P-51A from TF red box kit - Page 3 - RC Groups
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Jan 18, 2013, 04:58 AM
Cohen the Barbarian

Got a bit done

The hatch has been shaped and the cockpit frame cut back at the correct angle (according to the plan).

I am working out how to mount the tail retract, thinking to mount a couple of spruce rods that the retract mounts to, with the load carried to the fuselage sides by load spreaders made of light 1/4 or 1/8 balsa.

I hope the scale purists will forgive me if I do not immediately put tail and inner gear doors on, and even when they are added, I am intending to have the inner gear doors stay open when the gear is down, for simplicity. I have read that the A had a latch which held the inner gear doors closed even pressure was lost from the system after shutdown.
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Jan 18, 2013, 08:34 AM
Registered User
I admire your dedication to using balsa, and as a many-years kit and plan builder (and a couple of scratch builds as well), I have turned my share of blocks to dust as well. But you may want to consider using pink insulation foam to fill in some of the huge spaces and shape that to match, especially if you plan to fiberglass the whole thing anyway. I now routinely use foam in place of balsa, particularly where structural strength is not a concern but lots of shaping is. The amount of time you can save is a lot, and the foam can be shaped very much like balsa, and has 1/4 to 1/5 the density, making weight savings an automatic. You can also get it in thicknesses varying from 1/2" to 2", and of course can hollow it out if you want even more of a weight reduction or need to make room for other components. I can't help but think of all the time you would have saved if you had used foam blocks for the nose sections, instead of layers of built up balsa, and the amout of time spent sanding and shaping would have been a fraction of what it was (I have a complete set of PermaGrit tools, which I highly recommend--but get them at an expo if you can because they are pricey).

Of course, looking at where you are (which I just did), you may not have the availability of materials that I am lucky to have. However, foam really is a nice alternative to balsa at times...Pete M
Jan 19, 2013, 01:40 AM
Cohen the Barbarian
Hi all, will be away for another few days.

Pete - thanks for the tip, I will look into sources. Country living has many advantages, immediate access to all kinds of resources is not one of them. When I find a source I may redo the nose - shouldn't take too long.

In the meantime I will be working on mounting the tail wheel and making the aerofoiled tailfeathers.
Jan 19, 2013, 07:10 AM
Registered User
Ldm's Avatar
looking good !!! Interested to see how you take on the tail shape

Pete i have used pink foam as well , even started a thread that said " is it me or is foam hard to sand" , yes an experienced builder with all types of kits i have tried everything from foam to flite skin .
Foam is great but needs to be sanded very carefully ,sanding to fast , too strong or too rough of a grade can make the the foam porous.
Would love to see your tools and methods perhaps in a separate thread .
Jan 21, 2013, 02:04 PM
I eat glue
Nice work.
Scale purists be damned! I'm going to be converting a Model Tech P-51 (65") to electric and it's not going to have a retracting tailwheel let alone gear doors!
If I want all out scale I'll build from Brian Taylor's Mustang plans.
Jan 21, 2013, 06:27 PM
Cohen the Barbarian

A note from exile...

Hi guys, still away from home, so I am not making any actual progress in a physical sense, but I have been putting some thought into the tail feathers etc.

Baldguy - I agree about scale in general, although I do get rather manic with my projects (so does my whole family). The primary concern with this build is to develop new skills and get a (hopefully) great flying bird to hone my warbird flying, prior to building the Gold Edition Spitfire Mk IX, Seafury and a Brian Taylor Hurricane, all of which kits are on my storage shelves, waiting.

Having said all of which, I have done some research, which indicates (please let me know if I am wrong), that the only difference in the wing planform was the extension to fit the landing gear. With apologies for the quality of the image, the picture below shows the two wings overlaid, black outline is A,B,C and orange is D. I feel it will be pretty easy to change the wing to the correct shape, probably 4 wing ribs and a bit of filling at the fuselage, a small price for a big gain in accuracy.
Jan 21, 2013, 08:29 PM
Scale Builder
[QUOTE=GenghizCohen;23885369]I have done some research, which indicates (please let me know if I am wrong), that the only difference in the wing planform was the extension to fit the landing gear.[QUOTE]

So far as I'm aware that is the only planform difference Genghiz. Not sure if you're incorporating them in your build or not but the clam shell gear doors on the A/B/C are shaped a bit different than those on the D as well.
Jan 23, 2013, 05:50 AM
Cohen the Barbarian
Life has been ten kinds of chaos recently, should settle down in a few days.

Thanks for the confirmation re wing form Chad, it really helps. I am thinking about the inner doors, but may leave them until the major assemblies are done.

Looking at the tail feathers, the supplied parts are basically planks and weigh 3.25 oz., so there is not a huge scope for lightening, but great possibilities for appearance. Also the horizontal stab supplied has warped in the time it has been waiting since manufacture.

I am considering two approaches, an may well make both to see which serves the purpose better.
Option 1 starting with a 1/8 core, build the airfoil section with riblets, then sheet and sand. Using light balsa this should save a little weight, but I feel it may not have the reserves of strength I would like.
Option 2 is similar, but has a spruce spar set into the core at the deepest point of the cross section. There would be some scope for recovering the added weight by cutting holes in the core.
Neither option seems particularly problematic in execution.

Any suggestions or thoughts?
Jan 23, 2013, 10:51 PM
Cohen the Barbarian
Hi all, got the blank for the stab made. My calculations show it should wind up weighing almost exactly the same as the original, maybe a touch less. The big advantages are appearance and the replacement is straight. When completed it will be a 3 layer structure, so should stay straight
Jan 24, 2013, 01:51 AM
Registered User
Ldm's Avatar
it may be too late but I have an unusual suggestion for you as another option .
1)cut lightening holes in your stab strategically not to weaken it .
2)Add depron foam to both side , sand to shape leaving the area that attaches to the fuse bare (no depron).
3)lightly sheet the depron with thin balsa , cover with coverite and your ready for painting .
The advantage is , airfoil shape , light weight , no real loss in strength and your weight changes may even be lighter
Jan 25, 2013, 03:10 AM
Cohen the Barbarian

Getting back on track...

at least for the moment.

I have ordered the depron, it will probably be a week or so before it arrives (the shop is fast and efficient, Australia post... not so much. Great suggestion, thanks LDM.

Blanks for the stab and fin are taking shape, fin will have a king post set into it to help lock the structure together accurately. The stab sits on its saddle squarely, although the picture doesn't seem to reflect this.

Having overlaid the scale 4 view wing onto the plans, the centre section is actually correct, what they have done is to stretch the wings by about 10%, giving a straighter leading edge and exaggerating the step in the middle. While it would be easy to correct this (just take about 1/2" out of the spacing between each pair of ribs outboard of the step) I am not prepared to sacrifice the wing area and potentially the whole aerodynamic integrity of the model.

Some redesign of the wing structure will be necessary to fit the retracts - the wheel wells will cut into the lower main spar. I will try to determine correct strut lengths and location, rather than simply rely on the plans. HK do some nice looking scale struts which may be usable.

The spinner in the pic belongs on my GE P40 (the ARF version), so it is a little oversize for this bird. The prop does not have the correct profile, but is almost scale size, and works well with the P60 on 6s - I have used this combo before on my Elder bipe to get quasi aerobatic performance. It looped ok, but forget about rolls...
Jan 28, 2013, 08:53 PM
Cohen the Barbarian

At last

I have received the 3 and 6mm (1/8 and 1/4") Depron. Progress can resume.

As an aside, I have added some pictures of my previous e-conversion, a red box Top-Flite Elder Biplane.

I would not recommend this as a first build. After a near tragic maiden, it underwent a major redesign involving enlarging the lower ailerons, adding ailerons to the upper wing, setting thrust lines accurately (about 3 words in the manual about this), correcting angles of incidence and decalage (no information at all in the manual, extensive research on the net required), upgrading the undercarriage, adding a (steering) tail wheel instead of a skid and giving it a name.

That being said, the conversion side of things worked well straight out of the box (so to speak) and as modified it flies well with its light wing loading (10 pounds, 10.5 sq feet of wing). It's a bit sad that in the time it took to build and rebuild, my flying interests moved on.
Jan 29, 2013, 06:42 AM
Cohen the Barbarian
Got a little done - lightened the stab core (not much gain), applied Depron and placed it between two flat surfaces held down by lead while the glue sets.

To those in the know - what is the best sort of glue to use to hold the Depron on? I am thinking gorilla glue for the skin for strength, but is there something lighter/better to use on the interior joins?
Jan 30, 2013, 06:17 AM
Cohen the Barbarian

A few impressions

For those who have not tried working with Depron.

Firstly it is very light, about half the weight of the lightest /softest balsa I can find.

It is similar in rigidity to the cross grain strength of light balsa.

Cutting - make like you are cutting cross grain very soft balsa, very sharp knife, multiple light cuts. It will cut quite well with a scroll saw.

Any other thoughts?
Jan 30, 2013, 07:05 AM
Registered User
Well, as one who has worked a lot with both:

Depron (the white) has a density of 2.45 lbs/cu ft, making it 1/3 to 1/4 the weight of balsa (~8 lbs/cu ft), unless you are using really light contest balsa (4-6 lbs/cu ft).

A 1/4" thick Depron rib is about the same weight as a 1/16" balsa rib, but is stiffer and has lots more gluing area.

You can put a curve in Depron using a heat gun, use a round tube to help shape it, but be careful of melting it. This one takes a little trial and error to find your technique.

My best substitution has been to use 3mm Depron for wing/fuselage sheeting (suitably supported by ribs or a balsa framework), to give a really light structure.

A good followup is to use silkspan and WBPU on top the Depron, to get rid of the foam grain appearance. Lightly sanding gives you a surface finish that will be super for either a film or even better, a painted finish. You can also use very light weight/low temp iron on covering directly on the Depron to give a smooth gloss finish, but will not necessarily hide the grain. However, using one of the heavier films (I recommend Econocote or similar because it is cheaper, lower temp, and since you have "solid" Depron surface underneath, you don't need to worry about sags, and it looks a lot like a balsa model).

When using 3mm (or 2mm) for wing sheeting, it is easiest to use a false leading edge of Depron, followed up with a balsa LE to give ding resistance. See drawing below for better explanation.

I use CF tubes for center wing spars, easy to stack the Depron ribs and then use a prepared brass tube (file saw teeth into one end, sharpen, and then twist it through the ribs). Just be careful to keep it vertical, may want to use a jig.

A SHARP knife is your best friend, don't be afraid to sharpen the blade often or change them out. Otherwise, wood files work ok, it will sand well with blocks, and PermaGrit tools are superb.

Lightweight white spackling will quickly become a staple. It can be used to fill in minor dings/imperfections, fill in seam cracks, and also can be used to completely cover a Depron part if you want to hide the grain even more--just be careful not to sand it all away when you go over the part (want to use 320 or 400 grit paper for this). If you have bigger holes or problem areas, just cut it out and put in a new piece, and reshape according.

I have found Depron a fairly good substitute for balsa for anything non-structural. Anymore I don't use1/8" or 3/16" solid balsa fuselage sides, or 1/4" thick flat balsa tail surfaces. Instead, I make a framework of balsa and then cover both sides with 3mm Depron, much as you did above. If doing an airfoiled tail, then depending on thickness or size of tailplane, would either add ribs and cover, or use 6mm Depron on both sides and then sand in the airfoil. Get a nice long sanding block for this. Again, to ensure stiffness, I will often use an appropriate CF tube in the framework or as part of the trailing edge..

However, it does not have the give of balsa and can be brittle, much like lite ply. But on the plus side, it's very easy to repair--either glue the pieces back together or do a sectional cut and replace.

For an example of how I used Depron to good advantage in place of balsa, see my North Star or Cutting Edge threads:



Pete M

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