Flag Wire Landing Skids

Hello everyone,

The weather here in Western PA is turning lousy so I am returning back to the buiding board.  I pretty much fly and repair in the summer.  I design, build and write articles through the off season.  This is my first article of the season.

This is an article showing how to build landing skids from flag wire.  The main landing skid is made from a single length of flag wire.  It produces 2" to 3" of ground clearance providing good protection from most objects found on the average grass or hard surface landing field.  Spring tension is built in so there is some absorption of the landing forces within the gear.  The gear distributes the landing impact to three points on the aircraft which are reinforced to take the forces applied.  They are very good at protecting hand launched belly landing airplanes from being damaged while landing.

I have used these on a wide range of hand launched airplanes from compact one-sheet designs like my Simple Fold Delta with a 26"wing span up to my larger Two Sheet No Waste Twin Tail Boom #1 and #2 (not yet published) with 60" wing span.  As long as you can hand launch it these skids should work and will help to avoid those belly scuffs and prop damage common to belly landings.  

Since they are made from fairly thin wire they are barely noticeable while your airplane is in the air.  There is also almost no wind resistance and negligible impact on CG.

Here is a picture of my Simple Fold Delta - EDF sitting on its landing gear.

This is a one sheet design with a wing span of 20".  The two hoops at the nose are the main gear.

Here is a side view of the main skids:

The skids keep the under-slung EDF from acting as tne  main gear.

Here is a shot of the bottom side showing main and tail skids:

The tail skids are mounted on foam board triangles, one each side:

You can see how the 90 degree bend is embedded in the faom triangle.

Here is my 60" Two Sheet No Waste Twin Twin #2 (not yet published) with the skids installed:

I am using this as an example because it was in need of landing skids. 

Here is how they are made:

MAIN SKID BENDING:

Take a marking flag with 21" wire ( from Lowe's) (Home Depot's wire is shorter) and remove the flag with a knife.

First find the center.  It should be 10 1/2" from either end.

Make a 90 degree bend at the center.

Find a point 1 1/2" from the first bend and make a second 90 degree bend.  The direction will be rotated 90 degrees from the first.  With the first bend laying flat on the table bend it straight up.

Do the same thing on the other side.

Since I have done a bunch of these I can angle the legs out as I do the second and third bends.  On your first tries you will bend mostly straight up.  These are actually much closer to parallel than this picture looks.

This is what it should look like after the first three bends. 

Bend the legs apart till they are splayed out 30 degrees on each side

Next you have to curl the first half of each leg back for about half its length.  One side is already done here and the second side is started.  Start with the pliers against the 90 degree bend and make a series of slight bends moving the pliers the width of the jaws each time.  It is OK if the ends come toward each other or even cross.  You can bend them apart later.

This is about 3/4 of the way done.

This is what it should look like after bending both sides. If your legs are closer together just bend them out till they are about parallel. 

Next you have to make the bends where they insert into the wing sockets.  The first bend is the width of the widest part of the jaws and goes straight up.  Like this:

The second bend is the width of the narrowest part of the pliers and bends back down.  It is just an over sized "Z" bend.

It looks like this.

Here it is with all bends made.  From here the rest is just shaping.

MAIN SKID SHAPING:

Here I bent each side out further till the total spread is about 6" to match the socket locations on my model.

When the base is held down the tails should be about 2" off the surface:

The total length should be about 7".

Play with it till it is uniform and looks like this.

MAIN SKID INSTALLATION:

Here I am measuring 5" forward from the leading edge of the wing on the bottom of the fuselage.

I used a plastic triangle to square the marks across.  (It is hard to see black on black.)

Position the skid like this on the marks and centered on the width of the fuselage, then trace it.

You should end up with marks like this:

Take a hobby knife and make single score cuts centered between the trace marks on both sides.  Like so:

Use a skewer to open the slots like you would for installing a control horn.

Trial fit by pushing the base down into the slot.  Make any adjustments needed till it will go all the way down to the paper on the other side.

Remove the skid.  Fill the slot completely with hot glue and then reinsert it.  Refill the slot if necessary and squeegee the surface smooth.  (This looks messy because of the paint.)

Next measure for the wing anchor positions:

Measure 3" from center both ways on the bottom of the leading edge of the wing.  You can see the mark on the left.

Here is a pair of anchors ready to install.  They are made from flexible cutting board (see related article below):

This picture shows all the steps to make the anchors. First cut a 1 1/4" strip of cutting board. Then cut a 1 1/4" square.  Then cut the square diagonally into two triangles.  Then drill the centers of the triangles with a 5/64" bit (or 1/16" reamed out a bit). 

Here I removed the paint where the anchor will glue down because the paint does not adhere to the tape well.  I should have done the skids before painting.

Here both anchors have been glued on.  It is hard to see detail because of the white background but the holes were already drilled in the centers.  They are set in a full bed of glue while trying to avoid any squeezing out (tricky). 

Drill through the holes in the anchors at about 45 degrees to make holes through the lower wing foam board.

Flex the end of the skid wire and insert it into the drilled hole.  There should be considerable spring tension on the wire.

Inserted:

 The "Z" bend acts as a stop and the spring tension keeps the wire firmly in place.

Finally cut a piece of 1/16 plywood (or 1/8" lite ply or gift card if that is all you have) to reinforce the area and to spread the load to the foam board.  This one measures 1 1/2" X 2 1/4".  Different models may require different shapes (see the second picture from the beginning above).  Set in a full bed of glue.  The glue on this should be in full contact with the glue in the slot.  

Here is a bottom view of the completed main landing skid:

TAIL SKIDS:

The example installation has a twin tail boom structure so there are two tail skids.  They consist of a foam board triangle with a bent wire skid attached.  They look like this: 

The size of the foam board triangle varies according to the size of the airplane. This model has a wing span of 60".  These triangles are 1 1/2" X 3 1/2".   

The wire portion is made from the same flag wire as the main skid 

This picture shows the sequence for forming the wire skid: 

First cut a 4" length of wire (length can vary according to model size).

Then make a 90 degree bend at 1/2" from one end.

Leave 2" straight, and then make a gradual sweeping bend over the last 1 1/2". 

Make a series of slight bends similar to what was done on the main skid.

Make a second one to match the first (if you have a twin tail or other arrangement allowing two tail skids).

 It does not matter as much the exact shape of the curve as much as that they match as closely as possible.

The triangle should have been installed as part of building the model. If not, cut and install them now.

Then measure 2" from the back edge (smaller for smaller models) and make a skewer at least 1/2" deep at 90 degrees to the bottom surface of the triangle.  I like to pierce the first layer of paper on the surface the where triangle is mounted to help tie things together.

Next melt a slot in the bottom of the triangle from the hole to the back edge with the tip of the glue gun.

Fill the skewer hole and the melted slot with hot glue.

Insert the 90 degree bend into the skewer hole and the straight part of the wire into the glue filled slot.

Immediately smooth the glue with a wet finger.

Build the glue up over the wire by running another pass over the full length and smoothing it down as well. 

The completer skid should look like this.

On some of the smaller FT models there is already a triangular foam board tail skid in the plans.  If there is the wire skid can just be added to it.  Adjust the size as needed to fit.

Well, that is all there is to it.  You can have this done in an hour or two with some practice.

DESIGN CONSIDERATIONS: 

The position of the main skid should be well forward of the normal landing gear. Otherwise you will have a tendency to nose over.  The anchors should be at the leading edge of the wing.

On wider fuselages the anchor points can be on the fuselage.

The base area of the main skid can be adjusted to fit the model but avoid getting too big.  Otherwise you will not have enough length in the skids.

The very back tips of the wire on tail skids should be slightly off the ground.

PERFORMANCE:

The main gear takes the brunt of the landing force.  Over time they can become bent so that the rear straight portion becomes curved.  If that happens just take them out of the anchors, straighten, reshape and reinsert.

The tail skids hold up pretty well over time and do not take any maintenance.  Once in a while you may have to re-glue a triangle that got bumped loose. 

These skids definitely save a lot of wear and tear on the bottoms of hand launched airplanes.  They also often save the nose in a landing that is a little too steep and fast.  They help avoid wingtip damage as well because they keep the model at least 2" higher off the ground.  They also help to minimize contact with moist grass. 

They add very little weight and very little drag.  They are mostly unnoticeable on a plane that is in the air since they have no wheels and the wire is only 1/16" thick.

I have used these on more than a dozen of my designs with great success.  I hope you find them useful as well.

It is time to start building up your hanger for next year. Good luck with your off-season building.