Fuel Pressure loss on Loop upline

[SMO]

Originally Posted by F1R

Where did you mount the aerolab relative to the exhaust tips?

Without looking/measuring my guess is 12 - 16" aft of the exhaust. It is in the stick well.

I am also guessing on take off, below 500ft, you have your boost pump on?

True.

[SMO]

Originally Posted by AlexPeterson

Approximately what is the frequency? I've noticed this at about 2400 RPM as monitored by the G3x Touch. I believe it may have to do with the sampling frequency of the engine monitor vs pressure pulses in the mechanical pump. Unverified, but the oscillation goes away when I dial the rpm down from 2400 to about 2350. The period of the osc. I have observed is about 5 seconds, and it will vary between maybe 21 and 25.

Frequency between the top and bottom is ~5 seconds, between tops ~ 10 seconds, 2300 rpm. Same EFIS/EIS as you, sampling rate is 1 per second.

[SMO]

Ok, I have a few things to check but will have to wait till next week now. Any further flights will stay within gliding distance of the airport till I get this sorted out.

[TS Flightlines]

Mark---can we assume some of the other flights after the changes also were up to 3g, and no incidents? So this indication just started? And boost pump on during loops?
Just asking--
Tom

[Tom Martin]

Mark
I am not sure which servo you have but the two that I am familiar with have an internal filter that needs to be cleaned. Symptoms of a partially clogged filter, that I have experienced, is a rough running engine at high power settings and a high angle of attack.
Just another thing to check off the list.

[DanH]

Originally Posted by SMO

Without looking/measuring my guess is 12 - 16" aft of the exhaust. It is in the stick well.

On the belly centerline? As F!R's suggests, a hot canister is a fine way to develop vapor bubbles....and since that particular gascolator is riveted into the belly skin, the entire belly is conducting exhaust heat to it.

Remember, vapor pressure rises with temperature. When vapor pressure exceeds local pressure, bubbles form. A 3G pull drops local pressure as stated previously.

So, is your canister actually hot? Can you fly with the top off the center console so you can reach it?

[RV7A Flyer]

Originally Posted by SMO

Ithe other change I made at the same time was replacing my Andair gascolator with and Aerolab.

Why did you change it? Can you change it back (to remove one variable from the problem)?

[AlexPeterson]

Originally Posted by SMO

Frequency between the top and bottom is ~5 seconds, between tops ~ 10 seconds, 2300 rpm. Same EFIS/EIS as you, sampling rate is 1 per second.

I'm guessing if you change the rpm some, you'll see the oscillations go away. Regarding the sampling rate, the 1/s is probably what is saved to your flight log. I wasn't able to figure out what the GEA-24's electronics do to sample/condition the raw signals. I see the same oscillations on MAP at 2400 rpm also.

I'd bet your oscillations are measurement artifact. The drop on pulling g's is likely not. I've seen that on occasion for 17 years FWIW, but I've never had any power loss associated with it.

[AlexPeterson]

Originally Posted by DanH

So check it for an air leak. Seal the inlet and pull suction on the outlet, see if it will hold a vacuum.
SNIP

Be sure all fuel is dried out before you do this, or it will simply boil and behave like a leak.

[SMO]

I have done some more research into my data logs and have also flown a test flight.

Test flight confirmed this is a "g" related problem, not fuel flow. I can fly 20 gph without the boost pump on and pressure stays up fine. I flew at 14 gph, pulled up into a loop and by the top of the loop fuel pressure was down to 6.8 psi and my boost pump had kicked in. Fuel flow dropped a little to 11 gph and came back up to 14.

Prior to the test flight I opened my stick boot so I could use my IR temperature gun to check the temperature of the gascolator during flight. Max temperature I saw was 100* F. My exhaust is turned down on the ends so should limit the amount of heat applied to the gascolator. Measured distance from the end of the exhaust to the gascolator is 18". Gascolator is offset from the centerline so it doesn't interfere with the stick (located in stick well).

Looking at my logs, it appears EVERY aerobatic flight since I changed out the transducer and the gascolator had at least some level of fuel pressure drop (below 18 psi) associated with increased g force, though not always enough to trigger my boost pump to kick in (11 psi). Previous flights show some corresponding drop (~5 psi) but nothing as dramatic as I am seeing after the gascolator and transducer changes.

Tom, I checked the internal screen on my RSA servo during the annual this winter and it was clean, although certainly worth checking again.

I changed out the Andair gascolator because I had to remove it in order to access the filter. The concept of the Aerolab was appealing because of the ease of access to the filter.

Besides checking for air leaks, the next test I will do is to bypass the gascolator (using an inline filter) and see if the symptoms go away. Next week's project.