Today's Message Index:
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1. 06:03 AM - Re: Fuel System Manager (911pete)
2. 06:48 AM - Re: Fuel System Manager (wsimpso1)
3. 07:40 AM - Re: Fuel System Manager (911pete)
4. 04:58 PM - Re: Fuel System Manager (wsimpso1)
Message 1
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| Subject: | Re: Fuel System Manager |
Eric
The HPFS can stay open in Auto. It just needs to be closed in the off position.
That is what is used during engine start.
Im using the Garmin G3X with a GAD27. The documentation says its for 12 volt systems,
but the garmin engineers I spoke with say it will handle 24 volts. I can
also use the GEA24 which is part of the system as well for the warning input.
It is rated for 24 volts. Either of these units can be configured for the warning
to be active high or low.
Pete
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| Subject: | Re: Fuel System Manager |
I have several points that I feel must be made. Maybe I am talking to Pete, maybe
I am talking to other folks.
Please, please, please prevent your system from turning into a Greek Tragedy. Pay
attention to what Bob Nuckolls has been preaching, page 17-14 of The AeroElectric
Connection:
First Law: Things break
Second Law: Systems shall be designed so that when things break, no immediate hazard
is created
Third Law: Things needed for comfortable termination of flight requires backup
or special consideration to insure operation and availability
Forth Law: Upgrading the quality, reliability, longevity, or capability of a part
shall be because youre tired of replacing it or want some new feature, not
because it damned near got you killed
We are talking Experimentals. Usually that means every individual airplane you
see is unique. The differences may be small but they can matter. This counts in
RV-7s where there are thousands, so it certainly counts in the Viper Jet where
Wikipedia indicates seven (7!) have flown. More complete Apollo systems flew
to the Moon and every one of them was an individual. I strongly suspect that
Petes Viper jet is not a one of a line of well proven birds, but is different
in a bunch of ways from the seven that have already flown.
Petes Viper Jet is not being built to the plans and with the intended and only
modestly demonstrated fuel transfer system. Pete is changing the design. He does
not have the computer nor the valves specified, and that also means he is most
likely changing the plumbing and wiring to accommodate the new pieces. Given
this collection of design changes, I seriously wonder how many other changes
are being made The different pieces may or may not be completely interchangeable.
You should have a plan up front, criteria to check, a decision process,
then check the criteria comparing new to old, and stick to the decision process.
If you need a different valve or bigger tubes or less restrictive connections,
so be it.
In the valves, your going in assumption might be that the original design was OK.
Was it? With only seven ever flown, I do wonder just how well it worked. I
really do not expect any info on how well it aged. Has anybody commented on the
main drawing down in some flight modes? How many emergencies have been declared?
I would want to know the pressure drop across the valves vs flow rates through
them. Find out what the intended valve did and what the new one does. The manufacturers
or the internet or maybe the Wayback Machine can all be checked to get
headloss vs flow rates in these parts. The specified 150 psi valve might have
been schemed out to lose only a couple psi at your max burn rate, but a 3000
psi valve might not be so free flowing - and work just fine for its intended
use. If the new valve is more restrictive, you may find the system can not keep
up. I think it is better to make sure your jet pump and solenoid valve and
plumbing will keep the main full while you are still on paper.
Back to whole system design. You are changing that too. Different computer, different
wiring, different plumbing. Maybe no big deal, maybe the original system
was just fine - until something broke or wore out or caught some crud on the
finger screen the others did not. Seven flown with a different design gives little
comfort Typical design guidance in homebuilts is system must flow 150% of
max climb flows. This gives margin for reality. In your case it allows for climb,
and then cycling draw from the selected wing tank to top up the main. Maybe
you want more than 150% max flow from the transfer system. Whatever you decide
is needed, make sure you have it.
Then we get to system reliability. There are a lot more T-51s out there and many
have had what I would call fatal flaws in electric system design. Single thread
(no redundancy) schemes and cascading failures (one failure causes other failures)
have been seen. I expect that this airplane has some poison waiting for
you somewhere. I advise you to get critical on everything electrical. Well
crafted primary and backup systems for flow of electrons and fuel are terrific
for turning potential catastrophes into a comfortable trip to an airport with
an FBO and nice topic for the EAA chapter meeting.
At least do your Failure Mode Analysis to spot the things that can go wrong and
where you should be scheming out the backups. Then you will be able to look at
it as an FMEA and see which backups make sense and where you think you have
acceptably low consequences and probabilities. Even a simple review would have
prevented at least one (perhaps two) T-51 engine cutoff accidents, where the
landing gear electro-hydraulic pump issue cascaded to electric loss Luckily no
injuries in these accidents, but major damage to beautiful hand crafted airplanes.
We can do better... And I hate to read an NTSB report about your bird.
Billski
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| Subject: | Re: Fuel System Manager |
Billski
Thank you for your comments/concerns.
As I said earlier Im a builder and Im building this as designed. The description
of the operation of the fuel transfer computer in the original post is from
a document that Viperjet supplied. I didnt make it up. The plumbing hasnt been
changed. The jet pump is the one specified by Viperjet, the fuel level sensors
are as specified, the system switches are as specified. The high pressure solenoid
is as specified. The only thing not as specified is the controller and
the low pressure solenoids. Im still trying to get the specified solenoids.
They are Dukes part number 5825-00-1 if you have any suggestions/solutions for
this part of the problem.
Now what can go wrong. Remember this is a fuel transfer system. It doesnt need
to supply 150% of max power fuel flow. The main 80 gallon tank does that. It
is also built as designed. Pretty simple, just a tank with a shutoff valve then
a boost pump and fuel filter into the fuel inlet on the engine. About 18 long
path of 1 inch (-16) fuel line and fittings. You could take off and safely
fly with empty wing tanks, but it would be a short flight 25-30 minutes. So
if the fuel transfer system fails inop the wing tank fuel is trapped and you land.
If the fuel transfer system fails operational you vent fuel and have some
extra time until the wings are empty. You still land in that case.
Pete
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| Subject: | Re: Fuel System Manager |
Two things wrong with the last statement:
Changing parts means it is potentially a new design.
With only seven of these ever having been flown, the original design is not necessarily
a proven design.
It is your butt in the seat. You have a relatively high risk bird, a potentially
riskier fuel system than as designed, and you are making like it is all OK because
a small number have flown. Yeah, but this is your version of the bird and
your butt in the seat during a forced landing. Please take the perspective
that changing the valve might do something bad and that the designers might have
stuck you with some single thread designs in the rest of the bird when some
carefully chosen redundancy would be relatively easy to look for and include.
My concern over the valves was their headloss. The valve supplier lists Cv as 0.86,
which means at 1 psi across the valve, it will flow 0.86 gpm of 60F water.
Kerosine is about 1.5 times as viscous as water, drop it to 30 degrees and it
roughly doubles again. You cited 2 gpm in non-takeoff modes. To refill the tank
quickly, you probably want at least double that, so 4 gpm from the wing tank.
That then becomes 1.5*2*4/0.86 = 14 psi of head loss in valve. If you want
quicker refill of the main tank, you will have even more headloss for that jet
pump to overcome.
You add up the head loss at 4 gpm of the plumbing, the valve, any filters, and
compare that to the jet pump pressure at that flow. Maybe it is OK, maybe you
need a less restrictive valve... I strongly suggest you check the Cv or headloss
from the previously specified valve.
As for the rest of the system, I do suggest that you get into chapter 17 of AeroElectric
Connection and then take a serious look at your electrics. Consequences
of an off airport landing or dark cockpit need to be considered in your failure
mode planning.
Billski
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