By Jeff Struthers
This has been a fun project, giving a new “Lease on Life” to an old airplane.
The aircraft in question is a Multiplex Sonic Liner and was only available years ago. I liked the look of it then, but as I was just starting out in R.C. Flight, I knew I would crash it. Eventually Multiplex took the aircraft out of production. Bob Stinson and I think this was around 2007.
I pretty much forgot about this airplane until early this year, when I was in Discount Hobby. I decided to walk back where the new boxed aircraft were to see what was there and wow, here was a Multiplex Sonic Liner. Still in the box, with a $ 46.00 price tag on it!
We have all felt the rush of finding a cool aircraft that fits within our budget, but how about for $46.00? Way cool. So, box in hand, I found the store owner John and asked about this. He said a customer had it sitting around for years, unbuilt and it was missing the battery / upper hatch cover. The cheap price was reflective of the missing part which John and I both thought could be made from Home Depot styrene insulation foam.
John also thought that replacing the old 6 volt brushed motors with E-Flight “6-Series” inrunners ( 2000 Kv) would fit in the existing motor pods without modifications. So, those were put on order.
I took the model home and started dry fitting it together. It was originally intended as a hand launch aircraft, no landing gear. The thought of this unique airplane belly flopping on our dirt runway didn’t do much for me, so E-Flight retracts were also ordered. I decided to have the mains rotate 90 degrees when stowed, as the wing is not very thick. There was also a concern about how far aft of the C.G. I could place the mains, as this is a pusher propeller system. On lift off rotation, these props are going to come close to the ground. The maximum size propeller recommended for these motors is a 6 x 5.5. That gives me three inches of clearance.
I elected to make the main gear struts 4 inches in length and install them 1 ½ inches aft of the C.G. This still places the counter rotating props 18 inches behind the main landing gear. Roll out is a bit long and I still have to keep the rotation shallow to avoid a prop strike.
Now because there are two servos for the ailerons, I have configured flaperons to get airborne with the hope of minimal rotation.
The original wing spars were about seven inches in total length and while I still installed them, I also installed a 1/8th carbon fiber slat from the wing root to just shy of the wing tip. This reduced some nasty flexing noted after the wings were glued on.
Also a concern, now that I was adding landing gear, was the flex of such a long fuselage. A single carbon fiber slat was epoxied along the battery compartment spanning the tricycle landing gear. I chose only one slat as I didn’t want to develop blind spots for the receiver. This seems to have done the trick rigidity wise, as my first few landings were less then optimal, shall we say?
The construction of the battery hatch involved gluing four slabs of 1 x 4 x 29-inch styrene together. It took some time, but a hot knife, sanding block and light weight spackle have produced an ok looking hatch (If you don’t look to closely). A vacuum cleaner to pick all the dang styrene dust was helpful too. That crap sticks to you via a static charge.
The finished airplane has wing span of 50 inches and a length of 50 inches. The two E-Flight inrunners fit into the pods just fine, leaving room for the 40-amp ESC’s to fit in as well. It’s a snug fit, but a Dremel tool carved some room and ventilation slots for the ESC’s and motors. I found small air scoops at “Park Flyer Plastics “, (Miscellaneous Section) to supply inflight cooling air.
The motors are configured to counter rotate and spin 6 x 4 props. These motors are only rated for a 3-cell battery, so with a fresh 3S 3700 I’m seeing 45 amps total current and 540 watts total power. All up weight is 3 lbs. 12 oz. so that’s around 144 watts / lb.
I know what you’re thinking, I have had the same thought. What would it be like with a four-cell battery? Well, faster and heavier. It currently flies scale and is quick enough.
This aircraft is loosely modeled after a picture I saw of a SST concept air liner study done by NASA and Boeing Aircraft. I did the blue paint and Callie Graphics supplied the decals.
So how does it fly anyway? Well, different but in a pleasing way. Remember that part where I talked about configuring flaperons? When I first engaged the flaps, I did it at altitude where I could see what would happen. The elevator on this airplane is up front, on the canard. The main wing and flaps are in the back, behind the C.G. So, flaps deploy down, wing lifts and the nose goes down, hard down. I had to add about ¾ of back stick just to fly level (different).
I really should have seen this one coming. The fix was to add elevator mixing with the flap switch, duh. With the elevator in front of the C.G., it’s just the opposite of a standard tailed aircraft. To go up, the elevator deflects down. To go down, the elevator deflects up.
Now when I deploy the flaps the elevator also deploys downward, creating lift both fore and aft. And that take off rotation I mentioned is now greatly reduced. Only a slight amount of back stick is needed to initiate lift off. The aircraft will rise almost horizontally from the runway, keeping the props clear of the ground.
For landing, the Sonic Liner needs to be flown to the ground. On final approach with full flaps, power is held at 50%. There is a slight, flat sink rate, so once I’m clear of the soft rocky patch on the east end of the runway, I’ll reduce power by 1 or 2 clicks on the throttle. The sink rate will increase so I must be ready to add those clicks of power back. It will take a second or two for the sink rate to change because this is a pusher configuration. I land it like a jet, adjust altitude on final with power not the elevator (pleasing).
In flight, the Sonic Liner likes to move along and seems to have a more solid feel at full power. As you can see, the wing chord gets short moving away from the fuselage. That may explain the soft feel at half or three quarters power.
This is an interesting airplane to fly and it looks pretty striking in the air. For me, making all the adaptions for motors, strength and flight control have been the fun part. Learning how to take off and land a canard aircraft was a challenge, but this is how we learn in our hobby.
Learn something and push yourselves a bit, it can be a pretty cool result.
p.s. Bob and Vince took some still pictures and videos of this aircraft flying and posted those on You Tube: