When I first started test flying my friend’s new RV8 all seemed well with the fuel flow on the Dynon Skyview. The numbers were sensible and just what I would expect from an IO-360. The fuel transducer is the classic Red Cube and the default pulses settings returned normal readings.
A very brief hiatus in testing while I went to work for a couple of weeks. During that break my friend changed the fuel filter for a brand new factory unit. He mistakenly believed the whole filter unit had to be changed, as opposed to the screen cleaned, so we know the thing he fitted is extremely clean. The fuel lines were purged afterwards. He also tried to chase down a problem with the oil temperature sender (which was sending us readings that were too low). Nothing was changed with the fuel system but the oil temp sender (a new one and the existing one) were both connected directly to earth to prove we didn't have a ground problem with them. While the battery was disconnected he installed another ground strap from the airframe to the battery tray, which is connected via 6 bolts to the airframe to act as the ground return, with a rather thick bit of cable twixt the battery tray and the battery negative.
I resumed the testing program (25 hours in Oz) and noticed that leaning did not give me the same fuel flows as before. They were higher at all RPM settings , even when the same parameters were set as far as altitude, RPM and temperature lean of peak. Fuel dips showed the high readings were only reading high and that the engine was still burning fuel at the previous, original, rates. In other words Skyview was over-stating the fuel flow.
Now the strange bit: when the booster pump is turned on the fuel flow goes DOWN and, I think, to the actual rate at which the engine is burning the fuel. Fuel pressure increases slightly, as you would expect.
We thought that some sort of air lock may have formed, causing the flow meter to read too high and that the increased pressure from the booster pump was enough to ‘squeeze’ the bubbles into solution and return the transducer to normal. But the pipes had been bled and my friend disconnected the Red Cube and made sure that there were no blockages, binding of the impeller or any other reason for it to not work physically.
We considered electrical load causing some spurious readings on the Skyview, so I left the pump off and simultaneously operated the electric flaps and the electric pitch trim. Maybe not the same current draw as the fuel pump but it made absolutely no difference to the readings; booster pump on or off.
When the pump is turned on or off the change to the Skyview readings is pretty much instant. The fuel flow doesn’t ‘jump’ to the new reading but it does start to change straight away and takes only 2 or 3 seconds for it to stabilise at the new numbers.
We’ve discussed the possibility of a ground loop being introduced with the extra wire from the airframe to the battery tray but really don’t see how this is a possibility, especially it being so close to the battery and so far away from the engine (the battery is behind the rear seat).
Obvioulsy, if the fuel flow dropped when the pump was turned off we’d be less confused but for it to drop to sensible readings when the pump is truned on just doesn’t make much sense to my mind. Just as an example, yesterday at 2300RPM, 5000’ and lean of peak by 6 degrees, I was getting a fuel pump on reading of 29 Ltrs/Hr and then 38 Ltrs/Hr with the pump off.
SO, kind people, do you have any ideas of ever heard of such a phenomena?
I look forward to the instant answer to all our woes
……so that we can concentrate on getting capacitance plate fuel levels to sensible readings
A very brief hiatus in testing while I went to work for a couple of weeks. During that break my friend changed the fuel filter for a brand new factory unit. He mistakenly believed the whole filter unit had to be changed, as opposed to the screen cleaned, so we know the thing he fitted is extremely clean. The fuel lines were purged afterwards. He also tried to chase down a problem with the oil temperature sender (which was sending us readings that were too low). Nothing was changed with the fuel system but the oil temp sender (a new one and the existing one) were both connected directly to earth to prove we didn't have a ground problem with them. While the battery was disconnected he installed another ground strap from the airframe to the battery tray, which is connected via 6 bolts to the airframe to act as the ground return, with a rather thick bit of cable twixt the battery tray and the battery negative.
I resumed the testing program (25 hours in Oz) and noticed that leaning did not give me the same fuel flows as before. They were higher at all RPM settings , even when the same parameters were set as far as altitude, RPM and temperature lean of peak. Fuel dips showed the high readings were only reading high and that the engine was still burning fuel at the previous, original, rates. In other words Skyview was over-stating the fuel flow.
Now the strange bit: when the booster pump is turned on the fuel flow goes DOWN and, I think, to the actual rate at which the engine is burning the fuel. Fuel pressure increases slightly, as you would expect.
We thought that some sort of air lock may have formed, causing the flow meter to read too high and that the increased pressure from the booster pump was enough to ‘squeeze’ the bubbles into solution and return the transducer to normal. But the pipes had been bled and my friend disconnected the Red Cube and made sure that there were no blockages, binding of the impeller or any other reason for it to not work physically.
We considered electrical load causing some spurious readings on the Skyview, so I left the pump off and simultaneously operated the electric flaps and the electric pitch trim. Maybe not the same current draw as the fuel pump but it made absolutely no difference to the readings; booster pump on or off.
When the pump is turned on or off the change to the Skyview readings is pretty much instant. The fuel flow doesn’t ‘jump’ to the new reading but it does start to change straight away and takes only 2 or 3 seconds for it to stabilise at the new numbers.
We’ve discussed the possibility of a ground loop being introduced with the extra wire from the airframe to the battery tray but really don’t see how this is a possibility, especially it being so close to the battery and so far away from the engine (the battery is behind the rear seat).
Obvioulsy, if the fuel flow dropped when the pump was turned off we’d be less confused but for it to drop to sensible readings when the pump is truned on just doesn’t make much sense to my mind. Just as an example, yesterday at 2300RPM, 5000’ and lean of peak by 6 degrees, I was getting a fuel pump on reading of 29 Ltrs/Hr and then 38 Ltrs/Hr with the pump off.
SO, kind people, do you have any ideas of ever heard of such a phenomena?
I look forward to the instant answer to all our woes
……so that we can concentrate on getting capacitance plate fuel levels to sensible readings