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

[Marc Zeitlin]

Certified airframe/engines deal with Avgas vapor pressure limitations by pressurizing the fuel tank... However the 'main' item that keeps them running (at least in IO or TSIO) is a vaned fuel pump designed to push bubbles out and back to the return tank...The point is simply that these problems are not trivial and simply making a claim that we are "going to 35K" without a lot of careful work is doomed simply to vapor lock and engine failure...

I appreciate the explanation of what TC'd aircraft do, but that doesn't explain how stock 100LL systems on E-AB aircraft, with no facility for dealing with vapor pressure issues, were able to work just fine up to 35K ft. (and even 47K ft., for BB) I'm missing some connection here...

Maybe Ross's graph above explains it all - as long as it's cold, the VP is low enough that all the fancy crap on TC'd aircraft isn't necessary? And it's always cold up high?

 

[rv6ejguy]

I appreciate the explanation of what TC'd aircraft do, but that doesn't explain how stock 100LL systems on E-AB aircraft, with no facility for dealing with vapor pressure issues, were able to work just fine up to 35K ft. (and even 47K ft., for BB) I'm missing some connection here...

Maybe Ross's graph above explains it all - as long as it's cold, the VP is low enough that all the fancy crap on TC'd aircraft isn't necessary? And it's always cold up high?

I think we often see overly complicated systems on certified aircraft like this and their clunky hydraulic wastegate systems is another example coming to mind. The E-AB solutions in actual, flying aircraft would seem to indicate there are often other simpler solutions which can also work.

 

[PMD]

The main problem with this on recips is that Jet A gells at -40C and Diesel at -8 to -40C depending on composition and additives so you'd have to heat it in the tanks up there. It might be challenging to divert enough heat from the coolant to do that. The good thing is that could potentially cut cooling drag. Ambient temp at 35K on a standard day is -65F.

Secondly, the pressure ratios (around 10) required by a CI engine up there also require a 2 stage turbo system and with that higher drag from the massive intercoolers required.

Of course, if one's diesel can run on JetA, it can run equally well on jetA1. I have run diesels on things you could not imagine - they can be the ultimate multi-fuel engine. BTW in the real world we start and run diesels down into the -50 range...without heating fuel or using additives. Refiners vary formulations with region and season. The days of D2 coming up from the deep South in a gelatinized ball are long gone.

You are right that in the diesel world today it is all about the turbo. Have seen an exercise to run one deep into the stratosphere and very much a sequential deal with huge first stage and intercoolers. Not sure where the exact limit is for one stage but VVT tech needed.

 

[blane.c]

It is simple, most of the time you will get away with it, all of the parameters necessary to cause a problem will not develop. However when the Gods unite against you what are you going to do about it? Crash and live the rest of your life in a wheel chair drooling into a cup? Or have a better plan?

 

[Marc Zeitlin]

It is simple, most of the time you will get away with it, all of the parameters necessary to cause a problem will not develop. However when the Gods unite against you what are you going to do about it? Crash and live the rest of your life in a wheel chair drooling into a cup? Or have a better plan?

That's not an explanation.

Posit a set of parameters that are physically possible (not 110F at 30K ft., for example) that could cause the problem that @lelievre12 indicated were going to happen above 22K ft. I'm not saying that it's not possible, but neither he (nor now you) have stated what those conditions are, particularly given the vapor pressure graph that @rv6ejguy posted. Show me why Bohannon and Price's flights (and my friend's, when he's above 22K ft) are a crapshoot, and not an acceptable one. I'm happy to be shown it's not a good idea, if that's the case, although I've never had my plane above 16.5K ft.

And if I'm above 22K ft. and my engine fails, I've got at least a 47 NM glide radius and 6,940 sq-mi (75 NM and 17,600 sq-mi at 35K ft.) within which to glide to somewhere reasonable. Unless I'm over the mid-Atlantic or Pacific, I'm pretty sure I'll be able to find something on which not to crash and drool (into a cup or otherwise).

 

[BBerson]

I don't know much about high flight but it might be possible that as the airplane ascends some of the fuel evaporates and is vented and the latent heat of evaporation cools the remaining fuel in the tank. Easy to see on a propane tank in use.
I have poured liquid propane into an open coffee can. About half of it boils off but eventually it cools to -44F and almost stops boiling.

 

[blane.c]

That's not an explanation.

Posit a set of parameters that are physically possible (not 110F at 30K ft., for example) that could cause the problem that @lelievre12 indicated were going to happen above 22K ft. I'm not saying that it's not possible, but neither he (nor now you) have stated what those conditions are, particularly given the vapor pressure graph that @rv6ejguy posted. Show me why Bohannon and Price's flights (and my friend's, when he's above 22K ft) are a crapshoot, and not an acceptable one. I'm happy to be shown it's not a good idea, if that's the case, although I've never had my plane above 16.5K ft.

And if I'm above 22K ft. and my engine fails, I've got at least a 47 NM glide radius and 6,940 sq-mi (75 NM and 17,600 sq-mi at 35K ft.) within which to glide to somewhere reasonable. Unless I'm over the mid-Atlantic or Pacific, I'm pretty sure I'll be able to find something on which not to crash and drool (into a cup or otherwise).

I don't know the peculiarities of the operations involved especially to the fuel system that you describe, I know that I have been to 13,000 feet in a cub with a gravity fed fuel system and no problems I also know at the same altitude in a Douglas on occasion especially when it was unusually warm on the ground prior to, the fuel system would begin to cavitate (for lack of better word) when switching off the tank centrifugal fuel pumps to run solely off the engine driven fuel pumps and having to turn the centrifugal pumps back on for an extended period of time. There is a procedure and a chart and graph in the operations manual of the Douglas for how long to leave the boost pumps on and it was designed originally to fly around 25,000 feet although ours were modified with the blowers locked in first stage so those altitudes were not available to us. So from experience I know you can have fuel vapor problems in the Douglas at much lower altitudes than those being discussed, so it stands to reason they would become more prevalent at higher altitudes.

 

[rv6ejguy]

By the time it takes the average turbocharged GA aircraft to climb to 35,000 feet from say a 5000 foot elevation airport having sat out all day on a 100F tarmac, the fuel in the tanks has dropped below any critical temperature. The EFI is recirculating something like 1 gallon a minute through the tanks and heat is being sunk off the tanks at a pretty high rate as the OAT falls throughout the climb.

Bruce's record to 47,000 feet shows just how much margin there is with avgas.

 

[Dillpickle]

Refreshing conversation to this point! It's wonderful to see a designer respond without vitrol. And if we don't see a tiny T-Rex with VB's name suggested on the prototype, I'm gonna be a bit disappointed.

 

[Flyfalcons]

Oh come on, LOL. Not today, Peter!

 

[blane.c]

World war two fighters and bombers went into the upper thirty thousand foot and into forty thousand foot elevations, so for a recip to do it ain't nothin' new. How gas bubbles in the fuel was dealt with was likely more than just thumbing their noses at the phenomena.

 

[PMD]

World war two fighters and bombers went into the upper thirty thousand foot and into forty thousand foot elevations, so for a recip to do it ain't nothin' new. How gas bubbles in the fuel was dealt with was likely more than just thumbing their noses at the phenomena.

Didn't want to introduce yet another factor into this discussion, but at this stage I guess anything goes (in the thread drift world).

ALL liquids in air have a certain amount (considerable in most cases) of air dissolved within. The simple act of pumping results in various locations where turbulence or work being done drops the pressure low enough to do one of two or both things: extract dissolved air and boil off the liquid into gas bubbles. The former suffers from being easily extracted by even a sharp edge on a flow path through a fitting, but will NOT go back into solution until reaching miscibility conditions - that will not be reached in a typical carburetor fuel supply system - but they WILL harmlessly escape while in the float bowl. Boiled gasoline (or any other fuel) on the other hand IF conditions are adequate (i.e. temp below & absolute pressure above the boiling point curve = RVP value under those conditions) it will simply give a bit of heat back to the fuel and condense. This can easily happen (especially in an injection system) AFTER a pump raises the fuel pressure in the line. The worry (as I think was made clear by the Douglas comment post) is that really bad gassing of fuel can overcome a pump on the inlet side by essentially "breaking the vacuum" that is feeding it.

I am wondering if the few very high altitude trips reported in this thread were done with the/a boost pump on?

 

[cheapracer]

Now you see some of the damage Raptor has done to future designers/offerings.

I know some of the stuff Aeromomentum has done that isn't public knowledge, so I wouldn't bet against this project, especially with Mark's industry experience and importantly, engine experience.

 

[tspear]

I am wondering if the few very high altitude trips reported in this thread were done with the/a boost pump on?

My Aerostar had a checklist item passing 10K to make sure the boost pumps were in low. If not, and hot soaked, the engines would start to gurgle and burp when you went through the low 20s; hit the pump on and the problem went away. (I forgot a few times as you can tell when I first got the plane). The boost pumps were somewhere down in or near the sump, I forget the details and I not going to find the diagram online.
Now the Aerostar fuel sump was on the bottom of the fuselage; about three feet below the engine top vertically and close to ten feet laterally. So the engine mounted mechanical pump had to pull the fuel a long distance!

I do recall an aeronautical engineer saying based on the position of the vent system in the Aerostar, the tanks were pressurized by almost 2psi when cruise around 230 KTAS in the mid 20s. No idea possible, or what....

Tim

 

[lelievre12]

By the time it takes the average turbocharged GA aircraft to climb to 35,000 feet from say a 5000 foot elevation airport having sat out all day on a 100F tarmac, the fuel in the tanks has dropped below any critical temperature. The EFI is recirculating something like 1 gallon a minute through the tanks and heat is being sunk off the tanks at a pretty high rate as the OAT falls throughout the climb.

Bruce's record to 47,000 feet shows just how much margin there is with avgas.

OK this is good and a long long way from the "lets take her up to 35K and what me worry?"

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

1. Reid Vapor Pressure curves are of course temperature related as the RV6EJGUY graph shows. Climb lapse rate ensures our fuel tanks don't boil dry as long as the fuel has time to 'cool' during climb and as long as the fuel was cool to start with. However aluminium wing tanks in conventional aircraft will have an entirely different cooling rate than composite wings would. So before you go putting a bunch of 75F avgas in your foam core composite wing, it might be worth testing the R factor and cooling rate before you charge up to 35K or 47K. I expect a composite wing tank will cool fuel during climb far more slowly than a aluminum wing would so this factor needs to be tested. Or if your tank is located under your seat in the cozy warm cabin, please don't fly above 20K!. If lapse rate is not externally maintained then fuel will boil giving up its heat of evaporation which in turn refrigerates the remaining fuel to maintain cooling. Up to 25% of the tank can be lost in the initial climb and more thereafter if fuel is heated. Most importantly, the most volatile constituents evaporate first which can alter octane of the remaining fuel.

2. Many (all?) fuel injection system run return lines to the 'tank'. Obviously if we are trying to keep the fuel cool, returning hot fuel to the tank is not a good idea. This factor needs to be considered too. On my own aircraft, fuel is returned to a pressurized header tank, and not the main atmospheric avgas tank. I have two header tanks, and can switch between them if one gets too hot and overwhelms my TSIO520 with bubbles. Header tanks are crucial for fuel systems with return lines.

3. Fuel entering the hot engine area is going to be immediately boiled unless it is pressurized. Whilst it sounds a simple matter to maintain pressurization, the dynamics of many fuel systems are more troublesome. As one example, a vaned fuel pump delivers fuel pressure in pulses. In between these pulses are moments of slack pressure where hot fuel will instantly boil. Once boiled, the fuel will not immedatly reabsorb the bubbles even if pressure is returned. The surface tension of the bubble takes time to reform to liquid. Hey presto, your fuel rate now has a % of vapor. It is therefore crucial that any fuel system be audited to ensure constant pressure throughout operation and accumulators or other devices added if required. The higher the altitude, the more troublesome the issue.

I can go on and on, however the fact that at least now we are all alive to the issue is enough for me. I am not trying to design this twin aircraft, just outlining the work needed before we clap and say "yay! 35K!".

For those interested in learning more about the issue, feel free to read the 1980 FAA report on "LIGHT AIRCRAFT ENGINES, THE POTENTIAL AND PROBLEMS FOR USE OF AUTOMOTIVE FUELS" . Its a great read and mandatory for anyone climbing above 20K with Avgas or 10K with Mogas.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a094154.pdf

And for Avgas boiling rates at altitude see: Reduction of Fuel-vapor Loss by Omitting Some of the Fuel Constituents Normally Lost During Flight - NASA Technical Reports Server (NTRS)

 

[BJC]

So before you go putting a bunch of 75F avgas in your foam core composite wing, it might be worth testing the R factor and cooling rate before you charge up to 35K or 47K.

The prototype Glasair Super III flew to 40,000 feet with no modifications to the fuel tanks. The fuel tanks are actually wet wings - wings with skins that have 1/2” foam cores.

Many (all?) fuel injection system run return lines to the 'tank'.

Not all.


BJC

 

[tspear]

As a general rule, I thought Lycoming has fuel return lines with injected engines, why CMI generally does not. But I could very well be mistaken.
I would suggest @rv6ejguy or @Dan Thomas or one of the many others on here could actually say if I am full of BS or correct on my anecdotal memory.

Tim

 

[rv7charlie]

Traditional Continental mechanical injection has a return line; traditional (Bendix style) Lycoming injection does not have a return line. At least one of the sellers of experimental Bendix style injection does offer a 'purge valve' option in its system design, but it is not intended as a full time return line. It's purely to purge the hot-side fuel lines of boiling fuel prior to engine start when doing a 'quick turn' requiring a 'hot start' (bane of most injected Lyc flyers). The valve is supposed to be closed prior to engine operation.

My experience (below nosebleed altitudes) running both carb'd and injected Lycs, on avgas, E-free mogas, and E-gas, is that carbs can be problematic in hot (>100F) weather if running 'winter' mogas (got the bent gear & busted prop to prove it), and with injected Lycs, the boost pump running at 20-30 psi easily overcomes fuel boiling in the lines going to the injector spider. There's almost no fuel in the injector lines at any one time (they are really tiny tubes), so it gets purged into the manifold within 5-10 seconds of engine start, in my experience.

My experience is in the hundreds (not thousands) of hours; most below 10k', but in temps that are frequently in the 90-100+ range. Others with more experience (and altitude) may have had very different experiences.

I have zero experience with the Continental system, except right seat time in others' a/c.

Return lines with electronic injection: All over the map. Most return from the fuel rail, but quite a few are flying successfully with the regulator in the cockpit (bypassed fuel back to the tank) and the supply line 'dead headed' at the injector fuel rail(s). I haven't run across anyone yet that's running the later iteration of electronic injection, with turbine pumps in the tanks, electronic pressure control and dead head lines at the rail, but I'd bet that it's likely some are already flying. With in-tank pumps running 40-60 psi and dead head delivery to the injectors, boiling fuel in the fuel lines should never be an issue.

 

[rv6ejguy]

1. Header tanks are crucial for fuel systems with return lines.

2. Fuel entering the hot engine area is going to be immediately boiled unless it is pressurized.

Point 1, nonsense. I have over 2000 customers flying our EFI with no header tanks, over 750,000 flight hours collectively. How come they're not falling out of the sky?

Point 2, we are talking about EFI here and they ALL feed pressurized fuel to the engine. With our top case mounted fuel blocks in ambient air, there is almost no heat picked up by the feed or return fuel. In fact, it's being cooled in flight if anything.

EFI fuel pumps are gerotor or roller vane types mainly, no issues happening like you describe. They just work.

I have composite airplane owners with Lancairs, Glasairs, Cozy's etc. using our EFI, some flying up to 25,000 feet.. No issues, lots in hot climates.

We DO worry about vapor lock BEFORE the pump on low winged aircraft using Mogas on hot days and high altitudes and have specific cautions about pump placement and eliminating 90 deg fittings prior to the pumps and to never use winter Mogas in the summer. We don't see these issues with 100LL.

I do believe there could be issues with Mogas at 35,000 feet. It will take flight testing to validate if that's feasible or not.

Really, before you post, I'd suggest you research what's already been done for years now...

 

[rv6ejguy]

I do recall an aeronautical engineer saying based on the position of the vent system in the Aerostar, the tanks were pressurized by almost 2psi when cruise around 230 KTAS in the mid 20s. No idea possible, or what....

Tim

Around 0.56 psi actually.