RE: Bob Cat Copy Falcon 120 Jet
Ok Guys, got mine up in the air 2 days ago, flew beatifully.
First, I am powering mine with my own built jet, which is heavier than a comercially available engine ( about 1.25 lbs heavier), so had to go on a diet. My model weighed 15 lbs 2 oz with a full UAT, but empty fuel tank.. Now it weighs 14 lbs 2 oz.
- I am using a carbon spar tube instead of the ally one, saving @ 35 gms. I fitted the tube fully in each wing recess and cut the tube to give me max penetration in each wing. The carbon tube is slightly longer than the ally one now.
- I removed the Dudro tank, which weighs empty, 185gms. I fitted a pop bottle, 2 liters in volume, which is 3.5 inchs square in section. This fits perfectly in the fuse and sits on top of the rear spar, which gives perfect access to the wing attachment bolts that fit from inside the fuselarge. Pop bottle weighs just @ 70 gms complete with all tubes and clunk.
- I fitted 7.4 volt 1320 mah lipo bats for receiver and ECU for the engine. Lipos weigh 65 gms each, old batts were 165 gms and 280 gms respectively. big saving here.
I fitted an Align voltage regulator which is made for Lipos, dropping voltage to 6 volts, and can handle 6 amps. It has a set of LEDs which display actual voltage of the lipo and gives indication of when not to fly, and recharge instead. Brilliant piece of kit. I fitted it directly to the receiver as it has its own switch, but remember, if you are using a 2.4 Ghz radio, then you need to carry the switch harness and the binding plug with you, as the switch harness has the additional 3rd wire built in the charge lead, to acctivate this fuction. At @ 14 pounds in UK it is excellent value. I mounted it inside, on the side of the fuse, and cut a small slot and fited a scoop to direct air over the cooling fins of the I.C. Looks good. Weight was about 30 gms, but worth the extra weight. (remember, if using some types of digital servos, you can not use 6 volts. My JR 821 servo can take 6 volts, others cant).
I am using ally bolts in all wing fittings and machined some ally washers to suit. Saves a reasonable amount of weight.
- I moved the UAT fwd , adjacent the nose gear plastic bay, along with the pump and moved the lipos (mounted on two carbon rods) right fwd as far as possible in the nose. The model only just balances in the c of g mark like this
- I bored out the internal steel pluger in the nose gear spring strut.
- I was worried about the loads imposed on the undercarriage, while taking off from grass. I made inquiries about which Jet-1A gears fitted the plane but have had no response, so in the interim, I fitted some JP oleos to the mains, which I found on a model shop shelf . With the excess spring wire cut off and these fitted, they added just 15 gms to each side, but gave me some kind of shock absordtion, worth the weight.
Right now to the flying, had a hell of a job finding anywhere to fly, that wasted weeks. The field I flew from was mown and grass quite short, but still offered quite a bit of resistance to the wheels. I used the supplied 2.75 inch wheels on the mains and a 2.5 inch wheel on the nose oleo. This trailing link oleo is from a Mick Reeves Reaper kit.
First I had problems starting the well tested and reliable engine. There was a stiff breez and I thought the wind might have been assisting the propane to exit the combustion chamer too easily. We tried putting the gas bottle in our coats (yes it was cold) and this helped.
First attempt led to flame out during the take off roll, as did the second. I started the engine and tried everything to stop the engine, holding the plane vetical and shaking it, all ok.I tried again, this time the plane attain a desent speed on the ground but resisted rotating, then hit some slightly rougher grass and stopped the plane. The nose wheel spring wire was bent. Remove the nose wire, strightened it and off again. This time the model appeared to be slightly faster, but again refused to rotate, then bang, nose wheel sheared off, screws being torn out.
I had seen it mentioned, not to use flaps for take off, and this was with the flaps up.
During the build, I checked the wing uc mounts, the nose uc mounts and the boom attachment points, and all were dry of glue. I had reglued all these joints and luckily the nose had survived, just the screws had torn out, damaging the beach blocks. (please check all these joints, before you fly, they were not safe on mine)
I cut out the damaged areas of beach bearer and inserted new sections of bearers. I then reinforced the bearers by fitting some 90 degree extruded ally angle (from B and Q), to the o Outside face of each bearer, so the other side of ally angle then sat on top of the bearers. I drilled holes in the ally side so glue could squeeze through them and improve the bond. The nose retract unit now sits on the ally angle and screws pick up in the ally first, then carry on through the bearers. This will help when flyng off grass, would have probably have been ok on tarmac.
Next I modified the nose oleo assemly to accept a 2.75 inch wheel.
I then fitted a slightly longer nose spring wire (10mm longer). The distance from the base of the retract mount to the nose wheel axel is now 156 mm, the nose wheel only just fits in its bay. I also fitted a stronger spring in the oleo, as was definately to soft. With the 5 deg down thrust on the engine, it is noticeable how much the nose wheel trys to dig in during the take off roll.
The main wires are as short as poss, just to allow the wheels to just fit in the bays without rubbing.
- Engine- i removed the glow plug, teased the element out a little further, as far as possible and adjusted the propane from 60% to 80% for the start.
I also adjust max rpm from 108,000 rpm to 110,000. At 108,000 I get just under 12.25 lbs. At 110,000, maybe 13 Lbs, will have to measure.
Next flight attempt.. Very strong cross wind at 70 degs across runway
Engine started first time, run up to 110,000 rpm was faltless, engine smooth and sweet.
Aircraft acceleration was possitive and with the nose now a little higher, wing grabbed the air better and she lifted off very quickly indeed. The wind caught the wing and tried to push her over, she settled on the ground then lifted straight off again. I held her down to gain speed, then pulled her up and tucked the gear away. The climb was really impressive and i continued to a higher height than normal, in case the engine stopped, to give me time to think.
She required about 7 clicks elevator trim to fly level, nothing on ailerons. I set the elevator level with the bottom of the stab, maybe as sugested, it needs to be level with the top of the stab. The plane is really gracefull in the air and flew incredibly smoothly in the really windy conditions. Most of the flight was about 1/2 throttle, giving a fair turn of speed. When you drop the nose the plane picks up speed quickly, the ailerons felt really nice and during a slow roll, i fed in full aileron and she rolled round rapidly and very axialy.
I did one tight turn and she went round unbelivably quickly. I had mixed in 40 % expo on the ailerons and the elevator. I have now reduced this to 30 % as i think, during the tight turn, the elevator was fed in gentely then really came in just a little to fast.
Not knowing how much time I had ( i had filled the 2 litre tank with @ 1.8 litres to help it get off the ground), I decided to try the flaps and land. I had mixed in a slight amount of up elevator with the 20mm flap suggested and a little more with 45 deg flap. With gear and flap down, 20mm flap selected, I needed to trim the elevator down around 4 clicks for striaght and level flight. I did not try full flap as the cross wind was so strong, I didnt wont those two barn doors pulling the nose round at the wrong time.
I flew down wind further than normal, selected idle and she flew rock steady all the way down. I kept on some extra speed for the crosswind and she handled it really well. I landed a bit further down than I would have liked, but didnt wont to go around again in case of fuel shortage. After touch down, the right oleo folded up and she came to rest on her right wing tip. No damage done, and I have since tested and cant fault the gear, it locks every time. Strange.
I prepared for another flight, but lost a winding on my starter motor, so the bendix would not throw in, had to canx flying for the day.
All in all, i am over the moon with the plane, it is a really stable platform and handles the strong wind well. It looks stunning in the air and you can just cruise around or open the tap and eat up the sky. Obviously i did not have time to test the stall or inverted, but after the first failed attempts trying to get off the ground i was disillusioned now I am dieing to get her up again, when I return from work in a couple of weeks. I feel a good set of Jet-1a Oleos would really help the pounding on the grass and now I am confident that any extra weight that is added by them, can be carried confidently by the wings. I think the higher nose up attitude helped the take off, but it would be nice to try again in calm conditions to confirm this.After landing there was still some fuel in the tank and the UAT was full, we were running for 9.5 mins, although throtlled back most of the time. (It seemed like 2 minutes ).
Hope this helps, thanx to everyone who has contributed, and Gary, what Jet-1a struts are you using, Boomerang intros?, do they fit straight on, or need modding?.