WW2-fighter and critical Mach speed

[pbehn]

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)


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.


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

3. In 1952, an atmospheric sampling flight went awry and a maximum speed of 0.94 was achieved, and the plane barely recovered. I'm not sure to what degree the plane took damage.

Zipper I think you are getting to hung up on numbers, as far as I know the first thing to be affected by compressibility is the instrument that measures the planes speed, indicated air speeds of mach 0.92 are a bit of a joke with a pitot tube.

When a planes prop drops off or its wing wrinkles after exceeding maximum permitted speeds I think this speeds were calculated quite well.

From my experience of civil (pipeline) engineering I draw the following conclusions.

A maximum mach number isnt the value that a plane will fall apart or lose control it is the value below which the plane will stay together and have some control, exceed it at your risk.

The values resulting from calculations are useful to make sure there are no serious flaws in design with the technology known at the time, to work on improvements to the design AND to set operating limits for the planes in service. It is all very well to talk about a Spitfire reaching mach 0.92 (or whatever) and landing without a propeller no one ever mentions the many planes under test or in service that broke up or simply went straight into the ground. High speed dives in WW2 aircraft are extremely dangerous in both combat and test scenarios, it is a leap of faith as to whether the plane holds together and pulls out of the dive.

A maximum or minimum limit in my work experience is based on the materials specified yield strength and specified minimum wall thickness. In practice the material is always at or above the minimum yield and the thickness is at or above the minimum. The calculation of a maximum mach number will therefore be a number that sets the limit of what is guaranteed or controlled. You can exceed that number if you are foolish enough but if you are in a military organisation and do it for kicks expect a court martial and if you are a civilian pilot expect to lose your license, if you survive of course.

 

[Zipper730]

Anderson's Introduction to Flight

Is "Fundamentals of Aerodynamics" similar?

 

[drgondog]

Y

Is "Fundamentals of Aerodynamics" similar?

You need a sound background in differential and integral calculus - it is an intermediate but rigorous text that will lead you to Computational Fluid Dynamics with Numerical Methods for solution of non-linear partial differential equations.

I would have to immerse myself for a couple of days to refresh 40 year memory and unused sections on my brain. EDIT - on second thought a couple of weeks minimum to be able to regurgitate the end of chapter tests.

As a rule of thumb, you need senior level math/engineering including advanced calculus, differential equations, matrix theory, finite element relaxation methods, advanced Fluid Mechanics and Aerodynamics to navigate effortlessly for John Anderson's "Fundamentals of Aerodynamics".

If you master this text you are ready for a high paying job.

 

[Zipper730]

So, that would be no then

 

[drgondog]

So, that would be no then

No, they are 'similar' but his Intro to Flight doesn't force knowledge of higher math to master the end of Chapter problems.

 

[Zipper730]

Ok

 

[Zipper730]

I was thinking about something: The version I got was for people overseas, what's the difference between the American and non-American version. It's in English...

 

[swampyankee]

The critical Mach number for the P-38 was .68/.70 one of the earliest of all the topline fighters in WWII. The wing was designed for Fuel storage and lift at a time before wing shapes like the Laminar flow wing on the P-51. It needs to be remembered that It was only criticle above 20,000ft and even the early P-38s were able to keep the enemy plane in sight through the dive if proper procedures were followed - close throttles and go to flat pitch on the props. The P-38 could always catch up on the level.

As I understand it the eliptical wing on the Spit was able to keep the critical speed up.

edit: Interestingly Warren Bodie in his research for his book of the P-38 (I recomend it for all WWIIaviation buffs) The P-38 had less fatalities due to compresability issues than either the P-51 or the P-47 the P-38 was higher profile and was sent to war over a year earlier with much development done in combat for everyone to see any problems.

I'm going to defer to my former co-worker, Dave Lednicer, who, among other things, designed one of the first cambered airfoils for a helicopter main rotor, the RAH-66 Comanche's fenestron, and moved from Sikorsky to AMI, one of the leading companies in the CFD business. He's one of the sharpest aerodynamicists in the business (just ask John Roncz)

"I think that the Spitfire's high drag divergence speed, relative to the Mustang is due to a combination of features, not just the wing or wing airfoil design." -- Dave Lednicer

(Dave Lednicer has performed and published a couple of articles on the CFD analysis of WW2 aircraft:

Lednicer, D., "A CFD Evaluation of Three Prominent World War II Fighter Aircraft," Aeronautical Journal of the Royal Aeronautical Society, June/July 1995.

Lednicer, D. and Gilchrist, I., "A Retrospective: Computational Aerodynamics Analysis Methods Applied to the P-51 Mustang," AIAA paper 91-3288, September 1991.

The quote is from The Spitfire (David Lednicer) http://www.yarchive.net/air/spitfire.html

 

[tomo pauk]

Quote from the linked article:

The Mustang has a 13.8% thick root airfoil and a 11.4% tip airfoil.

That would not be the case. Mustang's wing has the 16% thick rooot, not 13.8%. Listed at, for example, Gruenhagen's book on the P-51.

"I think that the Spitfire's high drag divergence speed, relative to the Mustang is due to a combination of features, not just the wing or wing airfoil design." -- Dave Lednicer

Right on the money, that one.
Spitfire was with rear view mirror, protruding cannon barrels & bulges (when cannons were there), the undercarriage was not fully covered until the lates marks, early marks were with performance-robbing carbs and exhausts + draggy external BP glass, windrscreen was more steep than on the P-51,sometimes the fit & finish left something to be desired for,while the addition of intercoolers and greater cooling capacity invoked far greater drag penalty than it was the case with P-51.
All of this is before we talk about the lightweight Mustangs that further improved sreamlining a bit vs. earlier types.

British have calculated that a Spitfire V with individual exhausts, uncluttered ram air intake, better windscreen, without cannons & bulges protruding (so only 8 MGs), better fit & finish and no rear view mirror would've done above 400 mph, while the Spit IX with similar treatment was supposed to beat 440 mph mark - on par with Merlin Mustang.

 

[Zipper730]

I'm going to defer to my former co-worker, Dave Lednicer

I've heard that name before... if I recall it had to do with an analysis of the B-24's wing.

"I think that the Spitfire's high drag divergence speed, relative to the Mustang is due to a combination of features, not just the wing or wing airfoil design."

What other variables if I may ask?

Spitfire was with rear view mirror

I didn't know that, but I do remember some P-51's had those in WWII...

protruding cannon barrels & bulges (when cannons were there)

Good point

undercarriage was not fully covered until the lates marks

You mean the tail-wheel? I always wondered why the British had those hanging out on so many of their aircraft when we pretty much always tucked 'em in.

early marks were with performance-robbing carbs and exhausts

I'm not sure I follow here...

draggy external BP glass

That's pretty weird, though I didn't notice it until I just did an image search: Why did they put the bullet-proof pane outside the canopy? Most designs seemed to have them forming the forward canopy piece...

windrscreen was more steep than on the P-51

Just to be clear you mean the angle was closer to 90-degrees up than the P-51? Last I checked angling it back would get the lowest drag...

the fit & finish left something to be desired

Was this an RAF problem as a whole?

the addition of intercoolers and greater cooling capacity invoked far greater drag penalty than it was the case with P-51

The spitfire had two radiators on the Merlin 60's (one under one wing which covered engine and oil; the other covering the intercooler) right?

British have calculated that a Spitfire V with individual exhausts, uncluttered ram air intake, better windscreen, without cannons & bulges protruding (so only 8 MGs), better fit & finish and no rear view mirror would've done above 400 mph, while the Spit IX with similar treatment was supposed to beat 440 mph mark - on par with Merlin Mustang.

Fascinating...

 

[wuzak]

What other variables if I may ask?

Tomo went through a number of them.

I didn't know that, but I do remember some P-51's had those in WWII...

Earlier version of the Spitfire had them externally.

You mean the tail-wheel? I always wondered why the British had those hanging out on so many of their aircraft when we pretty much always tucked 'em in.

Cheaper and easier to build.

I'm not sure I follow here...

Early marks of Merlin had carburetors which would cut out under negative G. They also had tight elbows from the carburetor to the eye of the supercharger impeller. Once that was opened out by Sir Stanley Hooker the full throttle height was increased several thousand feet.

That's pretty weird, though I didn't notice it until I just did an image search: Why did they put the bullet-proof pane outside the canopy? Most designs seemed to have them forming the forward canopy piece...

Because, when the Spitfire was originally designed it was not a standard fitment to fighters.

Just to be clear you mean the angle was closer to 90-degrees up than the P-51? Last I checked angling it back would get the lowest drag...

The Spitfire's screen was more upright.

The spitfire had two radiators on the Merlin 60's (one under one wing which covered engine and oil; the other covering the intercooler) right?

Spitfires with 2 stage Merlins or Griffons had two coolant radiators, on on each side. One side also contained the intercooler radiator and the other the oil cooler.

Joe Smith, who took over as lead designer at Supermarine after the passing of Reginald Mitchell, admitted that the Spitfire radiator design was no optimum, due to using 2 position flaps and having the outlet area too big.

 

[tomo pauk]

*

...
You mean the tail-wheel? I always wondered why the British had those hanging out on so many of their aircraft when we pretty much always tucked 'em in.

Mostly everyone's 1st monoplane fighters have had tail wheel made non-retractable, including the US types like WIldcat and Buffalo. Mid-war fighters, including the British types, including the Spitfire VII/VIII, were with retractable tail wheel. Fixed tail wheel is cheaper and faster to produce, thugh

<carb issues>I'm not sure I follow here...

The carbs used before 1943 on the Merlins were could not be heated, thus the need to protect them from the icing arose. The Protection was the metallic oval ring in front of the carb, called "ice guard". It messed with air flow, cost was 8 mph on the Spitfire V. The pressure injection carbs, standard issue on American engines, called 'fuel pumps' in the UK, could be heated, thus the ice guard can be dispensed with*. Problem was that there was few and far between Spitfire Vs (main fighter of the RAF in 1941 to 1944) with new carbs. The Spitfire IX was with better carbs pretty early, at least it is how I get it.

That's pretty weird, though I didn't notice it until I just did an image search: Why did they put the bullet-proof pane outside the canopy? Most designs seemed to have them forming the forward canopy piece...

BP glass was not on original spec. Battle experiences showed the need, so it was retrofitted in the most expedient way, and carried so with Spitfire V. The Bf 109 was also with external BP glass.
1st Spit with internal BP glass was the Mk.III, that remained on prototype stage. Serial installation started with Spit VII and IX.

Was this an RAF problem as a whole?

Most effected seems like the Spit V, cost was sometimes up to 11 mph vs. a well finnished example. The earlier and later marks were better, speed loss was under 5 mph for random Mk.IX checked.

added: end result of all of the listed changes the Spit IX got was that drag coefficient was about the same vs. Mk.V, despite adding the intercooler radiator and another coolant raiator

Don't know about other fighters' and bombers' fut & finish, though. Looks like Mossie was the best in this regard

*though someties the airframe and engine producers went to great lengths to insure that there is a dedicated manifold to heat the ram air - just in case?

 

[Zipper730]

Tomo went through a number of them.

True, but he outlined areas that limited the airplane's top-speed, not played a role in the high drag-divergence mach-number being high outside the wings. That's why I said that.

Cheaper and easier to build.

Expediency makes sense, as for cost I have no idea how much a Spitfire cost compared USAAF contemporaries.

Early marks of Merlin had carburetors which would cut out under negative G.

I thought that had to do with the float chamber, and a lack of pressure-injection?

They also had tight elbows from the carburetor to the eye of the supercharger impeller. Once that was opened out by Sir Stanley Hooker the full throttle height was increased several thousand feet.

So the tighter elbow basically negated ram-compression gains? How did the V-1710, R-1820, R-1830, R-2600, R-2800, and R-3350 compare in this regard?

Because, when the Spitfire was originally designed it was not a standard fitment to fighters.

Meaning it was added as the plane was either undergoing development, or was already in service?

The Spitfire's screen was more upright.

Gotcha

Spitfires with 2 stage Merlins or Griffons had two coolant radiators, on on each side. One side also contained the intercooler radiator and the other the oil cooler.

Joe Smith, who took over as lead designer at Supermarine after the passing of Reginald Mitchell, admitted that the Spitfire radiator design was no optimum, due to using 2 position flaps and having the outlet area too big.

So the greater drag didn't have to do with two radiators versus one, merely the cowl-flaps, and the size of the outlet area?

Mostly everyone's 1st monoplane fighters have had tail wheel made non-retractable, including the US types like WIldcat and Buffalo.

And the Seversky P-35

Mid-war fighters, including the British types, including the Spitfire VII/VIII, were with retractable tail wheel.

The carbs used before 1943 on the Merlins were could not be heated, thus the need to protect them from the icing arose. The Protection was the metallic oval ring in front of the carb, called "ice guard". It messed with air flow, cost was 8 mph on the Spitfire V. The pressure injection carbs, standard issue on American engines, called 'fuel pumps' in the UK, could be heated, thus the ice guard can be dispensed with*.

That makes enough sense

 

[wuzak]

So the tighter elbow basically negated ram-compression gains? How did the V-1710, R-1820, R-1830, R-2600, R-2800, and R-3350 compare in this regard?

The tight elbow increased pressure loss and thus negated some of the ram air pressure.

I am not sure how the radials fared, but I believe the V-1710 was poor in this respect.

Meaning it was added as the plane was either undergoing development, or was already in service?

Already in service.

So the greater drag didn't have to do with two radiators versus one, merely the cowl-flaps, and the size of the outlet area?

Having two radiator ducts of larger area certainly had an effect on drag. As did the inlet and exit areas, the flap mechanism and probably the internal and/or external shape of the ducts.

And the Seversky P-35

The P-35, along with several other early attempts of aircraft with retractable landing gear, exposed, at least partially, the landing gear when retracted. The P-35 had quite large fairings for the undercarriage.

 

[drgondog]

I'm going to defer to my former co-worker, Dave Lednicer, who, among other things, designed one of the first cambered airfoils for a helicopter main rotor, the RAH-66 Comanche's fenestron, and moved from Sikorsky to AMI, one of the leading companies in the CFD business. He's one of the sharpest aerodynamicists in the business (just ask John Roncz)

"I think that the Spitfire's high drag divergence speed, relative to the Mustang is due to a combination of features, not just the wing or wing airfoil design." -- Dave Lednicer

(Dave Lednicer has performed and published a couple of articles on the CFD analysis of WW2 aircraft:

Lednicer, D., "A CFD Evaluation of Three Prominent World War II Fighter Aircraft," Aeronautical Journal of the Royal Aeronautical Society, June/July 1995.

Lednicer, D. and Gilchrist, I., "A Retrospective: Computational Aerodynamics Analysis Methods Applied to the P-51 Mustang," AIAA paper 91-3288, September 1991.

The quote is from The Spitfire (David Lednicer) http://www.yarchive.net/air/spitfire.html

For what it is worth, I agree Lednicer's qualifications. AFAIK he is the first person that meshed a Navier-Stokes model with CFD in VSAERO to get closer model results to wind tunnel data..

 

[Shortround6]

The "bend" that Hooker straightened out (or opened up) was between the carburetor and the supercharger.
It had nothing to do with the ram pressure delivered to the mouth of the carburetor. Yes it was a choke point (on a number of engines besides the Merlin) but it affected overall airflow. We very often confuse pressure with mass airflow (volume) because there are times when they are dependent on each other or because with a given supercharger set up volume and pressure correspond rather well. Change the supercharger or change some aspects of it and the relation ship between pressure and volume no longer works. The "new" supercharger could flow more or less air at the pressure.
Very few, if any, superchargers, had good inlets before Hooker and some engines never caught up despite having several years to do so.

 

[wuzak]

The "bend" that Hooker straightened out (or opened up) was between the carburetor and the supercharger.
It had nothing to do with the ram pressure delivered to the mouth of the carburetor. Yes it was a choke point (on a number of engines besides the Merlin) but it affected overall airflow. We very often confuse pressure with mass airflow (volume) because there are times when they are dependent on each other or because with a given supercharger set up volume and pressure correspond rather well. Change the supercharger or change some aspects of it and the relation ship between pressure and volume no longer works. The "new" supercharger could flow more or less air at the pressure.
Very few, if any, superchargers, had good inlets before Hooker and some engines never caught up despite having several years to do so.

Mass flow and pressure are dependent on each other and temperature all teh time.

The tighter elbow causes a greater pressure loss than a wider elbow, and thus reduces mass air flow.

A smaller diameter will also increase the pressure drop across the elbow and reduce flow.

You cannot eliminate the pressure drop across the elbow, but you can minimise it.

 

[pbehn]

Great discussion, just saying.

 

[swampyankee]

One of the problems with the Allison V-1710, for a few iterations, was that it didn't have a very good supercharger. Centrifugal compressors -- that's what all the aircraft used for supercharging -- need an efficient diffuser to get best efficiency. For some reason, Allison had a lot more trouble getting their diffuser sorted out than did Rolls-Royce. I don't know if they used their own, or sourced it from a specialist vendor, but I remember reading that both Pratt & Whitney and Wright had become dissatisfied with the superchargers they were buying and transitioned to in-house design and fabrication.

 

[Shortround6]

And here we get back to the Army Air Corp and their well over decade long interest in the turbo charger.
Swampyankee is quite correct in saying that P &W and Wright were dissatisfied with the superchargers they were getting from General Electric and started to design their own.
What this meant was that up to the late 30s there was ONLY ONE supercharger company in the United States. And that company was putting more work into the turbo part than the charger part (the compresser). Any problems with the compressor were masked by 1. the abundant power of the turbo provided by the exhaust and 2. with 87 octane gas or less you couldn't use much boost anyway.
One of Hooker's claims to fame (among many) was that he very quickly realized that several of the mathematical formulas used to design superchargers had errors in them. Anybody (and that pretty much means everybody up to 1939/40) using those formulas wasn't designing very good superchargers.
I would note that the Allison supercharger was actually pretty good for 1939/40 being not that far behind Rolls-Royce (until Hooker and the Merlin XX) and ahead of DB, Junkers, Hispano Suiza, Wright and P & W (except for the P& W two stage and the early ones needed a lot of sorting out) being able to hold 42in (about 6lbs boost) of pressure to 12,000ft.
Allison, in the years when they were making 2 to 3 V-1710 engines per year, did a lot of sub-contracting work for General Electric making supercharger parts. Building parts from blue prints is certainly much easier than designing the supercharger but Allison was certainly starting not much, if any, further behind that P & W or Wright.

The Army believed that only liquid cooled engines would stand up to making high power at altitude using turbos due to cooling problems. Air at 20,000ft being much colder but with only 1/2 the mass per cubic foot cooling may have been doubtful. In the beginning they were probably correct but the liquid cooled engines took a long to develop and the Air cooled engines were improving almost by the year.
Wright for example in the eight years between 1931 and 1939 on the Cyclone 9 went from Cylinder fins pitched at 0.375 in to 0.21 in and the length grew from about .75 in to 2.25in with the cooling area of the cylinder head going from 600 sq in in 1931 to 2300 sq in in 1939. The change in total cylinder cooling area between the R-1820 G-100 series (1100hp take-off) and the R-1820 G200 series (1200hp take-off) was from 2800 sq in per cylinder to 3510 sq in.
Other companies around the world were doing similar things. Bristol for example went through at least 5 different cylinder heads in 10 years with the last one having 40% more cooling area than the next to last let alone where they started.
Rolls had been making superchargers since the late 20s and their racing experience may come into play with designing for high boost pressures.