Leading Edge Slats on the Me-109

[taildraggernut]

Originally Posted by Crumpp

Experts....have fun

So.....how exactly does the fact that the venturi effect caused by the 'slot' created by an open slat which re-energises the boundary layer of air over the wing maintaining smooth flow (sometimes called laminar http://en.wikipedia.org/wiki/Laminar_flow ) prevent you from understanding what I meant?

While we're discussing this so civilly perhaps you could explain to us all this magical phenomenon that prevents a slatted wing from stalling once it has gone beyond its allowable angle of attack?

[Crumpp]

I really can't believe you typed this publically, slats are NOT I repeat NOT anti spin devices, slats are not anti-anything devices, they simply allow you to hang on to laminar flow air at slightly higher angles of attack but if you exceed that angle a slatted wing is stalled just like any other and subject to the same pitfalls.

While we're discussing this so civilly

You are not discussing anything civilly...you are just making insults.

So.....how exactly does the fact that the venturi effect caused by the 'slot' created by an open slat which re-energises the boundary layer of air over the wing maintaining smooth flow

First of all, you do not understand the fundamentals of LE slat aerodynamics.

Quit using wikipedia as your source. It is not credible and in this case is just plain wrong as the author does not understand boundary layer mechanics.

In boundary layer mechanics, we have two portions, laminar and turbulent.

Laminar flow is the last thing you want on the outboard portion in a stall. Laminar flow is low energy. That is why it is low drag AND subsequently, low lift.

Slats work by increasing turbulent flow not laminar flow. Turbulent flow portion of the boundary layer is high energy and high lift!

Flow separation from the top of the airfoil, i.e., stall, results from the loss of the kinetic energy in the boundary layer due to viscous shear and an adverse pressure gradient. A turbulent boundary layer is better able to delay flow separation than a laminar boundary layer because of the higher energy associated with the turbulence. For this reason it is better to have a turbulent boundary layer over the airfoil. Vortex generator are put on the top surface of a wing for the purpose of forcing the early transition of the boudary layer to turbulent.

optfly.iaa.ncku.edu.tw/aftdesgn/lect16.pdf

The laminar flow is not the purpose of the slats, it is the increase in turbulent flow boundary layer which delays the onset of the stall.

No stall = NO SPIN! Hence, the spin resistance found in slats and the reason engineers used them as an early anti-spin device.






They are spin resistant because they allow for control inputs that would normally result in a spin. One can easily see this in the RAE report.

[Glider]

Originally Posted by Crumpp

No stall = NO SPIN! Hence, the spin resistance found in slats and the reason engineers used them as an early anti-spin device.

While there are a number of devices that delay the stall, in the end if you push it too far, you will stall.

Of course a stall by itself doesn't result in a spin.

It is very, very rare to find an aircraft of any type that cannot spin. I only have experience of one type of Glider that could fit that bill and am confident that aircraft of the 1930/40 era would have to be specially designed.
The Me 109 isn't one of those aircraft. I don't disagree when people say that it was a difficult aircraft to spin and that the model is wrong, but go to the extream and it will depart.

[taildraggernut]

Originally Posted by Crumpp

You are not discussing anything civilly...you are just making insults.

I am being very civil, if you have taken insult from that then I believe you are a touch over-sensitive......I believe that comes with instability

Originally Posted by Crumpp

First of all, you do not understand the fundamentals of LE slat aerodynamics.

Now that is an insult.

Originally Posted by Crumpp

Quit using wikipedia as your source. It is not credible and in this case is just plain wrong as the author does not understand boundary layer mechanics.

Oh come now, that 'Stop using Wikipedia as a source' is too cliche, used as a cheap attempt at discrediting in many internet debates, there is nothing wrong with that article.

Originally Posted by Crumpp

In boundary layer mechanics, we have two portions, laminar and turbulent.

Laminar flow is the last thing you want on the outboard portion in a stall. Laminar flow is low energy. That is why it is low drag AND subsequently, low lift.

Slats work by increasing turbulent flow not laminar flow. Turbulent flow portion of the boundary layer is high energy and high lift!.

optfly.iaa.ncku.edu.tw/aftdesgn/lect16.pdf

The laminar flow is not the purpose of the slats, it is the increase in turbulent flow boundary layer which delays the onset of the stall..

http://history.nasa.gov/SP-4103/app-f.htm

I suggest you read this article, it will help you understand boundary layers and the effect of skin friction and subsequent separation due to turbulence.

Extracts from the article......I hope NACA are a more credible source for you rather than wikipedia.

The flying qualities of wings can be enhanced in two ways, and boundary-layer control can help in both. The first is to decrease drag; the second is to increase lift. The most desirable way to decrease drag is to maintain laminar flow within the boundary layer and prevent a transition to turbulent flow.

Over a normal wing, the boundary layer remains laminar over only a small portion of the wing chord before breaking up into turbulent flow. The area of turbulent flow experiences significantly greater skin-friction drag than the laminar flow.3

Finally, however, the boundary layer on the upper surface breaks free of the wing altogether, reducing lift drastically. This is known as stalling. If the boundary layer can be kept from separating, the maximum lift of the aircraft can be increased, an important consideration in increasing takeoff-weight capacity and reducing landing speed. Furthermore, the same energizing of the boundary layer that delays separation can also help to maintain the boundary layer in fast laminar flow, increasing total lift even at low angles of incidence.

Originally Posted by Crumpp

No stall = NO SPIN! Hence, the spin resistance found in slats and the reason engineers used them as an early anti-spin device.

They are spin resistant because they allow for control inputs that would normally result in a spin. One can easily see this in the RAE report.

Spin resistance is not anti-spin.....anti would suggest there is complete protection and that is simply not the case.

in case you were doubtfull that slats are a boundary layer control device heres more stuff from NACA..

http://history.nasa.gov/SP-367/chapt4.htm

Slots.- The maximum coefficient of lift may be increased through the use of a slot formed by a leading-edge auxiliary airfoil called a slat. Figure 63(a) illustrates the operating principle. When the slot is open, the air flows through the slot and over the airfoil. The slot is a boundary-layer control device and the air thus channeled energizes the boundary layer about the wing and retards the separation. The airfoil can then be flown at a higher angle of attack before stall occurs and thus get a higher...

Boundary-layer control.- Another method of increasing CL,max is by boundary-layer control. The idea is to either remove the low-energy segment of the boundary layer and let it be replaced by high-energy flow from above or by adding kinetic energy to the boundary layer directly. Both of these methods maintain a laminar flow for a longer distance over the airfoil, delay separation, and allow one to get a larger angle of attack before stall occurs, and thus a higher CL,max The slot was shown to be one means of passing high-energy flow over the top surface of a wing.

Originally Posted by Crumpp

Flow separation from the top of the airfoil, i.e., stall, results from the loss of the kinetic energy in the boundary layer due to viscous shear and an adverse pressure gradient. A turbulent boundary layer is better able to delay flow separation than a laminar boundary layer because of the higher energy associated with the turbulence. For this reason it is better to have a turbulent boundary layer over the airfoil. Vortex generator are put on the top surface of a wing for the purpose of forcing the early transition of the boudary layer to turbulent.

Vortex generators are using a completely different method, the 'turbulence' they are creating is simply in the form of vortices to draw in energy to a portion of the airflow.

http://www.aerospaceweb.org/question...cs/q0228.shtml

The advantage of wing devices that create vortices is that a vortex adds energy to the airflow and increases its forward momentum. This momentum encourages the airflow to remain attached to the surface of the wing at higher angles of attack than it would otherwise. As a result, the wing is able to continue generating lift in conditions where it would have stalled. This behavior is particularly advantageous on high-performance military aircraft that need to be extremely maneuverable at high angles of attack in combat. The advantage for commercial airliners is increased safety since the plane is less likely to experience a wing stall during critical stages of flight like takeoff and landing.

The method by which these vortex devices work can be better understood by studying the above diagram of vortex generators on a wing. A vortex generator is much like a miniature wing perpendicular to the main wing. These generators are mounted at an angle of attack to the airflow over the wing so that each creates a vortex off the exposed tip, much like a trailing vortex created by a wing. The above example shows vortex generators aligned in opposite directions so that the vortices they create rotate opposite to each other. These vortices serve to increase the speed of the downstream airflow so that it is "entrained" to follow the sharp curvature of the deployed flap and remain attached to its surface. Otherwise, the airflow would likely separate from the flap causing a loss of lift.

Now hopefully you will be able to explain to us all exactly what are the mechanics involved in complete stall/spin avoidance once a slatted wing has been taken beyond it's maximum angle of attack?

[Hood]

Originally Posted by taildraggernut

Now hopefully you will be able to explain to us all exactly what are the mechanics involved in complete stall/spin avoidance once a slatted wing has been taken beyond it's maximum angle of attack?

I haven't read Crump literally.

I've read it as:

"Slats are an anti-spin device in that they extend to prevent the wing stalling, thus giving a pilot a chance to avoid a spin when non-operation of the slats would cause a stall and a possible spin. They also slow down the stall so there is a greater chance that the stall does not result in a spin."

Like ABS. It's a device to stop you skidding. It doesn't mean it'll stop you skidding in every situation though.

Who was the Finnish 109 pilot who said they don't know what happened in a stall/spin because they never stalled/spun?

And isn't this getting away from the original topic, which is do they have any effect in this game?

Hood

[taildraggernut]

Slats are or should I say were in the 1930's/40's primarily a device to improve low speed handling qualities and cannot be anti-spin, the only true form of anti-spin is propper handling of the aircraft, slats simply make the behaviour at stall more benign but their effectiveness has a cut-off point beyond which there is nothing to provide these protections, in high speed manouvering the chances of exceeding those limits are much higher.

I believe the relevance of these extended discussions to the original subject is to do with what some people are expecting from the effect of slats, the original question I believe is almost impossible to answer without being able to disable the slats, and that is subject to whether they are actually coded as separate devices as opposed to the FM being modelled with simple wings that reflect performance with slats.

[Crumpp]

Slats are an anti-spin device. What is so hard to understand about it?

If you have ever flown an aircraft with slats, you can immediately notice the difference in slow flight and stall behaviors.

Here is the slats in my old airplane:

http://www.youtube.com/watch?v=-vbqgfjyW2Q

[taildraggernut]

Slats are NOT an anti-spin device...what is so hard to understand about it?

I have flown aircraft with slats....have you?

you can immediately notice the difference in slow flight and stall behaviors.

Right, now ask someone to make a video of the same aircraft doing high speed stalls taking the aircraft beyond the limiting angle of attack and we can compare the results.

[Crumpp]

Now hopefully you will be able to explain to us all exactly what are the mechanics involved in complete stall/spin avoidance once a slatted wing has been taken beyond it's maximum angle of attack?

Now your trying to morph the discussion into something else.

You keep confusing "anti-spin" device with a spin resistant airplane.

Spin resistant airplanes employ anti-spin devices such as wing cuffs, LE slats, and slots to build spin resistance.

What you are missing is the ability to put it all together. If we were talking about the entire wing being able to stall at once, then the airplane will enter a spin.

It takes a lot of work to do that in a Bf-109 by design.

First of all, only the outboard portion of the wing receives the benefit of the slats in the Bf-109.

In any airplane, the most desirable stall progression is for the wing root to stall first and the tips to stall last.

This leaves the wingtip unstalled and the ailerons effective.

The next feature of the slats is the automatic deployment. Air pressure operates the slats and they will deploy to exactly the position the wing requires for a given condition. That is why in a skid, they will asymmetrically deploy. For some reason, gamers tend to think "asymmetrical" deployment of the slats is a bad thing, it is not unless there is a malfunction of the slats. Instead, the slats deploy to exactly what the wings need automatically and unless the pilot looks out and is somehow psychologically disturbed by seeing the slats out at different amounts, the airplane skids normally without noticeable effect.

The last feature in the Bf-109 is the elevator control is set up so that with the wing root stalled, the pilot cannot continue to raise the nose. The Socata Rallye is designed that way as are many aircraft.

The designer uses control design to keep the pilot safe by limiting the moment the elevator can produce about the CG. This way, the wing tips remain effective throughout the stall. Cessna does this in a C-172 as well. Again, it is a common feature in a properly designed aircraft.
Now, Mtt did have to demonstrate spin entry and normal recovery in the Bf-109. They did this by adversely loading the aircraft to its rearward CG limit and modifying the slats to be pilot controlled. In other words, the airplane was at its rear most CG limit and the pilot could lock the slats so they did not deploy.

[Crumpp]

I have flown aircraft with slats....have you?

That is my airplane and I am at controls in the film.