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Old Jan 29, 2013, 01:33 PM
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Serbia, Central Serbia, Belgrade
Joined Mar 2011
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Mr. Kakuta I want to cangratulate your magnicient work and speed of improvements that you implement! The first time I saw your Kestrel 17-1, I knew that I have to make one and here I am, I am building one.

Since I am going to make one with motor Turnigy 1811, Kv1500, one stage reduction of 13.25, wing span 85-90cm, total weight bellow 130g, I have few questions:

What is the max and mid current on your Kestrel? Do you recommend to build the articulate wings or regular (if you recommend articulate wings, please include more details, photos how to make one)? Where is the center of gravity on Kestrel and how critical it is? Your Kestrel has max 14.6 flaps/s and around 6 flaps when orni goes up. At which position of throttle is this? I am trying to determine if I am going to overload the motor. The brushless motors tend to have small slack as three phase asynchronious motors, which they imitate. The slack is usually no more than 10-20% under load. That is why I ask, how come that no load 14.6 flaps/s go down to 6 flaps/s, or this is less than half throttle? Do you recommend to make Sean style tail as oppose to your style tail, which one is more effective( so far I am leaning toward the Sean tail as it is easier for me to make)?

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. So far not bad in weight , body + motor + gears + bearings+ main wing rods = 63g.

Greetings!
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Old Jan 29, 2013, 04:40 PM
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Joined Sep 2003
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Jusjak,

Motor current depends on the rpm, rpm depends on the load. Its normal for rpm to decrease and current to increase as the load increases.
I have one of the 1811, 1500 kv motors. I havent checked the kv yet but I did measure its coil resistance as 1.44 ohms. At zero rpm its current draw would be close to 5.14 amps at 7.4v. the Hobby King spec sheet gives a max current of 5 amps. If your orni can reach 6 flaps/sec in the air its current draw will be around 3 amps at full throttle. It would draw more with fresh batteries and flap a bit faster. 3a X 7.4v=22watts with 13watts heating the motor.It will get hot. I read a 'rule of thumb' says 3 watts/gram motor weight max. Ornithopters don't have a prop to cool them so build light and make a 'test' covering held on with tape or other easily removable glue to see what your current draw is.
George
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Old Jan 29, 2013, 08:26 PM
K Kakuta
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Joined Aug 2006
1,198 Posts
Quote:
Originally Posted by jusjak View Post
Mr. Kakuta I want to cangratulate your magnicient work and speed of improvements that you implement! The first time I saw your Kestrel 17-1, I knew that I have to make one and here I am, I am building one.

Since I am going to make one with motor Turnigy 1811, Kv1500, one stage reduction of 13.25, wing span 85-90cm, total weight bellow 130g, I have few questions:

What is the max and mid current on your Kestrel? Do you recommend to build the articulate wings or regular (if you recommend articulate wings, please include more details, photos how to make one)? Where is the center of gravity on Kestrel and how critical it is? Your Kestrel has max 14.6 flaps/s and around 6 flaps when orni goes up. At which position of throttle is this? I am trying to determine if I am going to overload the motor. The brushless motors tend to have small slack as three phase asynchronious motors, which they imitate. The slack is usually no more than 10-20% under load. That is why I ask, how come that no load 14.6 flaps/s go down to 6 flaps/s, or this is less than half throttle? Do you recommend to make Sean style tail as oppose to your style tail, which one is more effective( so far I am leaning toward the Sean tail as it is easier for me to make)?

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. So far not bad in weight , body + motor + gears + bearings+ main wing rods = 63g.

Greetings!
What is the max and mid current on your Kestrel?
---I have not measured it.
Do you recommend to build the articulate wings or regular?
---Setting of Articulated wings is difficult.
I recommend that you make straight wings.
Where is the center of gravity on Kestrel and how critical it is?
--It is around 35% from leading edge of the wing.
A position of CG changes by form of ornithopter.
You actually fly, and set CG from the result.
At which position of throttle is this?
---In a horizontal flight, a throttle is central.
Do you recommend to make Sean style tail as oppose to your style tail, which one is more effective( so far I am leaning toward the Sean tail as it is easier for me to make)?
--I recommend invertedVtail or Vtail.
--I used Turnigy 1811 (Kv2900) before.
But, for an acrobatic flight, I changed it to Kv3800Motor because a thrust was not enough.
In Kv1500, a thrust may not be considerably enough.
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Old Jan 29, 2013, 08:30 PM
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Serbia, Central Serbia, Belgrade
Joined Mar 2011
49 Posts
Hi George,

Sure that motor current depends on rpm and rpm depends on load but in brushless motors this is around 15-20% (rpm full throttle no load vs. rpm full throttle with load), otherwise the motor is overloaded.
If it was different you could use one motor with certain Kv in different applications. You actually can, but than full power of motor is not utilised
RPM can be lowered with esc which in essence cuts the effective voltage, but you can not go to full throttle (full DC power) without overloading. I am aware of this from the beginning, just want some practical values to see how much I loose in performance by not using the ideal motor Kv and reduction.

Thanks for practical values you supplied; it shows me some boundaries in which to stay.



Quote:
Originally Posted by Plantoflap View Post
Jusjak,

Motor current depends on the rpm, rpm depends on the load. Its normal for rpm to decrease and current to increase as the load increases.
I have one of the 1811, 1500 kv motors. I havent checked the kv yet but I did measure its coil resistance as 1.44 ohms. At zero rpm its current draw would be close to 5.14 amps at 7.4v. the Hobby King spec sheet gives a max current of 5 amps. If your orni can reach 6 flaps/sec in the air its current draw will be around 3 amps at full throttle. It would draw more with fresh batteries and flap a bit faster. 3a X 7.4v=22watts with 13watts heating the motor.It will get hot. I read a 'rule of thumb' says 3 watts/gram motor weight max. Ornithopters don't have a prop to cool them so build light and make a 'test' covering held on with tape or other easily removable glue to see what your current draw is.
George
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Old Jan 29, 2013, 08:37 PM
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Serbia, Central Serbia, Belgrade
Joined Mar 2011
49 Posts
Mr. Kakuta, thanks for the answer.

With 2900Kv motor, did you use the same reduction ratio as with 3800Kv, which is 32, I beleive?
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Old Jan 29, 2013, 10:06 PM
K Kakuta
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Quote:
Originally Posted by jusjak View Post
Mr. Kakuta, thanks for the answer.

With 2900Kv motor, did you use the same reduction ratio as with 3800Kv, which is 32, I beleive?
Yes. Reduction ratio is 28.
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Last edited by Kazu-kaku; Jan 29, 2013 at 10:22 PM.
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Old Jan 31, 2013, 01:37 PM
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Serbia, Central Serbia, Belgrade
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Mr. Kakuta, can you please show how you make these 2 joints? Thanks.
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Old Jan 31, 2013, 07:34 PM
K Kakuta
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Originally Posted by jusjak View Post
Mr. Kakuta, can you please show how you make these 2 joints? Thanks.
Lily-yarn and PE line for fishing and super glue.
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Old Feb 10, 2013, 05:07 AM
K Kakuta
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Slow Hawk 2 with Articulated wings: Flight with slow flapping.

Slow Hawk 2 with Articulated wings: Flight with slow flapping.

Slow Hawk 2 with Articulated wings: Flight with slow flapping. (2 min 38 sec)


 Comparison that there is no Articulated wings.
The flapping number considerably decreased by Articulated wings.
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Old Feb 10, 2013, 05:28 AM
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Va.Beach VA.
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It flies like a bigger bird. Doesn't have to flap so hard or so fast.* Thats a big improvement!
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Old Feb 10, 2013, 06:02 AM
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i think that is great Doctor a massive improvement does it steer left and right ok now??? regards Brian
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Old Feb 10, 2013, 06:55 AM
K Kakuta
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Originally Posted by therealthing691 View Post
i think that is great Doctor a massive improvement does it steer left and right ok now??? regards Brian
Maneuvering is good by Sean's tail.
Because temperature is -2 degrees Centigrade, performance of battery deteriorates, and a thrust decreases.
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Old Feb 10, 2013, 07:01 PM
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i am glad to see that is possible as i would like to do this to my hand built carbon fiber bird with the wing span of 1930mm .I really love the slow flapping on your articulated SH and on my large bird it would be really slow.This will have to wait until the end of the year as i have much to do before i tour europe this year. It will be a great project to do when i get back though.Thanks for the inspiration Dr Kazu kind regards Brian
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Old Feb 10, 2013, 09:27 PM
DIY Mania from Taiwan
Taiwan
Joined Aug 2011
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Articulated wings performs much more like a real bird flapping action after watched the vid 3 times
thanks for sharing and great efforts
happy Lunar new year
best regards
Sam
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Old Feb 10, 2013, 11:40 PM
K Kakuta
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Slow Hawk 2 with Articulated wings: Good Flight in strong wind

Slow Hawk 2 with Articulated wings: Good Flight in strong wind

Slow Hawk 2 with Articulated wings: Good Flight in strong wind (2 min 23 sec)


Good Flight in strong wind 7 meter /second.
The flapping number considerably decreased by Articulated wings.
By Articulated wings, thrusts increase.
And an ornithopter becomes strong for wind.
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