Why don’t you do the experimental community a service and get RV-7 or -8 that you can experiment freely with, possibly making lots of new and nice solutions and products along the way? I mean seriously, there is no need to give up your IFR flying or anything either, people do both.
I could not handle two planes and one girl! It could be as complicated as one plane and two girls…
Also, taildraggers can’t fly in more than 5kt of crosswind 
I guess it is an off the shelf hydraulic gear pump?
It must be, though I have not yet started looking for the OEM part.
Do you have the hydraulic diagram of the system? and a picture of the internals of the pump?
PDF extract here
Pump images:
You can see the fine filters on what is presumably the inlet. Not much is going to get in through those.
I think the high current condition (not seen on the new pump) must be, as you say, a load increase, or a motor fault. I will send off the motor to a motor rewind specialist, for a look. Motors can do strange things if there is a winding open or short…
The condition seen during the UP cycle where the current drawn falls to nothing could be a faulty motor, or the pressure switch briefly opening, but this condition is totally non-repeatable when running off the aircraft battery.
There is actually a long story re the pressure switch (researched extensively by a former TB owner, and a bit myself too). The switch is a low quality job used throughout GA, presumably because it is an approved part… there are much better quality (aerospace) switches. But I see no indication that the switch is faulty. It does the job of pressing the gear against the upper (rubber) stops perfectly.
I am a bit suspicious re the S/N of these pumps:
Old pump
P/N 641634
S/N 318
New pump
P/N 641634
S/N 413
That’s not a lot between year 2001 and year 2015! Only 95 numbers. And consider my TB20 is S/N 2121 and the last one made was c. 2250 which is 129, but when you subtract the non-retract models you get very close to 95!!!
So the “new” pump could have been 14 year old stock, which would explain something else: for the first few cycles after it went in, there were long pauses during the UP cycle, ~2 seconds, zero current drawn. Like motor brushes not touching.
Of course, that would also suggest that nobody in the field has been buying gear pumps from Socata. It has to be, because I know for a fact of several which have been changed.
A google on
parker pump 108
produces loads of hits… example (suggesting the design has changed yet again)
The diagram is more like a sketch, but a few fault modes comes up. The check valves 8 and 11 must be pilot operated check valves (or they wouldn’t work according to the drawing). Are they mechanically or electrically operated (I guess electrical) ? Also the check valves 5 and 13 seems to be pressure operated with a sliding valve. I assume 8 and 11 is electrically operated, then on page 7. If 11 lose current for some 1/10 of a second or something. Then the retraction will stop, the pressure will increase in both lines. This should also increase current draw on the motor, but maybe not, it could also go to some protective mode for all I know. Now, if 8 suddenly (and erratically) receives current for some 1/10 of a second, then the retraction will also stop, but this time with zero load on the motor.
The pressure activated sliding valve connected to 5/13 could also fault, but this is maybe less likely.
Another curious thing is the emergency valve (17). In none of the diagrams does it show it’s operation. Page 15 does show emergency extraction, and this is a bit peculiar. Emergency extraction will draw oil or possible air into the system, the wrong way through the filters because the volume in the system increases when extending. (Assuming the valve 17 work as I think it does, but is not drawn).
The emergency gear release valve just establishes a bleed across the two ports of each of the three slave cylinders, allowing the gear to fall under gravity.
I think the whole system is very simple.
On the way UP, the pump just pumps till the pressure switch reaches 1400psi (IIRC) and then it stops. At 1400psi, the gear is well up against three rubber bungs. During flight, some pressure obviously leaks past the slave cylinders, and at (IIRC) 1200psi the pump comes back on again, for about 0.5 second. This happens every 30-120 minutes, on a good system. If you left a TB20 on jacks for 10 years, no power, the gear would be down eventually.
On the way DOWN, the pump runs in reverse, till some combination of switches is operated, and then it stops. The gear is then past the locks and can’t go anywhere, ever.
So the gear up/down switch just runs the pump one way or the other.
I think any check valves are inside the pump.
The pump has 3 wires. One is the ground and as power is applied to one or the other of the other two, that makes it run one way or the other, AIUI. That is rather strange… I would have reversed the pump using relays. Making a 3-wire pump is a waste of copper. There are two relays, expensive ex-TBM700 ones. Pre-GT TBs used cheap relays which didn’t last so long.
On the way DOWN, the pump runs in reverse
OK That explains a few things. Then I guess the 5/13 also are inside the pump? When retracting this system actually creates an hydraulic lock and the low pressure valve, 12, has to open to let oil out (the cylinders are not balanced). When extending, more oil needs to enter the system. This means 11 and 7 will only open when fully retracted and the pump is still running, but not else. 12 will limit the pressure during extending, bit will also create excessive pressure during retraction. This leads me to the pressure switch, 3. What exactly is the function of that?
The pressure switch cuts out the pump at the top of the UP movement, when the pressure reaches (IIRC) 1400psi. This happens when the gear squeezes some rubber stops by a certain amount. See my comments above.
Searching this for “pressure switch” gets you the details of the Eaton one.
If you listen to the sound of the pump, you can hear how it changes from high pitch to lower pitch, right before the nose gear judders. This suggests that the motor is working against higher pressure and therefore is slowing down. In the video where there is no nose gear judder (normal operation), the sound changes also to lower pitch, but less so.
The pressure switch cuts out the pump at the top of the UP movement, when the pressure reaches (IIRC) 1400psi. This happens when the gear squeezes some rubber stops by a certain amount. See my comments above.
OK, and
On the way UP, the pump just pumps till the pressure switch reaches 1400psi (IIRC) and then it stops. At 1400psi, the gear is well up against three rubber bungs. During flight, some pressure obviously leaks past the slave cylinders, and at (IIRC) 1200psi the pump comes back on again, for about 0.5 second. This happens every 30-120 minutes, on a good system
So this is simply a switch allowing current to the pump below 1200 psi, and stops the current at 1400 psi. Could explain it all in my opinion. The number 12 valve is sat at too high pressure along with too much pressure drop in the lines to/from the nose gear. Another way to look at it is the pump runs too fast when retracting. Then again, the pressure switch could be faulty.
the pressure switch could be faulty.
That could explain everything – IF it was in any way consistent. But it isn’t.
Also the “hesitation” happens way before the gear gets anywhere near squeezing the rubber bungs. At the point where the nose gear hesitates (the main gear never does) the system is moving very freely. Each of the three gear components can be moved easily with one hand – all the way up to meeting the rubber bungs.
So if the pressure switch is faulty, it means that
To summarise, the new pump runs fine, but only if running on the aircraft battery. I don’t have an issue with that – I can easily see a working system will draw peaks over 40A. So I consider the problem solved.
Imagine the issues in a 12V aircraft. Every problem to do with current is 2x worse.
BTW I emailed some Parker distis to see if they can locate the pump.
The coincidence in serial numbers and TB airframe serials is, ahem, way beyond “interesting” especially knowing how old some of the stock at Tarbes is.
Imagine the issues in a 12V aircraft. Every problem to do with current is 2x worse.
Or the same, only at half the speed Transient pressure peaks in hydraulic systems are not exactly news. Whatever is wrong with your system, it is not the pump itself (since you changed it), but has something to do with how the pump work in relation to the rest of the system. Normally an hydraulic system has a pressurized accumulator that among other things will handle things like this. To keep it simple is of course important.
A bit of data in case anybody wants to pick this up one day. Just got this reply from a Parker disti
I can’t see why the pump is special. It is awfully similar to other 108 series pumps. I also think (as written above) that all/most of the pumps installed on TB20/21s since 2002 were not sourced via Socata anyway…
One would need to find out the flow rate, but that can also be measured.
