p38 dive recovery flaps revisited

[Skyeho]

Cartoon P38G pilots don't need dive flaps!

Cartoon Pilots don't have scores.

[Guppy35]

Cartoon Pilots don't have scores.

What score? 

[df54]

http://www.kazoku.org/xp-38n/articles/p38info.htm

Pilots reported that when deployed in level flight, the nose would "pop up" very quickly, followed by a steady decrease in airspeed.


  Is this statment fact or fiction. 

[Dobs]

Holy Thread Revival.

Dive Recovery flaps on the P38L were designed to move the center of lift back and help get out of compressability.  There was an associated nose pitch up with it, but varied depending on how far you were into compressability.

" The P-38's dive problem was revealed to be the center of pressure moving back toward the tail when in high-speed airflow. The solution was to change the geometry of the wing's lower surface when diving in order to keep lift within bounds of the top of the wing. In February 1943, quick-acting dive flaps were tried and proven by Lockheed test pilots. The dive flaps were installed outboard of the engine nacelles, and in action they extended downward 35° in 1.5 seconds. The flaps did not act as a speed brake; they affected the pressure distribution in a way that retained the wing's lift.[40]"

From
PILOT'S FLIGHT OPERATING INSTRUCTIONS
FOR ARMY MODELS
P-38H Series, P-38J Series, P-38L-1 L-5 and F-5B AIRPLANES
"DIVE RECOVERY FLAPS.—P-38L and Later P38J
airplanes are provided with dive recovery flaps to improve
the dive recovery characteristics of the airplane.
As described above, the airplane without these flaps becomes
very nose heavy and starts to buffet above placard
dive speeds. This condition is caused by a high speed
stall and a consequent decrease in lift in the wing producing
the nose heavy condition. The dive recovery flaps
which are installed under the wings between the booms
and the ailerons, restore the lift to this portion of the
wing and thus cause the uncontrollable nose heaviness
to occur at a higher speed. The flaps also add some drag
to the airplane, which in conjunction with the higher
allowable dive speed, permits safe dives at a much steeper
diving angle. The dive recovery flaps should be extended
before starting the dive or immediately after the dive
has started before a buffeting speed has been reached. If
the airplane is buffeting before the dive recovery flaps
are extended, the buffeting will momentarily increase
and then diminish. With these flaps extended, the nose
heaviness is definitely reduced but the diving speed
should never be allowed to exceed the placard by more
than 15 or 20 mph. With the dive recovery flaps extended
before entering the dive, angles of dive up to 45° may
be safely accomplished. Without dive recovery flaps extended,
the maximum angle for extended dives is 15°.
Diving characteristics are better with power off than
with power on.
WARNING
Although the dive recovery flaps greatly improve
the diving characteristics of the. airplane,
dangerous buffeting and nose heaviness will
still be encountered at diving angles above 45°
if the diving speed is allowed to exceed the
placard limits by more than 15 to 20 mph."

[Randy1]

An oldie but a goodie.  A simple way to look at it is the dive flaps don't solve the flow problem across the upper wing surface they just make the bottom wing surface have the same problem as the upper wing surface.

[FLS]

...

Dive Recovery flaps on the P38L were designed to move the center of lift back and help get out of compressability.  ...

The center of pressure moving back was the problem not the solution. You probably meant the center of lift moves forward.

[Ack-Ack]

An oldie but a goodie.  A simple way to look at it is the dive flaps don't solve the flow problem across the upper wing surface they just make the bottom wing surface have the same problem as the upper wing surface.

No, the dive flaps didn't "make the bottom wing surface have the same problem as the upper wing surface".  The problem with the P-38 in a high speed dive was that the center of pressure was moving back towards the tail (creating turbulence over the tail surfaces).  The dive flap solution changed the geometry of the wing's lower surface when diving in order to keep lift within bounds of the top of the wing. 

[Randy1]

No, the dive flaps didn't "make the bottom wing surface have the same problem as the upper wing surface".  The problem with the P-38 in a high speed dive was that the center of pressure was moving back towards the tail (creating turbulence over the tail surfaces).  The dive flap solution changed the geometry of the wing's lower surface when diving in order to keep lift within bounds of the top of the wing.

Exactly,  the formation of a high pressure region disrupts the flow of air over  the top of the wing surface .  The bottom wing gets its flow disrupted mechanically but the results are exactly the same.  These compression issues were fist discovered by the propeller designers  a few years before the P-38 problem and is limiting factor on fast you can go with a propeller driven plane.  The original P-38 problem was  nailed down after discussing the problem with a propeller designer.  Can't remember his name.