Me 109 mediocre roll rate at speed

[elbmc1969]

In its summary, it states: "A summary has been made of the available information on lateral control. A discussion is given of the criterions used in lateral-control specifications, of the factors involved in obtaining satisfactory lateral control, and of the methods employed in making lateral-control investigations in flight and in wind tunnels. The available data on conventional flap-type ailerons having various types of aerodynamic balance are presented in a form conveniant for use in design. The characteristics of spoiler devices and booster mechanisms are discussed. The effects of Mach number, boundary layer, and distortion of the wing or of the lateral-control system are considered insofar as the available information permits. An example is included to illustrate the use of the design data. The limitations of the available information and some of the lateral-control problems that remain to be solved are indicated".

It clearly states what data is calculated and what is from testing. There is absolutely no requirement for test dates or airframe numbers in this particular type of document, so I honestly don't know what you are on about.

I haven't read this document and I can't judge it, but M seems to be saying that it's a reference document for engineers.

The CRC Handbook doesn't include sources for the vast number of tables in it, and no one expects it to. There may be a parallel.

 

[GregP]

Do you mind providing a cite for this claim? The main reason for the Spitfire stick being hinged laterally near the top was to allow large lateral deflections without the pilot's knees interfering with the stick. This design feature was used on other aircraft also, even as late as the Hawker Hunter, which has a hydro-mechanical control system so stick forces would not have been a consideration on that design. I have never before heard your explanation for why the Spitfire stick was hinged laterally the way it was - so perhaps a reference to a book or journal paper for this alternative explanation? I must admit it sounds very suspicious considering there are much better ways to increase stick force, such as effective gearing design practically anywhere else in the control system, control surface hinge placement and aerodynamic approaches like aileron leading edge design.


What is wrong with that report? It carefully looks at how different design considerations affect lateral control to help engineers with control surface and control system design. I have read that report and referred to it many times throughout my career as an aeronautical engineer. There is very little in that report that compares different aircraft of the period, except perhaps Figures 46 and 47, but those graphs were made with a fixed 50 pound stick force input and not a full deflection. The purpose of those two graphs was to show a spread of aircraft with different aileron designs and how the different devices can all lead to good lateral control handling, with some special mention of the sensitivity of Frise ailerons specifically to various design parameters. There is nothing in that report that is meant to compare the performance of one particular aircraft to another - it is a lateral control design guidance document.

The British pilots I have spoken with and the documents I have read over 60 years all say the stick was to allow hard rolls without involving the stick being interfered with by knees. I know 3 or 4 US pilots who flew Spitfires or fly them currently, and all LIKE the British stick design. It didn't prevent roll, and was effective up to the F.21, after which boost tabs were used on that and later marks due to loads at higher speeds.

 

[Mfezi]

I haven't read this document and I can't judge it, but M seems to be saying that it's a reference document for engineers.

The CRC Handbook doesn't include sources for the vast number of tables in it, and no one expects it to. There may be a parallel.

Apologies for my post-and-run: I was oversees for a few weeks on a big project and just had no time for posting. I've been reading these forums fairly regularly and find the more technical discussions quite interesting (although a little frustrating sometimes - like the one that I responded to), but unfortunately I don't have much time to post on discussion forums. Please excuse the highly belated response.

Yes, the report quoted by "wrathofatlantis" is aimed at engineers and meant to provide guidance when dealing with various lateral control problems. During WW2, the aeronautical sciences developed rapidly, but much of it was done in secrecy or it was done internally within the various manufacturing companies and design organizations with simply no opportunity to publish or reflect on the lessons learned. Institutions like the NACA and other government departments, as well as their equivalents in the UK and other countries, ran various programs or committees to compile relevant data, methodologies and other guidance material to ensure that engineers didn't have to reinvent the wheel on each new design. These would be done topic by topic, and the theme of this particular report was therefore a summary of the latest (non-classified) lateral-control research. It was presented in a way so that flight control engineers can use it as a guidance document and it is an excellent document for that purpose - as reflected by the fact that it is still occasionally used today, such as one of the programs that I worked on as described in my earlier post. The report consists of a number of chapters:
I. Criterions used in lateral-control specifications
II. Factors involved in the lateral-control problem
III. Testing procedures and application of experimental results
IV. Characteristics of lateral-control devices
V. Booster mechanisms
VI. Structural aspects
VII. Application of equations and design charts
VIII. Status of lateral-control research

From the above, you can see that it is a handbook or guidance document for engineers. It is not meant in anyway to try and win a "who had the best plane" argument.

There are two charts that appear in the document which is apparently what "wrathofatlantis" had a problem with:
Figure 46: Variation with indicated airspeed of helix angle pb/2V obtainable with a 50-pound stick force. Altitude, 10,000 feet.
Figure 47: Variation with indicated airspeed of rolling velocity obtainable with 50-pound stick force. Altitude 10,000 feet.

These two charts show roll performance for a wide variety of WW2 fighter aircraft, but it doesn't dwell on specific models or serial numbers as it is not relevant for what these charts are meant to convey. It therefore helps to read what the purpose of these charts were (and note, they are not depicting maximum roll performance - the data shows the performance achieved with a 50-pound lateral control deflection):
"Data have been collected on the rolling-performance characteristics of a number of fighter airplanes of American and foreign manufacture. Pertinent details of the wing-aileron arrangements of these airplanes are given in table VI. All the balancing devices that have been discussed are represented. Comparisons are made on the basis of the helix angle pb/2V and the rolling velocities obtainable at 10,000 feet altitude with a 50-pound stick force. An accurate rating of the balanced ailerons is not possible from the data presented. The only conclusion to be drawn perhaps is that good performance can be obtained from ailerons having any of the various balances, provided sufficient care is exercised in the design and development. The wide variations in the performance of airplanes having Frise ailerons may be an indication of the well-known fact that Frise ailerons are extremely sensitive to each of a large number of design parameters".

The "table VI" that the document refers to is a table that lists for each aircraft tested the geometry of the wing and aileron, the type of aileron, the wing span, the location of the aileron as a fraction of the semispan (effectively its moment arm), the fraction of the span occupied by the aileron and the chord fraction of the aileron.

The graphs effectively show how well an aircraft rolls with a 50-pound lateral input. If it rolls slowly, it means the aircraft has fairly heavy controls, but it doesn't necessarily mean it has bad performance. It also shows how some aircraft's controls become very heavy at high speeds. The document basically says all these aircraft had satisfactory lateral control forces (except where noted otherwise - like the zero that had ailerons that became very heavy at high speeds), but it doesn't show the maximum roll rate achievable. For one aircraft, a 50-pound stick force may reflect a full stick deflection, or even be an extrapolation, where for others the stick may barely move with that control force input.

The document cites 91 other documents to support the material presented. But these are also not meant to compare one aircraft against another. They all deal with lateral control design, aerodynamic and structural modelling and analysis techniques, and so forth. For example, they have titles like "Estimation of Stick Forces from Wind-Tunnel Aileron Data", "Charts for the Determination of Wing Torsional Stiffness Required for Specified Rolling Characteristics or Aileron Reversal Speed", "The Compuation of the Critical Speeds of Aileron Reversal, Wing Torsional Divergence, and Wing Aileron Divergence" - all relevant topics in aircraft design (with much more modern techniques available to solve them these days), but perhaps less exciting for a historian or game designer trying to know which airplane was better.

Wrathofatlantis, in his post, made comments like "continuous rolls up to maximum rolling velocity achieved (therefore meaningless)": The "therefore meaningless" comment, unfortunately, shows a complete lack of understanding of aircraft design, handling and performance, which is maybe what triggerred my original response, along with the strange claim of why the Spitfire stick was designed the way it was, when lots of other aircraft of the period (Hurricane, Typhoon, Tempest, even post war jets) had the same style of stick.

Anyway, I got some real world technical problems to deal with, so I'll leave it at that...

 

[GregP]

This part of your post requires a separate response. The above quote is somewhat true, but not really for relatively stiff and highly manoevrable aircraft like WW2 fighters. The problem of too heavy controls, especially at high speeds, was much more prevalent than the problem of controls that were too light. A lot more effort therefore went into devices that assisted the pilot in applying the necessary hinge moment to overcome the opposing aerodynamic and inertial moments (making the pilot stronger) than the other way around. During WW2 this was done with servo tabs and other clever mechanical and aerodynamic design, but eventually even that was not sufficient and by the end of the war hydro-mechanical systems were being introduced to assist the pilot with overcoming the control forces. In the period following the war, more and more powerful systems were implemented, to the point that the control systems became fully non-reversible and springs and other devices were added to recover some feedback via artificial feel systems. Eventually it progressed to the development of fly-by-wire and the introduction of hydro-electrical or fully electrical servos to move the control surfaces. The big problem to be solved as aircraft became faster and heavier continued to be to assist the pilot with overcoming the associated heavy control loads. Nevertheless, it is true that the control forces of any manned aircraft are tweaked during the design process (made heavier or lighter) to give the ideal handling qualities throughout the envelope - it is just that trying to make them lighter is more common on high speed aircraft such as WW2 fighters than deliberately making them heavier.


Now, this claim is simply ridiculous, and I will explain why in a moment. If you saw this claim somewhere in the literature, I guarantee you that the author had no idea what he was talking about. The reason is simple: Introducing elasticity into a control system to fix a control force issue is, frankly, stupid. Firstly, it will immediately affect flutter susceptibilty: More elastic systems reduce the flutter frequency, making it both easier to excite flutter and causing any oscillations to take longer to dampen out. You can see the effect by considering an equivalent spring-mass-damper system, where the natural frequency can be calculated as omega = sqrt(k/m), with k the stiffness and m the mass of the system. Reduce k and you reduce the flutter frequency. You almost always want to increase that frequency, which implies making components lighter and stiffer.

Secondly, more flexible systems introduce a lag in the control response, which severely degrades handling qualities. It makes precision manoevering tasks like formation flying, air combat and weapon aiming much more difficult. There is no evidence of these defects being present in the Zero - on the contrary, most pilots really liked its handling.

No engineer in his right mind would deliberately reduce the stiffness of the control cable system - and that to achieve something that is relatively simple to achieve through effective gearing, control surface sizing and control surface leading edge shaping or a host of other, much safer methods. I can guarantee you that, unless the designers of the zero were complete idiots or skipped their undergraduate engineering classes on dynamics, they would not have deliberately designed an overly elastic control cable system to tune the handling qualities of the aircraft.

The claim is also not supported by the literature. For example, there is a memorandum published by the Army Air Forces Material Command after evaluation of the zero, titled "Pilots Comments on Handling Characteristics of Japanese Zero Fighter", dated 24 November 1943. The report says: "Handling and Control at Various Speeds - The Zero handles well at all speeds up too 300 mph. Both stick and rudders are sensitive to touch and plane responds readily to changes of controls; however, as the speed is increased the aileron control becomes progressively difficult and at speeds of 300 mph or more it is necessary to use both hands on the stick to perform rolls."

The above characteristic would usually be described by pilots as a "stiffening of the controls at high speed". This is effective stiffening - the aerodynamic hinge moments is what fights the pilot's inputs. As the speed gets higher, the moments get higher, and the pilot experiences it equivalent to a spring that gets stiffer and stiffer, up to the point where the stick cannot be moved. This behaviour is different from what you would experience if the control system cables were overly elastic. In that case, the pilot would describe the response as lagging the inputs, typically using phrases like "sluggish to respond". Furthermore, rather than the system feeling like it stiffens up at high speed, the pilot would still be able to deflect the control stick, but the ailerons wouldn't move and the aircraft wouldn't respond. This would be very obvious and would immediately be noted both by test pilots and operational pilots. Instead, reports on the Zero's handling qualities like the above one generally use phrases like "the controls getting heavy" or the "aileron controls stiffening up" at high speed.

Since the stiffening up of the Zero's ailerons at high speed could theoretically be fixed by various means, it is indeed possible that the Zero's designers left it this way to protect the structure at high speeds, but they definitely didn't do so by deliberately building elasticity into the control cable system.

I notice I forgot to respond to this:

I suggest you check again - the Hurricane also had a control stick of which the upper section was separely hinged for lateral control, despite having "massively rigid wings", as you say...

I have seen a supposed "report" stating the elevator control cables in the A6M were reduced in diameter to allow "springiness" to prevent the pilot from over-pulling and busting the g-limits. I saw it somewhere in this forum in the past, but I also recall the "report" was not cited. Rather it was quoted in autor-written text form. I have never seen or heard of an original report on it.

At the Planes of Fame, we have an A6M-5 Model 52 Zero that is flyable ... at least when we get the 3 leaky cylinders fixed it will be again. Something like 10 - 12 years ago, we did a major overhaul of the airframe. All the pulleys and flight cables were replaced with new ones. The cales that came out were original, as-captured in 1944. Our aircraft is serial 61-120, and it was the Zero used in the 1944 Fighter Conference trials. It was the ONLY aircraft in the entire trial that never broke in testing. During the conference they put something like 180 flight hours on it. Everyone who flew it, including Charles Lindbergh, liked it, but not especially at higher speeds.

During overhaul, when we removed the flight control cables and replaced them, there was nothing remarkable about the cables we removed. They were about the same as control cables in P-51s, etc. We replaced them with more or less equivalent cables. Usually, there is nothing wrong with old controls cables other than they are usually dirty from years of dust on them. We replaced ours only because we had the Zero apart down to smallest component parts and there was no reason NOT to replace cables that were, by that time, about 70 years old give or take a few years. The pulleys we used were the same sizes as the ones that came out. Nothing unusual in them.

For clarity, when I say "we" above, I mean the guys at Fighter Rebuilders, not me. The sum toal of my work on the A6M during overhaul was to remove the flight conrol surfaces, strip them of fabric (I was asked to remove the original fabric from all surfaces in one piece so they could be preserved), and prep them for recovering. While I have been helping with restoration for 20 years, I am not a certified A & P, so my work is checked over by an A & P when I do it, always. There have been a few occasions when I had to re-do some things that were, shall we say, less than optimum. It's the nature of learning how to to fabricate Aluminum on hand-built airplanes.

When you get to some things, it is less than straightforward to form the items needed. Think of the gun ports on the nose of, say, an F7F Tigercat. They are not simple to make and have them come out looking good and matching the sheet metal as they should. It takes a few tries with the help of someone who has done it to get one right. I didn't make one, but I watched a couple of guys make one, so I at least have a good idea of how to do it, should it ever come up as a task. Right now, we have some interesting repairs to make on the elevator of a C-47 that will require some sheet metal skills. So, I guess we'll see if we can do it or maybe need some guidance on how to get it done correctly.

They don't teach that stuff anymore in A & P school. What they DO teach these days is mostly about working on in-service jet airliners ... none of which helps you with an old WWII-era fighter with fabric-covered control surfaces.

 

[special ed]

Years back. about 1968, a friend who worked on crop dusters was asked & hired by Ozark Airlines because he could do fabric covering properly for their DC-3's control surfaces. When Ozark was absorbed by another (can't remember who) he moved to Chicago with them and I lost a modelling buddy.