We all seem to be going off into a more "aerodynamic" direction now. So let's keep it open and see if we can keep the semantics lawyers at bay....

If you look at the pressure distribution diagrams for just about any airfoil at some positive angle of attack which is generating a significant quantity of lift it quickly becomes apparent that the area under the charted line for the pressure DROP from the top side is way more than the pressure rise on the bottom side. It may be that the pressure DIFFERENCE is what is supplying the lift but it's pretty clear to anyone looking at those diagrams that the pressure drop over the top surface plays a bigger part than the pressure rise on the lower side.

Something we can play with to get a feel for this is to make sure you Java is up to date and then go play with Foilsim. In particular the pressure distribution readout and how it alters with changes from a very low Coefficient of Lift or Cl value up to the Cl's we get at around 8 to 10 degrees of angle of attack. It's quite the eye opener.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html

Hey Lnewqban,

These are cool pictures! It's like the airplane is literally being sucked up into the area of vacuum.

Just to introduce a little perspective, an average model with a wiingloading of 20 oz/sq.ft would be supported by a pressure rise averaging say 0.02% over the underside, and a pressure reduction averaging 0.04% over the top surface. That is hardly pulling a vacuum, just a slight perturbation of the standard atmospheric pressure of 14.7 psi

For decades, it was believed that air molecules traveling a further distance over an airfoil had to accelerate in order to 'rejoin' with the molecules they were separated from. It wasn't until the early 80's, IIRC, that research at UND proved this did not happen, that there was no mysterious attraction between molecules, and that they rejoined more randomly than believed up to then.

So vacuum cleaners don't suck, huh. Tell that to Dyson. Unless you are an Aerodynamics Engineer with at least a Master's degree you have only an inkling of knowledge of what you are talking about. Aerodynamics is extreemly complex with many, many factors involved. Sometimes simplicity is the best answer.

Kinda like God? lol

Guys, I left the semantics based posts on page one since they came before the request to move on. Let's not dwell on this whole "suck" and "vacuum" thing. As I said earlier move on to some aerodynamics or this'll simply get closed.


I ran this same sort of calculation on a 747 one time. With all the square inches of that massive wing and despite the gross takeoff weight it came down to an average difference between the lower and upper surface of a hair over 1psi to achieve flight. And as Alasdair suggests our models only need a few oz/sq inch even for the biggest and heaviest.

One of my free flight rubber models has 150 sq inches of area and weighs 5 oz ready to fly with the motor all wound up. So that's only 5/150= 0.033 oz/sq-in difference over the wing's area to fly.

why do rc flyers worry about wing lift and loading the planes fly ok right from the box> i dont think model plane flyers do any thing to there models>> if your that smart go work for the air force

Because a lot of us enjoy designing out own models that push the limits of performance in any number of ways. The only way to do that is to learn about stuff like how and why airfoils lift. Which is why we have an aerodynamics forum so folks can ask, discuss and learn about the science of aerodynamics and how it pertains to our smaller size models.

Was this much ado about nothing or little info about a lot of things?
The conjecturing / defining of " lift" ,is endless.
The most simple answer is usually the the best.
It is simply pressure differences at work
Mother Nature abhors a vacuum
But she loves a Bissel----


(look it up)

It's simple. It's called the dumbing down of America. My grade school science books explained Bernoulli’s principle back in the 60's and it really was not that hard of a concept to understand. Back when the rear windows of cars actually went all the way down I used to experiment with my hand all the time. And why do you have to lock a thread down just because you don't like what is being said? What ever happened to the free discussion of thoughts and ideas? No one being ugly here that I have seen, you can always hang out somewhere else. I agree with the OP I prefer to see my planes and spacecraft lift off the ground and not suck off.

Very interesting tool indeed, BMatthews !!!

The abrupt change in pressure after the stall is very evident, as well as how camber influences all.

If switching to velocity, those big peak values show that most of the work that the wing does over the air to obtain the reaction of lift is done next to the leading edge.

Somehow different for the little perturbation that our tiny wings represent, but the concept is the same.

I wish they had a 3-D distribution of the pressures on a real wing, in order to see how the absolute value of top and bottom pressure also decreases along the wingspan and how some air flows span-wise (bottom flows faster than top) and induces the tip vortex.

It would be great that the museum could add some demonstrative pictures to that placard that we have discussed here.

It seems that anytime any innocent asks " what is lift?"
the urge is to trot out all the old explanations of on various ways of measuring lift .-or the efficiencies of lift.
The museum did it right - tho they over did it a bit -in my book
A wing is just a plate travelling at an angle which produces a pressure difference -
specifically a higher pressure below and lower pressure above the wing -
How it does it - can and does fill reams . most of which would put the casual observer to sleep. (a better use of their time)

I never worry about lift, I place my efforts into the lowest wing loading I can possibly achieve, and the lift takes care of itself.

Think about it...

Bob

The suggestion that the thread might be locked was based on the first few posts that did nothing but lament over the use of the term "suction". The forum is about aerodynamics and how it pertains to our model flying. Not about semantics of word usage. The issues of what word was used is no more pertinent to this forum than discussions about retractable landing gear or how to mix the correct shades of colour for WWII scale models. It was simply off topic for THIS forum. And that was why I suggested a new direction for the thread or closure. Not being nasty, just trying to keep the discussions for this forum area on topic.

I believe that the wing loading is pretty close to the actual average pressure differential between bottom and top surfaces of the wing needed for sustaining level flight.

The value of the wing loading is actually a little lower, due to the air leaks in the hinge lines and around the tips and edges, all of which increase with high AOAs'.

Example: wind load of 21 ounce/sqft = 0.009 PSI relative to atmospheric pressure

Am I correct?

Like I said, with low wing loadings, available lift is not an issue and will alway take care of itself at just about any AOA. I know this to be correct trough my own practical experiences...

Bob

Your math checks out when I run it. .009psi as the average difference needed to fly with that wing loading.

OK - but what does this have to do with a sign -directed at casual visitors of the museum????
"

Great, thanks, BMatthews.

Low wing loadings means that we have plenty of area and that our wing needs creating less disturbance to the air in order to sustain the weight of the plane and any G forces associated to maneuvers.
Less disturbance means either less velocity through the air or less AOA or both, which directly relates to less drag (every type of drag).

That also means that any force coming from the air (gusts and thermals) has more influence on the airplane with low wing loadings, due to bigger aerodynamic areas and less inertia (high area to weight ratio).
That is a bad thing in the case of gusts for sport models and a good thing in the case of thermals for sailplanes.

http://www.youtube.com/watch?v=aVdoF4VbCFQ

Not much!

It seems people are trying to make the case that higher pressure under a wing is just as responsible for lift as lower pressure above a wing.

Of course pressure differential is required to produce lift but the upper surface of a wing is most critical. That airflow over a wing produces a pocket of low pressure above it and the wing moves to fill that void. It's like it gets sucked up into the area of vacuum just like the AF sign says. Destroy that pocket of low pressure and you fall out of the sky.

At work (I'm a corp/charter pilot) we have operational specs that state we must remove any ice or frost from the wings/tail before flight. I cannot even polish frost so that it's smooth. It must be removed. That's how critical the top of the wing is. If I have frozen condensation below the wing (usually near the wing root where the fuel is sits and it's below freezing) I do not have to remove that. It's just not as critical.

-

The NET difference (bottom to top) is the key to having the lift required .
the wings on your bird -IF the loading were low enough- would work even with the ice added - ignoring weight gain from the ice
real world requires better efficiencies of full scale craft - I realize-
as to which surface is the most critical- you have to consider speed involved
bottom line - both sides have to contribute and the contribution changes with speed and loading

The principles of flight are the same for RC as it is for full scale aircraft. If you want to test the importance of the upper surface of a wing spray some wall texture (to simulate ice) on top of one of your models, let it dry and go fly it. My guess is it won't fly well due to airflow separation over the wing.

Another way to think of it is to take a simple flat bottom trainer wing. The bottom is flat and the top has a camber to it. The air passing below the wing is relatively unmodified whereas the air over the top is modified because it travels a greater distance causing higher velocity and lower pressure as a result. So clearly the wing is a lifting device, not a pushing up device.

Yet another example is is to blow over a sheet of paper while it lays flat on a table. As you blow over the sheet of paper you are creating a low pressure area and the paper will lift off the table. Nothing underneath it to push it up.

In our small sizes -I doubt the critical shape issue will show up
On full size - the weight of ice and turbulance are issues

It's not the weight of the ice but the airflow disruption that is the issue.

I agree, RC models have much lighter wing loading and tend to be grossly overpowered but the principals remain the same.