Gap Seal Mods: Improved Handling

Initially promoted as a speed mod, aileron and flap gap seals should give a small improvement in cruise, but the major benefit is in roll rate and handling..

Pilots have been looking for ways to make their airplanes faster and more efficient since there has been airplane ownership. One of the long-targeted spots for aerodynamic clean up has been the gap between the trailing edge of the wing and the ailerons and flaps.

The idea was that reducing the flow of high pressure air from under the wing, through the gap, to the low pressure area above the wing, should increase overall lift and allow the wing to operate at a slightly lower angle of attack, making it faster. It proved to be generally true and gap seals became a stalwart part of virtually all speed mods.

Aileron gap seals proved to have an unexpected benefit, discovered as early as 1937—a N.A.C.A. (predecessor to NASA) Technical Note that year reported on testing—sealing the gap improved rate of roll.
We looked at who is offering gap seals for what airplanes, what claims are being made and whether gap seals do increase performance. We came to a few conclusions: that the big benefit to gap seals is not widely recognized—improved handling in roll and rate of roll; the mod shops make speed claims in MPH because it sounds better than knots; there will be some speed increase, but don’t count on more than one or two percent—with potentially some increase in rate of climb and flap gap seals do not adversely affect stall speeds.
Gap seals are not something that a developer can just put on the market. They require Supplemental Type Certification (STC) testing. However, the FAA testing is aimed at assuring that the airplane’s handling as we’ll as stability and control are not degraded, not measuring performance.

Lasar Aero Styling
Lasar offers flap and aileron gap seals for M20B-M20G (flap gap) and M20B-M20K (aileron gap) Mooneys. The material is aluminum. The seals are installed with pop rivets. The flap gap seal is essentially a model of the changes Mooney made in 1977 with the Model 201. According to Lasar’s Paul Loewen, its 2.5 MPH speed increase information published is from Mooney. Customers have reported an increased rate of climb in before and after flights. There is no change in full-flap stall speed, according to Loewen.

Price for the flap gap seal is $325. Lasar estimates installation time at 12 hours.

The aileron gap seal uses Mylar chafing tape to protect the skin of the aileron and prevent an increase in control forces due to friction. Lasar only claims a 1 MPH speed increase. Price is $180—estimated installation time is two hours.

Founded by Roy LoPresti, his eponymous company has been making speed mods for decades. We spoke with company president, Curt LoPresti who told us that they concentrated on Pipers when it came to gap seals. They designed, tested and received approval for aileron and flap gap seals, but eventually decided to only market the flap gap seals, which it makes for the PA-24, -28, -32, -34 and -44. The seals are composite with a Teflon strip bonded to the composite during fabrication, making the seal effectively a lifetime unit.

The seal is attached to the wing skin forward of the spar with blind fasteners. The seal itself has a small spring force upwards, so it stays tightly against the flap in service.

The claimed speed benefit varies among the airplane types, however, LoPresti said the improvement tended to be about 3 MPH with some boost in rate of climb.

Expressing a concern about the effect of the seal interfering with the airflow over the flap when it is deployed, LoPresti said testing revealed no change in flaps-deployed stall speeds.

In our discussion, LoPresti talked about the lift distribution on a wing and the expected speed benefit of gap seals. He said to visualize an airplane head on—the lift distribution is such that the most lift is generated inboard of the ailerons. Consequently, stopping the flow of high-pressure air under the wing, through the gap has more effect inboard. In testing, the expectation matched the reality—the speed increase from aileron gap seals was almost negligible.

We were quoted a price of $1149 for flap gap seals for the PA-28 and -44, with an installation time of three to five hours.

It seems Horton has been making STOL kits and speed mods for Cessnas since Clyde built his second airplane. It offers aileron and flap gap seals for a wide variety of Cessna singles, all strut-braced, through the 1986 model year.

Aileron gap seals are the buyer’s choice of aluminum or. fiberglass. The seal is attached to the rear spar of the wing. The Horton representative we spoke with told us that the company’s experience was that the aileron gap seal gave at least a 2 MPH increase in cruise speed, but that the big improvement was in roll rate and low-speed handling, particularly on landing.

The flap gap seals are aluminum with silicone adhesive and are secured using blind rivets to the rear spar, flap support brackets and flap we’ll bulkheads. The Horton representative did not claim any increase in cruise speed. He noted that because Cessna used Fowler flaps that moved aft and down during extension, that there had been concern about an increase in stall speed with flap gap seals. He said that testing by the company and the FAA for the STC revealed no change in stall speeds.

Price is a total of $349 for both kits. Installation is estimated at eight to 12 hours.

Growing from the work of Jim Bradshaw, its late founder, in making the Piper Twin Comanche some 20 MPH faster, Knots2U has expanded into speed mods for a wide variety of singles and twins. It provides various gap seal kits for airplanes in the Piper, Beech and Cessna lines. The seals are aluminum with an anti-chafe strip made of ultra-high-molecular-weight (UHMW) plastic. According to Knots2U’s John Bailey, the strip should last 10-15 years and is easy to reinstall.

For the Piper line, aileron gap seals are available for almost all airplanes in the PA-24, -28, -30, -31,-32, -34 and -39 series lines. The seal is aluminum and is installed inside the aileron we’ll with screws. It is easily removable for inspection and maintenance. Prices range from $235 to $381.60—for the Twin Comanche the price is $658 for a combined flap and aileron gap seal kit; Bailey told us that installation takes about five hours.
The flap gap seal designed by Knots2U is also aluminum and installs inside the flap well. Prices range from $229 to $254; installation is estimated to take four hours. Bailey told us that their testing showed a reduction in stall speed and an increase in rate of climb of between 100 and 125 FPM.

According to Bailey, the performance improvement with flap, aileron and stabilator gap seal kits installed is 4 to 5 MPH.

For Beech Bonanzas and Debonairs, Knots2U has a combined aileron and flap gap kit priced at