[SARPLT]

As I did not see any info on the FlyFly MB 339 I thought I would start a build thread. Although I had looked at this ARF a few times, I had no intentions of picking one up until I saw that HK had it listed for $69 a few weeks ago during their February sale. As I have spent much more than this on 50mm EDFs, I figured for a 90 mm EDF it was pretty cheap so I ordered one to give it a try. Even with the significant shipping cost and exchange rate it arrived at the door for under $200. I was expecting it to come in a larger box but given the fact that the fuselage is in four pieces (2 front halves and 2 rear halves) they have nicely packed it in a moderate size carton. The model came undamaged which was likely due to the fact that the carton is honeycomb construction and somewhat indestructible which was demonstrated by the delivery system, lots of exterior dings and dents but fortunately no damage. As there is an in depth thread on this model on RC Groups, I will keep my comments to my own experiences with this build.

This is a rather large EPO foam 90mm EDF. As this is my first FlyFly model, my initial first impressions of the quality, detailing and finish are very good, particularly considering the price. I have a spare 100 amp Turnigy ESC and Turnigy 3553-1800KV (6S) motor sitting around so my intent is to build this model with this setup. My first step in the build was to check to see if the stock fan unit was suitable with this motor or whether or not I needed to buy another EDF unit. The fan appeared to be of lesser quality and had some burrs on the route of a few of the blades, likely from the forming process. After cleaning these up I balanced it and then mated it to the motor before running it up on a test stand. I must admit that I was expecting this thing to come apart but it ran nice and smooth for about 2 minutes producing 1.6kg thrust on a 5000 mah (6S) battery at 65 amps. The model is supposed to weigh around 2.6kg so we will see how this works out for performance.

The next task was to look at putting the wing together. This is a very unique wing style as it has a large clamshell intake located right where the wing halves are joined. There are several reported wing failures in-flight due to this design and it’s lack of supporting structure in this area. It appears that the manufacturer has tried to stiffen the wing by installing a spar along a portion of the trailing edge of the wing and adding four small plywood stiffeners to go across the bottom of the wing where the bottom clamshell cover is attached to the underside of the wing but it quickly became apparent that this would not be sufficient. I therefore decided to add a 6mm CF spar through both wing halves out to the aileron servo location. This spar is located so it comes in contact with the air scoop on the clamshell cover to help stiffen the spar from any flex. Once I glued the wing halves together I realized that an aft spar was also required. It was a little more difficult to install after the wings were attached but never the less I managed to get one in without too much problem while still preventing any twist in the wing. This aft spar does not go out as far as the forward one. All the flying services use double taped hinges. I was going to replace these but from other forums it appears these are holding up well as a hinge except for operations in high temperatures where the tape begins to lift. Also as I do not want to paint the model, if I remove the tape it will likely pull all the paint off so I will give them a try. For some reason the rudder has not been cut out of the vertical stab so that will require a little bit of cutting and carving to mount a rudder servo. The ailerons and elevators all have servo cut outs that HXT 900s fit nicely into so that is my servo of choice. I must admit that I have yet to have one of these servos fail which I cannot say the same for some of the various Turnigy and Hitec metal servos that I have used. Although there are control rods provided, I replaced them as I do not like using the wheel collar type connectors on flying services. The model comes with fixed landing gear with plastic housings and metal oleos. As I cannot image going without retracts, I will be installing the HK 514s into the wing and the matching HK nose retract into the fuselage. The 514s fit nicely in the mounting wells but do require a bit of foam to be removed to widen the hole in the clamshell. I am currently waiting for some servo extensions to be delivered so I will have to wait to join the clamshell to the bottom of the wing. I have also ordered some more robust gear struts as the ones supplied are pretty flimsy.

The front portion of the fuselage went together fairly easily. I had to modify the nose gear mounts to take the HK retracts so this included adding a couple of hardwood mounts . Contemplating whether or not to add a bulk head as well. I used 5 min epoxy to glue the halves together rather than PU glue as I did not want the glue squeezing out of the seams ruining the paint finish. I should have used a slower setting epoxy as there was not much working time to get everything lined up. Hopefully I will able to get going on the aft portion of the fuselage soon.

Cheers
Marty

[4stripes]

Marty, everything sounds great, except your power and thrust readings given. I would have expected closer to 1800 watts and more than 2kg thrust. Is it possible your battery is weak, or perhaps your throttle not calibrated with the ESC?
1.6kg thrust will drop by around 25% in the plane.
Cheers Eric

[SARPLT]

I decided to run it again with a freshly charged 5000 mah 35C battery (6S) to confirm the numbers and I got a similar thrust at 1.58kg, voltage drop was from 25.2 down to 21.92, current was higher at 81 amps and 1812 watts. The motor was getting warm this time. As the Turnigy motor is only rated for 62 amps and 1500 watts this is pushing it well above its recommended limits. I also calibrated the throttle travel on the ESC with no change to the numbers. The stock fan unit has seven blades.

For fun I decided to compare these results with that of another EDF unit that I have setup for another model (HET 5 blade fan with a Typhoon 1680kv). Although this setup is for 8S I ran it using the same battery (6S) and Turnigy 100 amp ESC as above. Surprisingly I got more thrust 1.84kg with less amps, 57 amps. The voltage drop was also less at 22.74, understandable due to the reduced current draw. Wattage was 1335. I had expected all these numbers to be less then the first fan due to a lower kv motor and lower blade count of the fan but I am not quite sure why more thrust was produced. Could this be because of a better quality fan/motor.

I may have to do some further testing and consider other options as my original plan of using the stock fan is no longer viable. Possibly use the HET/Typhoon setup or maybe a CS fan/ 6S motor combination. I will check to see if I can fit one into the mount without too much carving.

Cheers
Marty

[airbourne]

The thrust to watts ratio seems very low with the stock included fan in your MB339 kit. As a comparison, most 70mm fans pushed to 1700watts or so deliver 2.4 kg of thrust on the bench.

I don't normally recommend it, but this sounds like a job for the much talked about ChangeSun 90mm 12blade. It's cheap, will hold up to the power levels you are looking at, and should give you near 1:1 thrust with a fandrive 1500kv motor, or you can also try running your existing motor on 5S.

[j-c terrettaz]

The MB 339 are super flyer. I have a turbine version and its a stable plane with no bad habit!
@ our club Rob was flying the same model like your and the only weak part as expected was the landing gear, way to soft for a 90 mm plane. You should upgrade with something more solid or bungee the jet.
Maybe the trust look weak but they fly on little power...Mine weight 25Lb and flew with a 13Lb trust turbine!no problem.
This year I upgraded to a 20 lb trust motor..

[SARPLT]

Glad to hear the MB 339 is a good flyer. Hopefully I can figure out a suitable power plant. Last night I tested another fan, a 89 mm Haoye fan from HK which I had picked up as a spare. This is a very similar fan to the stock FlyFly one also with seven blades. It is however a little better quality and stiffer so I was hoping for better results. Using the same Turnigy 1800kv motor and 5000 mah 6S battery I got almost the same thrust but a higher current draw and wattage, 1.51kg thrust at 91 amps and 2033 watts. Since I had the test stand setup I also decided to pull my CS fan out of my Starmax F18. This is the model that I had a fan explode in while flying last fall (Non-contanement of an EDF thread). To finish off the flying season I installed a CS fan with the existing motor. As I got this motor with another fan unit, I am not sure exactly what make it is. It appears to be a Warhead possibly 1350 or 1500kv. Unfortunately during the installation I did not have a test stand to measure the performance and my watt meter had also failed so I did not know what the numbers were. I had a few flights with this setup which had awesome performance. Now I know why because this fan is creating 2.35kg of thrust at 110amps and 2033 watts. It is surprising that I did not burn this motor out. I guess I will have to look at possibly replacing this motor before next flying season as well.

I have attached a photo of the four fans that I have tested. From left to right, FlyFly stock 7 blade fan, Haoye 89mm seven blade fan, HET 5 blade fan, Changesun 12 blade fan. As you can see the three fans are very similar in length 58mm and the CS is 90mm. I trial fitted the CS fan into the MB 339 fuselage and although it could fit it will be rather close to the aft CF spar that I installed. I will continue the search to determine what will be the best setup for this model, keeping in mind I am not expecting this model to be a speed demon. I just want something that has a reasonable power consumption and can produce at least 2kg thrust. I will play with the CS fan again tonight to see if I can fit it in without inteferring with the spar.

Marty

[SARPLT]

I played around with fitting the different fan units into the aft portion of the fuselage. The CS fan will fit but it will have to be pushed back into the foam to enable the fan mounts to line up and to prevent the wing hold down blocks from being covered. This also means that the thrust tube would also have to be cut back and likely require another former to secure it. It also complicates access to the motor wires to hook up the ESC. Would likely have to cut a hatch in the fuselage to gain access to the wires. In comparison to the other stock fan you can see that it fits nicely without any mods. So to not complicate this build as my intent was to keep it straight forward, I think I am leaning towards going with a fan that is the same length as the stock FlyFly one with a housing length of 60mm. I was thinking of possibly trying a Dr Mad Thrust 90 mm chrome plated 12 blade fan 1600kv. It seems to have a reasonable wattage and current rating 85 amps and 2100w. It also is not very heavy (370g) in comparison to some of the other fans. I am trying to keep this model light. Anyone have any experience with these chrome plated fans. I have had good results with all my other Dr Mad aluminum fans.

Cheers
Marty

[4stripes]

I use a hacksaw to cut the standard CS shroud to the same length as the Flyfly shroud. I also run the motor wires out the side rather than the back end in most cases. Makes installation easy.

[4stripes]

Regarding the Dr MT Chrome plated fans...
I do have the 70mm version and the shroud was also Chrome plated. Very poorly put together and not balanced properly. When it was taken apart to be reassembled properly, I discovered the motor was attached to the plastic mount with tapered screw heads causing the mounting holes to split. The motor would have shaken loose had it been run for any length of time. After substituting another shroud, it was reassembled and balanced. For the money, it was OK but I won't be buying any more as the CS units and TAFT fans are better quality IMHO. Lander AKA DrMT has quality issues in just about every item they sell.

[airbourne]

Originally Posted by SARPLT

I played around with fitting the different fan units into the aft portion of the fuselage. The CS fan will fit but it will have to be pushed back into the foam to enable the fan mounts to line up and to prevent the wing hold down blocks from being covered. This also means that the thrust tube would also have to be cut back and likely require another former to secure it. It also complicates access to the motor wires to hook up the ESC. Would likely have to cut a hatch in the fuselage to gain access to the wires. In comparison to the other stock fan you can see that it fits nicely without any mods. So to not complicate this build as my intent was to keep it straight forward, I think I am leaning towards going with a fan that is the same length as the stock FlyFly one with a housing length of 60mm. I was thinking of possibly trying a Dr Mad Thrust 90 mm chrome plated 12 blade fan 1600kv. It seems to have a reasonable wattage and current rating 85 amps and 2100w. It also is not very heavy (370g) in comparison to some of the other fans. I am trying to keep this model light. Anyone have any experience with these chrome plated fans. I have had good results with all my other Dr Mad aluminum fans.

Cheers
Marty

Unless you must have the extra bling from the chrome, i would stay away as these are known to be of low quality. I would rather assemble a unit from all the parts you currently have unless you need another fan?

If you're shelling out the cash for a new fan you may want to look into a jetfan as well. This fan will power many types of jets for you down the road.