Veloce 600, 6 seat pressurized twin auto-converted engines.

[rv6ejguy]

Errrr. We are talking about using auto engines at 35,000 feet and NOBODY is doing that now nor ever did. Even the 'high altitude' B29 Stratofortress was only certified to 32K.

The research and papers I presented clearly show that there WILL be serious issues with using volatile fuels at 35K and its simply dreaming to expect Mogas with a vapor pressure of up to 14PSI could ever operate at 3PSIA. It wont. It will boil away. If it was easy, everyone would have done it and you would have seen B29's flying at 40K. Its not easy which is why Avgas is specially formulated to have a higher vapor pressure and even with this high octane, still is altitude restricted.

Of course when flying at 25K/7PSIA or lower, Avgas and maybe even some Mogas is stable and wont boil. At these altitudes EFI systems work just fine. We are not debating that. We are looking at what happens when you go higher.

If you think you can simply port all the hot return fuel from your EFI regulator back to the main tank and shoot straight up to 35K, then good luck with that. Any heating of the fuel at 35K will simply boil it away faster than the lapse rate already will. And any pump, even auto EFI pumps will have a suction side trying to take a feed from that boiling fuel. Since none of these have vapor handling systems its not unreasonable to expect trouble.

Nope again. I guess you're not aware that a B-29 holds the USA manned piston absolute altitude record at 47, 910 feet, set in 1946...

Properly designed EFI DOESN'T add any significant heat with the fuel block exposed to ambient air as shown below. It will be COOLED by -65F air here.



Bruce Bohannon's record flight also set time to climb records in class at the same time so if anything, heat soaked fuel would have been pronounced here due to the rapid ascent. It wasn't, the fuel system performed just fine, even at 47,000 feet which is a LOT higher than what you posted originally. Quoted from your post- "As a start, it's not possible to use Avgas at above 22,000 feet easily as its Reid vapor pressure range is only 5.5 to 7 psi At 35,000 feet, standard pressure is only 3.5 psi so all your Avgas just boiled out of the tank and turned to vapor." Flyin' Tiger wasn't using any special pressurized tanks as Bruce said himself. I've explained why the fuel doesn't boil up there- very cold temps- not 100F where VP is measured. Vigilant1 brought this second point up previously.

Your theory has been shown to be wrong by real-world examples. What more can I say? Read my signature quote by Richard Feynman...

Mark never mentioned using Mogas in this thread that I can see. Maybe I missed that part. A cross country plane like this won't be using mogas because it has limited availability at airports. Mark knows this. It may use unleaded avgas if that comes into use, but that will have similar characteristics to 100LL.

 

[Richard Roller]

Found the pump schematics in my old textbooks.

View attachment 115125

View attachment 115126

The fuel is centrifuged to remove any bubbles. I had to look it up and review it. Last studied it a long time ago. Any vapor generated by the pump can still get into the injection system. The bypass valve at the bottom opens if the pump fails, letting the boost pump provide fuel under pressure.

And that boost pump, at least in the Cessnas, has a variable-speed function controlled by a couple of adjustable power resistors in the supply circuit, with a microswitch on the throttle body to short one of the resistors when the MP goes past 19" or so, and the other resistor is shorted by the Emergency side of the boost pump switch. Getting those resistors adjusted right is a pain, as is setting the low and high unmetered fuel pressures. The Lycoming/RSA is so much simpler, since its mechanisms sort it all out.

I worked with these two systems for many, many years. Personally I found the Continental system simpler and easier to maintain. They just worked, very little fiddling with them. It seemed that they were easier to start with a hot engine. Lots of run down batteries with pilots who couldn't seem to get the feel for starting the Bendix system with a hot engine.

 

[Dan Thomas]

I worked with these two systems for many, many years. Personally I found the Continental system simpler and easier to maintain. They just worked, very little fiddling with them. It seemed that they were easier to start with a hot engine. Lots of run down batteries with pilots who couldn't seem to get the feel for starting the Bendix system with a hot engine.

I had the opposite problem. The Lycs in the Cessnas worked well, while the Continentals in the older airplanes were frequently out of calibration. Had one Lyc in a T206H that was hard to start and ran badly right after startup. Finally the system was overhauled. Had a 172ST that was hard to start cold, and traced it to a sticking poppet in the flow divider. PA had machined the poppet with too little clearance and when cold the casting would shrink just enough to trap it shut. Precision came out with an SB on it.

Continentals out of calibration: Often it was the boost pump resistors out of whack. Then Cessna couldn't get the Dukes pumps anymore and supplied Weldon pumps instead, which drew less current while giving the same volume and pressure. Less current meant smaller voltage drops across those resistors and we couldn't adjust them high enough to compensate, so the pump ran too fast and the pressures were too high. That could be lethal, causing much too rich mix in an emergency. Got hold of Cessna and showed them the math and they agreed and sent us new resistors to match the new pump. Fixed it, but never saw an SB on it.

 

[TFF]

Lear Romec pumps are used on turbo Lycomings. Not enough pressure for EFI though, about 24 psi plus differential.

 

[Flyfalcons]

What's the plan for RVSM certification on this?

 

[tspear]

Of course when flying at 25K/7PSIA or lower, Avgas and maybe even some Mogas is stable and wont boil. At these altitudes EFI systems work just fine.

I flew an Aerostar at 28K for a few hundred hours, and it was certified to 30K. (RVSM limited).

Tim

 

[Marc Zeitlin]

What's the plan for RVSM certification on this?

I don't believe there can be one, as:

https://www.faa.gov/air_traffic/separation_standards/rvsm/documents/AC91_85B.pdf

does not seem to have any facility for RVSM approval for Experimental aircraft, at least as far as I can tell. Although I've been to 52K ft in Proteus, we did so inside the restricted areas R-2515 and surrounds. I know that Proteus has been to altitudes above 29K outside of the restricted airspace, but is not RVSM approved (I don't think Scaled ever figured out a way to get it approved, or else it was just too complex and expensive) so the flights were (IIRC) approved on a case-by-case basis, mostly because they were over oceans and there was no controlling agency (again, IIRC).

So, maybe technically capable of flight above 29K ft., but it's not clear whether legally capable will be achievable with reasonable work/cost.

And I assume that was the point of your question.

 

[tspear]

Read 5.5.1 Paragraph #2. would be the only method to do so. As for if it is worth it? For the kind of flying I do, nope.
To qualify, you would need at least two RVSM qualified altimeters, RVSM qualified A/P, tied to said altimeters, along with Nav system.
Based on current equipment, I believe that you mean dual, GTN750s, dual G600 TXi, GFC 500 at a minimum; and that means your avionics are likely more than the rest of the plane before you even start talking to the FAA.

Tim

 

[rv6ejguy]

I think it's safe to say that most homebuilts will be cruising below 29K for a variety of reasons. If you're not doing over 300 knots, ATC probably doesn't want you up there anyway, adding to their workload, mixing with jet traffic.

 

[TarDevil]

I've been to 52K ft in Proteus

If you cared to start another thread describing such and similar adventures I promise to read every word.

 

[C Michael Hoover]

RE: Header tanks for Fuel injected aircraft. Bellanca Vikings with Lycoming engines (IO-540 K (or G) 1E5 do not have a header tank, but all fuel is returned via a ganged fuel tank selector to the tank from which it came. The turbo Vikings, with dual Rajay turbos show altitudes up to 25K in the POH, but I never had mine that high.

 

[Dan Thomas]

RE: Header tanks for Fuel injected aircraft. Bellanca Vikings with Lycoming engines (IO-540 K (or G) 1E5 do not have a header tank, but all fuel is returned via a ganged fuel tank selector to the tank from which it came.

Yeah. That plumbing is a nightmare. I've seen it. Must be $500 in fittings alone in that system.



Some Cessnas have a similar system.

 

[rv6ejguy]

EFI is simplicity by comparison.

 

[PMD]

View attachment 115251
EFI is simplicity by comparison.

I don't see a check valve in the parallel pumps.

 

[Dan Thomas]

I don't see a check valve in the parallel pumps.

They'll be inside the pumps. Oscillating pumps can't operate without check valves. Aircraft boost pumps are sometimes rotary pumps and they have the checks built in.

 

[Dan Thomas]

View attachment 115251
EFI is simplicity by comparison.

Unless we start looking at its computers. Not simple at all.

Your diagram services two tanks. The Bellanca diagram services five: the two mains, two aux wing tanks (not shown; their lines are shown just outboard of the mains) and the aux fuselage tank. Outlets and returns for all, plus venting. A real nightmare.

 

[rv6ejguy]

I don't see a check valve in the parallel pumps.

Built into the pumps.

 

[rv6ejguy]

They'll be inside the pumps. Oscillating pumps can't operate without check valves. Aircraft boost pumps are sometimes rotary pumps and they have the checks built in.

These are gerotor pumps actually.

 

[rv6ejguy]

Unless we start looking at its computers. Not simple at all.

Your diagram services two tanks. The Bellanca diagram services five: the two mains, two aux wing tanks (not shown; their lines are shown just outboard of the mains) and the aux fuselage tank. Outlets and returns for all, plus venting. A real nightmare.

No moving or wearing parts in the computers though, MTBF is hundreds of thousands of hours from our stats.

 

[wsimpso1]

The discussion is now focused on the the real issues for Avgas at altitude and solutions thereafter. I am glad at least there is debate.

Yaaay! I shall add some more to the discussion.

There are actually a bunch of reasons that our airplane fuel systems work with a lot less trouble than you might think from just looking at Reid vapor pressure curves. Some important issues:

• The air at the surface of the fuel in your tanks is covered with air that is coming in from outside, temperature of which drops rapidly as you climb. Usually this starts the flight as a very small fraction of tank volume, and as you go up, the air added gets colder quickly;
• The air above fuel on a single vent tank - typical of gasoline systems - is generally at saturation with fuel vapor and well above upper explosive limit for the fuel mix in vapor form. Once you get to saturation in the air above the fuel, evaporation slows way down;
• The fuel vent pressurizes the air above the fuel. Yeah, at 100 knots indicated and sea level air density, it is only about .24 psi. At altitude, where you may only have 7 psi from the atmosphere, even that low number is a significant help in reducing evaporation;
• The fuel being use is not evaporating from the surface into the air by some some monolithic process. On a molecular scale, you have a mix of small through large molecules, and each type is a mix of low through high velocity. The highest velocities and smallest molecules, when they are close to the free surface, zip right through the surface and are then fuel vapor. The rest - predominantly larger and lower velocity (cooler and harder to evaporate) stay in the liquid. This is why evaporation cools liquids- the hot molecules are leaving in much larger fractions than the cooler fraction;
• Vapor pressures of our fuels are dominated by the lowest molecular weight fractions - loss of even modest amounts of the lighter fractions of our fuel quickly reduce the vapor pressure of the remaining fuels, so evaporation slows down as we lose those light fractions. To use vapor pressure curves to model fuel evaporation at a surface, you would need to do it with a range of fuels with lighter fuel fractions that reflect the evaporated fuel near the surface with the air;
• As fuel evaporates from the surface, the fuel near the surface becomes both cooler and more resistant to evaporation. Yeah, there is some mixing, but it is not high order in most gasoline tanks, and so the fuel remaining becomes more resistant to evaporation, and the fuel at the surface even more so;
• Evaporation is a powerful cooling effect on mixed liquids - wide ranges of molecular weight - and so will cool fuel quite strongly with even modest evaporation.

I seem to remember that there are more effects on fuel boil off, and I will add them if I remember them… In any event, these effects are all in the right direction to allow us to fly substantially higher than we might think we can given the vapor pressure curves we can record with whole fuels. The effect is strong and does account for a lot of the very high altitudes actually achieved vs what one might expect if we look at one effect alone. Add in a modestly pressurized header tank, and your fuel metering system actually works pretty darned well too as you go up.

Now if we get into road diesel and Jet A, things get murkier:

• The air above fuel on a double vent tank - now recommend by some for diesel/jet fuel outfits is constantly turning over the air above the fuel to stay below the lower explosive limit for the air fuel mix - and so the air temp above the fuel tends to reflect the outside air temperature more effectively, further reducing evaporation by methods described above;
• The tendency to use the huge amount of returned fuel as the source for jet pumps moving fuel from tanks around the airplane to the header tank stirs the fuel more removing much of both thermal and molecular stratification advantage in gasoline systems;
• Most components of diesel/jet fuel are heavier than in gasoline, so ALL of the vapor pressure effects are much smaller;
• In total heavy fuels have less issues with vapor pressure...They still had to make special jet fuels with higher molecular weights for very high altitude use. The U-2 and SR-71 and their brethren come to mind.

One other point - mention of fuel evap at localized low pressure spots. Yes, it is cavitation. The bubbles tend to exist only a very short time downstream of their generation, usually popping back to liquid very shortly after the bubble leaves the low pressure site of its formation. These discontinuities in flow and thus in pressure are why cavitation makes so much noise in a cavitating spot in a system.

Ain't engineering great?

Billski