Q-talk 112 - Gear Leg Fairing Construction and Other Go-fast Ideas
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by Doug Humble
- Hits: 3200
Dave Richardson found this article in the Central States Association newsletter (EZ Guys) and is entertaining some of the fairing ideas on his Tri-Q. The article was edited by Terry Schubert and reprinted with their permission.
Gary Hertzler (AZ) - If I had to list all the changes I have made to improve my Vari-Eze's cruise efficiency and top speed, I would prioritize the top 5 in an order something like this:
- A straight well built airframe with accurate aerodynamic surfaces
- Low drag cowling with inboard exhaust pipes
- Well designed wheel pants properly installed and faired
- An efficient prop that is matched to the airframe/engine
- Main gear fairings that align with the line of flight at cruise attitude
One additional item on top of the speed improvement list does nothing for efficiency. Replace the plans specified firewall mounted air filter with a ram air box similar to that described in a recent CSA newsletter. However, when we look at the more practical side of time and dollars expended for mph (efficiency) increase, the priority listing assumes a different order. The top item becomes gear fairings and wheel pants.
The purpose of this article is to describe 2 entirely different methods of creating fairings for main gear on either Vari-Eze, Long-EZ or Cozy. The fairings will increase top end speed 5 to 6 knots. I installed gear fairings on my VE over 12 years ago but the speed increase got lost in the many changes that I did at the same time This speed increase has been documented on 2 different installations that I have been directly involved with.
Don Jones submitted the first method to me. It was described mostly in his words with a few coordinated changes and is a construction more reminiscent of our EZ flying surfaces, but with the foam core removed prior to installation. The second method will consist of adding foam and glass to the existing leg and shaping it to a new airfoil. For those EZ owners that want to take a short cut, LightSpeed Engineering does provide prefab fairings.
If you are like me, there is a great deal of satisfaction seeing the payoff of personal efforts. The objective of this effort is to construct a symmetrical airfoil around the existing gear leg that is in alignment with the line of flight in a normal cruise condition. If you are a real speed freak and are interested in only going faster than your other EZ buddies in an all out race, then the fairings should be aligned with the line of flight at that race attitude. Since I figure that I'm in a cruise condition 99+% of the time, that is the attitude I selected. At any rate, the deck angle difference between these two conditions is not so great that the results would change much.
The best way to find this deck angle is to acquire a digital Smart Level which reads out in 0.1 degree increments. Do not use gravity pendulum angle finders, as the accuracy is not sufficient for this job. If finding a smart level isn't possible, a 12" level with a feeler gage pack will work. Fly in a cruise condition at your normal cruise attitude and place the Smart Level on the armrest and note the angle of attack. It should be between 1.5 and 2.0-degree range if the arm rest top surface is parallel to the longeron. If you are using a 12" level continue to add feeler gages under the aft end until the bubble is centered.
If you aren't savvy with trigonometry, a rule of thumb for determining small angles would be "in 1 inch length there is a 0.017 inch rise for each one degree angle change". This means for a 2-degree deck angle using a 12" level, it would take a stack of feeler gages 2x12x0.017 or 0.41" total in order to center the bubble. Record this number, as you will need it to set your airplane in this "flight" attitude when manufacturing the fairings.
Select an area with a flat smooth floor where you can leave your plane for several days while the fairings are under construction. Place plenty of weight in the nose and set the plane on all three wheels. Ideally it is best to place supports under the wing spars to relieve almost all the load off the main gear, but just setting the gear to relieve strut stresses will also work. The point here is to have the gear in an undeflected, in-flight condition. Place shims under a wing jack if you use them or under either of the main tires to position the aircraft level across the longerons. Then shim under the nose wheel to duplicate the in-flight deck angle you had determined previously.
Now drop a plumb from each side of the fuselage at both the instrument panel and firewall and make a small mark on the floor with a permanent marker. You will need to check these marks throughout the construction process since it is so easy to bump the plane out of alignment. Now, snap a chalk line centered between these two pairs of dots. That gives you a projected centerline of your aircraft on the floor. Now that you have the fuselage set up, you are ready to start constructing the fairings.
With the gear strut set, or ideally the weight off the landing gear, trace the outline of the top of gear leg bow on a piece of cardboard. Measure the max thickness of the gear at the top where the gear intersects the fuselage and at the bottom where the wheel pant will meet the fairing. Measure the distance from the top to bottom of the strut along the top of the gear leg. Cut a form set/fixture (use the cardboard template for shape) from plywood as shown in the photo. To reduce construction time, fabricate two form sets or fixtures so that both gear fairings can be worked simultaneously. Make this cut as smooth as possible as it will have an effect on your final product. Construct feet and fasten them to the convex form using drywall screws so that the fixture will be steady when laying up the fiberglass.
The cores are made from left over blue Styrofoam, if available. Otherwise, a pink 2"x4'x8' sheet of Styrofoam from Home Depot's insulation department will do. Make the hot-wire template about 0.1" greater in max thickness than the dimension taken from the gear leg. This will allow for the wire burn into the foam core. Scale up the coordinates for the symmetrical NACA 65025 (25% thickness to chord ratio) airfoil given in the following table. Plot the coordinates and manufacture hotwire templates for the sections at the wheel pant intersection and fuselage intersection. if you are new to airfoil plotting, a good review on the subject can be found at: http://www.mh-aerotools.de/airfoils/hdi_plotairfoils.htm.
The table below is given in percent chord length. If for instance, you have a gear leg thickness at the wheel pant intersection of 1.2 inches then the chord length for this 25% thick airfoil would be 1.2/0.25=4.8". This 4.8 inches represent 100%.
To calculate the rest of the numbers multiply the figures by 4.8. As an example, the x and y coordinates for the 50% chord point are x=0.5x4.8=2.4 and y=0.1185x4.8=.57.
Hot-wire the cores as you did the wings. Draw a line with a straight edge down the leading edge of the foam core. You now have established a leading edge cut line for the knife trim after the lay-up.
|x (%) + & - y (%)|
Use the bottom or convex wood form first, as that will leave the top exposed for the lay-up.
Bend the foam over the form with the foam core contacting the form at its max thickness point. Max thickness for this airfoil is at 40% chord. Cut the foam, chordwise, in successive relief cuts about half way through. Space them about %" to M>" apart so that the foam core will bend to conform to the curve of the wood more easily.
Screw a thin 1/8" piece of wood to each end of the foam core and then screw it again into the ends of the wood fixture. This will eliminate the need for gluing the foam to the wood. Check that the core is not twisted when it is secured down against the form. Verify that the chord lines are parallel to each other at both ends and a couple of places along the airfoil.
Be certain that you are constructing one left and one right side fairing. Apply duct tape or silicone caulk to the foam core to act as a release agent. Lay peel ply along the center 1M>" of the core and the last inch along the fairing's trailing edge. This will later assist both the bonding of the cuff to the gear leg and also bonding the trailing edge close out.
Lay up 3 plys bi-directional cloth with a 45-degree fiber orientation. Knife trim along the leading edge line. After cure, sand the leading edge almost to knife-edge tapering back about an inch chordwise along the entire length just as was done when the canard or wing's leading edge was built.
Position the other half of the wood form on the fiberglass and Bondo it in place. Remove the original wood form and using the feet from it, reattach to the new core/fixture assembly.
Remove the foam from the aft % " of the airfoil down to the peel ply. Duct tape or silicone caulk the raw foam surface and duct tape over the trailing edge peel ply. Position peel ply along the center 1 M>" and trailing edge close out full length just like was done on the topside.Apply 3 layers of BID with 45-degree fiber orientation lapping around the leading edge.
Once cured, remove the cuff/fairing from the core and peel ply and sand the leading edge overlapped glass to feather out the 3 plys. If done properly, the contour in this area will be correct.
I squeegee on a micro mixture (somewhere between wet and dry) to the glass lay-up after a partial cure. This fills the weave of the cloth and greatly assists the finishing process.
Carefully trace a fine line along the center of the leading edge. With the fuselage aligned properly, slip the cuff/fairing over the gear leg and trim as necessary to mate with the fuselage and wheel pant. Temporarily clamp the trailing edge with small spring clamps. Place a straight 2-foot piece of 1x4 on the floor along one edge of the aircraft centerline, which was established in the fuselage alignment procedure. Weight or temporarily bond this 1x4 to the floor.
Using an accurate steel scale, measure the distance from the corner of the 1x4 to the leading edge line and trailing edge in several locations along the fairing/cuff. For a properly aligned fairing, the distance from the leading edge line and trailing edge to the edge of the 1x4 will be identical at each station along the gear. In other words the chord line of the fairing's airfoil must be parallel to the aircraft centerline.
Once you feel comfortable with this process, remove the cuff/fairing and apply a bead of medium dry micro along the max thickness of the gear leg top and bottom. Spread the cuff open and install over the micro. Before squeezing the trailing edge shut, apply flox to the glass to glass closeout. Clamp the trailing edge, align the cuff/fairing as described and let cure.
Fill in the trailing edge joggle with dry micro and finish per standard processes.
This procedure involves the application of foam to the gear leg, contouring a new airfoil and glassing the added foam. Sand the paint off the gear legs if this is a retrofit on an existing airplane. Cut a dozen foam blocks, scrap blue wing Styrofoam is ideal, into approximately 1.5"x6"x6" pieces. Spray glue a strip of coarse sandpaper to the top aft of the gear leg and sand the coutour into the foam. The leading edge of the foam block should be on or slightly forward of the max thickness point of the gear leg. Sand deep enough so the foam overhangs the trailing edge. Bond the blocks to the gear leg with wet micro. Be careful not to get micro between the blocks. Hardened micro doesn't sand nearly as
nicely as raw foam and will leave an uneven surface. These joint lines can be filled later with dry micro just prior to application of the glass.
After the micro sets up, sand the foam on the bottom of the gear leg to form an extension to the existing airfoil. This extension should be essentially a flat surface. Slurry the raw foam and apply 2 layers of BID with 45 degree fiber orientation to the trailing edge.
This next step is very important in achieving a good airfoil in alignment with the line of flight. Measure the max thickness of the gear leg in 8 or 10 places along the bow and record the dimensions. To avoid confusion I found it convenient to write the dimension with a felt tip pen directly on the gear leg. Construct a vertical reference fixture from plywood making the width just enough to fit between the inside of the tire and the fuselage. This fixture will be used to provide a vertical reference for accurate placement of the trailing edge of the new gear airfoil.
On one leg of the T shaped fixture, install a wood screw in the corner to serve as leveling adjustment. With the airplane aligned in position as described earlier, place the fixture on the floor touching the leading edge of the gear. Using a level, adjust the leveling screws so that the reference surface is vertical. Square the reference surface with the aircraft centerline using a carpenter square. Place a square against the lower surface of the gear leg at a location where the airfoil max thickness is known. Slide a 6 inch steel scale along the square until the distance between the square edge and the newly glassed bottom surface is exactly % of the airfoil max thickness at that point. Place a dot at this location. This will establish the trailing edge location of the new airfoil. Follow this same procedure at the remaining locations along the gear leg and connect the dots. Cut through the glass and foam along this line, thus forming the new trailing edge.
Sand the foam to coutour on the top surface. It is helpful to have a contour gage to match the top and bottom contours. Remove of the foam from the trailing edge down to the glass, as done on wing trailing edge close outs. Slurry the foam and fill the trailing edge wedge with flox. Glass with 2 layers of BID at a 45-degree fiber orientation.
The gear leg/fuselage intersection fillet is very critical for low intersection drag. For installations with a NACA inlet, this intersection is very close to 90 degrees and will require little or no fillet.
Notice in the photo of Jamsek's completed gear fairing, this fillet is integral with the leg fairing and slides against the fuselage side. There is no fillet radius ahead of the max thickness point on the gear leg airfoil and only a small fill aft of this point.
The gear leg/fuselage intersection is not as ideal (acute angle) on the original pitot type (P-51) inlet and requires a larger fillet extending back to the firewall. A good example of addressing this intersection is on my Vari-Eze. Even though there isn't a P-51 scoop on this installation, the intersection angle is the same with all the same interference drag issues.
There you have it, two different gear fairing methods. Choose the one that most suits your needs and experience level and enjoy the added efficiency.
You can order a printed copy of Q-talk #112 by using the Q-talk Back Issue Order Page.