Vapour lock? problems with new installation Rotax 912 ULS

[druines]

Anyone got any advice on how to deal with what appears to be recurrent vapour lock problems?

We have checked that fuel pipes route OK. Have put in 30% avgas (100LL) to reduce problem and have a vapour return that is working. Symptom is that when you apply full power after a bit of taxying (so the small space under the cowls is pretty hot), coolant at 105 degrees C, at about 5000 rpm there is a sudden loss of power.

We thought we had done everything we needed to get rid of this problem but it persists. Could it be anything else?

Help required....

[scooter boy]

Druines, I had a similar experience about a decade ago when the aircraft I owned at the time exhibited identical symptoms.
We also had a Rotax 912UL (great little engine BTW) with the vapour return lines fitted and in flight at high rpm, experienced sudden rough running which spontaneously got better after a few seconds but caused worry on more than 1 occasion.

Just as you have we did all the obvious things, new hoses, new filters, even expensive professionally done fuel flow rate checks - but all drew a blank.

We were thinking carb icing, vapour lock etc...

Eventually we asked some friends to look at the problem themselves and they did all the same things as we had and drew a blank - until they opened the fuel tank up.

The installation of an electonic fuel sensor into our tank had required some drilling. Some of the swarf from the drill had become saturated with fuel over the first couple of years of use of the aircraft and was intermittently partially occluding the coarse filters on the inside of the tank. This is why it was intermittent and worse at times of high fuel demand!

It sounds to me far more like a mechanical blockage than vapour lock.

Never experienced any further problems with it since.

Good luck,

SB

[Them thar hills]

Druines
What sort of aircraft ?
Any fuel pressure gauge info ?
What does your Inspector think ?
Is your fuel beyond doubt ? Why not try pure Avgas.
Is there a carb-heat system on board ? (I know not usually)
How do you know its vapour-lock ?
Has anyone watched what comes out of the exhaust ? Over-rich ? Black smoke.
I once had a Rotax 912 in a Rans S6 go very lumpy on me. The prop was a carbon-fibre ground adjustable one. The prop blades had no pitch on the part that swept the cowl opening, so on the ground there was no prop-driven airflow through the cowlings. So the plumbing all got a bit warm when taxiing. Using 100LL Avgas cured that problem.
Regards
TTH

[tangovictor]

i thought, the vapour lock problem on the 912 only happened, during very warm weather ? I seem to remember, that if the fuel under pressure is vented back to the tank, ie like a car, then it can't vapour lock ? may be Im remembering wrongly here so don't take as gospel.

[ChrisVJ]

BIG question.

Do you have an auxiliary pump or are you relying on the Rotax mechanical pump.

In USA ultralights do not require auxiliary pumps. Rotax pump draws gas from tank so pressure in lines is lowered and where they pass next to hot parts vapour lock forms. The Rotax pump is self priming but of course when the vapour lock reaches the pump it can not pump the vapour forcefully enough to maintain the gas pressure at the carb and draw gas from the tank. The carb overflow should release the vapour and allow gas flow to the carb but if the vapour block is big enough the level in the float chamber will drop until she stumbles while this is happening. An auxiliary pump at a low point of the system should fix this.

Another possibility is if you already have an auxiliary pump the pressure is too high. The 912 is very sensitive to overpressure and the recommended is 2.1/2 to max of 4 lbs/sq inch. (The problem being that of course the pressures are aggregated by the two pumps and can overcome the float valve.) We use Facet pumps for auxiliary.

According to Rotax the carbs do not suffer from carb icing however there were some pictures on the internet from one of the suppliers of third party deice systems showing a layer of ice in the throat of a bing carb. One damp Autumn My engine quit when I throttled back to idle after landing. After a couple of minutes she ran fine so I chanced a circuit and the same thing happened. I guessed that a thin layer of ice formed in the carbs but they were fine until the throttle was closed when the ice just filled the gap. I have not had the problem since.

Another thing I have found is that the 912 is very sensitive to plugs. Mine ran a bit rough on starting a couple of times and putting new plugs in cured it straight away. Jst happened at 49 hrs after that last plug change. I use the recommended but I have been told that results are much, much better if you get the Autolite equivalent, (But not in your warranty period!)

[druines]

Thanks for all your posts.

Aircraft is a Dyn Aero MCR01, Club variant (bigger wing than the VLA and separate flaps). They are, apparently, susceptible to overheating on the ground but mine seems to have it particularly bad. The cowls are a tight fit (by design) so this doesn't help.

System has facet aux pump at low point in system. Inspector's suggestion is that engine temps get too hot while taxying, against similar aircraft. No solution though. I have fitted the "saucers" under the carbs as this is recommended by the kit manufacturer, although I have wondered whether these become heat radiators after the space inside the cowls has cooked.

I don't have a fuel pressure gauge fitted, but the fuel flows have been checked professionally at full chat and meet whatever the regs say - 4 times fuel required being delivered?

The problem seems to occur if the airplane runs, then stops, allowing the temps to rise inside the cowls, and then runs again. Yesterday, was not too hot for a flying day - 17 degrees and when this happened it must have been cooler as it was at 5.00 p.m.

The mechanical blockage is an idea I will pursue, just in case.

Once airborne the airplane performs fine, with temps and pressures fine, but the loss of power at high revs on the take off run is enough to make anyone squeamish!

There is a carb heat system, though it is rudimentary. We have not tried it and will later today.

There is no black smoke coming out that is visible, anyway. I may try pure avgas if only to try to isolate the problem as fuel may be an issue. Aparently they don't have this problem in France! I was hoping not to have to use Avgas as it has implications for the Rotax on maintenance quite apart from cost and the fact that it is likely to be taxed out of the market.

[shortstripper]

Rotax hate avgas so if it comes to that can't you install a couple of very small airscoops to direct propwash onto the carbs? It does sound like they're getting too hot which can happen on tightly cowled rotax aircraft. Even my friends Zenair 701 can suffer on vey hot days, so he leaves the oil filler flap open on such days and nips out to secure it just before power checks.

SS

[boguing]

Had this with a car once. My Dad (an RR jet engine engineer) suggested wrapping all of the exposed fuel line components in many layers of aluminium foil.

Worked a treat.

[tangovictor]

might be worth asking Rod1 he has a similar aircraft, although I think he has an electric fuel pump for take off's !
I had a wonderful trip with him last year

[ChrisVJ]

Your problem does sound like vapour lock. Should be in the line rather than in the carb. In the carb it would be pumped out pretty well instantly.

Wrapping fuel lines in vicinity of engine sounds very intelligent. Why not try that?

[druines]

The book says that max oil temp is 130 degrees C. Normal operation is 90-110. Depends where the oil temp is measured of course. Temp, even in running in shoudn't exceed this, but the Faeta looks as though it is tightly cowled and this is where problems begin if it is left running on the ground or, if hot and shut down for a few minutes between flights (refuelling say) the fuel cooksd in the lines and starts to boil.

[druines]

Thanks for all your posts.

We have now conlcuded that it might be ignition problems, probably associated with the heat. The fuel pipe is already lagged with fire hose although I have coinsidered the silver wrapping idea and may yet try this as it may help.

There is no misfiring at 5000 rpm, just the engine hunting with revs suddenly fluctuating between 5200/5300/4900 and the throttle fully open. Temps and pressures are within limits albeit at the high end, except I don't know what the fuel pressure is and I understand that Rotax are sensitive here and some engines (though in theory not mine) have a mechanical fuel pump problem.

Yesterday we also had rough running on the left ign circuit so we are wondering whether this is connected.

[Rod1]

druines

I have the same aircraft. You have a PM.

Rod1

[druines]

Sorry Rod1, not sure what a PM is?

[Them thar hills]

Druines
I can think of a couple of 912's in local aircraft where the rubber coupling has split between a carb and the engine, causing mixture problems. Worth a look.
An obvious one, do you have 8 out of 8 sparks ? Have any plug leads been fried, the MCR01 installation is very tight.
Do any fuel lines have high points in them before they descend to the carbs ?
TTH

[Rod1]

A PM is a Private Message; see top right after you get in. Alternatively my Email is [email protected], send me your number and I will give you a call.

Rod1

[Mariner9]

From your descibed symptoms, almost certainly sounds like a fuel problem to me. Heat shouldn't effect the ignition. That only leaves the fuel system.

Vapour lock is dependant on many things. Basically, vapour lock should not occur provided a net positive suction head (NPSH) is maintained between your fuel pump(s) and the tank(s).

Without going in to the ins and outs of a complex NPSH calculation (I do these for a living on oil tankers BTW), the simplest way to check if vapour lock is a problem would be to disconnect your fuel tank from the fuel pump, then rig up a temporary connection to a jerry can secured above (ideally >0.5m) your fuel pump (outside the cowling for obvious safety reasons), and try running from that when the engine/cowling's hot. If the engine then runs fine, vapour lock's the problem.

If it is vapour lock, then the problem might be cured by reducing heat in the cowling/insulating fuel lines. In addition to the excellent suggestions above, are your exhaust outlets insulated with heatproof bandage? If not do so, it'll make a huge difference to your cowling temps on the ground.

Running on Avgas does help the NPSH, but not to a large degree (the difference between Avgas and UK Summer Mogas vapour pressure specs is not that large)

[Rod1]

On the MCR01 there are a number of things you can do to help this problem. I have operated in France in very high outside air temps with no problem, but some of the UK machines have things setup in a non standard way which makes the problem much more likely to bite you. The only big scare I have come across on the MCR in the UK involved an aircraft with a faulty electric fuel pump.

I have seen a number of aircraft which would not start until the engine had cooled down, but a short investigation soon showed why this was the case on some aircraft and not on others. In this case I have offered my services to try to help, but I have no knowledge of this particular aircraft, so have refrained from speculating. The MCR installation is, done right, not at any more of a threat from VL than other similar aircraft, maybe less, as 450 are flying, mostly in hot parts of the world.

Rod1