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CoolGuy
Topic Author
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Upside Down Aerodynamics

Wed Jun 13, 2007 1:35 pm

I read this on Yahoo Answers:

Some can briefly. As mentioned the prototype 707 did a couple of barrel rolls. Theoretically any aircraft can do a barrel roll because it is a positive G maneuver. Most aircraft could also do an aileron roll (typically a zero G maneuver) as well.

The exceptions would be the A320, A330, A340, A380, and the 777 because the flight control laws for their fly by wire software will not let them do it.

No airliner can fly upside down legally.

http://answers.yahoo.com/question/in...paid=answered#NbUvWjS6VTiYuYNOqRIj

What does a positive G maneuver have to do with it? (I wonder how many G's are experienced doing something like that?)

For the second part, what does the software restrict? No more than 45 degrees?

Legally, is it 30 degrees when operating under Part 121?

I never thought about these questions but it is interesting I think.
 
KELPkid
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 2:32 pm

Quoting CoolGuy (Thread starter):
What does a positive G maneuver have to do with it?

Sadly, engines, landing gear, fuel systems, and gyroscopic instruments all expect gravity to be acting downward...  Sad Piston engines in particular will lose lubrication if operated inverted-not good. If I were to try and roll a Cessna inverted (let's say for brevity's sake that I'm not maintaining positive G's), the limiting factors would be the gravity-fed fuel system (no fuel pump  Wink ) and the oil pump in the engine cavitating.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 4:28 pm

Quoting KELPkid:
the limiting factors would be the gravity-fed fuel system (no fuel pump ) and the oil pump in the engine cavitating.

And all the dust, bits of potato chip, assorted washers, pens and fluff coming off the floor and blinding you.  Smile
Todos mis dominós son totalmente pegajosos
 
Tristarsteve
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 5:33 pm

Quoting KELPkid (Reply 1):
the limiting factors would be the gravity-fed fuel system (no fuel pump )

In my younger days I remember flying in a Chipmunk, and doing a roll. The engine stopped, then restarted when you went the right way up again. Very disconcerting to a 16 year old who didn't expect it!
 
B737200
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 6:27 pm

Sorry if I'm going a bit off topic but I always wondered this.

During normal flight the curved upper surface of the wing creates an area of negative pressure which creates lift. Now if a aircraft were to fly upside down, not in a roll or anything, wouldn't the area of negative pressure act downwards towards the ground (sort of like the inverted "wings" on formula 1 cars designed to keep the cars on the ground) ? So then how would the aircraft stay aloft ?

I'd imagine that in such a case the combined forces of weight and suction from the wings' pressure differences both acting downwards and the lack of anything to counter it would cause the aircraft to fall to the ground.

So can anyone enlighten me?

As usual thanks.

[Edited 2007-06-13 11:28:09]
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BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 6:28 pm

Quoting TristarSteve:
The engine stopped, then restarted when you went the right way up again.

This is why aircraft need to be modified to fly in the southern hemisphere. See, they're made right-way up in the northern hemisphere, but when they go down under, their engines would stop if they didn't have special fuel and lubrication systems.

Of course, it's the other way around for Embraers.
Todos mis dominós son totalmente pegajosos
 
BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 6:36 pm

Quoting B737200:
Now if a aircraft were to fly upside down, not in a roll or anything, wouldn't the area of negative pressure act downwards towards the ground (sort of like the inverted "wings" on formula 1 cars designed to keep the cars on the ground) ? So then how would the aircraft stay aloft ?

It's all about angle of attack. Although Bernoulli has his place, it is the angle at which the wing meets the flow of air over it which generates most lift.

You can try a simple experiment while a passenger in a car - stick your hand out of the window and feel what happens as you angle it up and down. Your hand might be somewhat airfoil shaped, but you'll feel it's the angle that you hold it at which makes the biggest difference. Make sure you don't slap any cyclists while doing this. Or lamp posts.
Todos mis dominós son totalmente pegajosos
 
B737200
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 6:50 pm

Thanks for the prompt reply, I never thought of the angle of attack, never crossed my mind.

By the way does anybody (I'm assuming one exists) know the equation that gives the lift generated by Bernoulli. I'm assuming that the forces given by the deflection of the air due to the angle of attack is:

Force=2 x Density x Area of wing x Velocity of the air flow incident to the wing surface squared.
which is derived form the equation: Res F= Change in momentum/time.

I'm assuming that if the deflection of the air is to keep the aircraft aloft is must be equal to the combined weight and suction of Bernoulli right?

Hope I didn't confuse anyone but we don't study the behaviour of fluids at school so I'm left trying to put the pieces together.

Thanks once again.
Lady Guinness is ready to fly...
 
BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 8:28 pm

Quoting B737200:
I'm assuming that if the deflection of the air is to keep the aircraft aloft is must be equal to the combined weight and suction of Bernoulli right?

You mean when the aircraft is inverted? Yes, I expect so. Essentially, the pressure effect would be on the opposite side of the weight/lift equation from where it normally is.

I could be totally wrong here, but I expect that if you attempted to fly inverted for any length of time (assuming your motors will take it), you would find that to maintain altitude you'd need a higher angle of attack than normal to counter Bernoulli pulling you down.

I'm not really clear on what would happen to the airflow on the "top" of the wing at this point, (which is the normal bottom of it, of course). Flying normally, the upper surface of the wing is designed to encourage laminar flow and maintain the boundary layer in a predictable shape. The underside is not going to be as good at this and I would think you would have turbulence (with a corresponding increase in drag) and maybe even a higher clean stall speed.

I only hope that when a proper aerodynamicist weighs in here, they go easy on me.  Smile
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BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 8:35 pm

Quoting B737200:
By the way does anybody (I'm assuming one exists) know the equation that gives the lift generated by Bernoulli.

This might be interesting;

http://www.grc.nasa.gov/WWW/K-12/airplane/bern.html
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B737200
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 8:50 pm

Thanks a million, I'll have a look at that link.
Lady Guinness is ready to fly...
 
Tristarsteve
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 9:06 pm

Quoting BAe146QT (Reply 8):
You mean when the aircraft is inverted? Yes, I expect so. Essentially, the pressure effect would be on the opposite side of the weight/lift equation from where it normally is.

Aircraft that are designed for aerobatics have symetrical wings. i.e. they work the same upside down as the right way up.
 
oly720man
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 9:12 pm

Equation for Lift is 0.5*density*wing area*velocity^2*lift coefficient.

Lift coefficient varies linearly with incidence over a reasonable incidence range from maybe -10deg to +15 deg, assuming no high lift devices (flaps and slats).


The following NACA report shows the aerodynamic charactersitics of a fuselage with a number of different wings.

http://ntrs.nasa.gov/archive/nasa/ca...asa.gov/19930091947_1993091947.pdf

On page 6 of 17 in the pdf, figure 5, the variation of lift with incidence and Mach number is shown. The graphs are offset to the right for each increasing Mach number.

This graph shows that the lift coefficient at zero incidence is around 0.1 and zero lift is at around -1deg.

As an example, say we have a plane of mass 500kgs (or 500*9.81N) and wing area 10msq flying at sea level (density = 1.23kg/m3) then with a lift coefficient of 0.1 the aircraft would have to fly at a velocity of 90m/s,i.e. the aircraft would be cruising at 90m/s at zero incidence. If the aircraft was to fly inverted under these conditions then it would need a lift coefficient of -0.1 and this would be at an incidence of -2 deg, i.e. inverted the stick would be pushed forward.

In both the cases above, flying slower would mean a higher lift coefficient (upright) and a more negative coefficient inverted. For lift coeffiicent = 0.2, speed = 63m/s and incidence approx 1deg upright and -3deg inverted.

For aircraft that habitually fly inverted, e.g. aerobatic planes, the wing cross section is symmetrical so that stall angle and lift characteristics are the same regardless of the attitude of the aircraft.


The Bernoulli equation, in the link above, basically links pressure and velocity.
wheat and dairy can screw up your brain
 
BAE146QT
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 9:37 pm

Quote:
Aircraft that are designed for aerobatics have symetrical wings. i.e. they work the same upside down as the right way up.



Quote:
For aircraft that habitually fly inverted, e.g. aerobatic planes, the wing cross section is symmetrical so that stall angle and lift characteristics are the same regardless of the attitude of the aircraft.

Thanks chaps - I meant to mention that for exactly that reason. The problem was that when I composed my post, I had a picture of a 747 in my head.
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B737200
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 10:08 pm

You guys seem like you really know your stuff, can you suggest any textbooks on aerodynamics I can order (I don't really enjoy looking this stuff up on the internet)? I was going to order one a while ago but I wasn't sure which one I should get and I just forgot about.

I'd love to read up on this, I actually started playing around with equations for fun this morning which is quite surprising considering I'm still "recovering" from my exams.

Thanks.
Lady Guinness is ready to fly...
 
oly720man
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 10:42 pm

Quoting B737200 (Reply 14):
can you suggest any textbooks on aerodynamics

Unfortunately aerodynamics textbooks can be expensive.

http://www.amazon.com/Introduction-F...ref=pd_sim_b_3/103-3477413-4745446

http://www.amazon.com/Understanding-...ref=pd_sim_b_3/103-3477413-4745446

http://bookshop.blackwell.co.uk/jsp/...tals_of_Aerodynamics/9780071254083

Malta University does have a dept of Mechanical Engineering and it's possible that any academic bookshops there could have the above books, or people who have been on the course could be selling them.
wheat and dairy can screw up your brain
 
B737200
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 10:46 pm

Thanks alot, I'll give them a looking at, my mother should be willing to help me pay for one since its educational.

Once again thanks.
Lady Guinness is ready to fly...
 
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Starlionblue
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RE: Upside Down Aerodynamics

Wed Jun 13, 2007 10:53 pm

Quoting CoolGuy (Thread starter):

The exceptions would be the A320, A330, A340, A380, and the 777 because the flight control laws for their fly by wire software will not let them do it.

Does the 777 really limit this? I don't think so.

Quoting B737200 (Reply 4):

During normal flight the curved upper surface of the wing creates an area of negative pressure which creates lift. Now if a aircraft were to fly upside down, not in a roll or anything, wouldn't the area of negative pressure act downwards towards the ground (sort of like the inverted "wings" on formula 1 cars designed to keep the cars on the ground) ? So then how would the aircraft stay aloft ?

As some have stated, angle of attack is key. As also stated above, aerobatic aircraft (and many fighters) have symmetrical wings and derive no lift from camber and so forth.

Quoting B737200 (Reply 4):

I'd imagine that in such a case the combined forces of weight and suction from the wings' pressure differences both acting downwards and the lack of anything to counter it would cause the aircraft to fall to the ground.

Just a point of order. It's not really suction from the lower pressure above. It's the higher pressure below the wing that pushes up. Not quite the same thing.

And the weight (that is, the force of gravity acting on the aircraft) is of course the same whether the aircraft is inverted or not.


Bernouilli is a convenient but flawed way of explaining a complex process in the space afforded by a schoolbook page. While it has merit, it is, as has been stated above, flawed. As a matter of fact, any "simple" explanation for lift is flawed since lift is not simple. For an excellent summary of lift and why Bernouilli and Newtonian explanations are somewhat right but also have flaws, see here: http://www.howstuffworks.com/airplane4.htm. Keep going in the article.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FredT
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 12:52 am

Quoting B737200 (Reply 16):
Just a point of order. It's not really suction from the lower pressure above. It's the higher pressure below the wing that pushes up.

For your typical wing in the typical regimes of flight, the delta between ambient pressure and the low pressure on top of the wing is significantly larger than the delta between the ambient pressure and the high pressure below the wing. In addition the area on top of the wing will be larger. This means that it is indeed correct to say that lift (mainly) sucks!

Quoting B737200 (Reply 14):
You guys seem like you really know your stuff, can you suggest any textbooks on aerodynamics I can order (I don't really enjoy looking this stuff up on the internet)?

If you are after an introduction, John D. Andersons "Introduction to flight" linked above does a nice job. "Aerodynamics for naval aviators" is another good read, if you can get your hands on it. One of the best reads for a beginner should be

http://www.av8n.com/how/

If you do not want to read on the net under any circumstances you could consider having it printed.

Quoting B737200 (Reply 7):
I'm assuming that if the deflection of the air is to keep the aircraft aloft is must be equal to the combined weight and suction of Bernoulli right?

As you read on you will find that it is the pressure differences around the wing which deflect the air. These are consistent with Bernoulli's theorem (at least until you get significant compressibility effects, at M.3 or so), so it is not one or the other acting one way or the other but always the two going hand in hand.

Yes, a wing designed for non-inverted flight will be less efficient when flying inverted. Most wings will do it though. Aircraft are required to be certified for negative Gs (except when dirty, but that's another discussion) for starters, meaning they are able to generate negative lift (negative lift coefficient). It is just a question of just how much airspeed you need to generate enough negative lift to equal the weight of the aircraft. If that airspeed is above Vne, then you are not able to fly inverted.

Rgds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
David L
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:05 am

Quoting Starlionblue (Reply 17):
Does the 777 really limit this? I don't think so.

I didn't think the 777 FBW had any hard limits, either.

Quoting CoolGuy (Thread starter):
For the second part, what does the software restrict? No more than 45 degrees?

The bank limit with Airbus FBW is 67o, I believe.

Quoting BAe146QT (Reply 8):
I'm not really clear on what would happen to the airflow on the "top" of the wing at this point, (which is the normal bottom of it, of course). Flying normally, the upper surface of the wing is designed to encourage laminar flow and maintain the boundary layer in a predictable shape. The underside is not going to be as good at this and I would think you would have turbulence (with a corresponding increase in drag) and maybe even a higher clean stall speed.

As explained above, the wing would be less efficient when inverted, even with a net positive angle of attack, so a higher angle of attack is used to compensate.

But, in a barrel roll, the aircraft is supposed to lose some altitude when inverted anyway - it's inverted at the top of the spiral and the right way up at the bottom of the spiral.

Of course, I'm not one of those guys who really knows his stuff so I'd take it with a pinch of salt.  Smile
 
FredT
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:24 am

Quoting David L (Reply 19):
But, in a barrel roll, the aircraft is supposed to lose some altitude when inverted anyway - it's inverted at the top of the spiral and the right way up at the bottom of the spiral.

(My boldface)

Not necessarily. You can make a climbing barrel roll and never once lose altitude. The requirement to maintain a non-negative nz does hovever imply a downwards acceleration during the inverted part of the manoeuver.

Do I get the 'nitpicker of the week' award now?  Wink
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Starlionblue
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:37 am

Quoting FredT (Reply 18):
For your typical wing in the typical regimes of flight, the delta between ambient pressure and the low pressure on top of the wing is significantly larger than the delta between the ambient pressure and the high pressure below the wing. In addition the area on top of the wing will be larger. This means that it is indeed correct to say that lift (mainly) sucks!

Well I guess it's one of those "how many angels" questions. As I see it, the molecules in the high pressure areas are trying to get to the low pressure areas, not the other way around. The process pushes the wing up. It's not the lower pressure "calling" the molecules to it.

But I may be wrong.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
David L
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:41 am

Quoting FredT (Reply 20):
Do I get the 'nitpicker of the week' award now?

Yeah, fair enough. It would be hypocritical of me to complain.  Smile
 
David L
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:50 am

Quoting Starlionblue (Reply 21):
The process pushes the wing up. It's not the lower pressure "calling" the molecules to it.

Yes. While "suction" is a force we understand, technically the high and low pressures are both trying to push but the high pressure wins. The only things allowed to suck in physics are exams.

Who was it who asked "How many nits could a nit-picker pick if a nit-picker could pick nits?"  duck 

Sorry, but it's been a while since we've had a good nit-picking contest.  biggrin 
 
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Starlionblue
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 1:58 am

Quoting David L (Reply 23):
The only things allowed to suck in physics are exams.

Black holes too surely? Big grin


Which reminds me of the following bash.org transcript:
#99835 +(15877)- [X]
Hey, you know what sucks?
vaccuums
Hey, you know what sucks in a metaphorical sense?
black holes
Hey, you know what just isn't cool?
lava?


For those who don't know, bash.org collects funny or outrageous IRC (Internet Relay Chat) transcripts. Before you click this link, make sure you have an hour to kill and a penchant for chatroom humor. IMHO one of the funniest sites on the web http://bash.org/?top
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
David L
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 2:28 am

Quoting Starlionblue (Reply 24):
Black holes too surely?

No, still not allowed... unless you feel you're being sucked on to the surface of the earth.  Smile

However, since I've just had my first Robinson* Martini for several years, I'd better take a break and give this a go...

Quoting Starlionblue (Reply 24):
Which reminds me of the following bash.org transcript:

* Named after the person who introduced it to me:

1 part gin,
1 part Vermouth, extra dry,
1 part Vermouth, Bianco,
as many green olives as you can fit on a cocktail stick (I don't think there's a limit on the size of the cocktail stick).

Repeat until you sense an impending a.net ban.  drunk 
 
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Scooter01
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 3:15 am

I was going to try to make an intelligent posting, but it's after dinner and I'm getting drowsy so this looked like a better idea:

Quoting David L (Reply 25):
1 part gin,
1 part Vermouth, extra dry,
1 part Vermouth, Bianco,
as many green olives as you can fit on a cocktail stick (I don't think there's a limit on the size of the cocktail stick).

a bit different than my usual version which is:

Two Spanish olives on a cocktail-stick in a martini glass
Cover the bottom olive with dry vermouth (Noilly Prat will work fine)
Add one or two drops of brandy
Top up with Tanqueray from the -18C freezer

Intelligent posting will come tomorrow unless someone else beats me to it.
(Must be from someone who is qualified on building and flying U-controlled model airplanes)


Scooter
There is always a good reason to watch airplanes
 
FredT
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 3:58 am

Intellisc... entille... intelle... ah, drats... smart posting I cannot provide.

But I will contend that suction does exist in certain contexts. Such a context would be an environment with an ambient pressure exceeding zero. A lack of pressure relative to ambient in that environment can, and often for the sake of convenience will, be (drumroll) designated suction.

And the fact remains that the relative lack of pressure on top of the wing (suction) provides more of the lifting force than the relative increase of pressure below the wing. My pointing out that the pressure reference was ambient was to make the context clear. I have all my bases covered... suckers.  Wink

As a sidenote, you'll find that American and British WWII aircraft had a built-in feature to confuse pile-its converting. The Brits tended to use atmosphere pressure referenced manifold pressure gauges while the Americans used absolute pressure manifold pressure gauges. If you didn't know, you could be tempted to deduce that the British engines sucked while the American superchargers blew. Big grin

And Starlionblue... for posting that bash.org link... you owe me 30 minutes of my life! Thanks man, got a few good laughs there. I used to be a computer geek before I converted to aero engineering (and became a dual purpose geek).

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
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Starlionblue
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 5:43 am

My martini is quite barbaric but it works:
- Put ice cubes in cocktail shaker.
- Pour small amount of extra dry vermouth onto the ice cubes.
- Close up and shake.
- Siphon all the vermouth out, leaving the ice cubes in. This will leave a coating of vermouth on the ice cubes and nothing more.
- Pour gin (kept in the freezer) as needed into shaker.
- Close up and shake.
- Pour into glasses, leaving ice in shaker.
- Garnish with olives as desired.

The resulting martini is very very cold and very very dry. Just the way Iike it.

Quoting David L (Reply 25):

No, still not allowed... unless you feel you're being sucked on to the surface of the earth.

I know.  Wink

Quoting FredT (Reply 27):

And Starlionblue... for posting that bash.org link... you owe me 30 minutes of my life! Thanks man, got a few good laughs there. I used to be a computer geek before I converted to aero engineering (and became a dual purpose geek).

Only 30? Don't forget to move on to the runners up (100-200)  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
N231YE
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 6:04 am

Quoting Starlionblue (Reply 17):
Just a point of order. It's not really suction from the lower pressure above. It's the higher pressure below the wing that pushes up. Not quite the same thing.

I was told to shy away from the "high below pushes, low above 'sucks'" when I prepared to take my Oral exam for my PPL. I was told the correct response is, "the relatively higher pressure (in relation to the ambient surroundings) below the airfoil and the relatively lower pressure above the airfoil interact to generate lift."  wink 
 
David L
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 6:55 am

Quoting FredT (Reply 27):
and often for the sake of convenience will, be (drumroll) designated suction.

Yes, agreed.  Smile

Quoting N231YE (Reply 29):
I was told the correct response is, "the relatively higher pressure (in relation to the ambient surroundings) below the airfoil and the relatively lower pressure above the airfoil interact to generate lift."

That's OK in my book. The point is that the air that's allegedly "sucking" the wing upwards is actually doing the opposite by pressing downwards, just not as effectively as the air underneath is pushing the wing upwards.

Quoting Starlionblue (Reply 28):
Quoting David L (Reply 25):

No, still not allowed... unless you feel you're being sucked on to the surface of the earth.

I know.

Ah, OK. Just a security check - you can't be too careful  Smile

Quoting Scooter01 (Reply 26):
Cover the bottom olive

I try to cover the ice.
 
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Starlionblue
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 10:03 am

Quoting David L (Reply 30):
Quoting Starlionblue (Reply 28):
Quoting David L (Reply 25):

No, still not allowed... unless you feel you're being sucked on to the surface of the earth.

I know.

Ah, OK. Just a security check - you can't be too careful

I am a SciFi buff you know.  Wink

As evidenced by my very geeky and low quality review site http://books.rosboch.net
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
CoolGuy
Topic Author
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 10:11 am

For commercial airliners, is it a rule that aircraft cannot bank more than 30 degrees?

Also is there anything else that the FBW systems do not permit? Like a very quick climb? I remember hearing that the system will automatically prevent a stall by pushing the stick down at high angles of attack, though a pilot can try to override it.
 
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Starlionblue
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 10:47 am

Quoting CoolGuy (Reply 32):
For commercial airliners, is it a rule that aircraft cannot bank more than 30 degrees?

Pretty much. If nothing else you want to keep g loads within limits conducive to passenger comfort. The plane may be able to take in excess of 2 G, but the pax wouldn't like that very much.

Quoting CoolGuy (Reply 32):

Also is there anything else that the FBW systems do not permit? Like a very quick climb? I remember hearing that the system will automatically prevent a stall by pushing the stick down at high angles of attack, though a pilot can try to override it.

Well, that's a question requiring a rather long-winder answer.

First and foremost: FBW (Fly By Wire) simply means that signaling from the yoke/stick and rudder is electronic instead of mechanical. FBW in itself does not imply "limitations". Any limitations built in are in addition to this electronic signaling. For example, the Airbus FBW system incorporates computers that take control inputs from the pilots and translate them into outputs for the surfaces. While in a Cessna 172 moving the stick gives an exactly proportional response from ailerons and elevators, in a FBW Airbus, the control surfaces may move in a manner not completely related to the stick movement, except that they will produce the exact EFFECT desired by the pilot. An example of this filtering by the computers is that, under normal law, moving the stick to one side commands an exact roll RATE. This is different from a Cessna 172, where moving the yoke thus would just command the ailerons to move, with resultant roll rate depending on airspeed, aircraft weight and other factors.

The Airbus flight control system includes what is known as "envelope protection". This is a set of rules built into the system to make it harder for the aircraft to be flown in such a way that it will be damaged, stall, and so forth.

In "Normal law", the normal operating condition of the flight control system, includes the following protections:
- High AOA Protection.
- Load Factor Limitation (no overstressing of the aircraft).
- Pitch Attitude Protection
- High Speed Protection (no overspeed).
- Flight Augmentation (Yaw)
- Bank Angle Protection.

In "Direct Law", the operating condition where the computers are malfunctioning, the system reverts to functioning just like a Cessna 172. That is, control inputs are not filtered, but simply result in directly proportional responses from the surfaces.

More detail here: http://www.airbusdriver.net/airbus_fltlaws.htm
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
ThirtyEcho
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RE: Upside Down Aerodynamics

Thu Jun 14, 2007 7:16 pm

OK, Tex Johnson's famous roll of the 707 was a 1G maneuver; the airplane never "knew" that it was in a roll. Had you been a passenger in the airplane in IMC at night, with Tex at the yoke, it would have felt like straight and level flight. Even in the daytime, with the window shades open, it would have seemed that the planet Earth just did a roll around your perfectly straight airplane. Inflight meal service would have been normal, throughout.

Tex was THAT good.
 
FredT
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RE: Upside Down Aerodynamics

Fri Jun 15, 2007 4:35 am

Quoting CoolGuy (Reply 32):
For commercial airliners, is it a rule that aircraft cannot bank more than 30 degrees?

No. But above 60 degrees of bank and +/- 30 degrees of pitch it is considered aerobatics, IIRC, and I doubt you'll be able to find an airliner certified for aerobatics.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
futureuapilot
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RE: Upside Down Aerodynamics

Fri Jun 15, 2007 5:19 am

Ponder this for a second...

A wing passes through the air.. the air passes over the top of the wing. The faster the air travels, the less pressure there is. The less pressure helps pull the airplane into the air. That's all fine and dandy.

But what physics rule makes the the air pass over the top quicker? Air molicule A and B are right next to eachother. A wing comes through.. A goes Above, and B goes Below. What law of nature says they have to meet back up together at the trailing edge of the wing? If that's the case, I understand why the air goes faster.. more distance to cover... but why does it have to go faster.. it could go the same speed as the air passing underneith the underside of the wing.. and just not meet up with molicule B at the same time.

Just some food for thought...

-Sam
The Pilot is the highest form of life on Earth!
 
FredT
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RE: Upside Down Aerodynamics

Fri Jun 15, 2007 5:56 am

The air goes faster as it is accelerating into a low pressure on top of the wing. The low pressure exists due to the air accelerating inwards to follow the curvature of the wing (Coanda, centripetal force etc). Acceleration and a negative pressure gradient always go hand in hand, and the opposite holds true for a positive pressure gradient and deceleration.

That is in fact exactly the point of Bernoulli's theorem. One does not create the other, but one cannot occur without the other.

Through the Coanda effect we impart an acceleration to the air, thus lowering the pressure.

Two air molecules separated at the leading edge do not meet up. The molecule going over the top of the wing gets to the T/E significantly ahead of the molecule going below the wing.

I'd also like to point out that I had my foot solidly planted in my mouth in some of my previous posts in this thread. For a typical wing in typical flight conditions there is no high pressure below the wing, as compared to ambient. The pressure will be below ambient below the wing as well, only less below ambient than on top of the wing. Positive pressure, compared to ambient, will only be found around the stagnation point on the wing L/E.
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MrFord
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RE: Upside Down Aerodynamics

Fri Jun 15, 2007 12:13 pm

Quoting Starlionblue (Reply 24):
For those who don't know, bash.org collects funny or outrageous IRC (Internet Relay Chat) transcripts. Before you click this link, make sure you have an hour to kill and a penchant for chatroom humor. IMHO one of the funniest sites on the web http://bash.org/?top

While I'm definitely drifting waaayy off course, you own me 1 hour of my life.. for now  Wink
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OldAeroGuy
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RE: Upside Down Aerodynamics

Sat Jun 16, 2007 3:40 am

Quoting CoolGuy (Thread starter):
The exceptions would be the A320, A330, A340, A380, and the 777 because the flight control laws for their fly by wire software will not let them do it.



Quoting Starlionblue (Reply 17):
Does the 777 really limit this? I don't think so.

Starlionblue is correct. The 777 can be rolled inverted in normal mode but the wheel forces to do so would be very high.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
A320ajm
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 1:35 am

Quoting CoolGuy (Thread starter):
The exceptions would be the A320, A330, A340, A380, and the 777 because the flight control laws for their fly by wire software will not let them do it.

The fly by wire system ca be turned off on the aircraft mentioned and can do barrel roles. I saw an Airbus A320 do a barrel role in a flight simulator. I have it on DVD.
Thanks
A320ajm
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Starlionblue
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 4:12 am

Quoting A320ajm (Reply 40):
The fly by wire system ca be turned off on the aircraft mentioned and can do barrel roles.

Well, FBW cannot be turned off since FBW is used to signal the surfaces whatever law is in use. However, envelope protection can be turned off (I guess) so that the control logic does not stop certain maneuvers (in this case by limiting bank angle).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
AirSpare
Posts: 570
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 5:34 am

Quoting BAe146QT (Reply 5):
modified to fly in the southern hemisphere

Don't props spin in the opposite direction also? uggh, sorry about that one, flame away.

Quoting ThirtyEcho (Reply 34):
Tex was THAT good

damn, I'm trying to remember the name of the WWII pilot that performs airshow demos in a Shrike Commander, I saw a video of him rolling the Shrike while another guy poured a glass of ice tea.

He is an excellent pilot, I talked with him for 10 in Reno, nice person also. How could I forget. Ah yea, Bob Hoover, in his big floppy hat.
Get someone else for your hero worship fetish
 
474218
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 6:08 am

Quoting AirSpare (Reply 42):
Quoting BAe146QT (Reply 5):
modified to fly in the southern hemisphere

Don't props spin in the opposite direction also? uggh, sorry about that one, flame away.

Maybe thats why the P-38 was such a good fighter during WWII. It's props turned in opposite directions, making it a efficient in both the European and Pacific theaters.
 
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Starlionblue
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 11:05 am

a Shrike Commander, I saw a video of him rolling the Shrike while another guy poured a glass of ice tea.[/quote]

Quoting AirSpare (Reply 42):

damn, I'm trying to remember the name of the WWII pilot that performs airshow demos in a Shrike Commander, I saw a video of him rolling the Shrike while another guy poured a glass of ice tea.

He is an excellent pilot, I talked with him for 10 in Reno, nice person also. How could I forget. Ah yea, Bob Hoover, in his big floppy hat.

Bob Hoover in action: http://www.youtube.com/watch?v=9ZBcapxGHjE

Quoting 474218 (Reply 43):

Maybe thats why the P-38 was such a good fighter during WWII. It's props turned in opposite directions, making it a efficient in both the European and Pacific theaters.

 rotfl 

This reminds me of when Andrew Zimmern of the Travel Channel was in Quito, Ecuador. At the equator monument, they let people balance eggs on nails. "Since we are farther from the center of the Earth here, gravity is less. So you can do this". Well, first of all, the earth is flattened to the equator is slightly closer to the center than the poles. But that's neither here not there. Just like the "balance an egg on the equinox" thing, this one is a myth.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
BAE146QT
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 4:59 pm

Quoting Starlionblue:
Well, first of all, the earth is flattened to the equator is slightly closer to the center than the poles.

Are you sure about this? The Earth is an oblate sphere, but the bulge is at the equator. Think about it - centripetal force means that the bulge has to be perpendicular to the axis of spin.
Todos mis dominós son totalmente pegajosos
 
keta
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 6:07 pm

Quoting Starlionblue (Reply 44):
Bob Hoover in action: http://www.youtube.com/watch?v=9ZBca...xGHjE

 eek  eek  I couldn't believe it when I saw it! He's got not only great flying skills, he's gotta be a good waiter too, pouring a glass upside down without spilling some!  tongue 
Where there's a will, there's a way
 
David L
Posts: 8551
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 7:12 pm

Quoting BAe146QT (Reply 45):
Quoting Starlionblue:
Well, first of all, the earth is flattened to the equator is slightly closer to the center than the poles.

Are you sure about this? The Earth is an oblate sphere, but the bulge is at the equator.

I have to agree with BAe146QT on this one. I seem to recall that the force of gravity is about 9.81 ms-2 at the equator and about 9.83 ms -2 at the poles.
 
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Starlionblue
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RE: Upside Down Aerodynamics

Tue Jun 19, 2007 9:20 pm

Quoting BAe146QT (Reply 45):
Quoting Starlionblue:
Well, first of all, the earth is flattened to the equator is slightly closer to the center than the poles.

Are you sure about this? The Earth is an oblate sphere, but the bulge is at the equator. Think about it - centripetal force means that the bulge has to be perpendicular to the axis of spin.



Quoting David L (Reply 47):
I have to agree with BAe146QT on this one. I seem to recall that the force of gravity is about 9.81 ms-2 at the equator and about 9.83 ms -2 at the poles.

I had a major brainfart here. You guys are of course correct.

However this doesn't change the fact that balancing an egg on a nail isn't any easier at the equator. CoG is CoG wherever you are.

For more about balancing eggs, I refer you to The Bad Astronomer here: http://www.badastronomy.com/bad/misc/egg_spin.html. This website is a marvelous place where Phil Plaitt (The Bad Astronomer) debunks various scientific misconceptions http://www.badastronomy.com/bad/misc/index.html and does scientific movie reviews http://www.badastronomy.com/bad/index.html. Yes, he is a real astronomer.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FredT
Posts: 2166
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RE: Upside Down Aerodynamics

Wed Jun 20, 2007 12:07 am

Quoting Starlionblue (Reply 48):
However this doesn't change the fact that balancing an egg on a nail isn't any easier at the equator. CoG is CoG wherever you are.

I'm verry sorry. I seem to be contradicting you in every thread as of lately. That includes this one. Big grin

It is in fact easier balancing an egg on a nail if the gravitational acceleration is smaller. The egg will accelerate less rapidly once it starts tipping, giving you more time to react and stop it from falling off the nail.

Completely theoretical of course, when going from 9.81 m/s^2 to 9.83 m/s^2, but if you were to try it on the Moon you'd find it a lot easier.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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