PDA

View Full Version : Multiplex Twin Jet question


thebaldone
07-29-2005, 10:24 PM
First electric attempt and it seems to be a disaster! Bought a Twin Jet complete with 2 400 motors. Built it fine as per instructions and put in a 3000mh Nimh 8 cell pack. Problem is that it just won't get off the ground! Hand launch it and it simply flies in a downward direction and lands about 20 feet away! The shop where I bought it from recommended the battery pack but for some reason it just doesn't seem to have enough power. Fitted with a 35A ESC.

Any suggestions? What should the all up weight be for one of these?

TIA

Reformed Nitroaddict
07-29-2005, 10:30 PM
I know this is a very silly question - But being that it's a pusher prop, are you sure the propeller isn't on backwards? Not trying to offend you in anyway, but have seen this happen so many times it's worth mentioning.

thebaldone
07-29-2005, 10:39 PM
Props are on the right way as it blew load of stuff around my kitchen when I tested it!

Reformed Nitroaddict
07-29-2005, 10:46 PM
Hmm. That being the case - do you know what the AUW weight is, ready to fly? Those things are pretty quick, so it should take off with authority unless something is robbing the engines of power, or the thing is a brick.

thebaldone
07-29-2005, 11:06 PM
What should be all up weight? Any ideas? Would the fact that I'm using the fitted Tamya connectors cause loss of power?

johnhay73
07-29-2005, 11:21 PM
How long a runway have you got and are you running into the wind?
Also, days ago when I got my RTF I had about 6 or 7 hand launch nose dives much like yours until I got my throwing angle correct. After that I have had many nice hand launches. I was certain at first that the plane was the problem, and I was very frustrated. Now, happy as a clam.

thebaldone
07-29-2005, 11:26 PM
Well as this is my first attempt at electric, I'm just throwing it from just above my head! Are you suggesting that I need to get a fair bit of speed up before I launch?

What is the correct throwing angle?

thebaldone
07-29-2005, 11:58 PM
Just checked the instruction book and the AUW should be between 1100 and 1200 grams and I've just weighed it at 1120 grams so it obviously isn't too heavy ! Suggestion elsewhere is to dich the props supplied and go for 5 x 5 instead?

hexonxonx
07-30-2005, 03:03 AM
what about a test glide?thake the props off and get everything turned on,and try a level firm forward launch.it should be a controlled glide to the ground.if thats ok then try again with the props on and see if you get better results.

petej
07-30-2005, 04:42 AM
To second the suggestion by hexonxonx, try test guides. You might need to adjust trim (elevons slightly up) and/or center of gravity. Look in the instruction manual to check for suggested trim and where the CG should fall. You might need to add a little weight to the nose or (less probably) the tail. The test glides will also check on whether you're throwing it correctly. It should glide for about 20 feet or more without power.

thebaldone
07-30-2005, 09:19 AM
Thought about that BUT the booklet says Do Not Test Glide this model !

Andy m
07-30-2005, 09:48 AM
On theTwinjet you can not get the CG wrong as there are 2 raised pimples under the wing to mark it, check that you have not got downthrust on the motors, if anything you want a touch of upthrust with a pusher.

I have flown a Twinjet for years and find that it needs a good strong throw to get away. do not pull a lot of up elevator right away or it will just slow down and stall, let it sink a bit to gather some airspeed. Something else you might try is to add about 3 or 4 mm of up trim for launch.

Also, if I were you I would ditch the Tamiya connectors, they may be ok when new, but sooner or later they will give you problems as they wear at an alarming rate and they are not designed for continual connection/disconnection and are not really very good for high Amp applications.

I hope this helps to get your Twinjet flying well, you will enjoy it! :)

thebaldone
07-30-2005, 09:57 AM
Andy, thanks for the info, going to ditch the Tamya connectors. Did you change the props at all? Suggestion has been made that I go for 5x5 and use prop adaptors -
http://www.sussex-model-centre.co.uk/Prop+Adaptors-1203.htm so I can use better props?

thebaldone
07-30-2005, 11:19 AM
Another quick question -- do you use a seperate rx battery?

Andy m
07-30-2005, 05:27 PM
thebaldone,

To answer your questions, no I did not change anything the model is all standard out of the box. I use an ESC with BEC no RX pack, again all standard stuff.

Mine flies well on 8 CP1700's, better on 8 1950 FAUP's and great on a 3s 2000 Ma Kokam lipo, lighter is always better. :)

thebaldone
07-30-2005, 06:08 PM
Ok thanks for all the replies

I took out the RX battery pack and used the ESC to power it - Changed the standard push on props for 2 prop adaptors using the standard props and slighter increased my throw angle - verdict.... It flew !

Only problem I had was that that the motors seem to be giving me alot of height when at full throttle - The faster they go the more it pulls up ? The motors are down in side the recess but it still pulls up!

I have a Futaba FF7 which has alot of fuctions - does anyone know if it will mix throttle and evelvator so it would add in a little down elevator as the throttle stick is increased ?

Watts up
08-07-2005, 08:01 PM
once you mix out that you will have a ball......have fun

Madmarl
08-10-2005, 06:49 AM
I found out the hard way with mine that the original props work as long as the flat side goes forward. The instructions say lettering to the front. That's backward.
I drilled out my props to fit prop adapters. Worked fine.

Try formation flying with other Twin-jets. They can touch and still fly away!

I'm wondering about a brushless conversion. :rolleyes:

fdix
08-11-2005, 11:57 PM
I flew mine with permax 480 motors, cam speed prop 5,5x5,2 and 8 NiMH 3300mAh cells.

Going brushless is IMHO not the best idea, this because the airfoil and the structure in general produces lots of drag. Drag is not linear to speed but exponential, so you run into a wall.
I flew with various twinjets, some of them equipped with brushless motors and lipos. The climb rate is definatley higher and the sound scarier on BL, the speed in a pass tough was not much higher.

The investment for buying 2 brushless motors and a big pack of lipos is not really justified. I would instead buy a fun pylon racer.
A Turn left from Simprop with a brushless motor and a good pack of NiMH batteries will definitely keep you concetrated on your flying :D

Madmarl
08-12-2005, 05:38 PM
Yeah, I was just wondering if it would be worth it.
I'm having alot of fun with mine the way it is. I've recently picked up some 5.5 x 4.5 APC's to try. I got tired of buying the originals after the shaft would spin the hole bigger.

thebaldone
08-12-2005, 08:20 PM
Anyone know where there is an article on beefing up the node on the Twin Jet? I'm sure I've seen one somwhere but I can't find it again!

Madmarl
08-13-2005, 04:26 AM
I think you mean nose. I have snapped mine off too many times to count! Any soulutions? Other than not landing nose down?:o

thebaldone
08-13-2005, 10:46 AM
Yep I did mean nose and it's nice to see that I'm not the only one doing this! Assume you just glue back on and away again?

thebaldone
08-13-2005, 10:47 AM
Sure I saw an article somewhere showing how to strengthen it with fiber glass tape and resin?

Madmarl
08-13-2005, 05:13 PM
A little CA, and it's ready to go! I found with the nose gone, I don't hook it in the ground on landing. But it doesn't help the looks. I've been haveing so much fun flying it, I haven't painted it.:D

thebaldone
08-13-2005, 05:33 PM
Mine's the older type so I have to use 5 minute!

Madmarl
08-15-2005, 05:59 PM
I have the 'Blue Shark'. with 8 cell 2400 nicads and a 50 amp controller. I thought it was good and fast till I went to fly it yesterday. There was a Stryker with a brushless motor and 10 cell 1100 mah pack. Flew circles around mine! At half throttle! Considering it was about a third the weigh of mine, it went like a rocket. Gonna haveta jump on the bandwagon someday I guess....

pikeman401
01-28-2006, 10:55 AM
Just got mine a short while back and am still putting it together. I looks like they may have made some upgrades in the plane since you guys got yours. The plane now comes with stock 480 motors. I hear that makes a big difference. Also there is a balsa spar that runs from the tip of the plane to about 2/3 back in the botom of the fusalage. The wire harness that they sent me for the mtors looks really whimpy. I hate to change it because it plugs into the speed controller that I ordered from them but I seriously think that the small diameter wire may be limiting the amount of amps delivered to the motor. Just my opinion I may be wrong.

Madmarl
01-28-2006, 05:40 PM
What really limits it is the directions on installing the prop. I followed them and had trouble flying it. Then I looked at the prop carefully and saw they were on backwards. I put the flatter side to the front, and away it flew!
I used the wire supplied and didn't have any problems with my Twin Jet.
What size of speed controller are you using? I put a 50 amp in as a few of my buddies cooked thier 35 amp ones. Haven't had any trouble with it. Other than 'Dumb Thumbs'!
Good luck with yours. It's great to get a couple and do some formation flying. No damage when you touch!!!:)

pikeman401
01-29-2006, 02:35 AM
I ordered the speed controller that hey mentioned inthe instructions. The 400 DUO BEC. Has continuos rating of 30 amps and 40 amps burst for 2 min. When you say the flat side I assume you mean the side that has no writing. That is what they recommend in the instructions now. Did you also wire the motors up backwards? I have on 10 cell pack that I plan on using. Hope I don't burn the controller. And to Madmarl yes we will both have to jump on the bandwagon some day. Right now with my flying skills I think this will go fast enough for now. Let me get used to it with alittle less power before I strap on the turbo chargers.

pikeman401
01-29-2006, 05:38 AM
How did those APC props work? What type of prop adapter did you use? Anyone think that it may not be a bad idea to cut some ventilating holes in the battery compartment for some vent for the battery and other elctronics?

Madmarl
01-29-2006, 08:02 AM
Those APC props are louder, but seem to be about the same thrust. I used a prop adapter that fit the motor shafts.
I've been using 2400mh 8 cell nicads in mine. 7 cell didn't have the jam the 8 cells have. My cousin is using 8 cell 3300 nimh in his. Seems to fly the same with his packs or the 2400 nicads.

My packs get fairly warm after a flight. I usually am at WOT most of the time. But they are still cool enough to handle comfortable after. I think a cooling vent would help, especially when it's +30c out. I've heard that a plastic spoon cut in half and glued to the outside of the hole works well. Don't forget to leave an exit hole that is a bit larger that the entrance. Send some pics of it. I may even modify mine if it ever get +30c here in the frozen North! :D

Magnum9
02-11-2006, 09:31 AM
Hi guys, thought you may be interested in what happens when the twinjet is taken to the extreme - http://www.rcgroups.com/forums/showthread.php?t=471080

Jeremy Z
02-11-2006, 04:20 PM
TwinJet info can be found here: http://plawner.net/4/twinjet/twinjet.html

If you follow the links at the bottom of that page, you will find information about mods. There is also a video section on the TwinJet elsewhere on Patrick's site:

http://plawner.net

They say that for jets, you should have a bit of up elevon trim when you launch it. Also, you should launch them up at a bit of an angle, which you wouldn't do with a conventional plane.

So before you take more drastic measures, launch it with full throttle (being careful that your hand clears the props) with some up-elevon trimmed in, at about a 20 degree angle upwards.

The mods for that plane are numerous. Patrick did a brushless mod with his using dual Kontronik motors and speed controls. That is Expensive, with a capital E. His buddy converted one to a monojet, but it required the purchase of a rather elaborate prop setup. (follow the links and read all about it)

It is too late for you, but there's a replacement for the TwinJet in the works, called the Funjet. It's going to be about the same size, but powered from a single motor and a different shape. Not as slick or jet-like, but every bit as tough-looking, IMO.

Jeremy

Madmarl
02-11-2006, 06:10 PM
Thanks for the links.
I launch mine level and WOT. I've missed catching the props on the way by. So far. Mine has the stock 480's. The new props seem to work as well as the stock pushon ones.
Thinking of going to an out runner setup. Just need the Lipos.. and $$. Am trying out a depron foamie. Still in the cutting stage.

Jeremy Z
02-12-2006, 06:04 AM
Thanks for the links.
Thinking of going to an out runner setup. Just need the Lipos.. and $$. Am trying out a depron foamie. Still in the cutting stage.

You don't need LiPos to go brushless. Just set your ESC for NiMH/NiCd. Once you have the motor or motors, you'll be set. When you buy the motors, buy them with higher kv values that are appropriate for the lower voltage of your packs.

I had a depron foamie. My problem was that it was a 3D foamy, and I didn't have the slightest idea of how much expo and dialed-down rates it required to fly "normally". I crashed it 4 times in short order. At the end of it, I was just getting it figured out, but it was pretty much beyond repair at that time. Damn. It was only $20 (the Yak 55 from Hobby-Lobby) but it is the building work that I dread repeating... My advice on that topic is to set up with much lower throws than 3D to start with. If you have a computer radio, program in the lower dual rate setting at about 50% throw, but set it up mechanically for full throws. You will probably still need expo, assuming it is a 3D type foamy. That, and be patient for a low-wind day. It is critically important on these profile planes. I fly my other planes in wind against recommendations, but will never do it again with a profile plane.

Madmarl
02-12-2006, 06:23 PM
I'm cutting out a Super Bandit from Parkjets.com. Should have a bit more zoom than my TJ.

LuckyArmpit
02-13-2006, 08:35 PM
One thing that hasn't been suggested yet. The 3000 mah battery pack. If it is a nimh and its new, these need to be cycled 5 times or so. The first charge should be at 1/10 C or, 300 milliamps. This is called a setup charge. Then, discharge it and then charge at 3 amps. Do this for like 5 or 6 cycles.
I also have a twin jet. Mine is the blue shark with the 480 permax motors.
I fly mine with 5.5x4 MAS props and 8 cell 1800 mah nimh's. It needs to be launched on full throttle and into the wind with a hard straight throw. Do not immediately add up elevator or it will stall and drop like a brick.

Dave...

Madmarl
02-13-2006, 10:00 PM
My cousin has the white one with 400's. He doesn't notice any difference between a 2400 nicd 8 cell or the 3000 nimh 8 cell. He thinks the nimh has a higher internal resistance so it performs similar to the nicad packs. He has about 50-60 flights on both. And a well beaten TJ to boot!
I agree with the hard throw and easy on the elevator. A soft throw with lots of elevator is a recipe for disaster.
Are your props grey? I think mine are APC. They are a bit louder than the stock ones, but don't seem any faster. I really like the fact they don't fall off in flight. Mine flies like a wounded duck if that happens!

jonnyjetprop
02-16-2006, 03:38 PM
I have a twinjet powered by 2 mega 16/15/4, 2 CC Phoenix 25 escs, APC 6x4 eprops and a single 3s3p TP gen 2 pack. It even impresses the glow crowd:D

John

AEAJR
02-16-2006, 11:30 PM
[quote=thebaldone;835]Ok thanks for all the replies

Only problem I had was that that the motors seem to be giving me alot of height when at full throttle - The faster they go the more it pulls up ? The motors are down in side the recess but it still pulls up! [quote]


This could be a balance problem. If you are nose heavy, then you probably have a little up trimmed in to over come this. This will show up with the behavior you describe.

Try moving the CG back a touch which will follow with a little down trim to remove that excess up. You should get a more even handling response.

Madmarl
02-17-2006, 01:15 AM
That TJ must really scoot!!
Maybe the thrust line? Motors could pull the tail down at WOT, causing it to pitch up.

pikeman401
03-03-2006, 05:01 AM
Almost done with my TJ. I want to use prop adapters but don' know which ones to get. Would love a little help here. Will use whatever adapters and props that work, just need to know what to get. My TJ has the 480 motors.

Madmarl
03-03-2006, 08:09 PM
Try 5 x 5 APC's for the props and I can't remember the size of adapter I used. Take the motor into the hobby shop and try them out. Make a difference when the props don't go flying off mid flight!

pikeman401
03-26-2006, 05:04 AM
Looking for Madmarl. How did those APC 5.5x4.5 work? I got a pair and hope to use them. Some people are using the MAS props. What size and how are they working?

Madmarl
03-26-2006, 07:30 PM
I think mine are 5.5 x 5.5. They are louder, but I haven't put a tach on them. They seem to fly OK.
Nice to have them stay on in flight. I had a problem with the push-on type coming off in flight. Once they spun on the motor shaft, they didn't stay on long.
I have a brushless rig I've been wanting to adapt for it. I don't think the top end will be much higher, but the vertical should improve.

modflyer
08-19-2006, 12:23 AM
Hi there, I too have one of those Twinjet things. Mine too used to dip on launch too, it needs a good heave ho, it about pulled my arm out of joint one day throwing it.
Mine is non standard. The first thing to do is get rid of the extra weight!
I.e loose that niHm go for a lipo mine has a flight power 3700mHa gets me about 15mins to 20mins its alot lighter now after shes gone on a diet and flies real nice, but stick some expo on it else it gets too twitchy round the gimble. Gonna go brushless with it soon I have seen vertical performance of one of these and it's Awsome and very very fast not for beginners but remember use 2 esc's one for each motor else the motors under load will not syncronise.
Also for a dead easy and spectacular launch bolt a tow hook on I used an old allen key epoxyed through the fuz drill through about level with the front of the wing and through the bearer stick some glasscloth around the area and you dont need to go mad with the glassfibre cloth.
NEXT Make a 15ft long bungie the type used for hi-start and some fishing line about20ft with the highest breaking strain you can find and a doggie tiedown stake and split ring the beefier the better.
Do not put a paracute on, (like I did on my first flight) just some ribon the smaller the better.
I tried a parachute the first time (have you ever seen a radio control/control liner before) real funny.
Well there you are TWINJET TIPS have fun let me know how you get on.