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High Altitude flying and fuel selector question


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These may seem like pretty basic questions, but here goes. For long distance high altitude flying, whats the most common way to supply pressurized air? Do most people just use bottles or is there a way to run a compressor off the engine (or electrically) to a small tank? I ask this because I see people talking about long duration (say NY to GA) flights. Also, I see there are alot of suggestions for fuel selectors, and fuel management. While I'm years away from selecting an engine (hope the delta hawk becomes more affordable), Im curious as to why there cant be a fuel selector like in general aviation... left, right, both, or off. Whether its fuel injected with return lines or carberated, wouldnt this be the simplest in terms of pilot workload? My experience is limited to high wing, general aviation designs (152's to Caravans). Is the complexity of the cozy/canard fuel system due to the low wing layout? Thanks for letting me pick your brains :D

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.... For long distance high altitude flying, whats the most common way to supply pressurized air?

For what? Breathing? Pressurizing a cabin? Are you talking in general, or about COZY's in particular?

 

COZY's are NOT pressurized (and cannot be). If you're talking about breathing in an unpressurized aircraft, then above 12.5K ft., you need oxygen, not pressurized air.

 

Do most people just use bottles or is there a way to run a compressor off the engine (or electrically) to a small tank? I ask this because I see people talking about long duration (say NY to GA) flights.

It sounds like you're talking about breathing at high altitudes, so since you need O2 for that, pulling compressed air off the engine (either from a turbocharger or some other compressor) will be useless - you need pure O2. I know of no GA aircraft that have O2 generators on board - everyone (myself included) uses O2 tanks, which need to be refilled on occasion.

 

 

... Im curious as to why there cant be a fuel selector like in general aviation... left, right, both, or off.

The standard COZY fuel selector is L-R-Off. No Both. Same as in low wing GA aircraft, like a Piper Warrior. There's no magic about COZY fuel system.

 

Whether its fuel injected with return lines or carberated, wouldnt this be the simplest in terms of pilot workload?

"Both" only works if you can guarantee equal fuel flow (and fuel return, if there's a return line). This is more difficult in low-mid wing planes. Hence the L-R-Off.

 

Some folks (Steve Wright in particular) have put in LARGE cross flow tubes so that he CAN guarantee equal flow, and he also has a single point refueling port, with only an "on-off" valve to the engine. This is the simplest solution (but of course, means that there's only one fuel tank, in practice, so contamination or leakage leaves you with no backup. This is not a common problem, however, so the increased risk is small IMO).

 

Is the complexity of the cozy/canard fuel system due to the low wing layout?

The COZY fuel system is no more complex than any other low wing GA aircraft.
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Marc, or anyone else,

 

I have a few follow-up questions for you. Several years ago I got an open water scuba permit. I was surprised at that time to find out that scuba tanks hold regular pressurized air, not oxygen. Lately I have read a lot about oxygen for aviation applications and have been a little surprised to find that it must be oxygen, not just pressurized air. I really don't understand the reason it's OK to breathe pressurized air on a comercial jet, on a Lancair or in a scuba tank, but not out of an aviation bottle. Can you elaborate?

 

Also, on the Cozy, is the reason it can't be pressurized because (a) too many holes, it's impossible to seal, (b) the structure isn't built to withstand that type of outward force, © the materials (foam vs. molded Lancair), (d) some combination of the above, (d) none of the above, something else?

 

Thanks,

Steve

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Well, I believe that you could use pressurized air when flying. But, you will be carrying a bottle that is full of 79% nitrogen, which is a big waste. You will need a MUCH larger bottle. There is plenty of nitrogen available at the altitudes we fly, so why carry it along?

 

For scuba diving, you can't breathe 100% oxygen, and there is no nitrogen available, so you have to take it along. For some dives, nitrox is used, which is 50% oxygen and 50% nitrogen.

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....I really don't understand the reason it's OK to breathe pressurized air on a comercial jet, on a Lancair or in a scuba tank, but not out of an aviation bottle. Can you elaborate?

In order to be able to ensure a reasonable level of O2 in the bloodstream (generally, saturated O2 levels of 90% or better for the brain to work properly, and over 70% or so for it to work at all and for you not to die), you need to have a "partial pressure" of O2 that's above a certain level. Since at all altitudes the O2 is about 20% of the air, as the air pressure goes down with altitude, so does the O2 partial pressure (this is why you get less O2 as the altitude climbs). Pressurized aircraft deal with this issue by ensuring that the pressure inside the cabin is always high enough so that the partial pressure of O2 is always within safe levels (commercial airliners pressurize to an equivalent of 5K to 8K feet, usually).

 

Now, in an UN-pressurized aircraft, unless you're wearing a pressure mask or astronaut space suit, you'll be wearing a cannula or regular mask (like airliners have), so as soon as whatever gas is in the bottle comes out, it'll be at the ambient pressure. If it's just AIR in the bottle, then you haven't raised the "partial pressure" of the O2 going into your nose any, and it won't help your saturated O2 levels in your blood. If the bottle holds O2, then the gas entering your nose will be mostly O2 (along with some ambient air diluting it), so the "partial pressure" of O2 in your nose will be a lot higher than the O2 PP further away. This is good for your SpO2 (saturated O2) levels in your blood.

 

SCUBA has the opposite problem - as you descend, the pressure rises, and if you use pure O2, you'll have TOO MUCH O2 in your blood, and it'll poison you. On the other hand, if you use plain air, then you have to be very careful about going too deeply, or the nitrogen can come out of solution as you ascend, and you'll get the bends. Gas for breathing in SCUBA gear has to be tailored carefully depending upon how deep you're going, and your ascent rate has to be slow enough to let the dissolved gasses come out slowly, if necessary.

 

Also, on the Cozy, is the reason it can't be pressurized because (a) too many holes, it's impossible to seal, (b) the structure isn't built to withstand that type of outward force, © the materials (foam vs. molded Lancair), (d) some combination of the above, (d) none of the above, something else?

a) correct

b) correct

c) nope - hand layups COULD be designed to withstand the pressure - they just weren't in this case

 

Hope this helps.

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When I teach this subject, I have a box full of ping pong balls. Lets see if I can do it without my usual visual aids:

 

I have a box that represents the human lung. In the box is 100 balls, 80 are white and represent Nitrogen, 20 are Blue and represent Oxygen. The 100 balls represent the number of air molecules in a given area that will be found at sea level (15psi). Lets say that when you inhale at sea level, you will take in the full 100 balls

 

When you inhale, your diaphragm creates a sort of a vacuum, and air is pulled in, at the ambient pressure. So. at sea level, you'll get the full 100 balls, But more importantly, you get 20 Blue balls.

 

In order for you to function, your human internal combustion engine needs a blood Oxygen saturation level above 95%, it will take 15 to 18 blue balls to maintain this level.

 

Now we get in our plane, and start a climb. As we climb and pass through 8500 ft (11 psi), the atmospheric pressure drops and there are are now only 75 balls, 60 white and 15 Blue. When you inhale, you now only get the 15 blue balls.

 

Continuing our climb through 18000 ft (7.5psi), there are now only 50 balls. 40 white, and 10 blue. Remember, you need at least 15 to function, and your only getting 10. (thus the requirement for supplemental O2)

 

As you continue to climb, you will reach an altitude of 34,000 ft (3.6psi). At this altitude, when you inhale, you will only pull in 25 balls, 20 white, and 5 Blue ones. NOT GOOD

 

In order to survive at this altitude, you would need to be breathing 100% O2. In other words, all 25 balls need to be Blue.

 

PRESSURE BREATHING.

 

Pressurizing the air will not help, because your lungs will only inflate to the outside world atmospheric pressure. So, if your at 18,000 ft, your lungs will only inflate with the ambient 7.5 psi.

 

With that said, there are aviation breathing regulators that will in fact, FORCE air into the lungs, This is called pressure breathing. Normally, your diaphragm "works" to inhale, and "relaxes" to exhale.

 

With pressure breathing, when you relax, BLAM, air is "forced" into your lungs, and you lungs are blown up like a balloon. To exhale, you need to exercise stomach muscles, like squatting to take a $hit. Your Diaphragm doesn't have muscles to "exhale", only inhale. All this is just the opposite of normal breathing, and is very uncomfortable. Pressure breathing starts to kick around 37,000 ft (3psi), and will go up to about 55,000 ft (1.5 psi).

 

(This is the reason the High Altitude flyers , U2, SR71, wear a presurized suit. The suit is pressurized to about 3 - 5 psi, so the person inside the suit doesn't need to pressure breath. I can't imagin anyone pressure breathin for hours!)

 

When the Pressure Regulator kicks in, it normally forces the 100% O2 in your lungs at 3 psi, regardless of altitude.

 

I don't recall the exact number, but I seem to recall that you do not want to pressurize the human lung by more than 1.5 to 2 psi, as it will easily rupture. ALSO, Most people would suffocate, as they wouldn't be able to continue to forcefully exhale against the 2 psi pressure. This is a lot of work and requires training and exercise. Ask any Air Force Flight Crew Member who has done high altitude work.

 

So, With the magic number of 1.5 psi as the maximum allowable differential pressure the lung can take, lets see what happens if we pressure breath with normal air, instead of O2.

 

Lets go back down to 18,000 ft were we have 40 White, and 10 Blue balls. If I increase the pressure by 1.5 psi, I would effectively be breathing the same air density as if I were at a lower altitude, in this case, 1.5 psi increase would be the same as being at 14,000 ft. If you only look at the altitude difference, this theory looks plausible (Breath compressed air vs compressed O2). But remember, in order to make this work, you must be "pressure Breathing".

 

Take a look at

 

http://www.iflyez.com/ConvTables.pdf

 

There are several tables in this document, but look toward the back (page 18)for the Altitude vs Pressure tables.

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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If you did choose to pressure breathe compressed air at up to 18,000 ft as you suggest, then it would open the possibility of not using a bottle of compressed air at all. You could make your own on the fly using an engine driven compressor, or bleed from a turbo or supercharger if you have one. It is best if it is oil-free though!

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If you did choose to pressure breathe compressed air at up to 18,000 ft as you suggest, then it would open the possibility of not using a bottle of compressed air at all. You could make your own on the fly using an engine driven compressor

I think what you may save with your experiment you will spend on the pressurised mask & regulator equipment. I asked a RAAF F-18 pilot how they can use their mask unpressurised to FL500 when an ordinary mask is only rated to FL250 and the difference was... Umm, the words got long here, but it added up to PRICE.

 

I would be no more keen to pressurise my lungs than to pressurise a Cozy!

 

An oxygen system with an cannula sounds much more comfortable to wear & use to me.

Mark Spedding - Spodman
Darraweit Guim - Australia
Cozy IV #1331 -  Chapter 09
www.mykitlog.com/Spodman
www.sites.google.com/site/thespodplane/the-spodplane

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My main point about "Pressure Breathing" was, This is very, very uncomfortable, its hard work, and it does present medical risks. This Technique is a last ditch survival breathing method and is NOT used as a normal procedure.

 

I guarantee, you will not arrive at your destination rested and comfortable, You'll fell like you've been in a bar fight, and the winner used your chest and stomach as the punching bag.

 

I don't believe I would skimp on the correct pressure regulator or breathing apparatus. Fully approved regulator with Helmet and full rubber face mask.

 

If you are a licensed pilot, you can sign up and receive free high altitude training. This is a one day course put on by the USAF, and includes an altitude chamber checkout (they limit the altitude to 25,000 ft). The only negative I can think of for this training is, They select the date, you do not have an option.

 

Waiter

F16 performance on a Piper Cub budget

LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract

visit: www.iflyez.com

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Waiter ... thanks for the good info.

 

I originally assumed it would not be a problem to breathe the compressed air with a pressurized mask. But, I was not aware that the lungs cannot hold much above 1.5 psi over ambient, or of the comfort level in doing this type of breathing.

 

So, to breathe compressed air comfortably you would need a pressurized suit, and I am sure that would be expensive and uncomfortable to wear too.

 

Sounds like there is no comfortable solution other than pressurizing the complete interior of the plane with compressed air.

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I don't recall the exact number, but I seem to recall that you do not want to pressurize the human lung by more than 1.5 to 2 psi, as it will easily rupture.

Since you guys have addressed the physics and pathophysiology of this pretty well, I am not going to waste bandwidth restating what has been explained at least twice.

 

My contribution will be to clarify the magnitudes (or lack of) involved in the lungs when they breath.

 

Sea Level atmospheric pressure is 14.7 PSI/760 torrs/29.92 inches of mercury.

 

If you were to take a glass column of mercury and hold it such that an open end of the column were in a bowl of mercury, and the sealed end was at the vertical end, the atmospheric pressure would push the mercury in the column to a height of nearly 30 inches.

 

Now.. Imagine we are using water.. the column of water would be 33.9 FEET of water.. 14.7 psi of sea level standard day air pressure would be THAT tall... even at say.. 10,000 feet, where you have 1/3'rd less atmospheric pressure, that column of water is 22 feet.

 

Now.. translate this to the ventilator patients I deal with on a daily basis in the intensive care unit. We measure airway pressures in CENTIMETERS of water.

 

The "otherwise healthy" average patient's inhalation pressure and exhalation pressure, UNASSISTED (there are vent modes that do this) are on the order of 20-30 CENTIMETERS of water.. When they have severe disease, it may take pressures of 60 or more CENTIMETERS of water to force air in... when they cough it may reach 100 CENTIMETERS of water pressure. This translates to less than 3.5 feet on a water manometer in a worst case scenario and is less than 1 foot in normal breathing. This translates to 0.44 psi at sea level.

 

The body deals poorly with large differentials in actual pressure. More than about 1 psi and you cant hold your breath against the air trying to leave your body.

 

To address the original poster, you cant simply pressurize the mask to successfully breathe AIR at high altitudes.. and in the case of VERY high altitudes, you cant use pure ox in a pressurized mask. You have to pressurize the entire pilot - either a suit.. or a cabin.

 

As Marc and Waiter have already addressed, its all about the pressure imposed by the oxygen content in the inspired air.

 

Dave

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ive and uncomfortable to wear too.

 

Sounds like there is no comfortable solution other than pressurizing the complete interior of the plane with compressed air.

Just how high are you wanting to take your unpressurized, piston powered aircraft?

 

You can wear a mask with high concentration oxygen "comfortably" into the lower flight levels and be able to breathe normally.

 

If you are observant you can even get an MBU regulator, mask and blinker off ebay or from a surplus store.

 

Dave

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