Well, so far the discussion has identified several concerns[B)]: machining actual gears in this scale is quite difficult, most electric motors spin too fast for scale prop speed, and some sort of offset is needed to maintain a prototypical thrust line for many inline-engined aircraft. Also, the use of belts in lieu of gears was suggested.
So, here is another idea. Instead of gears, this plan uses o-rings as both friction material and a belts to transfer rotation. Neat thing here is that gears REVERSE rotation, while belts MAINTAIN the rotation direction.
Per a request for a larger image:
NOTE: By maintaining the diameters A=A and B=B, prop speeds will be identical.
The yellow gears form a gear reduction set. If the motor speed is unimportant, or can be controlled by another method, the yellow gears can be eliminated and the orange cluster gear driven directly.
While I haven’t actually engineered anything (yet), I estimate that the entire mechanism could be created to be 1/4" wide and 1/2" tall (or about 6.5mm x 13 mm).
Another interesting approach… will look forward to seeing your prototype! There would be all kinds of possible applications of something like that, even tandem contra rotating rotor blades, driven by a central motor.
It would be pretty easy to fix the speed of the motor, you’ld just need to put the right value of resistance in series with the motor, or a potentiometer (like what’s in a volume knob) so you could set the motor to a speed that looks right. I was thinking of one thing, if only one of the counter rotating props was spinning and the second was free of enough fricton, I wonder if it wouldn’t spin on it’s own from the air moving from the other? I don’t know if that would work or not, but if it did it’ld sure be a whole lot easier than machining gears in 1:48
toomanyslurpees- It’s true, you can easily adjust motor speed using resistors or other electronic means, and plus it would simplify machining. I’m not sure how well the free-wheeling prop idea would work, becuase I can say from personal experience how hard it is to keep a prop loose enough to spin like that, and then connect another one to a motor.
That’s not to say it wouldn’t work for some people, it’s just I’d feel more comfortable with both props “hard wired” (as it were) to something.
My efforts to get a working prototype are very crude, and limited in application. I succeeded in getting a working model using gears from a junk RC car and bits and pieces found around the house. My challenge is getting it down to a working size. (It was basically a copy of this idea, only the motor was inline with the props, and the gear/belt shaft was offset. And it was a little more stretched out.)
These are all really great ideas, several of which I’ve never have though of. These threads are all a great help for my ulitimate goal, a working 1/72 (!) XP-56.
I guess both props should be at the same rpm too, I thought about that later. Maybe just permantly glue a hair dryer in front of each engine, that should work great until the whole thing melts (even then it’ld be fun)
bdelov asks “Regarding the rotating speed, if A is different from B in diameter, wouldn’t that make the props spin at different speed?”
The short answer - No. As long as the drive gear (or pulley) is the same pitch diameter as the driven gear (or pulley), there is no mechanical advantage, and therefore no change in rpm. A and B are separate drive systems that coincidentally share common axis and drive speeds.
