WW2-fighter and critical Mach speed

INTERESTING EXCERPTS OF high Speed Dives.

Airplane stability and control: a ... - Google Books

Four things I noted as I read this thread.

1. 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. Later the P-51H was 'straight' with a smaller main gear, enabling the removal of the swept inboard portion.

2. Both the P-47 and P-38 had increased longitudinal stability reduced 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. The resultant movement of CP caused a 'pitch down' moment (or Mach tuck)

In both cases the problem was partially solved by integration of of wing/dive flaps aft of the 1/4 chord point.

3. The Mustang did not have the same effective quantitative change in nose down pitching moment and never exhibited the tuck as the critical mach number was reached. Stick forces to pull out were still high but not due to tuck under.

My theory on this is that the shock wave perhaps started aft of the 1/4 (aerodynamic center) chord as the max thickness of the Mustang laminar flow wing was near the 40% chord location. The Mcr of the Mustang wing was ~ .75M but the max (Flight tested) dive at ~.84M was limited to extreme vibration at this speed, implying structural failure beyond .84M.

If this is true, at the CL required of the wing to pull out, there was no resulting change in CMac causing it to pitch down.

4. The reason the low tail (below wing plane) became a standard supersonic adopted design i(for highly swept wing) is the positioning below the wing plane reduced the longitudinal stability increase through the transonic range..

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. 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.


The T-Tail on the other hand is out of the downwash flow behind the wing. So, when the swept wing starts to lose outer wing lift in the transonic range, the pitch up is not counterbalanced by a corresponding reduction in downwash over the tail. Both the MiG 15 and F 104 are classic examples of this issue

The F-100 and the Electric Lightning were early examples of the low tail design to counteract this swept wing transonic behavior.

Wing fences, boundary layer control, vortex generators, etc were design features to reduce spanwise flow and improve low speed stall characteristics of highly swept wings

Holy thread revival Batman!

IIRC part of the big issue with compressability in US new airframe testing in the late thirties was the fact design requirements and thus flight test protocols were still being carried over from the 1920's and hadn't accounted for the tremendous increase in speed capabilities of new experimental models like the XP-38. The USN for example had a flight test protocol in 1939 of zero lift altitude loss of 10,000ft in a power on dive (ie. full throttle, nose pointed at ground). This could kill the pilot of an XF4U but was perfectly fine to try out in an XF3F.
So all of a sudden compressability became the keyword for trying to convince the authorities to update their flight test requirements when speculating on new model prototypes.

To be honest I'm not sure if compressability was such a big issue for combat models following the selection process unless you're discussing boom and zoom attacks from 30,000ft on targets flying at 18,000ft. Then your P-47C and P-38J had issues, but the rest of the time they were good as rain. Thing was P-47 and P-38 a/c would actually cruise for engagement at those kind of altitudes so it was an issue, but most other a/c didn't and if they did many could get away with it easier for critical mach. I think most of the time you're talking about WW2 aerial combat at 15-25,000ft though and there just isn't big critical mach issues unless we start talking about ground attack, where you get the same issue again of large zero lift altitude loss in power on dives.

I don't think critical mach had much bearing on your average Spit vs Messer vs Mustang vs T-bolt vs Lightning vs Dora sort of aerial combat for the vast majority of cases really, I think we're talking about very specialised circumstances and a lot of hype that was related to prewar US flight test protocols and peviously unconsidered dangers when they're not continually updated by aeronautical engineers rather than bureacrats.

We might look at critical mach issues in the life of the T-bolt for example, Lindbergh personally rated the P-47C as unstable and outright dangerous to the pilot at speeds exceeding 450mph in a dive at medium altitude (15,000ft iirc), much less than the manufacturer rating of 550mph at the time, and this was well before and outside of any critical mach issues related to long dives from extreme altitude. It's not fast enough at that altitude to get to critical mach, which wasn't the T-bolt's problem.
Put it this way, I hardly think WW2 combat even in 1944-45 was a matter of fighters running around doing battle at 0.8 mach. More like 350-400mph with plenty of reserve in a tight squeeze and that's the very fastest, I think it's too easy to get confused by maximum performance specifications in publication and assuming that those aircraft could actually engage combat at anything like those speeds. Your 485mph P-51H is still going to be doing combat at 400mph at best, because the bugger still has to do things like turn and evade and deflection shoot, plus if he's really smart try not to blow the engine getting too excited.

vanir said:

Holy thread revival Batman!


To be honest I'm not sure if compressability was such a big issue for combat models following the selection process unless you're discussing boom and zoom attacks from 30,000ft on targets flying at 18,000ft. Then your P-47C and P-38J had issues, but the rest of the time they were good as rain. Thing was P-47 and P-38 a/c would actually cruise for engagement at those kind of altitudes so it was an issue, but most other a/c didn't and if they did many could get away with it easier for critical mach. I think most of the time you're talking about WW2 aerial combat at 15-25,000ft though and there just isn't big critical mach issues unless we start talking about ground attack, where you get the same issue again of large zero lift altitude loss in power on dives.

I don't think critical mach had much bearing on your average Spit vs Messer vs Mustang vs T-bolt vs Lightning vs Dora sort of aerial combat for the vast majority of cases really, I think we're talking about very specialised circumstances and a lot of hype that was related to prewar US flight test protocols and peviously unconsidered dangers when they're not continually updated by aeronautical engineers rather than bureacrats.

We might look at critical mach issues in the life of the T-bolt for example, Lindbergh personally rated the P-47C as unstable and outright dangerous to the pilot at speeds exceeding 450mph in a dive at medium altitude (15,000ft iirc), much less than the manufacturer rating of 550mph at the time, and this was well before and outside of any critical mach issues related to long dives from extreme altitude. It's not fast enough at that altitude to get to critical mach, which wasn't the T-bolt's problem.
Put it this way, I hardly think WW2 combat even in 1944-45 was a matter of fighters running around doing battle at 0.8 mach. More like 350-400mph with plenty of reserve in a tight squeeze and that's the very fastest, I think it's too easy to get confused by maximum performance specifications in publication and assuming that those aircraft could actually engage combat at anything like those speeds. Your 485mph P-51H is still going to be doing combat at 400mph at best, because the bugger still has to do things like turn and evade and deflection shoot, plus if he's really smart try not to blow the engine getting too excited.

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Vanir - it was a real problem at 25,000 feet simpy because a transition from max throttle level flight to critical mach occurred very quickly after a split ess or nose over to catch a diving fighter. The LW had a very difficult time learning that they could not escape a closely following Mustang or Jug like they could a Spit or Hurricane of earlier days, so that was first response to escape..

The .7-.75M was reached very quickly. In the case of the P-38 and fat wing it was in compressibility at ~ .7M

T 25K (STP) M=1 @ 692mph. .7 M= 484mph. If a Mustang is chasing at ~410 to 430 in WEP mode it will very quickly run past .7M.. ditto the P-38 and P-47

All P-47s until the installation of the wing dive flap, exhibited a tuck force causing a dive angle beyond 90 degrees if throttle not pulled immediately entering compressibility - not just the P-47C. The issue was the above discussed increase in longitudinal stability as lift was lost in the mach transition 'burble'.. until lower altitudes were reached and the a/c slowed enough to get full lift profile, the elevators were essentially ineffective - then they transitioned to extreme effectiveness - with serious potential to over stress the ship in pull out if pilot waited too long to release heavy stick force to pull out.

Ditto the P-38 and from what I understand - also the 109 and Fw 190 (not the severe tuck, but same longitudinal increase in stability (and stick forces) while near critical mach).

Fair enough, I guess I probably just find it easier to wrap my head around using historical case examples. Like I said I don't find it all that easy to picture aerial combat nudging much past 400mph in these warbirds unless we're talking about very specific circumstances, I mean note for example your average off the airfield 435mph rated Mustang has to get to optimum altitude, trim for the high speed condition, lay it down for a little while to get the speed up, perform a cooldown, then triple check everything is set and level just right, then do his high speed run and that's how he's rated for 435mph. It just sounds weird to me people are often talking about very high speed chases and critical mach when seeing 350mph during combat is a breakout.
But yeah I guess if we're mixing it up at 18,000ft and I decide to break off and run for the deck, full throttle and flat stick I suppose we're going to have compressability issues. With so few Luftwaffe fighters to get some aces stars with in 1945 I can see the wont to chase them down. But let's have a look at say the Eastern Front or Bodenplatte, in these types of aerial battles things like compressability would appear far less of an issue even among 700km/h fighters.

It was just the impression of absolute maximum speed performance in the high speed trim under perfect conditions often with detailed flight preparation, being seemingly used as the standard measure of combat performance which has been weirding me out lately. Say for example, if critical mach was being achieved quite so routinely during regular sorties you'd expect pilots to be dropping like flies.

The buffeting in the p38 was eliminated in a leading edge fillet at the junction of the cockpit and wing. Raising the horizontal stabilizer did nothing.
This buffet was reached before compressibility.
The highest mach no. for any Spit was .9 and the test pilot lost the prop crashed with injuries to his back.

Hello Fibus
first time when Martindale achieved Mach 0.9, or was that M 0.91 or 0.92, he lost the prop with reduction gear of his Mk XI but made normal wheels down landing on the base. His back was injured during a later high speed dive, when IIRC the supercharger exploded, and during a emergency landing he had at last stage evade power lines and he crashed. in spite of his back injuries he managed to save the flight recorder from his burning Spit.

Juha

I am a newbie, please plyasnite means that your phrase (OKM tested).?

The Spitfire's airframe is no stronger than many other WWII fighters and less strong than some. It does have a high critical Mach number relative to many other WWII planes, probably the highest but once the critical Mach number has been reached, the effects of the shock waves render any controls and surfaces not designed for supersonic flight less than good. Since the center of lift shift rearward, the tail is not usually capable of handling the downward force required to stay level. The propeller on a Spitfire would never be able to operate supersonic either.

Level flight at Mach 1.3 doesn't generate much in the way of stress and almost nothing in aerodynamic heating as is the case at faster speeds. The aircraft is still experiencing only 1 g. The challenge is controlling the nose-down moment with the horizontal tail when the center of lift shifts rearward on the wing above Mach 1.

Since the Spitfire never got to Mach 1.3 and supersonic wind tunnels in WWII were virtually nonexistent, where does the claim come from that the Spitfire could handle Mach 1.3? When supersonic wind tunnels were more available, they were used for research into new aircraft, not to study WWII propeller planes at speeds they would never see. So I am wondering how anyone comes to the conclusion that the Spitfire could handle Mach 1.3 when it can never get there?

There were some articles published about the aerodynamics of some WW2-era fighters, using modern CFD techniques. See

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.

Have fun, and send any criticisms to the authors, not to me.

If the Sitfire reached 0.98 and fell apart, it rather obviously exceeded the critial mach number. At the real critical mach number, things don't become a pile of junk.

GregP said:

If the Sitfire reached 0.98 and fell apart, it rather obviously exceeded the critial mach number. At the real critical mach number, things don't become a pile of junk.

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Critical Mach number isn't a structural limit, but an aerodynamic one. For airfoils, it's sometimes defined by a sudden increase in airfoil drag. It's dynamic pressure or flutter than makes airplanes break.

swampyankee said:

Critical Mach number isn't a structural limit, but an aerodynamic one. For airfoils, it's sometimes defined by a sudden increase in airfoil drag. It's dynamic pressure or flutter than makes airplanes break.

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That's the Drag Divergence Mach Number.

Critical Mach Number isn't really even a limit, if considering the whole airframe. The airflow can reach supersonic at points other than the wing (over a canopy is one place, or the hump on a 747), which would be the Crit Mach Number for that airframe.

Or that's my understanding, anyway.

the lancaster kicks ass said:

apparently if the spit could be powered to such a speed, the airframe could withstand the stress of mach 1.3...........

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the best a Spit could do was 086.And still retain control. Sqn Ldr Martindale at the A.R.D.U. GOT TO 0.92 when his spinner came adrift. It is still the highest speed recorded by a piston engined plane..

delcyros said:

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

Spitfir MK IVX: 0.89*
Hawker Tempest: 0.83*
F4U: 0.73 (windtunnel tested)
P-51B: 0.84*
P-38: 0.65
P-47C: 0.69
P-47N: 0.83*

*) All Mach figures, for planes with an airscrew in front of the plane, above 0.80 are highly debatable. It could happen, that a prop driven plane exceed this Mach number but the airscrew would render most controls useles (maybe except the wing). That would result in terminal dive. (terminal means not unrecoverable, but the normal use of controls is out of order. Inverted controls, buffeting, no response and other aspects)For example, the original MiG-9, the soviets first jet fighter had only a critical Mach number of 0.79. Later versions had Mach 0.81.

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all the figures above 0.80 were in fact done regularly at Duxton by the ARDU team which included Eric Brown who flew and tested every Axis and Allied fighter.

Anonymous said:

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

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. Also, the shock wave from the wings and fuselage directly impead the function of the tail fin. Basically the P-38 was designed before any knowlege of mach was understood, and they just got unlucky in this one respect. Raising the tail plane to the top of the fin might have relieved a lot of this problem.

=S=

Lunatic

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Both Axis fighters had a tactical mach of 0.75. P51 0.78, P38 0.68. P47 0.72 Spitfire Mk XI 0.84/86 Tempest 0.82/83 all recorded at Duxton as the British were the only ones there with a machometer.

redcoat said:

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

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that was Tony Martindale at ARDU and the speed was 0.92

drgondog said:

Four things I noted as I read this thread.

1. 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. Later the P-51H was 'straight' with a smaller main gear, enabling the removal of the swept inboard portion.

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Actually there was quite a substantial change in form between the P-51B and P-51D. Consider that the B did not have enough room to mount its guns upright and the P-51D did. Another factor was that the structure of the P-51B would not hold up in a high speed dive because the upper panels would balloon upwards and cause a great increase in lift (and remove the wings).
The test pilot Tommy Hitchcock died in this manner when testing a high speed dive.

The problem with the 0.65 Mach limit on the P-38 was because of buffeting, nose tuck, and loss of elevator control starting at about 0.67. The 0.65 Mach limit was not insignificant because at higher altitudes, the Lightning had a maximum speed pretty close to this.
At 30,000 feet, 0.65 Mach is about 450 MPH True Air Speed.
At 27,000 feet, the maximum speed of the late model Lightning running War Emergency Power was about 445 MPH TAS.
Not much margin at all.

Nice Thread Revivals.....
- Ivan.

I'm not sure that late Lightings were faster than 430 mph on any power setting. 420+ mph was a rare occurrence.
It was predicted for the P-38K that it would better the 440 mph mark, since it was to use new engine, new turbo, new props and anti-detonant injection. The altitude was to be close to 30000 ft, and that's begging a question of how well such a P-38 would've behaved when entering the Mach limit in level flight.

Ivan1GFP said:

Actually there was quite a substantial change in form between the P-51B and P-51D. Consider that the B did not have enough room to mount its guns upright and the P-51D did.

Misconception - the only airfoil change made to the P-51 from XP-51 through Mustang I, P-51-1-NA, Mustang IA, A-36, P-51B/C/D/K and the NEW designs (XP-51F through P-51H) was that the new Designs went to the 66-(1.8) 15.5 from the earlier NAA/NACA 45-100 (A/B/C/D/K)

Reflect that the P-51-1-NA/Mustang IA had the Hispano 20mm - a bigger gun than the Browning M2 .50 cal. The canting was done to make it easier to feed over the top but in the end, caused more problems than that solved. Same gun bay essentially, exactly the same wing thickness, and slightly different storage arrangement from B to D.

Another factor was that the structure of the P-51B would not hold up in a high speed dive because the upper panels would balloon upwards and cause a great increase in lift (and remove the wings).
The test pilot Tommy Hitchcock died in this manner when testing a high speed dive.

The gun bay door was stiffened between the early B/C to middles production blocks and the gear up locks were re-designed for the main gear to prevent them from crashing through the wheel covers while pulling out of a dive.

The problem with the 0.65 Mach limit on the P-38 was because of buffeting, nose tuck, and loss of elevator control starting at about 0.67. The 0.65 Mach limit was not insignificant because at higher altitudes, the Lightning had a maximum speed pretty close to this.
At 30,000 feet, 0.65 Mach is about 450 MPH True Air Speed.
At 27,000 feet, the maximum speed of the late model Lightning running War Emergency Power was about 445 MPH TAS.
Not much margin at all.

Nice Thread Revivals.....
- Ivan.

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Having said the above, the thinner P-51H wing still accommodated six upright M2's.

The Primary differences between the NACA 23016 airfoil over the NACA 45-100 Laminar flow airfoil is that the transonic shock started at 25% Chord (Approx the A/C) and moved aft whereupon the CMac went major negative forcing nose down pitch whereas the a/c for the P-51 NACA 45-100 was about the 40% Chord and the shock wave formed and remained at the same basic point with no material change in CMac forcing a major nose down pitching moment. The Spitfire and P-47 both had similar airfoils (but thinner) as the P-38 so while the shock wave would migrate chord-wise to about 50% chord - the shock wave was delayed because of a thinner wing.