# Dive limits



## tomo pauk (Mar 24, 2014)

Was thinking of making a dedicated thread for aircraft dive limits. Hopefully, we should have plenty of prime source data for many of the A/C of ww2. To start the ball rolling, here is data for the P-38, along with some other related stuff:

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## Juha (Mar 24, 2014)

Pilot's notes usually has the max permissive dive speeds (IAS) and here is an old thread:
http://www.ww2aircraft.net/forum/flight-test-data/ww2-fighter-critical-mach-speed-802.html


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## tomo pauk (Mar 24, 2014)

Thanks. The idea is to post the excerpts from manuals and tests here.


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## Aozora (Mar 24, 2014)

tomo pauk said:


> Thanks. The idea is to post the excerpts from manuals and tests here.



P-51B/C/D Technical Orders:










P-51D/K Flight Restrictions:


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## Aozora (Mar 24, 2014)

From Westland Wyvern S4 Pilot's Notes -
Weapons Release Limitations:





Other Flight Limits:

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## tomo pauk (Mar 25, 2014)

Thanks 

P-47N dive limits. That variant was without dive flaps, and still dived great, despite the 'old' S3 profile shared with granpa P-47B. And P-43??

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## tomo pauk (Mar 25, 2014)

Dive limits for P-47D-25 through -35:


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## FLYBOYJ (Mar 25, 2014)

Great stuff guys!!! I love to see actual FM data as opposed to what is written in "a book" by an author who probably never flew a plane or read a flight manual. You're gonna find the ability to bust many a myths by researching actual data.

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## tomo pauk (Mar 25, 2014)

Thanks.
Some other A/C: P-39L/M/Q, P-40 (later models) and Hurricane II/IV. Unfortunately, I don't have neatly listed altitude limits:

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## tomo pauk (Mar 25, 2014)

double post


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## tomo pauk (Mar 26, 2014)

Dive limits for the P-61. Unfortunately, the person that draw the diagram mixed up the 10000 and 20000 ft lines.


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## OldSkeptic (Mar 28, 2014)

tomo pauk said:


> Dive limits for the P-61. Unfortunately, the person that draw the diagram mixed up the 10000 and 20000 ft lines.
> 
> View attachment 257531



Love the P-61 one... its maximum dive speed was less than a Mossy XXX maximum level speed....


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## FLYBOYJ (Mar 28, 2014)

OldSkeptic said:


> Love the P-61 one... its maximum dive speed was less than a Mossy XXX maximum level speed....



Remember that's Indicated Air Speed - you'd have to calculate True Air Speed, even then it still might be slower.


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## tomo pauk (Mar 28, 2014)

This is from the picture from pots #1 of this thread:






So the P-61 can dive at 10000 ft and under at 415 + 5x10 = 415 + 50 = 465 mph

Keen eye might also see that the P-38 is at red line at 30000 ft when flying at 440 mph, and while at ~35000 ft when flying at 430 mph; seems like that expecting from the (X)P-38K to be topping 450 mph above 30000 ft might be a tad too much?


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## tomo pauk (Mar 31, 2014)

Dive tests found at Williams' site:
Fw-190 (early model)
Bf-109 (9.3 MB pdf, in German, but numbers should be self-explaining)

('Vw' = Wirkliche Geschwindlikheit = TAS; 'Va' should be IAS ; 'Machzahl = Mach number)

Many thanks, Mike al 

added: dive limits for the Bf-109F - table - should have bearing on problems with wings of the Bf-109F


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## tomo pauk (Apr 1, 2014)

Dive limits for the Spitfire XIV/XIX:


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## tomo pauk (Apr 1, 2014)

Dive limits for the Spitfire XIV/XIX:

View attachment 258182


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## tomo pauk (Apr 1, 2014)

Air speed limits for the Sea Fury 10 and 11 - same limit would be for dive speeds?


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## tomo pauk (Apr 1, 2014)

Dive limits for the Spitfire IX and XVI:






Speed limits for the Mosquito FB.VI:


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## FLYBOYJ (Apr 1, 2014)

8)


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## Mike Williams (Apr 2, 2014)

tomo pauk said:


> Dive tests found at Williams' site:
> Fw-190 (early model)
> Bf-109 (9.3 MB pdf, in German, but numbers should be self-explaining)
> 
> ...



Here's additional material on Me 109 dive:






From Bf 109 G-2, G-4, G-6 Bedienungsvorschrift, page 26 





P-51

Compressibility Dive Tests on P-51D






Spitfire

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## Aozora (Apr 3, 2014)

De H Vampire F Mk I operating limits dive speeds; what was Mod 148?:











Douglas A-26:












Grumman F7F:


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## tomo pauk (Apr 3, 2014)

Great job, people


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## mhuxt (Apr 3, 2014)

tomo pauk said:


> Speed limits for the Mosquito FB.VI:
> 
> View attachment 258185



Those are the post-war limits, 25 mph lower than the wartime equivalent. Will see if I can find my Aussie docco from December '44. I keep trawling around looking for a WWII pilot's notes, never can find any for the VI.


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## thedab (Apr 5, 2014)

What the dive limitations for the 190?


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## mhuxt (Apr 9, 2014)

Found the Aussie doccos I was looking for re: wartime Mossie FB.VI dive speed limits. 

Date on the first page, speeds (in IAS) on second two.

Edit - date is on the second of the three attachments, sorry.

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## OldSkeptic (Apr 12, 2014)

mhuxt said:


> Those are the post-war limits, 25 mph lower than the wartime equivalent. Will see if I can find my Aussie docco from December '44. I keep trawling around looking for a WWII pilot's notes, never can find any for the VI.



Great work everyone. But this one is great, been chasing the mach limit of the Mossie for some time now. Not bad at all. Probably the best of any WW2 twin.


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## mhuxt (Apr 12, 2014)

OldSkeptic said:


> Great work everyone. But this one is great, been chasing the mach limit of the Mossie for some time now. Not bad at all. Probably the best of any WW2 twin.



I think I also have another docco which gives the results for the Mossie of dive tests, expressed in mach number. This is different from the limits imposed in the pilot's notes, I suppose for obvious reasons.

From memory, the maximum test results were around 0.7 mach, or somewhere just north of that.

Again, it's "somewhere on my hard drive", will see if I can find it.


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## mhuxt (Apr 20, 2014)

Found the mach number stuff.









I believe the note at the end about diving limits being 350kts is incorrect, post-war, as above, it seems to have been 370.


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## thedab (Apr 20, 2014)

Tempest V


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## thedab (Apr 20, 2014)

Sea Hornet

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## thedab (Apr 20, 2014)

Corsair I-IV


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## Mike Williams (Apr 20, 2014)

thedab said:


> What the dive limitations for the 190?

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## FLYBOYJ (Apr 20, 2014)

Thanks Mike!!!!


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## thedab (Apr 20, 2014)

Cool thanks


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## Aozora (Apr 20, 2014)

F-82E dive/flight limits:


















Westland Lysander:

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## tomo pauk (May 17, 2014)

Dive limits for the P-63A:

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## tomo pauk (May 19, 2014)

Limits for the F4U:

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## daveT (May 19, 2014)

Great thread on fighter plane dive limits. How about the big bombers?
Anyone got the limits for the B-24, B-17?


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## thedab (Aug 17, 2014)

any one knows,what the dive limit is for the FW190D-9


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## Greyman (Aug 17, 2014)

daveT said:


> How about the big bombers? Anyone got the limits for the B-24, B-17?



I can't help with the American bombers but I have the Pilot's Notes for the following:

Stirling I, III, IV
_(pressure head under fuselage)_ - 325 mph IAS
_(pressure head on mast)_ - 295 mph IAS
_(ASI connected to static vents)_ - 310 mph IAS

Halifax III, VII
_(w/o mod. 1423 or 1448)_ - 280 mph IAS
_(w/ mod. 1423 or 1448)_ - 320 mph IAS

Lancaster I, III, VII, X
360 mph IAS


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## tomo pauk (Jun 17, 2017)

NACA report on the YP-38 full-scale wind tunnel test, along with recomendations to increase the 'speed limit' that goes along with delay of the onset of compressibility:

https://crgis.ndc.nasa.gov/crgis/im...d_Diving_Tendencies_of_the_YP-38_Airplane.pdf


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## Greyman (Jun 17, 2017)

daveT said:


> Great thread on fighter plane dive limits. How about the big bombers?
> Anyone got the limits for the B-24, B-17?



Info from Pilot's Notes I've come across in the intervening years:

Liberator II, IV, V
310 mph IAS

Fortress II, III
_(w/o mod.161)_ - 220 mph IAS
_(w/ mod.161)_ - 270 mph IAS


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## Old Wizard (Jun 17, 2017)




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## daveT (Jun 21, 2017)




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## DarrenW (Dec 26, 2017)

From the wartime F6F-3/5 Pilots Handbook:







This is below the altitude of 10,000 feet. Also, the dive limits of later built F6F-5s (and those with Model F6 Service Change #75 incorporated) were increased by 10 Knots.


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## Zipper730 (Jan 3, 2018)

tomo pauk said:


> Dive limits for the P-61.


There's a lot of confusion: Some of the dive figures indicate around 0.60-0.62 mach, however 415 mph indicated at 25,000 feet indicate almost mach 0.84


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## Zipper730 (Jan 3, 2018)

So far, from analyzing everything I got the following except the Fw-190 because my table only works in increments of 2,500 feet and that doesn't perfectly work out for metric

Chance Vought F4U

275 KIAS @ 30,000 ft: Mach 0.706*
307 KIAS @ 25,000 ft: Mach 0.711*
340 KIAS @ 20,000 ft: Mach 0.715*
375 KIAS @ 15,000 ft: Mach 0.721*
390 KIAS @ 10,000 ft: Mach 0.689*
410 KIAS @ Sea-Level: Mach 0.6198*
I'd almost swear I remember hearing that it started to run into problems at 0.73...

De Havilland Mosquito

*FB.VI Airspeed Limits*

No external ordinance or tanks: 450 mph / 390 kts
External tanks: 400 mph / 388 kts
External ordinance/External fuel tanks half empty/Bomb-Door limit: 350 mph / 304 knots
*Overall Mach Limits*

Mach 0.75: Placard Limit
Mach 0.77: Maximum attempted
Hawker Tempest Mk.V

370 mph indicated @ 30,000 ft: Mach 0.823*
410 mph indicated @ 25,000 ft: Mach 0.823*
450 mph indicated @ 20,000 ft: Mach 0.823*
490 mph indicated @ 15,000 ft: Mach 0.819*
540 mph indicated @ 10,000 ft: Mach 0.829*
The plane's aerodynamic limit in wind-tunnel tests was 0.83, so it came pretty close to the potential

Bell P-63A

525 mph indicated @ Sea-Level: Mach 0.69*
500 mph indicated @ Sea-Level - 10,000 ft: Mach 0.657 - 0.768*
450 mph indicated @ 10,000 - 15,000 ft: Mach 0.691 - 0.752*
400 mph indicated @ 15,000 - 20,000 ft: Mach 0.669 - 0.732*
350 mph indicated @ 20,000 - 25,000 ft: Mach 0.64 - 0.705*
300 mph indicated @ 25,000 - 30,000 ft: Mach 0.604 - 0.669*
250 mph indicated @ 30,000 - 35,000 ft: Mach 0.558 - 0.621*
De Havilland Sea Hornet

350 KIAS @ 5,000 - 10,000 ft: Mach 0.571 - 0.618*
340 KIAS @ 10,000 - 20,000 ft: Mach 0.600 - 0.715*
260 KIAS @ 20,000 - 30,000 ft: Mach 0.547 - 0.667*
260 KIAS @ 30,000 - 41,500 ft: Mach 0.667 - 0.832*, **

Footnotes
*: Mach number is based on a table I have and rounded to the third digit
**: The aircraft only goes up to 41,500 feet if I recall and I have 40000 and 45,000 feet.


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## DarrenW (Jan 3, 2018)

Zipper730 said:


> So far, from analyzing everything I got the following except the Fw-190 because my table only works in increments of 2,500 feet and that doesn't perfectly work out for metric
> 
> Chance Vought F4U
> 
> ...




This is some very interesting stuff. I see that some flight manuals give a never exceed or max dive speed with no mention of altitude. I assume that if it's not stated then the speed is at an altitude below 10,000ft. Would make sense. The pilot would then have to calculate what that speed would translate to at any given altitude, knowing that IAS and TAS differ with changes in altitude.


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## grampi (Jan 3, 2018)

Where any fighters able to break the sound barrier in a dive?


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## Shortround6 (Jan 3, 2018)

The F-86??


No piston engine fighter could come close and no WW II jet could either. Even coming close (over 0.90 mach) would result in loss of control and/or the plane coming apart. 

A Spit once hit 0.92 but lost it's prop doing it and the pilot only recovered control after loosing thousands of feet of altitude. And then climbing thousands of feet due to being tail heavy





Wings were also bent slightly.
People may very well argue with the actual accuracy of the reported speed but no other piston engine aircraft came close.


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## Zipper730 (Jan 6, 2018)

Shortround6 said:


> The F-86??


That's right, even the prototype could


> A Spit once hit 0.92 but lost it's prop doing it and the pilot only recovered control after loosing thousands of feet of altitude. And then climbing thousands of feet due to being tail heavy


I think you might be mixing up several events

The Spitfire VIII was involved in numerous dive tests due to the new steel spar which theoretically allowed dynamic loads to something like 825 mph (I'm not sure if the wing was remotely aerodynamic to that point, it'd be around Mach 1.25), the fuselage, tail and prop could not take it even if the wing could.
Placard limit was 0.85 Mach
There was one dive where the prop tore loose and the pilot ended up gliding the plane in: The plane pitching up I don't recall, but I do remember the loss of the prop
One dive reached a successful speed of Mach 0.891
A Spitfire PR was modified with an all-moving tail to test the validity of the concept for the Miles M.52, and achieved 0.92 Mach
A Spitfire PR lost control at high altitude after stalling: The aim was to test environmental conditions at high altitude; the pilot ended up overspeeding the airplane but pulled out and later on it was found he had acheived 0.94. It was 1952 if I recall


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## fubar57 (Jan 6, 2018)

Spitfires Helped Lead the Way to Supersonic Flight

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## pbehn (Jan 6, 2018)

None of these high mach values are in any way verifiable as far as Mach number because they used pitot tubes in a dive. They were certainly going very fast but when your method of recovery is to scrap the propeller and therefore change the trim then you have survived a potentially fatal accident not set a record. The Spitfire was a great aeroplane, it doesn't need outlandish claims of breaking or coming close to breaking the sound barrier to gild its reputation.

Chuck Jaeger was dismissive of anyone claiming to have broken the sound barrier in WW2 aircraft and he more than most should know. For me I have never seen conclusive proof that any Spitfire ever broke its "placard limit", the one that lost its prop and bent its wings certainly proved why such limits exist, it may have broken it, but it was wrecked and that proves a point.

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## Zipper730 (Jan 6, 2018)

fubar57 said:


> Spitfires Helped Lead the Way to Supersonic Flight


It's honestly a good question who did break the sound-barrier first: Honestly I wouldn't be shocked if an Me-262 pilot did it in April 1945


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## Shortround6 (Jan 6, 2018)

Zipper730 said:


> It's honestly a good question who did break the sound-barrier first: Honestly I wouldn't be shocked if an Me-262 pilot did it in April 1945


 Most us would be shocked, Much like the bent and battered Spitfire any 262 that got that close to the sound barrier would have gone into lawn dart mode. 
The plane would have been uncontrollable by the pilot and either crashed or broken up in flight before actually making a sonic boom. 

There are several modern built Me 262s flying with General Electric J-85 engines (or commercial equivalent) and they have same placard never exceed speed as the WW II versions. ANY speed in excess of the placard speed and the owner/operator has entered the world of test pilot. 

Please note there were a number of jets built in the late 40s that were as fast or faster than the Me 262 and most of them are never reported as breaking the sound barrier, even in a dive.

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## pbehn (Jan 6, 2018)

Zipper730 said:


> It's honestly a good question who did break the sound-barrier first: Honestly I wouldn't be shocked if an Me-262 pilot did it in April 1945


Chuck Jaeger did. A Grand slam bomb which was basically an aerofoil weighing 10 tons didn't go supersonic dropped from 18,000 feet. The sound barrier is like an exponent, the closer you get to it the harder it is to get even closer and still harder to pass it. At my fittest I could run 100m in 12 seconds, some fast soccer players can do it in 10.5 seconds, the best sprinters go under ten seconds, how close are they to Usain Bolt at his best? In practice, not close at all, that last half second means they don't make the photo finish. Mach 0.9 or 0.94 in a dive measured by a pitot tube, is actually not even close.


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## Zipper730 (Jan 7, 2018)

Shortround6 said:


> Most us would be shocked


Okay, to be clear -- we're talking manned vehicles: Bullets have been going supersonic since the start of the 20th century if not earlier...


> any 262 that got that close to the sound barrier would have gone into lawn dart mode. The plane would have been uncontrollable by the pilot and either crashed or broken up in flight before actually making a sonic boom.


I'm just basing it on a claim that a pilot made back in 1945 involving a mach-jump effect he noted (well he noted the speed gauge hovered for awhile then increased), when it was discussed in the subject of supersonic flight, he mentioned the matter and with everything said he speculated he might have went through.

A recent study said it was theoretically possible but it only would apply under a very limited set of circumstances: I do remember it being stated that he pulled back on the stick and then used stab-trim to add to it. This could damage the plane, but I'm not sure if it would always happen.



pbehn said:


> Chuck Jaeger did.


And in level flight, which is still a bonus point.


> A Grand slam bomb which was basically an aerofoil weighing 10 tons didn't go supersonic dropped from 18,000 feet.


The idea was to drop it from 40,000 or 45,000 feet initially


> Mach 0.9 or 0.94 in a dive measured by a pitot tube, is actually not even close.


Actually the 0.94 figure was done with elaborate atmospheric testing equipment (remember how I said it was being used?), though this was in 1952.


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## pbehn (Jan 7, 2018)

Zipper730 said:


> The idea was to drop it from 40,000 or 45,000 feet initially
> Actually the 0.94 figure was done with elaborate atmospheric testing equipment (remember how I said it was being used?), though this was in 1952.


My point was that it had no appendages at all apart from 4 comparatively minute tail fins. If it was an aircraft with wings, a tail and pilot dropped from 40,000ft by the time it reached 20,000 ft it would have to be pulling out because it weighs ten tons.


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## Graeme (Jan 7, 2018)

Zipper730 said:


> Actually the 0.94 figure was done with elaborate atmospheric testing equipment (remember how I said it was being used?), though this was in 1952.



That was Edward Powles...

Edward Powles - Wikipedia

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## Graeme (Jan 7, 2018)

Zipper730 said:


> The idea was to drop it from 40,000 or 45,000 feet initially



From which aircraft?


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## pbehn (Jan 7, 2018)

Graeme said:


> From which aircraft?


I have read the same, I think it was from an aircraft Barnes Wallace would have designed himself, it was a theoretical or preferred height for maximum effectiveness of the bomb.

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## Graeme (Jan 7, 2018)

Ahhhh....thanks Pbehn.


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## pbehn (Jan 7, 2018)

Graeme said:


> Ahhhh....thanks Pbehn.


The Grand Slam and Tallboy were "earthquake" bombs, the idea was to explode deep underground and create a "camouflet" or void which undermined the target, to do this obviously needs a sort of armoured bomb and a minimum velocity depending on the target. In the case of the attack on the V3 cannons in the Pas de Calais the perfect strike was to miss narrowly all the concrete covering and undermine the whole construction, as with an attack on a bridge, not only was the above ground structure destroyed but the foundations were too so a quick reconstruction wasn't possible.


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## pbehn (Jan 7, 2018)

Graeme said:


> That was Edward Powles...
> 
> Edward Powles - Wikipedia
> 
> View attachment 478493


That is a great graph Graeme but, with respect, I don't believe it, for a start it is too damned neat.


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## Zipper730 (Jan 8, 2018)

Graeme said:


> That was Edward Powles...
> 
> Edward Powles - Wikipedia


Wow, I didn't know it was 0.96... yikes that's fast for a propeller driven plane (hell, I've never flown that fast and I've only been in a propeller plane once)!

Were there any aerodynamic differences to the Spitfire from WWII to this point?


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## Zipper730 (Jan 8, 2018)

Graeme said:


> From which aircraft?


Regarding the Grand-Slam bomb, that was a bomber designed by Barnes Wallace known as the "Victory Bomber", it was a six-engined aircraft of about the same length as a B-29, with beautiful elliptical wings of high aspect ratio, and a fuselage that kind of looked like a baleen wale: From above it was clearly gorgeous, from the side it was ugly as sin 

It's performance nonetheless was quite remarkable: While carrying a 22,000 pound bomb, it's radius of action was well in excess of 1,500 miles with a top speed of around 340-350 miles an hour. The massive surplus range would allow it to easily strike any target in Germany and, as a result, it would climb partially to altitude while in UK airspace; then it would continue to Germany: Bomb release would occur at between 40,000 to 45,000 feet and, being unburdened of 22,000 pounds of weight, it would continue climbing to altitudes of 50,000 feet or greater on the way back. It would start descending once out of enemy air space and land somewhere in the UK.

It was supposedly inspired by the high altitude Wellington proposals, and had some of the same problems. Due to the geodetic construction, they could not incorporate a pressure hull like the USAAF's B-29's. So they basically had pressure hull inserted into the fuselage with the forward fuselage wrapped around it (hence it's shape).

The pressure hull had a pair of bubble-canopies up top for the pilot and navigator, and a window aimed downward which was for the bomb-aimer (known everywhere else as a bombardier). From what I remember, there were problems on the high-altitude Wellington variants with frost forming on the windshield (this seemed to occur on the later Canberra as well). Since it's altitude was very high and, with a top speed approaching some fighters, it was felt to only really be vulnerable from behind: As a result, it only had a defensive turret in the tail. It was a quad 0.303 similar to that used on the Vickers Wellington but due to the pressure hull, it was remotely controlled. The system was sighted through a periscope mounted in a transparent enclosure in the tail.

Wallis wanted to get the design into the air as quickly as possible, so he pushed to design it around a single bomb: This didn't go over well with the Air Ministry, so he eventually agreed to propose other armaments (some went up to 29,000 or 32,000 pounds), but it was too late.


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## Greyman (Jan 8, 2018)

Yipes


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## grampi (Jan 8, 2018)

I'm surprised it was tried in a Spitfire...I would've thought the Mustang would've been a better choice...


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## pbehn (Jan 8, 2018)

grampi said:


> I'm surprised it was tried in a Spitfire...I would've thought the Mustang would've been a better choice...


I believe the Spitfire had a higher theoretical mach number but also in to post war years the P51 Mustangs under lend lease had been returned written off or scrapped.


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## grampi (Jan 8, 2018)

pbehn said:


> I believe the Spitfire had a higher theoretical mach number but also in to post war years the P51 Mustangs under lend lease had been returned written off or scrapped.


I guess I don't know what determines a plane's mach number, but wasn't the Mustang slicker (less drag) than the Spit?


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## pbehn (Jan 8, 2018)

grampi said:


> I guess I don't know what determines a plane's mach number, but wasn't the Mustang slicker (less drag) than the Spit?


Aerodynamics isn't my major but I have read here that although the Mustang had less drag at most speeds at the absolute limit the thinner wings of the Spitfire held an advantage. However at these speeds propellers start falling off and controls stop controlling.

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## DarrenW (Mar 9, 2018)

Can someone explain how the P-38 can have the highest acceleration during a dive but end up near the bottom of the list when comparing maximum dive speeds? I assume it's about the effects of compressibility but I'm unsure. If I'm correctly understanding what Dean is explaining in his book, at the start of a power dive the P-38 would initially leave the P-47 behind, but eventually the Thunderbolt would catch up and overtake it because it has a higher max dive speed and wouldn't have to throttle back as early as the Lightning to avoid structural damage. Is this it in a nutshell?

Also, why would he use data for a P-38G in one category and switch to a P-38 J/L model in another? Didn't the latter have dive flaps so it could have better control in a dive situation? I'm assuming the P-38G's dive limit is somewhat lower than the J/L model because it lacked these same flaps.




. Does this seem about right?


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## Juha2 (Mar 9, 2018)

DarrenW said:


> Can someone explain how the P-38 can have the highest acceleration during a dive but end up near the bottom of the list when comparing maximum dive speeds? I assume it's about the effects of compressibility but I'm unsure. If I'm correctly understanding what Dean is explaining in his book, at the start of a power dive the P-38 would initially leave the P-47 behind, but eventually the Thunderbolt would catch up and overtake it because it has a higher max dive speed and wouldn't have to throttle back as early as the Lightning to avoid structural damage. Is this it in a nutshell?
> 
> Also, why would he use data for a P-38G in one category and switch to a P-38 J/L model in another? Didn't the latter have dive flaps so it could have better control in a dive situation? I'm assuming the P-38G's dive limit is somewhat lower than the J/L model because it lacked these same flaps.
> 
> ...



Airfoil used in P-38 and how the junction between fuselage and wings was made were IIRC the main reasons of the low critical Mach number of P-38. The function of dive flaps was to enable the recovery from high speed dive IMHO they didin't rise the critical Mach number, simply allowed getaway from possibly fatal situation.

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## DarrenW (Mar 9, 2018)

Thanks Juha2 for your reply. I was wondering about the dive limit of the P-38. I have seen 420 mph IAS for 10,000ft and below but apparently this can be increased to 440 mph indicated with dive recovery flaps extended.

And I'm not sure if we are comparing apples to apples with regard to these max dive speeds of different aircraft because many placards will give a range of altitude (in the case of the P-38 they are in 10,000ft increments). If we are talking about the limit _at _10,000ft one could either quote the figure given for 10,000ft and below (420 mph) _or_ 10,000- 20,000ft (360 mph), but of course the limit would be 60 mph less if we use the latter approach.


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## tomo pauk (Mar 9, 2018)

DarrenW said:


> Can someone explain how the P-38 can have the highest acceleration during a dive but end up near the bottom of the list when comparing maximum dive speeds? I assume it's about the effects of compressibility but I'm unsure. If I'm correctly understanding what Dean is explaining in his book, at the start of a power dive the P-38 would initially leave the P-47 behind, but eventually the Thunderbolt would catch up and overtake it because it has a higher max dive speed and wouldn't have to throttle back as early as the Lightning to avoid structural damage. Is this it in a nutshell?



The P-38G-L will have more power than P-47C-D, for about the same weight. Talk 2650-3200 HP vs. 2000-2600, on 130 grade fuel. Thus it will have better initial accelration.
P-38 will also out-climb the P-47C/D for the same reason.

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## Zipper730 (Mar 9, 2018)

DarrenW said:


> Can someone explain how the P-38 can have the highest acceleration during a dive but end up near the bottom of the list when comparing maximum dive speeds?


The aircraft is fairly heavy and, until mach effects appear, fairly streamlined so air has little ability to impose significant resistance to keep the plane from picking up speed very fast with gravity pulling it down combined with the thrust of the aircraft's propellers and exhaust system (which is little as it has a turbocharger).

The propellers produce enough thrust to have allowed speeds of over 400 mph in the prototypes, around 395 in mass production models (extra weight, some drag produced by the bullet proof glass), and up to around 420 mph in later variants, gravity effectively provides (at least in a 90-degree dive) 1g of acceleration, turning every pound of aircraft into a unit of thrust pushing downwards at around 32 feet every second, with little resistance.

After the filleting of the wing/body junction, the critical mach number was around 0.65 (airflow hits mach 1 on some part of the wing) drag divergence mach is around 0.67-68 (airflow is now supersonic on some parts of the wing, shockwave strength reaches a point where it can drain enough energy out of the airflow to cause some separation), which causes an increase in drag, as well as a decrease in control effectiveness. The aircraft's loss of control occurs at around 0.74 mach. While gravity still is pulling it down, the drag from the airframe has gone up, and the efficiency of the propellers are affected by airspeed and mach numbers (which is why flying up high helps to a point, though mach effects do start to kick in and, past a certain point, it'll reduce the available net thrust) and by this point the net thrust of the propellers are less than the drag produced by the aircraft. I'm not sure if 0.74 is terminal velocity (a speed at which drag and thrust equal each other, gravity in this case is thrust as well), but going straight down without a means of recovery is quite terminal!

Since the speed of sound is affected by temperature, as the airplane reaches warmer air, some control effectiveness is restored, but propeller thrust also increases for the same reason (not much, but with gravity doing it's thing, it's enough to get you back to 0.74), so once the control is restored one would want to start pulling back, at first with everything you got, then progressively less (as the aircraft comes out of the dive, it will stop accelerating as gravity is being countered by lift to a degree, propeller thrust is fairly low, overall airflow over the aircraft is significant, and lift actually increases drag) to avoid over stressing the airframe (once it slows down enough, you'll regain full control and be well above the maneuvering speed, yanking the stick all the way back above the maneuvering speed risks snapping off the wings).

Long message short: The plane accelerates very quickly into mach-tuck! The P-38's problems with mach tuck had to do with two things

Thick wings: This is largely due to the high aspect-ratio (8.26), which favors a thick structure to avoid excessive aeroelastic flexing
The junction between the fuselage and the wings
The wings are convex on top, which causes the airflow to accelerate over them
The fuselage is convex in shape, causing the airflow to accelerate to either side it
The two combine together to produce an unusually high velocity airflow (I'm surprised this isn't thought of as an early example of area rule issues)

Prior to the filleting of the wing/boom junction, the critical mach number was actually lower (supposedly, all the way down to 0.59)

An airplane like the Spitfire has less mass, which means it won't pick up speed quite as fast, but it can ultimately tolerate a higher mach number, so it will accelerate a lot longer before it runs into mach tuck or airframe destruction: While this does sort of contradict what we were all taught in school that all objects fall at the same speed, this fails to factor in

Air resistance: A heavier object is not affected as much by the mass of the air. If not raindrops would probably kill us.
Gravity: This only applies with objects of significant mass, but technically they would both attract each other, and as a result would hit faster than if only one attracted the other.

BTW: Edited to properly include units that are consistent

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## DarrenW (Mar 9, 2018)

Thank you Zipper for the outstanding explanation. So I guess it depends on the situation that you find yourself in. If you just need to make a quick break away during a dogfight and are not being followed too aggressively than a plane like the Lightning would be what one would select. But if you are alone and fighting your way back to safer skies, with the enemy in chase and bent on your ultimate destruction, than a plane with a higher critical Mach number is needed in order to eventually outdistance your pursuer. If you start with enough of a lead of course.


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## DarrenW (Mar 9, 2018)

tomo pauk said:


> The P-38G-L will have more power than P-47C-D, for about the same weight. Talk 2650-3200 HP vs. 2000-2600, on 130 grade fuel. Thus it will have better initial accelration.
> P-38 will also out-climb the P-47C/D for the same reason.


 Thanks for the nice summation Tomo. Now it makes perfect sense to me.


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## Glider (Mar 9, 2018)

The one that gets my attention is the F4U1-D. It wouldn't take much of a dive to exceed 443 when things get risky


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## DarrenW (Mar 9, 2018)

Glider said:


> The one that gets my attention is the F4U1-D. It wouldn't take much of a dive to exceed 443 when things get risky


 I was thinking the very same thing. I believe that, like the F6F-5, the quoted speed in the book for the Corsair is for altitudes between 10-15k and at the high end would mean the aircraft would be traveling at a true airspeed of 543 mph or around 0.75 Mach. That's really clipping along!


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## pbehn (Mar 9, 2018)

DarrenW said:


> Thank you Zipper for the outstanding explanation. So I guess it depends on the situation that you find yourself in. If you just need to make a quick break away during a dogfight and are not being followed too aggressively than a plane like the Lightning would be what one would select. But if you are alone and fighting your way back to safer skies, with the enemy in chase and bent on your ultimate destruction, than a plane with a higher critical Mach number is needed in order to eventually outdistance your pursuer. If you start with enough of a lead of course.


Things generally go in a different direction, pilots were in the plane they were sat in, it wasn't a choice, from that the best choice of action for a given combat situation evolved. Individual pilots, squadrons and whole air forces developed tactics/gambits to maximise strengths and minimise weaknesses.


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## pbehn (Mar 9, 2018)

Zipper730 said:


> The aircraft is fairly heavy and, until mach effects appear, fairly streamlined so air has little ability to impose significant resistance to keep the plane from picking up speed very fast with gravity pulling it down combined with the thrust of the aircraft's propellers and exhaust system (which is little as it has a turbocharger).
> 
> The propellers produce enough thrust to have allowed speeds of over 400 mph in the prototypes, around 395 in mass production models (extra weight, some drag produced by the bullet proof glass), and up to around 420 mph in later variants, gravity effectively provides (at least in a 90-degree dive) 1g of acceleration, turning every pound of aircraft into a unit of thrust pushing downwards at a little under 9.81 m/s every second, with little resistance.
> 
> ...


Zipper, you obviously know quite a lot but please take more care with units and descriptions. You cannot leap so freely between weight, density and mass especially since gravity is such a major part of this discussion.

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## DarrenW (Mar 9, 2018)

pbehn said:


> pilots were in the plane they were sat in, it wasn't a choice



Yes of course. I was talking figuratively, not literally....


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## DarrenW (Mar 23, 2018)

I came across something interesting the other day. Apparently the do not exceed speed for the late-built F6F-5s was very similar to later model P-47Ds:

F6F Pilot Handbook 1 May 1946:






440 Knots = 506 mph

P-47D (variants 26 thru 35) Pilot's Instructions 25 Jan 1945:





Neither correction table in these two manuals had figures for 500 mph, but if they remained fairly constant from 300 mph the P-47D would edge out the F6F-5 by a mere 9 mph.

Now this of course got me thinking. Why would the P-47, possessing such a fearsome reputation in a dive, have a maximum do not exceed speed comparable to other high performance fighters of it's era? What set it apart from the herd?

Then I read this from an article called _Flight Test Comparison...Ending the Argument_, found in the June 1990 edition of the EAA's Sport Aviation magazine, and it seems that how an airplane _behaves_ in a dive may be just as crucial as it's maximum safe dive speed and overall acceleration:






In essence there was far less pilot input required with the P-47 than the other types during these tests which, besides instilling the pilot with extra confidence to push his machine even harder, allowed for a faster build up of speed (hence acceleration) as it was easier to keep the airplane under control as it dove in a straight line. I would think this is a huge factor why it is the first airplane people think of when discussing the diving ability of WWII airplanes.....

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## Zipper730 (Apr 20, 2018)

pbehn said:


> Zipper, you obviously know quite a lot but please take more care with units and descriptions. You cannot leap so freely between weight, density and mass especially since gravity is such a major part of this discussion.


I already edited the message to ensure that all the measurements are now in imperial units.

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## Zipper730 (Jul 4, 2021)

1. I should have really adjusted my numbers to factor errors in speed gauge with early calculations in dive performance
2. I have a mach/airspeed calculator that I'm printing into a table


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## drgondog (Jul 4, 2021)

DarrenW said:


> I came across something interesting the other day. Apparently the do not exceed speed for the late-built F6F-5s was very similar to later model P-47Ds:
> 
> F6F Pilot Handbook 1 May 1946:
> View attachment 487105
> ...


I just saw this. Prior to dive flap installation in both P-47D (-30) and P-38J (25) both ships experienced pitch changes (Delta Cm) due to traveling CP in transonic range. The P-47C/D experienced nearly the same pitch down/long recovery difficulty as the P-38 until the dive flap was installed. 

The Mustang experienced light 'pitch down' Cm changes due to the NAA/NACA 45-100 airfoil upon which the velocity gradient was lower (than more conventional P-47 and P-38 airfoils with T/C max @~ 26% from LE to T/C max =37%. The airfoil design for the Mustang was a dominant reason for delayed Transonic Parasite Drag increases @ .55M to 0.75M.

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## Zipper730 (Jul 4, 2021)

drgondog said:


> I just saw this. Prior to dive flap installation in both P-47D (-30) and P-38J (25) both ships experienced pitch changes (Delta Cm) due to traveling CP in transonic range. The P-47C/D experienced nearly the same pitch down/long recovery difficulty as the P-38 until the dive flap was installed.
> 
> The Mustang experienced light 'pitch down' Cm changes due to the NAA/NACA 45-100 airfoil upon which the velocity gradient was lower (than more conventional P-47 and P-38 airfoils with T/C max @~ 26% from LE to T/C max =37%. The airfoil design for the Mustang was a dominant reason for delayed Transonic Parasite Drag increases @ .55M to 0.75M.


From what I remember, the P-47 could dive somewhat faster off the bat: I'd almost swear they were able to get to 0.78 in the P-47B/C variants (it was outside the placard limit, but they could recover).


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## drgondog (Jul 4, 2021)

Zipper730 said:


> From what I remember, the P-47 could dive somewhat faster off the bat: I'd almost swear they were able to get to 0.78 in the P-47B/C variants (it was outside the placard limit, but they could recover).


Both the 51 and 47 were near equal in terminal dive. Yeager and Hoover made many such comparisons post War while stationed at Wright Pat.

The top recorded dive for 51D was .85M with significant structural damage that caused it to be salvaged after the flight.

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## Zipper730 (Jul 4, 2021)

drgondog said:


> Both the 51 and 47 were near equal in terminal dive. Yeager and Hoover made many such comparisons post War while stationed at Wright Pat.
> 
> The top recorded dive for 51D was .85M with significant structural damage that caused it to be salvaged after the flight.


Yeah, Greg's Airplanes & Automobiles listed a document (I think it was for the P-47, but I could be wrong) that specified 0.745 as a mach number. That's very very close to the P-51's placard limit of 0.75. I think that's 3-1/2 miles an hour difference at altitude


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## mig-31bm (Dec 11, 2021)

tomo pauk said:


> Limits for the F4U:
> 
> View attachment 263096
> 
> ...


Does the later F4U-5 has the same top speed limit for diving as F4U-1?


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## tomo pauk (Dec 11, 2021)

mig-31bm said:


> Does the later F4U-5 has the same top speed limit for diving as F4U-1?



I don't know.


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## DarrenW (Dec 18, 2021)

mig-31bm said:


> Does the later F4U-5 has the same top speed limit for diving as F4U-1?


The F4U-5 was the first Corsair with all metal wings (no fabric panels) and this obviously would affect the dive limit in a positive way. I'm sure if we could locate a F4U-5 pilot's manual there were be G limit charts that we could examine.


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## mig-31bm (Dec 19, 2021)

DarrenW said:


> The F4U-5 was the first Corsair with all metal wings (no fabric panels) and this obviously would affect the dive limit in a positive way. I'm sure if we could locate a F4U-5 pilot's manual there were be G limit charts that we could examine.


I feel like F4U-5 manual doesn't exist , i cant locate it anywhere though. Is there a photo comparison of F4U-5 and F4U-1 side to side


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## FLYBOYJ (Dec 19, 2021)

mig-31bm said:


> I feel like F4U-5 manual doesn't exist , i cant locate it anywhere though. Is there a photo comparison of F4U-5 and F4U-1 side to side













Vought Aircraft


Military Aircraft,Vought Aircraft,Aircraft Manuals




www.eflightmanuals.com

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## mig-31bm (Dec 20, 2021)

FLYBOYJ said:


> View attachment 652190
> 
> 
> 
> ...


I found this:
Is this the dive limit?


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## FLYBOYJ (Dec 20, 2021)

mig-31bm said:


> I found this:
> Is this the dive limit?
> View attachment 652262


Not sure - it mentions nothing about these values being indicated in a dive


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## pbehn (Dec 20, 2021)

FLYBOYJ said:


> Not sure - it mentions nothing about these values being indicated in a dive


I am normally OK with graphs, that one hurts my head about what it means.

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## FLYBOYJ (Dec 20, 2021)

pbehn said:


> I am normally OK with graphs, that one hurts my head about what it means.


Yeah, this one is a bit confusing, I've seen worse!


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## DarrenW (Dec 20, 2021)

The dive limit is 425 kts @ 10,000 feet. This is 35 knots faster than the F4U-1D at the same height and gross weight.


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## pbehn (Dec 20, 2021)

FLYBOYJ said:


> Yeah, this one is a bit confusing, I've seen worse!


Eureka!!!! A simple rotation by 90 degrees clockwise and it makes much more sense.

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## drgondog (Dec 20, 2021)

mig-31bm said:


> I found this:
> Is this the dive limit?
> View attachment 652262


This is Vne diagram which addresses banked turns +, negative G limits - axis, AoA loads +up 7.5G, and Q load limits resectively for 120IAS kts to 270kts Indicated at 30000 feet through 425IAS (kts) at 10,000 feet

This diagram works for all increased loads for Bank, Pullout (AoA load) and Dive Q limits for Limit Load of 7.5G design.

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## FLYBOYJ (Dec 20, 2021)

drgondog said:


> This is Vne diagram which addresses banked turns +, negative G limits - axis, AoA loads +up 7.5G, and Q load limits resectively for 120IAS kts to 270kts Indicated at 30000 feet through 425IAS (kts) at 10,000 feet
> 
> This diagram works for all increased loads for Bank, Pullout (AoA load) and Dive Q limits for Limit Load of 7.5G design.


Thanks Bill, once explained the light bulb came on!

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## drgondog (Dec 20, 2021)

FLYBOYJ said:


> Thanks Bill, once explained the light bulb came on!


Lol - Methinks the 'rounded edges on the Q limit were 'disconcerting' to your jaded eyes.

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## Zipper730 (Dec 21, 2021)

The important thing you'd have to factor in is the position & compressibility errors, but if I was to ignore that, I could provide some baseline estimates for the maximum mach number that could be achieved at high altitude.

*Altitude*_____*IAS*_________*TAS*_________*Mach*
10000'______422.5________491.5________0.770
15000'______388-389______489-490_____0.781-0.782
20000'______356__________488_________0.794
25000'______318__________475_________0.789
30000'______271-272______443-445_____0.752


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