I recently completed a modified build of the RC Powers Su27 V5/Super Series. Plans are available for purchase here as well as much more info on this plane that has been around since Oct 2015. I have built other Su style airplanes, but never an RC Powers Su, so I was eager to see how this plane handled.
I built using the tiled plans and when I printed them off I selected 88% on the scale to keep everything in perspective. By cutting on the very outside of the line for all the slots, I was able to get everything to fit together well. I had to widen the prop slot somewhat and trim some off the bottom of the nacelles, but nothing that weakened any structure. Rather than using the fixed leading edge flap per their plans, I decided to go with KF4 made from paperless Dollar Tree Foam. The only other performance mod I made was to double the size of the rudders, my rudders measure 6" x 1 and 3/4". My KF4S are 40% of chord measured at the wing root along the fuselage and 35% of chord at the wing tip. I extended the top KF along the LERX as you can hopefully see above.
For reinforcement, I used a combination of carbon fibre rod/tube and bamboo skewers glued into the foam with epoxy. Hopefully you can see all the reinforcements I used in this picture. The main spar along the back of the prop slot is 4mm carbon rod, very strong and stiff. The spar ahead of the prop slot and along the hinge line of the elevons is 3mm carbon tube. In the elevons and vertical stabs I used bamboo skewers. This setup seems to work very well, I did see a tiny bit of wing flex at the tips in high G maneuvers like pulling out of loops, power dives and full throttle turns, but nothing that worries me. It is important to approach the build with lightness in mind, the extra reinforcement is a fact of life or the plane will flex considerably.
I think in future it is going to be very important for designers and builders to keep park jets from getting too big if good quality hobby grade Depron is not going to be available. Otherwise, planes will get either too floppy or too heavy for most power systems to handle.
AUW with 2200 3S battery - 20.9 oz/592 grams.
Power setup - Gear Best A2212/5T 2700 kv motor, 6x4 APC prop, Turnigy Plush 40A ESC, Zippy Flightmax 2200 3S 40C battery.
Controls - 6 RC Timer 9 gr nylon gear servos for elevons, ailerons and rudders.
Elevons - 7/8" one way in the pitch, 1" one way in the roll
Ailerons - 1/2" one way
Rudders - 7/8" one way.
CG/PMI (Polar moment of Inertia) - My plane balances neutrally right in the centre of the black rectangle of the "CG spectrum" on the plans. The pen in the picture below points to where that is. My PMI (distance from the front of the battery to the tip of the bullet nut of the motor) is 9 and 5/8". All my electronics including servos are within this span, so over 60% of the weight of the plane falls within that distance. Below you can see a picture of what my electronics bay looks like ready for flight balanced on the CG shown.
I flew 18 flights in total in this first test session, so was really able to get a good feel for the plane and get the balance, trim and throws really dialed in. Conditions were ideal, very light winds for most of the flight session, increasing to about 10 mph in the last few flights. The RCP Su27 handled the wind with no problem at all.
If I had one knock against the flight performance of the RCP Su27 V5, it is that it flies "top heavy". As you can see in the picture below, there is a lot of foam above the wing plate. This coupled with the fact that the battery bay is not very deep (only 1/2" below the wing plate on my plane), this puts a lot of weight at or above the wing plate. This causes the plane to want to flip on it's back very easily in certain attitudes or maneuvers impacting a few areas of the flight envelope which I will touch on as I go along.
I launched at about 60-70% throttle, I saw slight torque roll, nothing scary, but it is there, otherwise the plane stayed at the attitude in which I launched it and continued straight ahead as I got it under control. Very well balanced, no diving, pitching up, easy to launch.
The Su27 was very well behaved in turns, tracked very solidly and consistently, but I had to be careful not to get too slow or too aggressive in the roll axis or it wanted to roll too far over or too quickly with it's top heavy characteristics. After a couple flights and getting my throws really dialed in, it softened this behaviour and the plane was very responsive and agile in turns.
I found the Su27 to be very responsive in the pitch axis, again it took me awhile to dial it in and not get too aggressive or too slow otherwise it wanted to flip on it's back too soon or spill out sideways from the loop. By keeping good steady speed and smooth inputs, it would loop very smoothly and evenly.
The Su27 is very crisp and axial in the roll axis, it rolls like there is a rod down the centre of the plane. I did find it needed more throw in the roll axis than some planes, I suspect due to the large vertical stabilizers and ventral fins.
Although I didn't fly in too strong a wind, I still found the RCP Su27 to be very stable in the yaw axis, holding it's track very steady in turns and in high speed runs. Rudder response was good, I tried several stall turns and it was smooth and true in the yaw axis, no induced rudder roll or requirement to input opposite aileron to keep it smooth in flat turns or stall turns. In fact, it is so stable, I could not get it to rudder roll at all. I would take it up and hold the rudder over full for 5 seconds or so and it would drop the nose slightly and turn in a flat turn, but no rudder roll as I have seen in other planes if I held the rudder over too far for too long.
I found I was able to fly around fairly comfortably down to about 40% throttle. It was quite well behaved, although I could feel the "top heaviness" as it got close to critical stall speed and had to watch for tip stalls as it wanted to roll over on it's back as I got slower and slower.
I did see a tiny bit of torque roll at high speed, but normally it was very locked in, just buried the nose and got moving forward. Very stable and well behaved at top speed.
The Su27 glided very well on approach to landing, it felt very balanced as it slowed down. To keep from digging the ventral fins in too hard on landing, I slowed down and pulled the nose up gently to a stall about 6" off the ground and let it come to rest. I put a thin layer of epoxy on the ventral fins and I think this will help keep them strong and durable over time.
Here are pictures of where I held the ruler to measure the throws I listed above.
Obviously I must caveat that your experience if you build it stock per the plans with the leading edge flap and of larger size will probably be different. As I mentioned early in this post, I built mine this way for very specific reasons, but overall I am very happy with the foundation upon which I made my modifications.
A job well done, RC Powers. :)