fus air intake fus air outlet motor intake enlarged shortened elevon horns
tape hinges and flattened elevons twin jet underside showing trim for orientation  

Written by: Bill Glover

Twinjet Configuration

Only change since initial setup presented bellow is 480Ls instead of 6V S400s, same 8x2400 and CAM 5.5x4.3 props.

This is a very early "mark 1" kit. The current TJ has some slight airframe differences . ... the wing is thicker (filled in) between the motor nacelles and the top of the fuselage, and the servo bays are slightly closer to the hinge line.

The present "standard" and "racer" TJ airframes come out of the same mould so are identical to each other apart from the color.

I'm using 8x2400s with the stock 6v S400s. My mods. are:

1. CAM 5.5x4.3 props
2. Tape hinged elevons
3. Air inlet under front edge of canopy (like Twin Star)
4. Hollowed out rear fus. to let air flow straight through and out at the back
5. Elevons bent flat (to remove reflex) - Jurgen's idea
6. Clear tape to protect fus. underside
7. Some USAF decals etc. from scrap box

Review

Initial Review. Extract from Ezone Thread - Click to read the full thread


All up weight is 42.0 oz, which is 10.5 oz over the quoted weight. my 8x2400s (instead of 7x1700s) account for 2.9 oz of that, the CAM props and adapters 1.5 oz at most ... your guess is as good as mine where the other 6 oz came from!

The control throws are exactly as stated in the instructions, I needed the outermost hole on the servo arms and second innermost on the elevon horns to get this. I'm using exactly the same mixer/throw settings on the TX as for my PJ, which is handy!

OK, now the good bit. This model is very impressive, I flew it at three IC sites over the weekend and the immediate reaction from most people was "I gotta get one of those"!

With the flattened elevons I have moved the CG back slightly so it balances just behind the bumps under the wing (with fingers just touching them). In flight it needs a little down trim (the thickness of the TE, maybe 2 or 3 mm) but there is no trim change between power on and off, so the standard thrust line is fine. With this setup the model is fast, smooth, and stable. Low fast passes are a breeze ... no "zooming". This model looks and sounds superb on a fast, low pass. I use some up trim for solo launching - with this the model just pulls out of your hand and climbs out. I reset the trim to neutral on climb out.

Inverted is rock steady, needs some 'down' held in (as you would expect) but flies inverted circuits and low passes with no problem. However, it will not do an outside loop, I think when pitching to higher angles of attack inverted the airflow breaks away at the rear of the wing and the elevons lose effectiveness.

Roll rate is excellent - at speed (and the model is fast) I would estimate 2.5 rolls per second on full stick deflection. But it is not too sensitive for general flying - no exponential or dual rates are required. Pitch response is fine, but not so sharp as roll. Loops, reversals, immelmans, etc. are no problem.

Slow speed handling is superb, there is no real stall and tight turns are completely safe. I was flying circles and figure eights at low speed just above the ground ... the ability to fly well both fast and slow was one of the things that people were most impressed with. It doesn't drop the nose in slow turns like my PJ does - feels really well sorted.

I managed to get the model to spin - pull sharply nose up, then just as it runs out of speed apply full up and left (or right). Because of the mixing one elevon goes fully up (45 degrees plus) while the other is neutral. If you time it just right the sudden drag on one side causes enough yaw to pull it into a spin! Couldn't get a stall turn though.

The glide is steeper than the PJ, and more height is lost in turns. This is a good thing as you can lose a little height without picking up too much speed. Landings are not too fast, it slows up well when you flare.

At the IC sites I had the timer set to 8 mins and didn't hit BEC cutoff. The packs came out only slightly warm - exactly the same setup (motors/cells/props) in the Twin Star caused them to get pretty hot ... that model wasn't fast enough to let the props unload in flight (too much pitch).

As you would expect, the TJ handles wind and turbulence well (better than the PJ).

The standard battery retention works well and seems completely secure, though it's a little fiddly getting the rubber band around the pack (I use a length of 3/16" dowel to pick up the band once the pack is in). You also need to slide the pack in with the model inverted .. otherwise it sticks on the velcro.

In summary, this is a great model, particularly with the elevons bent flat to reduce the reflex a little. There are a few QA problems - like the supplied elevon pushrods being too short - that Multiplex need to sort out (if they haven't already), and my personal view is that the standard "foam plus tape" hinges are too stiff.

I just bent the elevons flat using my fingers & thumbs. I had already cut them off in order to re-hinge with tape top & bottom, but you could probably do it with them still attached. Don't just bend the extreme TE down, you need to take the whole upward curve out of the elevon. That foam is pretty tough, if you've got any offcuts lying around, try snapping or tearing them!

If you go with tape hinges, make sure you clean the foam up first. Meths is completely safe for this. I experimented with masking tape first, and couldn't get it to stick to the foam at all until I'd cleaned the surface! So far the tape hinges are staying put, I will use cyano if they do start to lift.

The Gunther and CAM props both have the same stated pitch (110 mm) and are probably similar in performance on this model. However I think the CAMs are slightly more efficient .. the blades are rigid and don't flex like the Gunther ones. They are also a little quieter as they are further from the wing and fin TEs (due to the prop adapters). The Gunthers are very light (and free!) though. The howl they make sounds great at an IC site, but not so good in a local park. The CAMs still sound pretty good, a club member came rushing up from the car park yesterday to see what was being flown!

I think the motor cooling can be improved, to start with I opened up the intakes a little ... the rim around each is very thick, so I carved the foam away to streamline the airflow and let more in.

I also intend to try covering the open bottom of each motor cutout with a flat plate. This would force the cooling air to flow along /through the motor and exit at the rear. I suspect that a lot just spills out downwards at the moment. However the heat will dissipate quicker on the ground with the motor bays left open.


I spent 5 minutes flexing the foam hinges back and forward but the servos still stalled at full throw ... too stiff for me (just personal preference)!

One small thing I forgot to mention in the original post, on anything other than smooth grass the elevon horns were picking up a lot of debris .. so were obviously contacting the ground on landing. To avoid shock-loading the servos (and hinges) I trimmed the horns back to the pushrods (I wasn't using the outermost 2 holes). Of course this also reduces weight and drag and so makes the model fly faster