Source: 2022 Q2 Beartracks, Mark Scott
An empty winter basement with no projects underway is a dangerous thing.
It is great fun flying my “A Model” Bearhawk 4-Place to wonderful out-of-the-way places with short grass strips. Short field performance is great, especially with 260 hp installed, but I knew if the flaps were extended inboard it would do better. I applied my professional aerodynamics experience and calculated an expected 3 kt reduction in power-on stall speed with flaps extended inboard close to the fuselage.
Three knots might not sound like much. But landing roll distance is dictated by kinetic energy which is: ½ *Mass * Velocity squared. This is why brakes are often rated by energy dissipation capabilities. For example, a touch down speed reduction from 35 kts to 32 kts is a 16% reduction in kinetic energy which means a 16% reduction in landing ground roll for the same braking power. That would drop your 500 ft roll to 420 ft. I have a time integration program that calculates airplane takeoff roll that shows the same or slightly larger percentage reductions in takeoff roll.
The 3 kt power on stall speed reduction from only an 11 inch inboard flap extension is derived from three things. First the flap area is simply bigger which of course helps. The second and biggest benefit is the new flap area is in the propwash which generates a couple times more lift than if it were not. The third benefit is the close proximity of the inboard flap end to the fuselage creates resistance to the air escaping from the high pressure lower surface to the upper surface. This further increases flap effectiveness. Hence, inboard flap
area is very valuable. All the newer Bearhawk models as well as Maules, Huskies, and other high wing aircraft have flaps running close to the fuselage so I was pretty sure there would be no adverse directional stability or control issues.
Figure 1 shows the original flap configuration. The fixed portion trailing edge is slightly longer than at the hinge line.

This geometry makes the flap to fuselage gap decrease as the flap is deployed. The inboard portion of the fixed section is removable via Tinnerman screws in nut plates for access to the flap cable, guide, and pulley.
Figure 2 shows a true straight edge hardwood board clamped to the wing at the cut line.

I used an air powered grinder tool with a thin 3/32 cutting disk. I found the thinner disk made for a cleaner and easier cut. The outboard rib is cut and trimmed back to the spar as far as possible but the inboard rib is not cut. The plan was to leave the rib attached to the rear spar and bend it inboard parallel to the top fuselage longeron. You can’t run the flap immediately adjacent to the fuselage because of the flap cable guide and pulley.
Figure 3 shows the left side rib bent parallel to the fuselage longeron.

You have to drill out some wing skin rivets at the front to enable the bend. Figure 4 shows the completed much narrower fixed portion.

I was able to reuse the original fixed portion skin by trimming it to match the new rib location and drilling it to match the rib’s Timmerman nut plate locations. With the new narrow fixed portion location set I made a cardboard mock up of the flap extension. Cardboard is wonderful stuff for making aluminum mockups.
Figure 5 shows a trial fit.

This mockup was invaluable to laying out the new inboard flap skins.
Figure 6 shows the right flap extension in work.

Conventional aluminum design and fabrication techniques were used. The inboard rib and skins are .024 inch and .020 inch 2024-T3 respectively. A simple stress analysis on the attachment rivets found the .020 skin more than adequate. There is a .016 in thick center rib to eliminate potential thin skin oil canning vibrations that could develop in the pulsating propeller wake. The skin is one piece with a semicircular trailing edge to match the flap. I used flush 1/8 pull rivets throughout the assembly including the attachment to the flap.
Look carefully at the left flap extension in Figure 7.

This is where it got a little tricky. I reflex my flaps about 5 degrees up in cruise. This places the flap leading edge very close to the rear spar. With the mockup I found interference between the leading edge of the new inboard flap section and the rear spar. In particular a rivet head at mid-section, and the rear spar attachment doubler plate and a rivet at the inboard section. I had to bring 4.25 inches of the most inboard section in about 3/16 of an inch for clearance. I made two additional holes for rivet head clearance. The craftsmanship does not have to be exceptional here since it is always hidden. It just needs to be structurally sound and fit well.
Figure 8 shows the completed left flap extension riveted to the flap inboard rib.

It took a few tries to get the flap contour to nicely match the flap rib but it worked out in the end. It also took a couple of iterations with the flap extension clecoed to the rib to check flap clearance and trim to the new fixed portion. Once installed I carefully laid down lines for the dark blue stripe and made templates. Fortunately, a friend started painting his Kitfox and built a beautiful paint booth. I used that to complete the painting. The final result is shown in Figures 9 and 10.


So, were my calculations correct? It looks like they were. My power on stall speed dropped from 32 Kias to 29 Kias, plus or minus 0.5 Kias, at the same gross weight and CG. I have not been able to do takeoff roll measurements yet but it feels shorter. I think the flaps look a lot better too. I reduced my flap extension speeds by 5 Kias due to the larger flap loads.
The indicated stall speeds are lower than expect based on calibrated airspeed calculations. I suspect some kind of upwash at the pitot tube is going on since Cessnas have a large Kias to Kcas calibration factors near stall and I have a similar pitot tube installation. Subsequent and ongoing airspeed calibration runs with a yarn tuft rake shows a lot of upwash. This may the subject of a future Beartracks article. In
any event the difference in speed between the flap configurations should still be fairly accurate since my measured pitch attitude is very close for both configurations.
If you have interest in performing this modification I would be happy to talk to you about the details.