Chasing the issue - low fuel pressure on takeoff
The low fuel pressure on takeoff appears to be a design issue related to cavitation at the engine driven fuel pump. Controlling the pressure at the engine driven fuel pump appears to be the most direct fix.
The problem is not consistent. At times, the pressure at takeoff does not drop below 4-4.5 psi. At other times, the pressure can drop into the low 2s or even near 1 psi.
Often, I can lower the nose and pull back to 4000 rpm, see a pressure recovery, power up and continue the climb.
The two solutions offered prior were to use a blend of 100LL or just accept the low pressure if there is no engine falter. To me, neither is acceptable. I prefer, and the engine was designed for mo-gas.
Hopefully, the slightly higher rated Facet pump will boost the pressure at the engine driven pump to prevent the pump cavitation, if that is the issue.
The problem may only appear on the RV-12, at least according to two of the Rotax dealers and information from user?s groups or operators of other designs. Thus, it would appear to be a design issue related to the RV-12.
PRESSURE DROP IN SYSTEM:
There are several contributors to the lessening of the pressure at the engine driven pump.
Static pressure with the Facet pump: 3 psi
Pressure with engine and Facet pump running: 5.5 psi
Pull the c/b: 5.2 psi
At higher rpms such as runup: 3.5-4.5 psi, sometimes drops to 3 psi, rarely to 2 psi.
At takeoff, the pressure often drops to 1-2 psi.
Van's, Rotax and others suggested instrumentation or installation issues as the source of the indicated or actual low pressure problem.
1. Bad engine driven fuel pump: changed engine driven fuel pump, same problem. Result, both engine driven fuel pumps are ok.
2. Unusual blockages restricting flow: Using the Facet pump only, the static pressure was 3.0 psi. The pressure transducer is on the right forward side of the firewall, near the top. Flow was 33 gph at the engine driven fuel pump; 20 gph at the fuel pressure transducer; and 1.4 gph at the end of the fuel return line (you should also see 1.5 gph flow on the Skyview.). I did not check the flow to each carburetor as they only need 2.5 gph to each. Result, no unusual blockages.
3. Bad fuel pressure sensor: Static pressure about 3 psi and drops to 1-2 psi on takeoff. A new sensor was added and the results were the same. Dynon pressure was compared to a mechanical gauge on the ground, at takeoff and in flight (see later post for mechanical installation). The mechanical gauge was consistently about 0.1-0.2 psi less than the Dynon sensor. Result: Dynon sensor is accurate to demonstrate that the large pressure drop was real.
The system design also contributes to the pressure loss. It appears that the typical overall system loss is about 1.5 to 2.0 psi attributed to increasing fuel flow when and the rpm is increased from idle to 4000-5000 rpm (see above).
4. Contributing to the overall loss:
Long fuel line runs: 140 inches of 3/8 tube. The Facet pump in the middle confuses the issue for me.
Numerous components/fittings: 4 components and 10 fittings
Six 90-degree bends (plus two 45-degree bends)
5. Column pressure due to column height from tank to engine driven pump: (this is included in the overall system loss)
from full tank: 8 inches or 0.2 psi
from empty tank: 21 inches or 0.5 psi
Takeoff dynamics further contribute to the pressure loss.
6. Increasing column height due to pitching up is added to the overall system loss.
10 degrees pitch at 72 inches: 12.5-inch column increase or 0.3 psi
7. longitudinal acceleration adds to the overall system loss:
0.2 Gs longitudinal acceleration calculated from change in ground speed. With a 90-inch horizontal column, the 0.2 G longitudinal acceleration = 18-inch column increase or 0.45 psi pressure loss during acceleraton and/or initial climb.
The total loss is about 2.5-3.0 psi at takeoff. The portion of pressure loss due to the takeoff appears to be about 0.7-0.8 psi and may be the reason that the problem typically appears during the takeoff and initial climb phase of flight. The pressure drop at takeoff lessens the pressure at the engine driven fuel pump. Perhaps that is the primary trigger for cavitation at the engine driven fuel pump.
The higher rated Facet pump may fully offset the takeoff effects.
Testing the fuel pressure gauges
I followed advice from Van's, Lockwood, and Dynon. All were helpful. The following are not in the order of progress.
The fuel pump was replaced. The results were somewhat better for a few flights. The Dynon fuel pressure gauge was replaced and compared to a mechanical gauge. Both gauges were remarkable in agreement.
I inserted the mechanical gauge and new Dynon gauge into the fuel pressure gauge line as shown.
Then the mechanical gauge was routed through the hinge pin access port. The duct tape fairing was added and flight checked.
Some graphs show the pressure loss due to acceleration and pitch up
The following graphs were used to better define the fuel pressure drop at takeoff. Skyview captured the USER LOG DATA at 4 samples/second. A freeware program, DATPLOT, was used to create the graphs. The .csv files were converted to .xlsx, trimmed, manipulated, and equations added; and were converted back to .csv. DATPLOT requires that a value be in each cell of data to be graphed. That is, put a zero in each empty cell in the column of data to be plotted.
Fuel pressure has a 4-second lag, that has been corrected in four of these graphs.
The data titles may be hard to read.
Blue - fuel pressure; psi
Magenta ? fuel flow; gph
Green ? rpm; right scale
This flight had a pressure drop to 2 psi during the takeoff (4100). A second drop (4440) was introduced during a power up and acceleration while in flight.
The next two graphs:
Blue ? fuel pressure; psi
Magenta ? fuel flow; gph
Green ? column pressure due to longitudinal acceleration; psi - right scale
Red ? column pressure due to pitch component; psi - right scale
Black ? sum of the two column pressures; psi - right scale
The column pressure due to longitudinal acceleration and pitch up is presented below. The longitudinal acceleration was calculated using the change in ground speed. The column pressure due to pitch up simply uses the increase in height of the fuel pump. It appears that both fuel flow demand and acceleration/pitch up are necessary for the large fuel pressure drop (see 4409 and 4660). For example, at 4409, the fuel pressure drop occurs when the fuel flow and acceleration/pitch up (black line) are present. At 4460, only the acceleration is present with no fuel flow demand or loss of fuel pressure.
Blue ? fuel pressure; psi
Magenta ? fuel flow; gph
Just an interesting graph. The fuel pressure can be traced from the 2000 rpm group along the higher lines to the lower fuel pressure group at 5000 rpm, then back along the lower lines as the rpm decreases.
The first graph used fuel flow without the 4-second adjustment. The second graph has the 4-seconds correction applied to the fuel flow data. It seems to make more sense. the point is to be careful with the data, I am sure that we don?t know all of the anomalies in the system.
Low fuel pressure on takeoff and the Facet fix
Review of new Facet pump Notification 18-07-12
The previous posts may help explain why we are seeing the fuel pressure loss at takeoff and why the upgraded Facet pump could solve the problem.
Change to higher pressure aux fuel pump
--ES40105 vs ES40135
3.0-4.5 psi vs 4.0-7.0 psi
----30 gph vs 32 gph
I have been experiencing fuel pressure drops to low pressure on many takeoffs. Once I switched to the higher-pressure pump, I have not seen the pressure drops to the 1-2 psi range.
CONDITION --- Facet 40105 vs Facet 40135
engine off facet only ---- 3.0 psi vs 4.9 psi
engine idle facet on ----- 5.5 psi vs 5.7
engine idle pull facet cb - 5.2 psi vs 4.8
run up facet on ----------4.0 psi vs 4.9
takeoff facet on --------- 1.0-3.0 psi vs 3.6
cruise facet on ---------- 4.0 psi vs 4.9
Note; typically, the psi for each condition above varies a few tenths from flight to flight and day to day.
The higher-pressure pump appears to keep the pressure above the cavitation pressure at the engine driven pump. Thus, the fuel pressure does not dive into the 1s and 2s at takeoff. One concern was that if the pressure into the engine driven pump was too high, the output pressure would exceed the 8-psi limit. So far, the maximum pressure at engine idle with the new facet pump has not gone above 6.0 psi.
The pump is noisier. The noise does disappear at power up for takeoff. I must have gotten used to it as I no longer notice it when the engine is running during taxi.
An article that further explains fuel pump cavitation
I thought that this article explained the issue of low fuel pressure very well.
Wow John you were busy on Christmas Day ciphering fuel delivery problems.
I hear what you're saying but I know of at least six RV-12's (D-180, Skyview, Dynon, and HDX EFIS) that don't exhibit loss of fuel pressure on take-off. I have personally experienced vapor lock on a summer day after restarting a heat-soaked engine. My procedure now is to power the master switch and let the electric pump recirculate cool fuel through the fuel distribution hoses above the warm engine. Before restarting a heat-soaked engine I'll run the electric pump for several minutes. Haven't had a problem since. Also, I run 93E10 exclusively...
What year is your Rotax 912 ULS. This is on a 2015 Build RV-12, correct?
The airplane first flew in 2016.
It is good to hear that some using mogas are not having a problem.
I'm in phase 1 of the first of our two 12's.
During the 2 hour flight which we need to do, I spent some time in the pattern - 15 circuits :(
During the climb on crosswind, the fuel pressure was dropping, but it recovered as I levelled off and throttled back.
I assumed it was just a characteristic of the model and engine :D
This is excellent information, thank you for sharing.
Reading all of this history has me asking myself a serious safety question.
In general the intent of an electric fuel pump in this design is to be a backup incase the engine driven mechanical fuel pump fails, noting it adds other side benefits such as helping prevent vapour lock. From what I interprete (possibly incorrectly) above, a failure of the electric fuel pump on takeoff could cause engine loss of power even with the mechanical pump operational due to too low fuel pressure? If this is in fact the case, our electric fuel pumps are no longer a backup but a critical device in which a single point of failure could cause a loss of thrust on takeoff...if this were true (please jump in and correct me), then should we not have 2 electric fuel pumps?
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