We need a technical and logical discussion.

Any topic with a chance of polarization - Not for the easily offended.

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David Redszus
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Re: We need a technical and logical discussion.

Post by David Redszus » Wed Aug 23, 2017 2:47 pm

So what is the huge minus pressure in a carburetor venturi? Numbers please, numbers for the top of the wing as well. Venturi is inches of water very low pressure, enough to suck some gas out of the float bowl.

And to answer the venturi question, its the positive pressure in the float bowl that pushes the column of fuel towards the minus pressure, or the lower pressure caused via the venturi.
My guess is that you would like to understand and evaluate the fluid flow through a carburetor.

If so, we should realize there are several factors that affect both air flow and fuel flow mass rates.
Among them are: area, flow coefficients, gravity, density, viscosity, surface tension and pressure ratios.

The best explanation of fluid flow, using Bernoulli, is given by C. F Taylor, Volume 1, Chapter 9.

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Re: We need a technical and logical discussion.

Post by Zmechanic » Wed Aug 23, 2017 3:01 pm

exhaustgases wrote:And right on again. That is the first argument flying upside down. I say a plane with a flat wing could fly as well. Its all about the positive pressure.

It drives me nuts when I see a post on an airplane site that says its the lift on the airplane propeller that sucks the plane through the sky, makes you wonder what makes the huge wind storm behind the plane.
I haven't browsed this whole thread, but you should check out the P-51's laminar wing. It is much more symmetrical than the classical wing profile. It required high speed and higher angles of attack, but that was also its advantage, it could handle much higher angles of attack.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Wed Aug 23, 2017 11:17 pm

The carb is a great example it has about 15 psi outside pushing in on the case around that venturi that has an ever so slight vacuum I don't remember how many inches of water they normally are, the vacuum is ever so slight. So question answered the low pressure area on top of a wing is very slight compared to the bottom. P-51 wing? Also you can use the F104 wing.

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Re: We need a technical and logical discussion.

Post by David Redszus » Thu Aug 24, 2017 11:01 am

exhaustgases wrote:The carb is a great example it has about 15 psi outside pushing in on the case around that venturi that has an ever so slight vacuum I don't remember how many inches of water they normally are, the vacuum is ever so slight. So question answered the low pressure area on top of a wing is very slight compared to the bottom. P-51 wing? Also you can use the F104 wing.
No sir. The pressure along the top of ANY wing is much lower than along the bottom surface. I posted the numbers earlier; they haven't changed.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Thu Aug 24, 2017 7:37 pm

David Redszus wrote:
exhaustgases wrote:The carb is a great example it has about 15 psi outside pushing in on the case around that venturi that has an ever so slight vacuum I don't remember how many inches of water they normally are, the vacuum is ever so slight. So question answered the low pressure area on top of a wing is very slight compared to the bottom. P-51 wing? Also you can use the F104 wing.
No sir. The pressure along the top of ANY wing is much lower than along the bottom surface. I posted the numbers earlier; they haven't changed.
The negative pressure is ever so slight at the top of the wing as compared to the high pressure at the bottom of the wing. That is what I meant to say. You could put a flat plane on the top of that wing and it will work just fine, again its the big plus pressure at the bottom that keeps it up there.
You can just reduce surface area with a curved surface at the top and maintain the lifting ability of the wing. And again old fabric covered wings don't come back with all the fastener points at the top ripped out and or torn fabric from a high top of wing suction, it just does not happen.

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Re: We need a technical and logical discussion.

Post by David Redszus » Fri Aug 25, 2017 12:30 am

The negative pressure is ever so slight at the top of the wing as compared to the high pressure at the bottom of the wing.
Not even close to being true; the opposite is true.

Earlier I posted some data for a NASA airfoil with pressure coefficients. The first column is the location along the wing chord, the second the Coefficient of Pressure under the wing, and the third column is the Cp above the wing. The actual lift forces are a function of and closely coincide with surface pressures. To obtain actual lift force we must multiply the coefficients by velocity squared.

As can be seen, the pressure coefficients under the wing are much lower than above the wing and they are negative in direction. That means under wing pressures are creating a force trying to move the wing downward, not upward as would be the case above the wing.

5 deg attack angle. lift coefficient 1.088
0.1...-.017.....0.323
0.2...-.014.....0.313
0.3...-.016.....0.282
0.6...-.031.....0.165
0.9...-.056.....0.014
You could put a flat plane on the top of that wing and it will work just fine, again its the big plus pressure at the bottom that keeps it up there.
NO, not true. Lift is a function of airspeed over (and under) the wing, not static pressure. Actually it is the difference in velocity above and below the wing that creates a difference in pressure and therefore in lift.
You can just reduce surface area with a curved surface at the top and maintain the lifting ability of the wing.
Yes, but only if you increase the attack angle and velocity. If you increase the attack angle you will reach a stall condition in which airflow above the wing becomes detached (flow separation) and lift is lost. Under wing pressure, small as it is, is maintained.

Thin wing shapes are very common in high speed military aircraft. But they must be flow at high velocities and attack angles. They are subject to stall conditions if attack angles are exceeded.
And again old fabric covered wings don't come back with all the fastener points at the top ripped out and or torn fabric from a high top of wing suction, it just does not happen.
It certainly would happen if the air foil were moving at higher velocity. The lift force, including fabric removal forces, will increase with the square of the velocity.

As an experiment, try attaching hoses to the upper and lower sides of a wood board connected to pressure sensors or gauges. Stick it out the window of a fast moving car and tilt the board to various attack angles. Note the pressure above the board is always many times greater than below the board.

Or, you can save yourself a lot of work by simply reading Dr. Joseph Katz book, Race Car Aerodynamics, Bentley Pub. ISBN 0-8376-0142-8, Chapters 2 and 4.
Or any other aerodynamics textbook.


Those who do not read have no advantage over those who cannot read.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Fri Aug 25, 2017 3:05 am

So for you to say the pressure under the wing is trying to pull it down must mean the wing is in a dive.
On a fan blade the angle of attack (or if you wish the angle of incidence) pushes the air down or in the direction from high point to low of the pitch angle in the correct rotation direction. This is really simple stuff. My fan gives a rip about lift coefficients. Its actually like a machine tool cutter, the fan or propeller is cutting that air and forcing it back just like a drill or a screw conveyer. In all cases its a positive pressure that does the work.

My paper airplane just broke the laws of aerodynamics, its wings are flat on top and bottom.

Those pressure coefficients must be a mistake of some sort or apply to a Russian or Chinese plane.

Besides they are just columns of numbers with no meaning, no labels, no definitions etc. I don't understand what they mean.
Nothing points to this is the top of wing pressure and this is the bottom of wing pressure? For this discussion we need the pressures, not coefficients.
And don't need fancy, like this condition or that. Just pick a flight regime and a choice of wing and go for it. A P51 would be a good one.
Last edited by exhaustgases on Fri Aug 25, 2017 3:33 am, edited 3 times in total.

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Re: We need a technical and logical discussion.

Post by joe 90 » Fri Aug 25, 2017 3:11 am

Hydrofoils?

The old sort come right out of the water.
The newer more efficient ones stay under the water.

Absolute values of pressure don't matter.
Lift is caused by the difference in pressure.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Fri Aug 25, 2017 3:22 am

joe 90 wrote:Hydrofoils?

The old sort come right out of the water.
The newer more efficient ones stay under the water.

Absolute values of pressure don't matter.
Lift is caused by the difference in pressure.
I agree, but more plus pressure on the bottom than what the minus pressure is on top. A powered paraglider is an air scoop of sorts, similar to a parachute. Only minus pressure on top is from the area being evacuated as it falls, the canopy fills with air.

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Re: We need a technical and logical discussion.

Post by David Redszus » Fri Aug 25, 2017 10:41 am

exhaustgases wrote:So for you to say the pressure under the wing is trying to pull it down must mean the wing is in a dive
.
No, there will be a small pressure under the wing, trying to pull it down even when in level flight.
My fan gives a rip about lift coefficients. Its actually like a machine tool cutter, the fan or propeller is cutting that air and forcing it back just like a drill or a screw conveyer. In all cases its a positive pressure that does the work.
Its true that most fan blades can't do math very well. And it does not care about coefficients, only actual pressures. Remember that for any action there must be an equal and opposite reaction. What is the opposite force to your fan positive pressure?
My paper airplane just broke the laws of aerodynamics, its wings are flat on top and bottom.
No it has not. You could fly a sheet of plywood if you had the right attack angle and velocity.
Those pressure coefficients must be a mistake of some sort or apply to a Russian or Chinese plane.
They are not a mistake. They apply to Russian, Chinese, French, German, British, Italian, Korean, and US aircraft as well.
Besides they are just columns of numbers with no meaning, no labels, no definitions etc. I don't understand what they mean.
The data columns were labeled clearly. Yes, you don't understand what they mean.
Nothing points to this is the top of wing pressure and this is the bottom of wing pressure? For this discussion we need the pressures, not coefficients.
Sure it does. The first column is chord position or location along the surface of a wing from front to rear. The second is underwing pressure. The third is wingtop pressure. Since actual pressure is determined by velocity squared and surface area, the data has been normalized by dividing the pressure by velocity and area. It now applies to that wing shape regardless of wing size or velocity.
Just pick a flight regime and a choice of wing and go for it. A P51 would be a good one.
That's very easy. Just give me the NACA airfoil number for a P-51 wing shape and I'll give you all the lift and drag numbers.

Question: If underwing pressure is the cause of lift, then why does a wing stall at high attack angles? It still would have plenty of positive underwing pressure.

Check out Hanley Innovations .com

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Re: We need a technical and logical discussion.

Post by David Redszus » Fri Aug 25, 2017 1:44 pm

Just pick a flight regime and a choice of wing and go for it. A P51 would be a good one.
OK, lets use the P-51 Mustang airfoil as an example. Technically, it is part of the NACA 6 series of airfoils (P51D BL27.5),
originally used on the B-24 bomber and quickly adapted to the P-51. It is a elongated teardrop shape with curved top and bottom surfaces, laminar flow, with low drag for high speed applications (like a fighter plane).

The lift to drag numbers are a function of attack angle as follows:

Angle...Lift...Drag
2........0.380...0.0078
4........0.596...0.0086
6........0.721...0.0107
8........0.906...0.0137
10.......1.085...0.0176
12.......1.259...0.0208
higher attack angles will cause the wing to stall. Underside pressure notwithstanding.

With regard to pressures on the lower and upper wing surfaces, it depends on where you
measure them. The percent chord length location is shown in the first column, followed by lower and upper
wing surfaces. For actual pressures, simply multiply by velocity squared.

Chord...Lower...Upper
.10......-0.047....0.419
.30......-0.064....0.306
.60......-0.093....0.140
.90......-0.119...-0.070

The lower wing surface shows very little pressure compared to the upper wing surface.

While much is made of the Rolls-Royce Merlin engine, much of the improved performance can be credited to the drag reduction of the wing design.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Fri Aug 25, 2017 3:21 pm

Wing stalls at high angle of attack? Its like the rock skipping on the surface of a lake. The airflow either slows too much to cause the moving high pressure area or it acts like a flat plate in the wind ie something that just stops the flow of air under the wing and causes eddy currents of air that disrupt the flow as well.

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Re: We need a technical and logical discussion.

Post by exhaustgases » Sat Aug 26, 2017 4:16 am

David Redszus wrote: While much is made of the Rolls-Royce Merlin engine, .
It was kinda lacking in some areas, Packard made some improvements, but still lacking.


At 7:50 no Bernoulli affect at the top, only nice positive pressure at the bottom.
https://www.youtube.com/watch?v=q_eMQvDoDWk

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Re: We need a technical and logical discussion.

Post by exhaustgases » Sat Aug 26, 2017 9:06 pm

https://www.youtube.com/watch?v=vzheFCZLtv8

And great title "Wings don't suck" learn about the down ward pressure from a wing...........

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