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Old Mar 23, 2011, 04:54 PM
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Old Whitebelly's Avatar
USA, CA, Newbury Park
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DeHavilland Canada DHC-4 Caribou pre-build log

I've made a few mentions elsewhere about how I want my next scratchbuilt to be a DeHaviland Canada DHC-4 Caribou, one of my utter all-time favorite airplanes. I have a friend who flew the real ones a lot (and with whom I'm starting the restoration of a real Bellanca soon), and I'd like to make the model in his honor, if he won't accept it as a gift.

The 'Bou is one of the most beloved by people who worked with them, and one of the most laughed-at my those that didn't, it seems. It was slow-it was said to be the only airplane to suffer a bird strike fro behind. But according to DHC's specs, it could take off at Gross (32k lbs) in 725 feet, and I've heard firsthand stories of doing it in less space with overloaded airplanes. It has flown for many Air Forces, large and small, not to mention the UN, and has done a fair amount of work for organizations that don't tend to talk alot, you might say. For some really interesting data (not to mention a truly cool old brochure) look at this-

1962 DHC DHC-4 Sales Brochure

I've started my plans, and it will be a foam-balsa-carbon hybrid. Sorry I won't be able to offer CAD or similar things as this moves along; I'm an old paper-and-pencil guy. I Like foam building best, but some parts of the plane would need the stronger materials just to hold together. Because I am not after true dimensional scale, but I want flight scale performance as much as possible. So, chord will be larger, control surfaces will be a greater percentage of the fixed plane they attach to, etc. There will be a working cargo door for air drops, natch.

I'm likely to build it with fixed gear, since I'd like to play rough with the model once complete, just like the original was treated. I may try and include the ability to fit retracts if i can think of some clever way to do it. But since I have a 6-channel radio, I'll need to keep channels for all those flaps and flaperons that make the plane what it was; retracts just may not happen until I move up to a more sophisticated radio.

This is early days; NOTHING is set in stone, not even scale. Around 96 wingspan is the goal, however. But not so big that I can't play with with it. So this isn't a Build Log, not yet...it's a brainstorming and designing log. And, I hope, a place where other "bou lovers can add their own comments, about the real plane, this project, or best of all, their own models! As long as it's DHC-4 related, it should be okay.

To start the ball rolling, I want my plane to be capable of a maneuver that VERY few airplanes can handle, least of all transport craft-the wheelbarrow.

YouTube video of DHC-4 Wheelbarrow

So obviously, some serious flappage will be needed, not to mention some monster elevator control!
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Old Apr 04, 2011, 05:43 PM
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Sounds like a good project!
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Old Apr 04, 2011, 11:58 PM
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B sure to check this one out also

http://www.rcgroups.com/forums/showthread.php?t=1124466

The Twins is a crazy flying twin fer sure !
----------------

I remember seeing Caribous being rebuilt at the Bangkok airport every time I flew in. 1969-1970 USAF


That distinctive Upswept tail end is hard to forget or mistake.

Are you going to do a BENT wing or just a simpler Flat wing ?

edit later:

I took the liberty of downloading the PDF from above
and extracting the 3-view and then Tracing over it in CAD
to 96" span.

The provided PDF is from the CAD file to show sizes of things
relative to the span.

Enjoy.

Looking forward to your build
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Old Apr 06, 2011, 03:02 PM
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Old Whitebelly's Avatar
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Birdofplay-thanks so much for the PDFs! All info is welcome, and especially I need info on the shape of the skin itself. I'm on the prowl for things like old plastic model kits etc as well for that. There are some curves, especially at the rear of the fuse, that I am told by people with experience on the real Caribou that never seem to show in photos, and one goal is to impress these guys, not just have a fun airplane. Drawing that CAD file for me was very nice of you-people here like you make it one of the best places on the net.

And I've built several Twins so far-at the moment. one of them is my basic essential flyer; the one-you-bring-to-every-session sort of airplane, and holds a spot next to my Slow Sticks; in my world, that's an honored spot. My best F&T Twin weighs around 40 ounces with batteries, and I often stick a 2.5 pound barbell weight in the cargo area to prove to people that a foamie can move some cargo. And that is with no more than a pair of CHEAP Hobby People 2812/09 motors and a single Hobby People 2200MAh 3-cell, and pulling about 30 amps total. F&T have really created something that I think will be iconic twenty years from now. I'm being serious-people will look back on it as a hobby-changer, I'm convinced.

My 'Bou will indeed have the inverted gullwing; I want it to appear as scale as possible in the air. This means I plan to sacrifice some on-the-ground scale details to help it fly more like the real things. So, as it's evolving, the chord is going to be a bit larger than scale (for more lift), the outer wing panels possible extended beyond scale a few inches (same reason), obviously a different airfoil will be used, (in fact, different foils inboard the nacelles), and the control surfaces will make up a larger percentage of the total area of their respective flying surfaces.

Since I've found that it's very easy to get almost silly power from pretty cheap motors these days when you run more than one of them (thanks again, F&T!) I am even thinking that the oubtboard wing panels will be highly undercambered-drag be damned, if it adds lift. This Caribou will be about 50% larger than my Twin, so doubling the power (which is an easy goal) should let me pull a TON of flap around.

My main concern is going to be flap actuation-the real item had absolutely massive flaps, and could drop them nearly vertical. Enough so that the change in incidence let the plane fly at nose-down attitudes over 5 degrees, while climbing. I don't think I will want to have that much motion moved by a single linkage or servo, so the actual flap design will likely be unique to this model, rather than patterned off of the big one. I'm thinking also that I'll want a radio that allows me to lower the flaps with a pot, not a switch; that could simplify so much, and avoid having huge changes in airfoil happen quickly. (Maybe this would be an excuse to try one of those ridiculously cheap 9-channel Turnigy radios?) These are normal challenges for guys who build the big scale jets and the like, but part of my goal is to keep this in the "cheap" side of things. Sometimes, it is possible to substitute a bit of cleverness for money. Sometimes.

I have started building the center wing section, but it's going to go through many rebuilds, I am sure-I have a crazy idea that I want the wingbox to have a hinge in the middle, so that particularly hard landings don't snap landing struts. The part I'm calling the wingbox would actually extend to the nacelles, and handle the twisting force from the inboard flaps as well. I'm aiming for about 8 degrees of movement, with a spring. Once I'm happy with the basic box (made with birch dowel at the moment, I'll build a final version with carbon tube. I'm using a spring so that I can change the resistance to bending as needed, rather than relying on flex in the carbon (which is what my Twin wing relies on).

But as I said at the first, this isn't the "build" log yet, it's the "brainstorm" log. As it's growing in my head (and on the 100' roll of red rosin paper, my favorite drafting medium) it will have this carbom wingbox, bolted to a plywood section of the fuse. The rest of the fuse will be largely foam formers and skin, with bits of balsa, birch, or carbon as needed locally. Wing spars carbon, ribs and skin foam. I'll use various types of foam for various areas; my favorites are black Elmer's where rigidity is needed, and $Tree where I want something softer, like skinning the fuse. I will defo build in spaces and hardpoints for retracts, but the initial build will have fixed gear, with plenty of shock travel (I have some nitro buggy struts in hand for the purpose, actually.) I assume the build log will stay in the scratchbuilt foamie section, in spite of some "cheating" with the wood etc.

Any more Caribou or DHC lovers out there?
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Old Apr 07, 2011, 02:20 PM
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Go for it... I think 96" is about right for this build. What were you thinking?
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Old Apr 07, 2011, 08:28 PM
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Whitebelly, I love the Caribou so much . . . . .lets put it this way you better get started building or I'll start building one LMAO!

Great project subscribed big time!
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Old Apr 07, 2011, 10:27 PM
A Day @ a Time - Matt. 6:25-34
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North AL, USA
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An amazing airplane... saw one on TV at an airshow and it was amazing what this bird could do. The pilot did all kinds of tricks that you never see other airplanes do.
I'll be watching this one...
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Old Apr 08, 2011, 12:46 AM
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Crazy Caribou Pilot (1 min 19 sec)
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Old Apr 08, 2011, 02:40 AM
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I am thinking that a Caribou Tail End could easily be blended with The Twins Fuse.

and a New more intricate and heavily FLAPPED wing could be used.

Make the door open IN with the hinge at the rear of the door.

This way we have a proven fuse.

Now as for the wing.

I looked at the NOSE view and decided that a spar could be nestled straight across
from engine and still accommodate the bent wing if the ribs are drawn accordingly.

I may have a go at that notion when I get a chance in CAD.

After seeing my Twins going vertical today I'm pretty sure that a 60" Caribou
could be a good size for transport and pocket book.

More l8r

Bob
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Old Apr 08, 2011, 07:37 AM
A Day @ a Time - Matt. 6:25-34
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Great idea, who gets the prize for the first wheelbarrow?
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Old Apr 08, 2011, 11:54 AM
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We need some good , cheap, lightweight ideas for SERIOUS FLAPS !

Have you seen how far down the flaps go on the full sized version ? wheeew

They not only extend but curve down significantly.

I've been considering the notion of using some SLIDER TRACKS.
Meaning a rigid guide on each side of the center flaps.
One track at of hidden in the Nacelle and the other right at the fuse.
a servo IN the nacelle might be able to drive the whole thing.
Kinda like a window shade pull down.

Even though we ARE here inthe foamys forum I was thinking 1/8 ply for the tracks.

input ?????
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Old Apr 08, 2011, 11:58 AM
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Oh and another thought re the SHORT landing would be to use a
REVERSABLE CAR type ESC.

Probably would require special cooling considerations.

Differential thrust, Reversable thrust, Wheel barrow, aerobatic, LARGE - wheeeee !
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Old Apr 08, 2011, 12:56 PM
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Here is an overlay of the DH and The Twins.

The DH was enlarged until it was the same 7" fuse height as The Twins.

This is done only to show relative sizes and for discussion purposes.

As it turned out the DH would have a Span of ~ 71"
if it's fuse was ~ the same as the Twins.

comments ???
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Old Apr 08, 2011, 02:00 PM
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Interesting mod on the back of the fuse of the twin.
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Old Apr 08, 2011, 02:37 PM
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Larry:
I tried to keep the CG's of both in alignment to show the relative sizes
of nose and tail.
It looks like the DH wuld have a much larger RUDDER authority and stability too.
Not that it;s needed on the Twins.

With a big servo hidden in the rear of the nacelle and a long arm on it
we could get a 90 pulldown of the flaps.

Here is the track idea in a simplistic side view.

comments
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