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Old Mar 21, 2014, 05:00 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
5,112 Posts
Mini-Review
WLtoys F939 4-Channel Airplane RTF Review



Introduction

I received a WLtoys F939 RTF as my first micro RTF airplane. As it is a pretty good airplane, I thought of sharing my thoughts of this airplane here. I hope thatís ok as I canít seem to find airplane reviews in this forum.

I am a beginner to airplanes - I only had a HobbyKing Cessna and an Nine Eagles Xtra 300 before getting the F939; I also did not read through the Parkzone Polecat thread, which the F939 bears so much resemblance to. As such, I may write about really obvious, beginner-level stuff so please bear with me.

Iíd like to thank the nice folks at Tmart for sending me a F939 for review. Here goes!




The Package

Below is a video of me unboxing the package.

Unboxing WLtoys F939 4-Channel Airplane (FlySky) (3 min 9 sec)


It comes with 2 x 1S 3.7V 130mAh batteries (MCPX-styled connector), 1 x spare prop, 1 x USB charger, 1 x transmitter, and other miscellaneous stuff.






Build Quality



The micro airplane is small and feels light. The tail section gives me the impression that it is weak and fragile.
To see the internals, I removed the 4 pieces of clear tape then removed the top cover.



The control surfaces were neutral out-of-the-box except for the two ailerons. I had to shorten/length the push rods a little. Overall, the range of movement for the control surfaces suggest to me that this is going to be a highly agile airplane.



I noticed that after applying aileron inputs, the control surfaces do not return to neutral. I have to apply the opposite direction then it goes to neutral. This is going to negatively affect flight.

I have no idea where the center of gravity is so I guessed it is at the line on the main wing where the red area meets the white area. Based on that, I adjusted the position of the battery to achieve a balanced aircraft.

The prop is 13.2cm / 5.2 inches long. Iím not sure of its pitch.

The thrust angle is 0 degrees up/down, 0 degrees left/right.


(I drew a blue line indicate the horizontal axis)



Transmitter

It looks exactly like the V911, V929, V939, V949 transmitter. There are two D/R available - low and high. And there are trims for aileron, elevator and rudder. But there is no way to add expo, which I found it to be really important for this plane.

The build quality of the transmitter is obviously toy-grade and servo movements feel a little lower in resolution. How do I know? As the airplane works with my Turnigy 9X (FlySky), I could see slightly smoother servo movements with my 9X.




Flight Characteristics



I tried to fly it at a carpark by the beach. It was windy and that proved to be a really bad decision. I was unaware how badly this micro airplane would fly in windy conditions.

I started with the stock TX, low rates. I gave it 75%-throttle and a little bit of elevator, the airplane lifts off effortlessly then almost immediately, it stalls. I tried a couple of times and found that even at low rates, I had applied too much elevator. I need to reduce the rates even further as well as to dial in some expo.

As most of you may have guessed, there is no fun flying this plane in windy conditions. I was fighting the wind all the time. I have to extremely careful with the elevator and throttle as it stalls very easily. You may see from my videos how I struggled to keep the plane in the air.

I switched to my 9X (ER9X firmware) with the following settings:

Aileron (CH1) Expo 20, Rate 85%
Elevator (CH2) Expo 20, Rate 80%
Rudder (CH4) Expo 10, Rate 70%


The plane was more manageable but I was still not having fun fighting the winds. Landing was extremely unpredictable as I smashed the plane onto the ground several times. The tail landing gear took the impact so hard that it broke the entire vertical tail fin. I donít think having the tail landing gear attached to the rudder control surface is a good idea. With a broken rudder, I went home for a quick fix with CA and tape, then search for a less windy place.

I took it to my neighborhood soccer field. The wind was gentler but there isnít a flat ground for take-off and landing. I had to remove the landing gears which I think is a good idea for a beginner like me.

Over arm and under arm hand launches were easy. I used 50% throttle for my hand launches.

The lack of strong gusts made flights more enjoyable. I managed to pull off some figure-8s, loops, rolls, and inverted circuits. As the rudder was just repaired by CA and tape, I did not try knife-edge.




Issues



At high throttle (around 60%), the plane pitches up. If I gave it full throttle, it would pitch up so much that it does a loop. I had to compensate with forward elevator to keep it from stalling. As my elevator control surface is neutral / 0 degrees when the elevator stick is neutral, I suspect the pitch up has to do with the thrust angle. As a side note, I did not experience this during inverted flight. I may need point the props downwards.

The aileron control surfaces are fragile. They must have broken off from the main wing in a bad crash. Each piece is held to the wing by 4 pieces of tape. While they respond to the servos, they lacked the precision that they had before they broke. How do I repair the broken ďhingesĒ of the two aileron control surfaces?

While the rear wing has a carbon rod to maintain its rigidity, there isnít any for the vertical tail fin. I had a couple of inverted crashes and that broke the fragile vertical fin. It may be a good idea to reinforce the fin with carbon rods, or avoid crashing inverted.




Conclusion



WLtoys F939 is a fun plane to fly under gentle or windless conditions. The ability to work with FlySky transmitters overcomes the imprecision of the stock transmitter as well as the lack of configurable D/R and expo. I donít think beginners will find it easy to handle this plane. On the other hand, I believe advanced pilots with FlySky compatible programmable transmitters may appreciate this airplane.




Pros

+ Supports 9X
+ Nimble, agile, capable of aerobatics
+ Works with popular MCPX-styled batteries (tried MCPX batteries and they work)


Cons

- Difficult to fly in windy conditions
- Fragile
- Pitch up issues
- No adjustable rates or expo on the TX



Links

- Photo Gallery (high-res): http://www.flickr.com/photos/fyresg/...7642701372483/
- WLtoys F939 4-channel Airplane RTF: http://www.tmart.com/Wltoys-F939-40c...d_p238628.html
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Old Mar 21, 2014, 05:06 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
5,112 Posts
Flying around at a windy carpark and then at a soccer field.
WLtoys F939 4-Channel RTF Airplane (FlySky) (8 min 26 sec)


Low Flight.
WLtoys F939 4-Channel RTF Airplane - Low Altitude Flight (3 min 45 sec)
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Old Mar 21, 2014, 06:12 PM
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United Kingdom, London
Joined Jun 2012
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Thanks for the review Fyre. I have been wanting to have a look inside & you did not disappoint.

I think a lot of the issues you are facing are just general fixed wing troubles, those micro foamies (without stabilization) will normally only fly in calm conditions & dont take hard crashes well! It wont take a beating like a micro heli. The tiny servo's rarely center perfectly & its something that happens with a lot of servos. I also use fiber tape for control surfaces, strong & sticky .

Its never easy to tell by looking at pictures but it seems to be setup with the elevator & main wing on the thrust line with zero incidence angles? If so I would setup the plane with a flat elevator (no trim) & adjust the CG incrementally so that it flies level both inverted & right way up at full throttle, either that or slightly nose heavy so it dives a tiny bit when you release the elevator.
My guess is you may have it a bit nose heavy & counteracted this with the elevator trim at low-ish speeds. On this type of plane I normally trim the elevator so that it behaves the same inverted.

Best of luck with the videos . Hope the weather calms down for you, its been terrible here .
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Old Mar 21, 2014, 06:24 PM
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Joined Feb 2013
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Thanks for the review FyreSG. well written as always.

I'm still very much a noob when it comes to planes. Was given a Super Cub for Xmas which I love so am interested is something a bit smaller.
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Old Mar 21, 2014, 10:45 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
5,112 Posts
Thanks for the compliments. I'm glad my review was useful. I added a video of my flight at post #2. Please check it out!
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Old Mar 22, 2014, 05:54 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
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Repair broken aileron control surface

Any suggestions on repairing the broken control surface? I tried tape but that restricted its range of movement.



Thanks.
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Old Mar 23, 2014, 04:29 AM
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France, PACA, La Valette-du-Var
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Thanks Fyre Great review. This bird remind me the NE YAK54 I got ... and more or less the same Pro & cons point.... Anyway this one is FrySky compatible instead of J6pro for NE planes.

Some BNF versions should be at least proposed. What about the Lipo battery bay ? Can a larger Lipo be installed ?
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Old Mar 23, 2014, 11:36 AM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
5,112 Posts
Quote:
Originally Posted by SeByDocKy View Post
Thanks Fyre Great review. This bird remind me the NE YAK54 I got ... and more or less the same Pro & cons point.... Anyway this one is FrySky compatible instead of J6pro for NE planes.

Some BNF versions should be at least proposed. What about the Lipo battery bay ? Can a larger Lipo be installed ?
Thanks, glad you like it. This one is FlySky compatible, not FrSky. FrSky is very different protocol.

I proposed to both Tmart and BG about a BNF F929/F939. They will look into it.

Yes, it accepts the larger V922/FBL100/MCPX battery. I'm using them now. It makes the plane much heavier, but it flies. The heavier plane flies very differently.

Do you know if the newer V911 batteries have MCPX-styled connectors?
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Old Mar 23, 2014, 03:13 PM
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Fyre, is it using a geared 8.5mm motor? (same size as v911). If so I would probably fit a new stye v911 connector, its smaller than the mcpx connector & its all it really needs. All of the UM parkzone planes use the same connector so there is a wider selection of lipos of the correct size.
It wont tolerate added weight very well, a larger lipo may need to be slightly relocated to get the CG back to normal. Normally with these geared 8.5mm motors a good combo is the GWS 5030 prop with a small bag of nanotech 160mah lipos .

Tape only really works well when the chamfer is on one side & not both, you could try some hot glue on those hinges. I would only glue one side & be very sparing with it, you only want a thin layer so its still flexible & the ailerons can move freely. Tape a box around the hinge so you dont go spreading the glue all over the place & spread it lightly up to the tape. Let it dry, pull off the tape & you have a nice rectangle of hot glue . It works well on larger foamies but you may have to experiment a little to get the right tension on the joint, I have never tried with tiny servos.

EDIT: Also dont use the tip of a very hot glue gun to spead the glue on foam, it will melt holes in it very quickly

Also just looked & the New style v911 connector is actually the mcpx connector & not the parkzone UM conector! Its probably easier just ordering a few v911 lipos
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Old Mar 23, 2014, 03:52 PM
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UNGN's Avatar
Southlake, TX
Joined Jan 2008
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Quote:
Originally Posted by FyreSG View Post
Any suggestions on repairing the broken control surface? I tried tape but that restricted its range of movement.



Thanks.
I use 1/16" strips of mylar cut from a 3.5" floppy disk. Slot them into the wing/control surfaces (where each foam hinge was located) with an exacto and hold them in place with a foam glue like UHU por.

This gives a low resistance, but positive return to neutral hinge.
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Old Mar 23, 2014, 05:48 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
5,112 Posts
Quote:
Originally Posted by limmacslim View Post
Fyre, is it using a geared 8.5mm motor? (same size as v911). If so I would probably fit a new stye v911 connector, its smaller than the mcpx connector & its all it really needs. All of the UM parkzone planes use the same connector so there is a wider selection of lipos of the correct size.
It wont tolerate added weight very well, a larger lipo may need to be slightly relocated to get the CG back to normal. Normally with these geared 8.5mm motors a good combo is the GWS 5030 prop with a small bag of nanotech 160mah lipos .

Tape only really works well when the chamfer is on one side & not both, you could try some hot glue on those hinges. I would only glue one side & be very sparing with it, you only want a thin layer so its still flexible & the ailerons can move freely. Tape a box around the hinge so you dont go spreading the glue all over the place & spread it lightly up to the tape. Let it dry, pull off the tape & you have a nice rectangle of hot glue . It works well on larger foamies but you may have to experiment a little to get the right tension on the joint, I have never tried with tiny servos.

EDIT: Also dont use the tip of a very hot glue gun to spead the glue on foam, it will melt holes in it very quickly

Also just looked & the New style v911 connector is actually the mcpx connector & not the parkzone UM conector! Its probably easier just ordering a few v911 lipos
I just sealed the hood so I can only check the motor size next time I open it.

The hinges are at the top side of the wing. I discovered that I cannot apply tape at the bottom while the original one is on the top. Adding tape on top seems to work fine. I'll try the glue trick. I have some art and craft glue; should be soft and gentle for the hinges.

If you're really sure that the new V911connectors are indeed MCPX-styled, I'll get them asap! I can't really tell from the pictures.

To be honest, I don't really mind the added weight as I'm just gliding the plane around instead of racing it through pylons. One small issue is when the heavier plane exits from the loop, I need to ease off throttle earlier and pull back on the elevator more.
Quote:
Originally Posted by UNGN View Post
I use 1/16" strips of mylar cut from a 3.5" floppy disk. Slot them into the wing/control surfaces (where each foam hinge was located) with an exacto and hold them in place with a foam glue like UHU por.

This gives a low resistance, but positive return to neutral hinge.
Thanks for the suggestion but I think it's hard to find mylar strips and harder to find floppy disks! Do you think small stripes of duct tape at the broken hinges would work?
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Old Mar 23, 2014, 08:05 PM
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Fyre, I always thought the new v911 connector was the same as the parkzone one (also on the 9958, MCX, MSR). I use these connectors on my v911 powered fixed wing stuff, the socket is smaller.


The F939 looks like it possibly came from the same mold as the parkzone polecat which uses this connector.

There's a good pic of the new v911 connector half way down this review.
http://www.rcgroups.com/forums/showthread.php?t=1773059
It does look exactly like the connectors on the v922/mcpx so I think you are in luck. I guess this is why Wltoys chose to use this connector on the F939.
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Old Mar 23, 2014, 08:21 PM
KDS450QS(ZYX4),V977,CX-20
FyreSG's Avatar
United States, CA, Monterey
Joined Apr 2012
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Ok got it! I'll get some. Thanks for researching! I just had a flight each with each battery. Somehow I find that the heavier plane is more resistant to wind. More inertia maybe?
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Old Mar 23, 2014, 08:48 PM
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Aircraft with higher wing loading in my small amount of experience are generally better in the wind & can be quicker but have a higher stall speed. They can be more difficult to handle & slower to recover after stalling etc.

I have a Sbach 3d foamie with lots of wing area & not too much weight to it, in strong winds its like flying a rc kite. On the other end of the spectrum a friend has a Hobbyking Rarebear (awesome plane), its a tiny thing & quite heavy. Its a smooth plane & is also quite impressive in the wind considering its size.
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Old Mar 25, 2014, 12:55 PM
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Hi FyreSG,

Will this plane bind with the stock Tx that comes with the V911?
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