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 aux fuel
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 custom tank
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 the standard
RV-3 simply was never designed for the target speeds. Alan's RV-3 with our
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 long of
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 to incorporate
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 install in
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. The assumption
is that the tip tanks will require separate vents and if that is the case it
seems as if  the 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 not aware
of such a lightweight aircraft quality valve that  is actuated by very low
pressure.  Folks who have put tip tanks in RV's or other ac may have a line
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 suck fuel
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



How about a facet pump with check valve to transfer fuel from each
outer to each main?  These pumps are pretty light.  Run the engine
from each main for long enough to be sure that it has enough room for
the volume contained in the tip tank, turn on the transfer pump for
long enough to move all of the fuel out of the tip into the main
+1minute.  I believe these pumps won't have a problem running dry, but
you could put a low fuel indicator light for each tip tank on the
panel to indicate when the transfer pumps should be turned off.

I think that this is how the tip tanks installed in single engine
Cessnas are operated.  They don't have much dihedral to get fuel to
transfer by gravity.  Incidentally, the "tip" tanks are actually
installed between the tip rib and the next rib inboard, not like tip
tanks on Bonanzas.   I believe I have read of a group of RV builders
that figured out how to add cylindrical tanks to the RV by using a
hole saw to cut into ribs from the tip.  I think they were able to add
something like 5 gallons per wing.  If this were done in addition to
actual tip tanks, you'd have significantly more capacity.

I know this is essentially how the baggage compartment tanks in the
Cessnas are used - transfer pump with check valve.


Regards,

Matt-