"The Big Guinea" - 180% DTFB FT Guinea Pig

Hey everyone,

I wanted to post a project that a co-worker and I built this past summer, I think it shows just how much you can do with DTFB and it turned out pretty well.  I know its a long read, but I’ve tried to add as much detail as I could about how we did everything just in case anyone wants to try something similar. 

 The "just in case it crashes" pre-maiden flight photo.

 The completed airplane.

 Video compilation of the maiden flight and several more recent flights.

This project was something that a co-worker and I thought up one day while we were goofing off at work.  One of us suggested “Hey wouldn’t it be cool to make a huge Guinea Pig?”.  After some back and forth to hash out the details, we decided that we’d try it.

We decided that the basic goal would be to build a 180% FT Guinea Pig out of DTFB in the most simple and affordable way possible. 

We wanted to retain the overall look of the FT Guinea Pig, but were not opposed to making changes for simplicity, practicality, or in a few cases preference.  We focused on trying to use materials and electronics that are commonly used for the regular size FT aircraft, partially to prove that you can build something pretty big using your park flyer equipment, and partially so that if this thing just crashed and burned, we would be able to put all the electronics back in our other airplanes!   This is by no means a 100% replica of the FT Guinea Pig, nor is it the prettiest build if you look closely; however, it is a fairly quick and easy way to build a huge plane that looks a lot like the Guinea Pig. 

The Design Process

As I mentioned above, one of our priorities was to make the plane simple and as cheap as possible.  With that in mind, we made one big design change to the classic FT Guinea Pig right up front.  We decided it would have 4 engines instead of 2.  By doing that, we could use the same D2830-11 motors that we use on several other park flyer sized airplanes that we fly, including a few of the FT planes.  We also tried our best to use HXT900 servos wherever possible.  That prevented us from having to buy electronics that we could only ever use on this plane.  

Before we started building we had to do a little bit of planning to figure out exactly how big we would make the plane, and to make sure we could actually get it to fly at that size. The first thing we did was to download the free plans for the FT Guinea Pig and import then into a CAD system.  I recommend using Draftsight for personal use because it’s mostly the same as AutoCAD, but it’s free (see the link below).  Once we had the plans in CAD we measured the pieces and scaled them up as much as we could without them becoming too big to fit in my truck.  That ended up being about 180% scale to the stock FT Guinea Pig.

https://www.3ds.com/products-services/draftsight-cad-software/free-download/

Once the size was defined, the next step was to do a weight estimation.  To do that, I traced out the basic shape of all the scaled up pieces from the plans in CAD, and then calculated their area.  Due to the size of the build, I also looked at them and tried to guess which ones would need to be doubled for strength, and then added that area as well.  I then calculated the weight of foam we would use based on the weight per square inch of foam board.  I used a value of 145g per sheet (.24g/in^2) which is a little bit inflated to try and account for glue.  Obviously this is a very very rough estimate (and it turned out several pounds low in the end...), but its better than nothing.  We then added in the weight of all the electronics we expected to use to get a ballpark figure for the final weight of the plane. We made sure that the 4 motors we planned to use would be able to power a plane that heavy by calculating the Watts per pound that we would have.  We came up with an estimated 85 Watts per pound, which looked pretty good and gave us some room for error (the completed airplane ended up having only 63 W/lb and it still fly's fine). Since everything looked okay, so we started building.

I should apologize right now that I didn’t get more pictures of the build, I always mean to do that, but I always seem to forget.  I have added pictures from the completed airplane that I think should make everything clear.

The Wing

The first thing that we built was the wing.  We decided to make it three pieces, which would allow us to put the wing joint farther away from the center of the wing, so that it would be under less stress than if it were in the center.  The center spar was built out of two .5” square wooden dowels from Home Depot, which were then shear webbed with balsa wood on both sides. The outboard wing spars also used the .5” wooden dowels, with two dowels sandwiching a single dowel which then fit into the opening in the center spar.  We then added a screw that goes through the spars where they overlap to hold the wings together.  We did almost exactly what Josh did on his Monster FT Cruiser from the 400^th episode build.  We also added a small wooden secondary spar towards the trailing edge of the wing to make sure the sections did not twist relative to one another during flight.

https://s3.amazonaws.com/assets.flitetest.com/article_images/full/monster-build-tips-11-jpg_1396472251.jpg

How the wing segments slide together.

The rest of the wing was built in a method similar to what Experimental Airlines does on his Armin wings so that we could get a smooth airfoil.  We left the bottom of the wing flat, then cut a score for the leading edge bend, and removed the paper from the inside of the foam on the top surface of the wing so that it could bend smoothly.  Due to the size and thickness of this wing, we added ribs every 4 or 5 inches inside the wing, but their main purpose was just to hold the shape of the airfoil over such a large span.

 http://flitetest.com/articles/Easy_Foamboard_Airfoil_the_Armin_Wing

The smoothish profile of the wing, this is the very outboard wing tip. 

One thing we did not do, which we really really should have, was to pre-cut holes in all those ribs for the motor and servo wires to go through.  We ended up having to make a makeshift drill out of a 36” long wooden dowel, and a .5” DIA brass tube with some teeth cut into the end of it.  I was able to load this into a power drill and drill holes all the way through our wing for wires, although I DO NOT recommend doing it that way, it was a huge pain. Cut your holes before you glue the wing together…  

The makeshift drill bit, this was a huge pain, don't do it this way...

We decided to use two HXT900 servos for each aileron.  Not only did this allow us to stick to our common electronics, but it also allowed us to use a single thickness DTFB sheet to make a huge aileron.  Since there are two connection points on each aileron, the aileron will not flex nearly as easily as it would have with a single servo connection point.  You may also notice that the ailerons stick out behind the wing almost as if they were an afterthought.  There are two reasons for that; the first is that I think it’s easier to build with this type of wing.  You can build the wing, then just tape on the aileron.  The second is that by having the larger chord at the aileron, you cause the wing to have a tendency to stall first at the center, this means that when the wing starts to stall, you should still have aileron control and will be less likely to have the plane go into a tip stall. 

Two servos per aileron.

Other than the spars, and some plastic that we used to keep the foam from getting dinged up, the entire wing is made out of DTFB and hot glue. 

Plastic sheeting to keep the rubber bands from crushing the wing.

The “Power Pods”

I say “power pod” very loosely here; this could have been done better, but we were trying to be fast.  We could have used a legitimate swappable power pod here since it would have fit, but we ended up just building a permanent wing pylon that kind of looks like a power pod. They are functional, but not pretty. 

 Center wing section with a "power pod".

The Fuselage

This was definitely going to be the biggest fuselage that either of us had ever built, so we weren’t sure if it would really work out made of DTFB alone, but we figured we’d try. We joined several pieces of DTFB together on the living room floor until we had a single huge board.  We then measured the scaled up plans in CAD and drew the pieces onto to board and cut them out.  We tried to build the fuselage almost exactly like how the regular size FT Guinea Pig is done.  We added an electronics tray in the top 2 inches or so of the fuselage, several square foam bulkheads throughout, and we doubled the DTFB on the walls of the fuselage in the payload area for extra strength.  We also added some paint sticks along the bottom edge of the cargo bay because we were worried that a hard landing might crack the foam there.  Right on top of those paint sticks, we added a removable floor to the cargo bay, it just slides in and rests on the paint sticks (see photos below).  The removable floor allows cargo to slide out without hitting any of the servos or wires that are in the area. The nose was also built in the same way as the nose for the FT Guinea Pig, although we did add an internal battery tray in the nose just in case it was needed. 

The completed fuselage.

 The completed nose section. The wooden blocks on top are for little pins that hold the nose onto the fuselage.

 Looking into the cargo bay from the back.  Note the doubled walls and the paint sticks with the removable floor on top of them. 

The Cargo Bay Door

The cargo bay door was one of the more complicated parts of the build, and frankly there’s probably a better way to do it, but here’s what we did.  Our goal was to be able to carry fairly heavy payloads, but for aerodynamics and looks, we still wanted to be able to shut the door when we were done with the drop.  The two most common ways of doing a cargo door are to either put a servo on the back of the door, and use it like a latch, or to add a servo to the inside of the door and move the door like a control surface.  We initially went with the latter of those methods.  We put two servos right on the inside of the door and they worked great to actuate the door up and down, however, as soon as any load was put on the door, the servos stripped out.  Any amount of weight on the door was just too much for the materials we were using.  To fix this we did two things.  One was to go back and add a third servo on the back of the door which functions as a latch, the second was to add a way to secure the load and keep it from sliding back and hitting the door.  To do that, we added a rubber band inside the cargo bay.  This rubber band is fixed on one side of the floor, and attaches to a servo on the other side.  This allows us to secure large objects such as water bottles so that they don’t roll around while the plane is in flight.  That prevents extra weight on the cargo door, and perhaps more importantly, it prevents load shift which could significantly change the CG while flying. There have been many full scale crashes because of load shift, so we were really hoping to avoid that problem.

The way this all works in practice isn’t elegant, but it works.  When we get ready to drop something while we are flying around, we hit two switches.  The first one releases the rubber band and it unlatches the cargo door, the second one lowers the cargo door. Once the drop is done we just close the cargo door and leave the latch where it is.  It worked out pretty well in the long run, although I’m curious to know if anyone has found a better way to do something similar. 

The two internal servos controlling the cargo door.

The external latch servo on the cargo door.  

The Main Landing Gear

We built the main landing gear a little differently than the FT Guinea Pig, mostly because we were kind of lazy. The way the gear is attached to the airplane is similar to the Guinea Pig though; we sandwiched a piece of foam with the landing gear wire between two paint sticks, and then glued the whole thing to the bottom of the fuselage. Thats pretty much all we did . . .  We were almost done with the plane by then and decided that the wheel pants and double wheels were unneccessary, even though they would have looked cool. 

The main landing gear.

The Nose Wheel

One of the bigger design changes we made to the FT Guinea Pig was to add a steerable nose wheel rather than using differential thrust to steer. We thought that with a plane this large, we might not have the power, or the structural strength, to do a lot of aerobatic maneuvers, so in this case differential thrust didn’t seem like it would give us any advantage in the air. We also wanted to be able to steer more precisely on the ground. First we built the double nosewheel nosewheel assembly.  To do this we used 4 pieces of wire, two straight pieces and two smaller L-angle pieces.  They were then secured together by wrapping them with copper wire and filling in the voids with solder.  This is a great method of joining steel wire together, it ends up being a really strong joint.  We had the nose wheel strut go through 2 layers of foam and 3 layers of plywood, then we added a traditional nose wheel control horn on the top of the strut.  A servo was installed right beside the protruding nose wheel strut and hooked up to the control horn.  We added a few wheel collars to secure the strut, and we were done. After the maiden flight we found out that this was a place that we really needed a standard servo, as the HXT900 we had originally installed was stripped out. Since that has been replaced, the setup has worked great. 

The nose wheel mechanism, you can also see the electronics tray. 

 The nosewheel strut, copper wire and solder is a great way to make a wire joint.

The Tail Section

The horizontal and vertical stabilizers were pretty easy to build for this project.  The only major change we made from the FT Guinea Pig was to double the thickness of the foam.  We used two layers for each major surface to give it a little more strength.  We installed two HXT900’s for the elevator (just like we did with the ailerons), and we used a single standard size servo for the rudder.  Since the rudder itself was huge, we used a piece of credit card style scrap plastic (I think it came from an advertisement in the mail) to re-enforce it. 

The vertical tail. 

The horizontal tail. 

The Electrical System

The way we wired everything together is simple, but it is a lot of wire and a lot of soldering.  We had two XT60 connectors run in parallel with 12 AWG wire to a single point where all four ESC’s plus our UBEC connected. This point ended up having fourteen wires that all met each other (7 positive and 7 negative).  The system is wired so that either battery can power all 4 motors, however if you actually did this at full power, the current drawn would just barely exceed the ratings of both the 12AWG wire and the XT60 connector, and the system would quickly overheat.  That’s why it was important that both of the 12 AWG battery lead wires had full contact with all four ESC wires.  Since all of the ESC’s are located right by the battery, we then ran 12 more 16 AWG wires from the ESC’s all the way out to the motors, with a few connectors added in wherever we needed to break the plane apart. I know thats a little bit confusing to describe, so take a look at the wiring diagram below. I haven’t had problems with anything electrical so far, however, it is a lot of wiring.  Each time we take it out to the field, there are a total of 24 wire connections that have to be made  before we can fly (including battery and servos).

Power system wiring diagram.

 The 4 ESCs and receiver in the electronics tray. 

Specifications

Wingspan: 110”

Length:  76”

Motors:  4X Turnigy D2830-11

ESCs:  2X 25A Turnigy Plush, 2X 30A Turnigy Plush

UBEC: 5A from RMRC

Receiver: Lemon RX DSMX 10 CH with Satellite

Batteries:  2X 5000 mAh 3S

Servos:  10X HXT900 (or similar), 2X HK Standard Size

Weight: About 11 lbs with Batteries

Power: Around 700 Watts at Full Power

Current Draw: Around 60 Amps at Full Power

The Maiden Flight

After a few months of sporadic evenings working on this, we were finally ready for the maiden flight.  I was shocked when it went almost exactly as planned!  The plane flew great.  It definitely feels like you’re flying a school bus, but it maneuvers well and is incredibly stable.  When it was time to land, the plane settled right in with a little bit of power and made a smooth landing really easily. 

The wing is big...

So many wires to hook up. 

Low pass. 

Photo pass. 

Painting

Once we knew the plane would fly, we went ahead and gave it a paint job.  First we put a coat of polyurethane on the whole thing, then we found a friend with a backyard and a garage, and we went ahead and spray painted it.  We were hoping it would look something like Fat Albert of the Blue Angels, but it might have ended up looking more like Donald Duck, I’m still not sure. 

The finished product after paint, with the Champ just for contrast. 

Following Flights  

The plane has flown quite a few times since its maiden and it’s still doing pretty well.  We tried to do a candy drop on the third flight for a local fly-in that my RC club was holding; that was when we first learned how weak the cargo door servos were.  The plane went bouncing down the runway with candy spilling out everywhere, and as soon as it took off all the rest of the candy fell out right there on the runway.  The kids didn’t complain, but after that we went back and added the latch servo that I described above in the build.  So far the heaviest payload we have tried to fly with and drop is 2 lbs of water bottles, the plane handled that fine though, so eventually we might try a little more.

The video at the top of the page shows bits and pieces of both the maiden flight, and a few other much more recent flights.  The parachute that we dropped in the video was holding a single 16 oz water bottle.  I hope to get a good flying day soon and try to get some better air to air footage of it flying. 

In conclusion, I think that overall this project was pretty successful in doing what we originally set out to do, which was to build a huge FT Guinea Pig out of common and inexpensive materials.  I think we both learned a lot over the course of this build and despite the problems that we had, the plane ended up flying far better than I would have guessed.  It goes to show that you really can build some pretty incredible stuff out of something as simple as DTFB and cheap electronics. I wish I had a set of plans to put in this article, but we never made any formal plans, and I doubt too many people will be trying to speedbuild this one anyway. Despite how happy I am with the outcome of this project, I don’t think I’ll be doing anything this big again anytime soon; I’m not sure I can fit another one of these in my apartment!

Thanks FliteTest for the design to copy, and an even bigger thanks for showing us what a great material DTFB is!  

Update: I have flown it a few more times and so far I've gotten the payload up to 4 lbs. Based on that, I'm pretty sure it would drop a 2 liter bottle, so I think thats what I'll try next.  I also got some better flight videos and parachute drop footage that I added below.