The gap seal system that we have developed for the EMG-6 electric motor glider is a substantial improvement over the traditional gap seal system used on most ultralights. The most common gap seal type system involves sewing a tab with Velcro onto the trailing edge of the wing and having a tab with the reverse Velcro sewn onto the leading-edge of the aileron. This is a great job at preventing the airflow from transitioning from the high-pressure lower surface of the wing to the lower pressure surface of the upper wing. In this case that’s the primary job of the traditional ultralight gap seal. However one of the downsides to this methodology is that it only deals with the problem of the transitioning airflow from the bottom to the top. The surfaces that the airflow is transitioning over at this juncture between the aileron in the wing is a fairly substantial gap as a result of the typical design of aluminum tubes spars join together with fork and eyebolt hinge system. These large round aluminum tubes create a substantial amount of drag even with the gap seals install. Our system that we have developed deals with both the transitioning airflow as well as the streamlining and drag reduction at this intersection between the aileron and the wing. We create a gap seal that completely fills the cavity between the aileron spar and the trailing edge spar. In addition our polyethylene foam gap seal is designed to be able to move with the aileron allowing for a smooth transition of airflow from both the bottom of the wing and the top of the wing independent of aileron position. The weight of the polyethylene foam gap seals is negligible. The gap seals are easy to install and remove and replaceable if needed. The gap seals are waterproof as they are manufactured from solid polyethylene foam pool noodles.
This is a video That we did some time back on the process of cutting the Gap Seals. The dies, technique, and the new solid-core pool noodles improved process substantially from when we made this video. That being said, this is very close to how we cut the gap seals today.
Aileron gap seal
The shows the right wing of the EMG-6 with a section of polyethylene foam gap seal installed in between the number 2 and number 3 hinge.
Gap seal close-up
This gives a little better close-up of the fit of the aileron gap seal. The standard procedure is to cut a small section out of the center core of the gap seal and slide it over the top of the aileron hinge. This makes it possible to completely hide all of the hinges along the entire length of the wing.
Gap seal profile
This is a picture of the end profile of the aileron gap seal. The horizontal and vertical stabilizer gap seals are identical except both of the arcs cut into the ends where the spars interface are the same diameter on the tail gap seals. In this case here we have a 1.75 inch diameter rear spar and a 1.5 inch diameter aileron spar.
The aileron gap seal was designed and tested in SOLIDWORKS before we ever started making the cutting dies. This is not the 1st set of gap seals that we have manufactured however it is the 1st time that we've manufactured them from a solid pool noodle without the center hole. This allows us to have a much cleaner gap seal without having to compensate for the a giant hole in the center of the pool noodle.
Gap seal cutting tool
1st step in making the polypropylene foam gap seals is to manufacture the cutting dies that will be used for each of the gap seals. In this case here you see us precision CNC machining the cutting dies using a .125 ball nose end mill. Takes a total of about 8 hours to finish both the aileron and the tail gap seal tool cut outs.
Gap seal cutting tool
In this picture we can see the cut out for both the aileron and the tail gap seals. There are 2 different profiles and as such, require separate dies for each of the profiles.
Cutting tool assembly
The basic layout of the cutting tool involves plywood guide blocks with a spacer in between the cutting tool and the plywood to keep them from overheating and catching on fire. In the center you can see our cutting die. And on the far right side is a 1 inch thick block of aluminum to act as a heat sink.
The purpose of the guide blocks is to guide the pool noodle through the cutting dies very precisely. We have found that about 3 inches of guide block is about ideal.
In this picture here you can see the layout of the cutting dies in relationship to both the guide blocks and the heat sink. The heat sink is quite a bit larger diameter than the pool noodle to prevent it from burning as it comes through the die.
Preheating the cutting dies
We have in the past use a propane torch to heat up the cutting dies and heatsink but the amount of energy that we can get out of a propane tarts requires almost 20 minutes of preheating before we can start cutting. By using a oxyacetylene torch with a rosebud or even just the end of the cutting torch we can heated up in about 5 minutes.
In this picture here you can see the heat sink is placed directly against the aluminum dies to be able to transfer the heat back into the cutting dies as quickly as possible. Aluminum is a great material for transferring heat quickly.
Preheating the heatsink
We want most of the heat to be in the heatsink before we start the cutting process. Primary reason for this is to prevent the die from distorting from overheating.
Heating almost complete
After about 5 minutes of heating we are ready to cut the pool noodles. The heatsink will not have to be reheated until after about 10 minutes of working time. Normally this is enough to do all of the gap seals for a single airplane.
Polyethylene foam blank
We found a source for black, solid pool noodles. Where much happier with the quality of these pool noodles than some of the Chinese pool noodles that we cut in the past from Walmart.
Pulling the extrusion
In this picture here you can see that by using tension on the pool noodle it folds it perfectly straight as it extends through the guide blocks and then into the cutting dies. By holding relative position the cut becomes absolutely perfect.
Pulling the pool noodle
This is a close-up showing the 5 individual pieces that come out of the dies through the extrusion process.
There is a lot of scrap with the pool noodle as it comes to the extruder. We often use the scrap for packing material.
You can see that with the right amount of heat and the right amount of speed little finish comes out looking really clean. The surface is melted as it is cut and therefore leaves a slight skin over the outer surface of each one of the extrusions.