Posted Wednesday, January 3, 2007 @ 00:56 AM  

Loops and barrel rolls are fairly easy if you don’t let the nose drop and you don’t mind too much about their shapes. But when you fly a slow roll, or a four-point roll, or a Cuban eight or a reverse Cuban, the engine stops. Or at least, it stops producing power, although the propeller keeps turning pretty fast. It does that in an inverted glide, too. And the oil pressure drops to nothing. Surely, that’s bad for the engine?

Yes, it is. Do that for long, and you will wear it out very quickly.

British VW expert, Barry Smith, said his first symptom was loud clattering from the valve train. Barry then spent many years designing and perfecting a fully inverted lubrication and fuel-injection system for VW aero engines, and sold them as the Acro VW. Barry is now in semi-retirement, and no longer sells those engines. I cannot even persuade him to sell the drawings, although he ought to be able to make a nice, steady income from that. So I had to find another way to keep my engine lubricated while grossly mal-treating it. Especially since I’d cobbled together a means of helping the engine keep producing power for a few seconds under negative G.

There is an oil additive called molybdenum disulphide (that’s probably disulfide for Americans). It’s black, and is promoted as reducing wear, internal friction etc. All the usual guff. But they market this stuff by putting it into a motor, running it for a while, then unscrewing the sump plug and letting all the oil run out. The engine then continues to run for a couple of minutes. Magic!

I asked an old buddy who once worked in BP’s automotive research department. He said BP tested it, and reckoned it wasn’t worth adding to their lubricants. After all, who would be daft enough to run an engine without any oil in it? Answer? Me!

Three years ago, I poured molybdenum disulphide into my oil. Since then, I’ve flown over 150 hours, with no perceptible change in oil pressure. I take off at full throttle, climb at full throttle and fly ten minutes of aerobatics at full throttle (I don’t even ease it back… ever). Then, when the engine and I are both at red line, I throttle back gently, cool off and land. That’s broadly one hundred fifteen-minute aerobatic sorties, with at least 30 seconds of inverted flight on most of them.

I am NOT recommending that you do this yourselves. I am merely telling you what I have done. It works for me, so far. But, remember, I’m actually trying to wear out this engine, so that I can fit the new 1,750cc one that’s sitting on an oil drum in the corner of my hangar. If and when my little engine wears out or fails, I will tell you. Meanwhile, the additive SEEMS to be beneficial.

Another word of caution. Do not use it when breaking-in your engine. It will probably prevent the cylinders from bedding in properly. But then, you wouldn’t fly aerobatics while breaking in your engine, would you?

Happy (inverted) Fournicating to you all.

Posted Wednesday, October 15, 2008 @ 01:54 AM  

Sam

Ps dont laugh but ive gotten my 1/8 rolls up so i can do them cleanly.

Posted Friday, October 24, 2008 @ 10:05 PM  

I've just realised that my inverted carb mod won't work on your airplane, because you don't have the standard Zenith 28RXZ carb.

The principle is that, when you turn upside-down, all the fuel in your float bowl falls out through the vent (there is always a vent above the float, to allow for changing air pressure). My mod merely moves that vent to immediately below the venturi (so fuel will trickle down into the venturi when you're upside-down), calibrates it precisely to allow the correct flow of fuel for full-throttle operation, and replaces the vent with another with a U-tube to prevent fuel from leaving the float bowl in any attitude.

The big trick is to get the correct size of vent hole (supplying fuel at 15 litres per hour when inverted). Since you can only enlarge a hole, not make it smaller again, that took dozens of test flights, removing the carburettor and adjusting the hole size after each one.

I'm nearly there now, but that's not much use to you, unless you want to go throught this whole process on yor carb.

Yours, Bob

Posted Thursday, November 20, 2008 @ 04:56 AM  

Yes, soldering works well on the jets, beause they are brass.

I tried to get solder to adhere to the carburettor body, but it is a different kind of metal (monel?) and I could not get it to stick.

Because of that, I have been very slow and cautious with my investigations.

Where I have had to seal a hole in the carburettor body, I have used Araldite and/or Loctite bearing retainer.

Does anybody have a better idea, because I think I have now made my hole too big.

OOps

Posted Friday, January 29, 2010 @ 10:43 AM  

Hello again Barry,

It is great to hear from you again after all this time.

Like you, I know that merely being able to lubricate your engine when it flies upside-down is not enough, and I very well remember your PopFly article(s?) about your Acro motor and the all-attitudes dynamometer you first had to design and develop.

With all this in mind, plus the knowledge that I have neither the time nor the skills to do the same sort of thing, I thought laterally and added molybdenum disulphide to my oil instead.
Of course, being the kind of guy I am, I didn’t do this without a lot of research. Among others, I approached you and a friend who at that time worked in the fuels research department of BP. I got him to talk with the lubricants guys, and got back the telling reply… ‘There’s no point in adding MolySlip to your oil unless you’re intending running the sump dry. It’s expensive and pointless.’

However, since what I was intending to do was precisely like running the sump dry, and since I remembered the old Motor Show MolySlip demonstrations of a Morris 1000 having its oil all drained and then the motor kept running, I tried it.

That was six years ago. At that time, my Australian Fournier’s Rectimo engine was only 1200cc and nearly forty years old, with 1,006 hours on the clock and no sign of overhaul in either the logbooks or on the motor itself. It still has round rocker-shaft pillars and 1:1 rockers in its single-port heads, plus the original Rectimo data plate riveted to the 31 VW 000.101.101A single oil pressure relief valve crankcase so you know what I’m talking about, but I did increase the bore with second-hand 83mm cylinders and pistons to make it 1400cc four years ago.

I have just gone back through my log-books and maintenance releases and established that, since then, I have flown a total of 310 hours in it, of which 260 hours and 460 sorties have all been increasingly full-on +6/-3g aerobatics (well, as full-on as one can fly in an RF4). This includes lots of upward and downward quarter vertical rolls, plus a few 180-degree half vertical rolls.

I know it’s cheating, and not making a proper engineering job of it, but actually the MolySlip works for me, and there is still no sign of wear in the engine, nor of any performance loss. I religiously time the climb to aeros height every time I fly, and always make 600 feet of climb (from 300 to 900 agl) at aerobatic weight (depending on temperature, thermals etc, of course).

I have now flown around 100 hours and more than 100 aerobatic sorties in my English RF4 as well, with its engine (rebuilt by you many years ago) similarly treated with MolySlip. Again, there has been no sign of premature wear.

I have also adapted their carburetors in the past couple of years so that the engines will continue running for a few seconds of negative G – enough for me to round out a loop, make a Derry turn or fly a four-point roll without the engine stopping. Indeed, I can now fly a rolling circle in my British Fournier without the engine stopping. The Aussie one’s carb is playing up, so it cuts out a bit. See my YouTube clip at: [url]http://www.youtube.com/watch?v=vV0wJhEq8go[url/]

I can also lurch around a full 360-degree negative G rotation, although you cannot call it a proper outside loop. See: http://www.youtube.com/watch?v=M53Bhowf_cg
There’s a 180-degree downward vertical roll at the end of that!

For the spectators’ view, see http://www.youtube.com/watch?v=BnTPTKgipxI

Thank you for all your information and advice over the years, and I would love to meet up with you and see you fly your Acro Advanced. If I can persuade the organisers of the Sunderland or Windermere displays to use me again, maybe I can pop over to Redcar and meet up afterwards?

Happy flying Barry, and may your skies continue to be blue whenever possible.

Yours, Bob