RV-8 Project N804PT

Extra Capacity Fuel Tanks

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Note: Construction of the tanks is now complete, but it will still be some time before flight testing will determine if the design works as planned. I designed the tanks and made the drawings myself, with some input and help from friends who work as engineers. If things work out well, I will credit them for the help at that time (I don't want anyone else taking heat for my mistakes). The success (or lack thereof) of this project will not be determined until the airplane actually flies!

 

Many of the best features of the RV series of aircraft are realized because the airplanes are very simple and light. Therefore, any changes which tend to make the aircraft heavier or more complex should be viewed with a good deal of skepticism. A careful cost/benefit analysis of any contemplated changes should be made.

I wanted increased fuel capacity, but I did not launch into this part of the project lightly. There are at least two after-market extended range fuel systems currently available for the RV-8, and with an already constructed wing, one of these systems is undoubtably the best way to go. I started planing this modification before my wing was built, so my plan will allow extended range tanks to be built into the wing during the original construction.

A comparison of my modification with the two best aftermarket installations I am familiar with will demonstrate why I chose this system:

  Tuckey Tanks SafeAir1 Tanks Flymore Tip Tanks
Added Fuel Capacity 34 US gallons 9 US gallons 25 US gallons
Added Empty Weight 8 lbs 10 lbs ?
Extra Cost (exclusive of installation time) ~ $300 US in materials $1725 US $4000 Aus + tax and shipping (from Australia)

It should be noted that this comparison does not include the cost of any required labor for the installation of the tanks. The Flymore tanks are basically a bolt-on application replacing the original wing tips, with only the plumbing being at all involved. The SafeAir1 tank installation is slightly more complicated, but still very easy compared to the effort required for my built-in tanks. My tanks will add considerably to the time required for the construction of the wing, and cannot be added to an already completed wing.

Although contruction of the auxilliary tanks is part of the wing construction and is documented there, since the actual construction methods are not part of the Van's construction manual, details about how I actually built the tanks can be found at the link below:

Tank Construction Details and Pictures

 

 

EXTRA FUEL CAPACITY PROJECT FOR RV-8 N804PT

PROJECT OBJECTIVE

The RV-8 has a standard fuel capacity of 42 gallons. Although this is an adequate capacity for the typical aerobatic or short VFR cross country mission, on an IFR flight of more than a few hundred miles there is insufficient fuel for reserve and alternate purposes. The purpose of this project is to increase the fuel capacity for longer range IFR capability, without sacrificing the light-weight handling qualities of the RV-8 on flights where the extra fuel capacity is not needed.

PROJECT OVERVIEW

The RV-8 uses very conventional semi-monocoque riveted aluminum construction. The wing is built around a standard I-beam type main-spar at approximately the 30% chord station. The aft body of the wing section is conventionally built up from stressed aluminum skins riveted to the spar and to aluminum ribs. The forward portion of the wing section is a D-tube. The outer section of this D-tube uses an aluminum skin over aluminum ribs, with the skins riveted to the main-spar and thus forming the aft portion of the D. The inboard section of the D-tube has an additional tank baffle slightly forward of the spar which finishes the D section as well as forming the rear wall of the fuel tank. The tank is removable for repair or maintenance, being attached to the wing by machine screws fastened to nut plates riveted to the main spar and outboard leading edge section.

This project adds a tank baffle and tank ribs in the outboard portion of the wing leading edge D-tube, thus forming an additional fuel tank in each wing of 17.4 gallons capacity. Unlike the standard, inboard fuel tank, the outboard tanks are not removable.

In order to meet structural and operational considerations, the outer fuel tanks are to be used only on longer flights where additional fuel is required. Aerobatic flight is prohibited with fuel in the outboard tanks, and landing with fuel in the outboard tanks is only permitted in case of emergency.

CONSTRUCTION DETAILS

The standard RV-8 outboard leading edge consists of a .025" 2024-T3 aluminum skin, with 6 ribs. The spacing of the ribs varies, with the inboard ribs being spaced 10.5" apart, and the outer ribs spaced slightly farther apart, with an escalating distance between ribs progressing out to the wing tip.

The standard RV-8 wing tank consists of a .032" 2024-T3 aluminum skin, with 7 ribs spaced at 9" intervals (5" for the last rib at the fuel filler cap). The floor of the tank is stiffened with two runs of .75" angle stiffeners running spanwise across the bottom of the tank. The baffle, which forms the aft wall of the tank, is .025" 2024-T3 aluminum. Aluminum angle brackets transfer loads between the baffle, ribs and spar at each rib station.

This modification leaves the structure of the wing tanks completely unchanged. The outboard leading edge retains its original skin and the outboard-most and inboard-most ribs. This leaves the attachment of the tank to the wing, the tip fairing to the wing, and the outboard skin to the wing spar completely unchanged. The 4 intermediate ribs are removed, being replaced with 6 tank ribs. The spacing of the outboard ribs is thus reduced to the same 9" interval as the tank ribs. The floor of the new tank is reinforced with the same skin stiffeners as the original tank. A baffle made from the same stock as the standard tank forms the aft wall of the new tank as well, with similar angle brackets to transfer loads. The entire modification adds approximately 3 lbs of structure to each wing.

To see a larger version of the wing plan, Large Wing Plan

STRUCTURAL CONSIDERATIONS

The following areas of concern have been identified:
1 - Wing bending moment on the spar center section
2 - Bending and shear load on the wing spar in the area of the new fuel tank
3 - Floor loading on the new tank required to support the weight of fuel in the tank

1) In normal flight, the bending moment on the spar center section is actually reduced by carrying fuel in the auxiliary tanks. This bending moment is increased in the case of ground operations, but since the aircraft is restricted against landing with fuel in these tanks, and the bending load for taxi and takeoff is not significant when compared to the landing loads, no adverse effects are anticipated.

2) Analysis of both bending loads and shear loads on the wing spar of the modified wing shows no increase in either load for any station along the spar. Instead, over much of the spar, both bending moment and shear stress are significantly reduced with fuel in the auxiliary tanks.

3) The standard tank is rated to 6 g's with its .032" skin. Since all other structures are identical or stronger than the standard tank, the auxiliary tanks will be good to at least 4.6 g's with their .025" skins. This is more than adequate for non-aerobatic flight.

WEIGHT AND BALLANCE CONSIDERATIONS

The standard recommended gross weight of the RV-8 is 1800 lbs, with a maximum of 1600 lbs for aerobatics. With an anticipated empty weight of approximately 1100 lbs, the useful load is 700 lbs. A 76.8 gallon fuel load (460 lbs) would leave 240 lbs available for cockpit and baggage loading with no change in the standard gross weight.

Since the additional weight of fuel in the outboard section of the wings adds no significant structural load to the aircraft in normal flight, allowing the maximum gross weight to be increased to 2000 lbs in the case of a maximum zero fuel weight of 1540 lbs or less would guarantee that any weight in excess of standard limits is limited to the outboard wing tanks, and thus keep the same structural margins for non-aerobatic flight as the standard aircraft. This would, however, adversely affect the performance of the aircraft as noted below.

The allowable CG range is 15% MAC to 29% MAC for normal flight, and 15% MAC to 26.5% MAC for aerobatic flight. The arm of both the inboard and the outboard fuel tanks is 80.0" aft of datum, which works out to be at 17.2% MAC. If the zero fuel CG is within limits, it is not possible to move the CG out of limits by adding fuel.

Since the additional fuel is effectively on the longitudinal CG of the aircraft, there will be no adverse effects on the pitching moment of the aircraft. However, additional fuel out in the wings will increase the angular moment of inertia of the aircraft around both the roll and the yaw axes. This is anticipated to result in detrimental effects to handling as noted below.

 

PERFORMANCE CONSIDERATIONS
(180 hp engine)

With gross weight restricted to 1800 lbs, no significant performance considerations apply.

In the case of 2000 lb operations (with zero fuel weight of 1540 lbs or less), the following estimates of performance apply (1800 lb performance with 42 gal fuel in parentheses):

Top Speed 183 kts (184 kts)
Cruise (75% @ 8000') 159 kts (160 kts)
Cruise (55% @ 8000') 153 kts (155 kts)
Stall Speed 53 kts (50 kts)
Takeoff Distance 690 ft (575 ft)
Landing Distance* 560 ft (500 ft)
Rate of Climb 1300 fpm (1650 fpm)
Ceiling 19000 ft (20500 ft)
Range (75% @ 8000') 1335 nm (695 nm)
Range (55% @ 8000') 1590 nm (816 nm)

*Landing weight above 1800 lbs permitted only in emergency

HANDLING CONSIDERATIONS

With no fuel in the outboard tanks, the handling and stability of the aircraft will be identical to a standard RV-8, so no special considerations apply.

When fuel is carried in the outboard tanks, however, the added moment of inertia about the yaw and roll axes will have several implications. Although the roll control will be slightly affected, this is not anticipated to be a major concern in non-aerobatic flight. The added yaw momentum could, however, affect the aircraft's spin characteristics. Because of this, a restriction against any aerobatic flight, including spins, is placed in the aircraft's flight manual in the case where any significant fuel is present in the outboard tanks. Ground handling may also be affected, but since this is typically more of a problem on landing than take-off, a restriction against landing with fuel in the outboard tanks is also placed in the flight manual.

PLUMBING DETAILS

The details of the standard wing fuel sumps and porting are entirely unchanged. The regular wing tanks are vented to the auxiliary tanks, however, instead of the standard system of running the vent through the fuselage. The auxiliary tanks are then vented overboard through NACA type scoops on the underside of each wing. An additional fuel port is located at the low point of each auxiliary tank. An electric low pressure fuel transfer pump can be used to ensure fuel transfer from the auxiliary tank to the main tank.

Because the main tank vent is ported to a low point in the auxiliary tank, as fuel is drawn from the main tank, it will be replaced with fuel from the auxiliary tank until the auxiliary tank is emptied. Due to the dihedral of the wing, there is no significant pressure head (suction in the vent line) in coordinated flight. During uncoordinated flight, the pressure head is actually less than that of the standard fuel vent arrangement, even not taking credit for any positive pressure generated by the NACA scoop of the auxiliary tank vent, when the auxiliary tank is empty. If the auxiliary tank is full, the pressure head could be somewhat higher in uncoordinated flight than with the standard system. Operating the transfer pump would not only eliminate this pressure head, but provide positive vent pressure instead. For this reason, operation of the transfer pump is required for takeoff with fuel in the auxiliary tanks.

 


FUEL MANAGEMENT AND OPERATIONAL CONSIDERATIONS

When there is fuel in the main tanks and no fuel in the auxiliary tanks, a check valve on the transfer pump prevents fuel from flowing to the auxiliary tank. In this case, fuel management is identical to a standard RV-8.

If there is fuel in one or both auxiliary tanks, normal operations will result in fuel from the auxiliary tank replacing fuel burned in the main until the auxiliary tank is emptied. The only changes from standard RV-8 fuel management are to ensure the auxiliary tanks are emptied before landing or conducting any aerobatic flight, and operating the auxiliary transfer pumps for takeoff when fuel is present in the auxiliary tanks.

When fueling the aircraft, the main tanks must be filled before the auxiliary tanks are fueled. Opening the main fuel caps could result in fuel overflowing out the filler neck if both the auxiliary tank and the main tank are full or nearly full.

 


EMERGENCY OR ABNORMAL OPERATIONS

If an obstruction or damage to the fuel system prevents fuel from transferring automatically from the auxiliary tank to the main tank, the fuel transfer pump may be switched on. Since the fuel vent in the main tank provides a return line to the auxiliary tank, there is no disadvantage to operating the transfer pump even if the main tank is completely full.

If the aircraft is landed with fuel in one or both auxiliary tanks, an overweight landing inspection is required before further flight.

In the case a take off is aborted at high speed with the aircraft gross weight above the standard 1800 lbs, considerably more than normal braking energy will need to be dissapated. For this reason, the brakes on the project aircraft will be upgraded beyond the typical RV braking capability.

The standard Cleveland brakes on the RV-8 have a kinetic energy rating of 117,500 foot pounds. Assuming an aborted take-off at 1800 lbs, this allows the abort to be started at a speed of up to 54.3 knots with reserve braking left over. The project aircraft will substitute Grove 56-1A brakes which have a kinetic energy rating of 162,316 foot pounds. This will allow a 2000 lb abort at up to 60.5 knots with the same braking reserve as the standard airplane with normal Cleveland brakes.


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