RC India

Hey everyone!

I'm going to attempt to build an ornithopter. I've already been through a couple of websites. But, like it is with hovercrafts, there is no place that tells you exactly how to build an ornithopter step-by-step.

Here are the points/ questions I've decided to consider before building:

Wing Design:
1)   How flexible should the leading edge spar be?
2)   Triangular vs. elliptical wings.
3)   Is there a need to set an angle of attack (and how much) or does the flexing of flapping wings automatically handle it?
4)   How to set the up and down flapping angles?

Gearbox and power system design:
1)   Low kv BLDC with low gearing ratio vs. high kv BLDC with high gearing ratio.
2)   Clustered spur gears vs. worm-wheel gears.
3)   Circular to linear motion mechanism.

Please add any important points I've missed and share your thoughts about the above.

I request everyone to refrain from pointing to websites as I've already been through a lot of them. I need answers from personal experience and knowledge.
 
 

From the various sources I’ve been through, the following seem to be the optimal parameters/ mechanisms for a normal sized orni:
 
1)   Wingspan of 36 to 42 inches.
2)   Flapping frequency of 4 to 5 Hz (flaps per second).
3)   Weight between 350 to 450 grams.
4)   Up-flap angle of 30 degrees and down-flap angle of 25 degrees.
5)   Outboard wing hinge flapping mechanism.

Hmm…no orni enthusiasts. Not unexpected.

Anyway, as there is so less straightforward info available here, I'm going to post basic concepts and a detailed build log so that someone new to ornithopters won’t have to go through scores of papers on bird flight.


1] Ornithopter flight mechanism:
I used to wonder how the simple up-down motion of flapping wings could produce lift and thrust. Shouldn’t the two motions cancel-out each other’s effects?
Then I read (and re-read multiple times) Robyn Lynn Harmon’s paper on flapping wing motion tracking experiments.
Here’s an excerpt:

For steady level flight conditions the ornithopters flap their wings three to six times per second. There are two spars, one at the leading edge and another placed diagonally from the leading edge to the rear of the fuselage. This spar arrangement creates two regions in the wing, the triangular “luff” region, which is a loose membrane, and the “flap” region which is kept taught by a series of fingers that run from the diagonal spar to the trailing edge. The flexible skeleton-membrane structure allows for highly dynamic passive shape change as the wing moves through the air.
The large degree of bending in the wing is a result of the membrane adjusting its camber and pitch to maintain tension equilibrium throughout its surface. At the beginning of down-stroke and upstroke the inertial acceleration of the wing causes the leading edge spar to bend significantly. This results in a variation of the local stroke angle along the span and therefore a phase-lag between the wing root and wing tip during the stroke period.
Additionally, since the flap region is essentially hinged about the diagonal spar, it experiences a large deflection. A consequence of the flap deflection is that the flap’s force loading exerts a moment on the wing that increases the pitch into the flapping motion, so if the wing is in down-stroke, it will have downward or negative pitch. This pitch adjustment is important to maintain a relative angle of attack with minimal stall, whereas an untwisted rigid wing would experience accelerated flow separation due to the large inflow angles.


The wings have a triangular support structure. A main spar runs along the leading edge of the wing and a strut connects from the rear of the ornithopter's body to a point near the tip of the main spar. From this strut there are several smaller carbon rods that project to the edge of the wing which are somewhat free to move. This results in a fanning motion from the trailing edge of the wing that produces thrust while the leading edge is flapping up and down which directly contributes to a part of the lift in addition to the conventional lift coming from airflow over the wing.

Morals of the story:
1) In order to produce lift and thrust, the leading edge spar of the wing must be stiff (or rather have very little flexibility) while the rest of the wing should be relatively more flexible. 
2) The up-flap angle at wing-root should be a couple of degrees greater than the down flap angle.

Umm, i am by no means even entitled to speak on this topic but i did watch a TED talk a while back where they showcased an ornithopter built by the company Festo called the SmartBird. In the talk they demonstrated its working and the approach was a bit different than what you described above. The wing was divided into two sections by a joint, and during the downward motion the wing would be at the maximum extension thus exposing more surface area and generating lift. But during the upward motion the wing would fold in such a way that the surface area would lower and hence the lift was more and the bird could fly. I think video would explain it a lot better than i can.

I know you asked not to post any links to websites and i apologise in advance if you have already seen this.

http://www.ted.com/talks/a_robot_that_flies_like_a_bird (http://www.ted.com/talks/a_robot_that_flies_like_a_bird)

Thanks for joining in man! :)

I did see that video a few days after it was released. I have a habit of downloading TED talks. Got a huge collection now.

The mechanism you described is possible due to use of articulated wings. Most beginner ornithopters do not use articulated wings due to their inherent mechanical complexity. Not to mention, those guys have amazing resources (machines and materials) at their disposal. I haven't completed my education yet, so I'm gonna have to stick with whatever build material I can find in my recycle bin. 


Well, now you know why!   :P  ;D

Just kidding! Don't go thinking that I'm conceited.

Swapnil sir, am keenly following this thread of yours, as am also planning to build a micro ornithopter for a while now, if possible gimme your email, I'll share the data that i have regarding them

regards
Bilal
abbaaszaidi@gmail.com

Thank you! Micro ornithopters have a totally different set of parameters and mechanisms. I do have some material on them.

And don't call me sir, man! I'm your age (maybe even younger)!

2] Gearbox:
The gear box is needed to get the 4-5 flaps per second from high kv BLDC motors. The following equation is used to calculate the gearing ratio.
Max. flapping frequency = (kv of BLDC X LiPo voltage)/ (gearing ratio x 60)

For a 1800 kv BLDC with a 2s LiPo and 5 Hz frequency,
Gearing ratio = (1200 x 8 ) / ( 5 x 60) = 32

For such high gearing ratios more than one stage of gearing is required. Then one has to choose between a few stages of large clustered-gears and many stages of small clustered-gears.
The worm gears seem to be an obvious alternative as they provide similar gear reduction in a single stage. But, their frictional losses are extremely high and should be avoided. However, some people seem to have successfully used worm gears.

3] Flapping Mechanism:

The design of a gearbox is nowhere near as complex as that of the flapping mechanism. There are loads of options. Some are simple but inefficient while others are efficient but really complex and heavy. Here's a discussion thread on the various flapping mechanisms generally used.

http://www.rcgroups.com/forums/showthread.php?t=1842514 (http://www.rcgroups.com/forums/showthread.php?t=1842514)

I decided to start with the gearbox.

I realized it would be easier to use the gearbox of the geared motors used in robotics.

Below is the picture of the first gearbox I tried (resting on my first sweepstake  ;D ).

The problem with it is that it came with a 100 RPM motor and has a very high gearing ratio (50+) and that I don't currently have a high kv BLDC. It is , however, a very simple and lightweight option. If anyone wants to try this method, go with a 1800+ kv BLDC and the gearbox from a 300 RPM geared motor.

Initially I thought about designing and fabricating acrylic gears. It's really easy considering that there's an awesome gears plugin available for sketchup.

However, I happened to visit my favourite electronics hobby shop (Sharma Electronics) and to my surprise they had 3 boxes full of gears. I had to go through a lot of them to find the perfect ones.
Finally I came home with those shown in the pics below. The big black ones fit perfectly on the 6mm shaft of a geared motor. The smaller black one (in the 2nd pic, clustered with the bigger one) had a bit smaller shaft-hole and needed some modification.
I got the pinion gear from the final stage of a 24V 100RPM geared motor. Removing the pinion from the shaft was a bit tricky. I had to take care not to harm the gear teeth while using the hammer. The 3rd and 4th pics show the axle and pinion. 

Fitting the pinion on the BLDC was a bit complex as they had a diameter difference of 2 mm.

I had to first superglue a shaft-adapter ring (from the set that comes with an electric prop) on the BLDC shaft. Then, with the BLDC running at full throttle, I held a sand-paper against the plastic ring and trimmed it till it was 6 mm in diameter.

I also used the plastic rings as spacers as can be seen in the pics below.

I designed a rough CAD model of the gearbox before trying to implement it. Then I printed the design and cut the parts out of a copper clad PCB. I used plastic 'stand-offs' to hold everything together.

The PCB gearbox helped a lot in determining the perfect distances between the gear shafts and gearbox plates (I did do the measurements and the math, but it's the feel that really tells you).

However, this gearbox was flimsy and I needed accurately spaced shaft holes. This meant fabricating acrylic parts with a laser cutter.

After refining the 1st design and fabricating the acrylic parts (3rd pic) I realized the pinion and 1st gear were a bit too close (0.5mm). The 4th pic shows the corrected design.

Hahaha! This thread is turning out to be a lengthy n lonesome monologue.  :P  :)

I'm elaborating everything as I'm not using off-the-shelf or ready-made parts. Hope someone finds this useful someday.

@swapnil
Great job. Keep it up.
Took a while to respond, as i got interested enough to go through the thread again from the beginning.
I am a great believer in jugaad, and i feel proud of you.
How much does the gearbox weigh, and what is the final ratio?

Just saw the acrylic pic.
Excellent  {:)}

Iyer sir, thank you for your kind and encouraging words!  :)

I haven't weighed the gearbox yet but it's a bit on the heavier side. I'll have to do some more 'jugaad' to trim its weight.

I've attached the gearbox specs. Thanks for reminding me, I almost forgot! The gearing ratio is a bit low, but that's the most I could get without risking greater size and weight. I'll be using a 2s LiPo at half throttle.

Swapnil, I am following this thread with a fair bit of interest. Of course, not knowing much about Ornithopters, I chose to lay low and say nuffin, but learn something.

Please do continue, it is fascinating!

Only comment I have is I don't trust that plastic ring superglued on! Please look for a better solution, depending on what you have available  ;)

Regarding 'monologue', i was about to say 'wait till Sanjay sir sees this' !

Sanjay sir, actually I've used 5min epoxy. I've also epoxy-glued the metal pinion to the shaft using some elaborate 'jugaad'.
And the first stage of gearing doesn't undergo really high stress. Now all I can do is hope it doesn't give up.

Like I mentioned before, selecting and designing the flapping mechanism is much more difficult than the gearbox. The Scotch Yoke mechanism seemed stable enough and I decided to try it. It turned out to be really challenging to design and put together.

(http://www.robives.com/images/oct2013/sy-a01-ani.gif)

Also check out this vid: http://www.ornithopter-pilot.com/images/N1FlapMechTest10ScotchYoke.mpeg (http://www.ornithopter-pilot.com/images/N1FlapMechTest10ScotchYoke.mpeg)

First test video:

www.youtube.com/watch?v=2ZKjw-8TRS8&feature=youtu.be (http://)

Swapnil bhai, thanx for all those lengthy elaborations, am learning a lot along they way.
besides, 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.

one more thing, how are you gonna obtain differential flap angle with this mechanism??

regards
Bilal

why not use something like this??

Could you please elaborate that for me? I am an electronics engineer, don't know jack about mechanical stuff.

I chose the scotch yoke as it seemed much more elegant than the others.

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  >:D
4) small reamer

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

www.youtube.com/watch?v=kjf5AOPiw8M&feature=youtu.be (http://) (http://)

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.

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.  :thumbsup:
( 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 :)

@Swapnil

sir i like your hard work and your design work is inspirational many :hatsoff: you deserve
 your gearbox, flapping mechanism every effort is
 :bow:

Thank you so much for your encouraging words!  :)
I do hope someone gets inspired and builds a much better ornithopter.

My ornithopter build has been completed and it flies really well. I'm posting pictures from 2 weeks ago to retain chronology. The huge gaps between postings are due to my really busy schedule (academics, personal projects etc).

The first flight was powered by a 3s 900mAH LiPo and felt kinda heavy. I'll soon try it with a 2s 850 mAH. I'll post a wing flap test and flight video after posting all the build details.

Anwar sir was right. I felt much more satisfaction while doing this project than I would have felt after receiving a cash prize.

P.S: I forgot to post the wing design last time. Pic attached. If anyone needs the sketchup files, please do ask.

Do post your flight videos Swapnil. Will be eagerly waiting for it; by the way whats the total flight weight of your ornithopter?

Interesting build, nicely done.
You could save a few grams by replacing the metals screws, nuts and spacers with nylons ones where ever you can.

Low KV, high gearing. Torque to RPM
High KV, low gearing. RPM to torque
Right?

Also, did you check this site? http://www.ornithopter.org/ (http://www.ornithopter.org/)

good luck! :thumbsup:

I'm eagerly waiting to post a flight video too, but it's too windy and rainy here right now. I'll be posting the rest of the build pics and test videos soon (possibly tomorrow).

The total weight is around 380 grams.

@AnjanBabu:

I did try nylon screws but they couldn't handle the huge forces generated by the system. All ornithopters I've seen of this size use mostly metal parts for holding everything together. Weight is not an issue any more. The final flapping mechanism is significantly smaller and lighter than the first one. The final weight is comparable to Sean Kinkade's 'Parkhawk'.

I didn't understand your query regarding kv rating and gearing ratio. The statement in the quote is quite straightforward. 

 

For example, to obtain 10 RPM, you can either low gear a 20 RPM motor or high gear a 5 RPM motor.
Low gearing a low KV motor will further reduce the RPM and high gearing a high KV motor will further increase the RPM.
I was wondering what you were trying to do?

I'm dreaming about building an ornithopter since almost 7 years now. :giggle:

In the case of an ornithopter no one will think about increasing the rpm..
What we need is high torque and low rpm at the output..
Nice job man.. some suggetions
Go for a very very light wings you will know why when you attach the wings and make it flap for the first time..
All the best and waiting for the videos

@AnjanBabu:

There's a difference between 'high gearing' and 'gearing up'.

Ornithopters need a flapping frquency of 4 to 10 HZ (i.e 240 to 600 RPM) depending on the size. Given the high kv ratings of BLDC motors we need to 'gear down'. There are two ways this can be done. Using a low kv BLDC with low gearing down ratio or a high kv BLDC with high gearing down ratio.

So, when I say low or high gearing, it refers to the amount of gearing down. Most ornithopters this size use a high kv motor with a high gearing ratio. I used a low kv motor with a low gearing down ratio to get the required 5 Hz frequency.

What I failed to realize in my hurry was that high kv motors usually have a low current requirement. Hence, they need a smaller ESC and battery resulting in a much lower weight.

I'm considering redesigning the whole bird with a high kv motor at its heart seeing that the current flight characteristics aren't good enough.

@arun.sreelakam

The wings I made are really lightweight and strong considering their size. A 'parkhawk' or 'kestrel' of this size has a wing frame made of CF rods. So there's not much difference in weight. I guess mine are a bit lighter.

The first flapping test (done almost a month ago) was really amazing. It created huge gusts of wind. Felt like an eagle was pulling at my hand!

Good job

the wing designs is ok but i am worried about your gear box whats the weight ?

The gearbox was light for its size and worked fine for a few flights after which it gave up. Ornithopters like the 'parkhawk', upon which I based my design, use large gears. The gears in my gearbox were small and of poor-quality metal and hence kept getting chewed up.

If I were to do it all over again I'd go for a smaller gear box with fewer gears and a large gearing ratio. Also, I'd definitely go for a carbon fibre wing skeleton.

great  :hatsoff: can i see the video .............

I don't have any videos anymore. I deleted the whole project folder after I got bored with it. Ornithopters, much like multicopters, aren't much fun to fly; they just make a good science project. I've tried a lot of stuff but nothing trumps the joy and thrill of flying an airplane.

Ohhhhhhhhhhhhh.........is it so ! hahahahhahhhahahah

Yeah, I even deleted the hovercraft and quadcopter folders for being too boring.
The ornithopter videos were online till 2 months ago. I removed them from there as well. Sad, you couldn't see them.

Can i get ur contact number swapnil because i need guidance for rc ornithopter project

Few months back I also made by can find carbon fibre so project discontinued(https://uploads.tapatalk-cdn.com/20190208/5b86d0fee82e7a709569887adeabc71c.jpg)(https://uploads.tapatalk-cdn.com/20190208/ab93536977be295dd20175edf3c43e65.jpg)

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