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Old Nov 03, 2012, 12:41 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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A one way clutch integrated into a combination extension shaft and reduction would be a piece o' cake.

I can see it now... full throttle for takeoff, a slight redution once a hundred or so feet in the air then a couple of more clicks lower after a few more seconds down to about 75% revs for a "cruise climb" where you click it down a click every few seconds for the next half minute then a click down per second or so for the last half minute until the engine dies. This would pretty much emulate how a big rubber motor would run down except it would take around 70 to 80 seconds instead of the usual 30 to 35 for a rubber powered Sparky.

Then there's this "foo, foo, foo, foo..... " sound as something like a 24 to 30 inch prop freewheels around for the rest of the flight.

It would be BLOODY MAGNIFICENT ! ! ! !

Earl, to do the best with this and not suffer from too strong a drag penalty you want to run roughly a "square" pitch to diameter ratio. So that means about a 24x24 setup which obviously is going to want to run under power at a pretty low RPM. Likely something like 3000 tops.

How's that sound to you?
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Old Nov 03, 2012, 01:50 PM
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United States, MD, Elkton
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I'm experimenting with a G-62 and a 32"x12" pitch prop for my 1/3 scale Fairchild....The engine turns 7,000,the prop 4,000....The thrust is around 40+ lbs

Rather than a Sprague (?spelling) clutch,I'm considering an automatic clutch,off a chainsaw...direct drive.-The engine wouldn't be stalled,and the tension springs can be adjusted to keep the engine in a powerband.Then a very large prop could be used(Larger than a 14 x 6)to give a better impression of scale....We ain't done yet...
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Old Nov 05, 2012, 03:00 PM
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Orlando FL
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Maybe a Heli style power-train would work for you here. It would give you the ability of having the cluch (automatic), the one-way for easy start, and the gear for reduction. That way, even your OS 91 FS could run your plane in a very small RPM setting. The reduction would give you the toque needed to rotate a huge propeller. (Remember, an OS 50-H SX [2 stroke ringed] swings 2x 600mm blades at 2k RPM with up to 15 degree's of pitch, and 2 92mm blades at 5k [ tail ] in a 50 size heli.) Heli's use both indirect drive via gears only, and belts/torque tubes for tail.


I think it would actually be easy to put the transmission inside the fuse, using just the heli fan to cool the motor, and either a shaft in a OWB, a wire, or a belt to get the power to the front if you prefer to not have the weight there.

Just a thought though.
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Old Nov 05, 2012, 03:14 PM
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We had kicked that around early on,and still haven't reached a concrete decision...I like the cooling fan and the weight concentrated in the cabin area...

I have to consider that looooonnnng rubber power nose,which I refuse to shorten...
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Old Nov 05, 2012, 04:00 PM
Laser Cutter Guy
United States, PA, Greensburg
Joined Oct 2009
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Quote:
Originally Posted by epoxyearl View Post
We had kicked that around early on,and still haven't reached a concrete decision...I like the cooling fan and the weight concentrated in the cabin area...

I have to consider that looooonnnng rubber power nose,which I refuse to shorten...
Just curious....How have you determined that you definatly need to shift weight rearward? As I am sure you know, when scaling things up, it is not always a linear progression of weight. Just wondering if all this deliberation is needed.

I like the Heli clutch and gear idea. The old Byrons kits did something similar just no clutch.
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Old Nov 05, 2012, 04:40 PM
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The original CG was at 60 - 75% back on the wing,since the stabilizer carried some of the weight..

I know that I can get away with half that by shortening the nose....But I don't want to do that..

If I put the .91 in the nose,it takes another one at the tail to rebalance it ! That wouldn't be all bad,since the wing loading will still be low ,low , low...

It's not a contest contender...it's just for fun..
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Old Nov 05, 2012, 04:52 PM
Laser Cutter Guy
United States, PA, Greensburg
Joined Oct 2009
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Originally Posted by epoxyearl View Post
The original CG was at 60 - 75% back on the wing,since the stabilizer carried some of the weight..

I know that I can get away with half that by shortening the nose....But I don't want to do that..

If I put the .91 in the nose,it takes another one at the tail to rebalance it ! That wouldn't be all bad,since the wing loading will still be low ,low , low...

It's not a contest contender...it's just for fun..
My point was that the mass behind the CG now that it is scaled up is much larger (thicker, denser materials and such) and even with that long nose you may need the weight in the fwd position. I would wait till the wings and tail are built before finalizing on the engine placement. I understand it is being built for fun, but no sense in possibly having to do things twice.
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Old Nov 05, 2012, 06:37 PM
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You only have to do things TWICE? 'way cool !! Yeah-you're right-it's time to move,and stop the jabbering.....The fin and rudder are hinged,and the stab outline laminations are soaking,while I'm cutting wing ribs.....here we go...

We'll dummy it up,and I'm not afraid to relocate the C/G with radio control....
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Old Nov 05, 2012, 09:05 PM
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USA, NH, Madbury
Joined Dec 2007
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Quote:
Originally Posted by epoxyearl View Post
The original CG was at 60 - 75% back on the wing,since the stabilizer carried some of the weight..

I know that I can get away with half that by shortening the nose....But I don't want to do that..

If I put the .91 in the nose,it takes another one at the tail to rebalance it ! That wouldn't be all bad,since the wing loading will still be low ,low , low...

It's not a contest contender...it's just for fun..
Why not just lose the lifting tail, and build a conventional non-airfoiled stab thus moving the CG to a "normal" position? I'm not sure what the advantage of the lifting tail was, but it must not have been the greatest idea or we'd still be using them.
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Old Nov 06, 2012, 05:35 AM
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lifting stab

I'm not sure of the reason behind the lifting stab,but I believe it had something to do with trimming a free flight model.
Sparky was originally a 32" rubber powered airplane,designed to stay aloft as long as possible,both under power,and gliding.

I don't want to alter the design any more than necessary (WHAT ?-I know) and leaving the lifting stab is part of my plan.

I'm not going to defend or explain the 'why'.....it's something I want to do is all.

Modelling has changed significantly since the beginning,and I want to experience a part of it I missed...I don't see 'em so well anymore so bigger and R/C is the solution for me.
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Old Nov 06, 2012, 10:44 AM
AndyKunz's Avatar
Illinois
Joined Sep 2001
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It was a way to get more lifting area. Wing area and span was limited, so they would enlarge the tails (Sparky has about 40% of the main wing's area) and give them lifting airfoils. As a result the center of lift moved rearward, so the CG needed to move back or you'd have a nose-heavy condition.

I'd say he's on the right path. The idea about a heli drive might not be bad, but what to do about the large ring gear might be interesting.

Andy
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Old Nov 06, 2012, 01:30 PM
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United States, MD, Elkton
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opposite rotation

A donated chopper is giving up it's heart and soul for this project....May it rest in pieces.
I don't intend to work the workings nearly as hard as a chopper does,but then my sadistic friends suggested utilizing the variable pitch feature...

Which could be not a bad idea.....Gotcha!

I'm not afraid of hard work,or failure...I'm a stellar failure at some things,having worked at it for 70 -odd years...but if you want to come along for the ride,the door's open,come on in...

See?-it's always something !! The out put shaft rotates the wrong way,and that's gonna upset some people...But the engine is an old O.S.61V Rear exhaust
type...It has a removeable front bearing housing/carb mount.....Didn't we rotate that 90 degrees,and reverse the rotation of the engine ? Please say Yes !
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Old Nov 06, 2012, 10:04 PM
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Orlando FL
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Quote:
Originally Posted by epoxyearl View Post
...
See?-it's always something !! The out put shaft rotates the wrong way,and that's gonna upset some people...But the engine is an old O.S.61V Rear exhaust
type...It has a removeable front bearing housing/carb mount.....Didn't we rotate that 90 degrees,and reverse the rotation of the engine ? Please say Yes !
I would need to research about this specific engine, but generally speaking, the older engines that had the 2 piece crank case could be reversed by rotating the 90*, which basically changes its timing.
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Old Nov 09, 2012, 07:54 PM
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Found the can opener!!! (worms)

The .60 size helicopter drive assembly is being investigated. The drive reduction is 8.5 turns of the engine,to 1.0 of the output shaft..

If I build a 43" prop,with the proper spars,built up ,sheeted and fiberglassed,much like a formula 1 Racer's wing,I think there's an outside chance it will work..

I can build in a bunch of pitch and washout,with the potential of 1200 to 1400 hundred rpms.

I don't think my rubber powered one turned nearly that fast.
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Old Nov 12, 2012, 02:06 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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On the lifting vs "flat" tail section.

Each aircraft design has a neutral point where it is neutrally stable. This is based on the wing area, aspect ratio, stabilizer area and distance between the two surfaces. There's nothing in there about lifting sections on the tail or not.

What happens is that we place our CG slightly ahead of the neutral point and trim the angle between the wing and stab so that we achieve some small measure of positive stability. Again, there's nothing about the airfoils or not in all this.

So why the lifting tail on so many free flight models? That's because the tails are so big and distance to the tail so large that the overall airframe's neutral point is well back. So the CG placement to achieve the most desireable and easy to fly amount of positive stability is also way back. But because of this the tail is lifting up like the wing. And a more efficient way to do that is with a lifting airfoil on the tail.

So changing the tail section to a flat one will not allow us to shift the CG ahead. Oh sure, we CAN shift it ahead and the model will still fly. But it will become SO postively stable that it'll be harder to fly due to reacting strongly to every small change in flying speed. A model that is strongly pitch stable is constantly trying to "balloon" coming out of even the most slightly non-coordinated turn and wants to nose up strongly with even small additions of power or dive steeply when the power is removed.

So all in all it's best to stick with the CG back where the design originally intended it to be located.

Quote:
If I build a 43" prop,with the proper spars,built up ,sheeted and fiberglassed,much like a formula 1 Racer's wing,I think there's an outside chance it will work..
Earl, I like your thinking.... Instead of this style though and since you were considering using a freewheel clutch I'd consider the idea of stack laminations of veneers and balsa then carve the shape. I'd also suggest dropping the size down to around 36 to 38 inches to go with that sort of RPM. A 36 inch sort of prop would lend itself to this sort of "solid" construction without being overly heavy. A blend of harder balsa and dark hardwood veneer would provide good strength against the centrifigal forces. And a finish that uses something like 2oz glass cloth with the weave laid on a bias angle and epoxy done nicely would give a great finish and torsional strength to avoid twisting.

That would also aid in keeping the engine and gearing up to the sort of RPM where the engine is happy by using a 36x12'ish sort of diameter to pitch. The only thing then is that with that sort of pitch it's not going to freewheel all that efficiently. It'll tend to be more like a dive brake and steepen up the glide. This is the problem with freewheeling props where the pitch to diameter ratio is a lot less than "square" (as in pitch is close to the same as the diameter). The drag caused by "flat" pitch/diamether ratios was covered well by an article in one of the old Zaic Yearbooks as I recall.

Basically you know the rotor size and can guess at the sort of pitch which allowed the engine to run at pretty much peak RPM. And to allow the engine to still reach that RPM you can't make your new prop much, if any, bigger than that rotor. Now if you replaced the engine in that setup with your .91 four stroke you've got less RPM and more torque. So you can consider a prop which would be slightly bigger than the original rotor.

But really to achieve the rubber model sort of look which occurs with likely down around 700 to 1000 rpm of the prop you would likely need to go with an even higher gear ratio.... or live with lugging the engine down to less than it's peak power RPM.
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