Guest

Which Is It, High Or Low Pressure?

Wed Mar 07, 2001 8:52 am

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
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Which Is It, High Or Low Pressure?

Wed Mar 07, 2001 9:02 am

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
 
Guest

RE: Which Is It, High Or Low Pressure?

Wed Mar 07, 2001 1:03 pm

Hasn't anyone told you yet? Forget all of those other theories - it's "Lifties" that make an airplane fly.
 
MCPilot
Posts: 16
Joined: Wed Feb 28, 2001 12:58 am

RE: Which Is It, High Or Low Pressure?

Wed Mar 07, 2001 10:17 pm

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.
 
Guest

RE: Which Is It, High Or Low Pressure?

Thu Mar 08, 2001 2:40 am

Uh oh, sounds like there's an argument brewing here between newtonian lift and bernoulli's theory.
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

RE: Which Is It, High Or Low Pressure?

Thu Mar 08, 2001 3:23 am

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.
 
Guest

RE: Which Is It, High Or Low Pressure?

Thu Mar 08, 2001 7:11 am

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.
 
Guest

RE: Which Is It, High Or Low Pressure?

Thu Mar 08, 2001 7:20 am

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).
 
Guest

RE: Which Is It, High Or Low Pressure?

Thu Mar 08, 2001 7:31 am

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).
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

RE: Which Is It, High Or Low Pressure?

Fri Mar 09, 2001 10:31 am

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.
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Which Is It, High Or Low Pressure?

Fri Mar 09, 2001 10:21 pm

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.
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

Rocks, Pigs...

Sat Mar 10, 2001 4:16 am

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.
 
Guest

RE: Which Is It, High Or Low Pressure?

Sat Mar 10, 2001 8:52 am

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 ?
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Spoiler

Sat Mar 10, 2001 10:56 am

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 avatar
Bruce
Posts: 4934
Joined: Tue May 18, 1999 2:46 am

RE: Which Is It, High Or Low Pressure?

Sat Mar 10, 2001 2:02 pm

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
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Which Is It, High Or Low Pressure?

Sat Mar 10, 2001 3:25 pm

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.


 
Panman
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Which Is It, High Or Low Pressure?

Sat Mar 10, 2001 5:21 pm

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
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Spoiler

Sun Mar 11, 2001 11:16 am

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
 
Guest

RE: Which Is It, High Or Low Pressure?

Mon Mar 12, 2001 3:40 am

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
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Which Is It, High Or Low Pressure?

Sat Mar 17, 2001 4:42 am

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
 
Guest

RE: Which Is It, High Or Low Pressure?

Sat Mar 17, 2001 5:55 pm

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.....
 
Guest

RE: Which Is It, High Or Low Pressure?

Sat Mar 17, 2001 10:48 pm

Humm?? I thought if you doubled your speed your squared your drag??
 
Panman
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Which Is It, High Or Low Pressure?

Sun Mar 18, 2001 4:56 am

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
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Panman

Sun Mar 18, 2001 6:44 am

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
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

More Stuff...

Tue Mar 20, 2001 1:26 am

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.
 
Western727
Posts: 1418
Joined: Wed Jan 03, 2007 12:38 pm

RE: Which Is It, High Or Low Pressure?

Tue Mar 20, 2001 5:32 am

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 created by the pressure differential, and that lift created by striking the underside of the wing is a minor contributor. I thought that there was a general consensus of this concept among the aviation community, but I guess not. The texts don't lie.
Jack @ AUS
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: More Stuff? Spoiler, Please

Tue Mar 20, 2001 1:12 pm

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 this discussion. Please, when you have time, review my previous posts with links to various interesting sites discussing lift. Even a NASA supported site.

This is the last link I will add to this discussion as it has been beaten to death:

http://www.geocities.com/CapeCanaveral/Hangar/6544/

To pique your curiousity this latest link begins:
During most of the 20th century, much of the popular teaching of how wings work has been false. In part this has been deliberate. Many years ago, a most famous aerodynamicist, Dr. Theodore VonKarman, instructed his assistant: "When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth." (From Stories of a 20th Century Life by W.R. Sears)

From Gale Craig's book "Stop Abusing Bernoulli, How Airplanes Really Fly."

You describe your occupation as a student in your profile. Read and learn.

Regards,
JG
 
Guest

Mr. JG And All Others

Wed Mar 21, 2001 1:14 am

JG, the post you have suggested are wonderful and a wealth of excellent information. Everyone should thank you for your excellent comments and bringing to light some " new" information. Not quoting something from the bicycle shop in Dayton Ohio way back when. Some day hopefully, some of these kids will realize that most of the publications are not "Gospel". Thanks to all for sharing their ideas but I think I will stick with some of the listed links from JG. "Grumpy Max Power"  Smile
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

JG

Wed Mar 21, 2001 2:26 am

So, are you saying that the Jepp manual is inaccurate?
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

JG's Last Post

Wed Mar 21, 2001 2:35 am

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 how an airplane remains airborne. Now JG, I stood up for you in another area of this site. I refrained from commenting on the other, less delicate remarks you made in that post. I respect your right to disagree with me (as incorrect as you may be), but you have no right to make personal attacks.
 
spoiler
Posts: 47
Joined: Sat Apr 07, 2007 8:19 pm

RE: Which Is It, High Or Low Pressure?

Wed Mar 21, 2001 2:38 am

PS, Don't bother to respond to that last email, because I'm not going to click on this topic again.
 
Mr Spaceman
Posts: 2723
Joined: Thu Mar 15, 2001 5:09 am

Which Is It?

Wed Mar 21, 2001 2:46 am

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 Molecules" from High to Low pressure, searching for "Dynamic Equallibrium" to occure causes a "vacume" that litterally sucks the aircraft off the runway and up into the air. At this point, a "planeing effect" against the bottom of the wing comes into play. ---If the phenomenom of lift, caused by "Low Pressure", was Not so important, aircraft wouldn't need airfoils! [at least, thats what I think]. P.S. -- Dare I mention the word "Helicopter?"
"Just a minute while I re-invent myself"
 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: More Stuff? Spoiler

Wed Mar 21, 2001 3:29 am

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 inappropriate frame of reference and utilizing a particle view of air as opposed to a fluid. I do not know what you study in college or university but I am sure that a physics text is available in a library.

Not every pilot applicant has the knowledge base, desire, or even need to know more than is provided in the current texts in order to fullfill the requirements of their appropriate certificate. I do believe the message in the quote from one of the links that I posted:

"When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth." (From Stories of a 20th Century Life by W.R. Sears)

For the purposes of this discussion I believe some of the currently available texts do in fact "dumb it down" to a level appropriate for their audience. According to the NASA supported site I posed earlier the information provide by them, even the FAA's own test questions, are largely incorrect . What they say is not necessarily wrong, just horribly incomplete. Therefore the text you reference is inadequate for this discussion.

Previously, I have spent years instructing in light aircraft mostly in a university setting. I have read the text you reference. I found that I had more questions. I read and researched further. As in previous posts, I invite you and others to do the same. Even go beyond reading the links I provided. With this forum member's and your own high level of enthusiam for aviation no single reference is adequate on the book shelf.

I hope this explains my position.

Cheers,
JG


 
JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

RE: Spoiler And Spaceman

Wed Mar 21, 2001 12:24 pm

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. The quote from the site referenced addresses an attitude that some publishers and even your federal goverment takes to dumb down concepts because they think their audience/constituents can't handle the truth.

I tried all of 3 of the links included in my replies and they work for me. The first one is good and may be the one that caused problems. I also read news on http://www.avweb.com and must have the cookie hanging around. Go to that site and search for lift and read what what you find on "Lift Doesn't Suck."

Now, what are you talking about when you say you stood up for me on some other area of this site. At times I engage in a discussion, or provide info, sometimes I even provide an opinion. I don't say anything worthwhile in these posts that would require defense. What's up? Your last response in this argument was rather sophomoric. Just like me, you know you can't stay away from this site or this topic. I know you have read this, even if you do not reply. I will update my profile and provide my email address. Write to me directly if wish.

Mr. Spaceman check out the links in my other posts after my reply titled "RE: Spoiler." They are just information and interesting reading... that is all, no attacks... However, those links may not contain the answer you want to hear.

Enjoy
JG
 
chdmcmanus
Posts: 372
Joined: Wed Mar 21, 2001 12:53 am

RE: Which Is It, High Or Low Pressure?

Wed Mar 21, 2001 1:09 pm

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 rather have servo tabs which were moved by the yoke. The servo tabs are less than 1/8 the size of the control surface in which it moves! Now if you do the math (I can barely run it myself so I'm not even going to attemp to write it) the effect is, when movement begins, there isn't enough "bernoilli" to begin motion, and once it has displaced the aileron, there isn't enough "newtonian" to maintain deflection, but the surface moves smothly and relitvely effortlessly through all ranges of motion. In my opinon that's logical to assume that they work together to perform a desired action. The other side is supersonic flight. As an airfoil aproaches transonic speeds, the airflow begins to break away until at supersonic speeds a sharp angle forming a "shockwave" is in front of the leading edge, Obviosly using "newtonian", but the control surfaces and turbine blades are still effective! In the late 40's, before supersonic flight had been acheived, the most popular belief was that full control reversal would occour due to the loss of airflow, but what was discovered was there is enough airflow around the shockwave to use "bernoulli" on the flight controls, once again both working together. All of my engineering books refer to an airfoil as " any surface designed to obtain a desireable reaction from the air through witch it moves" It doesn't say anything about either principle because they can both be used to obtain a "desireable reaction". In my opinion, this is why there are such radicaly diffrent wing designs for varios acft, and why a pilot can maintain control throughout all regimes of flight.
"Never trust a clean Crew Chief"

Who is online

Users browsing this forum: LX015 and 25 guests