I have no data on the change from Fowler to split flaps on a Legacy, but the differences in principle are well known. Flaps have two functions, to add lift and to add drag. You need added lift for take off and landing, to reduce T/O and landing speeds. Added drag is only desirable for landing, to reduce landing distance and improve engine response time in the event of a go around, particularly on jet aircraft.
Fowler flap mechanisms are complex, but they result in much more lift than other types, because of the increase in effective wing area, and because the airflow is deflected downwards progressively with slots, delaying airflow separation on the upper surface. Split flaps do not add wing area, so the lift increase is much less, and they add a lot more drag when fully deflected, because of the turbulent airflow behind the flap; there is less difference at half flap. So it really depends on what you mean by "95% as effective". The difference in take off speed and distance would not be very marked, since you are only looking for more lift at half flap. The landing speed and distance would be considerably less for a Fowler flap, which is why virtually all airliners use them. For the same reason, stall speed with full flap should be noticeably less with Fowler flaps.
Most homebuilders avoid Fowler mechanisms because they are difficult to design and build. And yes, you will see a small drag reduction if you eliminate the flap fairings, but 6 kts sounds a lot. If you decide not to fit Fowler flaps, I would suggest using plain flaps, like the 360, rather than split flaps. Split flaps give you nothing other than more drag, and are stucturally tricky on such a thin wing section.
Jerry Fisher
On Thu, Sep 3, 2009 at 9:34 PM, Douglas Brunner <douglasbrunner@earthlink.net> wrote:
Spoke to a Reno Racer recently who substituted a simple flush carbon hinge for the normal protruding aluminum hinges to change the Fowler Flap on the Legacy to a split flap.
- According to him, this should be worth about 6 knots at Reno Speeds due to the decreased parasitic drag caused by removing the protruding flap hinges.
- Also according to him, the split flaps are 95% as effective as the standard Fowler Flaps.
If the above facts are correct and the stall speed/takeoff distance is not significantly affected, it would seem that changing the standard Fowler Flaps to split flaps would be a good "speed mod" for all Legacies.
I presume that this was considered during the design of the Legacy and wonder if anyone has more information about why the designer decided to go with Fowler Flaps. Also any predictions as to what changing from Fowler to split flaps would do to takeoff distances and stall speed.
D. Brunner
N241DB
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