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wsimpso1
Joined: 04 Nov 2018
Posts: 36
Location: Saline MI
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 Posted: Fri Feb 27, 2026 6:47 am Post subject: Re: Fuel System Manager |
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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 you’re 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-7’s 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 Pete’s 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.
Pete’s 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-51’s 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 catastrophe’s 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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911pete
Joined: 09 Aug 2018
Posts: 18
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| Posted: Fri Feb 27, 2026 7:39 am Post subject: Re: Fuel System Manager |
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Billski
Thank you for your comments/concerns.
As I said earlier I’m a builder and I’m 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 didn’t make it up. The plumbing hasn’t 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. I’m still trying to get the specified solenoids. They are Duke’s 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 doesn’t 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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wsimpso1
Joined: 04 Nov 2018
Posts: 36
Location: Saline MI
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| Posted: Fri Feb 27, 2026 4:57 pm Post subject: Re: Fuel System Manager |
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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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