ST Model Arcus M

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Gliders are often thought of as the purest way of flying in which the aircraft, using a minimal control system, breezes through the air. In reality, in order to optimize many phases of the flight and to manage the lift and speed, gliders are often equipped with systems not found in classic propeller airplanes, at least in their basic form. For one, most modern gliders have retractable gear to reduce drag in flight, and many use airbrakes to increase the drag and reduce the lift for landing. The Arcus M we are reviewing today, adds to these features by including a Retractable Motor System (RMS) used in take offs as well as to sustain the glider between thermals.

These options are rarely reproduced on RC gliders, mainly because the added mass would outweigh the benefit they provide. They are typically reserved for the large scale gliders, where the wing loading is easier to manage.

The Arcus M by ST model is here to change the status-quo by introducing high-end options in a compact and affordable airframe. The model comes out of the box with RMS, spoilers and a retractable main gear. The impressive achievement is to have been able to incorporate all of these features without having added much weight, and while keeping the price tag impressively low.

Let’s dive deeper into this tour-de-force airplane.

 

 

 

  • Getting an RMS, retract and airbrakes at less than $150.
  • An accurately scaled model, down to the wing tip
  • The ability to take off on from the ground or by hand launch
  • The glider appears to float at low speeds

 

 

 

  • Wings, rudder and elevator are not stiff enough for high speeds
  • Use of airbrake at high speeds can wrap the wings causing instability
  • The wing tips are really (really) fragile
  • The RMS cannot be maintained or replaced. If anything happens to the servo, the airframe is lost.

 

Product Details

 

 

Manufacturer’s Description:
The high-viz trim scheme is already applied. A power system, most onboard gear, and all hardware are ready to go, installed at the factory. And the only assembly that remains is easy and over with in about 15 minutes. Add a receiver and a charged 4S pack and your departure time is minutes after you get to the field. Give it a hand launch, and it settles down into smooth, easy-handling flight almost immediately. Blip the throttle and it?s suddenly downfield and disappearing fast. Line up for a low pass, push the throttle to the firewall and feel the rush that you’ve been waiting for.
Manufacturer: ST Model
Wingspan: 87.4 in (2220 mm)
Length: 39.5 in (1000 mm)
Wing Area:  444 in² (28.6dm²)
Weight Range: 24-25 oz (680-710 g)
Wing Loading:  10.3-11.7 oz/ft² (32.5-35.8 g/dm²)
Battery 3S 1300mAh LiPo battery pack
Requires:  6-channel radio (AIL-ELE-RUD-THR-BRK-GEAR)
Receiver
Battery 
Price: $150

 

 

First Look

 

 

The box is well designed and can serve as a storage or transport box. The banner on the side indicates ?2.4Ghz R/C System”, but be advised this does not mean the glider comes with a transmitter or a receiver. I actually don’t know why this is mentioned as I don’t see any reason why an FM radio could not be used with this plane.

 

 

One of the benefits of this glider is that there are few parts and for the most part, it is completely assembled and ready to fly. Before flying you must install the battery, the receiver and program the radio (these three components are not included).

 

 

 

The elevator is very thin and includes injected molded plastic to strengthen where it attaches to the fuselage. There is also a spar going through most of its length to enhance the rigidity.

 

 

The detachable cockpit provides access to most of the servos located in the fuselage, with only the RMS servo and the ESC left out of reach.

 

 

The many features of the Arcus M make the radio installation a bit more complex than usual. This schematic?captured from the manual? is a good summary of the different connections.

 

 

The manual is very detailed?maybe even too detailed. It covers the assembly of the kit version, which to my knowledge, is not available in the U.S. You can skip the build section and jump directly to the settings section.

 

 

The ESC comes equipped with the same type of connector as found on the recommended battery.

 

 

The RMS mixer is pre-installed and the connectors are zipped tied to ensure they do not move in flight. The board connects to the throttle channel of the receiver (the R7008SB shown here was only used for testing). The auxiliary channel can be used to directly control the extension of the motor in flight.

 

Used for the review

 

 

 

The 6 channels of the R6106HFC are sufficient for the control of the Arcus M, thanks to the RMS mixer board, which combines the motor extension and the throttle into one single channel. The ailerons are connected to one channel with a Y connector, and so are the air brakes. The remaining 3 channels are the gear retract mechanism, the rudder and the elevator.

 

 

We used the recommended battery: Flyzone LiPo 3S 1300mAh 15C.

 

Plane’s Features

 

 

The fuselage is literally packed with features: the gear retract mechanism sits right next to the RMS leaving just enough room for the ESC. The front hosts all the control servos, the RMS mixer and the battery. The servos are mounted on a plywood tray.The fuselage is literally packed with features: the gear retract mechanism sits right next to the RMS leaving just enough room for the ESC. The front hosts all the control servos, the RMS mixer and the battery. The servos are mounted on a plywood tray.

 

 

The Retractable Motor System is actuated by a servo. A four-arm mechanism conveys the servo rotation to the motor pod extension. The access doors are pushed by the mechanism, and held by two springs. That is a very neat mechanism. The only issue with this mechanism is related to the servo. Only a section of the servo spline engages to the servo arm. This small section has to sustain all of the torque coming from the servo to the servo arm, and if the mechanism were to block or be forced inward, the spline is likely to fail. The problem is that there is no way to access the servo without digging deeply into the foam. Be very careful with the mechanism, and make sure it extends freely, with no bind.

 

Full scale motor glider

 

Yes, some full scale gliders are equipped with a motor and propeller, just like their RC model counterparts! They may not qualify as real gliders for the purist, but they are none-the-less greatly beneficial for the peace of mind of the pilot who knows (almost) for certain that he or she will be able to fly back to the home airport. There are different kinds of motors and retractable mechanisms used in the full scale world of gliders:

 

 

Retractable propeller mounted on a mast behind the cockpit, like the Arcus M, are probably the most common type of retractable propellers. The motors are usually gas engines, but some electrical gliders exist.

examples: Arcus M, ASH 30/31/32 Mi, ASG 29 E, ASW 28-18E, ASK 21 Mi

 

 

The Stemme S10 has a propeller that folds in the nose cone when not in used. The mechanism is  something to see: : the nose cone extends forward, leaving an opening in the fuselage for the propeller blades to extend while rotating.

 

 

The most common mechanism used in our RC model gliders, i.e. the front folding propeller, also exists in the full scale world.  The propeller and motor are mounted in front of the plane. The centrifugal force extends the propeller when the motor is in used, and the relative wind pushes the blades against the fuselage when it stops.
examples: LAK-17b FES, Silent 2 Electro.

 

Flight Report

 

 

 

The compact and nicely designed Arcus M is without a doubt a head-turner, and a trip to the flying field quickly confirms that fact. If fellow RC pilots are not seduced by the elegant outline of this glider, the RMS is sure to attract the technical hearts inside of them.
The wing aspect ratio may not be as high as the real Arcus but it is quite close?at least much higher than the other 2-meter wingspan foamies we are used to see. This ratio of the wing length to the chord is indicative of a performance planform, as it creates less induce drag. The wing aspect ratio is rarely that pronounced for a foam glider, as it is more difficult to design a structurally robust wing when the chord is narrow compared to the length. The Arcus M?s high aspect ratio has a positive impact and just looks great.

We tried different types of take-offs since the model can actually take-off from the ground, thanks to the RMS and the retractable wheel. This is a very unusual feature for a small glider like this one as RMS are usually in larger models. Being able to pilot the plan to take-off on its own is great fun.

Hand launching the plane is also very straight forward and equally as fun. The RMS brings the propeller far from the eyes and hands of the person throwing the plane, which makes the operation that much safer. I must note that the plane had to be trimmed significantly to fly straight. With the elevator level, it had a tendency to dive to ground, so beware on the first launch, and have your thumb ready on the elevator stick.

The power is not over-abundant but sufficient to get the plane to altitude at roughly a 15 to 25 degrees climbing rate. The motor extending and retracting does change the CG location, but not to degree that the plane becomes difficult to control. The change is really minor and does not affect the flight behavior.

The Arcus M is happiest at lower speeds, and has a very good glide ratio considering the extra weight it has to carry. At 34g/dm2, its wing loading is higher than most models of that size (for example, the Flyzone Calypso is at 26g/dm2, the Radian Pro at 25g/dm2 and the Radian 22g/dm2). This leads to a plane that flies slightly faster. Its low drag wing helps keep the plane efficient.  At that speed, the glider shows very good ability to seek for thermals?making it quite pleasant to fly in this condition.

Here is the catch: the glider cannot handle higher speeds. The wings and elevator will start flexing more than they should, and any action on the aileron or the elevator will generate a bending moment that wraps the flying surfaces which could be fatal to the plane. The speed envelope is narrower than most gliders of that size.

The lack of the wing rigidity along the pitch axis makes the airbrake hazardous to use at higher speed. They should only be extended when the Arcus speed is under control, and they are useful when coming in at a steeper angle for landing. To be honest, the plane would do very well without the airbrakes, as landing without using them is already a simple task. But they do contribute to the “cool factor” of the Arcus M.

 

 

 

The limited speed capability reduces the ability of the plane to perform any aerobatic manoeuver. Of course, this wouldn’t be scale, but it is sometimes relaxing after a long session of thermal chasing to loosen up with a couple of rolls and loops.

The main wheel retract is also a very classy feature and coming in at a very shallow angle on the runway to let the glider roll is my favorite type of landing.

 

 

 

 

Summary

ST Model came up with a very accurate scale model of the Arcus M, down to the most advanced features including the Retractable Motor System, the spoilers and the main retract. It is debatable that these features have a truly measurable effect on the flying performance of the model, but it is sure that they bring a unique look to the glider. ST Model managed to keep the price extremely low for this model RTF despite its many options, which is impressive in itself.

The Arcus M is a model reserved to advance pilots. The plane does not have the inherent tolerance we want for a beginner plane, and if brought outside of its “comfort zone” it can become a handful to fly. The performance of the soaring glider is on par with other models of the same size, which considering the extra weight of all of its features, is not a small achievement.

The glider will please the experienced RC pilot looking for an unusual machine to fly, that has features only seen on large scale airframe, without spending the big money for them.

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