Question:

[B753]

Hi Everyone,

i stepped in inadvertantly. (that´s about 4 hours ago) ´cause it´s very fascinating what i´m reading here.
I like these little rubber things the most. And i do want to build one.
I already visited the shop from Nathan but i´m from Germany and i couldn´t finish the purchase there. Maybe there is no shipping to my country.

Anybody knows an alternative?

I also checked out the very interesting site of Patricia to get more basic information about the general working principle but the scetches from Mr Mack were very hard to read on my computer.
Is there already a thread in here which contains that kind of information such as working principle, C.G. difference between 2 and 4 wing concept? I´m working my way through for some hours now but i´m still on page 5 so... please :o

Sorry for such basic questionnaire but maybe one day i´ll be able to contribute something that´ll be interesting also for the more advanced ones here :)

Many thanks in advance,

Alexander

[John Mack]

Welcome Alexander!
If the obscure sketches were by me, can you remind me which sketches, please?
John.

[B753]

They´re called "Dragonfly Wing flapping mechanism" and "Asymmetric mechanism" but please don´t get me wrong: I´m a fascinated beginner for these constructions, so this may also be a major reason why i don´t understand the scetches. On one thread someone was asking for explanations and i guess Patricia referred to these pictures but i guess also to the written explanations (pdf files). So i visited the site hoping to find some beginner explanations... :)

I guess i´ll start building one today - and then i´m gonna ask further questions.

Alexander

[Chronister]

The Ornithopter Zone does ship to Germany. You can read more about this on the "policies" page of the online store. Or simply submit your order using the online shopping cart. You will have the opportunity to select your country when you check out. Let me know if you have any other questions. Nathan

[B753]

Thanks Nathan with a different browser it will work now.
In the meanwhile i have built two little ones - the first attempt was nothing. I tried to do everything out of cf but that didn´t work. The second attempt actually work but it only made a stretched circled glide. So i guess that´s still not what i´m looking for.
Since nobody really want´s to explain the basic principles again and again (i know i´m not the first one here asking for that sort of help) i will work my way trough - step by step. But nevertheless i´m gonna order a freebird and luna from you if you have them in stock. Don´t want to wait so long ;)

Alexander

[John Mack]

Hi Alexander, may I recommend the Design Manual CD for answering many of the Q’s you must have? It’s not an easy read! But (or because) it contains a WEALTH of information, distilled from years of practical experience. That would take you forwards, quickly and with more understanding, I’m sure. It also shows several varied designs for building. I can’t think why it isn’t recommended more often - for beginners and for more experienced builders, too!

I accept that my sketches aren’t clear!
The "Asymmetric mechanisms" are designed for providing more downthrust than upthrust to the wings, a) to balance the 1g lift and b) to save rubber power on the upstroke. Each flaps one wing only (though inertia and aerodynamics forces both tips to flap through equal amplitudes in flight).
The top one uses an articulated conrod, whose knuckle is guided by a link (instead of sliding on a guide). In the lower, the crankpin slides in a yoke which swings the wing.
The "Dragonfly Wing flapping mechanism" is complex because it seeks to: a) hide within the animal’s envelope, b) drive two pairs of wings in antiphase and c) get the motor hook as far forward as possible.
I hope that makes them a bit clearer. Please don’t ask me to draw them again!

John.

[B753]

John,

first thanks for the thorough explanation on your scetches. But please don´t get me wrong. I never wanted to say that they´re not clear or something. I think the main reason i couldn´t understand them well was mainly to my lack of experience for that topic. But it get´s better. I already build a third dragonfly and without any plans, just pictures of the other guys and a strong will to get one of these birds in it´s element - i´m improving. My goal is for sure to build one of these life-sized ones like you have, but i think even with the information given on your site it´ll be a long long way to get there.
I will ask Nathan for the CD. (That´s the one you meant right?)
Maybe after i have practiced my skills and worked with the CD for a while you may be willing to give me a little help in building a life sized dragonfly. I would be very grateful for that. Thank you.

Alexander

[John Mack]

Alexander,
Ha! I can TALK for ever, but one can learn only by thinking and trying, I think. You will like the CD Design Manual, I am sure.
I plan to revive the dragonfly project in a month or two and will be pleased to exchange ideas with you. They are fun.
Good luck!
John.

[B753]

John,
im defintely looking forward to that!
In the meanwhile i found a picture from one of your life sized ones (a butterfly lying on its back with real wings) but even if i´m taking the picture and magnify it, it´s kind of hard to see the technical and most interesting part of the construction due to the resolution. Is there any other picture available that shows a bit of the hidden technical elements in your life sized dragonflies? Or do i have to wait untill revival?

Sorry for this impatience :o

Alexander

[John Mack]

Alexander,
So you like mechanisms, too!

Please pardon the quality. I use the smallest file size to save dl time, partly by reducing the resolution. If I find the *.bmp's, I might post fragments to show the machinery more clearly.

Here is the swinging link of the butterfly. A Teflon roller runs in the yoke.

In this in-phase dragonfly the 4 wings flap down together, attached to adjacent side-by-side carbon torsion bars. Those counter-oscillate as they rise and fall with the conrocker. You can see the front of the conrocker engaging the crank, and its tail bearing behind the torsion bars. It also engages a forward-extension of one torsion bar (the other loops on just behind). The shoulder pins from the fore LE's "focus" through the tail bearing. The wings are of 0.2mm foam, loosely sown to the spars so they can twist aeroelastically. Tailess, trim is very sensitive. Best uration: about 13 seconds.

The counterphase model is even less clear. The torsion bars are stacked vertically so that the wings can be cross-coupled to them. You can see the shoulder struts. These have to spring sideways a bit as the wings flap. The cuts in the LE, between the stitches, are to reduce the torsional stiffness of the wings. For some reason, this model needed forward ballast. It never trimmed well.

Wire-loop bearings are ideal: light, can be nipped to almost zero clearance, are self aligning (enough for inaccurate kinematic geometries!) and NEVER wear. The rear of the torsion bars run in silk "bearings" tied to the conrocker.

John.

[B753]

John,

thank you very much for your pictures. Now i´m sitting here with my dictionary and your pictures, trying to understand your mechanisms - drawing and thinking how my version could look like. I want to build a small one that is at least (!) as small as yours. Wouldn´t that be a challenge for more than just us to build the smallest rubber powered orni ever? Challenge is good and important for inventions and building new things. Thanks again for the pictures.

Alexander

[John Mack]

Alexander,

Yes, challenge is stimulating. But there is nothing new! On Sunday I saw some truly exquisite 4-wingers, one only 22.5mm span.

I recommend you work your way down in span. These small models are VERY sensitive to trim. My smallest, of 50mm, has flown (19 seconds, once), but rarely does so. I am sure you could control them more easily once you have tamed Freebird and Luna. Ornithopters are contrary!

Good luck.

John.

[B753]

John,

that´s unbelievable. I think i have to come over to UK to see these. 22mm span and(!) rubber powered?
Yesterday i built a freebird with 15cm but the problem is, that the rubber untwists so fast. When i turn it to the maximum the bird will only flap for 3-4 sec. The rubber is as long as the fuselage itself. I will try to post a pic as soon as i find my cam again.

I´m wondering how you get such a long flighttime. I think mine is also a bit to heavy and with the resulting wingload it is very fast. But somehow i need the rubber to untwist more slowly. I don´t know.

Where was that, where you saw the tiny ones. Is there a web page for that ore something?

[Chronister]

You will get longer flights by doing two things: 1. use a thinner strip of rubber, and 2. reduce the weight of the ornithopter so that it can fly with the thinner rubber.

[John Mack]

Alexander,

Few small ornithopters are flown at indoor Meetings. These are listed in our model mags, but I think we’d have to organize a flocking of ornithopters, let alone small ones, before you visited!

The builder of the small ones, Peter, lives in the South. He has no webpage that I know of – but I’m working on him. You can see an old image of his 28mm one on
http://www.rcgroups.com/forums/showthread.php?t=238389&page=7&pp=15

My Freebird2 eventually used rubber loops of 150% of centre distance. I think I used 450 turns. By lightening and improved trimming, I was able to reduce the motor thickness (to one loop of 3/16”, I think), which reduced the frequency. But follow Nathan - he should know!

I think I tried a larger crank throw, too, to reduce the frequency by increasing the amplitude, but I can’t find any Notes – if I made any.

Be sure to have minimal clearance in all bearings and use a Teflon thrust washer. Try different membrane tensions by, eg, packing up the TE while dropping the spars on. An old trick is to iron the membrane (if tissue); you do not have to remove it. That revitalised mine, several times, though I do not know why. I think a taught membrane flapped slowly but gave less thrust, though I am not sure, now. Maybe you have done all that.

Come on, somebody, I am not the only one to have flown a Freebird! There must be so much experience out there. Let us hear it.

Keep at it! Ornithopters have Attitude; minds of their own. But I’ve seen many Freebirds fly very well - eventually. They are much easier than tiny ones, and excellent learning vehicles.

Onwards and upwards.

John.

[Peter Get]

Hi everyone.

Just a quick message to get myself started on this message board thingy & let you all know I'm available for questioning, and will no doubt have plenty of questions myself!

I've been out of it for too long now, but since meeting John (Mack) at Crawley last week I've been inspired to get going again (thanks John!). There's lots to do, starting with taking some decent pics of my collection etc., so please be patient with me?

BTW John is right, my smallest so far is a 22mm span equaliser-tandem (I believe that's the correct title for it?) and weighs approx. 3-4mg without rubber. It has flown, but not recently as it's waiting for me to pepare some new motors and then work up the courage to go for it. The trouble with such a small & delicate thing is that I risk accidentally trashing it every time I handle it!

I really have to get some sleep now, but I look forward to sharing info & ideas with anyone similarly inclined.

Cheers,
Peter.

[B753]

Thank you for your advices. I post two pics of my beginner self construction. Two pictures to show the two max angles the wings achieve. I don´t know whether theyr´re are right. Maybe this will help you to see where my mistakes are. I think the bird is too heavy (5g) and the rubber is too short.
Maybe someone will see any further wrong details.


John, i saw the picture and it is amazing. Hope to get there once. But i´m still waiting for the Freebird and the CD you recommended.

Any shiping estimate Nathan?

Alexander

[Chronister]

Hi Alexander. The shipping time is quite variable I'm afraid. It can take less than a week, or it can take several weeks. This is air mail but once in a while you would think otherwise.

Your ornithopter shown above has a really neat, kind of different mechanism, similar to what Patricia Jones Bowman has been working with if I'm not mistaken:

http://www.ornithopter.org/forum/showthread.php?t=14

Everything about your ornithopter looks like it should fly. If it doesn't, the rubber band may be at fault. Use four strands of 3mm model airplane rubber instead of what you are using. The difference between two similar looking types of rubber can be as great as the variability in shipping times. What happens when you try to fly this thing?

[B753]

Nathan,

thanks for the information on the shipping progress.

As a matter of fact my mechanisms i´ve done so far, are the result from what i think i understand from the pics i find in here. (strange sentence)
But i guess it´ll change when i receive the CD in six weeks ;)
If i gently throw it, it does a pretty nice glide, but with the rubber it seems to be uncontrollable. Very fast. I´m gonna change the rubber now and let you know about the progress.

Alexander

[John Mack]

That is a splendid model, Alexander! Just the sort of constrution I like.

Your amplitude looks fine. The tail appears to be rather small and could cause high drag if working hard. With a symmetrical flapper like yours, where the tail does not oscillate in roll, I have found that a higher aspect ratio tail works well.

Ornithopters seem to have minds of their own, so keep on experimenting. Sometimes you can get lucky and fly almost immediately, but it can also take weeks to find flying trim. There are so many parameters to vary that I have to keep notes - which is boring. I believe it is best to explore thoroughly one variable at a time. Your model looks good, so I urge you to keep on trying!

Nathan's right to advise model aircraft rubber. It can stretch 8:1 and is strong, too. Four strands of 3mm sounds too much for 15Cm span, though. (I use longer motors by extending the rocker, and also by using lengths up to 150% of the hook centres.) I dislike sliding pairs because of their high friction loss. Could you fit a roller? (I have now dispensed with the guide, relying on the springyness of the shoulder stays to confer near-vertical traverse. It appears to work well.) Is there a Teflon thrust washer?

Heavier models merely fly faster but, for duration, you need a low frequency. For that seek low weight - the paramount requirement for our type of model, I think. Can you change to a lighter membrane? Indoor modellers often use condenser tissue or very thin Mylar films. These last quite well, stuck directly to the spar. For the thinnest films I paint dilute PVA glue round the unprotected trailing edges. Try different slacknesses, too.

Lately, on a model with a similar mechanism, I have used a 1g balance "spring" to counteract the lift on the wings. A Lycra thread was stretched from cleats at about the middle of the spars and under the body. There seems to be an optimum tension (= length) for maximum duration.

There is so much to try! But I expect that your excellent model will soon be flying well.

Last weekend I saw somebody flying her first aeroplane, a Freebird. On its first outing it eventually worked up to over 30 seconds duration. (You can go off people!) Hope yours does, too.

Pity you haven't got the disc yet. It is a real treat. I think they should be distributed free, in breakfast cereal packets. Nathan.

John.

[Chronister]

You're right about 4 strands being too much. I didn't realize how small it was.

The tail might be the problem based on the description of it going out of control with the power on. On the other hand if it is simply nose-diving you could try moving the CG back or lengthening the connecting rod slightly.

John's suggestion of changing only one thing at a time is excellent advice.

[B753]

Thanks for tipps but i´m gonna change these things tomorrow cause today your package arrived! So it seems i was lucky to be a customer of the fast shipping process :)
Freebird: I built everything exactly according to the little instruction. With the double rubber and 100% length it will only do 2 turns out of 2m before touching the ground. Tried different trim settings and amount of rubber turns. No improvement. Sorry to bother you with that, but i´m wondering how the Luna orni will do it since it is even heavier than the Freebird.
Any tipps?

Alexander

[John Mack]

How many turns did you wing on, Alexander?

[B753]

90 turns. I´m wondering how long the rubber should be, cause you once wrote that you used 150% length on one orni. But the freebird manual suggests 100% if i´m understanding it right.

Alexander

[Chronister]

Alexander, you can use a longer motor for longer flight times. The kit comes with a short motor because it is easier to wind that way. With a longer motor you may want to use a winder so you can stretch the motor first and then let it shorten as you wind it up.

If your Freebird is flying in circles, it will probably fly much better if you trim it for a straighter flight. First try winding it the opposite way. This may straighten the flight path because slight variations in the crank geometry can cause asymmetric flapping. If that doesn't work, add a small amount of weight to whichever wingtip is on the outside of the turn. About 3 cm of the extra wire provided with the kit should be enough to overcome even a large imbalance in the wings or mechanism.

[John Mack]

Nathan’s right to imply that longer motors need to be “managed” more carefully.
There will be more bunching of the rubber. Here is what 700 turns looks like, with a ***mm loop between ***mm centres – ie, ***%. The model is 340mm span but lighter than Freebird, so the motor is one loop of *** x 1.0mm - and thinner motors allow more turns per metre. The rubber is TAN2, which will stretch 8:1, though I’m not yet sure how many turns it will take. It did not feel particularly “tight” at the end of winding.
Also, at high torque, the motor stick will twist. That can be seen here. Additionally, a thin stick will also bend due to motor tension. (This one is being thinned gradually, to promote bending, so that the initial rate of climb is reduced for limited-ceiling indoor flying while the nose is kept up during descent. So far, that has increased the duration from 73 to nearly 94 seconds on 650 turns.)
Though less on Freebird, these effects are inevitable, to a degree, but can be handled once known.
For single-handed flying I wind from the rear, using a three-loop wire link. The front loop for the motor, the centre is for the rear anchor, the third is for the winder hook. A hand-held electric winder with counter is best.

Off topic, despite the twist, the model flies at constant radius – and needs to, to keep between the lower roof trusses. Its consistency drew favourable comment at an indoor Meeting last night. Steering is by changing the relative root incidences.
The thread at the front is a lift-balance “spring” of Lycra, and was the subject of the night’s tests. There was an optimum tension.
The CF stays, extending forward of the LE’s just outboard of the shoulders, are intended to carry forward booms so that the membrane tension can be adjusted.

Hope you find the Design CD interesting – and useful!
Hope Freebird is progressing. (Two were climbing and circulating sweetly, last night. We had three O’s airborne simultaneously, a rare sight there.)
Best of luck!
John.

[John Mack]

Oops, Alexander, sorry, here are those *** dimensions!
The motor loop now measures 203mm, between 102mm centres. It is fairly new, so I do not think that differs much from when it was made (I’ve lost that record). The section measures 3.1 x 1.1mm.
One record: the previous loop of 210mm burst when old at 750 turns.
For comparison with your own, the model weighs 3.07gm, including motor. That has increased from when last weighed, mostly due to a longer motor I think.
John.

[B753]

I like the luna orni concept more. It flew much better than the freebird.
I´m working my way through the CD.
After i received some very lightweight film and a bunch of micro carbon profiles i´m gonna try to build a smaller and more lighter version of the luna orni.

...still trying to get there John ;)

Alexander

[B753]

Okay, finally it became a freebird concept again. It flies well but no chance for a one minute flight :(

I´m wondering whether that whole effort - to do all with cf - is false. Maybe there is no better material than the good old balsa.
It flies so fast... Mainly due to its weight i guess. Don´t ask me what weighs i don´t have such a precise scale.
But as you can see, i tried to minimize all diameters and choose the lightest film i could get.

Alexander (a bit disappionted)

[Chronister]

Alexander, what is the source of the rubber band? I've never seen white rubber before and I'm skeptical that it would have a high enough energy density to give you a one minute flight even if the machine were capable. I've never tried carbon for a rubber powered model. Balsa probably has a much better stiffness to weight ratio due to its much lower density. Carbon might reduce air resistance due to its smaller cross section, and that might partially explain your high flight speed. Also try a bigger tail on this ornithopter. (The post office might weigh it for you.)

[B753]

The rubber is actually made for rubber driven planes (or whatever they´re called)
I only have two sizes: one is flat at 1mm x 5mm and the other one is round with a diameter of 1mm.
I don´t know where to get other rubber. I was already happy to find those in my local store.
I noticed that the rubber that comes with the freebird was much more flexible and better than the ones i have.
Found a scale. AUW is 2g.
I´m gonna change to balsa again :) Thanks for comments though.

Alexander

[Chronister]

If that thing weighs only 2 grams, you're doing pretty well with the carbon!

[John Mack]

Your model looks fine! I expect it will fly well, eventually.

However trimming is often difficult so do keep making small changes! A larger tail or longer boom might help with that. Models with your layout often fly trimmed nose-up 10 or 15 degrees.

Slightly narrower tips sometimes seem to fly better – some models have flown with triangular wings, though I have not gone that far.

Carbon fibre and epoxy structures feel much more rugged and durable than balsa. However, although very stiff in bending, so it can be thin, carbon is poorer than balsa in twist for motor sticks. The only part that needs to be really tough, for the ROUGH handling which seems to happen with ornithopters, is around the crank bearing. I find that the mast can be slender near the top because the loading there is trivial – just mechanism, air and collision loadings.

I had not realized that yours weighed only 2g. That should fly well. The only difficulty that I can think of is that the air loads are not enough to twist the membrane. I saw that once on a Freebird with a stiff heavy tissue. Can you see it flexing appropriately?

Two rubber qualities are widely used by flyers: FAI and TAN2. I prefer TAN2 because it is softer and stretches more, but it is becoming more difficult to find. In UK I try Flighthook: (via e-mail to pauline@flitehook.freeserve.co.uk for an emailed price list), SAMS index: http://www.samsmodels.demon.co.uk/catindexpage.html and Mike Woodhouse at http://www.freeflightsupplies.co.uk/FFS-PRICE-LIST.PDF who all have a wide range of useful materials. There are many more suppliers.

Your model looks well designed and built. I feel sure it will become a good flyer. As we say here, keep plugging away!

John.

(I thought I had already this earlier. Please ardon if I repeat.)

[Chronister]

It's also possible the spar is too flexible, allowing the membrane too much twist. You could brace the membrane like in the Spencer Seagulls to solve that problem, if that is indeed the problem.

However, a bigger tail like John suggested might be the best thing to try first.

If the tail is too small, it has to be angled up a lot, and that causes a lot of drag and downforce. Also the small tail might not be keeping the model in correct trim. You did say it flies fast, and such a light model shouldn't. Beyond a certain speed, rapid flight consumes more power than slow flight.