Flap Mechanism

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The Wag Aero Sportsman 2+2 is designed as a four place aircraft, however the “+2” refers to two children or one adult for the back seat. When determining the routing of the flap cables, consideration had to be given on the location of these cables to keep clear of the back seat passengers, thus a side mounted cable system was needed.  The flaps are engaged by pulling a Johnson Bar Lever between the front seats that pulls the cable that run down the centerline under the floorboards until they reach behind the baggage panel and are then directed upward and over to the right side of the fuse where they terminated to a pivoting bell crank.  The bell crank cable was attached to a “Y” connection from cables routed from each wing.  As my previous post described, I found that this cable geometry limited the flap travel and it was later greatly simplified by removing the bell crank and using only one cable from the Johnson Bar.  The return springs in the wings provided the means of retraction and are assisted greatly by the thrust of the airstream.

The Northstar Wing instructions advise to set the maximum deployment angle to 52 degrees down. While that seemed extreme, the actual angle was between 40 to 45 degrees when the air load was applied.  The Northstar flaps are massive and one difficulty is being able to pull on that much flap. Your airspeed is very important and the slower the aircraft, the easier it will be to get that last notch of flaps engaged. The larger wing area and the Fowler like flaps help the aircraft fly much slower, so it takes some getting use to  as when you can pull on that last notch of flaps.

Deploying the Flaps

flap drawing

The Wag Aero wing plans use spoilers instead of flaps.  The spoilers have almost no similarity to flaps. They are very small panels above and below the wing that you deploy upward and downward by pulling a “T” handle just before you flare to land.  They are designed to kill the lift of the wing. The Northstar wing kit on the other hand has massive Fowler-like flaps.  I decided to use a lever to deploy the flaps manually using a standard Johnson bar flap handle located between the seats. The challenge was how to rout the cables and pulleys from the underside of the belly and back up to the ceiling of the cabin. I bought a used Johnson bar from an old parted out Piper and designed a mount and ratcheting system for the bar to attach.  The first set of 3 inch pulleys were welded offset to the centerline of the fuse to avoid competing with the push-pull tube that operates the elevator.  I made the cables turn at a right angle behind the baggage panel to keep the turtle deck opening clear and useable. Bringing the cables up and along the side of the fuse allowed for a solid and secure location for a welded pivot point for the bell crank to attach. The Johnson bar had a welded horn centered on the lower bar and the horn had two holes for cable shackles which led me to design a twin cable system all the way to both ends of the bell crank. The theory in my mind was that one of the two cables would always be pulling when raising or lowering the flaps.  It was not until I was attaching the wings during the aircraft’s final assembly that I discovered that the travel movement of the bell crank was not sufficient to operate the flap operating range of the wings. To correct this I used only one cable that attaches directly to the cables from both wings and removed the fuse bell crank location entirely.  The flaps have a very strong return spring on the rear spar that assure flap retraction as soon as the flap handle is lowered.