Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L

[oboe]

I wonder if they'd be open to correcting the information?

"Speed brakes" is almost worth a face-palm, especially on a site that should know better...

[nrshida]

Is there any footage of the Fowlers extending in say a ground test?

[oboe]

This isn't from the '38 but its a great animation of Fowler flaps:



You can see a rear-view of the Fowler's on a real '38 at the 8:42 time mark in this video:

[Drano]

FYI the Fowler flaps and dive recovery flaps are separate independent systems.

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[FLS]

I wonder if they'd be open to correcting the information?

"Speed brakes" is almost worth a face-palm, especially on a site that should know better...

41

I emailed both but I doubt we'll see a quick correction.

[Drano]

And I don't think the intention of the recovery flap was ever intended to slow the plane down at least not in a direct sense. If it were, surely it would have been larger in surface area and protrusion into the airstream. More intended to get back just enough control to get the nose pointed above the horizon so gravity would do the job.

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[FLS]

I got responses from both sites. The EAA will be correcting the information and the NASM is looking into correcting theirs.

[oboe]

That's good news - thanks for doing that, FLS!

[Randy1]

You might find it interesting that it was a prop designer that pointed out the air flow problem to Lockheed. The limit of a prop is due to the same effect.  They had faced that problem for some time.

In a dive the compression disrupts the flow over the top wing.  That makes the bottom of the wing a better lifting device than the upper wing pulling the nose of the plane down.  The faster the plane went the worse the nose tuck.  The dive flap, in a way, mechanically duplicated the problem on the upper wing now making the upper wing a better lifting device than the lower wing.

[Dawger]

You might find it interesting that it was a prop designer that pointed out the air flow problem to Lockheed. The limit of a prop is due to the same effect.  They had faced that problem for some time.

In a dive the compression disrupts the flow over the top wing.  That makes the bottom of the wing a better lifting device than the upper wing pulling the nose of the plane down.  The faster the plane went the worse the nose tuck.  The dive flap, in a way, mechanically duplicated the problem on the upper wing now making the upper wing a better lifting device than the lower wing.

The shock wave from the transonic flow on top of the wing physically moves the center of pressure (the point where the lift produced by the wing is focused). This causes a pitch down moment as the center of pressure is always aft of the center of gravity and increasing the moment arm aft increases the pitch moment downward.

The dive flap created an area of lift well forward, which countered the downward pitch moment. They may seem small but at high speed they would be extremely effective. I have flown jets with similar size and shape spoiler panels on top of the wing and fast deployment causes a very significant "bump", vibration and pitch change. It also creates some drag.

The AH dive flap on the P-38 doesn't do any of those things. It seems to just reset critical mach to a higher number.

[Krusty]

And I don't think the intention of the recovery flap was ever intended to slow the plane down at least not in a direct sense. If it were, surely it would have been larger in surface area and protrusion into the airstream.

I wouldn't be so sure. The days of the massive split-flap SBD dive brakes were obsolete and based on outdated thinking. You didn't need nearly as much of a disruption to massively slow you down. Look at Ju-87 dive brakes. Look at Me410 dive brakes. Look at the tiny brakes on the A-36 Apache. Almost the same size as the small dive brakes/flaps on the P-38s. Edit: Smaller, even!

[Devil 505]

I wouldn't be so sure. The days of the massive split-flap SBD dive brakes were obsolete and based on outdated thinking. You didn't need nearly as much of a disruption to massively slow you down. Look at Ju-87 dive brakes. Look at Me410 dive brakes. Look at the tiny brakes on the A-36 Apache. Almost the same size as the small dive brakes/flaps on the P-38s. Edit: Smaller, even!

And yet the A-1 Skyraider had 3 speed brakes each the size of a Mini Cooper. The A-10 uses a clam shell aileron/dive brake system very similar to the SBD. So the belief that bigger speed/dive brakes are more effective was still a school of thought well into the 1970's.

[colmbo]

The size of the dive flap would at least partly depend on what speed the designed planned to extend it at and what speed the designer wanted it to maintain.

The Stuka was basically a flying dive brake with the fixed gear, etc....might be it didn't need a lot of extra drag.

[Krusty]

Perhaps, though it was streamlined and still retained the dive brakes.

Also look at the Ju-88s, a rather streamlined and efficient design that was the primary type of bomber for Germany. Their dive flaps are a row of metal strips that simple disrupts airflow below the wing. The area is only slightly larger than those pictured on the P-38. Or the Tu-2, where many were removed because they weren't used, but they still were designed and implemented with a very small area. Or the F4U's dive brakes being the gear doors. Those aren't very large but have a profound effect when used.

Honestly it doesn't take a massive parachute to slow things down. Many planes' radiator flaps alone would drop a plane ~20mph. For cooling flaps!

[Randy1]

Just a reminder the drag is proportional to the square of the velocity.

https://www.grc.nasa.gov/www/k-12/airplane/drageq.html