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Feb 13, 2017, 08:21 PM
It's all about scale models!
MrSmoothie's Avatar
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Build Log

Martin Baker MB5 (from Alfa Model Mustang)


Welcome to my second Build Log. My first one documented my conversion of an Alfa Models P-51D Mustang into the famous RB-51 "Red Baron" unlimted racer, which held the world speed record for piston powered planes, just shy of 500 mph. The model turned out nicely, and flew much better than I could have hoped for. I really liked the contra-rotating props, and felt like I was getting into a groove with my techniques and decided to do another similar project, although even more involved in some ways.

Here's the RB-51 Red Baron thread in case you're interested, from start to flight videos:

https://www.rcgroups.com/forums/show...B-51-Red-Baron

One of the great things about returning to this hobby two years ago (after a three decade layoff) is that the many advances have allowed me to construct and fly planes I could only have dreamed of back in the 70s and 80s. The Red Baron was one such plane.

I'm pretty sure that my introduction to the Martin Baker MB5 came in the May 1971 issue of American Aircraft Modeler. There was a great article including scale drawings by Bjorn Karlstrom. My 11-year-imagination went nuts, wondering all the what-if possibilities if the 460 prototype had ever made it into production. It has been described as a "Mustang on Steroids" -- that phrase would have been meaningless in 1971, but today it's so true. I never forgot the plane and a few times I considered making a rubber scale model of it (when I was a competition F/F scale builder). But I never did.

As I was winding up the RB-51 conversion project, I got to wondering just how close was the MB5 in outlines to the P-51D? Just as I was about to get rolling, a fellow RCGrouper sent me a high res scan of scale drawings which are (to my knowledge) no longer available for purchase. The quality and detail was outstanding -- far more information than the Bjorn Karlstrom drawings (which were very good).

I brought the scale views into photoshop, and then superimposed a P-51D Profile over the MB5, and adjusted it until the rudder post to cowl ring length was the same. I was thus able to identify which areas would need to be modified.

I'm going to back-track for a minute. Among the many new things in the hobby since my return is the phrase "kit bashing". This made me laugh the first time I saw it, as I did this all the time when I was a kid. In particular, I remember my vast collection of Bachman "Miniplanes" -- sort of the airplane equivalent of Matchbox cars. I kept close track of new releases -- eventually I'd get tired of waiting for a particular plane to come out and I'd modify one that I had to become the one I wanted. Eventually I drew detailed instructions on how to convert ten or so, and mailed them to the company's US offices. Imagine my excitement when, a few weeks later, I got a package in the mail containing some two dozen Bachman Miniplanes -- all Japanese bombers and fighters which were only available in Japan, evidently too obscure to bother releasing in the US! I was ten or eleven. So I've always been intrigued with the idea of taking one thing and making it into another.

Now, I do want to say that I did not undertake this project to merely modify a P-51 a little until it sorta-kinda resembled the MB5. My goal was to see if I could get the outlines, say, 95% accurate -- fuselage, tail surfaces and wing -- and (as with the RB-51) power it with contra-rotating props.

So before you accuse me of being insane, check out the photos of the actual aircraft, scale views, and profile of the two types superimposed.

I love Alfa Models from the Czech Republic. They are nicely done, have accurate outlines, and fly great. The size means I can have more than a dozen in my space-limited home and not get into (too much) trouble. Their P-51 is a great flyer, and I already had one done in Iwo Jima 1945 markings belonging to a friend of mine here in Gainesville, who is about 92 now. The RB-51, in spite of being heavier and having clipped wings, proved to be a great flyer too, and quite different than the stock Mustang.
Last edited by MrSmoothie; Feb 13, 2017 at 10:59 PM.
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Feb 13, 2017, 08:49 PM
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Parkguy's Avatar
Cant wait to see this one
Cheers Parkguy
Feb 13, 2017, 10:03 PM
The sky is my playground.
Dora Nine's Avatar
This will be fun to watch.
Feb 13, 2017, 10:14 PM
It's all about scale models!
MrSmoothie's Avatar
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Let the Games Begin!


OK, so let's make a partial list of the differences that need to be dealt with in converting the P-51D into the MB5:

-- raise the stab height
-- increase the depth of the aft fuselage
-- decrease the height of the radiator and reposition further aft
-- raise the height of the turtledeck
-- increase the slope of the cowl ahead of the canopy
-- modify the rudder
-- modify the stab
-- smaller canopy and make fairing around canopy
-- different shaped wing (likely replace)
-- somewhat different lower cowl (may or may not bother)

Those of you who followed the RB-51 build log will remember that the P-51D arrived from Alfa with the fuselage broken in half, and the radiator and lower cowl crushed. Alfa replaced the fuselage and the build proceeded. But I saved that damaged fuselage, glued the parts together and waited for a good idea of what to use it for, until now.

I should point out that one of my self-assigned challenges with the RB and the MB was to do as many of the mods with the same construction materials as a stock Alfa -- if possible, not simply build new parts from balsa or foam sheet, even though there's a good chance that could be faster and easier.

I enlarged the scale drawings to be "Alfa-sized" -- again, going by the rudder post-to-cowl ring dimension as the starting point. Then I could print out any portion of the drawing in CorelDraw to serve as a 100% plan for the changes.

First step, cut off the elevator and rudder, saving both to be modified.

Next, raise the stab position 0.375" -- I cut the stab saddle off, inserted some EPS foam sheet strips, epoxied, and then spackled. Wherever I spackle, I'm adding a bit of Polycrylic and mixing, which makes it adhere better and sand smoother as well.

Then, deepen the aft fuselage. I cut slits almost all the way to the end of the radiator, and then spread the end the appropriate amount. Gap filled with elongated triangles of EPS foam, then spackled.
Feb 13, 2017, 10:37 PM
It's all about scale models!
MrSmoothie's Avatar
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Modifying the Tail Surfaces.


Much more to do on the fuselage, but I wanted to get started on the rudder and stab.

The rudder was fairly easy. I cut the top off, and then straight down the hinge line. I discovered that by sliding the fixed portion up, that increased the chord the necessary amount, and then that was trimmed flush. The top was extended with two thicknesses of EPS foam sheet, and later shapped. The bottom of the rudder was trimmed off straight and similarly extended. This was then filled with coats of Polycrylic with some spackle added, sanded and primed.

No, I'm not making the rudder movable. I find that I just don't use it on hand-launched planes -- no need to unnecessarily complicate things and add weight.

The stab was much more of a challenge. I had to increase the chord on the stabilizer at the root, narrow at the tip, increase the chord of the elevator at root, and then finally alter the shape of the tips and cut the balance tabs (is that what they're called). If you want to tell me it would have been easier to make a new stab from scratch, how would be the time. And you'd be right. I used elongated triangles of EPS sheet to extend the stab chord, and CA'ed balsa strip to the trailing edge of the elevator, and trimmed to the new angle (narrower at the tip).

Tip was cut and sanded to new outline, and then the hinge gap filled with triangle section balsa for 1" in from the tip, so that balance tabs could be cut.
Feb 13, 2017, 10:48 PM
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Major Radiator Changes


One of the things that make people think the MB looks just like the Mustang is the distinctive underside radiator. However, when you look at the double profile, you will see that the MB's isn't as deep, and is positioned considerably further aft -- about an inch at Alfa scale.

I gave this a lot of thought before I started cutting. After removing the section, I also cut the radiator apart at the central seam as it would need to be narrowed, since the fuselage is narrower as you move aft.

Moving it one inch aft, created a gap around .5 x 1", which I reinforced with 1/16" balsa on the inside and then filled with scraps of EPS foam.

This took a lot of effort, but I was happy that the parts all fit together as intended in the end. All joints were spackled, hit with some polycrylic to seal, and then sprayed with flat gray so I could see how I was doing. Things to touch up, but all in all, not bad.

On the RB, I had to be really fussy -- since it's a nice "new" raceplane, I couldn't count on flat paint and weathering to help obscure small flaws. It's nice to be doing a WARBIRD again -- I can let some little things go and not get too nutty -- after all, my goal is a flying model, not a plastic display piece.

Next, the turtledeck, cockpit and cowling...
Feb 14, 2017, 08:58 PM
Scale Builder
You're a man after my own heart Nick! First the RB-51 and now the MB-5, two of my very favorite birds. If the MB turns out anywhere near as good as the RB did then I am sure you are going to have a superb airplane here. I'm subscribed and looking forward to more updates. Onward and upward!
Feb 15, 2017, 01:24 AM
Registered User
Good job! How were you able to get ahold of the drawings?
Feb 15, 2017, 11:12 PM
It's all about scale models!
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Top of the Fuselage: battery hatch, cockpit, turtledeck


The differences to the top of the fuselage between the P-51 and MB5 may be subtle, but are important to creating the correct profile. In fact, that can be said of all the changes I've made (and will make) -- with the exception of moving the radiator back one inch, the other changes are .25 to .375" or even less. But they all add up in the end.

I sliced off the turtledeck starting at the battery hatch and back to the stab saddle. I used some balsa strips glued to the inside edge to strengthen the fuse, but also to provide something to glue against when I go to fill the space.

The front edge stayed the same, but the aft end was raised, so the top line is nearly horizontal. I cut long tapered strips of balsa (elongated triangles) and CA'ed to the bottom of the turtledeck, then this assembly was glued to the fuselage.

The MB canopy is much smaller than the P-51's, so I cut away the molded seat recess for the canopy completely. I went to my Alfa "scrap bin" and found an exhaust pipe salvaged from a crashed MIG-15, and found that a section of that could be cut (and then rolled with a dowel to increase the radius) to fit the space.

The original battery hatch presented a problem -- the rear edge would end up right in the middle of a cool fairing that streamlines the windshield. That would not do. So I decided that the rear edge should be right on the line between the windshield and the bubble canopy. To do this, I glued the battery hatch to the portion of the fuselage right behind it, and did so keeping a straight line along the top edge -- the MB5's fuse tapers UP all the way to the canopy, and doesn't go flat, as the P-51's. This meant more tapered balsa strips on the bottom to fill the gap going aft. Small bits of foam from the exhaust pipe were cut and glued to fill the gap between the hatch and the turtledeck, right at the separation of the windscreen and bubble canopy.

At this point, it becomes a matter of sanding, filling, sealing, priming... repeat. Eventually the flaws become fewer and fewer, and reach an acceptable level. I spent another afternoon doing this after the last photo shown was taken.

I actually have a game plan for the canopy and the fairings, which are a little unusual -- stay tuned. Note how small the actual opening is for the cockpit -- this is very close to what the scale drawings show -- sure seems like a tight fit. Yet test pilots seemed in agreement as to the cockpit layout and visibility.

The wing is also underway, but much to do, since I'm starting with a spare F4F Wildcat (!) wing. I would probably have been a little closer by starting with a P-51 wing, or even a P-39 wing, due mostly to aileron size and shape, but I don't have any spares of those. I won't post anything until I'm further along.

Finally, Himax products is expecting the repair parts for my twin C/R motor, originally purchased for the RB-51 but then either a defective part, or defective treatment by me. Regardless, I bought a second motor to to finish the RB, but will wait on the repair parts which are expected from China in a couple of weeks.
Last edited by MrSmoothie; Feb 20, 2017 at 12:01 PM.
Feb 18, 2017, 06:24 PM
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The wing is in-progress, mainly changing the span and chord of the ailerons. The tips get clipped and then part of the center goes away as well. Pics soon.

I can see that I'll have a bit of work ahead of my to fill the gap between the new leading edge and the underside of the cowl, as the Mustang's wing center section angles forward.
Feb 19, 2017, 11:02 PM
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The Wing...


I've been working on the wing, and have it pretty far along now.

Admittedly there are other wings which would have been easier to repurpose for this project. Even the Mustang wing would have probably worked better, and the P-39 wing had some advantages.

I purchased several spare Alfa wings some time ago, one of which was for the F4F Wildcat. I decided this was close enough, even though I could see it would take some added effort.

The biggest problem was the shape of the ailerons -- not narrow enough and not enough "span". The center section was too wide (thanks to the Wildcat's barrel cross section). The wing chord wasn't quite wide enough at the root and too wide at the tip.

I'd worked really hard to get as many of the outlines close on the fuselage and tail surfaces. But the wing would have a few compromises. I do think that once you get the span and general shape right, the nuances can be hard to detect if you don't hold the scale drawings in your hands.

A healthy chunk of the wing center section was removed. Then the wing tips were trimmed. The new span is just 28".

While I couldn't match the taper exactly, I did trim the trailing edge from the root to the new tip at an angle. Then I trimmed off the aileron and part of the wing that would be added to the aileron to get very close to the correct size and shape.

The trimmed aileron and wing section were CA'ed together and then trimmed to the new shape. Balsa was glued to the wing and the aileron to fill the gap.

Originally I thought I could use the Wildcat wing tips, but it became evident that wouldn't work. I tack glued balsa block to both tips and carved/sanded to the correct shape, then removed. .020 styrene sheet was vacuformed over both and trimmed flush with the tip base. Then I glued to the wings, leaving the balsa inside the styrene. This worked out really well and will be more durable than the foam tips.

If you've built a Flying Styro Kit model, you know the wing panels have to be attached with a spar joiner -- that's what I did here. After making a template with the exact scale dihedral angle, I cut the spar from 1/8" ply and epoxied to one spar, then the two panels were epoxied together.

The Wildcat has a couple of oil cooler scoops on the undersides, near the wing root. These were removed, revealing an indented panel in the foam. Rather that filling with spackle, I cut out an even rectangle and filled with a piece cut from the trimmed off wing tips. I took this trouble because I intend to cut out the wheel wells and insert a partial tire. The actual aircraft originally had inner wheel doors but these were removed early on, so part of the tire was exposed when retracted. I think this will be a nice touch and one I do on all the FSK models I've built.

Note that I removed the paint from the tape on the leading edge. This came off quite easily with blue tape. I do this on all Alfas I intend to repaint, as they chip so easily. I lightly sand the tape with 400 wet/dry to provide some tooth before painting -- enamel seems to adhere better than the Alfa paint.
Last edited by MrSmoothie; Feb 19, 2017 at 11:12 PM.
Feb 19, 2017, 11:06 PM
They call me Lipo...
Justwingit's Avatar
Absolutely superb craftsmanship Brother !

Following your build with great interest....
Feb 19, 2017, 11:10 PM
It's all about scale models!
MrSmoothie's Avatar
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How things are shaping up.


I decided it was finally time to tape things together and see how how it's starting to look. I can tell I have some work to do on the undersides of the wing center section (and the gap between the wing leading edge and the lower cowl -- the Mustang wing angles forward in this area).

I will be making a balsa mold and vacuforming a fairing to extend the rear turtledeck into the rudder.

The canopy shown is from an Alfa P-47 -- much smaller than the Alfa P-51 canopy and happens to be quite close to the correct size for the MB5. I will be adding the unique fairings to the front and rear.

Doesn't quite look like the MB5, as the cowl isn't on it, nor the long snout of a double spinner. But we're definitely heading in the right direction.
Feb 20, 2017, 01:01 AM
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kasra's Avatar
Nice work. Hope it doesn't get too nose heavy.
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Feb 20, 2017, 09:01 AM
It's all about scale models!
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Quote:
Nice work. Hope it doesn't get too nose heavy.
Thanks Kasra (and Justwingit)!

Kasra, if you saw my RB-51 Red Baron conversion, I was concerned about being nose-heavy. But in the end, to my surprise, balancing was easy and took nothing special to accomplish. The MB5 tail surface mods and aft fuselage work mean this will be more tail-heavy (empty) than the RB. The MB5 is basically the same airframe (different wing) and will use the very same electronic components.
Last edited by MrSmoothie; Feb 20, 2017 at 09:06 AM.


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