Uninterrupted fuel supply in high bank angle and negative G

[wantobe]

Should a normal category airplane (this is my idea of airplane, move from point A to point B quickly, not to do somersault in the air.) maintain continuous fuel supply under long-duration negative G condition?

My understanding is that a normal category airplane will not have long-duration negative G operation, thus the fuel system is not required to supply fuel under such operation. But I am not sure.

A related condition is high bank angle, or more precisely, high side-slip angle which moves the fuel to a side. What are the fuel system design requirement for a normal category airplane under such condition?

[Bob Dingley]

Sounds like a near aerobatic maneuver to me. An aquaintence wrecked a Baron in a somewhat similar condition. He did a hasty takeoff with the fuel selecters in "aux" position. Mains full, Aux's less than full. The airplane is placarded against this and its also covered in the AFM. The mains have two fuel lines, but the aux tanks only one.

He took the active accelerating while turning 90 deg to line up for an intersection take off. Centrifigal force was no factor for #2 tank, unfortunately the #1 tank unported during the turn. It ran just fine until he rotated. Then the air bubble hit #1 engine. You cannot believe how fast it hung a sharp left. Several witnesses. New airplane with less than 100 hrs. I'm pretty sure that the engine would have resumed normal operation on its own if only they were at cruising altitude. They made Barons for a long time. Single engine planes unport too. FAR 23.955. covers this for normal catagory.

Is this the sort of condition that you are describing?

Bob

[WLIU]

The short answer is no. Normal and utility category airframes will handle -1.5 G but a carbureted engine stops almost immediately if any negative G is encountered. Fuel injected engines will run a moment longer. The airframe design requirement is to handle turbulence. But the powerplant and fuel system designs are not required to function under negative G.

Hope this info is useful,

Wes
N78PS

[prasmussen]

Whoops! I thought fuel injection was the answer to inverted flight? Along with a dry sump and floppy tube what else could an old pilot ask for Christmas?

[wantobe]

... Centrifigal force was no factor for #2 tank, unfortunately the #1 tank unported during the turn. ... Single engine planes unport too.


I looked up "unport" in Merriam-Webster which said this word did not exist. What does it mean?


FAR 23.955. covers this for normal catagory.


FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).


Is this the sort of condition that you are describing?


The condition I am concerned is long-duration negative G.

Thanx!

[steveinindy]

I looked up "unport" in Merriam-Webster which said this word did not exist. What does it mean?

It means that the fuel moves (by gravity or the inertia of the fuel in a sharp turn) from the ports of the line leading from the fuel tank.


FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).

The most critical attitude is going to vary from aircraft to aircraft so it is up to the designer to define that. The FAR is necessarily vague about that. A reasonable person would read that as "If you're going to fly upside down or pull negative G, you have to make sure you won't kill your engine". It shouldn't be surprising that a category that covers everything from a Cub to an Extra 300 is not going to spell out every last minutiae detail.


The condition I am concerned is long-duration negative G.

"Long duration" (more than a couple of seconds) negative G also winds up often causing equally long duration mental impairment. Pilots- especially with the level of physical fitness of the average GA pilot- tend to poorly tolerate negative G and it can cause "red out" or full unconsciousness.


Fuel injected engines will run a moment longer. The airframe design requirement is to handle turbulence. But the powerplant and fuel system designs are not required to function under negative G.

Not being the most well-versed in advanced engine and fuel systems, how would one overcome this so if you were to encounter turbulence or something else that imparts dramatic but temporary negative G that you can keep your fuel injected engine running? As for the carburetor-equipped versions, I would not even consider such an engine for a modern aircraft because of the various problems (icing being the big one but apparently intolerance to negative G is another) with the system when better options exist

[martymayes]

FAR 23.955 does not mention negative G, only requires proper fuel flow in most critical attitude. This is a surprise for me since I expect it to impose fuel flow requirement under negative G for aerobatic airplanes (FAR 23 covers normal category and aerobatic category).

Might have to poke around a bit, like in 23.951.

[Bob Dingley]

Bravo Steve for explainining "unport." First time that I remember seeing it was in the accident report. Which recomended an ammendment to an AD thats been around forever:

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/C03B3364E8B0CED586256E0100546F2B?OpenDocument

(BEECH Models 95-55, 95-A55, B95A, D95A, E95, 95-B55, calls for a placard and flight manual ammendment)

No need to defy physics when designing systems. You just slap a placard on the panel, ammend the AFM and use common sense.

Bob

[steveinindy]

Bravo Steve for explainining "unport."

Thank you. The fact that you approved of/appreciated my explanation means a lot since I respect your opinion a great deal.



No need to defy physics when designing systems. You just slap a placard on the panel, ammend the AFM and use common sense.


That's kind of what I figured but then again my thinking has always been that if you can design a problem out with a practical approach, all the better.

[WLIU]

I will suggest that the statement "negative G also winds up often causing equally long duration mental impairment. Pilots- especially with the level of physical fitness of the average GA pilot- tend to poorly tolerate negative G and it can cause "red out" or full unconsciousness." indicates too much time spent reading and no time spent in inverted flight.

As an active aerobatic competitor, I can report that the compulsory Known programs include figures like inverted 270 steep turns that are not very short in duration. Outside snap rolls ARE short duration but can expose you to high -Gz. The Advanced and Unlimited guys come down with -7Gz on the meter some days.

The only situations where I am aware that we see "red out" are in ground based centrifuges.

For what its worth, what will put a pilot to sleep is transition from a period of negative G to high positive G. You will wake up woozy but that has not prevented some really competitive guys from turning back to the contest box and finishing their flight programs.

Getting back to powerplant behavior under negative G, the Czech Zlin has a carburetor that has a double float bowl, one upright and one inverted. When the airplane is rolled from upright to inverted, or back, the engine will cough as the carb switches between the float bowl that supplies fuel to the venturi and the fuel pump refills the float bowl. Very clever design.

A carburetor stops working at negative G as soon as the float moves to close off fuel flow into the bowl. Normally the boyancy of the float in a full bowl pushes the needle valve closed. Negative G also causes the float to move the needle valve to close off fuel flow. Plus the flow of fuel from the float bowl depends on +G. So the engine stops making power.

Fuel injection requires more fuel supply pressure to operate. But that higher pressure takes longer to decay when flow from the supply tank stops. So a fuel injected engine runs a moment longer under negative G. As noted above, if you have a flop tube in the fuel tank, you can keep fuel flowing and the engine making power. But you better also have an inverted system on the engine oil supply or making power just makes bad wear in the engine.....

Best of luck,

Wes
N78PS

[WLIU]

If I can hijack this thread for a moment, one of our more colorful aerobatic competitors had the comments below, posted on another forum, about learning to fly lots of negative G...


"(re: negative G)
"the key was to relax"
Correct. When you start doing aerobatics, the first thing you learn is to tense up before you do a maneuver, to keep the blood up to your brain.

That's good for pulling on the stick, but really bad for pushing on the stick. If you grunt before a push, you're really going to pressurize your poor skull with blood.

You want to set up the cockpit so that you can totally relax before a push, to keep your blood pressure as low as possible. The little jets that I fly, for example, have very nice stirrups to keep your feet on the rudder pedals under negative G. And you want your lap belt insanely tight, and your shoulder harnesses reasonably tight.

Tiny details, like your sunglasses and mike boom moving around under -ve G, need to be addressed. Your headset must be tethered in some manner. I like the perrone mesh "snoopy" helmets, but some people use chin straps. Heck, a line of duct tape will even do it!

Positive G is great fun. It's sort of like lifting weights. Once you get onto the "grunt", you can rapidly increase the +ve G that you pull: 2, then 4, then 6, then 8, then 10. A guy I know pulls +11 and pushes -7 on every flight. He's broken every part of his Pitts, and reinforced it.

Negative G is really strange. You must build the -ve G very slowly, to avoid really screwing yourself up. Your blood pressure soars to insane levels, so if you have any kind of flaw in your brain - aneurism, etc - you will find it right away. If I overdo it, I blow blood vessels in the skin on my face, esp underneath my eyes. Looks really weird. But the real danger of ramping up the -ve G too fast is tearing up your inner ear - the wobblies. No one knows much about them, because the western military doesn't do -ve G, and the civilians that get the wobblies don't want to talk about it, and have their medicals pulled.

Start with -1G. Roll upside down, a 2 pt roll, and stay there for a bit. Try to figure out - attitudes and movements - which pitch attitude you need to maintain altitude. Common error is to not push -1G and to lose altitude. A little bit of nose down trim can help reduce the stick push force. Obviously this is speed and aircraft type (flat bottom wing?) dependent.

Ok, so try to fly straight and level inverted for 10 seconds, then 20 seconds, then 30 seconds, hesitating during that 2 pt roll. Obviously you need to worry about aircraft systems limitations. Not all aircraft can tolerate indefinite -ve G.

You're going to get disoriented upside down. You are going to get lost. Seriously. Best thing you can do, is after taking off, is roll inverted and navigate to the practice area and back again. It's like learning to fly all over again.

Some level inverted turns are a good idea. Start with 30, then 45, then 60 degree inverted turns. Remember to co-ordinate with OPPOSITE rudder. It is really easy to slip/skip inverted turns. Some people install inverted balls (inclinometers). Watch the altitude! It is really easy to spiral dive out of inverted steep turns. 60 degree inverted turns is -2G, which is getting better. You want to start with -1G, then -1.5G, then -2G, then -2.5G, then -3G, etc. If you go right to -6G you are NOT going to be a happy puppy.

Another exercise, once you can fly inverted, is to push to 45 up, hold it for a moment, then level off inverted again at a higher altitude and slower airspeed. Pause, then let the nose drop to a 45 downline and push level again. This is really excellent practice.

One very simple maneuver I like is max airspeed at the surface, and pull for an inside 1/2 cuban-8. But instead of 1/2 rolling upright from the resulting inverted 45 downline, push to level inverted. Or, keep pushing to a 45 inverted upline, and do a nice 1 1/2 roll to the upright 45. Once you have mastered the inverted pushes to 45, time to take the next step to the outside 1/2 cuban-8. Dive for some airspeed. 1/2 roll inverted, and push through the vertical - lots of right rudder required, esp with a metal prop - to the 45 upright downline. Call it quits there.

Practice until you've got it nailed, no spiralling. If anything goes wrong, you are going up with decreasing airspeed which is good for the Vg diagram - you will "fall upwards" into some sort of snap or spin which is generally quite fun. Once you master the outside 1/2 cu-8, time to continue the push for the complete outside loop, which is interesting because your altitude is going away fast, the airspeed is increasing, and the negative G is building up fast. This is a much more dangerous maneuver than the outside 1/2 cu-8, and care must be taken with the top gate - is the altitude at least the minimum, and is the airspeed down below the maximum?

Not many people master the outside loop. If you do, you are in a pretty elite group of masochistic pilots!"

[steveinindy]

The only situations where I am aware that we see "red out" are in ground based centrifuges.

I have only experienced it during a ride in a military aircraft (a reward for helping their wing commander out....long story) and a centrifuge when I was a biomedical test subject. However, from what I have been told by a couple of the unlimited aerobatic guys I have talked to that you can easily red out at -4 or -5 G, lower if you aren't in good shape. There are few things in flying that actually frighten me but rapid onset of high levels of negative G is among them. Somewhere exists footage of me screaming (because of how much it made my head hurt) and then blacking out in the centrifuge.



Negative G is really strange. You must build the -ve G very slowly, to avoid really screwing yourself up. Your blood pressure soars to insane levels, so if you have any kind of flaw in your brain - aneurism, etc - you will find it right away. But the real danger of ramping up the -ve G too fast is tearing up your inner ear - the wobblies. No one knows much about them, because the western military doesn't do -ve G, and the civilians that get the wobblies don't want to talk about it, and have their medicals pulled.

That was what I was talking about. I just don't have that much first hand experience (twice as I mentioned) with it because I don't enjoy aerobatics that much outside of limited stuff for proficiency in upset recovery. Thank you for the insight you provided. I would love to ride with you sometime.


If I overdo it, I blow blood vessels in the skin on my face, esp underneath my eyes. Looks really weird.

Petechial hemorrhages or just petechiae....the same thing happens in extreme impacts (the crushing/compression of the abdomen and chest is the usual mechanism) and in people who are strangled. It is definitely odd-looking.

[WLIU]

I will encourage anyone who is interested in experiencing negative G in an aircraft that can safely fly that way to contact their local EAA/IAC chapter and offer to buy some gas. My chapter gets those sorts of requests and the pilots are very happy to share their experience and their sport.

The IAC web site can be found off the EAA web site, or at www.iac.org. Look under the Programs topic for the Chapter Locator. For example, In Indiana, IAC Chapter 124 is in Kokomo, and the chapter president is Michael Wild. His telephone number is listed on the web page.

Best of luck,

Wes
N78PS