Tempest and P 51

[ZachariasX]

1 minute ago, Panthera said:

Mainly due to weight & complexity, and the fact that the big Allied aircraft companies simply hadn't experimented as much with these devices on fighters as the Germans (& later Russians) had at that point

You seriously believe that? It certainly takes care of the argument.

 

Seriously, I don't know what to make of your postings anymore, now it comes across as plain trolling and I take it at that.

 

But as an advice if you want to convine anyone except thos who have no idea, this:

2 minutes ago, Panthera said:

That paper has so many figures wrong it can't really be taken too seriously ZachariasX, he doesn't even list the Clmax figures he used.

I said I don't like the paper, but he made a try do the right thing. I gave that example just for that.

 

He did two things right, he listed souce data and he calculated from there what you just claim. The fact that you were able to sanity check some of his assumptions is the good point of the paper.

 

For the sake of a good argument, you really have to do better when claiming *specific* (and what some can consider drastic alternate) realities.

 

13 minutes ago, Panthera said:

In a power off condition it means that the outer section is able to substantially exceed the Clmax of the inner section, preventing a tip stall and providing full aileron control up to and in the stall, which in this case results in the aircraft sinking instead of dropping a wing - as you see described by pilots flying the 109  on landing approach. 

You really maintain that the Bf-109 makes it fastest turn at a configuration, where the average wing lift would be 1.7? Show me a plot.

 

Besides, for any slow maneuvering where Wizzrad Willy spells super turn performance on the 109 by the grace of the slats, you seem to discount the idea that the Mustang pilot just deploys the flaps a notch and gets further lift as well. Your life is a better life if you understand that slats are primarily a landing aid. They can help slow speed maneuvering, but they do it in a way that is not universally liked.

 

You also have to understand (just in case that you are not just trolling here) that having a whole aircraft run at an AoA giving a theoretical lift coefficient of 1.7 at the slatted sections, then airframe drag as well as induced drag is getting immense. This is why you HAVE TO fly faster than that in order to make a fast sustain turn. It's not a good idea to hang around at 15° AoA. And this is why you are power limited in these aircraft and not lift limited. Everything the higher lift coefficient does is extending the doghouse plot to the left and the net gain will be at correspondingly slower airspeed and certainly not at 400 km/h.

 

The only serious thing I can take from your argument is the span load which is of very well known importance. After all, it is what makes the Dr.I a lower performer than the Camel thoughout the performance envelope (even with equal power), DESPITE the higher lift gven by the thick profile.

 

 

 

[Panthera]

I suggest you re-read my post, because either you didn't read it properly or you just didn't understand what was written. Furthermore I added a calculated scenario for you.

[ZachariasX]

2 minutes ago, Panthera said:

I suggest you re-read my post, because either you didn't read it properly or you just didn't understand what was written. Furthermore I added a calculated scenario for you.

Thank you, I just saw that and I appreciate that you added that.

 

At least we have established that the slats have little impact in a best turn scenarion.

[Panthera]

25 minutes ago, ZachariasX said:

Thank you, I just saw that and I appreciate that you added that.

 

At least we have established that the slats have little impact in a best turn scenarion.

 

Well that depends on exactly what you mean by "best turn scenario"?

 

Remember there are two types of max performance turns, the instantaneous and the sustainable. 

 

E.g.: If you want to turn as fast & tightly as you can (max rate), then that involves turning at your max lift coefficient, which will cause you to drop speed in the process. However if you want to turn at your highest sustainable rate (i.e. a turn you can keep up indefinitely without losing altitude) at any given speed (Ps=0), then you will be flying at a Cl that corresponds with your specific excess thrust. 

 

As such the 109's slats only really become a big advantage when a) you want to pull as tight & fast a turn as possible (minimum radius) & b) when you want to turn at low speeds.

Edited August 9, 2020 by Panthera

[Aurora_Stealth]

Hey ZachariasX,

 

Its very hard to take that paper at face value - its title immediately shouts the author's bias before an explanation or first sentence is even given... a research paper is meant to avoid opinion and instead share the author's research using scientific, mathematical or factual evidence with a conclusion then drawn. Because the author has made a conclusion in his paper's title before he has even presented his data, he cannot honestly be trusted to stay impartial. Panthera mentioned the technical issues in his calculations.

 

I have some answers to share regarding those test reports (sorry for this long explanation its complicated), the slats operation has been the subject of discussion for over almost a decade now. We had to conduct some investigations to get to the bottom of why these Allied test reports were contradicting so much from what is known today from operating these aircraft and pilot's experiences fighting them in WW2 also. This is also why its good to take all captured aircraft reports with a pinch of salt, as the best information will always come from the original manufacturer or operator who knows how to service them...

 

I'll try to explain, the USAAF reports were produced after the RAF had completed their own evaluations of the same aircraft (e.g. Bf 109 E-3 captured in France and Bf 109 G-2 captured in North Africa). Once testing was completed at RAF Boscombe Down these aircraft were then shipped to the US.

 

We know the same issue with the slats was mentioned by Eric Brown and others on these test aircraft (Eric Brown was one of the chief test pilots at RAF Boscombe Down), and that these slats were "snatching" and operating asymmetrically - they were in fact designed to open simultaneously to avoid causing this type of disturbance in flight or causing one slat or another to open or close before the other (causing the snatching effect) when they deployed. This also happened occasionally in Luftwaffe service during bad field conditions with catastrophic results and is mentioned in several accounts. If the slats we're poorly maintained and serviced - especially when dust and dirt accumulated in between the slat and the wing they would malfunction and open asymmetrically - sometimes on takeoff and landing. With a touchy aircraft like the Bf 109 G on takeoff, you can figure out what happened. There are a few accounts where Luftwaffe ground crew were ordered to inspect all their aircraft's slats to prevent this issue reoccurring, and then covered them up when when the aircraft were not in operation to prevent dirt accumulating in them... as some pilots were injured and killed this way. I do stress, this was hardly a regular occurrence but it is documented in some squadrons. You can read about this in the archives and pilots accounts in the Bundesarchiv in Germany but it may also be posted online, its also specifically stated in the biography of Erich Hartmann 'The Blonde Knight of Germany' for reference.

 

If an aircraft was picked up in a poorly serviceable state, or after a minor accident or crash landing like many of these captured aircraft (the Allies obviously didn't have spare parts for Luftwaffe aircraft) these issues could easily occur as these slats are sensitive devices operating with the pressure from two fingers opening and closing them (yes its really that light). So when issues occurred this could be a nuisance in the air especially in a turn and downright risky when you are trying to maintain a heading on a takeoff run. It appears this system was deemed unreliable in operation during evaluations by the RAF, rather than being interpreted as a malfunction for correction. The RAF had also flown the E-3 which used a less sophisticated slat system where as the F and later models used roller bearings and smoother mechanics which adds to the confusion over their operation. The British didn't use slats on their aircraft so were not familiar with their operation and relied on captured pilots for how they worked, this contributed to the misunderstanding and was overlooked it appears - despite the leading edge slat as a concept being invented by Handley Page - a British company but it was never really used.

 

It's unclear why exactly but the British tended to prefer aircraft with a lower wing loading and with more benign characteristics built in to accommodate pilots. Another problem is with aircraft like the Spitfire and some others, the leading edge is very curved, which makes adding a slat from an engineering perspective extremely difficult. The same argument can be said as to why they didn't introduce fuel injection systems as much (another story).

 

It's believed these slats were actually taped up altogether during some tests by the RAF / USAAF; thus negating the problem of snatching and also any benefit the aircraft had to its turning circle including issues when the aircraft approached its stall. Luftwaffe pilots as well as modern day warbird pilots will tell you these are the decisive aid in turning combat as the Bf 109 had a relatively high wing loading and relied on these to bridge the gap with aircraft which had larger wing area and lower wing loadings.

 

The fact this "snatching" effect occurred at all, as reported in Allied tests when attempting to use them is the tell tale answer why the aircraft seemed much less manoeuvrable than it really was in turns during these tests and in measurements from the reports. It's impossible to achieve the minimum sustained turning circle without loss of altitude with this constant snatching effect occurring. Which means it was happening and they realised it was an issue and then taped up the slats up to prevent it and retested it again.

 

Hope that sheds some light.

Edited August 9, 2020 by Aurora_Stealth

[ZachariasX]

5 hours ago, Panthera said:

Well that depends on exactly what you mean by "best turn scenario"?

I mean the scenario, where the aircraft completes a full steady state 360° turn in the least time possible, at respective altitude. I assumed your selection of parameters going in that direction.
 

One should also be more precise here:

5 hours ago, Panthera said:

If you want to turn as fast & tightly as you can (max rate), then that involves turning at your max lift coefficient,

Is just not true. The fatest (least seconds for steady 360° turn) is for fast aircraft usually a larger circle then the tightes possible turn / tightest circle (that takes more seconds to complete). The tightest turn is not max. turnrate. It is less than that.

 

You have to be really clear about what you are implying when doing some calculations. Having the cards open just as you did Panthera helps both understanding your reason, but also might have people find any error that happens along the way. Which is a good thing. It's also inspiring. Again, an honest thank you. You can sanity check many, many anectoal truths like that. But you should be aware of the limits of your assumptions. Hence I would never go as far in interpreting his results.

 

There are ways to circumvent that though. This is when you have "true" reference values to benchmark assumptions you have to make. I use that for instance when guessing flight speeds for WW1 aircraft (or any slow fixed pitch prop aircraft in general). You know the theoretical advance per revolution. You know how power requirement scales with airspeed. Then you pick just one steady state where your assumption can make sense, this is steady state cruise speed. Next, you look for any hard data on deviation from theoretical speed. There is. Similar aircraft will perform rather similarly. If you have significant changes of a variable, then you know you have to be carefull with your corresponding results. You can be awfully precise like this. But you still have to be careful. It's very simple, and if its used with care, it is also very precise.

 

I take your raw data values with a grain of salt and it is evident that drag vs. g load is something to be looked at. As I really don't care if one or the other  plane "is better", I'm still tempted to give Excel a run just to see what happens when one starts to correct your values. The more info you find, the more you cab factor in until Excel spits out a whole doghouse plot for your current assumprions. Also airframe drag can be compared by looking at power requirement vs cruise airspeed. Doing that might give me a better idea about that as just having a Cd value from somewhere.

 

While this is all fun and games, we should really be careful about how far we take our results. If not, it might lead to embarassing titles, same as that paper got.

 

 

4 hours ago, Aurora_Stealth said:

Its very hard to take that paper at face value

 

I repeat myself, I really don't. It is the method that I like. The author of that paper seemingly loves "his" P-51 as much as othery may love "their" 109. I don't have tp point fingers here at anyone in the current discussion, we all know others who do.

 

What I like is that "Mr. Layman" took the numbers he thought to be correct and did all the math aerodynamics requires (he thinks) and gets a result. The entirety, data (it is properly sourced) and math are verifiable and his results are reproducible. So if he did errors, you can see where. This is honest. This is good. It would be great if it all would be very accurate.

 

In this sense I very much liked Panthera for coming up with the math behind his rationale. And also here, it becomes evident where he starts making errors. Now, this is not bad, but you should be aware that you are making errors and better yet, you should be aware of the error you are making.

 

In Pantheras case, he keeps airframe drag constant, when in fact that rises along with increasing the AoA, meaning a relative reduction of total drag difference vs. assuming just the wings increase AoA and the airframe stays in perfect position (that is what he does).

 

Regarding the slats, the British knew them well. The Tiger Moth has them. They even made an aircraft that not only had slats deploy aerodynamically, but also the flaps with it. It makes the Lysander very interessting to fly.

 

There is only one good reason to use slats. This is using them as landing aid. It allows you to make a smaller wing for the same minimum speed you desire your aircraft to have. A smaller wing makes the aircraft have a faster top speed. This is all that ever mattered to Willy. And this is all that matters for the 109 regarding slats. You can use them if your aircraft is a total piece of work in maneuvers, as indeed, they can help you maneuvering where you otherwise had departed. Beyond that, you progressively go to fary tale land.

Edited August 9, 2020 by ZachariasX

[Panthera]

3 hours ago, ZachariasX said:

Is just not true. The fatest (least seconds for steady 360° turn) is for fast aircraft usually a larger circle then the tightes possible turn / tightest circle (that takes more seconds to complete). The tightest turn is not max. turnrate. It is less than that.

 

I'm sorry but this is quite simply incorrect, the fastest & tighest (min radius) turn involves turning at Clmax, that's a fact. Look at any doghouse plot and that's what it will tell you, the fastest & tightest turn is achieved at Clmax - and it is unsustainable.

 

 

 

And an F-15 plot with radius illustrated:

 

 

3 hours ago, ZachariasX said:

In this sense I very much liked Panthera for coming up with the math behind his rationale. And also here, it becomes evident where he starts making errors. Now, this is not bad, but you should be aware that you are making errors and better yet, you should be aware of the error you are making.

 

ZachariasX, if you don't mind me asking, do you have a background in aeronautical engineering or computational fluid dynamics? and secondly are these your own words or a mish mash of others you've cobbled together?

 

3 hours ago, ZachariasX said:

n Pantheras case, he keeps airframe drag constant, when in fact that rises along with increasing the AoA, meaning a relative reduction of total drag difference vs. assuming just the wings increase AoA and the airframe stays in perfect position (that is what he does).

 

First of all I made it pretty clear that it was an approximate, secondly the increase in drag coefficient you would would see from the exposure of the bottom of the fuselage is not easily accessible, and moreover it's not likely to really change the percentage wise difference between WW2 fighters. That said adding this in will if anything only hurt the bigger aircraft more.

 

3 hours ago, ZachariasX said:

There is only one good reason to use slats. This is using them as landing aid. It allows you to make a smaller wing for the same minimum speed you desire your aircraft to have. A smaller wing makes the aircraft have a faster top speed. This is all that ever mattered to Willy. And this is all that matters for the 109 regarding slats. You can use them if your aircraft is a total piece of work in maneuvers, as indeed, they can help you maneuvering where you otherwise had departed. Beyond that, you progressively go to fary tale land.

 

Again you're disappointingly going off on a misguided tangent here, despite by now several explanations as to how these things actually work.

 

Automatic LE slats are refered to as both high lift & maneuvering devices, not as mere landing aid devices, and placing them on the outer wing section of a piston engined aircraft with a centrally mounted propeller serves to improve both the maneuverability and landing characteristics of the airplane. On a jet (or a push prop design), which doesn't produce any prop wash over the wings inner section, full span slats are used (Me262, F-86, F-4, F-14 etc etc).

 

In short your claim that automatic LE slats are merely landing aid devices is mere conjecture, and something no pilot (esp. fighter pilots) or aeronautical engineer will agree with you upon (atleast not if he remembers what he was taught). I really suggest you read up on this, and I'm not saying that to be mean, you're statements are just incredibly misquided and confused on the subject.

 

Edited August 9, 2020 by Panthera

[ZachariasX]

39 minutes ago, Panthera said:

ZachariasX, if you don't mind me asking, do you have a background in aeronautical engineering or computational fluid dynamics? and secondly are these your own words or a mish mash of others you've cobbled together?

I don‘t and I don‘t need to. I‘m not claiming to have great new insights. Most sources states that the Mustang and the 109 have rather similar turn times, depending on conditions. I‘m ok with that. I‘m also ok with one better than the other, depending on conditions. You say the K4 is far better (your 2.5+ seconds is a lot in my world). Yet your math is not that convincing for making such claims.

 

You present a doghouse plot for an F-15 (theoretically) riding all the way at Clmax to 9G for your understanding of a „sustained turn“ of a prop plane. Well, ok.

 

Anyway, I should have stopped at where you considered people like Sydney Camm incapable of designing high lift devices.

[unreasonable]

Slightly OT - (as is all the 109 discussion), but leading edge slots are certainly primarily, (if not merely), T/O and landing devices in a Tiger Moth (the types that had them) - pilots can keep them locked closed with a cockpit control. They should be free on T/O and landing but locked before any aerobatics. 

 

 http://www.tigermothclub.co.za/Pilot's_Notes_for_Tiger_Moth_Aircraft_RAAF_publication_416_February_1944.pdf

 

Search and you will find some Moth pilots saying they hate them because of irregular operation, other saying that this is just poor maintenance and that properly serviced they are life savers. Sounds familiar...

 

Rudiger Kosin's "The German Fighter since 1915" quotes the test pilot at the trials in which the Bf109 V2, initially without slats, won out vs the He112 due to it's speed and acceleration. In short the 109's flight characteristics were awful during landing and looping - ie high AoA - due to separation. Three point landing was very difficult. Kosin says "The aircraft was subsequently fitted with slats on the outboard wings, designed to prevent early airflow separation, but separation still occurred before touch down, and still on one side".  

 

So according to this source, the slats were added to fix a particular fault, most evident on landing, which is of utmost importance to average pilots, and were not part of the original design. If the prototype had landed satisfactorily without slats, he might not have bothered.

 

Edited August 10, 2020 by unreasonable

[Aurora_Stealth]

Interesting details about the Tiger Moth - I'm surprised they used them in an aircraft like that. A fair number of modern owners seemed to have removed these fitted slats from the aircraft as they are apparently expensive to service... so there you go. On the other hand, the Luftwaffe wasn't exactly short on manpower on which to service their aircraft, but as the war went on no doubt this played its part.

 

I've read a bit about the RLM trials at Rechlin and need to find my book which details it all - they were pretty fascinating trials especially as the He112 shared some similar design traits with the later Spitfire.

 

I haven't heard of that particular issue regarding remedying issues at high AoA during takeoff/landing much before. With regards to the Bf 109 I think the benefits of the slats extend beyond just those specific conditions though. What confuses me is the V-1 (see image below - landing gear failure) used slats before the V-2, so not sure why they would have been removed for V-2 (?). Perhaps they were having issues with their deployment and needed further development? we know they improved the slat design again at the F model to make them operate slicker.

 

 

Quick and dirty extract out of wiki from Green (1980):

 

"Initially, the Bf 109 was regarded with disfavour by E-Stelle test pilots because of its steep ground angle, which resulted in poor forward visibility when taxiing; the sideways-hinged cockpit canopy, which could not be opened in flight; and the automatic leading edge slats on the wings which, it was thought, would inadvertently open during aerobatics, possibly leading to crashes. This was later borne out in combat situations and aerobatic testing by various countries' test establishments. The leading edge slats and ailerons would flutter rapidly in fast tight turns, making targeting and control difficult, and eventually putting the aircraft into a stall. They were also concerned about the high wing loading.

 

The Commission ultimately ruled in favour of the Bf 109 because of the Messerschmitt test pilot's demonstration of the 109's capabilities during a series of spins, dives, flick rolls and tight turns, throughout which the pilot was in complete control of the aircraft."

 

This may be my interpretation, and we know of the issues surrounding these slats under certain conditions but it's interesting in the quote that it doesn't explicitly state the Luftwaffe or RLM test establishment as having had a problem when the aircraft was flown; only from their end that it was an initial concern which sounds like it was dispelled after the aircraft being demonstrated. These concerns are not surprising if you consider people were used to the more conventional aircraft of the 30's including of the biplane era.

 

From what I have read in my book, the test pilot did something like 10 flat spins in one direction, corrected the aircraft then did another 10 in the other direction which is fairly astonishing as it was at fairly low level and in clear line of sight of people on the ground.

 

Anyway, sorry to divert OT

Edited August 10, 2020 by Aurora_Stealth

[Panthera]

Slats are a great if you want to maintain full aileron control near the stall, and prevent violent or excessive wing drop. For this reason they're a great aid during landing, however if your operating mechanism is unreliable under high G loads (which many of the early systems were) then you need a lockable system, which the Tiger moth has. The 109 uses a completely different roller design though, which is designed to work just fine under high G loads, hence there is no disturbance about any axis when they deploy. However if you didn't properly maintain the slats, you could risk them sticking, which if its just one of them is very dangerous. The Emil had reoccuring issues with this, which led to a redesign of the operating mechanism for the Friedrich. 

 

To quote Dave Southwood:

One interesting feature is the leading edge slats.  When these deploy at low speeds or in a turn, a 'clunk' can be heard and felt, but there is no disturbance to the aircraft about any axis.  I understand that the Bf109E rolled violently as the slats deployed, and I am curious to know the difference to the Gustav that caused this.

Edited August 10, 2020 by Panthera

[unreasonable]

3 hours ago, Aurora_Stealth said:

Interesting details about the Tiger Moth - I'm surprised they used them in an aircraft like that. A fair number of modern owners seemed to have removed these fitted slats from the aircraft as they are apparently expensive to service... so there you go. On the other hand, the Luftwaffe wasn't exactly short on manpower on which to service their aircraft, but as the war went on no doubt this played its part.

 

I've read a bit about the RLM trials at Rechlin and need to find my book which details it all - they were pretty fascinating trials especially as the He112 shared some similar design traits with the later Spitfire.

 

I haven't heard of that particular issue regarding remedying issues at high AoA during takeoff/landing much before. With regards to the Bf 109 I think the benefits of the slats extend beyond just those specific conditions though. What confuses me is the V-1 (see image below - landing gear failure) used slats before the V-2, so not sure why they would have been removed for V-2 (?). Perhaps they were having issues with their deployment and needed further development? we know they improved the slat design again at the F model to make them operate slicker.

 

 

 

Are you sure that is the V1? It had a Kestrel engine so the exhausts should be visible on a line higher than the prop boss,  and did not have that scoop just to the left of the gunports as we look. That looks like an inverted V engine, so the Jumo fitted version used in the trials some months after the Kestrel V1 crashed on ground loop -or even a later B model. 

 

If you got that picture from here https://www.thisdayinaviation.com/28-may-1935/ the previous pictures shows shots of "V1" and I think "V2" with Kestrel engines and propeller bosses without a gun port. Most of these sites just copy and paste from one another without any kind of checking. I think many of the picture captions we find in the internet are wrong, BTW. Indeed Kosin's book could be too - I know WM put slats on his 108, but who knows, without genuine primary sources it is hard to say. 

 

I seem to recall that when the GAF got hold of an intact Spitfire I they found it "childishly easy" to take off and land: the 109 with it's very high AoA at three point was a pig to land for the average pilot, even with slats - I do not think there is anything controversial about that.

 

[Aurora_Stealth]

I stand corrected, that's a good catch unreasonable yes it was - didn't have the time to open up all my books this morning as I'm still (technically) working.

 

Yeah that's very true, agree its hard to get reliable information.

 

They did, and nothing too controversial there... the '109 has its reputation for a reason and its wingloading and gear configuration can be a real liability in these situations. Well it certainly makes sense with regards to takeoff but I've read several accounts that the Bf 109 is actually fairly straight forward to three point provided you watch the rate of descent and don't allow any slip... although that was spoken regarding F and G models. Possible that the later design of rolling bearings may have helped a little there.

 

I had to find this, it was in this book (Bf 109) by Robert Grinsell he shows on the figure of the Bf 109 V1 leading edge slats which again added to my confusion. It's quite possible (again) that this is just a mistake and I'm willing to bet Kosin had better information. Don't really have much time to go further into it right now, but yeah its a minefield establishing facts.

 

Edited August 10, 2020 by Aurora_Stealth

[Panthera]

Yes the roller track mechanism itself isn't capable of sticking by design if clean and is meant to function perfectly irrespective of G load. However you still had to make sure the slat airfoil itself was well maintained, as any bending at the edges, which could occur if the leading edge struck debris in flight for example, could result in it getting stuck. But needless to say this would be a pretty rare occurence.

 

As for the 109's well known issues in terms of landing, this was all down to the narrow toe out landing gear.

[MiloMorai]

This link, http://109lair.hobbyvista.com/techref/systems/control/slats/slats.htm has diagrams of the slats.