Corsair vs. BF 109G,K or FW 190's

[Crumpp]

We cannot make a blanket statement that either aircraft design is superior to the other in horizontal maneuvering.

Hi Bill,

Certainly we cannot make a blanket statement as to specific performance. We can however predict general trends but only as to what the engineering will allow. We also have no method to factor the effects of pilot skill.

As I understand, low control forces are advantagous for high instantanous manouvers

Hi Kurfurst,

From the point of view of the pilot low control forces at high speed are advantageous. To the engineer they are dangerous and higher stick force per G is desirable to keep the pilot from killing himself.

Both the P51 and the Bf-109 exhibited stability and control issues when approaching their q-limits. This is common in WWII aircraft and IMHO, stability and control is where we find the largest differences in these aircraft. It was still a science in its infancy.

As I understand it, the Bf-109's developed a dutch roll when nearing the q-limits. A 13mm extension to the tail solved it allowing the q-limits to be raised in the "tall tail" 109's.

The P51D series exhibited longitudinal stability and control issues. The aircraft would begin to porpoise. Design changes to the empennage improved the longitudinal stability but did not allow for a q-limit increase.

The low stick force per G of both the P51 series and the FW-190 series is a very good way for the pilot to kill himself. Several Luftwaffe documents warn of this as well as the P51D series POH.

All the best,

Crumpp

[Crumpp]

I think the 109's is already posted in the thread.





All the best,

Crumpp

[HoHun]

Hi Al,

>Technical Instructions of the Generalluftzeugmeister, Berlin, 28th August 1942.)

Thanks a lot! Where did you find this gem?

Let me try to re-translate this sentence since there is a slight inaccuracy in the above translation:

"Sturzflug: Trimming so einstellen daß das Flugzeug durch Drücken im Sturzflug gehalten werden kann. Die Höhenruderkräfte und Flossenbelastungen werden bei hoher Fahrt sehr groß. Hemmung der Flossenverstellung muß einwandfrei arbeiten; sonst ist Selbstverstellung der Flosse möglich."

'Dive: Adjust trim so that the aircraft can be held in the dive by pushing on the stick. The elevator forces and tailplane loads become very high at high indicated airspeeds. The friction brake of the tailplane adjustment drive has to work flawlessly, else the tailplane can change incidence by itself [driven by the aerodynamic loads].'

>"Höchstzulässige Sturzfluggeschwindigkeit 750 km/h. Harte Querruder betätigung im Sturz und besonders beim Abfangen führt zum Bruch."

That's interesting, too. ('Maximum permissble dive speed 750 km/h. Harsh aileron deflection in the dive and especially on pull-out leads to airframe failure.')

I found a description of this effect (caused by wing warping induced by the aerocdynamic forces on the ailerons) in Mike Crosley's "Up in Harm's Way" - he was a Seafire pilot in WW2 and a test pilot after the war, and they found that aileron-induced wing failure was a reason for some then unexplained losses they had in the war.

I have always suspected that the Messerschmitt must have suffered from the same problem as it has the same single-spar wing construction as the Spitfire, but I couldn't find anything definite on this topic before, so thanks for confirming my suspicion!

Regards,

Henning (HoHun)

[Kurfürst]

Even the P-47 was prone to wing failures to such combined elevator+aileron induced stresses - there`s a NACA paper on the IIRC, I`ve read it a while ago. The paper also tells why : WW2 aircraft wings were not stressed for such twisting loads. Pullot yes, rolling yes, for rolling during pullout twisting the wings - no. The Spitfire neither - I wonder when Mike will palce that part below next to the 109 limitations he quotes.

This is tactical advise to the RAF Spit pilots, how to get around the negative G problems with a roll-and-dive manouvre. Seems to me exactly the same content as the German manual.



I am afraid Mike was a bit picky on that subject (too).

[AL Schlageter]

Originally Posted by Kurfürst

This is tactical advise to the RAF Spit pilots, how to get around the negative G problems with a roll-and-dive manouvre. Seems to me exactly the same content as the German manual.

Confused Kurfurst?

Neg G was with regards to the carb cutting out.

[Soren]

LoL, read the report before making stupid statements like that AL !

The report is about structural issues, it has NOTHING to do with the issues surrounding the carburators.

[drgondog]

[QUOTE=HoHun;290730]Hi Al,

>Technical Instructions of the Generalluftzeugmeister, Berlin, 28th August 1942.)

Thanks a lot! Where did you find this gem?

Let me try to re-translate this sentence since there is a slight inaccuracy in the above translation:

'Dive: Adjust trim so that the aircraft can be held in the dive by pushing on the stick. The elevator forces and tailplane loads become very high at high indicated airspeeds. The friction brake of the tailplane adjustment drive has to work flawlessly, else the tailplane can change incidence by itself [driven by the aerodynamic loads].'


That's interesting, too. ('Maximum permissble dive speed 750 km/h. Harsh aileron deflection in the dive and especially on pull-out leads to airframe failure.')

I found a description of this effect (caused by wing warping induced by the aerocdynamic forces on the ailerons) in Mike Crosley's "Up in Harm's Way" - he was a Seafire pilot in WW2 and a test pilot after the war, and they found that aileron-induced wing failure was a reason for some then unexplained losses they had in the war.

I have always suspected that the Messerschmitt must have suffered from the same problem as it has the same single-spar wing construction as the Spitfire, but I couldn't find anything definite on this topic before, so thanks for confirming my suspicion!

QUOTE]

HoHun - It was mentioned IIRC in Wagner's Mustang Designer or Gruenhagen's Mustang - I'll have to check. Both the 109 and Spitfire, (until the Mk XIV )had situations where the high aero loads on the ailerons caused the wing to torque (i.e the 'down' aileron caused a local twist changing the effective angle of attack to increase) The effect was twofold depending on speed and load - either neutralize or reverse the desired manuever or over stress the wing.


The 51 wing failures mentioned above by Crumpp had two root causes -
1.) the original design for wheel cover uplock failed at very high speed dive and the gear dropped, creating an immediate and fatal wing failure, and 2.) the ammo cover door in a high speed pullout deflected to unexpected point creating a local aeroynamic 'lift' at a critical point in the spar.

Both were fixed with the kits and TO's mentioned by Crumpp

The 51 also had some structural failures associated with rolls in dives. Both the horizontal stabilzer and tail had been designed with pre-war conventional methodology and was simply in adequate for the .80 dive with any aileron and rudder loads to add to the stress in the vertical stabilizer spars.

Not even the beefed up Horizontal stabilizer, metal elevators and tail design changes including reverse rudder boost tab and ventral fin and change in spar and rudder heighth fully solved the issues at those speeds - but the wheel door uplock kits and stiffened ammo doors fully solved the wing failure problem.

The H tail allowed the permissable max dive speed to increase slightly but even it required increasing and dangerous rudder input at max dive to keep from yawing too much.

I've often wondered whether the 109 was more or less of a problem in a similar way as many of the encounter reports talk about tail failures in the chased 109s during high speed dive...

PS - the information contained about the Spit and 109 aileron/wing twist issues were anecdotal but seem supported by the repective Manual discussions above.. I can personally vouch for the 51 in dive but I never pushed it to limit dive.. and the 51 Book is equally instructive about NOT using elevator trim to pull out of a dive.

Regards,

Bill

[drgondog]

Originally Posted by Crumpp

Hi Bill,

We cannot make a blanket statement that either aircraft design is superior to the other in horizontal maneuvering.


Certainly we cannot make a blanket statement as to specific performance. We can however predict general trends but only as to what the engineering will allow. We also have no method to factor the effects of pilot skill.

Crumpp - gentle LOL - I left your quote in bold as above to provide lead in to 8th AF general directives about relative merits of Fw190 and P-51. I quite agreed with it as well as your above follow on comment.

When I took my degrees in Aero, both aerodynamics and flight mechanics, while theoretically sound, were not an exact science in the 60's - by definition they sure weren't precise in the 30's and 40s when these ships were designed.

I imagine it is much better now with the compute power available but modelling is still an art as much as science.

Regards,

Bill

[Crumpp]

q-limits are also called the flutter limits.

Wing structures are akin to a 'tuning fork' extending from the fuselage. When a tuning fork is tapped the fork vibrates at a particular frequency, the stiffer the structure the higher its 'natural' frequency. The natural frequency of a wing or tailplane structure may apply another limiting airspeed to flight operations – related to structural instabilities: flutter and wing divergence.

When the airflow around a wing or control surface is disturbed by aerodynamic reactions, turbulence or pilot inputs, the structure's elastic reactions may combine as an oscillation or vibration of the structure (possibly evident just as a buzz felt in the controls) which will quickly damp itself out at normal cruise speeds. At some higher speed — the critical flutter speed — where the oscillations are in phase with the natural frequency of the structure the oscillations will not damp out but will resonate, rapidly increasing in amplitude. (Pushing a child on a swing is an example of phase relationships and amplification). This condition is flutter and, unless airspeed is very quickly reduced, the severe vibrations will cause control surface [or other] separation within a very few seconds.

Wing divergence refers to a state where, at the very low angles of attack of high speed where the nose-down pitching moment is already very high, pressure centres develop pushing the front portion of the wing downward and the rear portion upward. This aerodynamic twisting action on the wing structure, while the rest of the aircraft is following a flight path, further decreases the aoa and compounds the problem; finally exceeding the capability of the wing/strut structure to resist the torsional stress and causing the wing to separate from the airframe – with no warning! This could be brought about if a down gust is encountered at high speed.

http://www.auf.asn.au/groundschool/flutter.html

The limits of all of these aircraft are set for very good reasons. They are all subject to the same physics and have much more in common with each other than they do differences.

Both were fixed with the kits and TO's mentioned by Crumpp

Exactly. The design teams for all of these aircraft addressed the problems which arose. The failure is on the reader who looks at such difficulties and concludes "Ah-ha! my favorite airplane was better than that!"

Facts are it probably suffered from similar issues as well. These are simply the results of engineering at the edge of human knowledge. As the level of knowledge was about the same you can expect the problems to be about the same.

All the best,

Crumpp

[Soren]

Hows work progressing on the addition of the Bf-109 Crumpp ? (Looking foward )

Btw, I don't feel we finished our debate on the stalling speeds of the Fw-190 & P-51, whats your thought on this ?

[HoHun]

Hi Kurfürst,

>WW2 aircraft wings were not stressed for such twisting loads. Pullot yes, rolling yes, for rolling during pullout twisting the wings - no.

Actually, rolling pull-outs induce high stress for all aircraft, not only WW2. I dimly remember reading that at one point the civilianized T-34 fleet was grounded after an accident of this sort, for example.

However, the Bf 109 and the Spitfire with their single-spar wings were more sensitive against this than other WW2 aircraft. That both lose aileron effectiveness at high diving speeds is the first sign of wing twisting - for the Spitfire, Crosley points out that above some speed, the aileron response actually reversed.

So if you used aileron trying to correct in a dive, the aircraft would actually roll to the opposite direction. If you fed in more aileron intuitively in an attempt to correct, the wing would twist further, increasing the roll - tempting the pilot to add yet more aileron. If he did, the wing might come off, if he didn't, he would be diving at top speed at an oblique angle, partially out of control. Pretty bad situation ...

Regards,

Henning (HoHun)

[drgondog]

Crumpp - as an anecdotal aside I was an Aero (Fluid Mechanics and Structures) that worked at Skunk Works and Bell Helicopter before pursuing the all mighty dollar in Computer services.. but was the 'modeller' for the first NASTRAN airframe structural model (allegedly) accepted by US Army on the AH-1 Cobra in 1970.

The only reason I bring this up is that for every assumption I made about using rods and shear panels to simulate longerons/stiffeners and skins (instead of beams and plates in the NASTRAN model kit) I had to do the 'hand analysis' in report form so the contract officers could follow the logic..

As laborious as that process was I wonder how far the computer modelling has progressed in combining aerodynamic predictions in say, a relaxation methodology, with the matrix decompositions of flight mechanics - and then progress to account for aerdynamic deformations from limit to ultimate loads?

Your teaser about an indicial gust promted the stream of conciousness

Regards,

Bill

[Crumpp]

Hi Henning,

Off topic - I still haven't downloaded Open Office to check out your Hurricane analysis. I will get that done soon as I am very interested in your outcome!

As you say, asymmetrical loading is not a good thing for any airframe and is a problem all aircraft can experience.

Soren,

I don't understand what the debate is on the P51 and FW190 stall speeds

All the best,

Crumpp

[Kurfürst]

Originally Posted by HoHun

Hi Kurfürst,
...
However, the Bf 109 and the Spitfire with their single-spar wings were more sensitive against this than other WW2 aircraft. That both lose aileron effectiveness at high diving speeds is the first sign of wing twisting - for the Spitfire, Crosley points out that above some speed, the aileron response actually reversed.

Regards,

Henning (HoHun)

I don`t think this - aileron reversal due to wing twist - was a particular problem on the 109 at least. It had fairly high aileron reversal speeds, similar to the USAAF`s two-spar pursuit aircraft, noted in the report quoted below. Perhaps due to it`s box spar configuration, I can only guess, but generally a box spar is fairly good resisting twisting loads, effectively it`s a single, very wide spar in the middle.



RAE Technical note 1001 (comparison Mustang/Spitfire ailerons, August '42) notes a deduced aileron reversal speed of 510 mph Vi for the Spitfire and 820 mph Vi for the Mustang. The 850 mph IAS computed by Material Command for the 109F is rather close to the Mustang`s, ie. a two-spar design, taking into account the margin of error with these calculations.

On the Spitfire, I can see why it wing twist was such a problem with the original wing. The whole D-chamber torsion spar was in the leading edge, and strong as it may be, it provided almost no rigidity for the structure furhter aft, especially as the wing was fairly big and deep - which would explain why it had fairly low aileron reversal speeds until the wing`s redesign (did the 20 series have a new spar design?).

[Crumpp]

As laborious as that process was I wonder how far the computer modelling has progressed in combining aerodynamic predictions in say, a relaxation methodology, with the matrix decompositions of flight mechanics - and then progress to account for aerdynamic deformations from limit to ultimate loads?

Hi Bill,

Things are moving toward using computer simulation exclusively. There have been quite a few advances in computer algorithums for solving the Euler and Navier Stokes equations for example. It is still the realm of the big time firms though.

The little guys are still using saw horses and sandbags.

Do you have a copy of David Lednicer's computational fluild dynamics evaluations of the Spitfire, P51, and FW190 series? He used Vasero to analyze the designs with some very good agreement and insight. I can give you a copy if you do not have it already.

All the best,

Crumpp