General Forums => Aircraft and Vehicles => Topic started by: Ack-Ack on September 05, 2017, 04:29:27 AM
Title: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Ack-Ack on September 05, 2017, 04:29:27 AM
Had a discussion with Odee about the flaps and dive flaps on the P-38L and told him I'd help clear up his confusion by posting the difference of the two.
In short, the Fowler Flaps used on the P-38 did not, nor was it intended to serve the same function of the dive flaps. The Fowler Flaps, in layman's terms, extends the wing area by extending on rails or slots.
The dive flaps were intended to serve an entirely different purpose, and that was to aid the P-38 in recovering in a high speed dive. Again, in layman's terms, it essentially changed the flow under the leading edge of the wing to prevent air turbulence over the elevators that caused "nose tuck" and make it easier for the pilot to recover from the high speed dive. Basically, it helped resolve the compressibility issue with the P-38.
As you can see from the two different images, both systems were different and not used for the same purposes. In addition, the Fowler Flaps couldn't be deployed at high speeds so it would have been useless in aiding the P-38 in recovering from a high speed dive or when it has entered into compressibility.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Lusche on September 05, 2017, 04:40:50 AM
:)
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: DaveBB on September 08, 2017, 04:03:20 PM
Show an actual picture of the dive recovery flaps. It's amazing how small they are.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 08, 2017, 06:17:01 PM
Here ya go:
(https://i.imgur.com/yeXE5Bo.jpg)
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: pembquist on September 08, 2017, 08:24:09 PM
I thought the dive flaps moved the pitching moment of the wing forward so the plane would pitch up without elevator authority.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 09, 2017, 02:52:39 AM
Yeah, I'm not sure how they would change airflow across the elevator when they are outboard of the booms and the elevtor is in between the booms?
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: pembquist on September 09, 2017, 02:54:38 AM
Also I recall that the first batch of recovery flaps was sent out on a c47 or such to England and got shot down by the RAF.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Drano on September 09, 2017, 02:10:59 PM
Oboe, iirc what was discovered was the airflow over and under the wings and past the cockpit nacelle came back together at about the location of the elevator. Depending on speed and alt it made the elevator feel like it was stuck in concrete. A bad thing if headed downhill! The flap interrupted/redirected the airflow to largely correct the problem. Being on the underside provided a bit of a pitch up action as well.
FYI there's an excellent video on YouTube with Jeff Ethell about the 38 where he does a walk around on a newly restored 38L describing the systems. It does show the flap and it deploying and retracting. You get a perfect idea of how it works. Sadly he was killed a couple of weeks after filming it crashing that plane on final.
Here's the link :
https://youtu.be/Y3nddCJbcdI
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Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 09, 2017, 03:18:55 PM
The dive recovery flaps change the center of pressure on the wing, they don't affect the elevator.
When the P-38 is in a compressibility state, the air flow becomes turbulent over the tail over the P-38, making it very difficult for the elevators to work. Remember, compressibility is caused in the P-38 when the air hitting the leading edge of the wings breaks the mach barrier, which causes the turbulent air flow over the tail section. The dive flap changes the air flow so it is no longer turbulent over the tail section of the plane and allowing the elevators to function with much more authority in addition to the slight pitch up from the dive flaps.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 09, 2017, 07:46:43 PM
It isn't the air speed at the leading edge, it's the air moving faster over the top of the wing that causes the problem.
The airflow over a thick wing like the P-38 reaches compressibility at lower air speeds than a thin wing. The transonic flow moves the center of pressure on the wing back causing the mach tuck while the shock wave that forms moves back and locks the control surfaces. The P-38 is still subsonic when it compresses.
The dive recovery flaps disrupt the airflow and keep the center of pressure in the normal flight range.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Drano on September 09, 2017, 08:01:15 PM
Which is petty much what I said but thanks
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Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 09, 2017, 08:25:14 PM
The tail flutter caused by turbulence was fixed with the fillet where the wing meets the fuselage.
Even the NASM and EAA sites describe dive flaps as slowing the aircraft down to prevent compressibility. :bhead
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 10, 2017, 06:50:24 AM
That was a great video link, Drano. I hadn't seen that one before. Sure was a tragedy about Jeff Ethell.
Regarding the referred to NASM and EAA information, here it is:
Seventeen months passed before engineers began to determine what caused the Lightning's nose to drop. They tested a scale model P-38 in the Ames Laboratory wind tunnel operated by the NACA (National Advisory Committee for Aeronautics) and found that shock waves formed when airflow over the wing leading edges reached transonic speeds. The nose drop and loss of control was never fully remedied but Lockheed installed dive recovery flaps under each wing in 1944. These devices slowed the P-38 enough to allow the pilot to maintain control when diving at high-speed.
High-speed dives created turbulence and reduced elevator effectiveness, making it difficult or impossible to pull out of the dive – a phenomenon later understood as compressibility – the turbulence that builds up on a wing just before an airplane reaches the speed of sound. Lockheed engineers solved the problem with a speed brake that slowed the P-38’s dive and reduced the turbulence.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 10, 2017, 09:43:03 AM
Now people will be quoting that when they complain the P-38 "dive brakes" don't work.
Jeff Ethel wrote books for the NASM, you'd think they and the EAA would know better. I assume the web content was written by less knowledgeable volunteers.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 10, 2017, 10:17:09 AM
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...
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: nrshida on September 10, 2017, 10:20:09 AM
Is there any footage of the Fowlers extending in say a ground test?
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 10, 2017, 10:28:41 AM
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:
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Drano on September 10, 2017, 10:31:16 AM
FYI the Fowler flaps and dive recovery flaps are separate independent systems.
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Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 10, 2017, 10:53:15 AM
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.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Drano on September 10, 2017, 11:30:59 AM
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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Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: FLS on September 11, 2017, 05:23:12 PM
I got responses from both sites. The EAA will be correcting the information and the NASM is looking into correcting theirs.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: oboe on September 11, 2017, 05:39:43 PM
That's good news - thanks for doing that, FLS!
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Randy1 on September 14, 2017, 12:50:54 PM
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.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Dawger on October 24, 2017, 02:39:55 PM
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.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Krusty on October 24, 2017, 03:07:34 PM
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!
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Devil 505 on October 24, 2017, 08:28:20 PM
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.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: colmbo on October 24, 2017, 10:42:01 PM
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. :)
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Krusty on October 25, 2017, 09:08:29 AM
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!
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: Randy1 on October 25, 2017, 12:35:25 PM
Just a reminder the drag is proportional to the square of the velocity.
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: DaveBB on October 25, 2017, 08:42:27 PM
The dive break was made to fit the specific job. Some aircraft only dove at a 20 degree angle (glide bombing it was called), others at a 70 degree angle. Hence the different sizes. The SBD, SB2C, A-1, and A-36 all had big dive breaks. They were all designed to attack at a steep angle. Fighter bombers and other aircraft weren't designed for such aggressive bombing (though that does not mean they weren't used for that purpose).
Title: Re: Clearing up some confusion about the Fowler Flaps and Dive Flaps on the P-38L
Post by: colmbo on October 26, 2017, 09:53:25 AM
There were some aftermarket speed brakes for the Mooney GA airplane. They were small plates that extended from upper surface of wing, small enough that at approach and climb out speeds they had virtually no effect on performance but when used at speed during a descent they worked quite well.