WW2-fighter and critical Mach speed

On the subject of Critical Mach number, there are essentially three stages with the significant complications arising in the third phase.

Phase one - V local on any place on the surface of the airfoil < M=1.

Phase two - V = M=1 at some region on the surface of the wing and a mild shock wave forms usually at the max T/C of the wing. A mild separation of flow begins as the shock wave produces a large increase in static pressure behind the wave.

This is Critical Mach for the Wing.

Phase three occurs as the shock wave strengthens the supersonic flow region increases forward of the shock wave also forms on the lower surface of the wing. It is in this phase that large adverse pressure gradients form causing major separation and wake turbulence and noticeable changes in the aerodynamic forces. Some typical effects include MOVEMENT of the shock wave aft, changing the Center of Pressure location aft, changes the Moment about the a/c, decrease the downwash incidence on the elevator causing a decrease in elevator authority to resist the change in moment co-efficient ------------> P-38 and to some degree the P-47.

The Critical Mach number is in the velocity spectrum of increasing compressibility of air to the Force Divergence Drag number - or Drag Rise. Air is not an incompressible fluid and is only treated as such at aout M= .3. Somewhere for WWII era fighters there is a noticeable increase in parasite drag beginning around .55, some are delayed (like the Mustang) to .60-.62M. In this span the Parasite drag increases from a factor of 1 to 1.05 to 1.1 very rapidly - and described as Drag Divergence. Most of this initially is the formation of adverse pressure gradients and subsequent increase in flow separation and coupled form drag.

GregP said:

If it has zero to do with flutter, then my aerodynamics texts must be wrong. Stranger thongs have happened. I acutally have two math books with errors in them.

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I actually took the time to register just to post this:

You may own the texts, but certainly you do not actually know what inputs those equations represent in regards to flutter, or Limit Design Loads or Ultimate Design Loads or... Honestly, if I run into one more arrogant pilot who thinks just because he knows how to fly a plane and owns a book he knows how to design one, I am gonna sock his ignorant mouth.

Flutter has more to do with rigidity of the aileron, controls attached and the wing structure directly attached than velocity. Sure, ultimately it all ties back together, but which part of flutter are we going to address? The FAA etc throw this Vne around, but honestly it is pure bunk outside of an airplane that has been tested extensively. The Vne's one always sees and which the FAA accepts are often nothing more than "Rule-of-thumb" on civilian craft. Now a plane like the Cessna 172 or later slightly different models have had so many built, and so many crashed and investigated, that the atual numbers are fairly well known. The original flight design engineers who wrote the manuals certainly did not know. Pretty much outside of Boeing/Airbus in civilian airliners, they don't have a clue what the flutter speed is. Those GulfStream planes? They do not do the analysis on them. They just fly them and use rules of thumb of rigidity and extra rigidity to eliminate flutter, then slap a conservative number on their planes, knowing full well, they will hit Mcr first and therefore they do not have to worry about flutter at all by and large outside of some freak resonance which can easily be dealt with by adding weights to certain parts of the structure.

Don't want flutter? Increase rigidity.

Your models would probably be just fine if you replaced the typical crappy flexible floppy ailerons(foam/balsa) and corresponding lame hinges with something real. Just wrapping said balsa hinge in uni C-F super thin matte CF/Kevlar along with a hinge point further aft is vastly superior and very cheap to do. This easily allows for several hundred miles per hour with normal micro servos/horns with metal gears. If you really want to get fancy, place a damper on the servo arm. Of course such ailerons are much more expensive to build and therefore you never see them on an RC airplane or rarely outside of RC sailplanes, Q400 racers, and a few jets.

Relax said:

You may own the texts, but certainly you do not actually know what inputs those equations represent in regards to flutter, or Limit Design Loads or Ultimate Design Loads or... Honestly, if I run into one more arrogant pilot who thinks just because he knows how to fly a plane and owns a book he knows how to design one, I am gonna sock his ignorant mouth.

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This might be a good time to mention that many members on this forum are engineers qualified in various fields of aeronautics and may not be talking out their azz when posting info...

And I might also mention that several of these engineers are also combat rated pilots who can probably tear your head off at the shoulders and sh!t down your throat...so it might be a good idea to tone it down a little...

Relax said:

I actually took the time to register just to post this:

You may own the texts, but certainly you do not actually know what inputs those equations represent in regards to flutter, or Limit Design Loads or Ultimate Design Loads or... Honestly, if I run into one more arrogant pilot who thinks just because he knows how to fly a plane and owns a book he knows how to design one, I am gonna sock his ignorant mouth.

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Great first post - now tone it down or you'll get my boot up your @ss!!!

GrauGeist said:

This might be a good time to mention that many members on this forum are engineers qualified in various fields of aeronautics and may not be talking out their azz when posting info...

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As one of those Pilots/Aeronautical Engineers myself, if I say something stupid, my fellow Aero boys, with or without degrees, better kick my azz. No real engineer is worried about certificates. As if you can verify such baloney on the internet anyways. Science is science. Blowhard pilots be they civil or "combat rated" pretending to be engineers, are blowhards.

Many civil pilots without engineering degrees, I would trust their design accumen more than many of my co workers with Aero/Mech degrees as most engineering is via observation and insight on how something functions. It is not boxed into an equation or a chart. Degrees mean zilch. Generally, I have found that those folks with high hoity toity Piled higher and deeper degrees are the most dense when it comes to observation skills on how everything ties together. Such people are wonderful once the "box" has been defined, but until then... Logic rules.

Great forums. Read them for years. Learned many "new" ideas on how to design the planes I design today. Most things are not new under the sun.

Relax - I am inclined to agree with you with a couple of caveats. One of the best airframe design engineers I once knew was un-degreed but possessed excellent grasp of sound design for load paths and manufacturability once the design hit the floor, but -

Nobody is going to a.) analyze a wing, given the aero loads. external loads, etc with the detailed drawings in front of them and critical part by part analyze the load path, the geometry of the parts, apply the assumptions and then perform the detail analysis to derive the stress from the applied loads, part by part along the presumed load paths to get the MS without a combined in depth knowledge of Mechanics of materials, some applied theory of Aircraft Structural analysis - and b.) actual (and meaningful) experience under a mentor from the airframe business.

We can debate whether six years under a talented airframe structures guy can outpace a four year BS ME/AE with the requisite courses plus two years under the same tutor would yield the better airframe structures guy - but I would tend toward the latter.

Same for Aerodynamics and Performance analysis. The theories and the boundary conditions for applicability are in my opinion essential to drive 'good approximations' in what is essentially a non-linear world.

Just my opinion. Take what you wish and leave the rest.

drgondog said:

Relax - I am inclined to agree with you with a couple of caveats. One of the best airframe design engineers I once knew was un-degreed but possessed excellent grasp of sound design for load paths and manufacturability once the design hit the floor, but -

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If all we were going to talk about is the structure of the wing itself, I would agree with you. But, Flutter as a problem in search of a solution has a giant caveat. Its controls. Sure, an engineer can do load paths. But a by the book equations guy cannot intuitively observe the multiple variables, that are not "solvable". At some point intuition must be used along with creativity. Yes, the best engineers have both the ability to solve the equations, but also to creatively come up with a solution.

Generally I have found those with the creativity are those with their hands in the grease, nuts, and bolts, and NOT the engineer in question who sits on his ass behind a computer screen. I can almost guarantee you from first and second hand experience, that any engineer who does not get his hands dirty is at best a mediocre to craptastic engineer. Because they are clueless about operational, manufacturing reality.

Ok, enough blather, back to Mcr​ values of WW2 birds. Though the only M value one really cares about is its tactical Mach value which has far more to do with cg/chord %, CN​, NP, SP, TVC, etc (elevator authority) than a stupid Mcr​ value of the wing.

Spitfire early on had aileron flutter till they fixed it by simply adding rigidity to the aileron/wing and managing to increase the control throw angle allowing greater rotation rate at the same time. Yea, clipped wings helped and IIRC, larger ailerons. Would have had to go past the speed of sound to get flutter in the ailerons is what they WAG'd at the time. Good enough is good enough. Just proving that even in WW2, flutter, as a limiting load factor on wing structure is complete Baloney. Unless of course someone wants to say a wind up high positive g to low negative g turn is flutter... Very slow flutter...

I don't have to say much, Relax. Joe did it for me; thanks, Joe. I live in Rancho Cucamonga, CA. Come on over any time you feel like it and we can talk any way you like. Might be fun.

My RC's don't flutter below "flutter speed," and I don't fly them there anymore. I only fly the fast airframes fast. I DID notice that the flutter stopped instantly when I loaded the airframe. Of course, an RC can load to relatively high-g very quickly. Not what you might do in a full-scale aircraft, maybe without Gloc.

Since I fly full-scale within the POH limits, I am VERY unlikely to find a flutter limit on a Cessna or Piper or Mooney. When I owned a Cherokee and had it repainted, I removed the stab and made sure it was about 15% nose heavy (it was) ... and had no flutter up to redline. I checked ... at redline, in flight. No flutter in the dive.

Steve Hinton finds the same with our F-86 nd MiG-15 bis and CT-133 ... no problem if balanced correctly. Good enough for me even if you or anyone else disagrees. I checked the elevator for our A6M5 Model 52 when I disassembled it and it was the same; about 15% nose heavy. It doesn't flutter either when within the published the flight envelope to date. Neither do the ailerons or rudder, all balanced the same way.

Funny thing, ALL our WWII fighters balance the exact same way and have a perfect safety record from airshow to airshow and monthly event to monthly event and flight to flight, aerobatic or not.

If you elect to come this Saturday, we'll fly a Canadair CT-133 that, curiously, ALSO doesn't flutter at redline and is balanced the same way ... about 15% nose heavy, with a hydraulic system instead of a cable system. They start the National Championship Air Races every year with this aircraft in Reno. I assure you, it doesn't flutter.

I see a trend here ... and it works in real-world, actual flying, even for helicopters in the 4 or 5 companies I know that balance the blades themselves ... all about 15% nose-heavy. That includes Blue Sky Helicopters in Upland and the Edison Electric Flight Department who fly a LOT at Chino. So it works for ALL us local fighter, bomber, and chopper operators.

No flutter if below Vne / Vd. That might change drastically, of course, with ... maybe ice.

So Cal doesn't suffer too much from that, but I suppose it can happen almost anywhere. If so, it would NOT be from static balance issues. My RC flutter probably was, as I didn't balance at all. After I DID balance it nose neavy, no issues, even fast. Today, I just fly a bit conservatiely with the aerobats out of desire to not pay for them multiple times.

Cheers to you. Keep 'em flying.

Relax said:

If all we were going to talk about is the structure of the wing itself, I would agree with you. But, Flutter as a problem in search of a solution has a giant caveat. Its controls. Sure, an engineer can do load paths. But a by the book equations guy cannot intuitively observe the multiple variables, that are not "solvable". At some point intuition must be used along with creativity. Yes, the best engineers have both the ability to solve the equations, but also to creatively come up with a solution.

My comments were related to 'formally educated engineer versus experience gained but no engineering academic engineer'. While I understand the approach to determine normal modes and frequencies of a structure and generation of stiffness equations, I have Never solved (or been asked to solve) a flutter problem.. aero elasticity as a practical analysis tool before really advanced Computational capabilities was not in my toolkit - but I do understand the theories.

Generally I have found those with the creativity are those with their hands in the grease, nuts, and bolts, and NOT the engineer in question who sits on his ass behind a computer screen. I can almost guarantee you from first and second hand experience, that any engineer who does not get his hands dirty is at best a mediocre to craptastic engineer. Because they are clueless about operational, manufacturing reality.

I don't disagree with the latter and consider knowledge of fabrication and assembly processes on the shop floor as essential knowledge for the good design engineer.

Ok, enough blather, back to Mcr​ values of WW2 birds. Though the only M value one really cares about is its tactical Mach value which has far more to do with cg/chord %, CN​, NP, SP, TVC, etc (elevator authority) than a stupid Mcr​ value of the wing.

Mostly agree. Understanding why a laminar flow wing with with max t/c for a same thickness conventional NACA 23xxx wing not only has a velocity gradient significantly different but also why the shock wave forms at a higher Mach number as well as aft of the A/C (and Cg)to affect the CMac is important to your comments above..

Having said this 'the 'stupid Mcr' value between a P-51 and a P-38 was frequently a matter of life and death when the latter (i.e Bf 109 or FW 190) is being chased in a dive with a significantly higher Critical Mach.

Spitfire early on had aileron flutter till they fixed it by simply adding rigidity to the aileron/wing and managing to increase the control throw angle allowing greater rotation rate at the same time. Yea, clipped wings helped and IIRC, larger ailerons. Would have had to go past the speed of sound to get flutter in the ailerons is what they WAG'd at the time. Good enough is good enough. Just proving that even in WW2, flutter, as a limiting load factor on wing structure is complete Baloney. Unless of course someone wants to say a wind up high positive g to low negative g turn is flutter... Very slow flutter...

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Actually, IIRC the Spit primary problem wasn't so much flutter as insufficient wing stiffness that caused the wing to torse due to applied forces of the ailerons in the M= 0.5 range.

As an aside, Greg Pascal just referred to the 'practical' solution for flutter prevention in the Performance envelope - namely by location the inertial and flexural axes as close to possible to the 1/4 chord axis.. location the weight distribution forward is the easy test when a problem is encountered. This approach was well known for WWII a/c

Relax said:

I re-read your posts trying to figure out your perspective between 'creativity', 'analysis', the role of 'intuition', and the value of 'hoity toity degrees' with respect to Design and Problem Solving?

"As one of those Pilots/Aeronautical Engineers myself, if I say something stupid, my fellow Aero boys, with or without degrees, better kick my azz. No real engineer is worried about certificates. As if you can verify such baloney on the internet anyways. Science is science. Blowhard pilots be they civil or "combat rated" pretending to be engineers, are blowhards."

This statement seems to imply that if a pilot conveys knowledge of engineering without an Aero degree - that such actions also imply 'Blowhard'? How is that different from anyone representing a foundation of math and engineering analysis, but doesn't have the credentials?

Many civil pilots without engineering degrees, I would trust their design accumen more than many of my co workers with Aero/Mech degrees as most engineering is via observation and insight on how something functions.

Does this statement imply that Some pilots without the academic (or practical) background in the required Math and Engineering are back in your 'good side'? What does 'design acumen' mean in this context? Pilots suggesting different methods of mounting an engine to an airframe without performing the stress analysis? or assuming the airframe is 'over designed' and available for a few more unmolested "G's" than the Manual sez? or "let's put a Lear wing on a Mustang - it'll be better - without the personal ability to analyze the effect of the change?"

It is not boxed into an equation or a chart. Degrees mean zilch. Generally, I have found that those folks with high hoity toity Piled higher and deeper degrees are the most dense when it comes to observation skills on how everything ties together. Such people are wonderful once the "box" has been defined, but until then... Logic rules.

If your 'hoity toity piled higher and deeper' degree is a guy with focus in Quantum Mechanics confronted for first time with a Stability or Stress or Resonance problem on an airframe - I get that, or implying that non degreed A&P really understand how everything ties together - I get that also, or how some airframe engineers are not very good - I get that also.

So, let's pose this question. A really cool General Aviation product comes into your life and built by a really cool guy that you know, and you know he is a really good Mech (or pilot) but as far as you know does not have a degree or lead Designer experience in charge of all phases of design. It is very innovative. You have the money - you really want it. You gonna buy it and fly it?

But, Flutter as a problem in search of a solution has a giant caveat. Its controls. Sure, an engineer can do load paths. But a by the book equations guy cannot intuitively observe the multiple variables, that are not "solvable". At some point intuition must be used along with creativity. Yes, the best engineers have both the ability to solve the equations, but also to creatively come up with a solution.

This one also confused me. An good airframe, structures, aero, etc engineer is Never a 'by the book equations guy'. The good to great engineer is by training and education a Problem solver that understands the Results expected (Performance, payload, permissible load factors for Stress/ margins of safety determination, cost, maintainability, producability, etc), understands the existing art (or accepted process standards), understands the preliminary approach evaluation process, understands where he starts from (blank sheet of paper or modify existing design to be modified for new performance requirements), etc.- and can construct and follow solution approaches.

Flutter is a phenomenon. 'Back in the day' you could fix it by changing mass distribution, stiffness, hinge points for the control feature (aileron, elevator, etc) without diving into differential equations. Only rarely is it required to do a full blown frequency analysis or aero elastic modeling.

These attributes do not come naturally via 'intuition' and 'creativity' as Engineering is a solve/build approach based on accepted principles of Physics, Math, Engineering specialties combining both math and physics,and Experience. Those combinations, including experience, breed intuition and creativity in the very best Engineers - but Kelly Johnson's don't come along on a wagonload of turnips.


Generally I have found those with the creativity are those with their hands in the grease, nuts, and bolts, and NOT the engineer in question who sits on his ass behind a computer screen. I can almost guarantee you from first and second hand experience, that any engineer who does not get his hands dirty is at best a mediocre to craptastic engineer. Because they are clueless about operational, manufacturing reality.

No engineer that Only sits at a desk staring at a computer screen is any more that a CAD operator or at best a model builder/analyst in a very narrow definition of Engineer for That Job. Having said that, I pioneered NASTRAN at Bell and Lockheed precisely because I had demonstrated competence at airframe design and airframe structures - both of which, in my opinion and the opinion of my bosses, are required to decide on the Model attributes to mirror airframe construction (i.e. Rods to simulate longerons, Shear Panels vs Plates, etc)

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I'm not exactly sure of your definition of 'getting hands dirty' means. Both Bell and Lockheed mandated a two to three month tour working under the Production boss, reviewing and following Change EO's through tooling and production to visibly understand the difference between Design to the Drawing and Manufacturabilty of that Design.

The Shop floor teams were sharp about 'better ways to do it' and the smart Engineer learned from that. Everything from 'too tight' tolerances, to operations dispatched to a 3 or 5 axis NC machine could have been done as well with a different design approach on a Mill, etc... or conversely a multi process/machined part could have been done better in one stage on a 3 axis NC machine.

Having said that, the Production guys rarely (or not at all) had the requisite academic Engineering and practical experience to supervise a Structures or Aero group. Do you know of any?

drgondog said:

I'm not exactly sure of your definition of 'getting hands dirty' means. Both Bell and Lockheed mandated a two to three month tour working under the Production boss, reviewing and following Change EO's through tooling and production to visibly understand the difference between Design to the Drawing and Manufacturabilty of that Design.

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2 to 3 months... HA! As if this teaches you jack crap on manufacturing realities or maintenance realities.

I can tell from your reply that you were one of those sit your behind in the chair guys behind the computer screen. You think that by being able to shave an 100g off an existing structure, using FEA, makes you a good engineer. No. It makes you a good analyst. Not a good engineer.

An engineer views the ENTIRE project as a whole. He is not narrowly focused in structures. Structures is the minor field in engineering.

I can tell you 100% have no clue what I am talking about and never will as you do not have the framework built in your mind to visualize what I am saying as you are tied up in your own structures FEA world. GregP has a much better grasp at what I am talking about than you do in this case. Same goes for most of those production engineers on the manufacturing floor that you were deriding in your post.

Anyone can do FEA. Dime a dozen.

Lets just end it there.

Relax said:

2 to 3 months... HA! As if this teaches you jack crap on manufacturing realities or maintenance realities.

I can tell from your reply that you were one of those sit your behind in the chair guys behind the computer screen. You think that by being able to shave an 100g off an existing structure, using FEA, makes you a good engineer. No. It makes you a good analyst. Not a good engineer.

An engineer views the ENTIRE project as a whole. He is not narrowly focused in structures. Structures is the minor field in engineering.

I can tell you 100% have no clue what I am talking about and never will as you do not have the framework built in your mind to visualize what I am saying as you are tied up in your own structures FEA world. GregP has a much better grasp at what I am talking about than you do in this case. Same goes for most of those production engineers on the manufacturing floor that you were deriding in your post.

Anyone can do FEA. Dime a dozen.

Lets just end it there.

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As an A&P/IA CFII who has been in this business for almost 36 years and who wrenched on many an aircraft and flown stuff a bit more robust than bug smashers, I've come across many an engineer and wanna be engineers, worked with many and actually helped some design and redesign some "stuff". I've also run across some who vibrates their mouth in an attempt to show the world their aviation knowledge and mixes their message with arrogance and stupidity. I usually find these folks don't fly what they engineer, puke during routine airline flights and are real good playing video games. You my friend have shown me you're nothing more than an arrogant @sshole. Enjoy cyberspace and hope I never have a chance to shove my boot up your @ss, although it would be interesting to have your rectum pass through your vocal chords.

I thought aeroplanes were designed by engineers using science, I now realise what genius the great design team leaders had, not only to know how to design an aeroplane but to keep the wannabe prima donnas doing their job and not playing their own trumpet.

Now about the critical mach speed and flutter.

Not too sure what to say here except that whenever you go a few rounds with Bill Mitchell (Drgondog), you're gonna' lose at least half of them. While we may have occasionally crossed pens, he was mostly right. And, being at least somewhat of a gentleman, he has never threatened to come and fight about it.

Stick around, Bill. I like your brand of fighting much better. At least I usually learn something from it!

And Mr. Relax, Bill Mitchell can say he's flown a P-51 solo, and it wasn't a 2-seater. You might want to re-think your thoughts on his qualifications, both as an engineer and as a pilot. My aeronautical engineering is 30+ years old and I spent my career as an electronics engineer after initial aero training. Bill didn't. He worked as an aeronautical engineer for decades. I'm sort of embarrased to have gotten a back-handed compliment from Relax. I hereby disclaim any responsibility for it.

If anyone around here gets aerodynamics, it's Drgondog. If there's anyone in here who gets maintenance ... and gets his hands dirty at it, it's Joe (Flyboyj). He also crews at the National Championship Air Races for the jet class. Altogether a rather formidable combination to start trash-talking to right away.

You might possibly be a trifle aggressive there, Mr. Relax ... I'm not a moderator in here, and that probably works in your favor in this case.

GregP said:

Not too sure what to say here except that whenever you go a few rounds with Bill Mitchell (Drgondog), you're gonna' lose at least half of them. While we may have occasionally crossed pens, he was mostly right. And, being at least somewhat of a gentleman, he has never threatened to come and fight about it.

Stick around, Bill. I like your brand of fighting much better. At least I usually learn something from it!

And Mr. Relax, Bill Mitchell can say he's flown a P-51 solo, and it wasn't a 2-seater. You might want to re-think your thoughts on his qualifications, both as an engineer and as a pilot. My aeronautical engineering is 30+ years old and I spent my career as an electronics engineer after initial aero training. Bill didn't. He worked as an aeronautical engineer for decades. I'm sort of embarrased to have gotten a back-handed compliment from Relax. I hereby disclaim any responsibility for it.

If anyone around here gets aerodynamics, it's Drgondog. If there's anyone in here who gets maintenance ... and gets his hands dirty at it, it's Joe (Flyboyj). He also crews at the National Championship Air Races for the jet class. Altogether a rather formidable combination to start trash-talking to right away.

You might possibly be a trifle aggressive there, Mr. Relax ... I'm not a moderator in here, and that probably works in your favor in this case.

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It's Bill Marshal

Now I'm doubly embarrassed. Of course, I knew that .... must have been a senior moment.

Abject apologies, Mr. Marshall. Slap me.

Just re reading the posts here I cannot imagine how hot the air got between the various interests between Hawker and Supermarine and the thin wing versus thick wing camps. Even at the end I believe nothing was conceded and Hawkers said the Tempest was designed as it was just because the MOD would only buy a plane that "looked like a Spitfire"

GregP said:

Now I'm doubly embarrassed. Of course, I knew that .... must have been a senior moment.

Abject apologies, Mr. Marshall. Slap me.

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I don't think he minds and I know he's been called worse!

There have been a couple of times I thought worse myself ... temporarily.

I was wrong. I can admit that much and still laugh about it.

You know pbehn,

I've often wondered why, with national survival at stake, companies didn't seem to cooperate more.

Then I remember the only country that didn't have that problem was the Soviet Union. There, you cooperated or were shot or sent to Siberia or prison. It probably made for great cooperation ... at a large price.