|Apr 08, 2012, 05:44 AM|
Joined Sep 2010
Multiplex MicroJet Launch
Although this plane is no longer made I bought five this month, March 2012, and so there may be some coming to it afresh.
It's a LOVELY plane. It's not for beginners, though. It's too small, fast and maneuverable.
Most have been fitted with out runner motors and 1300 to 2100mah 3 cell LIPO batteries. These drive 2500kv motors with 4.7 by 4.7 propellors. So far, so good, 200 to 300 watts is more than enough for starters.
It flies really smoothly with long sweeping curves across the sky that look wonderful and will flatter any pilot. It will fly straight and true from one horizon to another, and it's rock steady on the approach and landing. It's exciting, smooth and quick.
BUT, lots of people have noticed that it has two foibles. I won't call them problems because this is a jet, so this is how they are. It doesn't like flying slowly so launching is tricky. And it rolls very fast so a very light touch is required, preferably no touch at all because it flies really beautifully on its own.
Foible One - Keep up the speed. Easy enough. Even during the descent, fly it down to the ground and fly parallel. Don't flare much.
Now for the take off, which can be very very tricky!!
Speed is the key. Without lots of air over the wings the plane is nervous and can snap roll. Add torque from the motor, and it will roll inverted instantly.
Some have used a catapult system using five metres of line and five metres of rubber, and fixed a hook under the nose 100mm from the front. A normal glider Hi-Start is too slow, you need speed. Videos are on UTube to help.
Others hand launch in the modern way by standing still, gently throwing the plane and letting the motor do the work. This usually leads to a rapid inverted dive.
Old timers will know the other way. You run into wind five or six paces as fast as you can, and launch the jet like a spear, upwards at 20 degrees or so. Do this with modest power, perhaps quarter, just enough to spin the propellor nicely and produce a good flow of air, but less that would be required to fly the plane.
Get it to flying speed at about four metres altitude and you should be OK.
Get your hands straight onto the controls and correct the left roll that is already developing, and feed in Up and gently add in climb power. Do not rush the power. Wait to get some altitude and flying speed.
Pulse the ailerons to level the wings, but avoid touching them again until you are properly aloft and ready to bring it round.
Foible Two - had you forgotten there was another foible? The roll is very rapid. It makes the elevator seem slow. The MicroJet can be flown on a basic radio control system, I fly one like that. But it's much easier with a computerized transmitter firstly because you can mix the elevon controls rather than use a separate mixer, but also because you can reduce the throws on surfaces more easily.
Try both reduced aileron (using the Dual Rates control) and Exponential. The latter will soften the control around neutral to make it less rapid. Worth explaining because I have been flying for decades but this is the first plane that I felt needed Exponential, even the TwinJet is fine without.
You might enjoy trying a gyro on the roll axis. It won't work unless there is air movement over the wings, so you still need to get some decent flying speed during the launch, but it will be useful in the first few seconds of climb. The Heading Hold type is best.
Finally, don't forget that it's small. Get your eyes tested and get some spectacles!!
If this has helped just one person avoid bending that lovely MicroJet nose, it will have been worth it. Hope you enjoy it as much as I do, it's Fabulous.
|Apr 09, 2012, 05:10 PM|
Joined Sep 2010
You are right, it's the short wings of many jet types that make everything happen so quickly.
Yet another experiment I would like to try is to put some momentum into the wings in the form of a little weight in each tip. It should slow the rotation. And twin, contra rotating props should be great too. From my modest experience of ducted fans, they don't have anywhere nearly as much torque.
Multiplex say that the FunJet is easy to launch. So how have they done that?
|Apr 10, 2012, 02:45 AM|
Joined Sep 2004
I built my MicroJet Lite. At 10.5ozs, it launched fairly straight forward.
But it sure was a pain to keep track of, being blue foam!
|Mar 29, 2013, 10:14 AM|
Joined Sep 2010
Since writing the above, I have over stretched my hurling arm by throwing the MicroJet, so can barely launch anything now!!
So, I have tried alternatives.
1 A vertical launch at full throttle worked. It rockets skyways, spinning as it goes until it falls off the top and heads away. I would like to say that I know where its heading, but its a bit of pot-luck. It does look spectactular when it works, but its very much a gamble.
2 A 20m length of bungee worked. It needs to be directly into wind otherwise the MicroJet turns rapidly. There are lots of different strengths of bungee rubber, so you really want the stuff that is 5mm or so in diameter, and practically doubles in length when stretched. I didn't use a line to extend the rubber since I just needed flying speed rather than height, but there is no harm in trying. Launch without power, gain height, then add motor.
3 A launch ramp is interesting, and can be just a few wooden battens, it doesn't need to be complex. The complication comes from getting the length of bungee just right, and you can do a lot of damage in the experiments. Point the ramp up at 45 degrees so you gain some height along with speed.
Just when the solution looks workable, the MicroJet decides to resist, so I cannot promise regular success.
Luckily, Multiplex built it strongly, and its easy to repair!!
|Mar 29, 2013, 12:48 PM|
I had this plane years back and was never able to launch her by hand.
I had as AF 4T 020 on 7cells and boy did she sure scream.
So, I built a wooden launch ramp.
0 degrees with about 10 meters of small tubing.
Worked great and was fun too.
Hook was half way between CG and nose.
The only trick I recall is not starting the motor until After the release of the bungee from hook.
|May 13, 2013, 11:18 PM|
I noticed that a year ago you got 5 white MJs. ARe any of them still new. I have two new in box Blue MJ airframes that I would be willing to trade for two white ones--just to balance out the MJ colors in my hanger.
Are you up for a trade?
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