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Old Jul 05, 2012, 07:25 AM
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United States, OH, South Lebanon
Joined Jul 2005
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Help!
Unable to reach desired speed

I recently built a built a 40" flying wing with a 2200kv motor and 6x4 prop and it was able to cruise around at 65mph.

Of course that was fun to get started with, but then I wanted to break 100mph, so I swapped the motor for a 3500kv helicopter motor, 50amp ESC and a 4.5x4.5 prop. Webocalc shows that a 4.75x4.75 prop should draw 32 amps on a 3s setup for 125+mph pitch speed. Unfortunately, my watt meter, has the smaller 4.5x4.5 pulling 38amps. I decided to give the setup a test flight, but I was only able to get the plane to the same 65mph speed of the smaller 2200kv motor. The motor came down warm, but not overly hot.

Am I overlooking something here? The motor spins up fine, but could there still be a timing issue? Does anyone know what the programming sequence is for the RCTimer 50a ESC with 3a BEC? The instructions I have don't seem to match the ESC's stick programming options.

Specs used for Webocalc:
AUW = 21
Wingspan = 40
Wing area = 400
Maximum prop size = 5
Top Speed = 120
Thrust = 21
Amp draw = 32
Motor kv = 3500
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Old Jul 05, 2012, 10:47 AM
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France, RA, Cormaranche-en-Bugey
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Hi,

your setup are around 300 to 450 W, you would probably go on a 1000 W setup in order to reach 100 mph, or cut down some of the wings.

Something like that motor : http://www.scorpionsystem.com/files/...data_chart.htm

spinning a 7x7 sport,
and 5S batteries

That should lead to a 60A setup.
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Old Jul 05, 2012, 12:27 PM
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United States, CA, Bear Valley Springs
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Part of it is the prop. You're trying to get the same amount of weight moving using a much smaller prop. Just because it's spinning faster doesn't mean it will translate to the thrust needed to get going. Go back to the 6x4 and try again keeping in mind amp draw and motor temp.

mw (Ritewing flyer)
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Old Jul 05, 2012, 12:29 PM
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United States, TX, Killeen
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You don't need a 1000watts to hit 100 mph especially on a wing. I was getting 117mph from my Cosmic Wind on 3s With a Hyperion outrunner at 620watts input. The Cosmic Wind is a pretty slippery plane but certainly not like a wing.
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Old Jul 05, 2012, 12:44 PM
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United States, OH, South Lebanon
Joined Jul 2005
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It still has plenty of thrust with the 4.5x4.5, probably 1:1, which is why I thought it would get more speed too. I realize that I can't get double the speed from double the power (200w to 400w), but I certainly expected 80+ from the new setup.

Is it possible that ESC timing with this high kv outrunner is an issue? I don't notice any issues with the motor spooling up, but are there other circumstances where timing adjustments could play a role?
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Old Jul 05, 2012, 01:16 PM
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Cosmic wind = 254 inē,

yours = 400 inē,

to x2 speed you'll almost need to x2^3 (x8) power.
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Old Jul 05, 2012, 03:37 PM
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Toronto Canada
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Quote:
Originally Posted by yomgui View Post
Cosmic wind = 254 inē,

yours = 400 inē,

to x2 speed you'll almost need to x2^3 (x8) power.
Just curious.

Because aerodynamic drag is proportional to area, to achieve the same speed as the Cosmic Wind would require 400/254 = 1.57 X the power.

For the Cosmic Wind to go 117 mph required 620W.

Therefore wouldn't the wing require about 1.57 X 650 = 973 W?
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Old Jul 05, 2012, 03:59 PM
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There is definitely some confusion here about wing area versus flat plate drag area. By the logic stated above if I remove the wing from my Cosmic Wind drag goes to zero and therefore watts required goes to zero as well. Would be nice if it worked that way.

My Super Sportwin is nearly 5lbs with 420 inches of wing area and does 107mph on ~720watts.
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Old Jul 05, 2012, 04:10 PM
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Yes, of course, but wing area has a big impact on overall drag. It's just a ballpark anyway, one can't really concluded with the numbers here.

I don't know what's the actual power of TimJC's setup (the 65 mph one), I think it might be 300W, going to 100 mph will obviously require much more power, logically near 1000W.
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Old Jul 05, 2012, 04:23 PM
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Is this the equation?

Double the speed = money thrown at it squared
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Old Jul 05, 2012, 04:38 PM
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Quote:
Originally Posted by flydiver View Post
Is this the equation?

Double the speed = money thrown at it squared
That's the one I'm familiar with.
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Old Jul 05, 2012, 04:57 PM
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Quote:
Originally Posted by yomgui View Post
Yes, of course, but wing area has a big impact on overall drag. It's just a ballpark anyway, one can't really concluded with the numbers here.

I don't know what's the actual power of TimJC's setup (the 65 mph one), I think it might be 300W, going to 100 mph will obviously require much more power, logically near 1000W.
Wing area has some impact on drag due to wetted surface area but the airfoil has alot more impact. Wing area comes into play much more when relating to induced drag such as when pulling around a pylon but straight and level the area plays a small part as wetted surface and profile drag.
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Old Jul 05, 2012, 05:12 PM
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Toronto Canada
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Quote:
Originally Posted by tsymonds View Post
There is definitely some confusion here about wing area versus flat plate drag area. By the logic stated above if I remove the wing from my Cosmic Wind drag goes to zero and therefore watts required goes to zero as well. Would be nice if it worked that way.

My Super Sportwin is nearly 5lbs with 420 inches of wing area and does 107mph on ~720watts.
Just some numbers for interest.

If the Sportwin weighs 5 pounds, that means that the lift is 5 pounds.

What's the wing lift-drag ratio? Maybe 3? Just a guess. It could be quite a bit higher.

That means that the wing drag is 5/3 = 1.66 pounds.

107 mph is 157 fps.

That means that Power = 1.66 X 157 / 550 Horsepower = 355W

Prop efficiency ~ 80%

Motor efficiency ~ 70% therefore motor power = 355/0.7/0.8 = 633W.


There will of course be more profile drag from the fuselage. If the lift-drag ratio were signifcantly higher (and it could be) that would imply that the profile drag of the fuselage is signifcantly larger.
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Old Jul 05, 2012, 05:15 PM
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Anyway, I don't think of a timing issue.
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Old Jul 05, 2012, 08:37 PM
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Moab, Utah, USA
Joined Apr 2003
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Quote:
Originally Posted by TimJC View Post
I recently built a built a 40" flying wing with a 2200kv motor and 6x4 prop and it was able to cruise around at 65mph.

Of course that was fun to get started with, but then I wanted to break 100mph, so I swapped the motor for a 3500kv helicopter motor, 50amp ESC and a 4.5x4.5 prop. ...I decided to give the setup a test flight, but I was only able to get the plane to the same 65mph speed of the smaller 2200kv motor.

Am I overlooking something here?
What you are missing is that a plane reaches its maximum speed when thrust equals drag. Therefore, increased speed, which increases drag, requires increased thrust. The thrust of a propeller changes approximately with the square of a change in RPM and approximately with the cube of a change in diameter. Going from a 2200 Kv motor to a 3500 Kv motor would increase thrust by a factor of around 2.5. At the same time, decreasing propeller diameter from 6 inches to 4 inches would decrease thrust by a factor of around 3. In terms of thrust, that's pretty much a wash. So it is not surprising the speed was about the same with both power systems.

Larry
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