Author Topic: p-38 compression below 12k (Read 334 times)

Shane · « **on:** January 15, 2002, 02:53:11 PM »

what's the justification for a p-38 (or any plane for that matter) hitting compression below 12k where the airpressure is thicker, as well as the mach nbr higher?

it's really getting annoying not being able to divebomb in a 38 without hitting compression at low alt, even with the "dive flap" deployed.

it's simply... wrong.

and no, i'm not confusing control stiffness with compression, stiffness doesn't cause screen shudders.

SageFIN · « **Reply #1 on:** January 16, 2002, 07:09:08 AM »

Ha, this time I'll be the fastest one: Where's your proof that it's wrong? We need numbers

K West · « **Reply #2 on:** January 16, 2002, 09:29:12 AM »

There is no such thing as a safe, compression altitude in any plane unless that plane cannot get to a certain speed.

I do not have any specific pilot anecdotes about the P-38 but even the Jug would enter compression if not careful from low alts during a dive bomb.

And I'm not saying that AH is correct. Just that it might not be wrong.

""...I decided to go in real fast to throw the AA Gunners off, so I split S'd down and
pushed full throttle forward, from 15,000 feet.
The next thing I knew my jug's nose went under, the stick locked and I couldn't pull it
back. Really scared now, I jinked my bombs off not caring where they went, and hauled back on the throttle. I thought to pull the stick back. It wouldn't move. My airspeed was over 600. Then, I remembered what an instructor had told me back in Richmond, how to use my elevator trim tabs in this kind of situation.
Desperately I cranked in positive trim. Nothing happened at first, then slowly the jug's nose started to come up. Praying now, I braced my feet against the dash and pulled back on the stick as hard as I could. The ground was rushing up at me at a furious rate and I had almost given up hope of ever pulling out in time when I felt the stick move back slightly. I pulled even harder and felt that beautiful ole jug respond, then, I started to "grey out" from the terrific G-force on my body as the plane slowly pulled out. I wondered: Would I pull out in time?

Now seeing how the P-38 is heavier and faster accelerating than the P-47 I do not understand how the P-38 would not enter compression in a dive. Even with fowler flaps out. And I did just read one post recently where it was stated that the flaps need to be deployed before the dive is begun. I cannot remember if that was for AH or real life but have you tried that? I've not compressed in the P-38 at low alts in a coons age. Even though I've rarely flown it the past several months.

Maybe for the sake of a discussion with facts, knowlegeable folks like Widewing, Wells and Badboy would chime in. IMO they are best outside sources and opinions for validating compression of the AH's P-38 (and other aircraft) at low alts available here.

Westy

F4UDOA · « **Reply #3 on:** January 16, 2002, 09:36:47 AM »

I actually happen to agree with this.

Anecdotally speaking I have never heard of a WW2 fighter reaching compression below 20K.

Remember compression was a new discovery when high alt fighters like the P-38 where developed. Before that the very fast diving fighters like the P-40 had been able to dive with impunity from lower altitudes.

This is not to say that a P-38 should not be able to reach speeds where the elevators get stiff or you cannot pull out soon enough. However the compression effects felt at higher alts would tend to subside at lower alts.

Widewing · « **Reply #4 on:** January 16, 2002, 11:27:23 AM »

Quote

Originally posted by K West
There is no such thing as a safe, compression altitude in any plane unless that plane cannot get to a certain speed.

I do not have any specific pilot anecdotes about the P-38 but even the Jug would enter compression if not careful from low alts during a dive bomb.

And I'm not saying that AH is correct. Just that it might not be wrong.

Have a look at the chart below. If you exceed redline limits at any altitude, the P-38 will buffet. Add some G loading and it will buffet at a lower velocity corresponding to the loading. The chart assumes 1 G for loading. Someone would need to do some offline testing to confirm that the AH P-38L conforms to the published limits in the manual. Also, the dive recovery flaps do not appear to function like those actually used.

Furthermore, it was always a good idea to deploy those flaps prior to diving to avoid the pitch-up associated with deploying them during the dive. In the actual aircraft, the operating switch was mounted on the yoke so that the pilot never needed to remove his hand to deploy or retract them. Most P-38 pilots will tell you that their usage became habit, especially those who had flown the earlier models and knew the terrors of a compressibility dive first-hand.

Gripen has studied this in greater depth than I have and can further everyone's understanding of the dynamics involved.

My regards,

Widewing

Tac · « **Reply #5 on:** January 16, 2002, 03:05:11 PM »

Have a box of kleenex tissues handy when you test it Widewing. I sure needed it

bolillo_loco · « **Reply #6 on:** January 16, 2002, 05:45:50 PM »

widewing, isnt that chart the same one that is posted in the pilots manual? if so I thought it was already discussed that it was not for 1g, but at 10k the speed was for a 5g pull out, at 20k it was for about 3 gs and at 30k it was for 2gs?? thanks.

Widewing · « **Reply #7 on:** January 16, 2002, 06:07:25 PM »

Quote

Originally posted by bolillo_loco
widewing, isnt that chart the same one that is posted in the pilots manual? if so I thought it was already discussed that it was not for 1g, but at 10k the speed was for a 5g pull out, at 20k it was for about 3 gs and at 30k it was for 2gs?? thanks.

There are two dive placards illustrated in the manual. Figures 25 and 25A. Figure 25 defines speed/loading/buffet. Figure 25A defines limits for a normal gravitational load of 1 G. My chart simply provides an easier to read and understand format, but tells you the same thing as figure 25A.

Bottom line: If you exceed critical Mach, regardless of altitude, the P-38 will exhibit compressibility. At low level, it takes speeds in excess of 500 mph to run up against critical Mach.

My regards,

Widewing

Zigrat · « **Reply #8 on:** January 16, 2002, 06:10:53 PM »

widewing is right

gripen · « **Reply #9 on:** January 16, 2002, 11:56:17 PM »

Well, I don't know if I have much to ad, but there is some stuff in the NACA server on the dive flaps of the P-38. Also original reports on RAE P-38 dive flaps tests, Kelsey's dive accident and NACA wind tunnel tests are available from the PRO.

gripen

ispar · « **Reply #10 on:** January 19, 2002, 12:27:33 AM »

In other words, yes, the P-38 will enter compressibility (not compression; compressibility and compression are very different. I'm assuming that people are referring to "shock stall" here) as it reaches critical mach at any altitude. However, as it enters thicker air, the critical mach speed is higher, and high enough that P-38 will be unable to reach that speed, except perhaps in unusual circumstances. The problem is solved by default. But the P-38 was not a low-altitude fighter, and that's part of why compressibility surfaced as a problem.

Widewing · « **Reply #11 on:** January 19, 2002, 02:24:58 AM »

Quote

Originally posted by ispar
In other words, yes, the P-38 will enter compressibility (not compression; compressibility and compression are very different. I'm assuming that people are referring to "shock stall" here) as it reaches critical mach at any altitude. However, as it enters thicker air, the critical mach speed is higher, and high enough that P-38 will be unable to reach that speed, except perhaps in unusual circumstances. The problem is solved by default. But the P-38 was not a low-altitude fighter, and that's part of why compressibility surfaced as a problem.

You haven't quite got it yet. Critical Mach does not change with altitude. It's the speed of sound that changes with altitude. Mach .68 is the placard limit regardless of altitude.

Actually, the P-38 turned out to be a very good low altitude fighter. It was at high altitude that the P-38 ran into trouble. This is due to its relatively low Mach limitation. At 35,000 feet, hitting the Mach limit means nosing over (at max level speed) just 15 degrees for, perhaps, 30 seconds. That's about how quickly the big fighter will begin a compressibility buffet.

My regards,

Widewing

ispar · « **Reply #12 on:** January 21, 2002, 12:20:25 AM »

Whoops!

Widewing, you are of course correct. I got my terms mixed up. The speed one has to reach to get to critical mach is what increases as altitude decreases.

Perhaps I should have been more clear. I should have said that the P-38 was designed as a high-altitude fighter (an interceptor, actually) and was used as such in Europe. It was because it was used at high altitude that the compressibility problem surfaced. I certainly didn't mean to imply that it was not a good low-altitude fighter - the records and history from the Pacific clearly show that that was where it excelled. The lower altitude allowed the Lightning to use its high speed, heavy firepower, and excellent turn radius to maximum effect. IMHO, it was the most important land-based American fighter in the Pacific.