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FW-190 and Bf-109 Longitudinal Stability and Control


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Here is a report on the elevator Stick Forces and influence of rudder position at various mach numbers.  The report defines the longitudinal stability and control characteristics of the FW-190 and Bf-109 series at .  Mach number can easily be converted to any velocity units desired.  The report gives diagrams at the listed CG positions for the stick forces over mach number as well as control deflection angles for the rudder and elevator.

 

This information is very useful to those familiar with stability and control characteristics.  Both of these aircraft were designed under stability and control standards and are considered to be some of the best examples of stability and control design even by todays standards.  That being said, neither aircraft is perfect as stability and control engineering was in its infancy during World War II.  They do represent very good examples and a yardstick to examine other World War II designs.

 

V101tom.....PM me if you have any questions!

 

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Here is a report on the elevator Stick Forces and influence of rudder position at various mach numbers.  The report defines the longitudinal stability and control characteristics of the FW-190 and Bf-109 series at .  Mach number can easily be converted to any velocity units desired.  The report gives diagrams at the listed CG positions for the stick forces over mach number as well as control deflection angles for the rudder and elevator.

 

This information is very useful to those familiar with stability and control characteristics.  Both of these aircraft were designed under stability and control standards and are considered to be some of the best examples of stability and control design even by todays standards.  That being said, neither aircraft is perfect as stability and control engineering was in its infancy during World War II.  They do represent very good examples and a yardstick to examine other World War II designs.

 

V101tom.....PM me if you have any questions!

 

Hi.

 

Thanks a lot. Very good material :)

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VO101Kurfurst

There's a lenghty Soviet report on the same subject (stability test results of the captured 109G-2). It would be great if someone could translate it, even partially. I can get on well with German, but my Russian is a bit rusty! ;)

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  • 2 weeks later...

Rolf Pingel delivered a nice shiny Bf 109 F to the RAF and in the report of his interrogation, under the paragraph Me. 109 F, it states:

"The outstanding disadvantage of the Me.109 F is that the wings are not so stable as they might be. At least two pilots, including the redoubtable Hauptmann BALTHASAR, Kommodore of the Richthofen Geschwader have been killed in the last three weeks by tearing the wings off their Me.109s when trying to follow Spitfires in a snaking dive. After a fast dive pilots have to pull out fairly gradually."

There was a perception in the Luftwaffe that there was and Pingel is not the only German source source for this.

As for the RAF, it seems to have cottoned on to this fairly quickly. Bader developed the tactic of half rolling and following the Bf 109 F in a dive, rolling back expecting his target to be ahead of him and pulling out much more gradually from the dive than he could in a Spitfire. This allowed him to pull his nose up through the target and make a shot. In his words the German pilots were " a bit porky on the old joystick". :o:

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The outstanding disadvantage of the Me.109 F is that the wings are not so stable as they might be.

 

Not so stable as they might be.....

 

General statement referring to stability and control engineering of an aircraft.  Not referring to the kind of stability required to get a gun site on target.  It is referring to the following know characteristic of the Bf-109 stability and control.

 

 

At least two pilots, including the redoubtable Hauptmann BALTHASAR, Kommodore of the Richthofen Geschwader have been killed in the last three weeks by tearing the wings off their Me.109s when trying to follow Spitfires in a snaking dive. After a fast dive pilots have to pull out fairly gradually

 

 

The Bf-109 when tested at high mach numbers (.55 to Vne) exhibited directional instability in the form of a dutch roll.  Gondola weapons increased this directional instability. If a pilot tried to correct it with aileron input under acceleration, an asymmetrical load condition would develop that could overload the airframe.   

 

This is a function of an incorrectly sized control surface for high mach number flight.  Like the Spitfire, the Bf-109 elevator was overly effective at high mach numbers.  A trade off for correct sizing at Vs.

 

The solution was in increase in the vertical stabilizer area to increase directional stability at high mach numbers.  The control system is designed to flown beyond the normal operating range without trim input.  If the pilot trims for the high mach number condition the airframe can also be overloaded.  The Operating Instructions specifically warn against trimming for high mach conditions.

 

This is in contrast to a poorly designed control system that allows the pilot under normal operating conditions to overload the airframe above Va or hold a steady acceleration.

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DD_bongodriver

This is in contrast to a poorly designed control system that allows the pilot under normal operating conditions to overload the airframe above Va or hold a steady acceleration.

 

 

 

This makes no sense at all, all aircraft are capable of overloading the airframe above Va and it has nothing to do with poor design, poor design is an aircraft that tears its own wings off because of dutch roll.

 

The solution was in increase in the vertical stabilizer area to increase directional stability at high mach numbers. 

 

From which model did they increase fin size?

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What 109, or 190, had vertical stabilizer trim?

 

 

The solution was in increase in the vertical stabilizer area to increase directional stability at high mach numbers.  The control system is designed to flown beyond the normal operating range without trim input.  If the pilot trims for the high mach number condition the airframe can also be overloaded.

 

Bongo, late war production 109Gs got a larger fin and rudder but it was not fitted to all a/c.

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 all aircraft are capable of overloading the airframe above Va

 

Yes, if the control system is not designed to prevent a pilot from doing it.  Some aircraft are much easier to overload than others.  The science and study of such control systems is stability and control engineering.

 

 

What 109, or 190, had vertical stabilizer trim?

 

 

Nowhere did I say either aircraft had a vertical stabilizer trim.  You know they did not and are just being difficult. 

 

Trimmed simply means moments about the CG are zero.

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DD_bongodriver

Yes, if the control system is not designed to prevent a pilot from doing it

 

 

No, Va is a limitation imposed on the aircraft because the control system 'is' capable of exceeding the limits and it is indicated to prevent conscious use of full and abrupt control deflection above it, much like a flap limiting speed, if the aircraft was built to prevent the situation happening it would in fact not be a limit.

 

Maybe this could help you.

 

http://flighttraining.aopa.org/magazine/1999/March/199903_Flying_Smart_A_New_Look_at_Maneuvering_Speed.html

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Nowhere did I say either aircraft had a vertical stabilizer trim.  You know they did not and are just being difficult. 

 

 

The 109 and 190 are the subject of the thread.

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No, Va is a limitation imposed on the aircraft because the control system 'is' capable of exceeding the limits

 

 

NO, Va is the speed the aircraft can experience structural failure at full control deflection

 

Not all control systems are engineered to allow the pilot to achieve full control deflection above Va.   Aerodynamics is all about the details and conditions, Bongo.  Without defined conditions misunderstandings are bound to occur.

 

Milo says: 

 

The 109 and 190 are the subject of the thread.

 

Yes and please read what I wrote.  Do not invent things to argue with yourself. 

Edited by Crump
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DD_bongodriver

No, Va is the speed below which the aircraft will stall before reaching maximum design load factor, beyond which caution must be excercised and full and abrupt control inputs must not be made.

 

Do not invent things to perpetuate arguments.

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For modern aircraft equipped with an electronic flight control system the loads and other parameters may depend on the system parameters.

 

Before the days of electronic flight control systems, one method the designers used to limit control deflection was stick force.

 

http://ftp.rta.nato.int/Public/PubFullText/RTO/AG/RTO-AG-300-V14/AG-300-V14-12.pdf

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DD_bongodriver

Before the days of electronic flight control systems, one method the designers used to limit control deflection was stick force.

 

http://ftp.rta.nato.int/Public/PubFullText/RTO/AG/RTO-AG-300-V14/AG-300-V14-12.pdf

Why a modern day document from NATO that doesn't even back up your argument?

 

Can you name an aircraft that employed this deliberate stick force limitation to prevent over stress?

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What is historically (technically) recorded and what one wants to believe in reference to modern aspects ... are not the same thing here it appears.

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DD_bongodriver

It is not my argument.  It is just a fact.

 

Bf-109.....

 

 

 

The Bf-109 when tested at high mach numbers (.55 to Vne) exhibited directional instability in the form of a dutch roll.  Gondola weapons increased this directional instability. If a pilot tried to correct it with aileron input under acceleration, an asymmetrical load condition would develop that could overload the airframe.   

 

This is a function of an incorrectly sized control surface for high mach number flight.  Like the Spitfire, the Bf-109 elevator was overly effective at high mach numbers.  A trade off for correct sizing at Vs.

 

 

At least two pilots, including the redoubtable Hauptmann BALTHASAR, Kommodore of the Richthofen Geschwader have been killed in the last three weeks by tearing the wings off their Me.109s when trying to follow Spitfires in a snaking dive. After a fast dive pilots have to pull out fairly gradually

 

 

I guess the cirquit breaker for the 109's electronic flight stability and control computer must have popped?

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Ok, for this thread, I leave my moderator's suit in the cloakroom.

Just taking a good chair, ordering popcorns and watching the show...

 

What's next?

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Only if the pilot was Hercules.

 

Right.  Hence the designers takes care of the pilot by preventing him from being able to overload the wings and destroy the airframe.

 

Of course designers cannot account for every situation which is why Operating Instructions are published.  If the pilot does not adhere to them, the result is generally catastrophic.

 

So, thanks for the anecdotal evidence of aircraft in completely unknown mechanical and flight conditions from a prisoner of war interrogation by the British to compare with measured results from Mtt and the RLM. 

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DD_bongodriver

 

Like the Spitfire, the Bf-109 elevator was overly effective at high mach numbers.  A trade off for correct sizing at Vs.

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Rolf Pingel (1 October 1913 – 4 April 2000) was a German Luftwaffe ace and recipient of the Knight's Cross of the Iron Cross during World War II. The Knight's Cross of the Iron Cross was awarded to recognize extreme battlefield bravery or successful military leadership.

 

On 10 July 1941 Rolf Pingel was forced to land in England after being hit by return fire from a British Short Stirling bomber that he was pursuing. He descended to low altitude but was harried by several Spitfires, and he crashed landed near St. Margarets Bay and was taken prisoner by a detachment of Home Guard.

 

Pingel's aircraft, Bf 109 F–2, Werk Nr. 12764, was returned to flying condition by the RAF and allocated the serial number ES906. It was briefly flown for evaluation testing until it crashed near Fowlmere on 20 October 1941, killing its Polish pilot F/O J. Skalski.

 

His Bf109F-2 was from WNr batch 12601-12978 produced between 11.30 - 06.41

 

Wilhelm Balthasar was killed during an aerial combat with RAF fighters over Aire, France. As he was diving violently in his Bf 109 F-4, the wing of his aircraft folded and he crashed to his death near Saint-Omer.

 

I can't see a Kommandeur flying a decrepit piece of junk. Bf 109 F-4 W.Nr. 7066 produced between 5.41 - 12.41 WNr 6999-7660

 

Right.  Hence the designers takes care of the pilot by preventing him from being able to overload the wings and destroy the airframe.

 

Right, the elevators were so stiff at high M, only elevator trim could be used.

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The Germans noticed how heavy the 109's untrimmable rudder was; it would be interesting to see both of the reports cited in this extract.

 

109vs190-002_zps0b7e84b4.jpg

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Two different statements Bongo.

 

Like the Spitfire, the Bf-109 elevator was overly effective at high mach numbers.

 

 

Both aircraft had overly effective elevators at high mach numbers.  The difference is the high stick forces of the Bf-109 help prevent overstressing the airframe.  A similar longitudinal stability standard is in place today in convention signers.

 

 

A trade off for correct sizing at Vs

 

 

 

The second sentence refers only to the Bf-109.  The Spitfire had an incorrectly sized elevator.  Start a new thread if you want to cover the Spitfire characteristics.

 

 

 

109's untrimmable rudder

 

Interesting.....

 

The facts say differently. 

 

1.  The Bf-109's rudder was trimmable.  It used the most common rudder trim method in aviation.....a fixed tab.  In stability and control engineering design, the joke is "a rudder is only there to fix the mistakes of the engineer". 

 

2.  The speeds discussed in the report were adverse behaviors occur are outside the normal operating range of the aircraft.  In fact, they are the defining behaviors for the limitations.

 

Here is the high speed dive report conducted by Mtt.  The behaviors are quantified and measured unlike the anecdotes presented above.

 

 

Diving_Test_109F_W.Nr.9228_ger_eng.pdf

 

 

 

 can't see a Kommandeur flying a decrepit piece of junk. Bf 109 F-4 W.Nr. 7066 produced between 5.41 - 12.41 WNr 6999-7660

 

 

And how many bullet holes did he have through the main wing spar when he started the dive? 

 

Did he follow the Operating Instructions? 

 

Did he asymmetrically load the airframe on recovery? 

 

Did he exceed Vne? 

 

Did he previously overstress the aircraft in earlier combat such that it would fail at a much lower load factor?

 

We do not know the conditions of flight or the aircraft to make any reasonable conclusions.

 

I am sorry but such anecdotes are just not credible for determining the engineering limitations of the design without measured data.  It makes an interesting story though and we can speculate multiple possibilities that have nothing to do with the stability and control of the Bf-109.

 

The full investigation based on these incidents is presented above in this post.  The conclusion is pilot error.

 

The report concludes overcompensation by the pilot using ailerons to recover from the dutch roll instead of rudder was the major cause of overstressing the airframe.  It was pilot error not the design.

 

The report also sites the type of grease used was not cold resistant and could freeze at high altitudes.  This has the effect of locking the trim jack screw preventing a pilot who trimmed for the dive from recovering.

 

Design changes to the empennage confirm the additional directional stability of the increased vertical stabilizer and the aircraft equipped with horn mass balanced tail completely eliminated the behaviors at all but the highest speeds.

Edited by Crump
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DD_bongodriver

The difference is the high stick forces of the Bf-109 help prevent overstressing the airframe

 

 

Interesting to note that in the very trial document you submitted they use the phrase:

Explanation of accidents in the front-line units. (Over-compensation of

the aileron controls and insufficient elevator authority at high mach

numbers).

 

 

now why would they consider it insufficient if they designed it that way because they had 'standards'?

 

interestingly the test conditions explain that it's actually ailerons that are being restricted to prevent over-compensation and that is for these tests only, later explaining it may be a recommendation, did they limit aileron throw on later production?

The Plane ME 109 F W.Nr. 9228 was used for these tests. An

ejection seat is build in as an additional equipment. To get an exact

documentation about the achieved figures, the instruments were

photographed and speed & altitude were recorded by an Askaniadevice.

To reduce the risk of pilot over-compensation, the control

movement was limited to 50% of the reference movement of the

ailerons.

 

 

Start a new thread if you want to cover the Spitfire characteristics.

 

No thanks, Spitfire is not relevant to this forum, only you have referenced it so far.

Edited by DD_bongodriver
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A G wing was used.

 

 

The report also sites the type of grease used was not cold resistant and could freeze at high altitudes.

 

At high speed the horizontal stabilizer trim is very easily moved in the "tail
heavy” direction but difficult in the “nose heavy” direction. At high altitudes with
correspondingly lower temperatures, the lubricating grease of the jackscrew
became stiff. Movements of the horizontal stabilizer were only possible with
a lot of force and were jerky.

 

It doesn't say it froze. It says the viscosity increased. If the grease froze, there would be NO movement at all.

 

The fixed trim tab was only good for a specific speed range > cruise. Above and below that speed the pilot had to use his legs, VERY tiring especially at high speed.

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DD_bongodriver

A G wing was used.

 

Yes I noticed that too, presumably because they anticipated a failure from an F wing.

 

The fixed trim tab was only good for a specific speed range > cruise. Above and below that speed the pilot had to use his legs, VERY tiring especially at high speed.

 

Dunno.... :scratch_one-s_head: maybe pilot fatigue was desirable and formed part of the 'standards'

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 insufficient elevator authority at high mach

 

 

Is because of the grease freezing at high altitude not the elevator sizing.

 

 

doesn't say it froze. It says the viscosity increased.

 

It does not say that either.

 

 

At high altitudes with

correspondingly lower temperatures, the lubricating grease of the jackscrew

became stiff.

 

 

 

rigid or firm; difficult or impossible to bend or flex: a stiff collar.

 

http://dictionary.reference.com/browse/stiff

 

 

 

Became stiff.....mmmm sounds like it froze up which is why they recommend a more cold resistant grease. 

 

 

 

Movements of the horizontal stabilizer were only possible with

a lot of force and were jerky.

 

Lots of force required to break the frozen grease and jerky movements from it breaking off in chunks.  

 

It is not really worth arguing but if you want to conclude the viscosity only increased, that is your business.  It is just insignificant minutia. 

Edited by Crump
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DD_bongodriver

Is because of the grease freezing at high altitude not the elevator sizing.

 

No, it doesn't say that, it says insufficient elevator authority at high mach, nothing to do with freezing grease, it also makes clear that the problem with the grease cooling was manifested on the horizontal stabilizer and not the elevators.

 

At high speed the horizontal stabilizer trim is very easily moved in the "tail

heavy” direction but difficult in the “nose heavy” direction. At high altitudes with

correspondingly lower temperatures, the lubricating grease of the jackscrew

became stiff. Movements of the horizontal stabilizer were only possible with

a lot of force and were jerky.

 

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I guess crump has never tried to do an engine oil change outside with the temperature at -20C.

 

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.

 

From everyday experience, it should be common knowledge that viscosity varies with temperature. Honey and syrups can be made to flow more readily when heated. Engine oil and hydraulic fluids thicken appreciably on cold days and significantly affect the performance of cars and other machinery during the winter months. In general, the viscosity of a simple liquid decreases with increasing temperature (and vice versa). As temperature increases, the average speed of the molecules in a liquid increases and the amount of time they spend "in contact" with their nearest neighbors decreases. Thus, as temperature increases, the average intermolecular forces decrease. The exact manner in which the two quantities vary is nonlinear and changes abruptly when the liquid changes phase.

 

http://physics.info/viscosity/

 

from your link crump:

not moving or working easily: The motor was a little stiff from the cold weather.

 

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VO101Kurfurst

interestingly the test conditions explain that it's actually ailerons that are being restricted to prevent over-compensation and that is for these tests only, later explaining it may be a recommendation, did they limit aileron throw on later production?

 

 

 

Nope, it remained the same. They however increased the Vne from 750 kph IAS to 850 later.

 

The 109F-K had changed to Frise ailerons and may have been that the instability found was a simple balancing issue. In fact they tested ailerons with Flettner tabs in 1943 and found that they could be deflected to 2/3s of their range even at 0.75 Mach, with no overbalancing noticed.

 

As for the 109, the reports posted clearly show that it had no elevator control problem with high Mach number and no limitation with its longitudinal maneuverability, as fairly tight turns were still possible near and at Vne.

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DD_bongodriver

Well no actually, the opening statement of Crumps test report clearly states it had insufficient elevator authority at high mach number, a condition backed up by a vast amount of combat reports and pilot opinions.

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BongoDriver says:

 

Well no actually, the opening statement of Crumps test report clearly states it had insufficient elevator authority at high mach number, a condition backed up by a vast amount of combat reports and pilot opinions.

 

 

The measured results of the reports I posted do not support your theory.  The first report clearly shows you are wrong.  Stop posting the same thing over and over.  I have presented two documented reports that show the facts. 

 

 

Kurfurst says:

 

They however increased the Vne from 750 kph IAS to 850 later.

 

Right, these behaviors discussed in the reports define Vne for the aircraft.  There does not seem to be much of an understanding of that in this thread.  These are not behaviors found in the normal operating envelope.

 

When they increased the tail size they increased directional stability which allowed for an increase in Vne.  Dutch roll is a directional stability issue and is corrected with rudder not ailerons. 

 

Attempting to correct it with ailerons simply sets the aircraft up for asymmetrical loading of the airframe. 

 

Combine that with the freezing grease which caused excessive force to be applied to an already oversensitive trim system at high mach and the "jerky" movements when the grease broke loose makes for the perfect set up to accidently asymmetrically overload the airframe to failure.

Edited by Crump
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DD_bongodriver

The measured results of the reports I posted do not support your theory.  The first report clearly shows you are wrong.  Stop posting the same thing over and over.  I have presented two documented reports that show the facts. 

 

How can I be wrong when quoting verbatim directly from a document you have provided? the reason it needs repeating is due to the lack of a comprehensive answer from you.

 

the facts are:

 

Spring 1943 the Germans conducted tests to find the causes of non-combat front line losses citing the causes as:

 

 

Cause: 1. Explanation of accidents in the front-line units. (Over-compensation of

the aileron controls and insufficient elevator authority at high mach

numbers).

 

 

using a modified aircraft to test for stability:

 

 

2. The proof of aircraft stability at high mach numbers with aircraft W.Nr.

9228. This aircraft is used by the DVL for high speed pressure

distribution tests on the wings.

Execution

of the test: The Plane ME 109 F W.Nr. 9228 was used for these tests. An

ejection seat is build in as an additional equipment. To get an exact

documentation about the achieved figures, the instruments were

photographed and speed & altitude were recorded by an Askaniadevice.

To reduce the risk of pilot over-compensation, the control

movement was limited to 50% of the reference movement of the

ailerons.

Condition of

the aircraft: For the first test flights the plane was in the standard condition of a

109F with G-wings, except for the movement limitation of the ailerons

and the ejection seat.

For tests above a certain speed (refer to results) stabilizer was

changed to a larger one. (This enlarged vertical stabilizer will be

incorporated in the 109G production series)

The elevator trim tab is enlarged in surface area by 100% compared to

the standard version. The horizontal stabilizer trim is limited in its

upwards range of motion to +1°15 by a stop unit.

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How can I be wrong when quoting verbatim directly from a document you have provided?

 

You are selectively quoting the test without regard to the end results.  The test aircraft was modified to prevent the elevator trim system from being able to overload the airframe if it was needed for recovery. 

 

The test explores the behavior of the aircraft BEYOND the estabilished Vne.  That means for safety, it had to modified with a larger fixed trim tab and stops where placed on the trim jackscrew.

 

The stops were moved in the test as the first placement at +1 degree 45 minutes was not sufficient.

 

 

 

 

 

 

 

 

 

In the test flights it became

apparent that the application of

horizontal stabilizer trim has a big

influence on elevator forces in a

dive. To reduce the influence of

trimming on the results (indicator

has a insufficient level of

resolution), a stop unit was build

in and the horizontal stabilizer

was moved until it reached the

stop.

During first flights the position of

the stop unit was at +1°45'. The

elevator forces at this stabilizer

position were not sufficient to

reach a dive angle greater than

60° at 100% power. Therefore,

the surface area of the static

trim tab was doubled.

In the following flights a force

reversion was noticed at the

reached speeds.

To pull out of the dive the possible steering force was insufficient, so it was

necessary to use the horizontal stabilizer trim (big impact of the horizontal fin).

But pulling out with the horizontal stabilizer trim is a potential danger (high g

acceleration increase in the pull-out) so dive recover should be achieved

without changing the position of the horizontal stabilizer.

(the development of forces in the dive is shown in flight report Nr. 901/274 chart

sheet 2 in the addendum of the report).

 

 

 

 

 

 

They changed the stop in the test aircraft to +1 degree 15 minutes for the trim point to enter the dive:

 

 

 

 

As a result the stop unit setting was changed to +1°15'. At the beginning of a

dive, greater force in pushing the control column forward was necessary with

this setting, but it decreased as the dive went on, until zero force was reached.

A force reversal did not appear in any further test. Pulling out of a dive was

possible without changing stabilizer position. (Flight report Nr. 901/274 chart

sheet 2 in the addendum of the report). All the values of the charts are flown

with this horizontal stabilizer setting of +1°15'.

 

 

While the report is correct, your simplistic blanket interpretation is wrong.  It applies only to the modified test aircraft with the stop setting of 1* 45'.

Edited by Crump
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DD_bongodriver

So when I highlight the stated fact that front line aircraft have a problem cited as 'insufficient elevator authority' and then highlight the test aircraft used to explore the issues was modified I am being selective?

 

yet you provide the test aircraft as a base of evidence for the behaviour of front line machines and that is not selective?

 

 

While the report is correct, your simplistic blanket interpretation is wrong.  It applies only to the modified test aircraft with the stop setting of 1* 45'.

 

My interpretation is neither an interpretation, wrong or simplistic, it is bullet point facts extracted from your own submitted documents

 

Yes the report is correct, it shows clearly the front line 109 was plagued with directional stability problems and an ineffective elevator, additionally it had aileron issues that compounded the problem arising from directional instability, as late as 1943.

the directional stability issue only being solved by the enlarged rudder and fin which was introduced on late G models, the ailerons apparently never needing modification due to an understanding that it was pilot induced errors.

 

all of this flying in the face of your opening to this thread proclaiming the 109 as a shining example of stability and control standards, all you have convinced me of is that the many combat reports and pilot accounts of 109's lawn darting from high speed dives is totally irrefutable.

 

Too much stability and no control.

Edited by DD_bongodriver
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