot. The upper corners of the seat back secure to the seat frame using quarter turn camloc fasteners. You can disengage the camlocs and pivot the seatback forward for easy access to the aft fuselage spaces.
With the seat installed, there is 38 inches between the surface of the seat pan and the top of the turtleback. And the sight line to the instrument panel and through the bubble windshield is much better now.
Weight came out to 6.15 lb added for the new seat pan, minus 1.60 lb removed for the old seat frame, 3.8 lb added for the new seat back minus 1.3 lb removed for the old .040 sheet aluminum flat seat back, resulting in a total weight gain of about 7.1 pounds.
Now it’s on to a new baggage compartment, baggage compartment door and shoulder harness attachments. Maybe I can gain back some weight savings there.
Mark
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