Hi Bo,
Its probably about time I posted an update on here, as quite a lot has happened over the last few months - or at least, until the weather closed in!
To answer Bo's questions first: The coupling between the motor and gearbox was a flexible one, as I wasn't sure how accurately the alignment between the gearbox and motor had turned out. It eventually failed - probably due to the "backfire" I was getting from the motor, and was replaced by a solid, cylindrical, clamp-on coupling.
This video of the first attempts at flight show the battery mounting. It fits in the floor of the radio compartment:
The backfire eventually stripped a couple of gears in the gearbox! Yes, it was that violent! Luckily, and old flying buddy had some spares, which he donated to me, but I had to rebuild the gearbox yet again!
The backfire eventually turned out to be caused by the speed controller overheating in the fuselage! On a sunny summer day, even a cool one, the camouflage paint job was making the fuselage too hot to touch! As the speed controller was velcro'd to the fuselage side and there was little air circulation inside, the speed controller would suddenly get "out of sync" and create a massive "backfire".
My son suggested fitting a cooling fan to the esc, but I've now mounted it onto the aluminium chassis plate near the motor, and this seems to have solved the problem. It is still in its heatshrink wrap, but the heatsink is now pressed to the chassis, and this seems to have solved the problem.
The motor barely gets warm! Bearing in mind I'm getting 10 mins airtime out of a 5AH pack, which gives an average current of 30A max. The motor and speed controller are both rated at 80A, so neither is working very hard! Indeed, because they are so lightly loaded, I found it hard to believe that it was overheating to blame!
If I was going to do it again, I would be tempted to use a prop-adapter on the motor, and keep the clutch! I have a Hirobo Lama which keeps the clutch, and if you program in a slow motor start, it sounds just like a gas turbine starting up! Mind you, the Lama is flown on collective pitch, and the motor set to constant speed in flight - not really suitable for a fixed pitch set up!
Turn the sound up!
I've now bought some new blades for the Cobra from Germany. The originals are not very straight, and difficult to keep in track!
Next up is to do the same modification to my DS-22! The HP 61 in that is well past its sell by date! This video was taken way back in 2002! That was before I realised that the engine doesn't like Nitro, and runs far better on straight fuel!
Tobe: That was very brave flying a Cobra that fast! All these old Schluters suffer from a runaway nose-dive if you exceed a certain speed! You can only recover from it by shutting the throttle, which then causes the nose to pitch up violently!
The original builder of my Cobra had this happen to him at a flying display back in the early 70s! He thought it was going in, so he shut the throttle, and the thing promptly stood on its tail! He hadn't been flying helicopters very long, and in attempting to recover it, crashed into the roof of a large marquee tent, doing significant damage to the tent! He had to pay for a sail-maker to repair the tent, as well as a new set of blades for the Cobra!
You can cure the nose-dive by setting the paddles at a positive angle - full-size Hillers have their paddles like this for the same reason. However it is difficult to get them both the same, and this can cause tracking issues! If you get the angle just right, the cyclic trim will not change with airspeed, but it is difficult to get just right!
I just live with it and keep the speed moderate!
--
Pete