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Sep 13, 2013, 06:20 AM
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Trevorh's Avatar
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P47 Thunderbolt (Brian Taylor 76in)


I chose this model as my 2013/14 Winter build with a view to replacing an electric conversion of the now-obsolete Air Loisirs MiG 3 which I put together back in 2006. However, once the P47 plan arrived, it became clear that the relatively modest increase in wingspan (62in for the MiG, 76in for the P47) somewhat understates the difference in size between the two models. Whereas the Mig is a bit Spitfire-esque, with a svelte fuselage and fairly high aspect wings, the P47 is a big brute of a plane with relatively short, stubby wings. Not to put too fine a point on it, the fuselage is HUGE, making a mockery of the idea that, by disposing of the MiG, I could make space in my overcrowded hangar to accommodate the P47.

No matter, by then I was mentally committed to the project and, some six weeks later, common sense still hasn't triumphed over ambition so, in spite of the fact that no balsa has yet been cut, the time has come to declare this project underway. To date, attention has been focussed on the non-balsa aspects of the project, in particular retracts, cowl and canopy. The power train ought to be in that list too of course but that depends on the likely all up weight and, at the moment, that remains the biggest unknown of all.

More later, but anybody with any tips, questions or other observations, please feel free to chip in at any time.

Trevor
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Sep 13, 2013, 09:54 AM
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vonJaerschky's Avatar
Great choice, Trevor. There are a few very large P-47's in our club, and they all fly great. They are gas powered, and all quite heavy but the large wing area and wide track landing gear results in a great handling model in the air and on the ground. I'm sure your version will be even better, with light weight and clever engineering! Are you modelling the bubble canopy or the Razorback version? Here's some inspiration for you, my friend's P-47 in action at our field last Sunday (sorry about that petrol racket!)
Stu's Top Flite 85" P-47 (5 min 32 sec)
Sep 13, 2013, 10:36 AM
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Trevorh's Avatar
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Canopy and Cowl


Wow, it certainly didn't take you long to find me Frank! Thanks for the video - that was some very smooth flying and showed just the sort of flight envelope that I am hoping for.

The Brian Taylor plan is for the razorback version and I have bought the canopy to go with it - past experience has shown that it is a good idea to have the canopy to hand before starting to build the fuselage!

I'm still undecided about the cowl though. Traplet do offer a fibreglass cowl but of course it's designed to cope with the vibrations of a big i.c. engine, so I was hoping to find a lighter solution. On the Chipmunk, I arranged to fit the motor through from the rear of the firewall, obviating the need for any front access, so the cowl was just built on as an extension of the fuselage, mainly from 1/16in balsa sheet. The P47 front end will be a bit more complicated but carving a plug would probably be just as much work.

As it happens, I plan to build the wing first, so there's plenty of time to consider options for the cowl. On the other hand, the retracts are right at the top of the list of things to be sorted, so. . . . .
Sep 13, 2013, 11:14 AM
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Retracts


My experience of retracts is pretty limited, albeit varied: The TopFlite DC3 which has Robart air units, the aforementioned MiG3 which has mechanical retracts driven by a central servo, the scratch-built Welkin with home made units, based on Ivan Pettigrew's design and, most recently, the DH88 Comet which has HobbyKing electric retracts.

Although all these systems have proven reliable, the Hobbyking electric units have been particularly impressive and much easier to set up than the others. A quick look at the Hobbyking site revealed that the units used in the Comet are now available with all-metal trunnions, so I ordered two of these.

While awaiting delivery of the retracts from China, the wheels arrived. These confirm what a beefy machine this is - they are 5in in diameter and, at just over 6oz apiece, don't feel particularly heavy for their size. However, when I put one of them on the end of a ruler and picked it up at a point to simulate the length of the u/c leg, doubts started to creep in.

Sure enough, when the retracts arrived they really couldn't cope with the task of hauling up those wheels on such a long leg. Whilst it might have been possible to rig up some counterbalancing springs, I really had to admit to getting it wrong - at least I now have a spare pair of retracts for the DH88!

After a bit of a hunt around for some heavier grade retracts, I happened across a pair of Robart mechanical units made for the original Hangar 9 150 sized P47. This model was slightly bigger than the Brian Taylor one (81in span vs. 76in) but I felt that such a slight difference could be accommodated so the order was placed.

As you can see from the second picture, these are very substantial units. They aren't too heavy but the main problem was that, when I fitted a pushrod and clevis to the mechanism, I couldn't raise the wheel, in spite of the help of the counterbalancing springs. I took one of the units along to a club meeting and, several sore hands and one severely bent pushrod later, the consensus was that they couldn't possibly work!

Undaunted, after a bit of research and cogitation, I ordered a pair of Spektrum A7000 retract servos and, after fitting some heavy duty servo arms (the originals just twisted under the strain) surprise, surprise we have lift off - or at least lift up!

The one remaining issue with these retracts is the mounting. There are two problems: Firstly, with the servo fitted, I don't think it will be possible to maneouver the unit in between two mounting beams and secondly, when tilted to the appropriate angle, the units don't quite fit within the depth of the wing.

At the moment my plan is to increase the wing depth by 6mm or so at the appropriate point, build the basic wing framework with the key ribs missing, and then mock up the mounting arrangement in situ in order to fine tune the mounting angles and assess the difficulty of fitting and subsequent access.

Here ends chapter one of what could yet be quite a long retract saga. Even if the mounting of the main gear goes smoothly, there's still the question of the fairings and doors - and of course the retractable tailwheel, but all that can wait a month or three.

It's just possible that I ought to start cutting out some wing ribs pretty soon.
Sep 13, 2013, 11:39 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Great video Frank, and a really good flight pattern by your pilot. I've seen the plan for this beast and as Trevor says - it's big! Are there any other Brian Taylor builders who could advise on retracts?
Sep 26, 2013, 11:37 AM
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Retract Mounting


I cut out a set of inboard ribs with the height increased a little to accommodate the oversize retracts. The tricky ribs were cut from card to make it easier to adjust the various mounting angles and cutouts without wasting too much balsa.

Before going any further, I clamped a couple of 3/8in sq basswood bearers as far apart as practicable and set about trying to manoeuver a retract into position with the servo already fitted. As expected, this proved impossible but with some judicious carving of the rearmost bearer, it eventually went in. I know this has weakened that section of the bearer but, since the only alternative was to leave it out of that wing bay altogether, I reckon that's a good compromise.

Anyway, after several sessions of fiddling and fettling, I now have a mockup of the retract in position, with the angles more or less sorted. I've shortened one of the oleo legs by about half an inch and nothing bad happened, so the retracted wheel now comes into the position marked on the plan. There's not much rib left over the top of the wheel bay though so I still might look for some other wheels - anybody have a good uk source for 4.5 - 5in wheels?

So, the scalpel and hacksaw are in use. In another day or two it should be time to break open the glue!
Sep 26, 2013, 05:08 PM
Gravity sucks.
mrittinger's Avatar
Retract angle = Degrees forward rake on the ground + Degrees rearward when retracted /2. Mount retract unit with equal tilt and caster, wheel will track perfect and fit flat into well.

Brians designs are first rate. Very nicely done, very detailed, and very nicely organized. I'm looking forward to seeing this one built, I'm sure it'll look great.

Mark
Last edited by mrittinger; Sep 26, 2013 at 05:15 PM.
Sep 26, 2013, 06:45 PM
Scale Builder
I have worked from most of the top designer's plans and Brian Taylor's are among the top two or three in both scale fidelity and parts accuracy. The only thing I'm not a big fan of is the fuselage box common to Taylor's designs. I prefer to build on a crutch. I have this same set of plans as well as the cowl and canopy and will be watching your build with interest. Good luck!
Sep 27, 2013, 02:30 AM
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Trevorh's Avatar
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Hello Mark and Chad. Good to have you both on board. The trouble I have with retract angles is not the theory but rather finding a suitable reference from which to measure. Even then, computing where to cut the apertures in the ribs to take the bearers would still be a challenge. So, for me at least, fitting retracts (especially ones not intended for the plane in question) always ends up being a 'cut and try' job. But then, one of my mantras for building is 'what you lack in skill must be made good with patience'!

You both mention the detail and scale fidelity of BT's plans and I think that this is one aspect that can make them a bit daunting to mere mortals like me. Hingeing of control surfaces is a case in point. The trailing edges of the wings are so complex that the plan view is very hard to interpret. Fortunately there are enough sectional drawings on the rib templates to work it all out. We each have to set our own standards and this is one area that I will be simplifying considerably.

Chad, is there any chance you could weigh the cowl for me? This is still an area where I have decisions to make. There's no rush - I can't see me starting on the fuselage much before Christmas!
Sep 27, 2013, 10:59 PM
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Quote:
Originally Posted by Trevorh View Post
Chad, is there any chance you could weigh the cowl for me? This is still an area where I have decisions to make. There's no rush - I can't see me starting on the fuselage much before Christmas!
Trevor,

I would be glad to weigh the cowl for you. However, I purchased mine second hand and I'm not sure if it is an original Brian Taylor cowl or if it is one of those offered by Traplet since they took over the Brian Taylor line. Thus, it may or may not be the same as what is currently available. I will be using gas power in my Jug and would gladly swap my cowl for yours should you happen to receive one that weighs more. Send me an email if you don't mind, otherwise I am bound to forget all about it!

[email protected]
Oct 04, 2013, 05:32 AM
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Trevorh's Avatar
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This aeroplane just got half a pound lighter! Retract servo current consumption has probably gone down by a factor of 10, too.

The main motivation for looking for other wheels, as mentioned earlier, was the limited space in the wing bay for the retracted wheel. The foam wheels are a little skinnier so should make the fit a little less critical.

The second set of inboard ribs is now cutout so the next step is another dry assembly, this time of the left wing panel. Then, once the outboard wing ribs are cut out, maybe some real assembly might begin. . .
Oct 09, 2013, 10:48 AM
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Once more, with glue!


Finally took the plunge and started glueing ribs in place. R1 is still not glued, pending finalising the dihedral brace arrangements. I am hoping that the structure this far will be strong enough to be lifted off the board for another trial fit of the retract so that the position of R6 (which is also not yet glued) can be finalised - I am still haunted by the fear that the retracts will be impossible to fit once the wing is built!
Oct 16, 2013, 06:20 AM
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Trevorh's Avatar
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A quick question: As you can see from the previous pictures, the wing ribs have 'feet' on them to help with alignment. These have worked well while constructing the framework but the wings have to be sheeted top and bottom with 1/16in balsa. Until the sheeting is applied, the framework is still flexible so I'm struggling to work out what the sequence of sheeting / foot amputation should be in order to minimise the risk of building in warps. Any thoughts?
Oct 16, 2013, 06:58 AM
I eat glue
Sheet the top first, then build cradles or jigs using the top of the wing ribs as templates. Take the sheet wing off the board, pin the jigs in place, then set the wing sheeted side down into the cradles and sheet away. Just be sure to allow for the washout twist that the feet have put into the wing!
Oct 16, 2013, 11:39 AM
Scale Builder
Quote:
Originally Posted by Trevorh View Post
A quick question: As you can see from the previous pictures, the wing ribs have 'feet' on them to help with alignment. These have worked well while constructing the framework but the wings have to be sheeted top and bottom with 1/16in balsa. Until the sheeting is applied, the framework is still flexible so I'm struggling to work out what the sequence of sheeting / foot amputation should be in order to minimise the risk of building in warps. Any thoughts?
The idea is generally that once the top sheeting is installed the structure will be rigid enough that the bottoms can be sheeted without a jig. This is pretty standard practice and modelers have been doing it that way for as long as their have been modelers. However, as baldguy mentions above, you can build a saddle in which to place the wing in order to be absolutely sure that no warps are introduced during the process of sheeting the bottom sides. I am a big proponent of making absolutely certain that my airplanes are built accurately and, thus, I almost always use a saddle as described. Just be careful to make it accurate so that the wash out is maintained. What's even more important is to make sure both sides are identical. Here are some examples.

Mustang wing in the saddle.



Here is the saddle for my Hellcat prior to placing the wing panel in it. Note the "fence" along the trailing edge of the saddle in order to support it and make sure that it stay perfectly straight as well.



And with the panel placed in the saddle.



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