Bf109 success, balanced fighter, or superior fighter?

[drgondog]

Hello Soren
As an engineer you should know that without the weight info saying over 12 G is rather meaningless.
And according to a message on another site by Ruy Horta a Rechlin test report dated 15-2-1939 stated that Bf 109E, flight weight 2200kg (very light weight for even E-1, if correct), could stand up to 8G, after which "deformation" would lead to permanent damage even if the a/c was built for up to 10,8G before actual catastrophic failure.

I agree with Kurfürst that 109 was a good fighter with excellent powerloading and acceleration and so well suited vertical manoeuvres and same time it had benign stall characteristics and fairly good horizontal manoeuvrability. Minuses were heavy control forces at high speeds, restricted vision from cockpit etc.

Juha

Juha - you are correct about G loads applying to a design weight - not a full combat load.

In the case of a Mustang the Limit load (i.e. load withing design elastic range) was 8g at 8,000 pounds and had an Ultimate load factor of 12g (1.5 x Design Load was industry standard - even for LW).

When the Mustang gross weights increased the structure was not correspondingly beafed up. So the actual Limit load for a 10,000 pound Mustang climbing out over the channel was closer to 6.5 with a 9.7 ultimate at that gross weight.

There were no major wing changes between the P-51A and B-K other than extension of root chord in D/K and dropping the wing about 7 inches to accomodate a smooth lower cowl transition with the new Merlin.

IIRC the 109F wing was only different from the Emil by virtue of cannon/ammo removed plus rounded tips? Did the G have a strengthened wing to accomodate the growth in weight - or remain the same?

 

[davebender]

Heinkel He 100 - Wikipedia, the free encyclopedia
My understanding (however faulty that might be) is the He100D-1 was the first model that could be mass produced as a combat aircraft. The He-100D-1 was armed and the unworkable surface cooling system was replaced wiith a radiator. This model became available after WWII had started so we are talking about late 1939. By that point in time Me-109F development was already underway.

If the He-100 had been available in 1936 rather then the expensive to produce He-112 then Heinkel would likely have won the Luftwaffe fighter competition. But the fall of 1939 was too late. A shame as it would be cool if the He-100 had been mass produced somewhere. Perhaps in China as most of their weapons were German designed.

 

[Soren]

Juha, until we have the document stating what you wrote then it's rather pointless really.

Friedrich reportedly had an ultimate load factor of over 13 G's. So fully combat loaded the 109 would've been able to take roughly the same or a little more than the P-51 for example, which is more than good enough. The light weitgh and thick wing of the 109 attributed to that.

The Fw-190 D-9 had a load factor of 6.9 and an ultimate load factor of 10.35 G by comparison.

 

[Kurfürst]

IIRC the 109F wing was only different from the Emil by virtue of cannon/ammo removed plus rounded tips? Did the G have a strengthened wing to accomodate the growth in weight - or remain the same?

What I know is the 109E had a much beefed up wing compared to the earlier Emils.

I can't say for the 109F wing. The general internal structure does seem the same or very similiar as on the Emil, but I don't know the details of the material strenght. However the planform changed, as did some important aerodynamic details (position and size of slats, introduction of Frise type ailerons)

The G-wing was beefed up, see Kurfrst - Leistungzusammenstellung Me 109 G. : verstärkter Flügel (Holm, Rippen, Beplankung)

The limit load for all late 109G/K is given as 6.5 G at 3300 kg (roughly full take off weight). Earlier G series were rated 6.7 G.

 

[Colin1]

I can't say for the 109F wing. The general internal structure does seem the same or very similiar as on the Emil, but I don't know the details of the material strength. However the planform changed, as did some important aerodynamic details (position and size of slats, introduction of Frise type ailerons)

How much difference would the radiator arrangement have made? It was given a significant overhaul in design on the F over the E.

 

[Soren]

Thanks for the info Kurfurst. The 109 was pretty close to the Fw-190 then in terms of strength.

 

[drgondog]

What I know is the 109E had a much beefed up wing compared to the earlier Emils.

I can't say for the 109F wing. The general internal structure does seem the same or very similiar as on the Emil, but I don't know the details of the material strenght. However the planform changed, as did some important aerodynamic details (position and size of slats, introduction of Frise type ailerons)

The G-wing was beefed up, see Kurfrst - Leistungzusammenstellung Me 109 G. : verstärkter Flügel (Holm, Rippen, Beplankung)

The limit load for all late 109G/K is given as 6.5 G at 3300 kg (roughly full take off weight). Earlier G series were rated 6.7 G.

At 6.5 for takeoff weight, the ultimate should have been ~ 9.7G (+/- ) at 3300..

Kurfurst - Does your report state how the G -wing was beefed up? The obvious approache (while maintaining same airfoil dimensions) would have been to either increase skin thickness over the wing torgue box (therebye effectively increasing beam cap area or increase the width of the existing beam flanges top and bottom - or both.

The only way a 51D would ever get to the design Limit Load (8G) range would be after burning much of the internal fuel and ditching the externals..


Soren, unless you couple your "13G" statement about Ultimate G load for the 109F with a corresponding statement of the design weight for that specific load, it by and of itself has no meaning..

A virtually empty 51D probably was around 13G also - but it wouldn't do that over Germany with a half load of fuel and full loadout of ammo..

 

[Soren]

The weight of the 109F I presumed was known to everyone, I apologize for leaving it out.

Bf-109F weight: 2,890 kg or roughly 470 kg lighter than the K-4.

So that's a ultimate load factor of 11.07 G's for the 109F. So the 13 G was most likely for an empty a/c and just the wings alone.

 

[Timppa]

At 6.5 for takeoff weight, the ultimate should have been ~ 9.7G (+/- ) at 3300..

AFAIK the Germans used safety factor of 1.8 (ratio of ultimate load/limit load), where as British /Americans used 1.5. So it is better to compare ultimate loads rather than limit loads.

 

[Juha]

Hello
being in the hurry this is completely IIRC, beefing up the F/G wing began during early F production, the skin thinkness at wing roots increased ca. 50% from F-1 to G-6 and late F-4 wings had ca same wing root skin thinkness as G-2 wing. Again IIRC also innermost ribs were beefed up.

Juha

 

[drgondog]

AFAIK the Germans used safety factor of 1.8 (ratio of ultimate load/limit load), where as British /Americans used 1.5. So it is better to compare ultimate loads rather than limit loads.

Perhaps just the opposite T.

Limit loads are set at elastic deformation point in yield.. Ultimate is 'failure well beyond elastic yield point. The US and Brit doctrine were to use 1.5x Limit and while somewhat conservative, many of the destruct tests demonstrated consistent failure points at close to 1.5.

Therefore - Limit Loads are the ones to comapre as the philosphy between Engineers on Yield points was the same.

 

[Timppa]

Perhaps just the opposite T.

Limit loads are set at elastic deformation point in yield..

Until you don't know the exact German definition for limit load, you are just guessing. Hence my suggestion for using the ultimate load.

 

[drgondog]

Until you don't know the exact German definition for limit load, you are just guessing. Hence my suggestion for using the ultimate load.

Are you suggesting the German engineers have a different definition from our engineers for Design Limit load? That it is different from the commonly accepted Mechanics of Materials definition of Yield point used by US and Brit engineers?

In this respect you are right - I AM 'guessing' that German engineers knew what they were doing with respect to airframe structures...If they used a Ultimate Load Factor of 1.8 over Design Load then they were designing well below the elastic Yield point to derive the Limit Load boundaries - That would result in VERY conservative advice to pilots with attendant reduction in sustainable performance.

I believe that their analytical methods and materials assumptions did not depart from the accepted methodologies for calculations of applied airframe loads, with the resultant shear, compression, tension and bending loads required to accounted for in the design of a safe airframe.

I suspect my 'guess' on this subject is a lot sounder than yours - but I will listen to yours. Please explain.

What is it?

Further - Your suggestion for Ultimate Load Limit is what. And your assumption is 'precise' (reference please)or 'guessing'?

 

[Timppa]

Are you suggesting the German engineers have a different definition from our engineers for Design Limit load?

Their definitions were probably the same:
-Limit Loads are the maximum loads expected in service.
-There shall be no permanent deformation of the structure at limit load.

Note that the line above does not equal to yours "Limit loads are set at elastic deformation point in yield".

-Ultimate loads are defined as the limit loads times a safety factor.
-The structure must be able to withstand the ultimate load without failure.

If they used a Ultimate Load Factor of 1.8 over Design Load then they were designing well below the elastic Yield point to derive the Limit Load boundaries

This is quite obvious.

many of the destruct tests demonstrated consistent failure points at close to 1.5.

I have actually read something to the contrary.

 

[Soundbreaker Welch?]

The 109 actually needed just one improvement to become a lot more lethal in the air, and that was a shift from steel wire control to push rod control like in the 190. That would've made stick forces much lighter at high speeds, and a 109 with light stick forces at high speeds would've been a nightmare for the Allies.

I doubt it would have caused them more trouble than the FW 190D, but I suppose the 109 was in some ways a better "dogfighter". You would still have the Spitfire to fall back on, one of the easiest to fly dogfighters in WWII. I would prefer over the 109, simply for it's graceful and agile flight, without many vices in it. Yes, it wasn't the best at high altitude, but the Griffon engine helped to offset that.

The 109 probably needed a complete overhaul to get it flying at faster speeds without freezing up; redesign the wing, fuselage, add more propellers; but Germany didn't have the time to rework it properly before the war ended.

It still performed remarkably well, even more so considering it was from the 30's, when the Allied fighters from that time were becoming obsolete, such as the Hawker Hurricane or P-36.

 

[Soren]

Welch,

With a climb rate of 5,000 + ft/min, a top speed of 719 km/h and a turn performance close to that of the Spitfire, the Bf-109 would've proven probably the best piston engined fighter in the air if the control forces had been as light as in the Fw-190.

 

[davebender]

Did the almost produced Me-209 solve the control force issue while keeping the positive attributes of the Me-109?

 

[Soren]

Maybe, but the Me-209 wasn't an ideal design from the beginning so it was cancelled.

 

[drgondog]

Their definitions were probably the same:
-Limit Loads are the maximum loads expected in service.
-There shall be no permanent deformation of the structure at limit load.

Then why did you say this to me? "Until you don't know the exact German definition for limit load, you are just guessing. Hence my suggestion for using the ultimate load."

Note that the line above does not equal to yours "Limit loads are set at elastic deformation point in yield".

Actually it does. When a material reaches elastic deformation point under load, it IS at permanent set... or permanent deformation" in point two above... and does not rebound to former dimensions. What are you arguing about?

-Ultimate loads are defined as the limit loads times a safety factor.
-The structure must be able to withstand the ultimate load without failure.

Then may we stipiulate that German, US and Brit structural design philosophy are essentially the same as I stated earlier?

And BTW there is no such thing as "MUST be able to withstand the ultimate load without failure" - it is "We have used 1.5 as a Hoped for useful factor which we pray will enable the airframe to survive this stress but there are no guarantees or warranties expressed or implied - bring it back if it fails and we'll take a look at it'>

>I then followed up with a comment to your "1.8" factor" for Ultimate by saying " If they used a Ultimate Load Factor of 1.8 over Design Load then they were designing well below the elastic Yield point to derive the Limit Load boundaries"<

>And you just said<

This is quite obvious.

So, which is "obvious"??

The Germans designed well below Yield for Design Limit Load - contrary to US and Brit practices? or,

the "1.8X" factor was a figure you regurgitated with no knowledge of the practices of German engineers,

or, it is obvious that airframe structures engineering is not your core knowledge base but you wished to sound knowlegable??

> I then said "many of the destruct tests demonstrated consistent failure points at close to 1.5." and you just said<

I have actually read something to the contrary.

There are 'contrary' reports and it is good that you have 'read some' and that the results were 'something to the contrary'.

So, what is your point? That many reports on Destruct Tests do not show failure modes close to the theoretical laod factor of 1.5, or that what you read suggests that 1.5 was not the 'industry standard', or??? What is your point?

Just to be clear regarding the Art vs Science of Mechanics of materials and Structural Analysis of complex airframe under load:

Failures occur at both lower and higher loads in desctruct tests for a variety of factors - namely test approaches that may not mirror the load assumptions the aero's and structures guys assunmed for their calculations, or the analysis was based on over/under conservative assumptions, or the science of structural analysis has elements of 'art' (it does) in the analytical approach.

The Science is somewhat imprecise, particularly when dealing with asymmetric loading conditions. Most (not all) destruct tests fall in three categories - wing failures for high G pull outs, wing/landing gear/tail failures due to high G landings (i.e. "drop tests") and service life destruct tests in which reversible loads are applied over a long period to note spar and other structural failure modes.

So the first two are the simplest but still subject to complex interactions in design phase (which is why destruct tests are performed). The latter (fatigue life cycle) structural analysis is more art than science

The A-10 wing tests recently resulted in letting out a contract to retrofit most of the fleet's wings and some carry through structure.. Helicopters are a notorious example of designing well Below Yield Point for design limits simply because of the sustained reversible loads applied by rotor systems - which results in fatigue failures early in the life cycle if pylon structure is designed to Yield for Limit Load threshold.

Timppa - you either have a background in airframe structural analysis or you don't. I suspect that you don't based on your comments but willing to be enlightened. Your comments follow along the lines of "uniformed but intelligent" so far.

 

[drgondog]

Welch,

With a climb rate of 5,000 + ft/min, a top speed of 719 km/h and a turn performance close to that of the Spitfire, the Bf-109 would've proven probably the best piston engined fighter in the air if the control forces had been as light as in the Fw-190.

Soren - I don't claim to know why the German engineers didn't provide boosted controls for the 109 but I am reminded that Lockheed did exactly that for the elevators on the P-38 to 'solve' compressibility dive recovery issue'.

Boosted elevators worked in the sense that control forces were manageably 'light' - but the airframe somehow lost it's tail in the process. I suspect without proof that neither the 109 aft structure or perhaps the wing structure were designed for high speed asymmetric load conditions that boosted controls would exacerbate.

I do know that Mustang designers provided REVERSE boost to rudder to INCREASE rudder pedal force requirements in dive and slow rolls - as the Mustang controls were too light, relatively speaking for those manuevers - and resulted in overstressing the airframe in certain assymetric load conditions.