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Which Is It, High Or Low Pressure?  
User currently offlineMax Power From , joined Dec 1969, posts, RR:
Posted (13 years 6 months 3 weeks 1 day 20 hours ago) and read 2082 times:

Ok I need some help with this one. All the books I read and folks I talk to say it's the low pressure over the top of the wing that gives lift. The air flowing over the top has to go faster than the air on the underside, there fore a low pressure area and lifting the wing up. There must be also a lot of pushing from the air on the bottom side also? I say this because for example, in a car at 50 mph and your hand outside the window, angle it up and the air pushes your hand up, just like under water. How much of lift is the upward push under the wing with a certain angle of attack. This has to play a big part in the aerodynamics of flying? No one ever talks about the underside of the wing. thanks

34 replies: All unread, showing first 25:
 
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 1, posted (13 years 6 months 3 weeks 1 day 20 hours ago) and read 1878 times:

The pressure differential around a wing accounts for only a small part of the lift. There is high pressure below, which wants to get on top where there is low pressure, thus pushing the wing up. However, most of the lift comes from the angle of attack of the wing. The angle of attack (AoA) is the angle between the chord line (imaginary line from the tip of the trailing edge to the tip of the leading edge) and the relative wind. When you stick your hand out the window of a car, you are feeling lift due to AoA. You turn your hand up, it has a positive AoA and is pushed up, you turn your hand down, it has a negative AoA and is pushed down. Airplane wings, however, rarely have a negative AoA.  Big thumbs up


09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 2, posted (13 years 6 months 3 weeks 1 day 16 hours ago) and read 1843 times:

Hasn't anyone told you yet? Forget all of those other theories - it's "Lifties" that make an airplane fly.

User currently offlineMCPilot From Dominican Republic, joined Feb 2001, 16 posts, RR: 0
Reply 3, posted (13 years 6 months 3 weeks 1 day 6 hours ago) and read 1827 times:

Max Power you are right i agree with you when the wing gets a certain angle of attack there is a lot of pushing from the bottom side.

User currently offlineMikeybien From , joined Dec 1969, posts, RR:
Reply 4, posted (13 years 6 months 3 weeks 1 day 2 hours ago) and read 1809 times:

Uh oh, sounds like there's an argument brewing here between newtonian lift and bernoulli's theory.

User currently offlineSpoiler From Spain, joined Apr 2007, 0 posts, RR: 0
Reply 5, posted (13 years 6 months 3 weeks 1 day 1 hour ago) and read 1803 times:

Yup, an argument. What Ralgha described is reffered to as 'dynamic lift.' But it doesn't account for that much of the total lift generated by wing. Most of the lift is, in fact, formed by the high and low pressure areas. Let's just pretend that it is angle of attack that determines the amount of lift produced. The higher angle of attack, the higher the amount of lift generated. Under those conditions, an airfoil could theoretically never stall, because the critical angle of attack could never be exceeded. Yes, dynamic lift does increas with angle of attack, but dynamic lift isn't a large producer of the total lift.

User currently offlineMax Power From , joined Dec 1969, posts, RR:
Reply 6, posted (13 years 6 months 3 weeks 21 hours ago) and read 1790 times:

Ok, but to Spoiler. lets say I just had a wing or even a 4x8 sheet of plywood. standing in the bed of a small truck, as I increase the angle of attack it pulls me up harder. Should I be able to keep increasing the angle the more lift until, it gets to lets say the vertical or 90 deg. position then it's pushing me back. I would think somewhere there it would be stalled. Thanks.

User currently offlineIFF/7000 From , joined Dec 1969, posts, RR:
Reply 7, posted (13 years 6 months 3 weeks 21 hours ago) and read 1787 times:

It's the underpressure that keeps the wing in the air. At a steady speed and an increasing AOA lift increases until just before the critical angle of attack.At this critical angel of attack the upper airflow starts to separate from the wing and you end up with a stalled wing. And like Spoiler says if you could still increase the AOA the high pressure would increase (but you would still stall).

User currently offlineIFF/7000 From , joined Dec 1969, posts, RR:
Reply 8, posted (13 years 6 months 3 weeks 21 hours ago) and read 1786 times:

Maxpower at 90° you do not feel lift but drag. Drag is the only thing we can miss on a wing. Airflow produces lift and drag with the AOA you increase or decrease both until imax. Check the lift/drag curve (CL/CD with AOA).

User currently offlineSpoiler From Spain, joined Apr 2007, 0 posts, RR: 0
Reply 9, posted (13 years 6 months 2 weeks 6 days 18 hours ago) and read 1770 times:

Ok, so I got a little curious about this one, and I went to my flight manuals to research. It turns out that at a higher airspeed, and lower angle of attack, most of the lift is caused by the pressure differential. At higher angles of attack, dynamic lift increases with the angle of attack. But the majority of lift is created by pressure differential. Dynamic lift is great for lifting boards out of trucks, or hands stuck out car windows, but we are talking about lifting airplanes here. Dynamic lift is enough to lift the weight of the board, or the weight of your hand, given it's surface area, but with airplanes, a lot more weight is lifted per surface area. Could you strap a couple sheets of plywood with the same wing area it's normal wing on an airplane and expect it to fly? If dynamic lift were responsible for the majority of lift produced, yes.

User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 10, posted (13 years 6 months 2 weeks 6 days 6 hours ago) and read 1761 times:

Well Folks,

After all of this, I still favor newtonian over bernoullian for the bulk of lift production. Afterall, we are not all flying straight Clark-Y airfoils or some GA NACA number. I was always told, if you put enough power on a rock it will fly... that goes for plywood, Concorde, Burt Rutan creations, and space shuttles alike.


User currently offlineSpoiler From Spain, joined Apr 2007, 0 posts, RR: 0
Reply 11, posted (13 years 6 months 2 weeks 6 days ago) and read 1748 times:

All you proponents of newtonian lift just keep proving my point for me. Sure, a rock will fly if you can get it moving fast enough, due to dynamic (newtonian) lift. Consider a small airplane such as a Cessna 172. There is no way that the airplane could stay aloft using dynamic lift as the major source of lift. It can't go fast enough. Why do you think we even have airfoils? What about flaps? I'm sure the next thing you are going to tell me is that flaps increase lift because the air flowing underneath the wing hits the flap and is forced down. How would you explain why almost all jet transports have leading edge devices (slats)? Have any of you newtonian guys ever even been to ground school? Come on! This is basic aerodynamics! I want you to do an experiment for me. Take a standard piece of 8-1/2" X 11" piece of paper, and hold it in front of you lengthwise, with two hands, so each hand is holding one of the corners. Ok? You with me? Next, I want to to bend the edge of the paper that's toward you slightly downward, so the paper resembles an airfoil. Next, I want you to blow over the top, not underneath, the top of the airfoil. The paper rises, and it isn't because of dynamic lift. And the newtonian theory still doesn't explain why an airplane can stay aloft with a low angle of attack, such as cruise flight (and I don't want to hear angle of incidence!) What about flight at minimum controllable airspeed (MCA)? The airspeed is low, so dynamic lift diminishes even though the angle of attack is large. How does the airplane maintain altitude? Have you considered induced drag? If airplanes have such a big drag penalty from induced drag, why even bother with airfoils if it is newtonian lift that is the major contributor? What about ground effect? It is the air coming over the wing, striking the ground that causes ground effect, not air deflected from the flaps (the air striking the ground is not newtonian lift the way that we've described it. It's ground effect - a newtonian phenomenon, but not dynamic lift). What about vortex generators? What about spanwise airfoil variations? I don't know how to make it any clearer. Why would any of this come in to play if bernoulian lift wasn't much of a factor.

User currently offlineIFF/7000 From , joined Dec 1969, posts, RR:
Reply 12, posted (13 years 6 months 2 weeks 5 days 20 hours ago) and read 1736 times:

Ground effect : pressure that build up beneath an airplane as it approaches ground-level. Where did you get that air over wing story for the ground effect ?


User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 13, posted (13 years 6 months 2 weeks 5 days 18 hours ago) and read 1744 times:

Wow, touched a nerve, sorry. Thank you for your passionate response. Tried your experiment, loved it. Could we agree that it is a little of both. I do not possess the drive, passion, or inclination (incidence) to battle over this topic.  Smile

My son, needs a diaper change and I am still trying to answer the heredity vs. environment question as I assess his behavior. I feel that it may take a lifetime to answer.


User currently offlineBruce From United States of America, joined May 1999, 5057 posts, RR: 15
Reply 14, posted (13 years 6 months 2 weeks 5 days 15 hours ago) and read 1727 times:

If we go with AoA lift, then what happens when the plane is in level flight? The wings are almost neutral right? then how would it have enough lift to stay up if it weren't for Dynamic lift!

Bruce



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 15, posted (13 years 6 months 2 weeks 5 days 13 hours ago) and read 1734 times:

I offer this simple article to amuse, entertain, enlighten, and present a different frame of reference. This article describes more of a system than a myopic belief in one or the other (Newton or Bernoulli).

http://www.avweb.com/articles/liftsuck/index.html

Enjoy and remember that I am not the author as those who disagree light their flames.




User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 16, posted (13 years 6 months 2 weeks 5 days 11 hours ago) and read 1730 times:

Bruce you just contradicted yourself. Where would the dynamic lift come from at a low angle of attack? Just put your hand (which is not an aerofoil) out of the window of any moving vehicle and keep it parallel to the direction of travel and you hardly feel anything but the air moving around it.

At low angle of attack it is most definately pressure differential that creates lift. In this instance speed is directly proportional to lift created and induced drag.

Panman


User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 17, posted (13 years 6 months 2 weeks 4 days 17 hours ago) and read 1714 times:

This site is for you... to peak you curiosity, round out your education and generally tame your passion. I have been to ground school, like you, been flying since my early teens (13) and have mucho experience under my belt... enough to realize that there is always someone who knows more than me.

Please read and enjoy this site with my compliments... there are many more available if you wish to pursue this further. There were so many questions in your post to address. Please do not treat a private pilot text on lift as gospel to fluid dynamics. I do appreciate your enthusiam, as misdirected as it may be. (observation not flame)
http://www.allstar.fiu.edu/aero/airflylvl3.htm


User currently offlineMax Power From , joined Dec 1969, posts, RR:
Reply 18, posted (13 years 6 months 2 weeks 4 days 1 hour ago) and read 1703 times:

Well, thanks for all the information, now I'm not so sure I want to fly. Smile Thanks to JG and the NASA site. Wish I understood it all! I appreciate everyone's imput. Now then, let's see, if a apple falls from a 30 foot tree and strikes the ground, would the same apple strike the ground twice as hard if the tree was 60 feet? and twice as long in flight?  Acting devilish

User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 19, posted (13 years 6 months 1 week 6 days ago) and read 1686 times:

Hey Max,

Here is another site that I found. Chapters 3 and 18 seem to be enlightening for you. Enjoy with my compliments.

http://www.monmouth.com/~jsd/how/htm/title.html


User currently offlineL_188 From , joined Dec 1969, posts, RR:
Reply 20, posted (13 years 6 months 1 week 5 days 11 hours ago) and read 1678 times:

Lets look at this logically

Newtoning lift is generated by excellerating a mass.
I.E. Force = Mass*velocity

Can we all agree that if you double the airspeed of your aircraft you increase life by 4 times.....

So if I double the airspeed not only have I doubled the force that the wings are hitting the air, I also hit twice as much air since I doubled my speed

So

2M*2V=4Force.....And the force a wing generates is lift.

It looks like mathmatically that force wins out over pressure.....


User currently offlineMax Power From , joined Dec 1969, posts, RR:
Reply 21, posted (13 years 6 months 1 week 5 days 6 hours ago) and read 1674 times:

Humm?? I thought if you doubled your speed your squared your drag??

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 22, posted (13 years 6 months 1 week 5 days ago) and read 1675 times:

No L_188 I can not agree with your hypothesis either, take a theoretical aeroplane with a Mass of 1 Kilo and a velocity of 1 m/s

Then the force comes out at 1 Newton. Let's now say the aircraft accelerates and is travelling at 2 m/s (it still weighs 1 kilo) then the force comes out at 2 Newtons, definately not 4 times.

Also the faster the aircraft goes the more induced drag is produced due to the pressure differential between the upper and lower surfaces of the wing. I can go into why if you want.


Max Power what you stated is related to the drag equation which is:

Drag = 1/2 (rho) v^2 S Cd

Where:
rho = air density
v = velocity
S = function of aerofoil shape
Cd = Coefficient of Drag

If you doubled your velocity the formula would be
1/2 (rho) 2V^2 S Cd

the 1/2 cancels out the 2 so you would get

(rho) V^2 S CD in effect squaring your drag.

Panman


User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 23, posted (13 years 6 months 1 week 4 days 22 hours ago) and read 1675 times:

Panman...

We had a term for your previous post this year during our US Presidential campaign and election... Fuzzy Math.  Smile Looking at your profile I can only assume that you are probably not a democrat and that you derived some entertainment from that recent event.

Be careful, in your equation 1/2 rho V^2 S Cd, whether for drag or Cl for Lift you may not cancel out the "2" in your doubling of the velocity discussion.

it is (2V)^2

it is not 2(V^2)

My long dead algebra teacher thrashed me for that, not to mention the occasional (+/-) error.

Proper handling of your expression should demonstrate to your that lift or drag (as appropriate) will increase by 4x when you double the velocity. Let us hope that the coefficient of lift is greater than the coefficient of drag for a given airfoil... my career depends on it. For your entertainment please enjoy the links to interesting sites that I have posted in previous replies.

Regards,
JG


User currently offlineSpoiler From Spain, joined Apr 2007, 0 posts, RR: 0
Reply 24, posted (13 years 6 months 1 week 3 days 3 hours ago) and read 1664 times:

I'll quote you the paragraph in the Jeppsen Manual where it talks about pressure differential being the major contributor of lift if you'd like.

25 Western727 : Looks like an exciting topic. Except I'm a little surprised that a topic this basic would stir up such a controversy. My texts tell me that lift is cr
26 Post contains links JG : Spoiler, Please put away your Jepp. reference, it will not satisfy this discussion. Your reference, as with many basic texts, does not serve you or th
27 Post contains images Max Power : JG, the post you have suggested are wonderful and a wealth of excellent information. Everyone should thank you for your excellent comments and bringin
28 Spoiler : So, are you saying that the Jepp manual is inaccurate?
29 Spoiler : I checked out that link, and it said "page cannot be displayed." Also, I resent you implying that I'm illiterate. Granted, I'm no engineer, but I know
30 Spoiler : PS, Don't bother to respond to that last email, because I'm not going to click on this topic again.
31 Mr Spaceman : Personally I believe that it is the "Low pressure" on top of the wing that contributes to most of the lift factor. I believe that the "Diffusion of Mo
32 JG : Spoiler, What I am saying is exactly what I have posted in previous replies. I believe the problem with the basic understanding of lift stemms from an
33 Post contains links JG : Spoiler, No, No, No, you have it all wrong. Nothing that I wrote was intended as flame or attack. In no way have I suggested that you were illiterate.
34 Chdmcmanus : Here's a monkey wrench for both arguments. The Boeing 707 is a good example. The ailerons on the 707 aren't actually moved by the control system, but
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