Continuing with the story...
The conrod was a bit flimsy and needed some metallic support. I drilled 2 holes in it, bent an umbrella rod into a u shape and epoxy-glued it into the holes. (pic 1)

The crank coupler hole has a plastic adapter (taken from a toy RC car) for the 2mm shaft. (pic3)

The flapping mechanism design needed a lot of refining. In the meantime I worked out how to get the up and down flapping angles.

This is it for today. Will continue with the story tomorrow!

Keep it Coming... watching this thread with interest...

@Bilal:
Did you check out the video from ornithopter-pilot.com I mentioned in #21? It works perfect! I've tried to replicate that to a good extent. Mine works well enough. It doesn't even vibrate (like in the youtube vid in #21) now that I've tuned it.

Please do suggest any more suggestions/ improvements regarding the Scotch Yoke mechanism.

Getting different angles for up-stroke and down stroke.

This can be done in a CAD program like sketchup. See pic below for more details.

1) Select a crank length.
2) Draw a vertical line twice the crank length. Set the respective angles at the end points using the protractor tool.
3) Complete the triangle. The 2 new sides give the min. and max. distances between the pivot and hinge (when conrod is at centre).
4) The distance between the altitude axis and the perpendicular from base centre gives the height difference between the pivot axis and centre-hinge axis. This distance is what causes the different up and down flap angles.

This is how the method described in #29 was implemented (see pics).

Tools used:
1) PCB drill
2) angle grinder with metal-cutting disc
3) surgical blades 
4) small reamer

The wing arms flapped perfectly in the test. Check out the video.

www.youtube.com/watch?v=kjf5AOPiw8M&feature=youtu.be

Time to make the wings!

I haven't used CF rods or plates before. How flexible and heavy are they?
I've tried making wing spars with epoxy coated ice-cream-sticks. They are really strong and sufficiently flexible. However, they seem to be a bit heavy.

I blew my budget on laser-cutting and don't know if I should invest in CF. Really need some advice from the seniors who have built ornithopters.

Good Luck swapnil bhai! making an ornithopter is a complex and expensive business. Hope you do complete it within budget and keep its weight low.

Thank you!

I don't think it's more expensive than building an airplane, but it certainly is much more complex.

I've been ignoring the weight issue so far in order to get better acquainted with the complex mechanisms involved. I do hope to solve the size and weight issues as soon as time and money permit.

Quote from: Bilal on April 11, 2014, 06:29:07 PM
... the scotch yolk mechanism is really tricky to design, the forces at TDC and BDC are exactly equal and opposite so they cancel each other leaving the mechanism with no force acting on it, this sometimes leads to a 'stuck' gearbox. To overcome this generally an offset of like 0.5 degree is included in the design, but "kehna asaan hai", so better watch for that.

Looks like Abbaas bhai (Bilal) is a bit busy so I'm asking this to everyone who understands mechanical systems. Why would the gear box get stuck? What kind of offset needs to be added? The gear box I made works smoothly but it would be risky to proceed before I solve this problem.

Searching around I found that the scotch yoke mechanism has the following advantages and disadvantages, but nothing about what 'Bilal' has stated.

Advantages:

1) Fewer moving parts.
2) Smoother operation.
3) High torque output with a small cylinder size.

Disdvantages:
1) Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.

Swapnil, Iam also building an ornithopter(will start a thread later) and have run into a lot of problems and believe me the major issues are caused by the gear box. Your gearbox setup looks like it has got integrity( parts wont fly out at high speed). A few suggestions-
a)Don't worry about the wear and tear issue if you can fly it for more than a minute even that is an achievement in itself.
b)You have the advantage of having a larger area to fix the CF rods(preferably 3mm or 4mm hollow) so wing fixing also will be easier(see pic)  hence use a straight wing(much simpler).
c)keep the frame of the ornithopter bare bones type by using CF rods.(see pic)
d)use a conventional tail.(see pic)
e)avoid using landing gear .
f)compromise strength if you have to but keep the weight low .
g)use nylon screws whenever possible.
e)use electronics which is just sufficient for the job.

Hope you complete it before you run out of cash or enthusiasm. 
( the pics are not mine )

@Fw190:

Thanks for your valuable input. I don't think I can invest in carbon fibre right now. I'm rapidly running out of resources and ideas.

I'll be waiting for your build thread, maybe it'll get me more enthused. 

Dude!!!
Perfectly executed!! subscribed..

CF

Thank you!

Although, like many people warned before, things keep breaking. Looks like I underestimated the forces at work in ornithopters.

The first design taught me a lot even though it was unnecessarily large and heavy. It was time to reduce the size and weight to within acceptable limits.

I already had a much smaller and lighter gearbox design with me (post #9 picture). I have loads of such gearboxes with different gearing ratios. This one, however, had a perfect value of 30.5 (see attached picture for more details).

I could have used a high kv motor but I didn't have one at hand at the time. So, I went with a 1400kv BLDC with a high W rating (205W) to generate the required torque. 

Then it was back to the drawing board (Sketchup). This time it took much longer as I needed to make the design compact and sturdy at the same time. The crank and yoke undergo extreme forces and need to be designed keeping that in mind.

Initially I used a plastic pinion (the on in post #9 picture). But, it wore out after a few tests.

I had an exact replica of the pinion in metal but it had a 2mm shaft hole. I couldn't just drill a larger hole this time. Instead, I reduced the shaft diameter from 3mm to 2mm. This was an extremely difficult task considering that the shaft needed to be perfectly cylindrical and symmetrical.

At first I was completely stumped as to how I was going to do it. Then I had a really good idea. I powered-up the BLDC (about 20% throttle) and brushed it slightly against the flat side of the disk of my angle grinder (running at half the max. RPM). I kept checking the diameter every second. Finally, the metal gear fit firmly on the shaft with the tap of a hammer.   

The most important thing to keep in mind while designing your own gearbox is 'proper gear meshing'. I had to redesign twice because of a '0.5 mm' error. Even a slight offset results in vibrations and heat.

The shaft of the geared motor comes with a 'C-clip' and is too wobbly. I had to fix a propeller O ring to make it work smoothly. Also had to reduce the shaft length.

Assembling the flapping mechanism was really tricky too. I had to place plastic spacers on the shafts for the arms to move smoothly.

Then it was time to design the next complex thing; 'the wings'. I'd have gone with CF rods but found a much cheaper and lighter option.

Bamboo sticks (purchased from a kite maker) coated with 2 layers of 5-min epoxy are as strong and flexible as CF rods of compareable diameter.

Transparent 'Gelatin' sheets were used instead of rip-stop. The wing turned out to be perfect.

Thumps up ... Great going

Thanks doc! More updates coming soon.

Eagerly waiting