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Old Jan 27, 2013, 01:04 AM
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reply to jusjack question

Yes it will work but I calculated reduction ratio 21.58 for 6 Hz flapping frequency which is more than enough for ornithopter 150 - 300 g and wing span about 0.8 - 1.2 m .
The reason for use of high KV motors is that they are more powerfull for the same size and weight but require higher reduction ratio so the gearbox going to be more complex and probably more heavy.

With 2 stages the gearbox will be lighter there is other advantages as well but the answer will become very long.

There is even an example how your ornithopter will perform approximately
https://www.youtube.com/user/injobho
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Old Jan 27, 2013, 04:13 PM
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Serbia, Central Serbia, Belgrade
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Thank you for your answer seagull10,

I included the original question at the bottom, for the reference, as it was posted in other forum.

Good to know that other people satisfactorily made an ornithopter with low Kv motor. I plan to make orni 90cm, 150gr. The reduction ratio I took from Kestrel design where the motor is kv3800, reduction 32, for 7.4V it makes 14.6 flaps/s.
I have kv1500, reduction 13.25, for 7.4V it makes 13.9 flaps/s.




Quote:
Originally Posted by jusjak View Post
Hi guys,

I have Turnigy 1811, kv1500 motor laying around. Do you think it is OK for Kestrel or other ornithopter, if I use one stage reduction of 13.25, which makes maximum of around 14 flaps/sec (no load)? Is there particular reason to go with such high Kv and 2 stage reduction? Vibration, maybe. No experience, this is going to be my first ornithopter. Thanks.
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Old Jan 27, 2013, 05:43 PM
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Quote:
Originally Posted by jusjak View Post
Thank you for your answer seagull10,

I included the original question at the bottom, for the reference, as it was posted in other forum.

Good to know that other people satisfactorily made an ornithopter with low Kv motor. I plan to make orni 90cm, 150gr. The reduction ratio I took from Kestrel design where the motor is kv3800, reduction 32, for 7.4V it makes 14.6 flaps/s.
I have kv1500, reduction 13.25, for 7.4V it makes 13.9 flaps/s.
I've moved the answer here, just to keep the Kakuta's thread out of congestion

Your references are to an early Kakuta's design . If you check the last ones the reduction ratio is more likely around 50( and IMO there is even more room for increasing it) with the same motor and on 7.4 V battery.
With your specification your motor and ESC will run too hot and most probably wouldn't reach the flapping frequency rate which it will be able to reach with the proper reduction ratio but will convert a good portion of your battery's' energy to heat instead in to mechanical energy which actually flaps the wings.
.
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Old Jan 27, 2013, 06:12 PM
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Yes, that is my concern too, Unfortunately, I posted the question too late, I now have done section with gears. I am going to try as it is, and see how it goes. I'll post results here. Still, a lot of work to be done, wing section, wing swivel mount, tail etc. If it is no go, I'll change reduction ratio or motor.
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Old Jan 28, 2013, 03:24 AM
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This may help you design your ornithopter:

The lift and drag produced by a flapping wing that rotates about an axis is strongly affected by flap frequency, flap angle (also called amplitude) and wingspan. Assuming the arc over which the wing oscillates has a fixed distance, only the time it takes for the wing to pass through that arc is what determines it's velocity. If the frequency increases, the time over which this distance is covered decreases and the velocity increases. As lift increases with the square of the airflow velocity, it can be said that lift the lift produced by a flapping wing also increases with the square of the flapping frequency.

t = 1/f
V = d/t = d*f
L ~ V^2 = (d*f)^2

where:

f = flap frequency
d = arc distance
t = time
V = velocity
L = lift

The wingspan also plays a similar role. The arc distance traveled by the wingtip increases with both wingspan and the angle of the arc. For some given angle, if the wingspan is doubled, the arc distance must double in order for the angle to stay constant. The same applies if the angle is changed. Thus the arc distance is the product of the angle in radians and the wingspan. This means the velocity is directly affected by the wingspan and as such affects the lift.

d = a*s
v = d*f = a*s*f
L ~ V^2 = (a*s*f)^2

where:

a = amplitude
s = wingspan


The lift produced by a wing is proportional to the square of the airflow velocity times the net wing area. The area of a rectangular wing is the chord times the wingspan. If the planform is non rectangular, a constant can be multiplied to both values in order to obtain the wing area. Since lift is linearly proportional with wing area times the velocity squared it, this is what happens:

A = s*c*K
L ~ A*(a*s*f)^2 = s*c*K*s^2*(a*f)^2 = s^3*c*k*(a*f)^2

where:

c = wing chord
K = planform adjustment constant

Thus it can be said lift is proportional to the square of flap frequency, amplitude and the cube of the wingspan. Since drag is also affected by these same variables, the effects are also true for it. If you decrease the wing span, the drag decreases, the wings can flap faster, the lift increases, but so does the drag again. In short if you increase or decrease the wing span in very small increments you can obtain the highest lift, for the lowest current from of your motor, that is assuming the chord and amplitude don't change. An easier way adjust the loading on your motor is to just change the wing chord. It affects drag linearly, so if you half it, you half the drag.

I can't really say which is the most optimum solution, but if you want the most static thrust, use the highest amplitude possible for the lowest flap frequency possible. This is due to inertial resistance created by the oscillating mass of the wing changing linearly with amplitude, yet increasing with the square of the frequency. Static thrust only matters if you want to have a slow flying ornithopter or a ornithopter capable of hovering. The thrust that keeps an ornithopter flying forward is more depedant on wing angle of attack than any other variable, and It's mostly affected by wing flexibility and forward velocity.

If you want a nice forward flying ornithopter (like a bird), my recommendation would be something with a 60 degree flap angle (+35, - 25, 5 degrees of dihedral), a 35 cm span for each wing (70 cm total wing span) with a 10 cm wing chord and a half eliptical planform. The relatively high aspect ratio would also give you a nice gliding ability.

If this overloads the motor chop off 5cm from the wingtips.

I Hope this long post helps you some
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Old Jan 28, 2013, 03:37 PM
Kjell Dahlberg
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I am missing the mass accelerating produced by the LE suction and the TE Jet-stream.
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Old Jan 29, 2013, 01:55 PM
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Joined Mar 2011
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Very informative XXXmags, I am sure it will be used.

So far I finished body, gear mechanism, main wing rods. Motor runs OK, it only takes 0.2A@7.4V to run rods and whole mechanism, meaning there is no excessive loss in the mechanics. I am still going to try with wing span of 85-90cm.

Off topic a bit, but still an ornithopter topic. The glue I am using is very good for strong joints needed for ornithopter. It is 2 component acrylate similar to this ( I bought mine locally so it is somewhat different package):

http://www.hobbyking.com/hobbyking/s...idproduct=5536

It is stronger than epoxyd and it is used in dental prosthetics. Sorry if I am repeating something that was already written, so far in posts that I've read regarding ornithopters nobody talked about the glue used. The glue is of monumental importance in this "business", it can speed the process of building and you can get away with not so ideal design.
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Old Jan 29, 2013, 04:39 PM
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I've used 2-ton devcon epoxy to build ornithopter drives and it has never failed.




The gray stuff is the epoxy
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Old Jan 29, 2013, 05:25 PM
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I've used epoxy before, but I find acrylite superior in strength especially on surfaces like plastic, carbon, fiberglass, metal. I still have to try it on slippery plastic like delrin. Once I tried to dismantle previously glued metal part to some plastic. I couldn’t tare it apart, ended up cutting the glue.

The trick is in releasing heat when this glue hardens, which helps glue coming into the surface of material. You can feel it by touch, you can even see little steam if you watch closely, after around 3 minutes.

Pros: Full strength achieved after 5-10 minutes (not the case with 5min epoxy although claimed), amount of components does not have to be 50% but close, it is not critical and does not impact final strength, better adhere to plastic, fiberglass and other non-porose materials, the glued material can be hold together for 3 minutes when hardening begins and it will stay in that position, which is not exactly the case with 5min epoxy although claimed.

Cons: Smells bad, worse than epoxy, comes only as 5 min glue

Please try it.
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Old Jan 29, 2013, 06:24 PM
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Yes the drying time is a drawback of using epoxy, especially the 2 ton stuff. It takes 8-10 hours fully dry. To get around this I've started to use sowing thread soked in CA to hold parts together. Although not as versatile as epoxy it it dries faster.

Next time I see some Acrylite I'll buy it try it out. I'm interested in what construction possibilities it has.
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Old Feb 18, 2013, 02:10 AM
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Quote:
Originally Posted by XXXmags View Post
This may help you design your ornithopter:

The lift and drag produced by a flapping wing that rotates about an axis is strongly affected by flap frequency, flap angle (also called amplitude) and wingspan. Assuming the arc over which the wing oscillates about has a fixed distance, only the time it takes for the wing to pass through that act is what determines it's velocity. If the frequency increases, the time over which this distance is covered decreases and the velocity increases. As lift increases with the square of the airflow velocity, it can be said that lift the lift produced by a flapping wing also increases with the square of the flapping frequency.

t = 1/f
V = d/t = d*f
L ~ V^2 = (d*f)^2

where:

f = flap frequency
d = arc distance
t = time
V = velocity
L = lift

The wingspan also plays a similar role. The arc distance traveled by the wingtip increases with both wingspan and the angle of the arc. For some given angle, if the wingspan is doubled, the arc distance must double in order to keep the same angle. The same applies if the angle is changed. Thus the arc distance is the product of the angle in radians and the wingspan. This means the velocity is directly affected by the wingspan and as such affects the lift.

d = a*s
v = d*f = a*s*f
L ~ V^2 = (a*s*f)^2

where:

a = amplitude
s = wingspan


The lift produced by a wing is proportional to the square of the airflow velocity times the net wing area. The area of a rectangular wing is the chord times the wingspan. If the planform is non rectangular, a constant can be multiplied to both values to obtain the area. Since lift is linearly proportional with wing area times the velocity squared it, this is what happens:

A = s*c*K
L ~ A*(a*s*f)^2 = s*c*K*s^2*(a*f)^2 = s^3*c*k*(a*f)^2

where:

c = wing chord
K = planform adjustment constant

Thus it can be said lift is proportional to the square of flap frequency, amplitude and the cube of the wingspan. Since drag is also affected by these same variables, the effects are also true for it. If you decrease the wing span, the drag decreases, the wings can flap faster, the lift increases, but so does the drag again. In short if you increase or decrease the wing span in very small increments you can obtain the highest lift, for the lowest current from of your motor. That is assuming the chord and amplitude don't change. An easier way adjust the loading on your motor is to just change the wing chord. It affects drag linearly, so if you half it, you half the drag. I can't really say which is the most optimum solution, but if you want the most static thrust, use the highest amplitude possible for the lowest flap frequency possible. This is due to inertial resistance created by the oscillating mass of the wing changing linearly with amplitude, yet increasing with the square of the frequency.

Static thrust only matters if you want to have a slow flying ornithopter or a ornithopter capable of hovering. The thrust that keeps an ornithopter flying forward is more depedant on wing angle of attack than any other variable. It's mostly affected by wing flexibility and forward velocity.

If you want a nice forward flying ornithopter (like a bird), my recommendation would be something with a 60 degree flap angle (+35, - 25, 5 degrees of dihedral), a 35 cm span for each wing (70 cm total wing span) with a 10 cm wing chord and a half eliptical planform. The relatively high aspect ratio would also give you a nice gliding ability.

If this overloads the motor chop off 5cm from the wingtips.

I Hope this long post helps you some
hello, this is helpful for me..you have made that formulas or from journal ?
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Old Feb 18, 2013, 03:14 AM
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I derived them for my own use after many unsuccessful ornithopters.
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