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04-07-2015, 06:32 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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I'm not necessarily proposing a solution...more a thought exercise. We tend to be soooo fixed in our thinking sometimes.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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04-07-2015, 06:59 AM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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Taking fuel pumps to the next logical step someone (Tom) needs to come up with an RV retro kit like the hot rodders have done...
http://www.chevyhardcore.com/tech-st...-installation/
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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04-07-2015, 07:28 AM
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Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
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Pumps in the wing root would push the fuel through the fittings and plumbing around the selector valve rather then sucking the fuel through same.
In my case, the fuel must travel through a 45 deg AN Fitting and (2) 90 Deg fittings before heading through more bent aluminum tubing to the pre filter and the boost pump. Then two more 45 deg fittings before getting to the mechanical pump. So, in my system the mechanical pump is sucking fuel through (2) 90s and (3) 45s, plus the usual tubing bends. I suspect this is more restriction than most systems, but it worked well in my Subaru days. (I am looking at ways to reduce the number of fittings or replace them with high flow types, maybe pull the return system and replace the valve with Van's etc)
When I built the plane, I wanted to have the Andair valve handle point to the tank in use, orienting it this way caused complications in the plumbing resulting in the arrangement described above. Mine is a 6 port valve with full return system that worked fine when the system had 35 GPH flowing back to the tank in use. Now that I have a dead end injection system perhaps the restriction from those fittings is enough to cause vapor lock, speculation.
Another possible approach to the problem would be to put a fuel pressure regulator at the spider and let the excess fuel flow back to the tank in use again... It worked OK before, but not sure that would work on this system. Comments? Anybody tried that?
One instance of loss of fuel pressure happened at 16,000' in below zero OAT, the other time was at 10,500 and 17F OAT. I have already made a few changes that may have solved the problem, and after reading through this thread I have learned that the need for boost pump use in climbs and high altitude cruise is not uncommon.
I would like to have fuel system with a little more margin to vapor lock situations so that I can run auto fuel as desired.
The problem with most of this business is I am making changes based on speculation rather than measurements. As stated above, I may have already solved the problem, but with this approach, only flight testing will tell and even then I may not be able to exactly replicate the situation that lead to the problem. (Just a little self criticism here as I probably should have measured fuel pressure on the suction side before making any changes and then measured after to see the difference, maybe somebody knows how much difference in pressure drop there is between an AN fitting and bent tubing? Enough to cause fuel to boil?)
FYI, I have eliminated a 90 Deg fitting from the outlet of the boost pump by bending alum tubing. I also ducted the cabin air heat down to the cowl exit and away from the mechanical pump.
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04-07-2015, 08:39 AM
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Join Date: Apr 2010
Location: Ridgeland, SC
Posts: 2,589
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Randy and others---
we may be in uncharted territory here in some cases. Altitude, type of fuel, a somewhat uncommon fuel system, OAT, all of this are factors. Randy, you may infact become the test pilot for this system.
Perhaps some of our other members can elaborate on some of this. I know Paul did alot of testing---Dan for sure, so possibly we can help Randy with his issue.
Tom
__________________
Tom Swearengen, TS Flightlines LLC, AS Flightlines
Joint Venture with Aircraft Specialty
Teflon Hose Assemblies for Experimentals
Proud Vendor for RV1, Donator to VAF
RV7 Tail Kit Completed, Fuse started-Pay as I go Plan
Ridgeland, SC
www.tsflightlines.com, www.asflightlines.com
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04-07-2015, 10:30 PM
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Join Date: Oct 2005
Posts: 1,516
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Quote:
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When testing the fuel vents, I was surprised at how hard it was to blow through them with the fuel caps off. It was difficult to move air through the tubing but I could get air to the tanks.
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This is very interesting, an unobstructed 1/4" vent line should make it very easy to get air into the tank????
A little fuel in the vent lines is certainly not uncommon especially with full tanks, but should not inhibit the free flow of air venting the tanks.
If in fact the tanks are under an even miniscule amount of vacuum would explain the occasional collapse in fuel pressure especially in those installations with multiple elbows and fuel line construction which impedes the flow of fuel.
In many installations the forming of vapor bubbles may only be a matter of a few degrees of fuel temperature or one more elbow in the fuel line or a tiny bit more draw needed to pull fuel uphill to the mechanical fuel pump.
Any of these 3 conditions, fuel temperature, fuel pressure and a streamlined fuel supply line can be addressed to eliminate a fuel vapor problem.
I have been burning mogas E10 for the last 300 hours without any trouble and this is the first year that I have observed a bit of fuel pressure fluctuation on climb out using mogas without the boost pump. As I previously stated, the pressure stays stable flying on avgas without the boost pump and stable with the boost pump on during climb using mogas. No boost pump required for cruise on either gasoline.
I have a constant flow return lijne plumbed out of the mechanical fuel pump, calibrated to return 6GPH @25psi.
I believe this installation provides a constant flow of cool fuel to the mechanical pump and addresses the fuel temperature issue.
I have not considered checking the fuel vent lines for possible obstruction but I have always noticed the air pressure building up during fueling when using a funnel. A forward facing fuel vent line should provide just a tiny bit of pressure
in the tank but certainly not a vacuum. Measuring tank pressure during flight might provide some more clues.
An electric boost pump in the wing roots would certainly go a long way in overcoming any of these issues and a good idea other than for the reasons I mentioned previously.
__________________
Ernst Freitag
RV-8 finished (sold)
RV-10 Flyer 600 plus hours
Running on E10 mogas
Don't believe everything you know.
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04-08-2015, 08:59 PM
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Join Date: Jan 2005
Location: Sedona Arizona
Posts: 349
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New fuel system
Since I posted about my fuel pressure loss at altitude etc. I thought I would post a follow up on what I have done in hope of eliminating the problem.
Well basically, I put in a whole new fuel system. I have pulled out my old feed and return lines and Andair 6 port valve and replaced them with the standard Van's fuel selector valve. (The old system was in place to facilitate the Subaru injection system, much like the EFII system with fuel returning back to the tank it came from.)
Before the changes the fuel left the tanks in the normal stock Van's manner. It then turned up 90 deg (bent alum)to get to the Andair selector valve, then a 45 Deg AN fitting, then an AN90 Deg, then coming out of the valve another AN 90 Deg. After that it droped down to the EFII pre filter and boost pump. Exiting the boost pump was another 90 Deg fitting and then two more 45 deg fittings to get to the mechanical pump.
I have eliminated (3) 90 Deg AN fittings, and (1) 45 Degree fitting, plus the tank selector valve now sits lower so the fuel does not have to be drawn as high (by about 1.5"). In addition, the routing of the fuel lines is much less resistant to flow as in fewer bends and elevation changes.
I am also trying out the Rocket style fuel vents (coils in the wing root) only I have increased the size of the coils to 3/8". The vent to atmosphere is located a few inches below the leading edge of the wing, in the wing root fairing. I went with three coils of the 3/8" tubing and used a 3/8" bulkhead fitting cut at an angle with screen glued in place like the originals under the firewall, only much larger. I suspect this area is a high pressure zone being just under the leading edge. I used an odd little adapter where the new vent line attaches at the tank, converting from 1/4" OD to 3/8" OD flares. The vent tubing inside the tank remains the same 1/4" OD tubing.
I only had parts around to install the new vents on one side so maybe I can figure out a way to measure the pressure in each tank and see how much difference the new vent makes, if any. It sure is easier to blow though. I think, in theory, the effort required to blow through the 1/4" tubing that bends all around inside the fuse, equates to less pressure reaching the tank.
So that is the new system. All new tubing in an arrangement that should allow a much freer flow, so less suction required and hopefully an improved venting system. FYI, I am leaving the old vent system in place and can easily hook it back up if I don't like what I have done for any reason. I cannot think of any easy way to measure the air pressure inside the tanks to compare the effectiveness of the vent change, any ideas? This would be a great time to measure since I have one tank vented in the stock manner and one with the coils in the wing root.
I intend to test the new system starting with topped off tanks and climbing to 17,500 and leveling off for a while to see how it does.
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04-27-2016, 03:01 AM
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Join Date: Sep 2007
Location: BC
Posts: 1,674
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It's been over a year...
Anything new or positive conclusion to this very interesting story?
Bevan
__________________
RV7A Flying since 2015
O-360-A1F6 (parallel valve) 180HP
Dual P-mags
Precision F.I. with AP purge valve
Vinyl Wrapped Exterior
Grand Rapids EFIS
Located in western Canada
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04-27-2016, 08:00 AM
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Join Date: Apr 2010
Location: Ridgeland, SC
Posts: 2,589
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I saw Bevans post to this, and thought, wow-----I forgot to play with this. (among other things, been busy, and OTHER issues.)
If Randy could chime in here at let us know the status, I think we all could learn something.
I re-read a few of the posts and was struck by the post saying the vent line was hard to blow through. So---is is -4 aluminum tubing, or hose, and IF its hose, what kind? The issue I see is that at Randy's base in Sedona, at 4300', its going to be hard to test things at 16000-17000 feet. IF it had hose as vent lines, and they were rubber hose, it is a possibility that at altitude the liner could constrict, or even collapse, creating an issue. If is is aluminum tubing, then it hard for me to imagine the tubing collapsing, and then opening back up as the plane descends. I guess crazier things have happened.
Hopefully Randy will give us an update and see if his changes have somved the problems.
Tom
__________________
Tom Swearengen, TS Flightlines LLC, AS Flightlines
Joint Venture with Aircraft Specialty
Teflon Hose Assemblies for Experimentals
Proud Vendor for RV1, Donator to VAF
RV7 Tail Kit Completed, Fuse started-Pay as I go Plan
Ridgeland, SC
www.tsflightlines.com, www.asflightlines.com
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06-10-2016, 06:58 AM
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Join Date: Jan 2012
Location: Weiz / Austria
Posts: 30
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am not sure if it will help, but thought I let you know about the problems I had
with the fluctuating fuel pressure on my YIO-360-M1B engine. Actually I
have solved the problem now. The cause for the fluctuating fuel pressure
indication was VAPOR LOCK in the fuel pressure hose. I have connected
the fuel pressure hose to the silverhawk injector which is located at
the front lower end of the engine. The fuel pressure hose I have routed
underneath cylinder 1 and cylinder 3 back to the firewall where the
pressure sensor is located. It seems that the fuel in the pressure hose
got heated up during longer flights at higher levels - thus causing
vapor locks in that hose and thus causing the erratic readings of the
fuel pressure. I have put an additional heat protection sleeve on the
fuel pressure hose now and re-routed it as far away as possible from the
hot areas near the exhaust pipes of cylinder 1 and 3. Problem solved, no
more erratic fuel pressure readings.
best regards,
Josef
__________________
RV-8A / YIO-360-M1B / 2 x P114 from Emagair
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06-10-2016, 07:45 AM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,145
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Quote:
Originally Posted by TS Flightlines
I saw Bevans post to this, and thought, wow-----I forgot to play with this. (among other things, been busy, and OTHER issues.)
If Randy could chime in here at let us know the status, I think we all could learn something.
I re-read a few of the posts and was struck by the post saying the vent line was hard to blow through. So---is is -4 aluminum tubing, or hose, and IF its hose, what kind? The issue I see is that at Randy's base in Sedona, at 4300', its going to be hard to test things at 16000-17000 feet. IF it had hose as vent lines, and they were rubber hose, it is a possibility that at altitude the liner could constrict, or even collapse, creating an issue. If is is aluminum tubing, then it hard for me to imagine the tubing collapsing, and then opening back up as the plane descends. I guess crazier things have happened.
Hopefully Randy will give us an update and see if his changes have somved the problems.
Tom
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In my 9A I've got the Tuckey-style outboard tanks and Rocket-style coiled 1/4" aluminum tubing (3 turns) at the wingtip, then vented to the bottom of the wing with the aluminum 45-degree fitting with screen pressed in it. My vent air comes into my outboard tanks and flows through them into the outboard tank fuel pickup, which is connected to the inboard vent fitting (flow-through venting). I've now got almost 5 hours of flight in Phase I at 16,500' and 17,500' without the slightest appearance of any venting issue going on. I would be more inclined to suspect vapor lock on the inlet side of the pump due to the reduced ambient pressure at this altitude, than a venting problem.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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