Besides the fuel supply I'm wondering whether the way the engines are mounted would allow such a maneuver. Static forces are quite different and I could imagine that the nacelles could fall towards the wings even if the whole assemby should be designed to withstand rough conditions in flight.

Shouldn't be a problem I think. If you look at the forces during takeoff, there's about 20k-28k of force trying to rotate the engine upwards, since it's in front of and below the wing. I don't think gravity will have any effect there.

Not to go off topic here but...

 

You are a very trusting person to display your full name, phone number, private email address, and city of residence on a public forum.

 

Back on topic:

 

You guys are over complicating things here. Sure a 737 can fly inverted...as can any other plane out there. Yes the engines could still operate in an inverted position. They don't know that they are upside down. As long as airflow into the engine is not disrupted, you would not notice a difference. Fuel flow would only be disrupted if performing high AoA, high speed (acceleration) maneuvers. Flying inverted would only produce -1g...which is nowhere near the acceleration required to not only overcome the tank pumps, but the EDP's as well. Certainly you would overstress the airframe long before you came close to starving the pumps.

 

The movie "Flight" is not realistic by any standards. It is actually laughable (and I did) at points. It is purely for entertainment, and shouldn't be analyzed with any hint of realism whatsoever.

In an airliner, in fact any conventional aircraft, the fuel supply is taken from the bottom of the tank.  In sustained inverted flight the fuel is lying on the top of the tank.  Unless the tank is completely full, the fuel pump inlets can only draw vapour.  So the only fuel left is what is in the pipe to the engine, which won't last long. Hydraulic systems will suffer a similar problem, but would last as long as flow demand was small.

 

@ flightwatch: I have been warned many times about this. So far I have not gotten any unwanted emails, calls, requests or anything from anybody. As long as it stays that way I don't see what harm it is doing. If I do however begin to receive unwanted emails, calls or mail I will without hesitation take that information off my profile. Thanks for pointing it out though Remy Mermelstein 777-300 FS Pilot

When it starts to happen it's already too late, your inbox will be flooded with spam and it will be hard to stop. Also openly advertising such personal details is a gift for identity fraudsters.  Still, it's up to you.  I do wonder why you want to advertise yourself in this way though.  Far better to set up a website if you feel you have a lot to offer the community.

 

I'd rather keep contact at arms length, initially at least.  If people want to contact me directly they can PM with their email and I'll reply with an email address.

 

Also due to the wing design of airliners having a distinct camber, a higher angle of attack may be required when inverted.

Airliner wings aren't especially cambered.  They have thin, aft loaded sections for low drag at high Mach.  Any camber tends to be mostly at the rear of the aerofoil. Not symmetrical, but the difference in lift between positive and negative alpha wouldn't be huge.

 

You are right that the wing is at an angle to the fuselage (typically 2 deg in a Boeing) so inverted the deck angle would be a lot more than for normal flight, so drag would be higher.  The stabiliser trim might hit the end stops before it could reach trim as well as that can move only about 3 degrees nose up but about 12 degrees nose down (nose up/down relative to the upright position). It's designed to counteract the generally nose up pitching moment from the wing, but inverted nose up becomes nose down.  So the crew might have to make a large elevator input to keep the aircraft level.

In an airliner, in fact any conventional aircraft, the fuel supply is taken from the bottom of the tank.  In sustained inverted flight the fuel is lying on the top of the tank.  Unless the tank is completely full, the fuel pump inlets can only draw vapour.  So the only fuel left is what is in the pipe to the engine, which won't last long. Hydraulic systems will suffer a similar problem, but would last as long as flow demand was small.

 

Yes and no.  There are 2 pumps per tank.  1 forward and inboard, and 1 aft and outboard and they are located in surge tanks to prevent the fuel from splashing about.  While you might notice a low fuel alert for the forward pumps, you would have to have very low tanks (-1000lbs) to starve the aft wing pumps in inverted flight.  If your theory was correct, normal maneuvering (30° banks) would starve a fuel pump with anything less than full tanks.  Also, the pumps are not located at the very bottom of the tank as to protect them from sediment and other impurities that may preside in the tank.  Hence why there is about 1-2% unusable fuel at any given time.

 

Anyways, it's a pointless debate because a commercial airliner is not going to ever try to maintain inverted flight for any length of time.  It's just not something that it was designed to do.  I believe that the op was attempting to recreate what was depicted in the movie "Flight" which was inverted flight for a brief amount of time.  Like I said before, this is getting way over analyzed.

 

Yes and no.  There are 2 pumps per tank.  1 forward and inboard, and 1 aft and outboard and they are located in surge tanks to prevent the fuel from splashing about.  While you might notice a low fuel alert for the forward pumps, you would have to have very low tanks (-1000lbs) to starve the aft wing pumps in inverted flight.  If your theory was correct, normal maneuvering (30° banks) would starve a fuel pump with anything less than full tanks.  Also, the pumps are not located at the very bottom of the tank as to protect them from sediment and other impurities that may preside in the tank.  Hence why there is about 1-2% unusable fuel at any given time.

 

Anyways, it's a pointless debate because a commercial airliner is not going to ever try to maintain inverted flight for any length of time.  It's just not something that it was designed to do.  I believe that the op was attempting to recreate what was depicted in the movie "Flight" which was inverted flight for a brief amount of time.  Like I said before, this is getting way over analyzed.

Please explain where you got your 1000 lb figure from.  Even allowing for an aft mounted pump in the thinner part of the wing the tank would have to be at least half full to get reliable fuel supply to the inlet of the aft pump.  Meanwhile all the other lines are sucking air.  See sketch below (apologies for my rubbish photoshop skills).

 

 

As for it being my theory, it's not my theory, it's basic physics. In normal manoeuvring, at a 30 degree bank the g vector will still be normal to the wings, i.e. vertical relative to the aircraft, assuming a coordinated turn. So the fuel in the tank will be located more or less exactly where it would have been with no bank angle. There are, as you say, baffles in the tank to prevent fuel slosh having too great an effect. But what baffles can't do is stop the fuel dropping to what was the top of the tank when in inverted flight.

 

This doesn't even consider dihedral. When inverted, what was the lowest part of the wing, where the pump inlets are, is now the highest part. So the wing could be almost full of fuel and both forward and aft pump inlets exposed to vapour.

 

Aircraft designed for sustained inverted flight need fuel systems specially designed to cope with this.

 

FDR data indicated that, by 1619:42, the airplane had reached its maximum valid

recorded airplane-nose-down pitch angle of -70°. At this time, the roll angle was passing

through -76° left wing down. At 1619:43, the first officer stated, ìmayday,î but did not

make a radio transmission. Six seconds later, the captain stated, ìpush and roll, push and

roll.î FDR data indicated that, by 1619:45, the pitch angle had increased to -28°, and the

airplane had rolled to -180° (inverted).

 

Further, the airplane had descended to 16,420 feet,

and the indicated airspeed had decreased to 208 knots.

 

At 1619:54, the captain stated, ìok, we are inverted and now we gotta get it.î

FDR data indicated that at this time, the left aileron moved to more than 16° (to command

right wing down), then, during the next 6 seconds, it moved in the opposite direction to

-13° (to command left wing down). At 1619:57, the rudder returned to the near 0°

position, the flaps were retracted, and the airplane was rolling through -150° with an

airplane-nose-down pitch angle of -9°. After 1619:57, the airplane remained near inverted

and its pitch oscillated in the nose-down position. 

 

At 1620:04, the captain stated, ìpush push pushÖpush the blue side up.î At

1620:16, the captain stated, ok now lets kick rudder left rudder left rudder.î Two

seconds later, the first officer replied, ìI can't reach it. At 1620:20, the captain replied,

ìok right rudderÖright rudder. At 1620:38, the captain stated, ìgotta get it over

againÖat least upside down weíre flyin. At 1620:49, the CVR recorded sounds similar to

engine compressor stalls and engine spooldown.

 

At 1620:54, the captain commanded deployment of the speedbrakes, and, about 1 second later, the first officer replied, I got itî At 1620:56.2, the captain stated, ìah here we go.î The FDR recording ended at 1620:56.3,and the CVR recording ended at 1620:57.1

 

Aside the CVR info I can't find any information on engine performance during their inverted flight, maybe its in the report but I'm not seeing it due to exhaustion. 

 

http://www.ntsb.gov/doclib/reports/2002/AAR0201.pdf

 

Information on Aerobatic aircraft fuel/oil systems.

 

http://www.airspacemag.com/flight-today/Upside-Down.html

Aerodynamically any aircraft in the air today will fly inverted. If you look closely at the base of the wings they are not flat bottom. It is what is called a Semi- Symmetrical airfoil. It is more fuel efficient than a flat bottom and when the angle of attack is increased from underneath it does create substantial lift. The two major factors that are the drawback are the fuel system, and G loading on the wings. The engines will stay running as long as they get fuel. This is just a matter of fuel pick up and the pumps. I am not sure on this. I would imagine that any airliner would be able to take up to two negative G's. If they were built any other way they would be falling out of the sky in some of these bad storms they fly through. When you hear of people being tossed against the ceiling that is a pretty good negative G hit. If a good pilot could fly a 737 at a constant level inverted flight in good weather I see no reason why the aircraft would not do it. There is nothing wrong with the flight model on the NGX. It just wont break.

 

Thanks,

Ron  

kevinh, on 30 Mar 2013 - 20:25, said:

Please explain where you got your 1000 lb figure from. Even allowing for an aft mounted pump in the thinner part of the wing the tank would have to be at least half full to get reliable fuel supply to the inlet of the aft pump. Meanwhile all the other lines are sucking air. See sketch below (apologies for my rubbish photoshop skills).

 

 

As for it being my theory, it's not my theory, it's basic physics. In normal manoeuvring, at a 30 degree bank the g vector will still be normal to the wings, i.e. vertical relative to the aircraft, assuming a coordinated turn. So the fuel in the tank will be located more or less exactly where it would have been with no bank angle. There are, as you say, baffles in the tank to prevent fuel slosh having too great an effect. But what baffles can't do is stop the fuel dropping to what was the top of the tank when in inverted flight.

 

This doesn't even consider dihedral. When inverted, what was the lowest part of the wing, where the pump inlets are, is now the highest part. So the wing could be almost full of fuel and both forward and aft pump inlets exposed to vapour.

 

Aircraft designed for sustained inverted flight need fuel systems specially designed to cope with this.

Being a former aircraft fueler, aircraft mechanic in the USAF, a ramp agent, somebody who has studied the inner workings of the 737 series aircraft since I was a kid, and a pilot, I kinda have a unique perspective on this subject. You are over-analyzing this. My 1000 lb was a guesstimate based on the location of the fuel pumps, the shape of the wing tanks, and how a liquid would react when put in that situation. You are failing to take into account wing taper...not only horizontally, but vertical taper as well. The tank is nice and fat near the wing root, but tapers considerably towards the end of the "wet" part of the wing. With that being said, the fuel would fill the smallest part of the tank...which would be the outer part. And since the tank in that area is less than a foot thick, you would need to be pretty low on fuel to starve that pump in inverted flight. In normal flight, the exact opposite would happen, as the tanks got low, the aft pumps would be starved for fuel Your theory would absolutely be correct if the tank was the same thickness...like in a 172, but it's not.

Being a former aircraft fueler, aircraft mechanic in the USAF, a ramp agent, somebody who has studied the inner workings of the 737 series aircraft since I was a kid, and a pilot, I kinda have a unique perspective on this subject. You are over-analyzing this. My 1000 lb was a guesstimate based on the location of the fuel pumps, the shape of the wing tanks, and how a liquid would react when put in that situation. You are failing to take into account wing taper...not only horizontally, but vertical taper as well. The tank is nice and fat near the wing root, but tapers considerably towards the end of the "wet" part of the wing. With that being said, the fuel would fill the smallest part of the tank...which would be the outer part. And since the tank in that area is less than a foot thick, you would need to be pretty low on fuel to starve that pump in inverted flight. In normal flight, the exact opposite would happen, as the tanks got low, the aft pumps would be starved for fuel Your theory would absolutely be correct if the tank was the same thickness...like in a 172, but it's not.

Matt,

 

I wasn't failing to account for taper, I'm well aware of it, but my Photoshop drawing skills don't extend to 3 dimensions.  Dihedral is very important too, and that means the inboard part of the tank, where the pump inlets are, is the highest when inverted.  

 

This web page shows the location of pumps for the NG and the Classic, which are significantly different. 

 

In the 737 Classic, which you seem to be referring to, all the boost pumps are located in the main tanks and the aft main pump is indeed further outboard than the fwd main pump.  I agree that the outboard part of the tank is very much smaller due to taper but the pump inlets are all in the thicker part of the tank.  Even with the more outboard location of the Classic's aft main pump it is still inboard of the engine pylon, so most of the tank volume is above it.  Without access to actual tank dimensions I can't be certain, but I very much doubt that the tank could be so nearly empty as you suggested before that aft pump inlet was uncovered.

 

In the 737 NG, all the fuel boost pumps are in the centre wing tank.  The fwd and aft main fuel pumps draw fuel from pipes leading from the inboard part of the main tank, the lowest part of the tank.  When upright, the unusable volume is that part of the tank below these feed pipe inlets. When inverted usuable and unusable volumes are reversed, so the usable volume becomes very small as a proportion of total tank volume.  Certainly very much more than ~1000 lb.

I think the simple answer to this question is yes,

The extended answer is Not for very long.

 

The Fuel tanks suctions are fixed bottom tank, as are the bypass suctions for Taking suction directly using the engine driven pumps.

So you get to use what's left in the pipework essentially.

 

The hydraulic resevoirs are not designed to work upside down, so expect to lose hydraulics rather quickly, once you have used anything in the accumulators that's it.

 

There are few other things that generally do not like being upside down, for instance the Engine Lubrication system.

Here's to hoping Sara's short summation is sufficient to put this thread to bed.

 

Wel form my knowledge of aerodynamics, the wings only provide lift due tofaster flow over the top of the wing,

I am really surprised that no one has corrected you on this yet. Lift has nothing to do with the airflow over the top of the wing. Basic physics 101, "For every action, there is an equal and opposite reaction", lift is the reaction to the wing pushing air downwards. If you want a really good primer on aerodynamics, all the way through to acrobatics, then I highly recommend http://www.flybetter.com.au/index.html

Err actually, It does have a lot to do with how the wing works,

introducing a shape (Usually curvature) on the top side of the wing means the air takes longer to transit the same distance as the air travelling below the wing,

This does not mean they meet up at the same time at the end of the wing either.

This introduces a low pressure on the top side of the wing essentially sucking the plane up once the force is greater than the weight it has to lift.

 

All the extras you can add merely alter this shape, in the case of an Asymetrical wing then an Angle of Attack needs to be introduced to make the wing generate lift (lengthening the top side).

 

This is also how Eductors / Ejectors work, essentially 2 wings opposing each other creating a low pressure after the curve, where suction can be taken from

Just looked at the book,

And yes the first example is correct if you are using a flat Asymetrical object as a wing,

However, when perfectly Horizontal it will not generate lift, only when an angle of attack is applied to the Aircraft.

An Aerofoil shaped wing when it is not Asymetrical as mentioned in my previous comment, will generate lift without introducing an Angle of attack to the Aircraft.

 

I am really surprised that no one has corrected you on this yet. Lift has nothing to do with the airflow over the top of the wing

 

Because Paul... one chooses his (implied her) battles.

 

Personally, I felt there was enough on the discussion plate with this inverted stuff (which no one in the world will ever do intentionally to begin with) without bringing up fundamentals of aerodynamics of flight.

 

Stuff can get deep quick and few really know or understand aerodynamic principles... let alone have the ability to try to explain them.  Even after having my first A&P Course in '86 and PPL ground school '87 and all the coursework since then, I still consider myself a novice in this area.

 

 

From Airfoil Lifting Force Misconception Widespread in K-6 Textbooks by William Beaty

 

    The popular explanation, PATH-LENGTH or AIRFOIL-SHAPE: wings do not deflect air: the air far behind the wing is flowing the same as the air far ahead. Instead, wings are essentially "sucked upwards" because the airfoil shape has a longer surface on top. Airfoils are curved on top and flat below, and therefore the air follows a longer path above than below. Hunks of air which get divided at the leading edge of a wing must join each other again at the trailing edge. Since the upper surface of the wing is longer, it causes the upper air to flow faster than the lower, which (by Bernoulli's principle) creates lower pressure above. Because lift is caused by the shape of the wing, wings can create lift at zero attack angle. They can create lift simply from path-length difference which leads to pressure difference, and no air needs to be deflected. After a wing has passed by, the air does not remain moving downwards. (This explanation is seriously flawed.) 

 

 

A book I would recommend, Understanding Flight by David Anderson and Scott Eberhardt... lots of pictures and diagrams.  A quote from the first chapter:

 

If one were to sum up how a wing generates lift in one sentence, it would be that the wing produces lift by diverting air down. This statement should be as easy to understand as saying that a propeller produces thrust by pushing air back.

 

For a fun tool that simulates airflow over a surface try NASA's Glenn Research Center FoilSim III Student Version

 

-Rob