|Wing Area:||400 sq. in.|
|Weight:||20 to 28 oz. (22 oz ready-to-fly as tested)|
|Wing Loading:||7.92 oz/sq. ft.|
|Servos:||4 micro (4x Hitec HS55 used)|
|Battery:||15-20A continuous capable lipo (Tanic 3s 2200 used)|
|Motor:||200-250 watt brushless (Axi 2808/24 used)|
|Prop:||APC 11x4.7 SF|
|Available From:||Steven’s Aeromodel|
When picking out an aerobatic 10 lb airframe, there are certain traits one would desire - namely, viceless handling, rapid and crisp control response, good energy retention, a powerful motor setup, and, perhaps above all, good looks. This Park Flyer took all of these properties and stuck them in a 20 oz airframe. It became the Groove. It combined flowing lines, reminiscent of much larger cousins, with the parkflyer ability to get into and out of the smallest of flying areas, the Groove is truly a little big plane - or perhaps, a big little plane. Yes, the specs table is correct. The Groove is a 45" wingspan aerobat that only weighs 20+ oz ready-to-fly!
Upon opening the box, I was delighted to see several wonderfully laser-cut sheets of wood among the rest of the kit contents. A glance at the wood showed superb cutting quality with little to no excess charring or un-cut joints.
In addition to the kit contents, the following was required in order to get the Groove airborne:
Before starting construction, in typical fashion, I did a read-over of the instruction manual. What I found was a very complete write-up, covering everything from the basics of construction right through the test flight. Black and white photos accompanied the text so as to leave no question about any method of assembly.
As with other designs, the fuselage is basically a box with a rounded turtle-decking on top. However, unique to Steven’s Aero’s designs, any open structures such as the fuselage used a style of cross-bracing called Truss-Loc. Basically, in each fuselage side, the outer frame was interlocking and the inner braces interlocked with one-another. On top of this, each of the cross-braces were labeled such that A goes with A, B goes with B, and so on. In the end, each fuselage side was a rather strong but remarkably light assembly. Once the fuselage sides were complete, they were mated together with interlocking fuselage formers, again labeled so as not to get them mixed up. Once the formers were in place, some stringers were added to help shape the turtle-decking, and voila; a completed fuselage.
The only decision left to be made before final sanding was the location of the rudder and elevator servos. The default location shown on the plans was inside the fuselage just underneath the canopy. However, the instructions do mention that for higher weight setups like the Axi, servos could be moved to the rear of the fuselage to help with the CG. For this build, I opted to put the elevator servo in the stock location and move the rudder servo underneath the fuselage.
Each of the tail feathers went together just like each of the fuselage sides. The outer frames go together and then cross braces were added between them. Made entirely from 3/32" balsa, they ended up very strong for their size and weight, or lack thereof.
What used to be the hardest part to any build, the Groove’s wing went together with great ease. The main spar came in two halves and was joined together with a uni-directional joint at the center. The ribs, complete with small tabs to help wing alignment, got pushed into slots in the spar. Beyond that, the trailing edge and leading edge were pushed into position and some turbulators were added.
The unique part to the Groove wing build came in the area of the landing gear. The landing gear legs themselves (pre-bent, I might add!) get built into a wooden blade of sorts and get pushed into a pre-arranged slot in the wing. This allowed quick and easy installation but also allowed quick and easy removal should repairs be necessary. Also, the wheel pants used a clever mounting method which utilized a small plywood plate as a retainer, and did not require any special alignment tools or wheel collars.
Somehow the bare wire of the landing gear looked like it needed a little something extra to me. So, I used some of the scrap wood from the kit and came up with some gear legs.
My covering of choice, typical to my other parkflyer builds, was Nelsons Litefilm aka Solite. However, with an airplane of this size, most any covering would work. A heavier covering like Ultracote may add another 1 oz on top of the All-Up-Weight – a negligible amount for such a large wing area. It all came down to personal preference more than any specific technical reasons.
As mentioned earlier, the rudder servo was placed underneath the fuselage in a pre-cut slot. This allowed a direct pull/pull connection to the rudder from the servo as well as quick and easy adjustments should the need have arisen. The elevator servo was located within the fuselage with the pull/pull routed through slits in the covering just ahead of the stabilizer. In both cases, the controls operated smoothly and allowed massive control movements. The aileron servos were placed in the assigned slots in the wings, allowing direct control linkages to the middle portion of each aileron. This minimized any control surface twist under load. With the servos in these locations, three servo extensions were needed – two for the ailerons and one for the rudder.
Battery (and ESC, for that matter) access was through a large hatch on the top of the fuselage. I opted to use small tabs of tape to secure the hatch, though any popular method would work such as hinging and taping, hinging with magnets, just magnets, etc.
The wing was built to be secured with two dowels at the leading edge and two small screws at the trailing edge. This was just one option as the alternative way (elastic bands) can also be done from parts in the kit.
I spent approximately four hours of casual building time to get the airframe ready for sanding. Sanding took another few hours followed by about 10 hours of covering labor. Add in another few hours to install the radio gear and the Groove was ready to fly. All told, about 16 hours were spent to get her airworthy.
Prior to the test flight, control throws were set up with two rates. The lower rate matched what was recommended in the instructions. The higher rate was no less than 45 degrees in either direction on all controls. Naturally, some exponential was added for the higher rate mode to deaden the control response for higher speed maneuvers.
|(distances/angles are either direction)|
|Control||Low Rate||High Rate||High Rate Exponential|
Equally as important as the control throws, a CG check was done before the first takeoff as well. Per the instructions, I placed the battery pack such that the CG would be dead on the main spar.
The initial test flight was done on low rates. After a quick control check to confirm directions, I slowly pushed the throttle ahead. The initial takeoff roll-out required only a breath of right rudder and the Groove gracefully took to the air in under 30 feet at less than half throttle. A trim check showed that I needed two clicks of right rudder to compensate for P-factor and two clicks of up elevator to compensate for the forward CG. Further takeoffs have shown that full throttle is never required, even for the shortest takeoff performance. In fact, with the listed power setup, takeoffs at ¾ throttle or above are akin to the space shuttle taking off! However, the real beauty in how the Groove performs on takeoff was the lack of the semi-typical parkflyer twitchiness. On the roll-out, the controls remain sharp and responsive, yet not overly so.
Further flights also allowed me to experiment with hand launching the Groove. There's really not a lot to tell since hand launching came as easily as taking off from the ground. I held onto the Groove by the fuselage just aft of the battery hatch, gave her some power, and with a light toss, off she went. It's as simple as that!
With a sub-8 oz wing loading, the Groove, as was expected, slowed down very, very nicely. On the low end of the power spectrum, the Groove showed no signs of aggressive wing drop. However, it did drop a wing ever-so-slightly in an aggravated stall as expected. Landings, I've found, were best done with a touch of power rather than power-off. With that in mind, the Groove liked to land in a 3-point attitude but still flying, rather than full stall.
Upon pushing the throttle ahead even further, I found the Groove was very willing to pick up and maintain speed. No trim changes were apparent, and the controls only proved to be more crisp, never becoming over-sensitive.
After an extended duration bout with circuits to get the feel for the Groove, I dove into some minor aerobatics. Translation: After a 30 foot roll-out and a quick trim check, I pulled vertical and performed rolls to a few hundred feet of altitude then split-S’ed the Groove back down. In all seriousness, this is what the Groove was made for – precision, IMAC-style aerobatics. The most remarkable part of aerobatic flying with the Groove was that, despite the large wing and immense control surfaces, the controls were always crisp and smooth with immediate starts and stops.
As the Groove had no right thrust built in to the motor mount, vertical lines required some right rudder to keep inline. That is just as I personally prefer it to be, as right thrust can work against an aerobatic airplane in certain maneuvers.
Let’s get to some specifics, starting with the basics:
With the test flight CG location (on the main spar), approximately half forward stick was needed to maintain inverted on low rates. However, when the CG was pushed back to around ¼” behind the main spar and rates were switched to “high”, only the smallest push on the elevator stick was needed to maintain altitude.
The basic loop and its derivatives varied for my style of Groovin’ from about four feet in diameter with flaperon mixing up to as large as around 300 feet in diameter with emphasis on constant speed and precision alignment and everything in between. With the grossly excess power onboard, the Groove was able to perform loops akin to a much larger aerobat such as the glow Extra I got used to in years gone by. When needed, rudder corrections in loops could be made with just the lightest touch on the sticks rather than having to throw the rudder stick all over to get results.
The aileron roll; a maneuver that, depending on the plane, can look utterly lazy or precise and crisp. With the Groove’s generous fuselage side area, even a non-corrected roll looks precise. A forward CG required a bit of elevator to keep the nose up while passing inverted, but once the CG was pushed back, this elevator compensation can be all but eliminated. Once a small bit of rudder-elevator mixing was added in (see “Knife Edge”, below), point rolls became dead easy, requiring ¼ rudder in each knife edge point and little-to-no elevator when inverted. Roll rates varied from just under 1 per second on low rates to just over 2 per second on high rates with a bit of speed.
Knife edge is a double-edged sword for the Groove. The rudder had massive authority and the generous fuselage side area of the plane allowed for some spectacular knife edge performance. The only down side to this came, as with all low-winged aerobats, in the area of coupling. There was a moderate amount of negative pitch coupling (the plane pulled toward the landing gear in knife edge) to be mixed out. As it turned out, mixing 18% up elevator to rudder was the key. This produces about 1/4" up elevator for the 3/4" rudder required to maintain altitude at cruise speed. Once this mixing was in place, the knife edge was dead easy with no additional coupling in any direction.
Two words sum up the Groove’s snap roll performance: Sheer beauty! From a slower speed, snap rolls could be performed with massive movement in all axes. From a faster speed, snap rolls were an absolute blur. Despite the low wing loading, when asked, the Groove very willingly departed a wing. This bad-when-you-wanna-be trait is ideally suited to an aerobatic ship of any size, and a rare find in parkflyers.
Carrying over from snaps, spins are total joy as well. The Groove loved doing spins of all varieties, from upright to inverted, flat to conventional, and even in knife edge. Spin entries were rudder/elevator only, inducing a true spin and not merely a spiral dive. Spin recovery was as simple as releasing the rudder and pulling out. There were no tendencies to remain in a spin once the controls are released.
I did what the plane was meant to do. I then had to do what it wasn't meant to do - 3D work. I found the Groove to excel in a few areas despite not being a dedicated 3D airplane. Hovering was surprisingly easy (despite my poor thumb work while torque rolling in the video) and walls were utterly amazing with no twist in either direction. I noticed some very minor wing-rock in harriers which quickly disappeared with a small amount of spoileron/elevator mixing. Blenders were also done with relative ease (see "spins" above).
After a few flights, I was able to get a feel for how long I could actually fly the Groove with the chosen power setup. I would routinely fly for approximately 20 minutes and put back around 1900 mah into the battery pack. Thus, flights close to 25 minutes would be attainable with the flying style shown in the video below. Results varied, of course, but 20+ minutes for my kind of flying was fantastic!
All flying shown in the video (including takeoff and landing) was done on high rates with the expo as noted above. To demonstrate the power of the axi setup, note that the vertical run at 1:10 was done at 2/3 throttle. The harrier at 2:22 was done without spoileron mixing. The large loop at 2:43 was done at a maximum of 2/3 throttle. The tight inside and outside loops at 3:37 were done with 50% flaperon mixing and without rudder compensation.
While not meant for the rank beginner, the Groove would be well suited to a pilot moving from their aileron trainer into something with some aerobatic prowess. With the rates low, the Groove was docile enough not to confuse someone not used to a higher roll rate and yet aerobatic enough to push the mental envelope. With that being said, kick up the control throws and the Groove would be equally well suited to someone with years of flying experience looking for the ultimate aerobatic parkflying experience. Is there a down side to this performance? Well, of course there is. With the Groove airworthy, I found all but my most addicting planes losing out in the airtime department. It seems I've been too busy getting my Groove on!
Special thanks goes out to my lovely wife, Christine, for taking the inflight pictures and video.
Thanks for a well done review! The thing I like about Martins reviews is that they are very readable and I can count on learning a little something even if I'm not immediately interested in building the plane. -Steve
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