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  #111  
Old 11-22-2020, 11:19 AM
pilotkms's Avatar
pilotkms pilotkms is online now
 
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Some more real world experience:
7A. IO360 Precision Airmotive Injection. Original Lycoming fuel pump. EZ Flo Fuel filters in wing roots. Began with 70 micron. Have reduced to 40 micron 250 hours ago. I take off without boost pump on, for at least the last 300 hours. Pressure stays at 25-27 thru all phases, either tank. Climbs to 12.5 no problem. (Hi as I go) I use boost pump for cold start only. 100LL AvGas only.
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  #112  
Old 11-22-2020, 12:31 PM
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rv8ch rv8ch is offline
 
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Default boost pump

Quote:
Originally Posted by pilotkms View Post
... I take off without boost pump on, for at least the last 300 hours. Pressure stays at 25-27 thru all phases, either tank. Climbs to 12.5 no problem. (Hi as I go) I use boost pump for cold start only. 100LL AvGas only.
Just curious, why not use the boost pump on takeoff? I also have mine on for landing - one less thing to worry about close to the ground.
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  #113  
Old 11-22-2020, 01:12 PM
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1001001 1001001 is offline
 
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Quote:
Originally Posted by Bill Boyd View Post
they are finer than spec for any injection system at 40 microns but don't have a very large filter surface. They are rated for plenty of flow, but that's at pressure, not on the suction side... At the time I was looking for ways to go finer, trying to block this crud from reaching the injectors. It seems to have all been flushed out now.

It is hard to overstate the problems associated with insufficient suction pressure on a pump. In my career, I have lost count of the times that I have found the root cause of an apparent pump performance problem to be actually poor suction piping design. The net positive suction head required (NPSHR) is an extremely important data point for any pump in a system, but oddly, is often not provided for pumps other than those intended for industrial service.

If you have the ability to install temporarily a pressure gauge in the suction line ahead of the pump you will likely see that the suction pressure is too low, possibly due to installing restrictions (filters) in the pump suction lines.

Especially for volatile fluids, ensuring adequate suction pressure is key for guaranteeing that a pump will perform to its design (or indeed at all).
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  #114  
Old 11-22-2020, 03:42 PM
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Bill Boyd Bill Boyd is offline
 
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Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

Steve and Tom are making hoses for me to plumb the left side with a flex line from the tank straight to a Holley paper element 10 micron filter under the seat and another flex line through the tunnel side to the Andair valve, eliminating two fittings, and with them the possibility of cracked flares.

In the meantime, I'll be flying with avgas or a high avgas blend - which I certainly hope is only a temporary necessity.
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  #115  
Old 11-22-2020, 05:38 PM
TS Flightlines TS Flightlines is offline
 
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What is alittle puzzleing about Bills situation, although easier to run down, is that both left and right sides have the same type of plumbing, filters, tanks, and I assume vents. Everything forward of the Andair selector valve is common to both subsystems, and there is no apparent issues with the right side. So that eliminates the pump forward. He is going to send us the original plumbing so we can inspect it, but quite honestly I'd be surprised if we find anything there.
Obviously cant take apart the tank, and I dont know the history. But eventually we'll get the root cause.

Tom
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  #116  
Old 11-22-2020, 05:55 PM
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Bill Boyd Bill Boyd is offline
 
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Tank built by an other owner before me (his second RV-10) and done well enough that it doesn't leak - although the other side has a slow leak through what I must assume is a loose B-nut inside on the vent line bulkhead fitting and out the vent, when the fuel level is high enough. The misbehaving side does not have this issue.

I've opened the under-seat filter on the left side and cleaned a tiny amount of tank debris out of the stainless mesh element before this issue started making itself known but not since. Will be interesting to see if there is anything new in there when Tom receives it as-is.
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  #117  
Old 11-22-2020, 06:32 PM
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Quote:
Originally Posted by Bill Boyd View Post
Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

Steve and Tom are making hoses for me to plumb the left side with a flex line from the tank straight to a Holley paper element 10 micron filter under the seat and another flex line through the tunnel side to the Andair valve, eliminating two fittings, and with them the possibility of cracked flares.

In the meantime, I'll be flying with avgas or a high avgas blend - which I certainly hope is only a temporary necessity.
After Phase 1 flight testing in my RV-10 I ran one tank of ethanol free mogas with the other tank 100 LL exclusively. I never experienced a difference in fuel pressure when I swapped tanks during flight. I did use a fuel pump cooling shroud and insulated the bottom of the tunnel to reduce tunnel heat. I am not a fan of paper media fuel filters at all in airplanes.
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  #118  
Old 11-22-2020, 07:04 PM
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Bill Boyd Bill Boyd is offline
 
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I did the mogas/avgas separate tanks thing for a number of years in my 6A while I developed confidence in mogas as a fuel - and never had issue one with it in the O-320. I can do so again.

I think the filter inbound from Steve is a stainless mesh 100 micron, but the Holley inserts are interchangeable I believe between that and 10u paper. I'd prefer to start with the looser mesh to minimize flow resistance.

My tunnel is barely warm to touch in flight.

Since the boost pump has been unable to prevent the pressure loss consistently when feeding from the left tank using mogas, I doubt the issue is amenable to a fix by cooling the engine driven pump, but I'm not ruling it out either.
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Last edited by Bill Boyd : 11-22-2020 at 07:07 PM.
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  #119  
Old 11-23-2020, 05:37 AM
Aircraft Specialty Aircraft Specialty is offline
 
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Quote:
Originally Posted by 9GT View Post
After Phase 1 flight testing in my RV-10 I ran one tank of ethanol free mogas with the other tank 100 LL exclusively. I never experienced a difference in fuel pressure when I swapped tanks during flight. I did use a fuel pump cooling shroud and insulated the bottom of the tunnel to reduce tunnel heat. I am not a fan of paper media fuel filters at all in airplanes.
As Bill mentioned below...the filters that we utilize are all high quality with a stainless mesh filter.
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  #120  
Old 11-23-2020, 05:54 AM
myv65 myv65 is offline
 
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Have you noticed anything with regards to the rate of pressure loss versus fuel consumption? You'll always see increasing vacuum as measured at the pump suction with increasing flow. If you've got a moderate obstruction on one side, you should clearly see a difference in the vacuum between the two sides, but also should find that the severity your problem varies with fuel flow rate. Clearly not a solution to the problem, but measuring that vacuum and how it responds to different conditions should confirm whether this is a restriction issue or not. If on the other hand the vacuum is lower (closer to ambient pressure) on the problematic side, you have a leak and you're drawing air.

When it comes to measuring vacuum, you also may want to consider using a differential gauge/sensor (other side open to atmosphere) rather than a single port. With single port you'll need to consider your current density altitude when comparing values.

Best luck getting to the bottom of this.
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