WW2-fighter and critical Mach speed

[drgondog]

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

Worth a reply. True regarding not enough time to graduate to Manufacturing/Process Engineer, not enough time to scratch a surface regarding tooling, machine tool set up or plant layout. Having said that, working with the manufacturing engineers reviewing Change Orders that slipped past Design review and caused issues with practical incorporation or best practices forces an airframe design engineer to re-think how his brilliant concepts cause issues on the shop floor. Maintenance has become a real discipline as post production labor costs and 'up time' for complex systems is a real problem - and the reason the F-14 was retired (among many other 60-s-70's designs. IMO you learn nothing (about Maintenance or Field Service or Spares Planning) on the shop floor except for better understanding of access to critical components when installed 'As designed"

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.

If you can tell that from your viewpoint, you may not a.) be an engineer, and b.) maybe not be an acceptable analyst


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

I believe I mentioned that in my post #70 above. The paragraph may have been a little too complex in citing attributes that I consider important to airframe engineering for you to grasp holistically - you failed to note that 'structures' is but a step in the process I outlined for you regarding my definition? IMO, based on your rants, you have toiled away in whatever career you have mostly irritated by folks that don't agree with you - and further, based on that observation, You STAY in a constant state of 'Irritation'?

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.

You have a severe reading comprehension problem - I never 'derided' production engineers, quite the contrary. I learned more from the Production Engineers and Shop floor supervisors as Program Manager for GE's AFCAM participation in the mid 70's by 'walking around and talking shop at Boeing, Lockheed, GD, MACDAC, GE Aircraft Engine Group and LTV than I ever learned as a design engineer - as USAF tried to get control of costs via synthesizing concepts from CAD/CAM and Drawing Classification systems.

Anyone can do FEA. Dime a dozen.

Ah, anybody with a sound background and experience can be trained to perform Finite Element Modeling tasks which are a.) reliable in concept, and b.) lead to sound detail design, but - hardly a dime a dozen.

Lets just end it there.

I suspect by the icon displayed under your username, that Joe has eased your way from this community. As a parting observation your rants remind one of Sam Kennison, the former 'comedian'. Maybe we will reconnect on some other forum and continue the dialogue?

I am really curious regarding your body of work and the experiences that led you to the rants you have posted?

 

[drgondog]

Greg - not a problem on the 'name'. We use our full names so seldom.. as to P-51 time 'and not in a two seater', well it was a two seater, which is how I learned to fly the damn thing. My father would not have given me a check out standing on the wing and wave 'bye'. Momma would have kicked his ass.

The value of in-air instruction, observation and coaching tips can not be overestimated for relatively low time pilot transitioning from AT-6..

 

[GregP]

Hi Bill MARSHALL,

I figured Mr. Relax might needed a dose of reality and took some poetic license. I think I was somewhat of a hair trigger some 2 - 3 years ago myself. It probably had a lot to do with being unemployed. Not so anymore. Busy teaching electronics at a college and find myself much more, if you'll pardon the expression, relaxed ... nothing whatsoever to do with Mr. Relax.

I don't have anything to prove at this time and I really DID get aeronautical training ... but haven't used it much since 1971. And I moved from a 2800 sq foot house into a 1000 sq foot apartment and only have two books out on aerodynamics, and neither one is a detailed treatise ... sort of basic intro stuff. I promise you I have 'em buried away, but I also don't exacctly recall the exact sequence of things or the entire set of concepts exactly since I haven't looked at it much over the last 40 years.

I'll probably just take your word for aerodynamics at this point until and unless I can get somewhere to unpack and study for awhile. When that happens, I'll probably just wind up agreeing with you anyway, Bill.

I started to do an aerodynamics spreadsheet from a Navy aerodynamics text and just ran out of time. Each and every class I'm teaching for the next year will be completely new to me and I'll probably spend most of my time just staying ahead of the classes. After the first time, it gets markedly easier, as I'm sure you know.

And way back when we DID do some FEA, but we called it "Numerical methods" and it took a large deck of punchcards to get what we can do these days on a spreadsheet, so it probably didn't go quite as far as today's software. It was the best we could do at the time with "double precision floating point numbers!"

The PC I just bought today has a 6th-gen Core i7 processor that would probably put all the computers in the world, all put together, in 1969 to shame. We've come a long way, baby ... and we can now get the wrong answer much faster, and with far more precision.

Sorry about the "Senior moment" there, Bill.



Senior moments:
View: https://www.youtube.com/watch?v=Xv1tMioGgXI

 

[drgondog]

Greg - you have zero to apologize for, now and in the past.

As to FEA using punched card stacks to define a model much less solve one boggles the mind. The numerical relaxation methods were barely emerging in your time at Purdue.

My Computer use career started during a co-op stint at NASA to help develop a low orbit atmospheric degradation algorithm using Bessel functions in the mid 60's - Fortran II and IBM 600 series with (IIRC) 16K of memory.

 

[GrauGeist]

...IBM 600 series with (IIRC) 16K of memory.

16K memory in the 60's? Lord have mercy, that must have cost a fortune!!

My late 80's i386 computer had a whopping 8mb (when typical systems had 2mb or 4mb) system memory with a Seagate ST225 hard drive (25Mb capacity) and this beast cost a paltry $2,750!

 

[drgondog]

Dave - I have no idea how much it cost, but it was somewhat self contained and you loaded the JCL, the data and the Fortran application to initiate the job. Sub routines and dimension statements had to be carefully examined for potential memory issues - either capacity or overlay.

 

[GregP]

That's hilarious. The first computer I used in 1968 at Purdue was an IBM 1130 programmed in Moron (no kidding .... LIBF $READ, etc .... which was library file read ...). The first language I ever used that was "high level" was Fortran II! We moved on to the PUFFT (Purdue University Fast Fortran Translator) programmed by Jerry Rubin, who I believe at the time was 16 years old and had just gotten his PHD. We still used punchcards since that's what the students were allowed to use at the time. I was in hog heaven when I got one of the first HP 67 calculators that would read little strips of magnetic tape and run a 200 or so line program! At least it didn't have nixi-tube displays. It was a red 7-segment LED display.

I seriously doubt that the calculations we did amount to a full-fledged FEA, but they seemed like a LOT of calculations at the time. Of course, Neil Armstrong, when he landed on the Moon in 1969, had 16k of CORE memory! My first "home PC" was a Commodore Vic 20. It was a real POS but we felt like kings when we made the TV set do some rudimentary programming at home. Later I got a Commodore 64. We called it a Commode Door 64. but at least you could play a game or two on it.

Enough of this thread hijacking.

None of those damned PCs had a critical Mach number anyway. Most would only travel faster than stationary-and-nailed-down if you dropped them or hurled them across the room in frustration. I dropped one out of a Cessna 172 and was immensely pleased with myself at the time. As I recall, my circular error of probability was huge since I didn't even aim except to be sure I was away from people and homes, and was way out in the Arizona badlands.

 

[MiTasol]

Greg - you have zero to apologize for, now and in the past.

I agree totally

At least you only have an occasional "senior moment" which is far better than suffering from CRAFT :<(
(Can't Remember A Flipping Thing)

 

[GregP]

Well, I sincerely hope I don't get there but, if someone does, at least they can't remember it.

I always called it CRS (can't remember sh*t).

I miss flying now (with the exception of the occasional warbird flight) but I absolutely don't miss paying for it. I'm planning on building an ultralight when I retire, but you never know. It might turn into a tractor to work the land. I DO plan on putting a CNC end mill in the shop for fun, but that could change, too, since I might not want to make airplane parts for restoration until I die. It might turn into a laser engraver or maybe a CNC router. I HAVE used one to make a beautiful instrument panel for an Embraer Tucano.

That might be a reasonable hobby with the potential for some income.

 

[Zipper730]

after a look-araouns I found some additional critical Mach-speed figures:
From Lunatic (compare P-47 vs. F4U):

Spitfir MK IVX: 0.89*

This doesn't look right, I know a Spitfire VIII was tested up to Mach 0.891, though that was pushing the airplane into a territory that was beyond the placard limit (0.85)

P-47N: 0.83*

I assume that's the MMO, right?

Recovering alone doesn´t proove that it was no terminal dive Mutke´s claim to break Mach 1 in a Me-262 dive (which I don´t believe) underlines that. He was able to recover from his terminal dive at very high subsonic speed (but not Mach 1, the airframe of the Me-262 couldn´t sustain the stress).

Actually, from what I remember there was a narrow set of conditions where the Me-262 could actually exceed Mach 1, and some recent tests corroborated it. It might very well put the plane very close to structural damage or worse (it'd likely require both stab-trim and elevator application which can sometimes overstress the aircraft): I can't say whether Mutke managed to do it.

Figures I have are 0.84 for for the P-51B, 0.82 for the P-51D because of the slightly thicker wing and the bubble canopy.

From what I remember the placard limit of the P-51D was 0.75, and was dove once to Mach 0.84 or 0.85 with skin buckling occurring. I'm not sure how much different the dive speeds were with the P-51B and D, though the -D might have had more drag due to it's canopy.

I also recently found the mach number of the Bf-109G was about 0.78, the 109F was about 0.80.

Was this the MMO or placard limit? I'm actually quite fascinated about the placard speeds for the Bf-109/Me-109 and Fw-190 variants: I do remember something to the effect that either the stabilizer or the rudder balance imposed some kind of flutter restriction that was eventually fixed and allowed dive speeds faster than the P-51.

The P-38 mach number is so low because it has a very thick (by proportions) conventional wing with the maximum chord well toward the front.

To some degree the thickness was made necessary because of the high aspect ratio: High aspect ratios produce more flexing of the wing, which require more thickness or more internal support members.

I'm not entirely sure why they used such a high-aspect ratio to be honest.

going off memory
The piston engined aircraft with the highest Mach speed is a late model Spitfire, which in the hands of a test pilot reached Mach 0.98 in a dive, though the aircraft almost fell apart, and its propellor fell off Fortunately the pilot managed to land OK

I think you might be mixing up several different scenarios.

1. Spitfire Mk.VII/VIII's were fitted with a stronger wing than the previous designs: The placard limit was around 0.85, though tests were done up to 0.891. During at least one of these tests, the propeller came off and the pilot managed to glide the plane and put her down somewhere.

2. During the development of the Miles M.52, they fitted a Spitfire PR variant with a stabilator: Dives were successfully done all the way up to 0.92 mach

The P-51 wing was the same until the P-51H. The only difference was the inboard leading edge angle depending on the wheel/gear uplock design changes made on the D/K.

The kink didn't affect the dive speed?

Both the P-47 and P-38 had increased longitudinal stability induced by the transonic separation - both by the reduced downwash on the horizontal stabilizer but also due to the change in wing moment coefficient as the shock wave moved aft past the 1/4 chord point of approximate maximum chord thickness.

I thought loss of control was predominantly related to the shockwave strength, turbulent flow, and loss of the downwash until you were nearly sonic, when the loss of the upwash shifted the C/L to 0.50% MAC

As the swept wing goes through the transonic region it will start to lose lift in the outer wing panels due to the spanwise flow contribution.

I didn't know spanwise flow would occur under these conditions. In fact I thought it was mostly an issue related to stalls.

When that happens, the wing/body lose lift and will pitch up causing the downwash over the low tail to Reduce- causing the tail to have positive lift - counterbalancing the wing-body pitch up.

I didn't think the tail produced positive lift, merely it stopped producing most of it's negative load.

 

[fubar57]

I'm grabbing some popcorn..........

 

[drgondog]

This doesn't look right, I know a Spitfire VIII was tested up to Mach 0.891, though that was pushing the airplane into a territory that was beyond the placard limit (0.85)

Was that the MMO or placard limit? Neither, that velocity was well past design limits, Placard speeds and Mcr.

Was this when supersonic flow appeared on the wing, when mach effects started to take hold (heaviness on the control column), or the maximum operating mach number?

Mcr is that freestream velocity at which the velocity over the surface (i.e wing, windscreen, etc) reaches M=1.

I wouldn't be surprised if that's the speed supersonic airflow started to form over the wing, but from what I was told around 0.68 was when control-problems began, with a total loss of control at Mach 0.74.

There is no constant Mcr for any airplane class or type, although aircraft with exactly the same wing (airfoil and T/C ratio) will be very close.

From what I remember the placard limit of the P-51D was 0.75, and was dove once to Mach 0.84 or 0.85 with skin buckling occurring. I'm not sure how much different the dive speeds were with the P-51B and D, though the -D might have had more drag due to it's canopy.

No difference in Placard velocity - basically wing related and the B had same airfoil as D. The drag on the D canopy was slightly less than the B. That said, the ammo cover doors were beefed up on the D due to visible 'balloon' effect during full blown shock wave formation as the airframe (wing) entered Drag Divergence regime. Drag Divergence is defined as the Velocity envelope at which a full separation and shock waves (indicating chaos in flow in which local shock waves are created) appear. In this velocity envelope the adverse pressure gradient exists in the region of shock wave formation - resulting in complete separation. This is the Transonic region and responsible for great turbulence downstream of the separation region. Selecting a 'Point' for Divergence was traditionally that 'point' at which the Total Parasite Drag increased 10%. Drag increased exponentially in this region.

In your example above - the .82-.84M velocity is Vne. whereas the .75M is the Placard velocity for the P-51. Different airframe manufacturers established the Placard velocity via test dives pushing the envelope then applied a safety factor for the Placard Speed. It (Placard) was the manufacturer's (and AAF/USN) way of saying " you should not go past this speed, but if you do you might survive it - proceed at your peril".

Placard velocity was near or slightly > Mcr, depending on the fighter, but less than Vne. Vne was determined by limit loads due to Dynamic pressure, whereas the Placard Velocity was lower than Vne with a margin of safety

Too many questions and repetitive assumptions to comment on past this point.

 

[Zipper730]

drgondog

Neither, that velocity was well past design limits, Placard speeds and Mcr.

Wait, if the plane couldn't even do Mach 0.83, why was that speed listed?

Mcr is that freestream velocity at which the velocity over the surface (i.e wing, windscreen, etc) reaches M=1.

Just to be clear, do you know what mach number the plane would either suffer control heaviness of a degree that recovery would be impractical/impossible, or that the tail would get blanked and tuck-under would occur?

Too many questions and repetitive assumptions to comment on past this point.

That makes sense because I was responding to several people

 

[drgondog]

drgondog

Wait, if the plane couldn't even do Mach 0.83, why was that speed listed?

Just to be clear, do you know what mach number the plane would either suffer control heaviness of a degree that recovery would be impractical/impossible, or that the tail would get blanked and tuck-under would occur?

That makes sense because I was responding to several people

Recommended reading:
1. Aerodynamics for Engineeers - Bertin and Smith
2. Introduction to Flight - Anderson
3. Fluid Dynamic Drag - Hoerner
4. Aircraft Design - A conceptual Approach - Daniel P. Raymer

 

[Zipper730]

Recommended reading:
1. Aerodynamics for Engineeers - Bertin and Smith
2. Introduction to Flight - Anderson
3. Fluid Dynamic Drag - Hoerner
4. Aircraft Design - A conceptual Approach - Daniel P. Raymer

Firstly, if you were able to identify which book would be the most valuable, what would you recommend?

Secondly, I'm not sure I'd be able to provide an answer if I read those books as math was never my strong suit (B/C average)

 

[pbehn]

Secondly, I'm not sure I'd be able to provide an answer if I read those books as math was never my strong suit (B/C average)

Why ask a question if you cannot understand the answer? On most aviation subjects drgondog can lose 90% of this forum, on his real specialities he (and some others) can lose 99% of the forum. You are asking for answers to very involved questions about aerodynamics which you admit you dont have the math to follow, when given a simplified answer for a laymans benefit you demand more detail, there is no short cut to knowledge, you cannot know the subject without the math(s), if you don't have the math(s) just accept a potted version because it isnt witchcraft.

 

[Zipper730]

Why ask a question if you cannot understand the answer?

I'm not sure that's entirely accurate

I'd like to re-frame it this way: Why would a person list Mach 0.83 as a number if it was beyond the critical mach number, the placard limit, a point where a loss of control would occur (if applicable at this speed) or in fact the aircraft's maximum mach number without structural failure? As I see it, this number is completely meaningless unless it was based on the shape without factoring in things like structural strength, which is purely theoretical and not data that reflects the facts.

On most aviation subjects drgondog can lose 90% of this forum, on his real specialities he (and some others) can lose 99% of the forum.

Yet some of what he's saying I can actually follow to some extent: There was an earlier thread I posted, then momentarily lost track of, and now I found again

WWII A/C: Maximum Mach Number & Airspeed in Dives

Some of what he was saying got me lost a little, but the basics of it I could actually follow on some level: A thinner wing tends to do better at a higher mach number, a lower camber wing tends to do better at a higher mach number, a wing with the crest further back tends to yield a higher mach number for the same t/c ratio.

Based on what he explained there is a point where the airflow starts to go supersonic on the wing and that's the critical mach number, and on older aircraft, the placard dive speed was a little bit over that to the point and outlined the maximum safe dive speed. The maximum mach number that was allowed was the maximum speed you could dive at without risking the destruction of the aircraft (failure of the frame or component).

You are asking for answers to very involved questions about aerodynamics which you admit you dont have the math to follow

i suppose it would be more accurate to say that if given the mathematics equation I'd probably require guidance on the more complicated stuff as mathematics has never been my strong-suit.

As for the textbooks: I usually would not refer a person to a textbook if I was capable of answering the question; if I could not answer it any other way, I'd list the books and upon request at least, tell them which one I thought was the easiest to grasp and/or the most comprehensive.

It would be wise to point out that it's preferable to read through one textbook over four, especially when you consider that it would be cheaper and I have a substantial book list

 

[pbehn]

As I see it, this number is completely meaningless unless it was based on the shape without factoring in things like structural strength, which is purely theoretical and not data that reflects the facts.
Structural strength is a fact based on the measured strength of the material under discussion and the use to which it is put.

Yet some of what he's saying I can actually follow to some extent: There was an earlier thread I posted, then momentarily lost track of, and now I found again

That is because you are being given a simple explanation boiled down so that a layman can understand

Some of what he was saying got me lost a little, but the basics of it I could actually follow on some level: A thinner wing tends to do better at a higher mach number, a lower camber wing tends to do better at a higher mach number, a wing with the crest further back tends to yield a higher mach number for the same t/c ratio.

i suppose it would be more accurate to say that if given the mathematics equation I'd probably require guidance on the more complicated stuff as mathematics has never been my strong-suit.

As for the textbooks: I usually would not refer a person to a textbook if I was capable of answering the question; if I could not answer it any other way,.

Please dont suggest that some people here cannot explain what you are asking, the problem is you cannot follow the explanations without knowing all the theory behind it and so just ask more questions.

For my tuppence, the limits are for the whole aircraft and while the wings are important there are other factors, a poster here did the calculations on a spitfire and it is the windscreen that is its worst feature.

 

[drgondog]

Anderson's Introduction to Flight

 

[Zipper730]

Please dont suggest that some people here cannot explain what you are asking

I didn't say they couldn't

the limits are for the whole aircraft and while the wings are important there are other factors, a poster here did the calculations on a spitfire and it is the windscreen that is its worst feature.

That would have never occurred to me...

Anderson's Introduction to Flight

Thanks