Slipstream modelling and the 109

[Chill31]

Generating lift creates the wake behind an aircraft. The more lift generated, the bigger the wake. Much like a ships wake. Big boat, big wake. The prop wash blends into the whole airplanes wake and is rather difficult to distinguish (if at all possible).

[Chill31]

With regard to slats, they are almost certain to deploy asymmetrically during a dogfight due to the high aoa and rolling involved in the fight.

[69th_chuter]

...

 

I think the slats on later Soviet fighters were added to an established wing design to cure unpleasant behaviour, maybe the 109 was the same, rather than designed like that from the outset, it wasn't so much a question of "Hey this will make our aircraft brilliant" rather than "Maybe if we strap this to the front edge of the wing it might stop the damn thing from falling out of the sky quite so often".

 

...

 

On the 109 the slat was there from the very beginning (as on all slat equipped Messerschmitts - he was a fan of the variable, but flat, airfoil).  On the V1 through the B the slats extended inboard to about the outboard end of the wheel wells.  With, I believe, the C the new wing guns required the clipping of the inboard end of the slats to clear the machine gun ports (just outboard of the later MGFF ports). 

Edited December 6, 2013 by chuter

[Kurfurst]

Indeed chuter. In fact slats featured already on the Bf 109s direct precedessor, the Bf 108 sports plane.

 

[MiloMorai]

Outward retracting, but vertical, u/c as well.

[Sternjaeger]

I talked to a friend who flies with the Bf108, and he actually gave me some interesting details: "the slats are really free moving and pop out quite easily, you don't feel them on the stick, just detect they're opening in your peripheral vision. In my experience I never had an asymmetrical deployment, and it's probably very difficult, as they extend across most of the wing. It's probably unlikely in the 109 as well, as it's really hard to put your outer wing in complete aerodynamic shadow. But even if you theoretically managed to, it wouldn't behave any different than an aircraft with a conventional wing entering a spin".

 

Whilst I was at it I also asked about the landing gear, but he said the geometry is not the same as in the 109, the only problem with the landing gear is that they're manually operated and they're a b***h to crank up!.

[HagarTheHorrible]

I talked to a friend who flies with the Bf108, and he actually gave me some interesting details: "the slats are really free moving and pop out quite easily, you don't feel them on the stick, just detect they're opening in your peripheral vision. In my experience I never had an asymmetrical deployment, and it's probably very difficult, as they extend across most of the wing. It's probably unlikely in the 109 as well, as it's really hard to put your outer wing in complete aerodynamic shadow. But even if you theoretically managed to, it wouldn't behave any different than an aircraft with a conventional wing entering a spin".

 

Whilst I was at it I also asked about the landing gear, but he said the geometry is not the same as in the 109, the only problem with the landing gear is that they're manually operated and they're a b***h to crank up!.

 

 

Eric Brown's quote was quite specific with regard to the circumstances rather than just being a general dislike or disregard for slats.  It was maybe something that really only pertained to a combat situation and wouldn't have been noticed, or even considered outside of that and even then, if it was a problem, it was only a matter of degrees rather than a show stopper and pilots just worked around it.  I would have thought that the size and power of the engine of the front aircraft would have had a big impact on what wake turbulence was produced and that 2-300 yards is closer than most aircraft, outside of guns combat, are likely to get ?

[Crump]

 

This is from the website of an aircraft manufacturer and are the words of the designer, who presumably attended some form of college to study stuff like this, maybe even has some credentials in the world of aircraft design and aerodynamic studies. http://www.zenithair.com/c-heintz.html

 

http://www.zenithair.com/stolch801/design/design.html

 

Yes, the designer is qualified and is correct. 

 

However, he is NOT accountable for folks misunderstanding the information being put out in the article.

 

 

This is a relatively simple system and not too heavy to design, but it has one big disadvantage: in gusty weather only one wing slat may be drawn out while the other stays in, creating a potentially major problem for the pilot who now needs full aileron just to keep the airplane level…!

 

http://www.zenithair.com/stolch801/design/design.html

 

In an wind gust there is asymmetrical application of dynamic pressure and differential wing section angle of attack by the very nature of wind gusting.  A rolling moment will be created in any airplane.  A slat equipped aircraft wing section has a higher maximum coefficient of lift than can be generated in a non-slatted wing section.  Therefore more control will be required to keep the wing the wing level.  A non slatted wing would simply be stalled.

 

This simply has absolutely nothing to do with slats creating a rolling moment.  It is the same as earlier confusion with the normal behavior of a swept low aspect ratio wing being blamed on LE slats.  They simply do not create a moment at all if functioning properly and only deploy as needed.

 

For example, we looked at lift production not lets address the "additional drag" of LE Slats.

 

The drag formula mirrors the lift formula when examining total drag.

 

Coefficient of Drag = Drag/[(1/2 density * Velocity^2) * Reference Area]

 

Drag equals Thrust in steady state flight.

 

Lets run some simple math to get an idea how the slats work regarding drag.

 

Fictional airplane

 

Thrust = Drag = 2500lbs

 

Wing Area = 150ft^2

 

traveling at a velocity of 100fps

 

Coefficient of Drag = 2500lbs / [(1/2*.002376sl/ft^3*100fps^2)*150ft^2

 

Units all cancel leaving us with a dimensionless ratio of Drag pressure to dynamic pressure = Coefficient of Drag = 1.4

 

We slow our velocity down to 90 fps. 

 

Coefficient of Drag = 2500lbs / [(1/2*.002376sl/ft^3*80fps^2)*150ft^2

 

Units all cancel leaving us a dimensionless ratio of lifting pressure to dynamic pressure = Coefficient of lift = 1.7

 

The slats open to increase the coefficient of lift over the wing section by energizing the boundary layer.  Coefficient of Lift and Drag have a direct relationship.  If one increases, the other must increase.  Therefore we see that our Coefficient of Drag has increased as the slats open as it should.  By their nature, slats do increase the coefficient of drag of any wing section.

 

Our Thrust force however has remained constant at 2500lbs and never changed at all!!!  No change in forces, no moment created.

 

Pretty simple.

[Crump]

 

Eric Brown's quote was quite specific

 

 

I just went and re-read his input on the slats.  He does not say a thing about the slats causing anything to upset the gun site picture.

 

What he says is aileron snatching.  That is caused by the trailing edge of the ailerons being too thick and rounded.  The thick trailing edge acts as an airfoil in its own right and accelerates the aileron in the direction of wing movement.  That may or may not be a characteristic of every Bf-109 as it is the result of fabric application not design and would be present in any maneuver as well as being noticeable when the slats deployed. 

 

What Eric Brown does say about the Bf-109G is:

 

Eric Brown Says:

 

It was simply a well conceived, soundly designed fighter that maintained during maturity the success that attended its infancy

[HagarTheHorrible]

I just went and re-read his input on the slats.  He does not say a thing about the slats causing anything to upset the gun site picture.

 

 

 

Yes he does, that's why I quoted it. 

 

My reading is that, regardless of manufacturing errors (although they may not have helped),  if you get a situation close to where the slats are going to deploy anyway and then add in some turbulent air that might be localized enough to only have one slat deploy, even momentarily, maybe not even fully and possibly several times in quick succession then it is going to cause the aircraft to oscillate due to lift and drag factors caused by the slat deployment.  This may not be overly dramatic for normal flying purposes, but for shooting it might be enough to frustrate a shot especially if it also coincides with the best attack option i.e slashing attacks (which in younger days I might have thought referred to standing on a foot bridge and weeing on pedestrians passing underneath rather than what fighter aircraft might do).

 

Your Eric Brown quote (in bold) is true but it isn't the whole truth, is it ?  It is the final summing up, that suggests, the 109 wasn't anything exceptional when compared to other successful fighter designs from the period.

Edited December 23, 2013 by HagarTheHorrible

[LLv34_Flanker]

S!

 

 I think the opinion about LE slats varies by the pilot. I heard from Günther Rall himself that he was not a big fan of them. Again Kyösti Karhila used them in his landing approach on short runways. But neither said they made the plane worse in combat. All I've heard is that they deployed near or at the stall giving a snap to stick, sharper on Emils less on F and G, plus an audible bang. Question was that does BoS model their eployment proper in different flight regimes? I hope they will do so and have an effect on the FM and not just being cosmetics like in original IL-2.

[Crump]

Hagar,

 

Eric Brown mentions aileron snatching occurring when the slats opened.  Yes, that would happen but the slats are NOT the cause of aileron snatching. 

 

 

Your Eric Brown quote (in bold) is true

 

Right, It is just what Eric Brown said about the Bf-109.

 

Hagar says:

 

you get a situation close to where the slats are going to deploy anyway and then add in some turbulent air that might be localized enough to only have one slat deploy, even momentarily, maybe not even fully and possibly several times in quick succession then it is going to cause the aircraft to oscillate due to lift and drag factors caused by the slat deployment.

 

Asymmetrical slat deployment will not cause any moments at all.  It is the turbulent air that causes the moments.  I just ran the math for you on the "lift and drag factors".  Notice the forces remain balanced.

 

Turbulent air causes the aircraft to move, not the slats.

 

regardless of manufacturing errors

 

 

It may not even be a manufacturing error.  If the captured example was bellied in and the ailerons required being re-covered, it is the a fact the German mill was much finer in their finishes and their dopes much thinner than ours.  I can easily see the thicker allied finish causing this to occur.

[Crump]

 

it wouldn't behave any different than an aircraft with a conventional wing entering a spin".

 

 

Sure, once it was forced to spin.  A slat equipped wing is much more resistant to entering a spin than a non-slat equipped wing.

 

Having the slats on the outboard wing section just covering the ailerons is even more resistant to spin entry than having full or a larger portion of the wing covered by the slats.

[DD_bongodriver]

Yes, it does exactly the same thing that wing 'washout' does by ensuring the inboard section of the win stalls before the outer.

[Crump]

Same concept exactly. Much in the same way a Hugo is a car just like a Bentley.

[DD_bongodriver]

Yep, both get the same job done but one is an overcomplicated compensation for an inherent shortcoming.

[Crump]

Based on the proliferation of modern fighter designs that utilize energy adders, the benefits far outweigh the additional complication.

[DD_bongodriver]

Yeah, it's almost impossible to find an aircraft that doesn't have half span Handley Page automatic slats these days.

[Crump]

Exactly and that is why so many fighters have them..........

[MiloMorai]

And some a/c did away with the slats >> F-86.

[69th_chuter]

Indeed chuter. In fact slats featured already on the Bf 109s direct precedessor, the Bf 108 sports plane.

 

 

 

 

And if you drop a plumb from the leading edge of the wing at the landing gear you'll note the relative position of the spindles (or contact patch of the tires).  The further forward the contact patch (or spindle, whatever) from the CG the less stable the plane is on the ground. 

 

 

On the F-4 Phantom shown those slats are part of the turn kit but they're not retractable on the outer wing (as in disappearing into the wing contour like the inner ones).  The outer ones have two extended positions, rocking at the attach point on the bottom of the slat: back for speed forward for flow control) and the kit includes a fixed slat on the stabilator (upside down, of course). 

 

Flat wings with slats have less drag and, consequently, a potentially higher top speed than a washed out wing but are heavier and more complex.  Boeing last used a flat wings on the 727 and 737 but those aircraft had different airfoil sections along the span.  Wings with washout have slightly less root bending moments and are simpler and lighter.  When racers clip the wingtips off of Mustangs they're basically removing a part of the wing that isn't producing much lift at speed (straight/level) anyway, just mostly form drag.  (Of course, at the slow end they come in handy, as do the rest of the ailerons - lol.)

[79_vRAF_Friendly_flyer]

A question to you gents:

 

The Westland Whirlwind fighter was designed and built with automatic slates. At some point the slats deployed asymmetrically and ripped off a wing of one of the fighters! After that, the slats were wired or nailed shut.

 

I have read this in several articles on the Whirlwind, but reading what you guys say, I find the story curious. Would this be possible at all? Could the wing have failed due to other circumstances and the slats being blamed?

[Sternjaeger]

Mmmh to be honest I really don't see how a slat could trigger a structural failure of that kind..

[LLv34_Flanker]

S!

 

 Would have been a very bad design of the wing if it could not tolerate the forces it was subjected to. Maybe there was something else wrong and the slat deployment was just one part of the chain leading to catastrophic failure.

[79_vRAF_Friendly_flyer]

Mmmh to be honest I really don't see how a slat could trigger a structural failure of that kind..

 

That's what I thought too, but perhaps the slats were very different. They were designed to allow the Whirlwind to land at lower speeds (it has a notoriously high landing speed), perhaps they provided a lot more lift than the slats on the 109?

[Crump]

And some a/c did away with the slats >> F-86.

 

Which has what to do with anything?

 

Completely different aircraft with different engineering requirements for transonic flight from a WWII piston engine subsonic design.  The F86F wing was redesigned in an attempt to improve the high speed sustained turning performance at high mach numbers for transonic flight. 

 

That is not something slats are designed to improve as they are not transonic devices but subsonic.  The F-86 wing was lengthened, LE extended, Chord changes and wing fences. 

 

 

The F-86F-25 was introduced with a legendary new wing designed in response to pilot reports that early F-86Fs in Korea could not turn as tightly as the Mig-15 in combat at high Mach numbers and altitudes. F-86s turning with MiG-15s had encountered aerdynamic forces that opened their leading edge slats, which normally opened only at slow speeds.

To correct this problem, North American test pilot George Welch suggested a change in the Sabre's wing during summer 1952 that led to the famous "6-3" wing. Welch recommended removing the slats and extending the F-86's leading edges instead. Three modified F-86E Sabres were tested. By August 1952, the 6-3 wing was perfected. This modification deleted the leading edge slats and added a new solid leading edge with six inches more chord at the root near the fuselage and three inches more length at the wing tip. Finally, a five inch high boundary layer "fence" was added to the wing's upper surface at 70% of the wingspan to direct air flow. These changes resulted in a lowerdrag cieffucuent and added seven miles per hour to the Sabre's top speed to 695 miles per hour. They also improved the maneuverability at high altitude by delaying the onset of buffeting.

The result of these changes was that the F-86F now turned inside the Mig-15 at high altitudes.The F-86's performance improved by 1.5g at Mach 0.92 at 30,000 feet before buffeting developed. The operating altitude also increased to 52,000 feet, maximum Mach reached 1.05 (in a dive), and the rate of climb increased by almost 300 feet per minute. The F-86F then became the unquestioned supreme day fighter in the world, but the trade off for the improved high-speed performance was that the Sabre's stalling speed was increased by 20 miles per hour. This required a faster landing approach (as there were no leading edge slats), and the low speed handling qualities suffered. Higher take off and landing speeds were needed to maintain controllability, but the improvement in combat far outweighed any negative aspects.

 

Later, as engineering knowledge of transonic flight was improved, the F-86 wing was redesigned again.  This time, the slats were added back in the final version of the wing.

 

 

The final "F" model Sabre was the F-86F-40. It was first built for the Japanese Air Self Defense Force. The F-86F-40 iis signifigant in that its outer wing changed again from the F-25. The FAO's wing was lengthened by one foot on each side and returned to using leading edge slats. The longer wing again reduced the stalling speed and allowed slower landing approaches and shorter takeoff distances. The result was an overall improvement over Sabres with the 6-3 wing. The conversion also improved the combat radius and the Sabre's high altitude maneuverability. Because of this, the United States Air Force converted all active duty Sabres to F-40 specifications beginning in March 1955 and added leading edge slats once again, although the Sabre's weight was increased by 200 pounds.

 

 

http://sabre-pilots.org/classics/v14killer.htm

 

The fact the F-86 designed experimented for a short time with removing the slats is hardly a condemnation of the use of LE slats.  

Edited December 27, 2013 by Crump