My dad has been hard at work this winter with our latest IMAC aircraft. We have trusted Carden designs for many years and have no doubt this new Extra 300 PRO will be as kind to us as the rest of Dennis’ designs. What’s different about this one you might ask? It’s our first big Electric, so read on for more info!
To give you some background into the PRO design, it has been an envoluation of the Extra line of aircraft, firstly the 118″ 260, then came the 118″ 300 and following this the availability of the DA200 , it gave birth to 126″ versions of the 260 & 300′s. Along with Dennis, Kurt Keolling multiple time US national champion work on enhancing the Extra 300 designs, building it around the DA200 to make the ultimate IMAC precision aircraft, namely the PRO!
The PRO differs only slightly from the previous designs and all the changes are subtle. There is a thinner wing profile, a slightly different wing plan form, this improving snaps, or more accurately exits of snaps. It also requires less power as drag is reduced, also meaning that 150cc engines are still very competitive for this aircraft. The fuselage is narrower & not just as deep, again reducing drag and side area, meaning crosswinds less effect the aircraft. The rudder is now counter balanceless, with modern servos, power/torque is no issue, the lack of counterbalance makes the rudder input more neutral and reduces the mixing required. The tail section is also thinner, drag being reduced here again. The fuselage construction removes some material but with extensive analysis and calcs carried out by Kurt who is a professor by profession, strength isn’t compromised. This all adds up to a less draggy, lighter and more neutral airframe, making IMAC pilots work less on the sticks so they can concentrate on presentation of their schedule. I have to stress though, these are small improvements, the original Carden designs are still among the best if not the best out there.
The build follows much the same principles as all Carden’s, it’s “proper” building, using traditional methods. The wings, tail and all decked parts are foam core with balsa skins. We have used the method of vacuum bagging the balsa to the core with epoxy skinning resin, the reason for this, is you can control the amount of adhesive used and with the pressure the vacuum creates, very little is used. This proven method keeps things very light.
The fuselage is made up from a CNC plywood engine box, which the motor, under carriage and wing tube are all supported from. The fuselage sides are a balsa frame work sheeted, giving a rigid and smooth finish. The sides are connected together with balsa braces and the foam parts are bonded to the sides with PVA glue.
At the beginning of the article I mentioned that this Carden PRO was powered by electrons rather than petrol. This didn’t alter the build significantly, we purposely built this plane with a wide range of power plants in mind, we can very quickly switch this plane back to the conventional petrol, DA 150/ 170, DA 200 or even the DA 250 four stroke when it gets released. We kept the bulkhead in the motor box in the forward position (DA 150/170) and built a standoff box from plywood, laminated under vacuum with fibreglass, to this an aluminium mount from Mejzlik was used which is specifically designed for our motor. The fuel tank tray is omitted from this plane, instead this is where the batteries locate. SO overall the aircraft is very much a standard build.
Some might be asking why go electric? Over the last 3 years we have saw a lot of flying sites lost due to noise, our big IMAC models are as quiet as they can be, but for some sites that is still too noisy, is it sustainable to keep flying our big models with a huge footprint and blindly ignore the potential problem? Probably not! We have found that it’s not the noise of the models, it’s the repetition. If we fly for a full day, back to back, week after week, maybe 3 days a week, that becomes an annoyance to neighbours. This Carden PRO project is an experiment that gives us the option to fly a fully competitive IMAC model at any club site with no noise impact. The advantage is I can fly at sites closer to home midweek and ultimately get more stick time. It will be used along side our ‘traditionally powered’ competition models and compliment them. We can reduce the petrol powered flights to 2 /3 spread out over the whole day, add to that say 3 -4 flights on our electric aircraft, suddenly you’ve flown the same, but exposed your neighbours to half as much noise over the same period, this is our experience anyway. Electric shouldn’t be seen as a replacement for petrol, but a way to preserve it in the future.
Our setup is as follows;
- Motor: Plettenberg Predator 37/6 EVO.
- ESC: Jeti Spin 300 PRO.
- Batteries: Eflite 5s & 4s 30c Packs wired in series to 14s and in parallel to give 15000mah capacity.
- Propeller: Plettenberg Spinner cooling system with 3 separate blades, giving a 31.5×13 3 blade prop.
- Radio Gear: Spektrum AR12200, Spektrum 2s 4000mah RX packs, guided by Spektrum DX18.
- Servos: JR 8911 HV all round (4 ail, 2 elev & 2 rudd).
- Covering & Hardware: Oracover & various Hangar 9 hardware.
The airframe with all equipment except batteries weighs 13.1 kg (29lbs), the packs themselves and support tray weigh 5.5kg (12.12 lbs), giving a total flying weight of 18.65kg (41.12 lbs).
We estimate that it is about 1lbs lighter than a fully fuelled DA200 with cans version of the same model, obviously the petrol powered model will get lighter as the flight goes on, but we are very happy with this weight. The fixed weight should also give more consistent trim throughout the whole flight.
The Predator 37/6 EVO, is capable of over 16kw, to compare that’s somewhere in the region of 20-21 bhp. During our ground tests we limited the motor to 80% power, it turned the 31.5×13 3blade prop at 6200rpm and drew about 140amps constant, this was obviously unloaded and only for a 10-15 secs burst, but the RPM figure alone is impressive. Remember that was limited to 80% of the motors max. Everything ran cool even without the air flow.
The Test Flight
After a number of ground tests at home we were comfortable all would work on the air. So on a windy and slightly grey Saturday afternoon we decided to give it a go. A range test was completed and all was fine, the ESC/motor was armed and was ready to go. We kept the motor limited to 80% and very quickly the airplane was in the air, no shortage of power. The plane felt superb from the off, no trim at all, the CofG felt about right, we didn’t push the model as we are slowly building confidence and knowledge in the motor setup. Without having even started the trimming/setup process, Rolls felt axial, Knife Edge seemed very flat, 4 pointers looked really smooth as a result. A few snaps showed that these would be very controlled and their exits precise. The power plant itself performed very well, it has a ball of power and is very quiet, in fact you can hear the air rushing through the airframe whilst in flight. We limited the first flight to 4mins and after the flight the onboard telemetry from the Jeti Spin ESC should, RPM was 6200, max ESC temperature was 47-deg. The test flight gave us some relief, although we had done a lot of research, 12 months worth before embarking on the project and got a lot of advice from the few people who are flying 40% electrics, there is still that element of un none that you worry about. We are still learning and it will take some months to build our knowledge with this plane. The 4min flight drew 800mah from each of the 3 14s 5000mah packs, that was a gentle flight, so I expect 8-9mins of unlimited flying is very achievable and still leave 15-20% in the packs.
I’d like to thanks Plettenberg for all their help and support, Bernd Berschorner for his experience as a competition pilot using this setup for many years, Jocke Goransson (wattsup.se) for his advise, David Nolan for his help with our power supply, Kees Blokland for a year of replying to my many ‘electrical’ questions, to Dennis at Carden for his help and support and to my dad for doing the hard bit of putting it all together. I very much get the easy and enjoyable part! I also thank Horizon Hobby UK for their continued support, it is greatly appreciated.
As I said before this project is an experiment, early results show it’s going to work out very well. And I’ll end with a comment I made earlier, “Electric shouldn’t be seen as a replacement for petrol, but a way to preserve it in the future”, we see that every day with our hybrid cars and if F1 can see electric technology working hand in hand with engines is the future, why shouldn’t aeromodelling get in on the act. A fully hyprid model airplane power plant might be a while off, but who knows what the future might hold!
Some pictures below.