P-38 Compressibility

[38ruk]

I was just rereading my favorite P-38 book and remembered asking about this in the past .....

In Warren Bodie's book, "The Lockheed P-38 LIGHTNING " , it reads , compressibilty was never encountered with dives starting below 25,000 ft .

In Chapter 7 "The Specters of War and Compressibility " PG:77 Pharagraph 2 SNIP: "For a number of reasons , it was virtually impossible to encounter compressibilty tuck in any P-38 types if the dive was entered at 25,000 feet or less . Numerous test flights confirm this " SNIP

Ive posted this before and never really got a answer either way about why the 38 here is easily compressed below 25k .

[Randy1]

If I remember right it was at 45 degree angle of attack.

[Ack-Ack]

I was just rereading my favorite P-38 book and remembered asking about this in the past .....

In Warren Bodie's book, "The Lockheed P-38 LIGHTNING " , it reads , compressibilty was never encountered with dives starting below 25,000 ft .

In Chapter 7 "The Specters of War and Compressibility " PG:77 Pharagraph 2 SNIP: "For a number of reasons , it was virtually impossible to encounter compressibilty tuck in any P-38 types if the dive was entered at 25,000 feet or less . Numerous test flights confirm this " SNIP

Ive posted this before and never really got a answer either way about why the 38 here is easily compressed below 25k .

And it's accurately modeled in AH, you can't enter into compressibility under 25,000ft. 

[FLS]

If I remember right it was at 45 degree angle of attack.

I assume you mean dive angle?

[Randy1]

I assume you mean dive angle?

Yes

[Triton28]

Power on, no control imput... probably gonna compress.  Power off and cross controlling on the way down, she's like a rock.   

[38ruk]

And it's accurately modeled in AH, you can't enter into compressibility under 25,000ft. 

I just did in a  0 mph stall at 15000 ft was compressed in a dive by 5k or so.

Edit: I think i understand my problem , compressibility and compressibility tuck are two different things . The nose doesnt try and tuck under from the lower alts where im seeing compression .

[FLS]

The book never explains the "number of reasons" they wouldn't compress so how do you compare without knowing what they are? 

The book says that M .65 is the start of compression and that's when you start to see high speed buffet in AH. Maximum tuck occurs at M .74  and you cannot reach M .74 from 25,000 ft so that may be one of the reasons the book didn't specify.

[Ack-Ack]

I just did in a  0 mph stall at 15000 ft was compressed in a dive by 5k or so.

Edit: I think i understand my problem , compressibility and compressibility tuck are two different things . The nose doesnt try and tuck under from the lower alts where im seeing compression .

What you're experiencing is the result of high speed airflow over the flight surfaces that makes stiffens up the controls and is not compressibility. 

Compressibility happens in the P-38 (and P-47) when the airflow over the leading edge of the wing breaks exceeds the speed of sound, and the higher altitude you're at the lower the speed is to hit that mach threshold.  Below 25,000ft, the air is too thick for the P-38 to reach compressibility, though you will experience the controls stiffen up because of the high speed airflow over the flight controls.

[JVboob]

I agree with akak on this. ^^

[Slade]

high speed airflow over the flight surfaces that makes stiffens up the controls

Some times we get too caught up in expecting everyone to know every flight control term and behavior.  Like many others I'd like to know the answer to this too.

Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?

Thanks for putting the answer to this in English for us P-38 novices.

[PR3D4TOR]

I agree with akak on this. ^^

That's a safe bet.

[Randy1]

Some times we get too caught up in expecting everyone to know every flight control term and behavior.  Like many others I'd like to know the answer to this too.

Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?

Thanks for putting the answer to this in English for us P-38 novices.

Yes, use the p-38l.  The ah models in aileron boosters the J in ah doesn't,t have.

[FLS]

Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?

It's not just a P-38 trait. It happens to every WW2 fighter. The speed where it happens varies by the wing design. Some P-47s have the same dive flap as the P-38L to delay the compression control lock.


BTW if you haven't noticed, your Mach speeds are shown on the clipboard E6b if you want to compare to historical max dive numbers.

[icepac]

Again.......the specificiation for the 25k remark was 45 degree dive angle.

Now if you're already going super fast as you pass through 25k and push over vertical, then it can easily happen below 25k.