K-4 Flettners

[agent 009]

While back I posted something about the flettners being nailed shut on K-4 cause the elevators were not tabbed & this caused turbulence. From a book called flying legends or something like that. Anyway , it could well be sci-fi, read on...  
 
K-4 fuselages, turning circle, and Flettner tabs
June 25 2004 at 7:08 AM Michael Gorman  (Login mrg22)
The 109 Lair Board Members
from IP address 62.188.48.139


Response to K-4 turn circle

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Hello all,

I have been following this thread with more than casual interest! As some of you may know, I am involved in a project to restore a '109K-4 to airworthy condition. The comments here have been interesting and relevant, and prompt me to offer some thoughts of my own.

Firstly, this question of a "lightweight, redesigned" fuselage for the K-4. I am not aware of ANY original reference at all to such an exercise, and if anyone has information to the contrary, I would be very pleased to hear from them! Further, we are working with major remains of three K-4 aircraft, and I can confirm that we have found NO evidence of any alteration from the standard "G-type" fuselage in the material which we have (apart from the obvious changes in location of hatches, filler ports, etc.). I suspect we may have another "urban myth" here.

Moving on the question of turning rate (or turning circle, actually somewhat different things), this is not the place for an aerodynamics lecture. However, in quick summary, an aircraft turns because some of its lift is directed away from the vertical by banking. For a level turn, the maximum rate of turn (in degrees/second, and hence the radius of turn at a given airspeed) depends upon the balance between total lift available and aircraft weight. If the external configuration of an aircraft (wing shape/area, aerofoil section, tailplane size, fuselage, etc.) remains the same (which the '109 essentially does from G to K), then the total lift available remains the same. Bottom line, a heavier aeroplane of the same configuration has a lower maximum turning ability, and vice versa, of course.

The issue of possible trim changes is a bit of a red herring here. It's true that changing the balance of lift between mainplane/tailplane would alter the turn, because the total lift would change, as A. Lake rightly notes. The aerofoil section on the '109 has a positive pitching moment, requiring a DOWNLOAD from the tailplane to balance it. This subtracts from total lift, which reduces possible turn rate (see above).

However, it's fairly clear that the longitudinal trim WASN'T different on the K-4. Specifically, the tailplane trim range was very similar to previous (roughly +2 degrees to -6 degrees), and according to the Flugzeug Handbuch, the setup for this trim adjustment is done to the same fuselage datum as previously. Given that the tailplane did not change size, this means that there was no change in available tailplane trim forces. As Lynn rightly notes, various things were moved around internally, but this was precisely to maintain the centre of gravity in the SAME POSITION, so as to avoid flying with excessive tailplane trimming loads. The D-Motor, larger oil cooler, wider prop blades, etc., all added significant weight forward, which had to be counter-balanced.

The question of the Flettner tabs on ailerons and rudder is an interesting one, but not directly related to turning rate. First, the tabs are not trim tabs in the usual sense of the term, but are known as servo tabs, which act to reduce stick or rudder pedal forces by applying an assisting force in the direction of deflection. Now, as Brent rightly notes, there is no photo evidence of their installation on K-4s. I have looked pretty carefully, but again, any correction would be much appreciated! Your point, George, re. the manual (Flugzeug Handbuch) is absolutely correct, it shows and describes the tabs, BUT in the Ersatzteilliste for the K-4 (of July '44), plain ailerons are shown. Also, at least one of our aircraft had plain ailerons installed.

One thing I have been trying to track down, in the "Wings of Fame" (Vol. 11) article on the later '109s, it is asserted that the Flettner tabs on K-4 ailerons were "usually locked shut", having been found to "cause a major discrepancy" in required stick forces between ailerons and elevators. I have tried in vain to find an original reference for this, and if we believe that K-4s didn't have Flettner-tabbed ailerons installed anyway, then this becomes yet another "urban myth".

Sorry if this post has been rather long, but you will understand that I find the subject somewhat interesting!

Regards,

Michael Gorman

P.S. I once had an article pass by me that mentioned even G-6's being field modified with flettners, but they caused wing failure. Wish I'd saved it. Could also be sci-fi, but it is interesting nonetheless. The Flettners were not factory standard on K-4's so I've read, which might explain why some didn't have em.

Any data? I know this is one of those "tough to track down" topics.

[HoHun]

Hi Agent,

>Bottom line, a heavier aeroplane of the same configuration has a lower maximum turning ability, and vice versa, of course.

Let me add that if we're limiting the question to sustained turns, engine power plays a role as well, and the K-4 might not actually turn worse than a lighter, lower powered earlier version in such a sustained turn. In instantaneous turns, the lighter aircraft wins of course.

>P.S. I once had an article pass by me that mentioned even G-6's being field modified with flettners, but they caused wing failure.

I believe that's highly plausible. The same experience was made with the Spitfire, which had a very similar wing construction (though a very different wing shape, of course :-) I always suspected it would be the same for the Me 109.

What happened to the Spitfire is that at high speeds, the aileron caused a force twisting the wing opposite to the aileron input, and the twisted wing caused as much or more lift than the aileron destroyed, negating the roll or actually rolling the aircraft to the opposite side. More aileron deflection to correct the unwanted roll would not help to correct that, but it could twist the wing to the point of structural failure.

With such an aeroelastic problem, you would not want powerful ailerons (for example of the Flettner type) because it would only help the pilot to accidentally break the aircraft. High aileron forces on the other hand would prevent such accidents.

My suspicion is that this was the reason the Flettner tabs were never (or rarely?) used for the Me 109.

Regards,

Henning (HoHun)

[Kurfürst]

I suggest you look at this picture, Gentlemen...



I think it shows a late production - appearantly captured in half-complete stage, control surfaces are not yet connected - 109K-4.

Special interests are the rod under the wing - it`s for the actuator rod for the Flettner, and cannot be found normally on Bf 109s. One can barely see a smudged line on the aileron, that could be the flettner - I am not sure, it`s too blurry.

The 109K-4 handbuch also notes the aileron Flettner tabs, with the comment they are locked at the moment, as well as many drawings for the 109K show them.

Now the next photo also shows the aileron Flettner on a late 109 wing, next to G-10.



The following is a 109G-6 of hungarian air force, photographed around oct-nov 1944, on the field.



Then there`s the picture of Graf`s 109G-6, photoed in 1943, it also clearly has Flettners.

So basically we have photos of aileron Flettners for G-6, G-10, and appearantly, a K-4. The servo tab was no standard by any means, but it was there and obviously intended for service from 1943 onwards.. why not put into widespread production, I guess manufacturing problems has more to do with than the boring old urban myth about the 'weak' 109 wings. After all, wheel well covers were intended for the 109G but only a handful got it, and there are 109K w/o as well. Simply, things take time to run up.

As for the wing twist problem of the Spitfire, it`s well known.  The NACA 868 notes the Spits wing caused 65% reduction in roll rate at high speed, as opposed to 30% of the P-47. It`s fairly logical to me, considering the Spitwing`s rigidity was provided by the D-shaped torsion box in the leading edge... forces that applied in the back on the wing should and were very able to twist such structure - hence the complete redesign of the Spit wing structure in Mk21.

I doubt the same applied to the 109. First the wing constuction is very different, a single main spar in the centerline, supplemented by torsion box spar behind. It shouldn`t be prone to twist, and in fact butch2k noted a good while that americans were surprised of the 109F wing rigidity, which was comparable to their own two-spar designs. Moreover, butch allagedly has some photos of 109s with aileron flettners.

[Kurfürst]

Note, the aileron Flettner tabs on the G-6 was not a field mod. They were installed in the WNF factory in automn 1943, and appearantly only there - hence the few examples of pictures with them.

[HoHun]

Hi Kurfürst,

>I doubt the same applied to the 109. First the wing constuction is very different, a single main spar in the centerline, supplemented by torsion box spar behind.

I believe that's a misunderstanding - the torsion box should indeed be the wing nose. I haven't looked at the exact spar placement yet, though.

>It shouldn`t be prone to twist, and in fact butch2k noted a good while that americans were surprised of the 109F wing rigidity, which was comparable to their own two-spar designs.

Hm, would be interesting to see that report. I haven't read much on aeroelasticity issues so far!

Regards,

Henning (HoHun)

[Kurfürst]

Hmm, maybe I did not use the right term... 'box spar' for the 109. That`s simple to execute, basically the main spear, the surface planting and the aux. spar forms a large box, stiffening the wing structure. Because it`s simple, it`s rather widespread. Interestingly, a hungarian book for flight mechanics gives the 109 as example for a box spar, among other things, certainly it`s a good example for prop with so many innovative features. I am not sure if it had used torsion box in the leading edge, though. Probably not, my mistake.

As for US report, I don`t have it, perhaps crumpp had seen something in US archives. Here`s what butch posted :

"Author:  butch2k  
Rank:  Over 1000 Postings  
Date:  10/03/03 11:39AM  
   
109 wing torsion is similar to what could be found in US fighter planes. No aileron reversal at high speed, which came as a surprise to US engineers in charge of evaluation the 109 aerodynamical properties."

[HoHun]

Hi Kurfürst,

Highly interesting, thanks!

I admit that previously, I was a bit confused by the aileron overbalance mentioned in the high-speed diving trials which, if I think about it now, would indicate the opposite effect of aileron reversal. That would indeed suggest a rather stiff wing.

Box spar - hm, that's surprising, I had always thought Messerschmitt to be the pioneer of the single-spar nose-torsion-box construction, and as far as I know, this applied to the Me 109 as well. However, I guess that doesn't mean you can't combine it with another torsion box including the auxiliary spar ...

Regards,

Henning (HoHun)

[agent 009]

Cool. now all we need is a pilots report on these.

[Karnak]

Kurfurst,

What number of Flettner tab equipped Bf109s made it into service do you think?

It is odd that the Bf109K, which was intended to unify Bf109 production into one standard instead of the mishmash of Bf109G models, was still not standardized.  It is understandable though, given the pressures Germany was under at the time.

Do you know of any comparitive flight trials for Flettner tabbed Bf109s and Bf109s without?  It's be interesting to see how much they improved performance.

[Wotan]

Butch replied to Kurfürst's thread over at AAW2 109K aileron flettner tabs....

butch2k, Jul 28 2005, 10:23 PM[/i]
In fact it seems that no K-4 were fitted with them, it's the WNF/Gyor produced a/c which got them as the flettner equiped ailerons were produced by diana, a major subcontractor of WNF.
IIRC the flettner tests took place late 1942 with some operational testing done by hte JG 50 in 1943 with flettner modded G-5 and G-6.

[Kurfürst]

Originally posted by Karnak
Kurfurst,

What number of Flettner tab equipped Bf109s made it into service do you think?

[/B]

I have no idea. There are some photos of G-6s with them, and appearantly they are coming from WNF factory, near Vienna.

It is odd that the Bf109K, which was intended to unify Bf109 production into one standard instead of the mishmash of Bf109G models, was still not standardized.  It is understandable though, given the pressures Germany was under at the time.[/B]

Yes, that`s what I think too. The 109K was to have retractable tailwheel and wheel well covers, but the latter are rare on early machines but can be find on later ones, ditto for the tailwheel but it`s harder to tell on the ground wheter it was just closed by the crew (mud etc.) or was fixed down.

As for the 109K, the Flugzeug Handbuch, Teil0, p13 states :

"Seitenruder mit am Boden einstellbaren Ausgleichruder (Flettner) und geschrankten Bugelkanteln versehen.

Querruderanordung links und rechts and den Tragflachen

Beide Querruder mit Ausgleichruder (Flettner) versehen). Ausgleichruder z.Zt. festgelegt."

I hope HoHun can give an accurate translation, but basically: the ailerons have Flettner, but they are at present time fixed.

Do you know of any comparitive flight trials for Flettner tabbed Bf109s and Bf109s without?  It's be interesting to see how much they improved performance. [/B]

Something was posted on this by George on the LEMB message board, about a test with Williemsen on some old test-hack 109G. Unfortunately due to some hack attack most of the board`s content was lost, and I don`t have a backup on that thread (anybody has?). What I remember clearly and that the ailerons could be deflected 2/3 at some very high dive speed,and that no aileron overbalance was observed with them.. 0.7Mach it was?

[Kurfürst]

Good, I have not grown senile yet! Found on another forum the original LEMB one from George :

"The following is from a test dive on 7.10.44 in good old W.Nr. 18550 (I say that because that a/c was used in just so much of the 109 testing): the test was made by "Willemsen" (he's not known to me. Does anyone have info on him?), on an a/c equipped with ailerons with Flettners, large stabilizer and rudder, a/c weight: 333o kg, and CofG at take-off: 24.6%.

At a corrected speed of 770 kph (.75 Mach at the test altitude), Test pilot Willemsen was able to get the ailerons to travel to 2/3 of their available range (no approx. force required is mentioned), forces were the same to either side, and there was no overbalancing observed. "

[Kurfürst]

More goodies :

[Kurfürst]

G-6s with aileron Flettners in WNF factor, automn 1943 (via radinger/otto)

[Kurfürst]

Originally posted by Wotan
Butch replied to Kurfürst's thread over at AAW2 109K aileron flettner tabs....

Wotan,

the trouble with butch`s latest position is that before I have seen him state that he had seen some pics of 109K with flettners on ubi boards... Moreover, the scale drawing of the 109K wing which shows flettners was originally posted by him.
I am not saying anything but I had butch change his position on several issues, so I`d rather go by the 109K picture I posted that appearantly HAS a flettner, regardless of everything. Otherwise, the fact that they were seriously intended is undeniable, they show up on drawings, the manual etc.