Twin Engine Coroplast FPV Wing

A bit of appreciation.

I would first like to start off by thanking Flite Test for putting on yet another amazing event.  I also would like to thank the Furey family for letting us all turn their property into an RC circus for four days.

 Catching your eye.

Now, let's talk about the plane.  Making a plane that I like to fly is one thing.  To make something that other people like is really special.  I can not tell you how many wonderful comments I received from people about how much they liked my plane.  I can't thank those people enough for their kind words.

This plane came about one night about a week before Flite Fest 2015.  My FPV plane at that time was a Crash Test Hobbies Grim Reaper XL (which is an excellent airplane by the way).  Unfortunately, the 62 inch wingspan posed an issue for me.  Because EAA AirVenture has decided to open up a flying field for rc planes every night, I wanted to fly a plane there and show off our amazing hobby to fellow aviation lovers.  However, we will be flying in a Cessna 210 up to Oshkosh.  This is where the problem lies.  The Cessna, while an excellent aircraft, is not all that big.  My Grim Reaper wouldn't fit into the Cessna without taking up headroom.  While I could probably convince my parents to let me bring it anyways, I wouldn't get the same satisfaction as I would flying something that I had created.  With my size constraint slightly limiting my imagination, I set out to build something eye catching.

Winging it.

I knew that I was going to have to build a wing.  The fuselage of a normal plane would cause transportation issues.  However, a normal flying wing is not all that eye-catching because they are so common.  My next thought was to try a swept forward wing like the Grumman x-29.

While forward swept wings are certainly eye catching, I did not want to have to deal with the instability in the yaw axis that forard swept wings are often plagued with.  So my last option was a straight leading edge.  While looking through a Cessna 210 manual, I noticed an interesting characteristic in the wings.

As you can see in the picture, the Cessna uses a trailing edge sweep.  This characteristic can also be found on planes such as the extra 300.

Even though I did not know how a flying wing of this design would react, I decide to throw caution to the wind and started sketching out different wing shapes.  I knew that I would have to keep the wingspan under 48 inches in order to fit in the plane and I went from there.  At AirVenture, the model airlanes that you fly have to be under five pounds.  Knowing that my plane was going to weigh somewhere around five pounds, I wanted to make sure that I had adequate wing area to achieve a good wing loading.  After some trial and error on my favorite flying wing website I came up with this:      

http://fwcg.3dzone.dk/?wing_span=48&root_chord=20&tip_chord=10&sweep_type=0&sweep=0&cg_pos=20&show_mac_lines=0

When plugging in the numbers for the tip and root chords, I did not take into account the elevons and so the actual wing area is a tad bit higher.

Once I had the plans for the wing, I began building.  Unfortunately, I didn't take any pictures while building the plane so I will demonstrate the concepts on pieces of coroplast.

Working with coroplast.

To start off, before you start cutting into coroplast be aware that when cutting diagonal to the flutes, the knife blade tends to be pushed around by the flutes making for not so straight lines.  To help with this issue, make multiple passes with the knife using light pressure.  This should help to create more straight cuts.

I started off using two 24x36 inch sheets of coroplast.  To attach them together, I used the standard technique as shown by flite test.  When doing this, use very hot glue to achieve a stronger bond.

Then smear the excess glue and you are done.

I attached the two sheets together to form a 48x36 inch panel.  However when gluing the sheets together, only glue 20 inches of the 36 inches. (this will make sense in a few minutes).  I then drew out the plans on the coroplast on the side with 20 inches glued together.  It should look something like this:

From here you can cut off the small triangles in the back and save them for scrap pieces later.  Next, make a slit in the coroplast all the way down the leading edge.  The slit should look something like this:

Next I made a slit in a flute 1.5 inches in front of the leading edge.  Then I made another slit 1 inch forward of the previous slit.  Then make slits every inch four more times.  There should be a total of 6 slits not including the leading edge.  Next fold back the coroplast at the leading edge and bevel both sides like this:

When you are done it will look something like this.  The left side of the leading edge being the bottom of the wing and the right being the part that will be folded over.

The wing formers.

Now it is time to make the wing formers.  First, I made a line on the wing for where I was going to put my wing formers.  I decided that the thickest point of my wing would be at 25% of my chord.  In this case the line should be drawn from a point 2.5 inches back from the leading edge on the wing tip to a point 5 inches back from the leading edge at the root of the wing as shown below.

I decided on a wing thickness of 8% of the chord.  This means that at the wing tip the former will be .8 inches tall and at the root the former will be 1.6 inches tall.  I took a single 24x36 inch sheet of coroplast and marked out 8 formers.  The formers need to be 24 inches long so orient the sheet accordingly.  If you alternate 1.6 inches and .8 inches up the sides you can save some material.  Once you have the 8 wing formers you need to laminate 4 together for each half of the wing.  (When using hot glue on coroplast, the glue needs to be really really really hot.  To the point where it would start to melt dollar tree foam.  The heat is needed to ensure a good bond.)  Now that the wing formers are done, you need to attach them to the wing.  Don't be afraid to use a lot of glue because it will only help to make the wing stronger.  Once you've applied ample glue to the former, place it along the line you drew at 25% of the chord.  Once done with both formers, add hot glue to where they meet in the middle to help make the wing stronger.  It should look something like this but again this is only a demonstration piece.

At this point, I added a carbon box tube behind the former for added strength.  Whether it is needed or not, I do not know but it gives me peace of mind to know that the wings won't fold. 

At this point if you've mock-folded the wings over you will notice that there is not enough material to fold over completely.  You need to attach additional material using the standard method that was shown torwards the beginning of the article.

Now that there is enough material to properly make the airfoil, you can fold over the wing.  This is probably the most difficult part.  You are going to have to be very fast to make sure the hot glue does not dry before the wing is folded.  Doing one side at a time will aid in this.  Put glue into the leading edge as well as all the slits that you made.  Then, put glue on top of the former and fold the wing over.  You will now notice the problem with making the wing thickness different from tip to root.  Some of the coroplast will hang over onto the other half of the wing, but it will be cut off in the next step.  Repeat the process for the other half of the wing.  When done, you will have an airfoil.  Now the overhang is very apparent.  I've found that the easiest way to get rid of the overhang is to draw a line straight down the center of the wing and just keep going over it with a razor until your through the coroplast.  Then any detail cutting can be done to make it fit better.

Once the fit is to your liking, you can attach the trailing edge of the wing.  To make this easier, you can cut off the excess coroplast at the trailing edge.  To do this,flip the wing upside down draw a line two inches back from the trailing edge.  Cut along that line so there should be just two hanging off the back of the wing. That will later become the elevons.  Then using really hot glue again, glue the trailing edge together like any other Flite Test plane.  When doing this, be sure to wipe away any glue that has been squished onto the elevons.

Now that the wing is glued you can cut the elevons.  This will be difficult because you will be cutting diagonal to the flutes.  Cut along the trailing edge using the lip created by the folded over airfoil as a guide.  Be sure to only cut through one side because the other side will be the hinge of the elevon.  After cutting halfway through, fold over the elevon and make a bevel for the elevon hinge the same way you'd make a bevel on a Flite Test plane or an experimental airlines plane.

Building the fuselage.

The next thing to do is to make the fuselage.  This is where you can get creative and make it fit your use.  I went for a larger fuselage to accommodate a bunch of gear.  Mine is 4 inches wide and 3.5 inches deep with an overall lenght of 35 inches. I recommend you do the same because you will need the long nose to achieve the correct balance.  As you can see in the picture below I tapered the tail.  I also made the front round.  I did this mostly for looks, but it makes landings smoother in my opinion.  The plane is able to slide on grass very well and it just makes landings look good.

I made the rear 20 inches of the fuselage 1 inch lower than the front portion to allow the leading edge of the wing to meet with the fuselage.  This allows for less stress on the hot glue joint between the wing and fuselage.  The 1 inch drop section is covered with a sheet of coroplast.  This is where the wing will be attached to the fuselage.  The only thing connecting this top sheet of the fuselage to the rest of the fuselage is the hot glue joints along the side.  If you want to add support so the wing won't seperate, you can although it is not needed. 

Now, if you have more time than me, you could build a better fuselage by cutting out flutes in the coroplast and folding it into a fuselage like Ed from experimental airlines does or like Flite Test does.  I was just short on time when I built mine so I opted for the quicker way.  The good news is that at some point I can go back and make a new fuselage because mine did take some abuse at Flite Fest 2015.  If I build a new fuselage I will try to add it to this article.

I left a large portion of the fuselage open on top so that I could later add a hatch as seen below.  The hatch was made by taking a small sheet of coroplast and cutting a slit in a flute to act as a hinge.  (you can see how the coroplast is a bit disfigured from the few mishaps I had at Flite Fest. Fortunately, it is still structurally sound)

Choosing the electronics.

Next come the electronics. Here is what I chose to use.

https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=70910

https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=70752

https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=65294

https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=58747

http://www.hobbyking.com/hobbyking/store/__14350__FrSky_V8FR_II_2_4Ghz_8CH_Receiver_HV_.html

The servos I chose are overkill, but better safe than sorry right?

The motors I chose are excellent.  They allow for unlimited vertical with 3s (using 11x4.7 and 10x5 props) and have the ability to fly on 4s.

The escs are my go to esc and I recommend them to anyone.  They have worked well on every plane that ive put them on.

The FrSky system is a wonderful system capable of going long distance and it is cheaper than most other brands.  I plan to upgrade to a Taranis soon instead of using my fsth9x with a module.

Installing the electronics.

Most of the electronics are mounted inside of the wing.  Only the battery and fpv equipment is located in the fuselage.  I put my receiver in the wingtip and ran all of the necessary wires to it through the inside of the wing.  The servos can be mounted anywhere really.  It all depends on the size of your servos.  I chose to mount them further forward for cg reasons.  I only suggest that you keep the control horn in the middle of the elevon because under heavy loads, the coroplast could warp if the control horn was on one end of the elevon.  Once I mounted the servos, I routed the wires to the wingtip containing the receiver.

The motors are mounted 9 inches away from the center of the wing.  I mounted them by cutting a portion out of the top of the wing and hot gluing a ft simple motor mount in.  Prior to actually gluing in the motor mount, I ran a y harness for the esc signal wire between the two holes I made.  I also made a power harness between the esc's.  To connect the battery to the power harness, I cut a small square in the middle of the leading edge so that the power harness could go directly into the fuselage.  Then, I plugged in the escs (with the red wire cut on one esc signal wire) and then proceeded to stuff them into the wing and then glue the motors and their mounts to the wing.  When doing this, use the leading edge of the wing to insure the motor and mount are perfectly square.  Here is what it should look like when you are done.

As a side note, If you can do differential thrust DO IT!!  If my plane had differential thrust, it's high alpha ability would be incredible.  (Thanks Ed for pointing out that the plane would be great at high alpha.)

Now for all the signal wires that are now showing up at the wing tip you will notice that there are servo wires coming from behind the wing former, and an esc wire coming from in front of the wing former.  I chose to put my receiver behind the wing former and so I cut a small channel for the esc wire to go through the wing former.

The final step, vertical stabilizers.

The last thing to do is make the vertical stabilizers.  This is where I had some problems at Flite Fest 2015. Thanks to a Penn State engineer who's name escapes me, I was able to correct the issue.  The problem was that at low speeds the plane would yaw uncontrollably.  It turns out the crosssection of the fuselage was so much greater than that of the vertical stabilizer, that the slightest bit of yaw would cause the air to catch the side of the fuselage and spin the plane around.  To fix this, I put large vertical stabilizers on the plane. Three large stabilizers to be specific.  While it may be overkill, to some degree I believe it adds to the stability and the ability of the plane to track so well.  You can make the stabilizers whatever shape you want.  I just chose to make them basic shapes.  The stabilizers can be seen in the pictures above.

Flight Characteristics

As many people noticed at Flite Fest 2015, the plane is incredibly stable.  I attribute this to its weight and the large stabilizers.  The plane maintains its stability at slow speeds which is very handy when landing.  The plane is also very stable at high speeds.  While in the current 3s configuration the plane is not all that fast, I am sure that with 4s it will continue to preform well.  The only issue that I have run into is that the plane likes to come in on approach on power.  Then just before landing, I chop the throttle.  Aside from that, the plane flys great.  Even though it is big and not designed for it, the plane can do loops, rolls, and inverted flight with ease.

 In conclusion.

The plane is great how it is now.  However, that is not to say that it could not be improved.  First of all, the plane would glide much better if it was a little lighter.  Coroplast is a much heavier material than foam board and this plane makes that apparent.  The fuselage is another part that could be improved.  While the fuselage I made is functional, it is not all that aesthetically appealing.  With some time and patience, a much better looking fuselage could be constructed.  The last thing that could be improved would be the motor mounts. While they are fairly strong, I don't believe flite test designed then to be used with such large motors.  I believe an aluminium motor mount would suit this plane a little better.

Once again, I would like to thank those who complemented my design.  I look forward to producing more planes that not only bring me joy, but bring joy to others.