Jumpin on the P-38 bandwagon

[HoHun]

Hi Bolillo_loco,

>If you have the pilots manual look at the section for diving. It clearly states that mach .675/.68 is the placarded limit with out dive flaps and that with the dive flaps extended it may be exceeded by 20 mph.

The AAF Manual 51-127-5 has similar information on the P-51D:

"It can be dived to beyond 75% of the speed of the sound before going into compressiblity.

[...]

As noted earlier, it is possible to come out of compressiblity safely i fyou didn't go into it too far. The most important thing to remember about this is that while in compressiblity, you have virtually no control over your airplane."

That's why the relative figures for comparing the P-38 to the P-51 are Mach 0.68 versus Mach 0.75 for a speed at which the aircraft is under control, or Mach 0.72 (according to Tony Levier) versus Mach 0.80. (Judging from Eric Brown's descriptions, Mach dives were conducted with small increments in Mach numbers until test results indicated the safe maximum had been exceeded. According to one of your posts, this figure was reached at Mach 0.81 when pullout - not the dive itself - resulted in airframe damage.)

>The test were the 38 with dive recovery flaps was tested against the P-47D in a dive levier stayed with the 47 or only lagged behind by a bit and then pulled right aside of the 47 after the dive shows that the dive recovery flaps broght it up to par with most a/c. P-47s also had dive recovery problems.

From all the single engined fighters that were used in numbers in the European theatre, the P-47 was the one with the lowest critical Mach number, and in fact it had a history of "graveyard dives" (as Eric Brown called them) not unlike the P-38.

Brown's tests revealed that the unmodified P-47D was limited to Mach 0.73, where it got out of control. If Tony Levier went to Mach 0.72 in the P-38, the Mach number difference of just 0.01 would match the observed results quite closely.

Just like you point out, later P-47D variants were fitted with a dive recovery flap similar to those of the P-38. That also meant the P-47 was ahead of the P-38 once again.

>Roger Freemans book points out several of the mustangs airframe weaknesses where several 51s had to be scrapped due to structural failures because of pull outs from high speed dives.

Structural strength is a topic only loosely related to critical Mach number - an airframe can be overstressed far below its critical Mach number. Of course, as a tactical move a dive is most useful if it ends with an undamaged aircraft in a safe place :-)

Regards,

Henning (HoHun)

>12-08-2001 08:34 AM

[Nwbie]

HoHun <S>

Glad to see you
Thats all  

NwBie  

[bolillo_loco]

Hi Hohun, you may be quite correct pointing out the 47 test against the lightning was one having a critical mach number of .73 w/o the dive recovery flaps because the book I have does not state which model 47D. It had to be a 47D with out the improved ailerons because the ones with out these the stick had a tendency to float about due to aileron ocilation at high speed if I am correct.

The pilots manual for the 38 however does state clearly that mach .68 is with out dive recovery flaps.

I still feel that any ww 2 type a/c was quite unsafe at any speed above mach .75 at an altitude above 15,000 ft. That is the opinion that I have gotten from what I have read anyway.

One thing I am convinced of is the P-38 is still quite a controversial a/c because all the threads get very long with many posts. I hope some day that we may get some more modern data from reliable sources.

I also would like to see hard original data for the 1,725 hp allisons for the 38L. I do not care if it is right or wrong. widewing said he knew warren bodie and perhaps he could get hard lockheed data, but as of yet he has not  

[ 12-09-2001: Message edited by: bolillo_loco ]

[batdog]

TAC... later versions of J had the dive flaps.


 xBAT

[akak]

P-38 kept it's maneuverability at high altitude, it just entered compressability easier the higher you went.  In real life if you started a dive above 20,000ft, you compressed but dives started below 20,000ft didn't compress.  That's why the P-38 excelled in the PTO and MTO because operational altitudes rarely went above 15,000ft.

 

Originally posted by gripen:


Well, the P-38 of the AH might do that but the real P-38 losed it's maneuverability at high altitude due to comressebility. The combat flaps helped at low altitude and at low speeds.

gripen

[ 11-28-2001: Message edited by: gripen ]

[Tac]

I know x-bat, the late j model was for all purposes an L with a J engine.

I want the non-dive flapped J model. Green is good!

"The combat flaps helped at low altitude and at low speeds"

Again, no. At high altitudes the combat flaps gave the 38 its greatest advantage. At hi alts fighters dont regain their E as quickly as in low alts, and if the battle became a turning fight at 250mph or so, the 38 would easily out-turn  and out-accelerate the 109's and 190's while having full control of the plane... a 190 or 109 trying to match it would either stall out or be forced to dive away. At low alts it also helped a lot, but it didnt give the 38 an advantage as big as it did "up there"

[ 12-13-2001: Message edited by: Tac ]

[gripen]

Ack-Ack and Tac,
Please consult manual and AHT. I wonder how many times I should point out that compressebility speed decreased when g load increased and Clmax decreased when altitude increased. These things are discused several times above too...

gripen

[Tac]

Gripen, im talking at 250 mph when the flaps can be deployed. At hi alts it does give much more benefits to the 38 than they do at low alts.

[gripen]

Tac,
Well, as noted earlier (several times) the Clmax  decrease when altitude increases because air density decreases and also because relative mach number decreases (for same reason compressebility speed decreases when g load increases, note relation between g load and Clmax). At low altitude the combat flaps (and contra rotating props) made high lift coefficient possible for the P-38 and at low speed below 140-150mph IAS it could out turn planes like the P-51, P-47 or Fw 190 (but not planes like the Spitfire or A6M), at higher speeds (between 150-250mph IAS) the combat flaps helped too but mentioned planes could still out turn the P-38 because they could tolerate more g load at these speeds despite what ever Clmax they could reach (while Clmax and g stall value for a given speed are related, you can't compare them directly). At high altitude the P-38 can't reach those high Clmax numbers because decreased air density and also because compressebility restricts Clmax, while those other planes can maintain their relative maneuverability better because air density nor compressebulity does not restrict their Clmax as much.

To put long story short: The turning performance of the P-38 at low altitude was based on high Clmax which it could not reach at high altitude.

gripen

[Tac]

[ 12-13-2001: Message edited by: Tac ]

[gripen]

Tac,
Well, I don't see anything which contradicts my points, or do you? No one has argued that the combat flaps do not help at high altitude (if compared to without combat flaps) but the relative maneuverability (if compared to others) of the P-38 decrease when the altitude increases. Please consult forexample following report "Tactical Employment Trials on the Republic Airplane P-47C" AAFSAT 16th February 1943 (turning comparison between P-47C and P-38G).

gripen

[Guppy]

From the charts I've got, at 10,000 ft the P-38 has G limits comparable to the P-51's up to about 230 mph. If the P-38's G-limits increased further with decreasing altitude, it would have a definite advantage below 10k.

However, how useful is the maximum G-limit of the airplane (due to accelerated stall, compressibility or whatever) in determining sustained turn performance? No WW2-era airplane could maintain a turn at 6G without losing significant speed or altitude. Up at 30k, I wonder if even 3G could be sustained. (Not that I'm saying better instantaneous turn isn't an advantage, but it's sustained turn which matters more in a classical turning fight.)

I seem to recall that those tactical employment trials between the P-47 and P-38 were conducted at 25-30 kft. At those altitudes, the P-47C had much more power available than the P-38G, which would most likely have been the dominant factor in sustained turning ability. Moreover, the P-47 was generally considered the best of the USAAF fighters at such altitudes.

Another point in favour of the P-38 at low altitude is accelerated stall behaviour. If a P-51 pilot presses the limits too far down low, he spins in (the P-51 apparently wasn't known for giving lots of stall warning). The P-38, by all accounts, had much better stall behaviour and could be pushed harder without risking a crash.

[ 12-13-2001: Message edited by: Guppy ]

[Tac]

Man.. then WHAT are we arguing about?   All I said was that it helped the 38 "up there" more than at low alts.. and in a turning fight (that meaning sustained turning), which in essence "up there" is a stall fight, the 38 has an advantage. Yes, the others may turn a bit better (and thats if they not turning against their torque), but not sustained.

They'd spin/stall out or very wisely extend to regain E.

I guess we're both saying the same thing but havent noticed.. lol.

[gripen]

Guppy,
Yeah, the P-38 could definately turn tighter (sustained and momentarily) than the P-51 at low altitude and at low speed, no one has argued otherwise here. But situation is different at high altitude because there Clmax limits the P-38  more than the P-51 and as noted earlier the P-51 (V-1650-3) has a lot better propeller efficiency and exhaust thrusts available, so at high altitude it has an advangate (sustained and momentarily) and the P-51 (P-51B with V-1650-3) also turned better than the P-47D at high altitude (sustained and momentarily).

BTW above about mach 0.6 the P-51 could reach higher lift coefficient than the Spitfire (according to RAE). About sustained turns it should be also noted that even at low altitude WWII fighters could not turn much more than 3-4g sustained, much less at high altitude.

Tac,
Well, I have been arguing that the P-38 was not particularly maneuverable at high altitude if compared to other high altitude fighters (like P-51 or P-47)  because aerodynamic limits of the airframe and propellers. It seems that you have have been arguing  that combat flaps helped more a high altitude, but no one has questioned that (actually I could do that if assuming high speed, but I don't care).

gripen

[ 12-13-2001: Message edited by: gripen ]

[Guppy]

Sure. I was just interested in defining "low speed." I think 140-150 mph is pretty conservative; I'd be more inclined to credit a moderate advantage below 200 mph (more like a sensible combat speed), with rough equality around perhaps 230-250 mph.

And at high altitudes, I have no problem accepting that the P-51B and P-47 had more power available up at 25-30k than the P-38 (particularly F/G and early H), which meant better sustained turns; just that G-limits don't necessarily tell the whole story.

By the way, I think this thread's going very nicely in terms of finding common ground...