Steve Beckham

Please put big tanks in it Steve so you can run the EAA 500 mile
races flat out. 220 HP at .5 BSFC is 110 pounds of fuel per hour
or 17.6 gallons an hour. 500 miles at 220 mph is 2.3 hours or
40 gallons. 20% more at .6 BSFC running it rich. How much
can you get in the wings?

Paul Lamar



Paul, the RV-3 wing tanks hold only 15 gal per side for 30 gal total, so
it
will take aux tank(s) to get to 40 gal.The simplest would be to make and
fit a tankin the baggage compartment which is right behind the seat.Will
have to see if the cg numbers work when a 10 gal rear aux tank would be
full. At least one of the two batteries was tentatively going back there
and
that is where the two nitrous tanks were installed in the first -3 for
climb
work. If cg works, could have fitments for both nitrous tanks and auxfuel
tank in same area--removing one or the other.Nitrous intended only for
climb work..

Don't relish the thought of an aux tank behind me in an off
field landing.Another option could be to make tanks out of the fiberglas
wing tips and use the dihedral to gravity feed to each wing tank.John
Johanson did that on his 'round the world RV-4 in conjunction with
fuselage
header tank and a tank that took up the space where the rear passenger
would
normally sit.But the wings and wing tanks on my -3 are already built.
Will have to see about drilling requisite holes in each wing tank and
installing sealed bulkhead fittings to plumb line to each wing tip.But
don't relish the thought of drilling off wing skins to route lines then
redoing all those skin rivets.

A fuselage tank was originally used in the
RV-3 so there is some roomin front of the pilot. But I modified the
fuselage cross section in that area (smaller) where the standard fuselage
tank would normally be installed.There may still be room for a customtank
in that area but probably would result in a nose heavy cg in conjunction
with wing tanks.

Am concerned about doing anything that adds weight.
Power-to-weight is the name of the game in climb.That would argue for the
tip tanks as most of the basic tank structure is already there except for
a
wall to seal off the fuselage side and when full in flight would keep the
load off that dinky spar.More than once I've climbed out of the RV-4 and
looked over at RV-3 on the gear with the wings off and am struck by the
difference between the two.

The RV-4 spar is a bridge truss by comparison.
This -3 spar has been strengthened in excess of standard, but thestandard
RV-3 simply was never designed for the target speeds. Alan's RV-3 withour
Superlight engine had no serious attention to drag reduction and it would
consistantly loaf along at 200 mph true, turning below 5,000 rpm, burning
9
gph With that engine or the Mazda conversion engine it would
cruise above VNE. At full chat, the aeroelasticity of that airframe is to
be taken serously. But bottom line is that you are right--for that longof
a cross country race, we need more fuel. Any ideas welcomed.

Steve Beckham

You need to go over 220 to get the $1000 E.H. prize Steve :)
Tip tanks are good as they reduce the bending loads on the spar.
I did a rough scale 3D of the RV3 wing tip.
This is a crude approximation of the volume
of a tip tank using the stock wing tip dims.

3779 cubic inches.. one gallon is 231 cubic inches.
That is about seven gallons per tank max.
Probably closer to 5 gallons in the real world.
You could make them a bit wider by a couple of inches
and add a lot more. I can do a better 3D if you need
one.

Paul Lamar


  Thanks Paul.Your notes are helpful reference.Will scope out the
  logistics of what will be involved in retrofitting the wings
toincorporate
  aux. tip tanks.Regarding tanks, are you aware of a source for a one-way
  check valve that requires very little pressure to actuate?....to
installin
  the tip tank gravity feed fuel lines to block reverse flow?It seems that
  putting in tip tanks will exacerbate the existing problem with standard
RV
  tanks of fuel running out the tank vent lines in steep banks.
Theassumption
  is that the tip tanks will require separate vents and if that is the
caseit
  seems as ifthe main tank would pee like crazy out the interconnect line
  and tip tank vent in a bank absent a check valve in the line.Am notaware
  of such a lightweight aircraft quality valve thatis actuated by very low
  pressure.Folks who have put tip tanks in RV's or other ac may have aline
  on them.The pressure side of the fuel injection system has one-way check
  valves but they are all require too much pressure to actuate.

  Steve Beckham

  Seems to me it all depends on the fuel pumps used surprisingly.
  A positive displacement roller vane pump like the Airtex, that can
suckfuel from
  a 7 foot head, would be better as it could generate a negative pressure
  in the tank over coming any spring pressure in the one way valve.
  The Walbro internal gear type may not be as good but I have not tested
  it yet. I have one and if I get time today I will test is
  as the rig is still set up. Do you have your fuel pumps yet?

  Here is a brass 1/4 NPT check valve with 1/2 psi cracking pressure.
  Not exactly light weight at .22 pounds but size helps when it comes
  to cracking pressure. Perhaps you can take the guts out and make
  an aluminum housing.

  One foot of water is 64/144 or .44 psi so this should work with a
  good pump. Gasoline is lower density than water so the head pressure
  of gasoline should be lower.

  http://www.valvestore.com/prodinfo.asp?number=537502

  Here is another but rather high buck.

  www.pegasusautoracing.com/pdfs/079.pdf


  Paul Lamar



Paul--I didn't do a good job of explaining the problem.The issue is not on
the suction side but simply gravity pressure draining fuel out the tank
vents on the low wing during steep banks. Problem is the worst the fuller
the tanks.It's more annoying than problematic normally, but need to keep
as much of the fuel on board as possible.

Steve Beckham

I hate to bring this up but given the nit pickers on here somebody will :)

In theory during coordinated turns there is no lateral load on the fuel.
Only in a slip which is a good thing that everybody should practice
as it can get you into a small field in case of problems.
I digress.

Worst case scenario is 90 degrees 20 foot head. That should not
over come a properly installed check valve according to these
dwgs. Air goes in the tank but fuel stays in IMHO. Am I missing
something?

Paul Lamar


Hi Paul,
You didn't miss something, Steve did. He misunderstood the placement
of the check valve and confused it with the check valves built into
some fuel pumps. As I understood your idea is to put the "half pound
cracking valve" into the VENT line. Then allow the suction from the
fuel pump to "crack" it open as fuel is withdrawn from the tank. In
an uncoordinated turn, the fuel sloshing outboard would create a pressure
in the VENT LINE greater than the half pound required to close the
newly installed check valve. The valve would then close the vent
(since the inboard fuel pickup can be fuel-less at that moment you
are hopefully on "BOTH" at the selector). The fuel pump reference is
only that the pump used must be capable "sucking" fuel with enough
suction to keep the VENT LINE CHECK VALVE opened. (Some of the centrifical
style pumps will not be able to do this, most positive displacement
pumps will). Any clearer Steve?   Dave M

Thanks Dave that is a great explanation of how it works.

The only thing not to use with a return fuel system is a "both valve".
That is a no no. It can draw only air from one tank being empty
and the engine will quit. IMHO that was involved in a fatal.

The valve was set at 45 degrees between both and one tank,
as I recall, causing the engine to quit I think. Use only left right
and off valves on return systems. Also make sure the detent
is strong as it appears the valve was inadvertently moved
by leg motion.

Paul Lamar


Dave, you are correct.  I didn't take my time reviewing Paul's first note
and misread.  In a quickie test to see if the standard Horner style RV-3
wingtips supplied with would take 5 gal I propped one up against the hangar
wall and filled with water.  Took 5 gal. easily.

Steve Beckham