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Old Apr 18, 2012, 08:52 AM
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Originally Posted by anlucas View Post
Check that the wiring and stuff in the intake duct doesn't disrupt the airflow to the fan. It can make a big difference.

Its about as cleaned up inside as its gonna get. I even moved the ESC up into the battery compartment



Flying at higher altitude means thinner air and thus lower amps.
Your right but flying at altitude or higher elevations does mean thinner air. But it also means I have to turn higher RPM's (more amps) to achieve the same thrust value's that you lower guys enjoy.
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Old Apr 18, 2012, 09:28 AM
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Originally Posted by SilverState RC View Post
Your right but flying at altitude or higher elevations does mean thinner air. But it also means I have to turn higher RPM's (more amps) to achieve the same thrust value's that you lower guys enjoy.
How high up are you?

If the 3300kv is pulling less than 75amps now, you could try the 3700kv CP.
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Old Apr 18, 2012, 09:31 AM
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I don't really know, but....

I would think higher altitude needs more RPM, because it needs to 'pump' more air volume to get the same thrust as lower altitude.
BUT, the resistance of thinner air is less.... and likely to all end up proportional in terms of power needed for that same end result. So whilst you NEED more RPM.... which means you need a bit more KV for a same fan system.... the load total is the same, and thus current draw will be the same.

eg If someone has a 1600kv at low altitude, you might need 1800kv. But the Watts (power) needed will be the same, for the same aircraft performance (if you get the KV matched to have that same flight performance as the lower altitude had).

But then there is lift..... probably a bit lower..... so you will need a bit more AoA to fly the same - probably a weeny amount, like 0.1deg or whatever. Which will rob a bit of power.

And all in all I would not expect too huge a power use increase.
But you would need to get that required KV increase worked out.
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Old Apr 18, 2012, 09:34 AM
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[QUOTE=anlucas;21365608]How high up are you?
5000ft


If the 3300kv is pulling less than 75amps now, you could try the 3700kv CP. QUOTE]


I dont know how many amps Im pulling as I dont YET own a testing tool to check what is going on with the motor. Im suspecting that I will have to invest in larger Mah batteries if I decide to go to a larger KV motor.
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Old Apr 18, 2012, 09:41 AM
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Originally Posted by PeterVRC View Post
I don't really know, but....

I would think higher altitude needs more RPM, because it needs to 'pump' more air volume to get the same thrust as lower altitude.
BUT, the resistance of thinner air is less.... and likely to all end up proportional in terms of power needed for that same end result. So whilst you NEED more RPM.... which means you need a bit more KV for a same fan system.... the load total is the same, and thus current draw will be the same.

eg If someone has a 1600kv at low altitude, you might need 1800kv. But the Watts (power) needed will be the same, for the same aircraft performance (if you get the KV matched to have that same flight performance as the lower altitude had).

But then there is lift..... probably a bit lower..... so you will need a bit more AoA to fly the same - probably a weeny amount, like 0.1deg or whatever. Which will rob a bit of power.

And all in all I would not expect too huge a power use increase.
But you would need to get that required KV increase worked out.
You may be correct in the fact that I just need more RPM's. But I think that what may be my limiting factor or factors is that Im using the CS10 fan which is known to be an amp hog. And because I need more RPMs because of the elevation that I fly at I need a larger KV motor. Which in turn is going to draw more amps. That and Im experimenting with a thrust tube which I believe will cause the motor to draw more amps because its having to push the air through a more narrow opening.
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Old Apr 18, 2012, 10:08 AM
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Silverstate,

the CS 10 blade is a heavier load than the other 4-5-6 blade rotors out there. At the same rpm, it generates more thrust than others. This is why we use lower kv motors.

In general, it needs slightly more amps to generate similar thrust levels to other fans.

If you spin any rotor in a total vacum then the motor will pull no more amps than the motor would pull without a rotor on it.

As the air becomes denser, the force needed to keep the rotor turning at a specific rpm rises.

In your case at 5000feet you will find that a motor with higher kv will need the same amps as a motor with less kv running at a lower altitude.

It is important to measure the amps you are pulling now because that will tell you how much more kv you need. You could also get away with using the same motor but with a 5S battery.
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Old Apr 18, 2012, 10:20 AM
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Originally Posted by anlucas View Post
Silverstate,

the CS 10 blade is a heavier load than the other 4-5-6 blade rotors out there. At the same rpm, it generates more thrust than others. This is why we use lower kv motors.

In general, it needs slightly more amps to generate similar thrust levels to other fans.

If you spin any rotor in a total vacum then the motor will pull no more amps than the motor would pull without a rotor on it.

As the air becomes denser, the force needed to keep the rotor turning at a specific rpm rises.

In your case at 5000feet you will find that a motor with higher kv will need the same amps as a motor with less kv running at a lower altitude.

It is important to measure the amps you are pulling now because that will tell you how much more kv you need. You could also get away with using the same motor but with a 5S battery.
While the thought of running 5S is very tempting Im going to stick with 4S and just run a larger Mah pack. I think that I may also try running a larger KV motor, after I get a watt, amp meter to test and see what Im actually running. Im currently running a 100a ESC so Im not to terribly concerned that Im going to cook anything.
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Old Apr 18, 2012, 10:29 AM
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These CP motors seem to be able to handle 80amps + without any problems. I have had peaks of 90 which on cool days are also not a problem.

I guess that at 5000feet, you guys don't have really hot days in the summer.
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Old Apr 18, 2012, 10:37 AM
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Originally Posted by anlucas View Post
These CP motors seem to be able to handle 80amps + without any problems. I have had peaks of 90 which on cool days are also not a problem.

I guess that at 5000feet, you guys don't have really hot days in the summer.

Oh, you are mistaken my friend. I live in the state of Nevada. We are considered high mountain DESERT. It gets plenty hot here during the summer. We have quite the spread in temperature here, during the summer. Anywhere from 20C up to 40C. But on average we will see 29C to 33C. But because I live in the desert the humidity is relatively low by comparison to where you live in Greece. Typically we will average 20-25% humidity, and sometimes as low as 10-15%. Which also makes our air less dense because of the lack of humidity.
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Old Apr 18, 2012, 12:46 PM
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Originally Posted by SilverState RC View Post
Oh, you are mistaken my friend. I live in the state of Nevada. We are considered high mountain DESERT. It gets plenty hot here during the summer. We have quite the spread in temperature here, during the summer. Anywhere from 20C up to 40C. But on average we will see 29C to 33C. But because I live in the desert the humidity is relatively low by comparison to where you live in Greece. Typically we will average 20-25% humidity, and sometimes as low as 10-15%. Which also makes our air less dense because of the lack of humidity.
Yeah i live in the low part of nevada in las vegas! Average summer day is between 100 -112 degrees! It gets hot all over in Nevada!
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Old Apr 18, 2012, 12:54 PM
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I know that it "doesn't feel right" but the LESS humidity the DENSER the air is.
Ken
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Old Apr 18, 2012, 12:58 PM
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I know that it "doesn't feel right" but the LESS humidity the DENSER the air is.
Ken
For some reason that just doesnt sound right. However you may be correct. That may be one of the reasons that speed tests are done land or air are performed in desert environments. Like Bonneville, and the Black Rock desert.

Edit: Ken you sir are correct. I just looked it up and found some interesting information.

Interesting information: Pilots use charts or calculators to find out how temperature and air pressure at a particular time and place will affect the air's density and therefore aircraft performance. In general, these calculations don't take humidity into account since its affects are so much less than the others. When the air's density is low, airplanes need longer runways to take off and land and they don't climb as quickly as when the air's density is high.

So knowing this information. Come summer time Im really gonna be screwed with my EDF's. Even though I live at 5000ft=thinner air, and in the dry desert=less humidity=more dense air. But the one thing that Im not sure if I can over come is what is refered to as density altitude. Based on the heat of the day that I see in Nevada during the summer, and the elevation the air becomes even more less dense. So lets say on a 90 degree temperature day at 5000 ft elevation. It would be like trying to fly at like 7000 or 8000 ft.

So I really need to figure out good hot motor/ESC set up with my CS10, to compensate for the environmental condition. Or only fly in the morning while the air is still cool and calm. Either way I want to figure out a good set up.
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Old Apr 18, 2012, 03:28 PM
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Now I have some new numbers, measured the CS70 fan in Hobbyking 70mm composite L39.

This setup:
- CS70 mm 10 blade -blades
- Red Brick 125A ESC
- 6S 5000 mAh 30C, just charged, good battery
- Turnigy 2860-2200 kV.
- Heatsinks from ExtremeRC (two installed on the motor)
- RC-lander metal fan housing
- Hobbyking Composite L39 70 mm (Pro-Design L39)
- No thrust tube yet since I have not figured out how to fit one in the congested space available in the rear of this plane which also contains elevator pushrod mechanics (and would poke inside the thrust tube even if there was a thrust tube).
- Straight intake in the fan (that seamlessly connects with the ducting of the plane)
- No cheater hole

Everything else is now ok except for thrust. Thrust is poor and thrust per watts is poor.

The numbers:
- 1380 W peak, 58A peak, motor stays cool/slightly warm (if power is stopped immediately)
- 1200 W sustained power, was reading something like 56 amps sustained, motor stays cool
- ESC gets slightly warm
- Electrically everything looks to be sane and okay and I am very pleased that the motor does not cook with this setup
But then:
- Measured with fish scale by hanging the model in the air. Having the power on or off only seems to have 450 grams difference. In other words, the fan produces 0.45 kg static thrust at 1200 W sustained power. That is kinda the worst propulsive efficiency I have ever come by.

By looking at it, there clearly is larger gap between the fan and housing than with the original RC lander blades. This will reduce efficiency, but I hope not this much.

I did not get reliable thrust numbers on stand without the plane, but I think I was getting more thrust than this. So the question is that is the intake or exhaust the problem. I will retest later with different fan on the same duct, to verify that it is not a problem with the fan.

The plane is supposed to fly with 3000 mAh 6S, I weighted it without landing gear to be about 1.8 kg. To have even somewhat jet-like performance, I would need more than 1kg sustained thrust. Should I make a cheater hole, add thrust tube (somehow)? Any suggestions?
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Old Apr 18, 2012, 03:34 PM
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Before you start cutting holes in the airframe for better intake I would say try it with the CS10 housing. If there is too much of a gap from the rotor tips to the inside of the housing, I think that will cause you to lose much thrust.
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Old Apr 18, 2012, 03:36 PM
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So the question is that is the intake or exhaust the problem. I will retest later with different fan on the same duct, to verify that it is not a problem with the fan.

The plane is supposed to fly with 3000 mAh 6S, I weighted it without landing gear to be about 1.8 kg. To have even somewhat jet-like performance, I would need more than 1kg sustained thrust. Should I make a cheater hole, add thrust tube (somehow)? Any suggestions?
You need a thrust tube. Composite airframes mostly cannot be flown without one unless a duct is already built in. Most of the air is bouncing around the insides and also pushed backwards around the fan housing. The fan outflow must be fed to the back.

It is quite easy. Just make the tube. Bend it in a U shape along the length and push it in through the rear - then tape it to the fan.
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