IBob

Okay, so I found this, which suggests that they are using a 2 wire current loop setup:
https://duckduckgo.com/?t=ffab&q=pinout+of+rotax+oil+pressure+sender&atb=v356-1&iax=images&ia=images&iai=https%3A%2F%2Fdesk.zoho.com%2FDocsDisplay%3FzgId%3D714941653%26mode%3Dinline%26blockId%3D1hflqdbff7005142a40f9ac47410e66c421a2

It says that
Pin A = n/a
Pin B = 8-24VDC (12V in our case)

Pin C = Signal
 

And here is how it is wired to the gauge:
https://duckduckgo.com/?t=ffab&q=pinout+of+rotax+oil+pressure+sender&atb=v356-1&iax=images&ia=images&iai=http%3A%2F%2Fwww.aviasport.com%2Fimages%2FConexiones_OIL_P_4_20_57.gif

Danny, I found that by searching 'pinout of rotax oil pressure sender'.

Agreed, Nev: if buying from a transducer manufacturer or supplier, it would all be on the data sheets. But Rotax have gone to their own numbered part (albeit manufactured by others) so unless we have a non-Rotax part number for the unit, we're guessing a bit. Unless Rotax have published details somewhere.....they may have.

Danny, as I recall, my engine came with the sensor/sender (we just used to call them transducers) fitted, with the plug and cable attached. The cable would have been marked, so no need to meter through to the plug pinout.
If all else fails, maybe you could find someone with a new uninstalled engine, and get them to meter out the cable/plug for you.

That's correct, Skippy. Think of it as a diaphragm, with one side to atmosphere, the other to oil (or fuel) and it is measuring the deflection in the diaphragm.
The key element is that oil pressure is atmospheric + pump pressure, so you are measuring that against atmospheric pressure.
So as you climb, the atmospheric pressure should decrease on both sides of the diaphragm, but the differential pressure (and diaphragm deflection) remains the same = accurate reading.
If, however, the atmospheric pressure falls as you climb on the oil/fuel side, but remains at ground level pressure on the other side (due to being sealed) then you get an increasingly inaccurate result.

Danny, these sensors are differential: they should measure the difference between the oil (or fuel) pressure on one side, and atmospheric pressure on the other.

Your oil pressure is atmospheric pressure + pump pressure, and is measured against atmospheric pressure.
So as you climb, the atmospheric pressure falls, but the differential pressure remains the same, so you continue to get an accurate reading.

But

If the sensor is sealed on the atmosphere side, then the atmospheric pressure is falling as you climb on the oil pressure side, but remains the same on the 'atmosphere' side. So you get an increasingly inaccurate reading.
And in the case of Skippy's fuel pressure reading it would make a huge difference.
 

Ah right, Skippy, that would be much more of an issue: the oil pressure unit is 0 to 10bar, mine sits 5 to 6,  but the fuel pressure is 1/3bar or less, so if the unit is not correcting for altitude, you would see little or no fuel pressure at 10,000'.
I have seen the same problem now, twice, with fuel pressure steam gauges, where the builder has not removed the shipping plug from the gauge body.
 

1 hour ago, skippydiesel said:

FYI - this sort of plug, with the silicone seal, has been causing intermittent erroneous readings on pressure sensors.

The reason seems to be, the seal is too good and does not allow for rapid changes of air pressure (like you might get in climb out) causing the sensor reading values to drop (in a climb).

If you are concerned with intermittent pressure drops, the test is simple - remove the seal and see if your pressures remain more consistent.

A permanent solution may be to cut a slot in the seal (still get some benefit from it) or if practical drill a small hole in the plug housing ie reduce the efficiency of the seal.

That's an interesting one, Skippy.
Given that 10,000ft sees approx 30% drop in air pressure, if altitude is the problem you would see approx 1/3 bar error at that height...not much, given that my oil pressure normally sits over 5bar.
 

Nev, could you explain about the rings not bedding in and so burning more oil?
I'm not disputing this, I would like to understand it better.
Thanks.

Hi Pat, it looks as though they made all the required changes to turn the Classic into a VG, as per ICP factory designs.
Perhaps this was necessary for certification.
I don't think that has much to do with removing the slats and adding VGs, though my understanding is that ICP made a small change to the wing leading edge profile when they took that step.
An excellent source of information would be John Gilpin at Stolspeed:

https://www.stolspeed.com/slats-v-s-vgs

It is what's called a current loop device, they commonly come in 2 and 3 wire versions and are an industry standard. Instead of behaving like a variable resistor, it puts out a current between 4 and 20mA. This one has a span of 10bar, so 0bar = 4mA and 10bar = 20mA.
The 3 wires are 0V and 12V to power it, and the third wire is the 4-20mA signal.

The reason for using current as a signal is that it can be connect with a short or a very long wire and it makes no difference. Voltage, on the other hand, will drop with a longer connection, due to the resistance of the wire, giving an inaccurate reading.
However, the reason for the change in these Rotax sensors was that the old VDO sensors had an unacceptable failure rate.

Hi Pat. Lots of good Savannah info here.
And no shortage of opinions, either!

Not sure why removal of slats would require structural reinforcement, will be interested to hear what you find out.

Note that if you have steam gauges, you cannot connect this newer type of sensor to the older style VDO gauge.
The correct steam gauge is this one:
https://www.aircraftspruce.com/catalog/inpages/cps15-06649.php?clickkey=64183

I pulled my windscreen back too tight at the top centre, Mark, with the result that it dented in there far too easily. The fix was just to let it out...a surprisingly small amount, like about 2-3mm at the centre, progressively less towards the sides...which then gave it a curved 'brow' and the right shape to withstand the prop blast. Fortunately, there was enough material there to do that......

BrenDan I did part of my training in Technam P2002s, they had a centre throttle and one on the left side.
(I flew on the left, but tended to use the centre throttle, as it was next to the trim switch, though trim may have moved to stick top buttons now. Then I built, during which I flew very little, and had to swap hands once the Savannah was finished.)
So you may find Technams out there with a LH throttle, maybe make a few calls?

Yes. If he's using the ubiquitous Ray Allen trim servo, the limit switches are built in.

What aircraft is this, RHTRudder?

RHTRudder the basic arrangement in microlights is usually a Double Pole Double Throw (DPDT) switch with 6 terminals.
This is the trim switch on the panel.
Internally it is two separate switches, mechanically linked (DP). The switches have a central neutral position and, up and down active positions (DT) with spring return to the neutral position.

They are wired so that when the switch is up, the two wires going to the trim motor one polarity (eg 12V/0V) and when down that polarity switches (0v/12V). This drives the trim motor in one direction or the other.
Here is how they are wired:

Tank flow conflicts are unlikely to be a factor at normal fuel feed rates.

Hi Rooviator, I suggest you also check for fuel line undulations, and filler cap seals.

MKennard, where two pipes are siamesed together, the flows collide. And depending on the dynamics of that, sometimes one flow beats the other one out, so the stable state is not flows combining, but one or the other 'winning' and excluding the other.
I would expect that effect to be far less, if at all, at low velocities. So it may be that draining the tanks as you describe is not what will happen at slower flow rates.

Mark, another option would be this, also makes for a more natural pull to full flaps, so we shortasses aren't trying to press the lever into our gonads:

I did some work on my Savannah to get more even fuel flows. Two things stood out:

1. The individual tank vents may deliver different pressures to the tanks in flight. The Savannah vents are rislan pipes sticking out under the wing, and normally cut at a 45deg angle. I was able to adjust these by altering the angle of the cut. A very small difference in pressure will make a big difference.
Cessna get round this by having just one vent, then cross-porting the upper tanks. Were I to build again, I would look at doing this.
2. Undulations in the fuel lines from the tanks. These can capture air at the high points, greatly impeding fuel flow. And this will change, giving varying results depending on the changing amount of trapped air. I went to some lengths to straighten my lines.
Note: I suggest you also check the filler cap seals: if they do not seal properly, then any pressure from the vent system will be lost, resulting in uneven tank pressure and fuel feed.
And finally, I am told by experienced pilots that most aircraft have some degree of uneven fuel feed.

And just to round out that conversation on fuel vents (as applied to the simple systems many of us fly.)
The received wisdom seems to be that a small positive pressure in the tanks is desirable.
But
Consider (also) this:
Your fuel tank is, say 500 wide x 500 long.
You fit a forward facing vent, and it raises the pressure in flight by 7kPa (1PSI).
The size of the vent doesn't matter, a smaller one will just take longer to get there.
The result is that the top and bottom of your tank are now bugging out due to an increased pressure over each of the top and bottom surfaces of 177Kg (390lb).

My tanks sit flush to the upper wing skin: after my short and far too interesting flight with fully forward facing vents, I looked up there, half expecting to see the skins bugged out and a few rivets missing......(
 

32 minutes ago, Marty_d said:

I'm building my 701 with a header tank, but will be plumbing the return line to one of the wing tanks.   This should avoid that issue and I believe Savannah specify return to the wing tank rather than to header, is that correct Bob?

Marty, yes, ICP run the return line back to high on an inboard tank.

Note also that the later Savannahs have also a vent line from the top of the receiver to high on the other inboard tank. This is because the unvented receiver has no reliable way of getting rid of air, and a bubble there could give false low fuel indications.
With that overall arrangement, I would think you can return fuel either to a wing tank or to the receiver, without any problems. The only remaining consideration (if returning to the receiver) is how to entirely isolate the fuel supply if necessary. One option is to put an isolating valve on the return line, but I choose not to do that, as the return plays such an important part in avoiding vapour lock EFATO. On balance, I chose to follow the recipe, and return to a wing tank.

Consider this:
If your tank is, say, 250mm deep (and full) then the pressure at the tank outlet due to the weight of the fuel is approx 2kPa (0.3PSI)
So an increase of just 2kPa (0.3PSI) due to a forward facing vent doubles the pressure of the fuel coming out.
So even a very small difference in how separate tank vents are positioned and facing can result in large differences in fuel pressure at the outfeed.
And as the tanks empty, that small pressure difference will result in increasing difference in flow.

We have seen a 4-tank setup, all with separate forward facing vents, where it was not possible to isolate the inboard tanks. And after about an hour of flight, the fuel levels in those tanks were all over the place.

If I could neatly and easily do it, I would cross-vent my tanks like the Cessna setup.
But it needs to be done from a high point in the tanks.