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Articles about scratch built RC model planes with custom design and models built from traditional kits (not rtf or artf models)

Flyfly Sabre F-86A build

Details

Sabre F-86A (kit FlyFly)

Sabre F-86A FlyFly LadyLuck color schemeBrand: Fly Fly Kit
Wingspan: 1420mm
Weight: 3kg  (6.6 lb)
Engine: Wemotec MidiFan balanced by Jepe
Batteries: Lipo 6S 4000
Electric retracts EPF Hobby DSR 46T
Radio Power: Hyperion Switch-Mode BEC
Radio: Futaba 2.4 Ghz
Construction: Polystyrene Kit
Finish: 25g fiber glass cloth and epoxy resin. Acrylic paint (Tamiya) airbrushed

The kit Fly Fly Sabre has achieved great success since its release. It lets you build an EDF jet  of the 90mm category for a very reasonable budget  (about 150 Euros for the kit).
The flying qualities are excellent and the imposing size of the model gives it a lot of presence in flight.
Of course we can quickly assemble the model and fly this bird after a few evenings spent in assembly. However, I chose a different approach. The kit reproduces quite accurately the shapes of the real size aircraft and I thought that spending a little more time on it you could obtain a nice semi scale model
I will not detail here all the construction points of the model but just a few changes I made from the original construction or important points not to be missed.
Note also a very active discussion (since 2007!) On the RC Groups forum about the Sabre.

Fuselage assembly

f86-back_shelves.jpgThe fuselage consists of four elements. Two front half shells and 2 rear half shells have to be assembled together to form the body of the aircraft. Nothing too complicated? Indeed the work is pretty simple. Except that you must install all the wiring BEFORE the assembly of the shells and there are lots of cables to pass. If you forget them, it will be impossible to add cables later!
It's in the aft fuselage that there are the most things to settle down (see left photo). you must provide:
- Two extensions lines for elevator servos (there are two elevator servos).
- Two ailerons servos extensions passing through a slot around the ducted fan.
- An extension for a steering servo. Originally the rudder was not functional on this kit. I think it's a shame to fly such a plane with 2-axis controls (elevator / ailerons). The change is very simple to make and I would recommend to anyone who build this plane to make the rudder functionnal.
- An extension for the controller (ESC), which comes under the fuselage (better cooling) and close to the fan.
About extensions: As it takes a lot of cable, it is cheaper to buy a roll of servos
cable and make your own extensions. This also allows to make them at the proper length and avoid unnecessary extra cable rolled up in a corner of the fuselage. The use of twisted wire with ferrites close to the receiver is better to prevent electrical noise over long distances, as in the case of the Sabre. Although with a 2.4 Ghz radio system, using ferrite is not essential.
- The power cables for the motor. We must install cable of good quality and sufficient gauge. The Wemotec Midifan draws about 60-65 amps on 6S, even more dependeing on the motor used. Too thin or poor quality cables
will overheat instead of supplying the amps to the motor. I've installed 4mm² gauge cables (AWG 11), which seems correct for this type of application.
- The air hose for the retractable landing gear. If installing an electric gear the air hose should be replaced by 2 servos extensions to control the electric gear motors. I did make the change after the plane was build, and the mod was not easy.

Before gluing the two parts together, we will also place the thrust tube for the fan. It's made of clear plastic and looks like a cylinder. But the front diameter is slightly larger than the output. You must therefore be careful to install it in the right position or it wont fit.

Once all these cables and air lines are in place in their respective slots and well secured we can proceed to the gluing of the shells together. I used epoxy glue, it holds very well. But this is in fact too strong for this application and unnecessarily heavy. Jepe recommends on his site using polyurethane glue. It is a good choice provided you do not overfill, because the PU glue expands when curing. Otherwise a good exterior wood glue will make the job too.
The assembly of he front shells is easier, since only the extension cable for the steering wheel servo and the air hose or extension cable for retracts have to be installed.

Wing assembly

F-86A wing detail

f86-wing-detail_02s.jpg Several rc fellows have seen the career of their Sabre end prematurely following the breakdown of the wings in flight! This problem is often mentioned on the forums. But when you look at how the wings are designed this is not so surprising.
The wings are supplied in two parts to assemble. They are molded polystyrene, On the underside there are two depron panels that allow to close the wing when all cables are installed. It is clear that without sufficient reinforcements, this wing will not stand strong load factors! Especially since everyone tends to install more powerful engines on this plane. Some modelers fly this Sabre on 8S lipos. The thrust to weight ratio is equal to 1, a power that has nothing to do with the Haoye fan supplied with the kit!
Again, consider the Sabre FlyFly as a kit, and not as an RTF model received on Friday evening and ready to fly on Sunday morning after a quick and dirty assembly.
The wing requires a careful work to fly safely. I followed the advice given on Jepe site and on RC Groups. The most important is to insert a carbon tube reinforcement that crosses the wing from side to side. I also strengthened the landing gear casing with two plywood plates glued with epoxy. The outer panel also receives a plywood backing glued against the foam "stringer". Simply make a small slot  with a drill bit and a Dremel, then paste the reinforcement with epoxy. Finally, two carbon strips are glued to the wing spar and sandwiched with the panel depron completing the assembly. The carbon strips are not essential. But it looks  "high tech" and as I had some left at the bottom of a drawer ;)
More seriously they give higher stress resistance to the wing. Polystyrene is not really known for its extreme tensile strength (neither compression by the way) and some reinforcements are more than welcome.
Thus reinforced and covered with 25g/sqm glass cloth the wing will be strong and still light enough.
I'm sure some people will say "oh yes, but all these reinforcements are extra weight aso.."
Yes, it's true. My model weighs 3kg (6.6 lb) ready to take off.
I think 3kg  is very reasonable for an airplane of this size. And (so far) my plane did not break in flight. It takes off and lands from a grass field and I haven't stripped of the landing gear after the first landing. the only question is whether you want to stay in the large foamy category or if you prefer to use this kit to make a model a little more ambitious.

 

Some other construction details

f86-tail.jpg f86-elevator.jpg

As it comes out of the box, the control surfaces are hinged with tape... I preferred to imagine that it was only intended to hold the pieces together for transport ... Seriously, how can we consider to fly a model with a 90mm diameter fan having the ailerons and elevators holding with scotch tape?
I replaced the tape by plastic hinges. They are inserted into a slot made with an X Acto knife and glued in place. The rudder is made functional and hinged in the same manner, We can distinguish in the lower left of the photo the servo well carved in the foam. 8 or 9 g
servos are sufficient on this model.

f86-aileron_servo_hatch.jpg

f86-aileron_servo-closed.jpgThe aileron servo wells are also modified to be more discreet in the wings, the original hole is enlarged, then I've put two wooden bits on each sides that will hold the 1mm plywood hatch on which the servo is attached.
The servos used are Hextronik HXT9000 well suited for this purpose.
Once closed and painted, the hatch is very discreet and the servo is better protected. Although, considering the low cost of a HXT9000, this is not really a problem if you ever have to change one.

Finishing and painting

f86-readytopaint.jpgF-86A 91129 The model is covered with  25g/sqm glass cloth  and epoxy resin, then airbrushed with Tamiya acrylic paint.
Once again I insist on the fact that epoxy resin is a toxic product that requires appropriate protective equipment when manipulated:
- Respirator with cartridge filter (a simple dust mask is completely ineffective).
- Vinyl gloves. Latex gloves do not protect when handling the epoxy.
- Safety glasses to protect eyes from possible projections.
It's also important to work in a room sufficiently ventilated to prevent buildup of harmful fumes.
The epoxy resin fumes can cause allergies. As for the long-term consequences .... I am not a doctor, but common sense makes me think it's best not to breathe this kind of products.
Jepe now uses acrylic products to cover with glass cloth. Although a good alternative, it won't be as resistant as with epoxy resin.
I used a profile from the booklet  "Walk Around n ° 21 F86 Sabre published by Squadron Signal to make the paint work. The full scale aircraft flew in Korea in 1952.
The pilot figure comes from HobbyCity shop .
I also added some elements in the cockpit to make it look a little more scale. I will not pretend it's a scale cockpit, but it's more convincing.
If you'd like to have a more detailed cockpit than the one that comes with the plane, Park Flyer Plastics sells a cockpit upgrade for the Fly Fly Sabre.
Most details and marking are airbrushed. Which requires a lot of patience to cut the different masks needed (somebody said I'm masochist?).
The pinup and the 334-FIS logo are printed on special paper transfer.
I discovered Lazertran paper that gives great results for printing decals with an inkjet printer.

f86-cockpit.jpg f86-masking.jpg f86-masking_02.jpg f86-cockpit_out.jpg

Some elements that you can use to finish your model

lady_luck.jpg 334-FIS.jpg Instrument panel Sabre F-86
Lady Luck 334 FIS emblem
F-86 instrument panel

Installation of EPF Hobby electric retracts

train-epf_01.jpg EPF Hobby control box DSR-46CIt is better to plan the installation of these retracts from the beginning rather than making the modification when the model is already built. As the dimensions are slightly different from the  FlyFly air retracts, this would require adapting the gear supports accordingly.
The air cylinder and the servo control valve is replaced by the EPF electronic control box which is lighter. The electric gear legs are slightly  heavier than the Fly Fly air retracts. Eventually the weight is roughly equivalent.
The EPF retracts can be powered by a separate
2S lipo battery  (probably most reliable) or draw its energy from the receiver pack or ubec if this one is powerful enough. As I've installed a switch bec able to sustain 8 amps I adopted the latter solution.
For security, the electronic control box automatically shuts off one leg if it jams during the sequence of entry or exit. This avoids draining the battery pack or overloading the bec with consequences always harmful to the integrity of the model.
The sequence of entry or exit takes about 5 seconds. It's great fun to operate on the ground and the effect is much more than a model train violently ejected from the slot as a devil out of the box!
The only difficulty to adapt the EPF retracts on the Fly Fly Sabre comes from the nose gear. We have to find a location for the steering wheel servo. I finally milled a housing on the side of the fuse where the foam is thick enough, and installed  the steering servo (
HXT9000) inside (see photos).
EPF indicates that the DSR 46 retracts can be used on planes up to 8 pounds (3.7 kg). It's probably a bit optimistic. But they seem to hold well with my  3kg Sabre taking off from a grass field.
They work very well so far, but they are few things to know to ensure reliable operation. It is imperative that the front wheel remains at neutral during entry or exit sequence. Otherwise the nose leg will jam half way. The sequence lasts about five seconds, so does it means we are not  supposed to operate the rudder while the retracts function?  I found a solution thanks to my Futaba
T12  transmitter:
The front wheel servo is assigned to the command "rudder 2". This command is mixed with the main rudder control  "rudder 1". When the retrracts go up the mixing becomes inactive and the front wheel immediately return to neutral. Mixing is activated again when the landing gear goes down. Moreover, with the T12 transmitter we can set a delay (max. 4 seconds) before activating the mixing. This is enough time to avoid jamming the nose wheel half way from its down position. Landing on two wheels are often quite challenging with a plane equipped with a tricycle landing gear. It is best to avoid getting stuck in this situation!

f86_nose_wheel_build01.jpg f86_nose_wheel_build02.jpg f86_nose_wheel_build03.jpg f86_nose_wheel_build04.jpg

Settings and balance

Checking balance on the F86f86_nose_weight.jpgThere have been many discussions on forums about balancing this plane. It seems that the value given on the construction manual is slightly too rear. Nothing dangerous, but the plane is more pleasant to fly when moving the balance point slightly forward. Once again I followed the advice given by Jepe on its site fto balance the Sabre.
Fly Fly manual indicates a balance point  located  215-220 mm from the notch at the front of the wing. Jepe for its part suggest a value of 207mm. Better a little nose heavy than tail heavy for the first flight, so I decided to follow Jepe recommendation.
Great! But my plane appears to be a  little tail heavy, and I wont be able to move the equipment any farther to the front.Batteries are already stcuk at the front of the cockpit, with the receiver on one side and the ubec on the other side.

Only option was  to add a few grams of lead.
But as I wanted to add only the minimum required, I milled a hole on the nose of the plane to fit the required amount (50g) for a perfect balance.
A Dremel to mill a small slot, a small hatch attached with two small screws (so I can remove the lead later if necessary), then a little touch up paint on the nose and voila. The Sabre is now perfectly balanced.
On the scale fully equipped with batteries, the weight is now  3.03 kg. I thought  that I could have used less filler, avoided the epoxy for Assembling the fuselage... And now it would weigh less. But the flights have proven that even with this slight overweight, the model has not lost any of its qualities.
Note: The photo at left shows the aircraft with a temporary gear. The FLyFly retracts had already broken and I had not yet installed the EPF retracts.

Ready to fly

The maiden flight was made with a 5S 4500 battery . The consumption is around 45 amps and the fan seems to deliver a lot of thrust. I'm pretty optimistic about this first flight. But the plane took almost all the lenght of the field before she was able to take off.
Once in the air, gear up the aircraft is flying correctly. The speed is fairly scale. The problem is that I'm almost full throttle all the time to keep a good flying speed. After several flying around, I decide to prepare for landing. Although I'm totally stressed, the plane is incredibely docile.
She touches the ground gently, run a few meters then the three wheels go back sharply in their wells. A little surprised, I check that I didn't touch the retracts switch when the plane was on the ground, but after a quick inspection of  the model I understand what happened: The nylon thread that holds the air cylinder on the front wheel broke. The landing gear didn't last more than one single flight. Okay the airplane is a little heavier than normal, but still. This is what motivated my decision to find another solution. I also decided to switch to a  6S battery for the fan.

sabre_jft-01.jpg sabre_jft-03.jpgsabre_jft-02.jpgWith a 6S battery pack, the consumption is now 60 to 65 amps, the Wemotec fan makes a higher pitched sound. With a little imagination you can almost hear a real turbine, and the thrust is really impressive!
Now the Sabre can cruise quietly at half speed. When I push the throttle to unleash the power, high speed low passes are impressive and the "jet" can keep climbing high at full thrust. Although I'm not certain that the full scale one that flew in MiG Alley in 1952 had this capabilities?
But flying like this, the electrons tank empties quickly! Especially since the 6S pack (actually 2 3S packs connected in series) is "only" 4000 mAh when the 5S was 4500 mAh. Adding a higher current at full throttle, the autonomy inevitably suffers. But the flight is so more pleasant, I prefer to lose a few minutes of autonomy and fly a plane with more character.

Inflight video of the Flyfly Sabre F86 with the EPF retracts and 6S lipo battery pack

If you fancy flying this legendary fighter, Jepe sells a "fast foam" version of the Fly Fly F-86 Sabre for about 145 Euros. This version comes unpainted (white eps foam) and has to be to be covered with glass cloth and painted as presented in this article. You can of course buy the standard version already painted and use the stickers included in the box. It certainly flies well, but the finish is not the same.