Official FlightLine 1600mm B-25J Mitchell PNP Thread

[tab28682]

Here are a couple of pics of my B-25 flying at the JMEFF event in St George, UT last month.

The background scenery there is simply beautiful.

[tab28682]

Picture trouble.

[davegee]

Yesterday I worked on a prototype somewhat scale version of the M40 para fragmentary bomb commonly used by units like the 345th Bomb Group in the Pacific Theatre in WWII. I took a picture of an orange-painted printed bomb with a weight on the nose a M5 16 bolt. The entire bomb weighs about 12 grams and is dropped out of the bomb bay as I did again yesterday with four of these bombs as a test. The real planes didn't drop a bomb that looked like this; this was just a test to see how well it drops from the airplane. I also drop these bombs from the bomb bay without the parachutes and they look pretty realistic.

In the photo below, the green and yellow bomb is a prototype for the M40. I'll add more details and probably make some changes on how it is constructed. I'm hoping I can get at least 4 of these units made up this fall before Real winter sets in and I have to wait until spring to do the tests.

I painted the bombs orange to hopefully see them better when they drop onto the ground in the weeds, dirt, and sage. I have several painted OD and are more scale looking, but I would probably lose every one of those in the weeds if I do drop them. They are too delicate to drop onto the runway, mainly the tail fins. They would break off every time. Still, looking to print them in a stronger material where they might survive landing on the runway without damage.

[davegee]

Today I did a couple more flights on my B-25J. I have 18 flights on it now, and 11 of them involved dropping bombs as a test. Today, I dropped four bombs I call my MK. 1 parfragmentary bombs which are orange-painted printed bombs with a dowel screwed into them with a snap swivel to attach a small 6 inch diameter parachute. I just carefully folded the plastic 'chure, loosely wrapped the strings around them, and carefully set them in the bomb bay. I wanted to see if they would get tangled, or what would happen with them. Turns out, they all dropped out quickly as soon as the bomb bay doors opened, and the parachutes opened immediately for a gentle landing in the field next to the runway. They fell in about a 10 foot circle, which worked well, and no damage, especially with using the parachutes.

This first test was just to prove the concept of these "parafrag" bombs and the weight of the printed bomb is about what I'll weigh the more scale looking bombs that I'll eventually fly on the plane.

I'll include a drawing of what they actually looked like. One half of it was the bomb that would explode on contact with anything on the ground or sea, and the other half was the parachute container that the parachute was pulled out somehow right after they dropped from the airplane. The whole point, of course, was to allow the airplane to get away from the bomb before it exploded, possibly crippling or destroying the bomber in the bargain. It seemed to work pretty well for airplanes flying about 200 feet above the ground/water and were devastating to whatever they hit.

I finished the venturi tube (non functional but looks like it works) that removed gun gases from the nose of the plane into the atmosphere. You can barely see it on the underneath of the fuselage, but it is there. These were installed on B-25s converted to low level gunships. For instance, this Cactus Kitten would have ten .50 cal gunfire power shooting forward with the top turret in on the action.

[davegee]

Yeah, it was right in front of the second table. With the wind being what it has been lately, I'm not surprised. I found a piece of what I assume was my Avanti as well and it crashed way to the west , across the road. We're thinking of calling the field the junkyard

Grossman56

Yeah, the wind must have carried at least that one foam bomb that you found quite a ways. I wasn't able to track at all the path of the bombs while simultaneously trying to fly my airplane, but I had a general idea where the wind had taken them, which should have been west of the clubhouse. I was surprised I couldn't find any of them, but they were expendable. I can print up at least 6 scale looking bombs at a time on my Prusa mini, and they fly much better. I'm hoping if we get a good day to fly when you're off of work, maybe we could meet up and fly. I need someone to take some pics as the bombs drop to see how they look.

Cheers

Davegee

[Grossman56]

Yeah, it was right in front of the second table. With the wind being what it has been lately, I'm not surprised. I found a piece of what I assume was my Avanti as well and it crashed way to the west , across the road. We're thinking of calling the field the junkyard

Grossman56

[davegee]

Hi Dan: I’m sure that’s mine. Flew them one time and couldn’t find any of them. You can keep it or throw it away. I’ve printed up a bunch of scale bombs with weights in the nose that drop realistically. The Flightline foam bombs drop like confetti.

I’m also experimenting on “parafrag” bombs that drop by parachutes.

Just curious, do you remember exactly where you found that foam bomb? I really searched the area, but couldn’t divert my gaze as I was busy flying the airplane.

thanks,

davegee

[Grossman56]

Hey Dave, did you lose a 500lb bomb at the field??
Seriously, I found a foam bomb and since your the only other fellow who flies scale at the field, I thought it might be yours.

Grossman56

[davegee]

Thanks, Steve. Just a preliminary first step in sizing how this will go together. Charlee confirmed for me that there was a "break" in the venturi tube at the forward edge of the nose wheel door. So, there are basically two tubes that make a fairly tight fit with the gear up and doors closed. The forward tube is bolted to the forward fuselage, and the rest of the tube is bolted onto the nose gear door itself.

Charlee explained that the reason for that blister being off to the right side of the nose gear door and not centered on the centerline as the Flightline folks designed and produced it, is because for whatever reason, the nose wheel when retracted is not centered on the fuselage but actually is angled off to the side, thus the reason for the blister being offcentered to accommodate that fact. I found that very interesting why it was designed that way. Nevertheless, that is why I will have to modify that gear door with the blister now off to the side. The venturi tube is curved to follow the shape of the relocated blister with the exhaust for the air just behind that blister.

Cheers

davegee

One addition to that email. Charlee later explained in a response below my entry is that the reason the nose wheel is offset to the right when retracted is that it allows room for a crawlway for a crewmember to get to the nose of the plane on the left side of the aircraft, below the pilot.

Thanks, Charlee!

Davegee

[davegee]

davegee looking like the picture. I like it. Best, LB

This is one that has to be mostly "eyeballed" since I have no design drawings, but I think I'm fairly close. That's about all I can do for what it is. I'm glad I put it on, though. Good to get the ol' noggin working to figure out how to do it. Not a big deal, but sorta fun, too.

[Elbee]

davegee looking like the picture. I like it. Best, LB

[davegee]

Dave, Nicely done. Best, LB

I've been working a lot the last day or two getting my venturi tube replica finished. It actually went pretty well, and I have essentially finished it now. For those who may not have read my other posts on this, the plane that I am detailing from the standard liveries available from MRC, is a B-25J called Cactus Kitten. These B-25s that were converted to low altitude (I mean REALLY Low!!)strafers and bombers. They added a ton of firepower in the nose sections of these airplanes multiples of .50 cal machine guns, and in some cases a 75mm cannon, the same as on a Sherman tank! Anyway, with the gunships they collected a lot of the gun gases when all these machine guns fired. They needed a way to evacuate the smoke from the nose section ASAP. What they came up with was a hollow tube that ran from the nose to the back of the nose gear door. This tube in flight would have high speed ram air running through it and would suck those fumes into the venturi tube and then out of the airplane.

Up until a couple of weeks ago, I didn't even know this device existed back then. I was really impressed with the head work and competence of whoever figured this out. My Cactus Kitten plane can be seen in one or two period photos with this tube on the underneath side of the fuselage. I just HAD to add it!

Below are some progress pics. I had to do a little "surgery" on the nose gear door to make this work and everything lining up. The model has this blister in the wrong place. I had to carefully cut it off with a razor saw, do patching, sanding, all that stuff, and eventually I got the look I wanted. I used Evergreen 3/16 OD plastic tubing, and some.010" sheet for other detail areas.

[davegee]

The tunnel is underneath and ends behind the pilot, near the lower entry/exit hatch. The greenhouse also had an escape hatch on the left side. For safety reasons, nobody manned (crewed) the nose and upper & tail turrets during takeoff and landing. That is why no additional crew members will be added to my PBJ-1J.

Hi Charlee: after I wrote you on this subject, I looked on the internet and was able to find a couple of short videos (less than 1 minute each) of people sliding through that crawlway on the B-25J that you mentioned underneath the pilot's seat and then into the nose. I guess it pays to be young and Very thin and agile back then to accomplish that, especially with your flight gear on!

I did get to take a flight on both a B-24J and B-17G with the Collings Foundation many years ago. My dad flew B-24s and B-17s in combat in WWII in Europe, so it was always of interest to me. On the B-24, they let us crawl around the plane for a few minutes and I went through the small crawlway on the side of the nose gear and into the nose of the bomber. You had to slide by the retracted nose gear to get up front, no easy task for someone over 60 like me back then. Somehow they did it, but with 26,000 aircrew lost from the 8th USAAF alone in WWII, coming home at all was no easy matter!

[Charlee]

The tunnel is underneath and ends behind the pilot, near the lower entry/exit hatch. The greenhouse also had an escape hatch on the left side. For safety reasons, nobody manned (crewed) the nose and upper & tail turrets during takeoff and landing. That is why no additional crew members will be added to my PBJ-1J.

[davegee]

The nosegear had to retract into the right side of the fuselage to make room for the crawlway that gives the crew access to the greenhouse nose. This also gave a space for the 75mm cannon that was mounted in the B-25G and B-25H, which wasn't even a thought in the head of the original design engineers.

Great info, Charlee. Where was the crawlway entrance to the nose? Did they have to get in before the pilots got in their seats by sliding the seats back, or was the crawlway underneath the pilots' floor? How did that work for the guy in the nose in an emergency like bail out? I'm still trying to picture how this aircraft was set up as I have not been in one or seen photos of that crawlway to the nose yet.

Thanks,

Davegee

[Charlee]

The nosegear had to retract into the right side of the fuselage to make room for the crawlway that gives the crew access to the greenhouse nose. This also gave a space for the 75mm cannon that was mounted in the B-25G and B-25H, which wasn't even a thought in the head of the original design engineers.

[davegee]

Dave, Nicely done. Best, LB

Thanks, Steve. Just a preliminary first step in sizing how this will go together. Charlee confirmed for me that there was a "break" in the venturi tube at the forward edge of the nose wheel door. So, there are basically two tubes that make a fairly tight fit with the gear up and doors closed. The forward tube is bolted to the forward fuselage, and the rest of the tube is bolted onto the nose gear door itself.

Charlee explained that the reason for that blister being off to the right side of the nose gear door and not centered on the centerline as the Flightline folks designed and produced it, is because for whatever reason, the nose wheel when retracted is not centered on the fuselage but actually is angled off to the side, thus the reason for the blister being offcentered to accommodate that fact. I found that very interesting why it was designed that way. Nevertheless, that is why I will have to modify that gear door with the blister now off to the side. The venturi tube is curved to follow the shape of the relocated blister with the exhaust for the air just behind that blister.

Cheers

davegee

[Elbee]

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Dave, Nicely done. Best, LB

[davegee]

I did some more test flights on my B-25J today. Did one flight with a bomb drop of 4 printed bombs from a bomb rack designed by Fast N Light and another person he knows where the original idea might have started. In any event, this was the best bomb drop so far, the orange painted bombs dropping in a line in a very realistic manner, from what I could tell with my peripheral vision as I concentrated on not crashing the airplane from a pretty low altitude of about 20 feet above the ground in level flight. I was able to retrieve all four bombs afterwards, undamaged.

At some point, I'll probably have someone who comes out to the field who can take some photos of the bomb drop to better judge how they are doing. I'm thinking this setup is pretty realistic.

The next step is to work on a scratchbuilt replica of the venturi tube that goes underneath the forward fuselage of the aircraft. This tube helped remove gun gas fumes from the nose section in flight. I'll take some more pics as I proceed with it. Not much to go on, but I think I can come up with a fairly believable replica of this part that was on planes like Cactus Kitten that I am working on.

I wanted to improve my landings, mostly due to the Robart nose tire not staying on the printed scale wheel properly. Using more E6000 for one wheel, and clear Gorilla Glue for another nose tire, I think I have that problem licked, and touchdown and rollout is now much more realistic and less jarring to the plane than before.

[davegee]

This is the factory drawings of the fuselage that the cross sections are related to. I don't think it has been printed anywhere else before. If it had, the existing three views and profiles of the B-25 would be more accurate. For the H & J models the horizontal stab was raised 7" and the fuselage widened starting at station 558.

Those are a real historical treasure, Charlee! Thank you for sharing them!

Davegee