Worth it starting a Scratchbuild Heli thread?

[niquenaque]

Francois,

Astute observation regarding two strokes.

Willi's machine at my last look seemed ok, not to forget that alot of his flying is done at low levels so there isn't that much 'off throttle' flying done, plus there is a lag effect of oil residue being removed from the piston slippers - there isn't as much going in to wash it away.. If you're using the right oil a temporary off throttle flight should be catered for it in the formulation, if you think of it, it isn't too different from driving a snowmobile down a big hill.

I've not flown a heli so correct this if it is technically wrong - doesn't the sprague clutch allow the rotor to coast and therefore unload the motor - if you've depitch the collective the load is not the same so the forces on the pistons are reduced, sure, same revs but not as much side loading on the skirts, crank pins or gudgeons as when full power is applied.. what do you think of this?

Cheers,

Nic.

[HobbyCAD]

Hi There Nick,

It's good to see that someone else is at least getting into the discussion. That's the aim of these open forums, to exchange info, and for all of us to find possible mistakes in our train of thoughts.

Willi has a CH-7, and there comes another angle on the dangle. Why do the CH-7's fly perfectly with a 582, and a lot of Mini-500's were getting cold seizures with it? Mechanically, very similar machines, the main difference being the open engine compartment of the CH-7, and the fully enclosed setup of the Mini-500.

I did not want to specifically mention types, because not all of a single type has the same problems, or lack of problems. I did not want to get bombarded with counter statements, so I generalised. You will see, I said, possible lubrication issue, coupled with another problem area, like lack of cooling, or enclosed engine compartment, can lead to a "sudden stoppage"...... of a 2-stroke !!!

Willi seems to have a Rotax 618 in his Angel, so he's not flogging the daylights out of his engine, to get his heli going. Even for those guy's with stock 582's in CH-7's, I think the open rear frame leads to much better cooling. No cold seizure problems there. They are a lot less prone to the Mini-500 seizure problems.

So yes, as I said, you can design around the "possible stoppage" problem of the 2-stroke engine. For me, the best option is to stay away from it, period. If the designers of kit's want to use a cheaper 2-stroke rather than more expensive, and more reliable 4-stroke, so be it. I just hate the idea that they, and in a way us consumers of their products, are willing to compromise safety, for the sake of money.

I see your point with the "delay in loss of oil", once coming off the throttle. You are absolutely right there. I always look at thing during the "extreme" portion of operation. When you are running at max power, the pistons are at max expansion, with the least amount of oil available between the skirt and barrel, everything around you is running hot, and you chop the throttle, I interpret that you then possible get a short couple of revolutions, where the high friction/low lubrication galls up the aluminium on the skirts of the piston, before it has time to "relax" after the high power setting. It does not seize there and then, but this scouring of the skirts adds up, until one day, it has just too much there, and it seizes on you. This situation will never happen on a 4-stroke. It does not rev as high, and the lube is always there.

My technical interpretation from normal lowering of the collective in flight is, you still need lift to stay airborne, or else you will be dropping like a stone. The rotor system is still being driven, it still gets drive from the engine, so the overrunning sprag does not freewheel in normal flight. Only when you seriously unload the drive input, and allow the rotors to keep their RPM, like in an autorotation or engine failure, does this sprag bearing do it's job. Also, the other coupling in the drivetrain, the centrifugal clutch, engages at a very low RPM, and will stay engaged right through the flight, it only disengaging if the engine RPM falls back to this low RPM. Some systems use belt tensioning, and has to be manually disengaged.

I stand to be corrected, your comments are welcome here.

Regards,

Francois

[bones]

I very much doubt the throttle would be cut back to the extreme in a heli, that it does in a gyro being thrown around, as the rotors need to be held at flight rpm, and here is a tit bit for you, most oil injection systems actually cut the oil mix back to 80-1 at fully noise, as opposed to 45/50-1 at idle, there is more to the cold sez thing than meets the eye(wallet)

[HobbyCAD]

Hi There Bones,

Yes, I'm aware that those little Mikuni oil injection pumps found on most of the oil injection engins pump less than required oil at high RPM, thus lowering the mix ratio at the top end of engine performance, where I believe it is needed the most in heli applications. Most probably the exact reason why airborne applications go for premix, not oil pumps. By the way, it's not that they cut down the mix ratio, they are just damn inefficient pumps at higher RPM's. I've seen some reports where they measure the output vs RPM, they hardly pump at low RPM, they pump well at middle RPM, and at high RPM, it only pumps a little more, thus a lowered amount of lubrication ratio.

Usually the first thing any hardcore PWC rider does is to disable the oil injection, and premix the oil in the tank. Ask me, I use to make big buck buying up seized Yamaha 1200 and 1300 jetski's from the Arabs in the Middle East, where they operate them in digital mode, either flatstick ON or OFF. So, it's mostly in that 80:1 ratio, they don't last long !!

Now you guy's understand why I want an open frame design, no cooling hassles, both for the engine, as well as for me. Need to stay cool while flying such a contraption !!

Regards,

Francois