R

Ron B.

Well Known Member
New to the low wing seen. I'm wandering , in a low wing with the selector on say left wing and you make a steep left turn with a low quantity of fuel ( not close to empty) would you not unport the fuel supply? Do you fly a low wing with fuel unporting in mind at all times? And lastly, reading the instructions that came with the ES Airflow pump ( as I understand them )that say not to run a tank dry as the ES pump will probably not allow the fuel from the other tank (once selected) to be picked up. How does one protect themselves from encountering this situation other than necer running a tank below say 1/2 tanks?
Still trying to learn
Ron
 
G-forces and inter-molecular hygroscopic bonding keeps the fuel in place. As for the fuel pump issue, can't say. I have run tanks in my RV empty multiple times until the engine stumbles and when I change the fuel selector the engine starts right up.
 
Ron, I'm hitting ya hard tonight!!

This was addressed on the matronics list this morning. Here is a great response by Tim Olson...I don't think he will mind if I paste his response here...Thank you Tim for your continuing input!!

"As for the un-porting of fuel, it's a real problem, which is
why many certified planes are placarded or have POH warnings
about minimum takeoff fuel and such. Coordinated approaches
are very important too, and while they teach you in your private
pilots course that you can choose between crabbing on final
and kicking the crab angle out at the last second, or slipping
to keep the nose aligned with the runway, you really have to
be wary of what you do when you're carrying less than 5 or
maybe even 10 gallons in a tank.

So that is why I personally love having just left/right/off,
and not both. I have a totalizer and two float gauges,
and can roughly mentally calculate my fuel levels that I
should have burned. Keeping them isolated helps me
ensure that I know more positively what my tanks hold.
Incidently, even in high-wing planes, I think the "both"
position perhaps promotes sloppy fuel management in some
cases. Myself, I don't have rarely....extremely rarely,
landed with less than 5 gallons of fuel in a tank. Maybe
only 2 or 3 times, and 2 of them where when I purposely
ran the tank dry having a near-full opposite tank. But,
when I am on a long flight, I alternate fuel tanks a couple
times during the flight and then as I get lower in quantity
I keep one tank set to arrive in the terminal area with
at least 10 gallons of fuel, whereas I don't mind at that
point if the other tank has only 5 or 6 gallons left.
But, being conservative, there have actually been very few
times I've landed with under 16-18 gallons, because I set
my high alarm to go off at 7 gallons per side. Fuel and
spark are the 2 most important things to making it safely
to the ground, as far as I can tell."
 
...you make a steep left turn with a low quantity of fuel ( not close to empty) would you not unport the fuel supply?
Click to expand...

You plan on keeping this turn coordinated or not? :D

If the answer is yes, then the fuel stays right where it was before the turn was started. Google "Bob Hoover" and "Ice Tea" for video proof.

If the answer is no, are you going to slip it or skid it? :p
 
Ron Lee said:
G-forces and inter-molecular hygroscopic bonding keeps the fuel in place.
Click to expand...

Whoa! Now there is a phrase I'm gonna Google!

I've landed (allegedly ;)) with only 3 gallons in one tank and the other was bone dry. I'm just not sure what all the fuss is about. :eek:
 
Last edited:
Not an issue

As many have already said, this is not an issue normally unless you fly in a real cavalier manner (not saying you or anyone does - but it is theoretically possible). To have a problem you would have to be very low on fuel (last gallon or two) and be flying in very uncoordinated flight. Its not something realistically pilots are prone to doing. I have deliberately run tanks dry when they needed service and its really uneventful. Switch tanks, hit the pump and the engine will fire up in mere (like 2) seconds, although it may feel like an eternity.

What can be an issue is running a tank dry in flight and not bleeding the air out of it before you need to rely on it. Don't trust a tank that's been run dry until you have run it long enough to bleed that air out. A worst case scenario (just an example) would be to say run the left dry, land on the right. Refuel and taxi out run up on the right and then switch to the left for TO. Your gonna get a momentary blip of air rather than fuel at a inopportune time and it will get your attention :eek: If a tank has been run dry, switch to it on the ground and run it for a while or do it at a high enough altitude to be safe.


Ron B. said:
New to the low wing seen. I'm wandering , in a low wing with the selector on say left wing and you make a steep left turn with a low quantity of fuel ( not close to empty) would you not unport the fuel supply? Do you fly a low wing with fuel unporting in mind at all times? And lastly, reading the instructions that came with the ES Airflow pump ( as I understand them )that say not to run a tank dry as the ES pump will probably not allow the fuel from the other tank (once selected) to be picked up. How does one protect themselves from encountering this situation other than necer running a tank below say 1/2 tanks?
Still trying to learn
Ron
Click to expand...
 
In coordinated flight, unporting is not a problem.

How do I know?

To do the service bulletin for safety wiring the fuel pickups, I flew burning fuel out of my tanks. (RV-6, Constant Speed, Carb) I only had one 5-gallon can, one 2-gallong can, and three 1-gallon cans. Once the fuel level got low, I made LEFT turns above the airport above the traffic pattern watching fuel pressure and fuel flow on my monitor. Once I saw the fuel pressure drop and fuel flow start decreasing, I switched tanks. Engine never missed a beat. I landed normally on the other tank and taxied to the hangar. I then removed the drain from the left tank and measure the fuel. I got about 6 ounces out. The other side I removed 9.5 gallons.

During ground testing before first flight, I jacked the tail of the airplane up, disconnected the carb fuel line, then turned the boost pump on till I could pump no more fuel. I could not get 1/2 gallon fuel out of each tank.

Flight testing now tells me that I have less unusable fuel than what my ground test determined.

The key to getting the fuel out it to fly coordinated. I have never run a tank dry in flight. The above example is the 2nd time in 11+ years and more than 2,170 RV-6 flying hours that I have tried to run a tank intentionally dry. The other time I was fighting headwinds greater than forecast. I had enough fuel for the flight with more than legal reserves but did not like the idea of 5 gallons in each tank at my destination. I chose to run one tank dry and have 10 gallons in the other tank. Did the same thing. Watched fuel flow and fuel pressure and when both started down, switched tanks. On the gound, the fuel pump said that the tank had no fuel in it based on the amount that went in.

IMHO, the calibrated fuel totalizer is the best safety item but not necessary instrument in the aircraft.
 
Gary, I can't say that actually running one dry is without risk but I did it to know what my usable fuel is and to experience the symptoms. The first time the circles I made over an airport became smaller and smaller for some undetermined reason.

I probably should get the fuel down to a gallon or so in one tank and try different normal maneuvers to see what happens. Realistically, I would have to make serious errors to ever land with only a gallon or so in both tanks.

I did make up the intermolecular term.
 
rwhittier said:
What can be an issue is running a tank dry in flight and not bleeding the air out of it before you need to rely on it. Don't trust a tank that's been run dry until you have run it long enough to bleed that air out. A worst case scenario (just an example) would be to say run the left dry, land on the right. Refuel and taxi out run up on the right and then switch to the left for TO.
Click to expand...

Good point. I taxi, do the run up and take-off on the same tank just for that class of problems.
 
Ron Lee said:
Good point. I taxi, do the run up and take-off on the same tank just for that class of problems.
Click to expand...

Since I'm sort of new at flying RV's (36 hours on mine) I've taken the above one step further just to be even more sure. I start-up, taxi and take off on the same tank I was on when I landed previously:D.
 
I want to point out that I posted this response from Tim's post on the Matronics list for discussion only...make sure you don't start flaming anyone but me...Tim is one of the RV-10 guys on the leading edge of getting the RV-10 building and flying with a ton of information. I value his opinion and all he has done for fellow -10 builders.
 
Ron B. said:
New to the low wing seen. I'm wandering , in a low wing with the selector on say left wing and you make a steep left turn with a low quantity of fuel ( not close to empty) would you not unport the fuel supply? Do you fly a low wing with fuel unporting in mind at all times? ...

How does one protect themselves from encountering this situation other than necer running a tank below say 1/2 tanks?
Still trying to learn
Ron
Click to expand...
Ron, there this is no bigger issue in a low wing than in a high wing. Slipping or skidding a high wing can unport their fuel ports as well.

To answer your question about protecting yourself from running a tank dry, you simply switch tanks every so often. There are many different schemes for this. In my case, right after engine start I start a 30 minute countdown timmer buildt into my EFIS and take off on the fullest tank. 30 minutes after engine start the EFIS notifies me via a prompt on the screen to change tanks and restart the timmer.

Not only does this keep me from running one tank dry but it helps keep the plane balanced. (Also, my engine monitor changes color when the fuel levels get into the yellow and red.)

As to why not a "Both" feature on the fuel system, do a search on this forum, there are lots of good replies. The basic answer is when pumping fuel up hill and one of the two tanks runs dry, the pump will suck air and not fuel out of the full tank. Cessnas don't have this problem because as far as I know, gravity has always worked and fuel will flow down hill, even with one empty tank.

Switching tanks is not a big deal and will quickly become second nature.
 
Thanks for all the responce, but still one comment. If when flying with the selector on one tank unporting is so rare an occurance why would the tanks unport if on both any sooner. I'm not arguing with anyone just trying to get a good picture. I do understand why a high wing would work better but fail to see why one tank at a time would be better, other than the senario of running the first tank dry and not loosing the reserve in that tank. I do understand if on both , on a low wing and one tank unports or runs dry that the engine and /or pump will not suck from the remaining tank.
Ron
 
This is

also and advantage of the wing root only fuel system..Assuming you have both pumps running on landing, if you do unport a tank you won't even know it..apart from the fact one of the pumps gets noisier...I've proved this at altitude.

reason is the unported pump will not generate enough pressure to overcome the opposite pump that is pumping fuel.

Frank
 
If when flying with the selector on one tank unporting is so rare an occurance why would the tanks unport if on both any sooner.
Click to expand...

Given a conventional low wing fuel pump setup (i.e. not Frank's :rolleyes:):
It's not that that the tanks would unport any sooner, it's that one tank would empty first, unpredictably so. It would do so because it would be ridiculously difficult to exactly balance the flow from each tank to the fuel pump. When the one tank ran dry, the pump would suck air and the big fan would stop.

Unpredictability is not a desirable feature in an airplane.
 
Thanks Ted I follow that one. Many times in my SuperCub I've noticed one tank showing empty on one site guage and 1/2 full on the other. A little while later they both would show 1/4 tanks. With 1/2 tank the other could unport and that big fan would stop.
 
Alas

Ron B. said:
Thanks Ted I follow that one. Many times in my SuperCub I've noticed one tank showing empty on one site guage and 1/2 full on the other. A little while later they both would show 1/4 tanks. With 1/2 tank the other could unport and that big fan would stop.
Click to expand...

We still continue to suck fuel rather than pump it..Using a gravity drain is almost the same thing...Not having a both position on the slector valve helps as long as you remember which tank is upwind..:)
Frank
 
Exactly!!!

Ted Johns said:
Given a conventional low wing fuel pump setup (i.e. not Frank's :rolleyes:):
It's not that that the tanks would unport any sooner, it's that one tank would empty first, unpredictably so. It would do so because it would be ridiculously difficult to exactly balance the flow from each tank to the fuel pump. When the one tank ran dry, the pump would suck air and the big fan would stop.

Unpredictability is not a desirable feature in an airplane.
Click to expand...

The only low wing airplane I have ever flown that had a both position on the fuel selector is a Varga Kachina. If you do an accident history search on Varga accidents you will find a high percentage of the causes are fuel related.
Adding a both position to an RV with a standard (as designed) fuel system will cause a higher probability of problems instead of reducing them.
I would also suggest to any builder that thinks about modifying the standard fuel system, to consider it very carefully. One of the common causes of RV accidents is engine stoppage for undetermined reasons with fuel starvation being a secondary suspect cause.
 
Last edited:
Also don't forget the tank does have some baffles inside to keep about a half gallon blocked into the pickup tube area...at least the -10 does.
 
rvbuilder2002 said:
I would also suggest to any builder that thinks about modifying the standard fuel system, to consider it very carefully. One of the common causes of RV accidents is engine stoppage for undetermined reasons with fuel starvation being a secondary suspect cause.
Click to expand...

Darn, I mentioned that last night on the "other" forum and darn near was burned at the stake...Thanks for some affirmation Scott!!
 
Rick S. said:
Darn, I mentioned that last night on the "other" forum and darn near was burned at the stake...Thanks for some affirmation Scott!!
Click to expand...

That's unfortunate, because it is true.
The original design fuel systems are about as simple as they can get in a low wing airplane. I have never seen any modification meant to further simplify or add safety to the RV system that didn't (in my opinion) add more complexity or failure points.

frankh said:
Knowing what one is doing is usually a good idea..:)
Click to expand...

I think we can all agree with that...the problem is, I have seen many instances were people were proudly telling everyone about there fuel system improvement they had made, feeling fully confident that they did know what they were doing, but it being obvious to many that they hadn't considered all of the possible issues. That doesn't mean they lack intelligence, it just means they lack knowledge in designing aircraft fuel systems. Not surprising since most of us lack any knowledge of aircraft design and construction when we start a project like this.

Personal motto...Confidence doesn't mean it is correct!
 
Indeed

I have to agree with you Scott, fuel flow problems on modified systems are a notorious problem and one that is very unforgiving..I.e if the fuel flow stops for whatever reason then so does the big fan up front.

One absolutely must view any suggested mod with a healthy degree sceptisim.

Having said that, the standard low wing fuel pumping system is also flawed and you won't find a system in industry designed to pump a high vapour pressure liquid thats is designed like it...I should know i design them all the time as part of my job. Basically sucking on fuel with a potentally hot pump is basically asking for Vapour lock.

The general principle of placing a pump at the lowest point of the system to avoid this issue is well proven, but certainly the method by which one gets there has many potential pitfalls.

As for me I will still keep the faith that there is a better way to do it by..well keep doing it..Two airplanes I have built have around 800 trouble free hours.

I do feel a lot better about making a take off from a high altitude airport running autogas, knowing my system is a whole lot better than the standard setup.

Frank
 
frankh said:
Basically sucking on fuel with a potentally hot pump is basically asking for Vapour lock.

Frank
Click to expand...

I agree. Which is why I would never think of making a takeoff in any conditions even remotely condusive to vapro lock without an operating fuel pump located on the cool (aft) side of the firewall pushing the fuel to the engine driven pump.
 
Which is great

as long as your elecrc fuel pump doesn't die on takeoff.

Unlikely of course.

Frank
 
This is why we are building our fuel system exactly as per the drawings. Till I read the above post, I thought the Airflow pump was for backup purposes only. Now I learn that having the Airflow on during take-off could be saving your life and we don't know it, i.e. the engine driven pump being hot could be experiencing vapor lock and the Airflow pushes the fuel thru this lock.
Thanks Ron
 
Exactly

The downside of the standard system is that in Hot and high conditions (especially with autogas) it is essentially a single electric pump system..I.e you cannot be sure the Mechanical pump will work.

Now electric pumps are VERY reliable but for my money I don't want to rely on a single pump and as I avoid 100LL like the plague in all weather conditions then two properly located electric fuel pumps is the way to go.

Many thousands of builders use the standard system and I can't argue with that kind of success...But it does have its limitations, just like the electric wingroot system does.

Its just a question of what risks your willing to live with.

Frank
 
frankh said:
The downside of the standard system is that in Hot and high conditions (especially with autogas) it is essentially a single electric pump system..I.e you cannot be sure the Mechanical pump will work.

Now electric pumps are VERY reliable but for my money I don't want to rely on a single pump and as I avoid 100LL like the plague in all weather conditions then two properly located electric fuel pumps is the way to go.

Many thousands of builders use the standard system and I can't argue with that kind of success...But it does have its limitations, just like the electric wingroot system does.

Its just a question of what risks your willing to live with.

Frank
Click to expand...

I agree that there is a level of risk with every designed system. The general goal is to design the level of risk to be as low as practical. Even redundant electric pumps don't design out all of the vapor lock risk....they require a steady flow of electrons to make them operate (another failure mode of a system relying on electric pumps).
As you mentioned, RV's with the standard fuel system have proven to have what I think is a very good safety record in regards to vapor lock related power loss, as long as the aux. electric pump is used.

I do believe that it is prudent for anyone using auto fuel, to install a cooling shroud on the engine drive pump and force feed it with outside air.
 
rvbuilder2002 said:
That's unfortunate, because it is true.
The original design fuel systems are about as simple as they can get in a low wing airplane. I have never seen any modification meant to further simplify or add safety to the RV system that didn't (in my opinion) add more complexity or failure points...
Click to expand...
When thinking about deviating from the plans one something like this, I simply remind myself how thousands of RV's are flying with the "stock" setup without any problems.
 
rvbuilder2002 said:
I agree that there is a level of risk with every designed system. The general goal is to design the level of risk to be as low as practical. Even redundant electric pumps don't design out all of the vapor lock risk....they require a steady flow of electrons to make them operate (another failure mode of a system relying on electric pumps).
Click to expand...

Good point. Does FrankH have a dual battery system? Dual alternators? Or just the typical single battery and alternator? My hangar buddy has dual Lightspeed ignitions and has dual batteries and alternators.

With an engine driven and electric fuel pump, you have different failure modes which makes it safer, unless you go redundant batteries/alternators (safety assessment my opinion).

If you really want to almost eliminate fuel starvation conditions, just manage your fuel and stop/refuel before it becomes an issue.
 
Last edited:
Correct

The electrical system has to be considered as well..

Just like if you are running twin electronic ignitions.

In my case I have duel alternators and a single battery..In my old Zodiac I had twin batteries and a single alternator.

The electrical system for the pumps is designed such that nothing relies on a single point of failure..as do my twin electronic ignitons.

That means both pumps are wired to the battery buss with two independant switches and fuses.

I fly in IMC which means that redudant electrical system is highly desirable in any case so no extra investment was required.

Not sure what you mean by managing fuel starvation issues Ron?..Do you mean put in 100LL before reaching a potential vapour lock situation..You can do that but of course the wingroot options avoids that..I can run any fuel I want in any condition.

The only reason this system will VL is if a tank vent became plugged.

Frankk
 
No Frank. The choice of fuel is not relevant. Ok, maybe it is if you are discussing vapor lock. I meant fuel starvation due to running out of fuel. I don't recall statistics but it may be a significant cause of off-airport landings/crashes. As such it is almost certainly preventable.

That raises another question Frank. If you avoid 100LL, how easy it is to get your desired fuel on cross-country trips?
 
Ahh I see

Yes your right something about most fuel exhaustion accidents occuring within 5 miles of the destination.

I avoid 100LL if I can..if not then I refill with 100LL..The engine doesn't care. For sometime I was running reduced ignition timing with mogas (limited to 25 deg max via a panel mounted switch wired to the E/Pmags) but it didn't prove to be necessary.

I also have a design in the back of my head for extended range tanks using the outboard bays that would be removable just like the existing tanks. With $2+ plus per gallon difference it would certainly pay for the materials quickly.

Hard for me to cut into a perfectly airworthy airplane though and in reality the RV has an admirable range as it is, thus most of the time I usually am home on a single fill up..:)
 
"This is why we are building our fuel system exactly as per the drawings. Till I read the above post, I thought the Airflow pump was for backup purposes only. Now I learn that having the Airflow on during take-off could be saving your life and we don't know it, i.e. the engine driven pump being hot could be experiencing vapor lock and the Airflow pushes the fuel thru this lock.
Thanks Ron"

So, given this information, when do you turn on and off the electric pump in your flying sequence? I have a buddy who runs his on takeoff to several hundred feet, and then again on descent. I presume that this does not harm the pump since it has a recycle line. Having never had such a pump on my previous airplanes, this is all new to me.

greg
 
Greg Arehart said:
"This is why we are building our fuel system exactly as per the drawings. Till I read the above post, I thought the Airflow pump was for backup purposes only. Now I learn that having the Airflow on during take-off could be saving your life and we don't know it, i.e. the engine driven pump being hot could be experiencing vapor lock and the Airflow pushes the fuel thru this lock.
Thanks Ron"

So, given this information, when do you turn on and off the electric pump in your flying sequence? I have a buddy who runs his on takeoff to several hundred feet, and then again on descent. I presume that this does not harm the pump since it has a recycle line. Having never had such a pump on my previous airplanes, this is all new to me.

greg
Click to expand...
Greg,

I leave my pump on until I'm at 1000' AGL for takeoff, when changing tanks in flight, and before entering the landing pattern.

With the pump on for landing and takeoffs I don't have to worry about the engine driven pump failing at a "bad time". Turning it on when switching tanks helps keep the pressure up in the event there is some air in the system.
 
Electric fuel pump ON times

1) After runup and before I enter the runway for take-off. Off at about 1000' feet or so. Unless I forget. Then when I check fuel flow and see it is too high I turn off the electric pump.

2) When entering the pattern on downwind prior to turning base.

3) Seldom do it when switching tanks.

I don't do T&Gs often but when I do I like to turn it off about the time I enter downwind then back on around base. Just a concern about running it too long which may be unfounded.
 
Ron Lee said:
1)

I don't do T&Gs often but when I do I like to turn it off about the time I enter downwind then back on around base. Just a concern about running it too long which may be unfounded.
Click to expand...

The standard pumps recommended and sold for RV's are continuous duty pumps and could be left on indefinitely (except as you mentioned...it typically causes some fuel flow error).
One good reason not to leave it on all the time is you would then not know if your engine driven pump is up to the task of running the engine at high fuel flows.
 
Ron Lee said:
1)
I don't do T&Gs often but when I do I like to turn it off about the time I enter downwind then back on around base. Just a concern about running it too long which may be unfounded.
Click to expand...

Go ahead and run the pump for the entire pattern. These pumps are routinely installed in automotive, marine, or industrial environments where they run continuously for years.
 
You must log in or register to reply here.