TF P-51D Gold Edition

[the-plumber]

Ok, I've lost the reference . . .

Once upon a time I had a link to a post (somewhere) which contained "the" solution to P-51 GE retracts vis a vis gear doors and fitting in the wing properly.

My copy of the kit has been gathering dust far too long, and I'm building a list of stuff to order.

Anyone know it the standard Robart or Century Jet gear accomodates gear doors, or do you really have to brew up your own based on the Violett T-33 mechanism ?

[markamerica]

I started this out on a Great Planes .40 P-51 kit, but have gone on to adapt it into subsequent P-51s. Daughter lost the digital camera, or I'd post some pics to speed this up. Hopefully, by the time we get closer to finished on her current P-51 ARM(All Ready for Modifications), discussed in another thread elsewhere, I'll take some pics of the mechanism if you're interested.

What I learned to do was to run the entire main gear operation, including timed main doors, off of one Futaba S136G lo profile 180 retract servo. No, I'm not making this up. Here's how it works:

First, using mechanicals, you want some with the sliding bar mechanism, like the hobbico types. (You can dress them up with struts, or strut covers either being available from Robart.) All of this is predicated on doing the layout at build time, when you're framing up the wing. So you need to create beam that will support the hinges and gear doors. This beam should run between the main spar joiner plate and the leading edge. This beam should be slightly recessed because you're going to slot and bevel a piece of balsa to actually be the center of the bottom surface of your wing, into which the gear doors will be hinged.

I'll skip through the whole layout and shaping of the gear doors, as I expect you can easily figure all that out without my help.

So back to your single servo: It must be set into the wing so that its long axis is in line with the airfoil, with the servo arm of course facing up. Here's where we get "tricky": Rig the main gear as normal with your pushrods. Get a second pair of screw-lock pushrod ends, and you'll want to use a four-point arm here, btw. So you have screw-lock ends in all the arms. Two, opposing one another, left to right, have rods for the retracts. Now take a pair of 3/4" screw to fit your unused screwlocks, and you're going to put nylon tubing on them, and a bar across the top, between the nylon tubing and the screw heads... So now you have an elevated bar, that when the retracts are fully retracted, are parallel to your centerline, and when the retracts are fully extended, are also parallel to your centerline, having moved 180 degrees.

With me so far?

So now you have the basis for your gear doors, because you need a timed motion that will take the doors down, and then up, all within the 180 degrees of servo rotation. Now, you make two rods. The rods will have a number of bends, and it is best here to think in terms of a crankshaft. The "rod" journal is going to be held against your nylon, because you are going to build the "crank journals" on either end of the span front to rear. They will be held against your nylon by elastic bands of some sort, from the sewing section of Walmart or whatever, On the rear side of the rear journal, the rod will bend and extend towards the opposite side.

Here's how to imagine this: As your servo rotates, this elevated bar with the nylon covered screws will spread these two "rod journals". As they do, the ends of the rod behind the rear journal will now travel downward, towards the wing. Those ends are ball-linked to rods in turn ball-linked to control horns set into the gear doors. As the ends of the rods travel downward toward the wing, the horns are pushed down, and thus the gear doors are opened.

Now, as the servo has rotated 90 degrees, the doors will be fully down, and the gear will themselves have travelled out of their wells sufficiently that now, as the gear doors begin to travel upwards(as the elastic bands squeezes the two "rod journals" back together against the nylon bearings), the struts and wheels are clear. As the servo finishes its 180 degree travel, the doors are closed again, but the gear is down. When you retract, the same process happens in reverse.

Now, the reason for the Hobbico slider-bar type retracts is this: They have as their last 3/16" of travel on both ends, the locking process, up or down. That helps you, because during the time it is unlocking, the doors are already in motion, and while they're locking at the other end, the doors are traveling that last bit. This is how you avoid an interference issue with the doors, in part. The doors are in motion all the time, but at either end of the slider-bar's travel, the gear are already up or down and the locking is occuring.

Like I said, a good picture or two would clarify this, and I know most of you guys like your air units, so it's probably not so useful to go further. Still, I like gadgets, and this was one heckuva solution back when I could afford the air pieces.

Regards,

Mark

[the-plumber]

[SNIP]
With me so far?



Wow, and, not really.

If I understood what I think you meant, you used direct drive mechanical retracts, and added an escapment linkage which cycles the inner gear doors in conjunction with the movement of the gear leg.

I looked at one of those once, and then set off to find some Excedrin.

As you said, a picture would help a lot, but don't make a project of it on my account. I plan to use pneumatics on the gear and gear doors, controlled by a UP sequencing valve (not cheap, but dead bang reliable).

I saved your post and will be curious to see the photos when you get around to posting them.

My concern is that I think I read where the Robart 605HD units don't fit into the wing without considerable nipping and tucking, and don't allow for gear doors because of the depth of the retract mechanism.

At least, I _think_ I read that somewhere.

What I'm looking for is a retract unit which can be fitted to the wing correctly and which is shallow enough to allow for inner and outer gear doors that work properly, _and_ which either has a functioning strut or can have a functioning strut adapted to fit.

My A-36 project won't start until I finish two giant models already on the work benches in the building shop. I'm at the paint and sand stage, and if I get lucky I'll be done with those models by the end of March.

Of course, I said that last year and the year before, and the year before that.

Work spoils a lot of fun, y'know.

Thanks for the explanation, though. I'm betting it has other applications as well, even if I can't think of any right off the bat.

[markamerica]

exercise in instruction as anything else. The daughter is learning. Thinking outside the box is part of the program... Yes, it is an Tylenol fest, at least it was the first time I built them. I have no doubt but that there are other apps for the overall concept.

As for your issue, the problem with outer gear doors always seems to be the question of sturdiness, as well as the whole attachment issue. Compromises are frequently made. Since you mention this, I think I'll make it a priority item to address on the daughter's P-51. I'll let you know what I come up with.

Most of the time, I really enjoy this tinkering more than the actual flying... LOL

The reason, I think, is because putting them in the air exposes all that gadgetry to potential disaster...

Good luck, in any case. My experience with pneumatics is relatively limited. They're nice, some of them, but oh boy are they expensive. I've got two TF giant scale P-51s awaiting my attention. I managed to get a pair of retracts for one of them on Ebay for just over $100. They're basically new, so I will consider myself having gotten a steal on them.

I still think you'd like that older GP .40-sized stang I have, what with its gimmicks like the gear doors, the custom exhaust, the .80 sized engine, and oh, yeah, did I mention that when rudder goes left, so does little pilot's head? And geeze, how did I forget the machine guns? Reciprocating in the wing driven by itty-bitty motors with timed LEDs flashing. (Another Tylenol deal).

Yeah, I love my gadgets...LOL

Regards,

Mark

[the-plumber]

Yeah, I love my gadgets...LOL


I always start out with a few grand ideas about a scale model, like turning heads. On the 33% L-4 I wanted to have the observer lift a camera and point it.

Alas, I also know that you never finish a scale model, you just get to the point where you stop working on it. At least, you stop working on it _as much_.

This far into the L-4 project, 4+ years, I've just got to the point of starting to look for the paint guns.

The observer will have to settle for a stiff neck. For now, anyway.

Maybe when I have nothing else to do . . .