EAA article
They have an article in the EAA mag, Sport Aviation. It reads more like an ad and installation log, but it has great background info. I would read it if you have not. I did research this and talked to them several years ago, just before they released the kit. I had a RV-4 at the time. I elected not to go with the kit, but not for technical reasons. I think the set up is sound.
Here is there site with all the info:
http://www.safeair1.com/HWA/er_tanks.htm
They have two designs, but they both transfer fuel from the aux to the main tank on the respective side. From there it is the standard fuel feed from main tank to engine. The original system used a gravity and a valve. The newer design for the RV-7/8 (which also will work on the RV-4/6) uses an electric pump to transfer fuel to the main. The original design works on the principal the aux tank is higher (due to dihedral of wing) and will drain into the main tank if its partially/mostly emptied. The reason for the difference or change to a pump transfer (using a typical Facet electric pump) is because of the brackets on the back of the RV-7/8 tanks, there is no (good) place to run the aux fuel line. I also suspect the original gravity feed took longer to transfer with just gravity, but than who cares, as long as it transfers fast enough to feed the engine.
Bottom line whether using the gravity/valve or electric pump they both aux tank setups feed into the main where fuel is feed to the engine just like usual.
He are the diagrams and description.
http://www.safeair1.com/HWA/er_system.htm
Van's aircraft made a comment in the last RVator saying they don't see a need for it (more fuel). Their logic is most of the time you don't fly for enough to justify it but always have to carry the weight. I have been Van's RV builder, pilot since about 1987. I remember when Van said constant speed props are too expensive and pilots don't want to spend the money. He changed his mind later when he put a c/s prop on one of his RV's many years ago. Well now all of Van's factory planes have C/S props, and yes pilots are willing to spend the money as the De facto prop choice.
In a word the extra fuel is a personal choice. This one (extra tanks) does have cost and weight implication. Other wise operationally it has no effect if you are not using them except the dead weight.
I have looked at their set up and it looks well designed, simple and almost fool proof. However as mentioned the difference between the RV-4/6 and RV-7/8 setup. The RV-4/6 setup uses gravity and a valve. The RV-7/8 uses a pump to pump into the main tanks. The latter has one caution. If you pump fuel from the Aux to the main too early (before there is room in the main tank) you will pump fuel overboard out the vent. You can opt for the RV-7/8 electric pump setup for the RV-4/6. It is not a big deal but it is not fool proof. You can imagine if you screw up and try for max range you may be in for a surprise. I think accurate fuel level gauges is a must, especially with the aux tank and max range flight.
I use to fly the Piper Apache/Aztec. It had two main tanks (36 gal each) and two wing tip aux tanks (18 gal each), for a total of 108 gal. With the O320's at 65% cruise I was burning 16 gal/hr. I had total endurance of close to 7 hours!!! at 160 mph true. Frankly on cross countries, after 4 hours I usually had enough any way and would land for a break. To go another 2-3 hours was possible but sometimes not desirable, at least from the stretching the legs stand point. I did the record flights sometimes and admit having fuel was great for local flights since I commuted about 70 miles round trip 5-6 days a week. I had to fill up less. So sometimes extra fuel is a blessing and a curse. The RV-6 will not fly the same with the mains and aux fuel fuel as with light fuel (no aux 1/2 main).
Sorry I have no first hand experience but I met the principals at Oshkosh when they first flew their RV-4 and where just starting to talk about selling it as a kit. To me it looks like a well engineered, clever and simple system that should be reliable. They do offer a pump timer that automatically shuts it down after the transfer starts, after a set time so you don't forget to turn it off later. Transfer takes from my guess about 10-14 minutes? The only other thing I can think of is pumping fuel overboard by transferring too early. Besides always carrying the weight and expense of the kit I see no big negative, but do you really need the extra fuel? To me going cross country landing at little strips, hanging out and soaking up the local culture (greasy spoon, petting old airport dog or walking around looking at cool planes and talking to old timers) is fun as going more miles in a day.
I have the RV-7 and see that the RV-4/6 would benifit more, but I flew from Seattle to Phoenix a dozen times round trip in my RV-4 and it took one fuel stop half way. One time I had a HUGE HUGE tail wind and over flew my Redding/Redmond California stop and continued towards PHX, flying another hour to Bishop California (in the Sierra Nevada Mountains). In theory with a little extra fuel, say 4 gal I could have made it to North Phoenix with no fuel. I don't do no fuel, but with an extra 9 gal it would have been doable in this one case with min reserves (I don't do min reserves either, at least planned). However with out the tail wind even with the 9 gal of extra fuel, I would always have to stop once, whether 1/2 way or 2/3rds, making it all the way with no fuel stop would be not possible, at least with out adding a lot more than 9 gal. So what is your mission?
I have a plan to add fuel with a small 10 gal drag racing fuel tank just behind the seats and tap into the main fuel selector. I know it will work. The plus part is it will be easy to install and remove when not needed. The negative is there is fuel in cockpit, it takes space and has affect on the CG and baggage. Good luck, let us know what you do, experience, pictures if you go for it. George
