Bf109 leading edge slats

[GunnyHighway]

I flew for the first time the Bf109 and was surprised by the stability feeling on take off and landing on concrete strips. What surprised me the most was the perfectly working slats. 

 

In order to avoid any unfounded controversial back and forth posts, I am citing the source up-front:  Haynes RAF, Royal Air Force Official Product, Messerchmitt Bf109 1935 onwards (all marks) Owner's Workshop Manual -an insight into owning, flying and maintaining the Luftwaffe's legendary single-seat fighter- by Paul Blackah and Malcom V. Lowe. Published by Haynes Publishing Sparkford BA22 7JJ, UK. ISBN# 978 184425 6426. They have published similar owners' workshop manual for Hurricane, Spitfire, Mustang, Tiger Tank, etc.... 

Website: http://www.haynes.com/products/sfID1/115/sfID2/125

 

The leading edge slats (based on a Handley Page Patent for leading edge slots) are referred to as "were sometimes more of a hindrance than a help", explaining that the slats would operate asymmetrically when the aircraft was pulled too early during take off. During air combat maneuvering when too much elevator was used (G-force), the slat would open asymmetrically. The ailerons would be prone to "snatching" (moving unexpectedly quickly or harshly) if the leading edge slats opened asymmetrically. The slats could also come open on occasion if the Bf109 was flown in the slipstream of another aircraft, which is not that good for a fighter which was supposed to come behind its quarry. This is on page 104 of the manual.

 

I would assume that everyone understand what happens when the slats operate asymmetrically, one wing has more lift than the other and the plane commence to engage into a roll. If engaging into a roll the ailerons also snatch simultaneously, I would think that the feeling a stability would vanish fairly quickly.

 

[JtD]

I would assume that everyone understand what happens when the slats operate asymmetrically, one wing has more lift than the other and the plane commence to engage into a roll. If engaging into a roll the ailerons also snatch simultaneously, I would think that the feeling a stability would vanish fairly quickly.

I would assume that everyone who understands what happens when slats open would disagree with that. Slats significantly increase maximum lift, not lift at a constant angle of attack.

[GunnyHighway]

Test pilots, engineers who flew those planes and restore them says it is a source of instability and I would have a tendency to trust their statements as their experienced it. I have tried at different setting and different situations, the slats do not deploy asymmetrically in the game, and the controls are harmonious.

 

The source are available, I did not make it up.

[SCG_Space_Ghost]

Test pilots, engineers who flew those planes and restore them says it is a source of instability and I would have a tendency to trust their statements as their experienced it. I have tried at different setting and different situations, the slats do not deploy asymmetrically in the game, and the controls are harmonious.

 

The source are available, I did not make it up.

 

Okay... Well that's interesting... You're citing a post-war manual... Next you're citing a Handley-Page document... And then referring to anecdotes about the 109 just below.

 

[Edited]

Edited April 14, 2015 by Bearcat

[Dakpilot]

Asymmetric slat deployment was a real issue, it would be interesting to see if it is modelled in the FM, if not it is certainly not a deal breaker as a trained Pilot would be aware of this situation and careful to avoid it 

 

Cheers Dakpilot

[GunnyHighway]

Asymmetric slat deployment was a real issue, it would be interesting to see if it is modelled in the FM, if not it is certainly not a deal breaker as a trained Pilot would be aware of this situation and careful to avoid it 

 

Cheers Dakpilot

I could not trigger it, and it is not a deal breaker, agreed.

[Venturi]

One thing I note - the 109 slats should deploy quickly, not gradually. New pilots were often startled by the sound they made deploying. "Bang!".. "Bang!"

[GunnyHighway]

Interesting detail.

[Leaf]

Wing leading edge slats - good or bad?

 

Information about slats

 

- The 109 included the leading edge slats starting with the very first versions which were powered by Kestrel engines, and it is quite funny that the very first version was equipped with a Rolls-Royce engine. It is fascinating that these slats were built under licence of the British Handley-Page. Up to the start of the war Germans paid license fees for every Messerschmitt to H-P. Also the oval wing employed in the Spitfire was originally conceived by Heinkel. Heinkel and Supermarine had a lot of co-operation before the war. Heinkel tried to buy a licence for the Merlin engine in Germany. 
- Lower wingloading does not automatically equals better turn rate, it is more a factor of actual liftloading. Depending on the wing`s design, it may develop more or less lift. As it stands, the P-51D had laminar flow wings, which lowered the drag, but this came at the cost of lower lift, especially under high G loads. The 109 had a conventional wing and was equipped with automatic leading edge slats. These opened out at low speed or at high speed under high G loads, and restored to airflow (=lift) which would have been long separated otherwise due to turbulance. That`s why it become so common on modern jets. This seems to be supported by the AFDU`s test, where they pitted the higher wingloading FW 190A vs. P-51B, yet they found their turning performance basically identical. I`d like to add that 109s were generally found to be the better turning machines in all German and Soviet tests vs. 190s. Also, stall characteristics of the 109 were very gentle and forgiving with plenty of warning, as opposed to the P-51. This also helped the pilots to push their aircraft to the limits of stall.

 

- At least in case of a Gustaf (which is the same as F-4, including the slat mechanism) the slats tend to droop out of the leading edge when the plane stands on ground. They however move in by pressing the leading edge with one finger, extremely easily. After having been pushed in, they either stay in or slowly glide out again. This is very sensitive to the plane's actual angle on ground, and may be affected even by the relative tire pressures of main and tail landing gear wheels. But in flight they bang in and out driven by the aerodynamic forces. There are no springs or such, both the movement out and movement in are totally driven by aerodynamic forces. Those forces are controlled by the wing AoA, and there is no "deadband" in the function at any AoA. The operating function has very narrow transfer from stable position to an another. This means that in normal flight it should be almost impossible to control the AoA with such precision that the slats would slowly slide in or out. I have talked about this with several Bf 109 veterans and none of them has claimed gradual operation, they all explain their working as slamming in and out. On the other hand none of them has descibed the slat extension to be associated with any sudden increase of drag either. Which is as it should operate also theoretically. Slats extend up the range of AoA where the airflow stays attached to the wing. Withouth slats the wing would stall at certain AoA, the airflow turning turbulent at the same moment with sudden enormous increase in drag. With slats the airflow stays non-turbulent for some extra amount of AoA, and there should not be any "stepped" increase in drag when the slats deploy, only at the point where even the slats cannot prevent the wing entering a stall. - Pentti Kurkunen, enthusiast

 

Pilot comments

 

Me 109 E:
"I was particularly interested in the operation of the slats, the action of which gave rise to aileron snatching in any high-G manoeuvres such as loops or tigh turns so I did a series of stalls to check their functioning more accurately. The stall with the aircraft clean, with half fuel load and the engine throttled right back occurred at 105 MPH (168 km/h). This was preceded by elevator buffet and opening the slats about 20 mph (30 km/h) above the stall, these being accompanied by the unpleasant aileron snatching as the slats opened unevenly. The stall itself was fairly gentle with the nose dropping and the port wing simultaneously dropping about 10 degrees."
- Eric Brown
- The author writes about an "unpleasant" event. Nothing catastrophic! Surely all of the planes of that time had features, that were unpleasant, just as well as many planes today have. Curtiss Hawk 75 was surely unpleasant to fly with the rear fuselage fuel tank filled, as flying acrobatics could get you killed. P-51 was at least unpleasant with fuselage tanks filled.

 

Me 109 E:
"The Bf 109s also had leading edge slats. When the 109 was flown, advertently or inadvertently, too slow, the slats shot forward out of the wing, sometimes with a loud bang which could be heard above the noise of the engine. Many times the slats coming out frightenened young pilots when they flew the Bf 109 for the first time in combat. One often flew near the stalling speed in combat, not only when flying straight and level but especially when turning and climbing. Sometimes the slats would suddenly fly out with a bang as if one had been hit, especially when one had throttled back to bank steeply. Indeed many fresh young pilots thought they were pulling very tight turns even when the slats were still closed against the wing. For us, the more experienced pilots, real manoeuvring only started when the slats were out. For this reason it is possible to find pilots from that period (1940) who will tell you that the Spitfire turned better than the Bf 109. That is not true. I myself had many dogfights with Spitfires and I could always out-turn them. 
One had to enter the turn correctly, then open up the engine. It was a matter of feel. When one noticed the speed becoming critical - the aircraft vibrated - one had to ease up a bit, then pull back again, so that in plan the best turn would have looked like an egg or a horizontal ellipse rather than a circle. In this way one could out-turn the Spitfire - and I shot down six of them doing it. This advantage to the Bf 109 soon changed when improved Spitfires were delivered." 
- Erwin Leykauf, German fighter pilot, 33 victories. Source: Messerschmitt Bf109 ja Saksan Sotatalous by Hannu Valtonen; Hurricane & Messerschmitt, Chaz Bowyer and Armand Van Ishoven.

 

Me 109 E:
"And there I discovered the first thing you have to consider in a 109. The 109 had slots. The slot had a purpose to increase the lift during takeoff and landing. In the air automatically it's pressed to the main wing. And if you turn very roughly you got a chance, it's just by power, the wing, the forewing, comes out a little bit, and you snap. This happened to me. I released the stick immediately and it was ok then. "
- Major Gunther Rall in April 1943. German fighter ace, NATO general, Commander of the German Air Force. 275 victories. Source: Lecture by general Rall.

 

Me 109 E/F/G: - The plane had these wing slats and you mentioned they pop open uneven?
"Two meter slots on fore wings.  The reason was to increase the lift during low speed take off and landing.  To reduce the length of runway you need.  In the air, if you make rough turns, just by gravity, the outer slot might get out.  You can correct it immediately by release of stick, you know? Only little bit, psssssssht, its in, then its gone.  You have to know that.  And if you know it, you prevent it." 
- Major Gunther Rall. German fighter ace, NATO general, Commander of the German Air Force. 275 victories. Source: Lecture by general Rall.

 

Me 109 G:
"- How often did the slats in the leading edge of the wing slam open without warning? 
They were exteneded always suddenly but not unexpectedly. They did not operate in high speed but in low speed. One could make them go out and in by moving the stick back and forth. When turning one slat functioned ahead of the other one, but that did not affect the steering. In a battle situation one could pull a little more if the slats had come out. They had a positive effect of the slow speed handling characteristics of the Messerschmitt.
- Could the pilot control the leading edge slats? 
No. The slats were extended when the speed decreased enough, you could feel when they were extended. "
- Kyösti Karhila, Finnish fighter ace. 32 victories. Source: Interview by Finnish Virtual Pilots Association.

Me 109 G:
"- In a battle, which was the case: did the pilot endure more than the Messerschmitt could do or vice versa? 
 The fact is that when you pulled hard enough the wing leading edge slats slammed open. After that the pilot could not tighten the turn. The plane would have stalled. I don't know, I never tried to find out what the plane would do after that. I never heard anybody else saying that he would have banked so hard that the slats came out. I did that a few times, for example once over the Isthmus I tried to turn after an enemy, banking so hard that both slats came out, but I had to give up. 
- How did the slats behave in such a situation, did they go in and out ? 
 It depended on speed, if you pulled more,they came out, then back in 
The slats came out completely, never half-way? 
 I never came to watch them so intensely. You just knew they had come out, you could see them and feel that the lift increased pretty much. 
- So the plane warned that now you are on the edge.
Yes, you knew the plane is about to spin."
- Antti Tani, Finnish fighter ace. 21,5 victories. Source: Interview by Finnish Virtual Pilots Association.

Me 109 F/G:
"- Did pilots like the slats on the wings of the 109? 
Yes, pilots did like them, since it allowed them better positions in dogfights along with using the flaps. These slats would also deploy slightly when the a/c was reaching stall at higher altitudes showing the pilot how close they were to stalling.....this was also useful when you were drunk "
- Franz Stigler, German fighter ace. 28 victories. Interview of Franz Stigler.

Me 109 G:
"As CL max is reached the leading edge slats deploy - together if the ball is in the middle, slightly asymmetrically if you have any slip on. The aircraft delights in being pulled into hard manuevering turns at these slower speeds. As the slats pop out you feel a slight "notching" on the stick and you can pull more until the whole airframe is buffeting quite hard. A little more and you will drop a wing, but you have to be crass to do it unintentionally."
- Mark Hanna of the Old Flying Machine Company flying the OFMC Messerschmitt Bf 109 G (Spanish version).

 

Me 109 G: 
"There was nothing special in landing the plane. It was heavy but the wing slats opened up when speed slowed down and helped flying in slow speed."
-Kullervo Joutseno, Finnish fighter pilot. Source: Hannu Valtonen, "Me 109 ja Saksan sotatalous" (Messerschmitt Bf 109 and the German war economy), ISBN 951-95688-7-5.

 

Me 109 G:
"It was beneficial to keep the throttle a little open when landing. This made the landings softer and almost all three-point landings were successful with this technique. During landings the leading edge slats were fully open. But there was no troubles in landing even with throttle at idle."
-Mikko Lallukka, Finnish fighter pilot. Source: Hannu Valtonen, "Me 109 ja Saksan sotatalous" (Messerschmitt Bf 109 and the German war economy), ISBN 951-95688-7-5.Source: Hannu Valtonen, "Messerschmitt Bf 109 and the German war economy"

 

Me 109 G: 
"We didn't have time for acrobatics but we weren't forbidden from doing them, though. Snap roll was fast and easy, and the engine didn't cough as in older planes. Immelman turn was splendid when you tightened the stick a bit on the top. The automatic wing slats did their trick and you didn't need ailerons at all for straightening the plane."
-Otso Leskinen, Finnish fighter pilot. Source: Hannu Valtonen, "Me 109 ja Saksan sotatalous" (Messerschmitt Bf 109 and the German war economy), ISBN 951-95688-7-5.

"Unexperienced pilots hesitated to turn tight, bacause the plane shook violently when the slats deployed. I realised, though, that because of the slats the plane's stalling characteristics were much better than in comparable Allied planes that I got to fly. Even though you may doubt it, I knew it [bf109] could manouver better in turnfight than LaGG, Yak or even Spitfire."
- Walter Wolfrum, German fighter ace. 137 victories.

 

 

All thanks to: http://www.virtualpilots.fi/feature/articles/109myths/

Edited April 14, 2015 by 19te.Leaf

[YSoMadTovarisch]

Funnily enough, German test showed that on the 109 the slats increased very little to no clmax at all, this was because the slats only spanned 2/5 of the wings, instead of the whole wing like the slats of standard conventional design. The slats main goal was to retain enough ailerons authority up to complete stall of the wings.

 

http://forums.eagle.ru/showpost.php?p=2371074&postcount=59

[novicebutdeadly]

Asymmetric slat deployment was a real issue, it would be interesting to see if it is modelled in the FM, if not it is certainly not a deal breaker as a trained Pilot would be aware of this situation and careful to avoid it 

 

Cheers Dakpilot

 

From what I have read the slat issue (asymmetric operation)  was an issue with the 109E series and earlier, the issue was addressed from the 109F series onwards when the wing was re-designed.

 

 

[novicebutdeadly]

I flew for the first time the Bf109 and was surprised by the stability feeling on take off and landing on concrete strips. What surprised me the most was the perfectly working slats. 

 

In order to avoid any unfounded controversial back and forth posts, I am citing the source up-front:  Haynes RAF, Royal Air Force Official Product, Messerchmitt Bf109 1935 onwards (all marks) Owner's Workshop Manual -an insight into owning, flying and maintaining the Luftwaffe's legendary single-seat fighter- by Paul Blackah and Malcom V. Lowe. Published by Haynes Publishing Sparkford BA22 7JJ, UK. ISBN# 978 184425 6426. They have published similar owners' workshop manual for Hurricane, Spitfire, Mustang, Tiger Tank, etc.... 

Website: http://www.haynes.com/products/sfID1/115/sfID2/125

 

The leading edge slats (based on a Handley Page Patent for leading edge slots) are referred to as "were sometimes more of a hindrance than a help", explaining that the slats would operate asymmetrically when the aircraft was pulled too early during take off. During air combat maneuvering when too much elevator was used (G-force), the slat would open asymmetrically. The ailerons would be prone to "snatching" (moving unexpectedly quickly or harshly) if the leading edge slats opened asymmetrically. The slats could also come open on occasion if the Bf109 was flown in the slipstream of another aircraft, which is not that good for a fighter which was supposed to come behind its quarry. This is on page 104 of the manual.

 

I would assume that everyone understand what happens when the slats operate asymmetrically, one wing has more lift than the other and the plane commence to engage into a roll. If engaging into a roll the ailerons also snatch simultaneously, I would think that the feeling a stability would vanish fairly quickly.

The reference is to the 109E and earlier and is 100% correct. When testing the captured 109E (the one that the French munted the engine) they found that if the pilot had to pull a turn tight enough to cause the slats to open, it would cause snatching.

 

This was one of the things addressed when the 109 was redesigned (F series and later), other things were giving the vertical stabilizer an airfoil to counter the prop/ engine torque effect meaning that the pilot had to apply little to no rudder to keep the plane flying straight (once airborne).

 

It would be interesting to see if they model this into the FM for the 109E in BOM, I actually hope they do (I fly mainly the 109) as it will add to the historical accuracy.

Edited April 16, 2015 by novicebutdeadly