LW planes compression

[bozon]

The 109 and p-38, both were based on old designes and suffered badly from compression at high speeds (and situation was worse as alt. goes up). At the time of the original design this problem was pretty unknown.

The problem was never really solved for either plane without the need to design a completly new one. But the americans made quite an effort to reduce the problem (like adding the dive flaps). Seems strange to me that t the Germans didn't try to do anything about it (as far as I know). The 109 went through the entire was as a main LW fighter  - didn't they try to come up with some improvments?

Do you know anything about such improvements?

Bozon

[whgates3]

maybe they just told the 109 pilots what they told the P-38 pilots before the dive recovery flaps were installed
"dont dive too fast"

[Shane]

i think the lw problem was more of control stiffening at higher speeds not so much as "compression" per se.

guess the engineers over-estimated just how strong their ubermensch pilots were.

[HoHun]

Hi Bozon,

>The 109 and p-38, both were based on old designes and suffered badly from compression at high speeds (and situation was worse as alt. goes up).

The Me 109 did not have a compressiblity problem. The Me 109F/G/K style airframe could dive safely to Mach 0.79.

Mach 0.80 was reached by a test aircraft with reduced aileron travel, but the aileron overbalancing encountered in that test probably would have proven too much for an unmodified aircraft.

The P-38 did have a compressiblity problem, with the placard limits being Mach 0.65 without and Mach 0.69 with dive flaps extended.

What the Me 109 had was a control force problem. At very high airspeeds, the elevator became relatively ineffective and the Me 109 would basically continue flight in a straight line.

By contrast, a P-38 exceeding its maximum safe Mach number encountered increasing nose-heaviness that steepened the dive, building up more speed resulting in higher (negative) Gs and finally in the self-destruction of the aircraft.

>Seems strange to me that t the Germans didn't try to do anything about it (as far as I know).

The P-38 was inherently dangerous (as was the P-47, which encountered the same problem, though at higher speeds), while the Me 109 was merely tactically limited by its control heaviness. The sense of urgency behind the P-38 and P-47 problems was much higher since these aircraft were killing their pilots without even needing the "help" of the enemy.

I'd also expect the speed and altitude dependencies of the compressiblity problems and the control force problems to differ as they had different causes.

I'm not sure yet what the difference between the dependencies is, which shows that your question was quite good :-)

Regards,

Henning (HoHun)

[whgates3]

did the Jug compress at low MACH numbers like the P-38 or was it dangerous just because it accelerated in a dive so well?

[Urchin]

Lol Shane... I got the funniest mental image of some scrawny little german yanking the hell out of the stick there.

[nuchpatrick]

Because the Jug's accel fast..Hince the same flaps come to bare on the P-47D-30.  If they could get the modeling done the P-38 and the D-30 would benifit from them, but at this point they're useless.

There are some stories where pilots would attempt to put their feet on the console and heave with all their might to pull the craft out of the dive..some lived to tell the story..others were not so lucky.

[Staga]

With is remaining speed Juutilainen dodged and pushed the Me in vertical dive. It was the only thing to do. He hoped that the enemy would not follow him if he exceeded the 950kmh limit - neither the Me nor the La were designed to withstand higher speed. He let the MT-222 fall vertically for 6500 m. The pilot's ears were buzzing like telephone wires, the speed was over 1000 kmh at 2000m altitude. The Me flew rock-steady.

He pulled the stick, which was nearly immobile and used the trim wheel. The Me returned to level flight at the altitude of 150 m, the speed was 900 kmh. No enemies were in sight. The pilot pulled the stick and converted his speed to altitude, then proceeded to make a "normal" landing without power.

The Me had not been damaged, neither in the battle nor in the dive. The enemy retreated before the other Finnish pilots had any chance to engage them.

Not a scientific data but food for thoughts anyway

[gripen]

The critical mach number of the Bf 109 (with F style wing) and the Fw 190 appears to be around mach 0,76 according to collection of German notes in the PRO. The Dive limits table for both planes support this. Compressebility effects were not so serious as in the case of the P-38.

gripen

[HoHun]

Hi Nuch,

>Because the Jug's accel fast..

Here's a description from Eric Brown's "Testing for Combat". Brown was test pilot at the RAE, the Allies' leading facility for the high speed research, and assigned to test the P-47's "graveyard dive" problem in March 1944:

"Before the next flight a Machmeter was fitted to the aircraft, and as instructed I climbed to 35,000 ft, carried out a 2 min level run at full power and trimmed the aircraft before pushing over into a 30 degree dive. At Mach=0.72 the aircraft began to buffet slightly and pitch nose down, requiring a strong pull force to maintain the dive angle. At Mach=0.73 the buffeting increased severaly and the nose-down pitch was so strong that it needed a full-blooded two-handed pull to keep dive angle constant. I had to hang on grimly in this situation, unable to throttle back until Mach number decreased as altitude was lost. The pull-out was not effected until 8,000 ft. Analysis showed that a dive to M=0.74 would almost certainly be a 'graveyard dive'."

Two other aircraft Brown found to have a similarly bad nose-down pitch were the F8F Bearcat and the Me 163. (In the latter it occurred at Mach 0.84 according to another source.) If he'd have flown the P-38, he'd probably have added it to that list as well :-)

Despite its lower maximum Mach number, the P-47 was generally considered to dive very well. Just as you pointed out, acceleration was the key! Even the seriously Mach-limited P-38 can make a serious claim to good diving capabilities based on its good power-to-weight ratio.

Confusing? Let me try to explain: Going into a dive from slow speed, it's powerloading that counts - aircraft like the P-38 and the Me 109 are very good in that regard. Once speed grows, the aircraft with better high-speed aerodynamics have the better acceleration - that's when the P-47 and the Fw 190 are in their element. If the speed finally gets really high, reaching critical Mach will slow acceleration seriously, and transsonic effects will tend to make the aircraft uncontrollable and force the pilot to slow down or pull out. In that regime, aircraft with high maximum Mach numbers are best, like P-51 and Spitfire for example.

So, when turning the fight into a diving contest, a fighter with low power-loading can outrun heavier enemies by going into a shallow dive from low speed, preferrably by unloading the aircraft. The heavier fighter can outdive the low-power loading one by diving steeply, shortening the initial phase were he is at a disadvantage to a minimum. (Perhaps you remember this from Air Warrior - to escape a P-38, you had to drop like a rock since it could catch you anytime in a half-hearted dive :-) And in a fighter like the Spitfire, you need a head-start before you go into the dive (and some good amount of altitude) since some time will pass before it reaches its final superior dive speed.

Regards,

Henning (HoHun)

[SFRT - Frenchy]

Start turnfighting a spit5 at 15K, end up pretty slow turnfighting. Decide to get out. While u coming head on the spit5, roll 40 degrees nose down and run. The spit5 can alf loop back in your 6.

When you hit the deck, 15k lower the spit5 will be 500y in your 6.

In Aces High, acceleration my ass.  How following a Me109 in a dive from the same 15k, see him pulling away then you reach the deck and catch up 2 sectors laters.

Or following a FW in compression at 550 and looking at him snap roll and dive away (my favorite).

I trust HTC and FM geecks to be aerodynamically coorect but I eard Gabresky rolling over in his tumb.

[niklas]

Beauvais did also some dive tests. From his experience:

"At the end of 40 the lw reported probrlems and accidents with the 109E during dives (nose-dives). So i wanted to dive up to 750km/h in Jan.41 from 9000m.
I did fly a half roll and pulled until the upper cockpit bar was vertical to the horizont. At 750 IAS i tried to pull out, but at first a little lake didn´t wanted to disappear out of my view. I couldn´t observe free moving and ineffective elevator. So i thought "case Jodlbauer" (Jodlbauers nose dives test in 37 ended as an impact in the river Müritz). Despite my concerns about the strenght of the control unit i decided to pull as strong as i can and in case of a failure to pull out with the stabilizer/trim. The force applied was sufficient. In about 4000m the aircraft was flying horizontal."

and

"The Messerschmitt-team offered me a 109 with an elevator equipped with a balance tab, which should enable the pilot to control a larger bandwith of COG. In "bahnneigungsflug" - dives with moderate angles, i think around 40-60°- nothing special occured. Then i entered a gust wind area and the aircraft tried to "get on the head" at around 650km/h. I had to pull a lot to change something. Suddenly it came very heavy around. I was pressed in my seat with such a force that i feared an aircraft breakage. I succeded in damping the pull out and especially to reduce speed. Later the wing showed a little crack (Riss im Flügelhauptbeschlag, wer das übersetzen kann nur zu...)

btw were any spits tested with Mach-indicators?

niklas

[bozon]

thx HoHun, niklas