Van's RV-4 lends itself very well to additional fuel for owners who use their aircraft for cross country operations. Hotel Whiskey Aviation found them lacking, due to limited fuel capacity, in cross country capability.  HWA designed ER Tanks to solve this problem.  It wasn't long before others wanted ER Tanks for their aircraft, and that's how it all got started!


ER Tanks for the RV-4 are $1650.00 + Crating, Shipping, and Insurance. Tanks picked up by the customer, or shipped to addresses within the state of Texas, are subject to 8.25% sales tax.  We DO accept credit cards for ER Tank purchases.  Personal checks, bank checks, or money orders are also fine.


  • Weight - Total System Weight = 10 pounds

  • Capacity = 8.2 Gallons (4.1 per side)

  • Method of Transfer = Gravity Feed

  • Wing Tips we can accommodate - Van's W715, W715-1, and Van's "Old School" Tips these were Van's original "flat top" tips sold with the standard RV-4 and RV-6 kits

  • Approximate Location of ER TANKS = 73.00" Aft of Datum


System Description / Operation:

ER Tank Systems are comprised of two tanks (one for each wing), tubing, fuel caps, hardware, and all the fittings required to perform the installation. The only component not supplied is a dollop of Pro-Seal. Each tank holds approximately 4.1 gallons, adding just over 8 gallons to the total capacity of the RV-4.


ER Tanks, for the RV-4, utilize a "Gravity Feed System,” This is Hotel Whiskey's preferred system because it is light weight, simple, has fewer moving parts, and is less expensive to purchase and maintain.  The "Gravity Feed" ER Tank System uses no pumps and requires no on-board electrical power or guages. An electric pump transfer option is available for $100.00.

Fuel flow is controlled by TRANSFER VALVES.  When the transfer valves are opened, fuel moves from the ER Tanks to the MAIN tanks. (Each ER Tank is vented individually)  There are other issues to consider, but when a system is properly installed, gravity will cause the fuel to transfer from the ER Tanks to the MAIN tanks.

If we were to feed the engine directly from the ER Tanks, which we don't, each ER Tank would require a sump and fuel indicator.  By pumping the fuel into the mains, we utiize the sump and fuel indicators from the mains.  The Installation Manaul goes into further detail but the gist of it is to transfer fuel the same time, every time.  By doing so, you learn what a normal transfer "looks" like.  Once you complete the installation and testing, you'll have total confidence in the system.

Normal operation (in flight) will require you burn enough fuel from the MAIN tanks to transfer the entire contents of the ER Tanks.  Fuel should be transferred simultaneously from both ER Tanks only during straight and level flight.  This prevents imbalances in the aircraft, and eliminates the requirement to keep re-trimming the aircraft.  If you transfer individually, one wing would keep getting "lighter" as fuel is transferred, and require constant trimming.

Below you can see how the ER Tank is located just ahead of the wing spar. This configuration minimizes changes to the empty and operational C.G. characteristics of your airplane.

The empty weight of the entire ER Tank System (Left & Right) is approximately 10 pounds but you will still need to re-calculate the weight and balance data for your aircraft and incorporate this information in your revised flight operations manual.



All ER Tanks are pressure tested for integrity before shipping.  No welding is required by the customer.  (Some images may depict ER Tanks during the fabrication process, and may not represent the finished product.)

ER Tank Systems include a SAMPLE Flight Manual Supplement and recommendations for testing and transferring fuel.  However, it is up to the owner/builder to perform proper flight testing, procedures, and insure proper in-flight ER Tank operation.

See the "Benefits" link from the horizontal menu bar above to learn why ER Tanks might be an excellent addition to your RV-4.


Aerobatics are not authorized when ER Tanks contain ANY amount of fuel.  Owner/Operator must stay withing Van's recommendations for aerobatic flight.  We have NOT conducted Spin Testing with ER Tanks, as a result, we DO NOT recommend spins in aircraft equipped with ER Tanks

Things to Consider:

If you're contemplating the installation of ER Tanks in your RV-4, you should consider purchasing a "Bung Kit".  Select the "Bung Kit" link under “tank accessories” from the horizontal menu bar above to learn why this might be a good idea.


ER Tanks must be installed in a completed wing.  If you install the tanks before the wing is finished, you may not have access to buck the wing skin rivets.  Of course, it's more efficient to install ER Tanks in a aircraft that's under construction but they are intended to be easily installed in flying aircraft, too.  We would estimate 25% of ER Tanks sold are installed in aircraft that are already flying.

Allow three full days for installation.  If you're aircraft is still under construction with the wing root fairings and wing tips removed, and no worries regarding paint, you're installation will go a little quicker.  However, nothing about installing ER Tanks is difficult, it's just takes some time.  For the most part, ER Tanks can be installed by one person but there are times when having two people can really speed the process.

ER Tank Kits are complete.  The only thing not included in the kit is Pro-Seal.  You'll only need a dollop (about the size of a golf ball) for the entire installation.  If you don't have any, check with builders in your local area or put a request out on one of the online forums.  (Make sure it's not expired!) 

Installation Services:

If you're not up to installing ER Tanks yourself, HWA offers installation services.  For more information, click on the "Tank Installations" tab of the horizontal menu above.

How to Order:

Due to the popularity of ER Tanks, and the expansion of our product line, there has been a backlog for ER Tanks for quite some time.  The current backlog is approximately 6 weeks.

If you want to reserve a slot, we ask for a $200 deposit.

Please click here to order.

Wing Tips: 
Two seat RV's have been produced with 3 different types of wing tips. We need to know which ones you're installing on your aircraft. I'll do my best with verbal descriptions, but I've attached an image to help clarify the issue.


The 3 types of wing tips most common to the RV-4 are:

  1. "Old School" - These are the original tips provided in the original RV-4 kits. The "old school" tips are flat on top, essentially extending the upper wing skin to the outer edge of the wing tip. The bottom surface of the fiberglass wing tip curves upward to meet the straight upper surface. The outboard edge of the wing tip runs parallel to the tip rib of the wing.  (The other two styles taper)  There were no factory cut outs for the lights. (that had to be done by the builder)

  2. Van's part no: W-715: The bottom of this wing tip is flat and the upper surface curves downward. The tip also gets wider as you move aft along the tip rib. These tips were originally introduced on the RV-7 and give the aircraft a slightly longer wingspan, as well as a "bat wing" appearance. The most distinguishing characteristic is the curved trailing edge as shown in the attached image.

  3. Van's part no: W-715-1:  At first glance this wing tip resembles W-715, but it has a straight trailing edge. It also has a larger cut out area for mounting landing and position lights.

The first step is to compare the upper surface of the tips. If it's flat, it's "old school". If it's not, it's one of the two shear tips. The attached image will show you how to quickly determine which tips you have. If you have any problems, please don't hesitate to contact me. We want to be certain since ER Tanks are not interchangeable with the different wing tip styles.



RV-4 ER Tanks have a critical dimension.  We can build ER Tanks so they fit aircraft "built to plans" but we prefer to build them to fit YOUR aircraft.  The difference is an easier installation and a perfect fit.  The dimension we'll need is the distance between two nose ribs.  You will be hooking your tape measure to the inside of one lightening hole and measuring to the same location (lightening hole) on another rib.

Which Lightening Hole?

The first lightening hole FORWARD of the main spar.

Between which Nose Ribs?

Measure the distance between the FIRST nose rib OUTBOARD of the main fuel tank. Be careful, there are two closely spaced nose ribs here.  Make sure you latch the tape to the one closest to the main fuel tank. The OUTERMOST nose rib on the wing. (wing tip nose rib)  Note the measurement and move to the other side of the aircraft.  Once complete, email the left/right dimensions, along with your wing tip type, to info@hotelwhiskeyaviation.com.

Is this Difficult?

No, only the wing tips need be removed for this measurement.  Use the widest tape measure you have. It will stay erect at greater distances and make the job a little easier.  A good flashlight or drop light is all you need.  The main fuel tanks can remain in place for this operation, though they will require removal for the installation of your ER Tanks.  With a buddy helping, I would expect the measuring operation to take less than an hour.  Running solo, maybe an hour and a half.