IBob

Hi Glen, I can only speak from the experience of my own build: 1. The engine was much easier to fit to the ringmount supplied, by first removing the coolant pump cover with it's coolant outlets. 2. After fitting the engine to the mount, the pump cover was easily refitted, but the position of two of the coolant outlets had to be adjusted to allow the hoses to pass through the ringmount. 3. This was done by holding the cover gently in vice softjaws, gently heating the required outlet and rotating it by pushing on a piece of dowel inserted in the outlet. I made these adjustments several times before getting them right, and was concerned that I may have leakage as a result, but there has been no problem. The pump outlets are a soft alloy with a fine thread secured with Loctite 243, which requires a moderate amount of heat. (The coolant inlets at the cylinder heads are a similar fitting with the same fine thread, but a higher temperature Loctite is used there, requiring considerable heat for removal. I have replaced one cylinder head port, it was a difficult job which took me three attempts to achieve a leak-proof installation.)

Suggestion: Before installing engine, remove rear engine cover and check ignition coil and trigger gaps are set correctly. This is best done with non-ferrous feeler gauges (some sets include a selection of brass gauges) as the rotor magnets will stick to steel gauges, causing the gap to feel snug even when it is not. In the absence of non-ferrous gauges, ensure the next size up is no-go. This is generally easier to check with the engine out. These coils provide both the power and the trigger for the ignition, and correct setting is critical to good starting. We have seen one instance of incorrect large gaps on an older engine causing poor starting, with resultant expensive clutch damage.

There are a couple of threaded holes in the top of the engine, approx 150mm back from the gearbox. A fellow builder lent me bolts with the bracket he made to go on there. The lift here is on that bracket on the shackles you can see. The strop is on there just in case, and under no load. You can see one of the holes in the second pic.

I think you've picked it about right, Jim. Regarding the minimum cruise speed, there is a general sense that the Rotax is best run at 5000RPM and above, so a combination of that and prop pitch (which is a compromise between takeoff and cruise performance) will tend to define your minimum cruise speed. Or that has been my experience.

Diaphragms and cuffs replaced before or after problem emerged? And when you said runs rough after takeoff, can you be more specific? Is this when you throttle back to cruise? And if so, how does it run on all other modes of flight and idle etc?

Have you weighed the carb floats as per the Rotax spec? Rotax seem to have gone through an astonishing number of float versions, possibly one of yours is marginal (though I would have expected that to show up at low power settings). It certainly sounds like a fuel problem, have the carb rubbers and seals been replaced?

The history question: Have you owned and flown the aircraft for a while? If so: Has it always done this or did it just start to do this recently? If it started recently, was it a slowly deterioration, or did it just suddenly start to happen? And, most important: What, if anything, was done by way of maintenance or adjustments in the the engine bay before or around the time that this began to occur? Also: Were the fuel filters changed at the same time as the rubber replacement? And are you seeing good (2psi+) fuel pressure at all times? (But bearing in mind that the current Savannah fuel filter arrangement has one filter before the electric pump, but 2 filters to the respective carbs, which are after the fuel pressure reading is taken.)

Hi Jim, I can't speak for the tailwheel version, but my tricycle S gets about 86kts (99mph) at 5000RPM. This is with a Bolly 70" prop pitched to give a good compromise between STOL performance and cruise. The Savannah will certainly go faster, but like most STOL designs it has a fair bit of drag, so power requirement and fuel consumption rises exponentially above this speed: at 86kts it feels to be cruising easily. This is with a 912 ULS. I believe Dan Tonner's VG XL (above) is a 912UL? You may like to visit John Gilpin's Stolspeed website: he has done a great deal of work with VGs different props etc on his Savannah VG: https://www.stolspeed.com/

What is the history, David?

By varying the pressure in the float bowls: The bing carb has a vent line, intended to keep float bowl pressure the same as incoming air pressure (so, for instance, where there is an airbox, the vent lines are plumbed to the airbox downstream of the air filter). Downstream from the carbs is a narrow equaliser line between the two inlet manifolds. A bit of vacuum is taken from here (via a needle valve) mixed with ambient air pressure, and fed to the carb bowl vents, so reducing float bowl pressure. Or that is how the less costly off the shelf setups seem to work. They're a bit mickey mouse in that any change to the throttle setting immediately upsets the setting of the leaning mechanism. So the drill when changing throttle setting is first go full rich/ then adjust throttle/ then readjust leaning. That's as I recall, if I've got it wrong I'm happy to be corrected.

Chups? Is this fush and chups or potato chups??

In fact, I see Mark acknowledges that in his initial post on this thread. So I'll leave it there.......

Wukfit, for the record, I'm sure Mark would be the first to acknowledge that this was a Kiwi innovation. What Mark has done is sorted out a very nicely engineered version for the benefit of those needing it.

I was in Germany when the NSU RO80 was introduced. It was a beautifully engineered car, futuristic for it's time. Part of the promotion was the claim that the faster the thing went, the more economical it became, a concept that new owners embraced with relish, there being no speed limits on the autobahns. They sold well initially, but not so much thereafter. NSU nearly went broke trying to maintain them, and owners took to holding up 1, 2 or 3 fingers as they passed each other, to indicate how many times they had had the tip seals replaced under warranty......

Markdun, I would suggest it is a matter of horses for courses, as they say: depends on the configuration of the fuel system and the pilot's understanding of that. So I have no opinion as to whether it is a good idea in your aircraft. In my own, it is a nice feature, allowing outboard tanks (which have no sight glasses) to be flown to exhaustion: they are less used than the inboard tanks, and would otherwise have stale fuel sloshing round in them. I n addition, I and others have written here about the benefits of having a 6L receiver tank between the main tanks and the engine: having a low level (its actually a not-high-level) indicator in that tank is the icing on the cake, giving assurance that we have the full benefit of the time that 6L buys.

Really enjoy that footage. And that's really cleaned up and fast for a Jodel. Very small air intakes......

Garfly, that's a really nice innovation! Also sort of mesmerising footage: they're certainly flying through the hills, not over them! I know another user here who always pops his oil filler hatch after landing. In his case he is concerned about the possible effects of heat on the ignition modules (as the earlier ones had a history of failing over time).

Yes, you keep saying that, kgwilson, and nobody is disagreeing with you. We are talking now about the solution......which for many thousands of happy users does not involve buying a different engine........)

Mike Gearon, I don't know what the stats say now, but it used to be that a significant proportion of forced landings were due to pilot error in not understanding or correctly operating the fuel system of the aircraft they were in. Some years ago I was at a BBQ in Napier when, to our astonishment, a Spitfire came through fast, low and banked. About 10 minutes later, the same aircraft was down in a plowed field: it transpires that Spitfire fuel must be pumped between tanks, and the pump is engine operated: the engine stops, you've got no way of transferring fuel. Unlikely that he'd have had time anyway, but the reason for the crash was fuel exhaustion with fuel still onboard. And somewhere in outback Australia there is a twin whose pilot lost an engine, then lost the other perfectly good one because he hadn't read the fine print saying not to put on multiple fuel pumps. So, those and other events, I've taken a special interest in the detail of my own fuel system, which is exactly as spelled out in the Rotax link within your link above: http://forums.matronics.com/files/912_fuel_fig_54_941.jpg but with the addition of a receiver tank taking fuel from the wing tanks, as mentioned previously. Regarding the conversation in your link, I think if I experienced heat related fuel pressure problems in normal flight (and that could not be fixed by changing pump and filters) I would be looking very hard at ways to reduce the heat on the fuel system. And while I would be adding a fuel return if that was not fitted, I would not be relying on that to fix the problem. Regarding my Plane Crash YouTube links above, the pilot has not thought it through. The reason his fuel has gassed in the pipes is the elevated temperature in the engine compartment. Ideally, his fuel system should have a return that moves that gas, and the heat it contains, out of the engine compartment (as per Rotax recommendations), replacing it with cooler fuel. But what he has put in is a short loop that simply recirculates gas and heat under the hood, because somebody has told him this will fix the problem. That's what's called magical thinking. Finally, since I wrote 'detail': note the fine fuel filter before pump and return etc in the Rotax recommended fuel system. This prevents not only carburettor jet blockage, it also prevents blockage of the orifice in the fuel return, and ensures the pressure relief bypass in the electric pump seats properly.

That certainly seems like a reasonable suggestion, RFGuy. In conversations like this I think we have to be careful that we are not chasing ghosts or tilting at windmills. For instance, Facthunter's assertion that a high mounted carburettor is not ideal placement (for at least two good reasons) is absolutely correct. However, we also know that many thousands of aviators have top mounted carburettors, which they cannot alter, but which fortunately do not seem to have resulted in a history of problems. I would be starting, if possible, by asking what observed problem are we working to overcome here? Or in the plant automation world where I used to work: what have been the practical experiences of others that I can benefit from? That does not mean we cannot use our own imagination and ideas, but it helps keep us grounded in the current realities.

Ops, that should have been: But it's not well presented: what he doesn't show is that he landed and turned off his engine before restarting and attempting to take off. You see a bit of that here: But at the end of Part2 he focuses on his fuel pump replacement and his (in my opinion inadequate) attempt at a fuel return, with only a fleeting mention of grounding running to cool the engine bay: which would probably have saved him the cost of a propeller etc.

Nev, I don't think anyone is trying to argue that having high mounted carbs is a good idea. The title of this thread is Prop Stopped, and I thought the discussion was around practical means for avoiding that. That said, I'm (also) not trying to argue that the Rotax fuel return answers all our fuel delivery prayers. But what I would suggest is that with the 912, it does a good job of reducing the possibility of vapour lock in a heat soaked engine bay. To dismiss that by saying the carbs shouldn't be high and that an orifice won't pass much liquid adds nothing useful in a practical sense to the conversation. Here is a heat soaked event......the EFATO is at the start, but the analysis (if you can call it that) is at the end. And to my mind the guy displays little understanding with the 'return' he has now piped in (that is not a return at all): https://mail.google.com/mail/u/0/#inbox/FMfcgzGpGTHLQrlpggWzXQjxQlPfnzsr?projector=1

All this theorising is all very well, but thousands of aircraft have a fuel return, via a small orifice, as per the Rotax installation spec. and it apparently works very well. Yes, it's a pity if it mucks up your fuel flow readings. No it's not difficult to arrange to vent it back to tank. And in a vapour lock situation (which you potentially have with a heat soaked engine bay after a recent stop) what you are expelling via the orifice is not liquid, it is gas......and so passes far more freely. How do I know? Shut down on mates strip, nobody home, set up for restart after 5mins, activate fuel pump and it chatters away for 10 to 15 secs before settling down to it's usual rate and fuel pressure is established. PS: in that scenario we also don't take off immediately. We allow a few minutes of engine running to lower the temperature in the engine bay. Or you could thermally insulate everything, and hope for the best???

Thruster88 are they, strictly speaking, vent lines? In my Savannah, which has an air box with filter, they are plumbed into the box downstream of the filter. I had the impression that the object was to have the pressure in the float bowl match the pressure of the incoming air. Though I am not sure what the arrangement is on engines with no airbox, and filters fitted directly to the carbs; maybe someone can clarify that?

Hi Steve..great to see, and congratulations! One passage ends, the adventures begin. And you get to pass it all on to those coming after you too.....)