Feb 28, 2012, 09:26 AM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by sammyc Also said both high and low speed though.
I was trying to be consistant. One full circuit at low cruise (no throttle adjustment) another circuit at high cruise power (again no throttle adjustment). The graph shows that the ESC/Motor/Prop was doing LESS WORK going down wind. Someone with an eLogger please repeat this experiment!!
Feb 28, 2012, 09:29 AM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by Hayabusa Heli When going down wind, there is less air drag on the entire plane, so you don't need as much throttle to go the same speed. If you had kept the throttle at the same setting, then the plane would increase speed when traveling down wind.
The plane did increase speed going downwind and slow down going upwind, but I did not adjust throttle. Being consistant and getting reliable data was the goal.
Feb 28, 2012, 09:34 AM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by Hayabusa Heli I could see how the prop would act like a "screw" advancing through the air as the plane moved through the air (see photo in Post #13236), which could possibly lessen the load (ie, current draw) on the motor once the plane is at top speed with the throttle at 100%. If you are cruising at say 50% throttle, and then hit the throttle to 100%, the current draw would spike up and then probably slowly decrease some as the plane approached terminal horizontal velocity. So the faster the plane goes with the motor at 100%, the less current it would draw due to the forward velocity of the plane causing the prop to screw through the air. Of course, the difference might only be 5 to 10% less compared to a pure static test ... just a guess without real numbers. In a static test condition (plane on the bench), the prop is basically acting like a fan that is moving static air to an exit velocity on the thrust side of the prop. No forward velocity doesn't allow the prop to screw through the air.
BINGO!
That is what my eLogger graph shows! From 25 down to 22 amps (~10%).
When I get home from work I'll try to overlay the GPS course data on the Amp plot.
Feb 28, 2012, 09:39 AM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by sammyc Yes, that's what I meant too lol, just using different examples. If the ground speed is kept constant, throttle will have to change downwind vs upwind, therefore if the current test was done to keep a constant groundspeed its not valid.

Constant (untouched) throttle per lap.
Only adjusted throttle AFTER completing a lap and to change altitude or speed for next lap. I was trying to be as accurate as possible.
Feb 28, 2012, 09:54 AM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by Simon2504 Im currently reading another thread where a lot of testing is going on in regards to props and current draw, (funfighter thread, where most people all trying to get 130mph+). I wont even pretend to know the physics of it all but it is showing 100% of the time bench testing does draw more current than in flight
Thank you for that!
I've been a R/C modeler for almost 40 years and from the beginning have been told that a motor works harder on the ground then in the air. The motor, gas or electric will "unwind" once in flight. Power requirements (amps) will be reduced at cruise speed for the SAME throttle setting. Works that way in my Cessna too. Before takeoff, when I apply 1600 RPM for the mag check the throttle is at about 1/2. At 1600 RPM on the downwind leg for landing it's only at about 1/3. Same power, less fuel. The only differance is that the 80 MPH WIND is assisting the prop.
Last edited by HiFlyer371; Feb 28, 2012 at 10:00 AM.
Feb 28, 2012, 01:33 PM
Registered User
United States, FL, Clearwater
Joined Aug 2011
1,556 Posts
Quote:
 Originally Posted by ajbaker Yes. See attached. The stock ESC is more than sufficient to handle the stock prop. It can even handle the 6x3 nicely, too. AJ
It have been very enlightening following all of the posts concerning motor current draw and wind speed. I now know more and I appreciate everyone's input on this subject.

AJ

I have looked at your data chart on the current draws vs props but I do not want to make any assumptions about a 6x4E prop.

Will the stock 20A ESC and brushless motor on my SkySurfer be able to handle a TGS 6x4E prop? I believe I had read that it would overload the ESC but I want to confirm that before I mount it. Thanks

Vince
 Feb 28, 2012, 02:45 PM Better then Sliced Bread! United States, CA, Arcata Joined Oct 2011 2,555 Posts If you really want accurate results you need a wind tunnel with accurate wind speeds. Latest blog entry: Team Skunk FPV Video: Across the Bay
Feb 28, 2012, 02:50 PM
Registered User
United Kingdom, England, Ripon
Joined Dec 2011
309 Posts
Dont you just hate it when you think the wind isnt so bad.....then you get it up in the air and have a battle just to fly lol
(and yes the camera is mounted straight, the plane just insisted on going sideways)
 Bixler in the wind (9 min 2 sec)
Feb 28, 2012, 03:18 PM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Quote:
 Originally Posted by NorCalMatCat If you really want accurate results you need a wind tunnel with accurate wind speeds.
That would be best in the laboratory but impracticable in the real world. The National Aeronautic Association requires 3 laps, over a closed course (upwind and downwind each) for establishing World Records.
Feb 28, 2012, 03:29 PM
Registered User
United States, WA, Seattle
Joined Oct 2011
195 Posts
Quote:
 Originally Posted by HiFlyer371 Thank you for that! I've been a R/C modeler for almost 40 years and from the beginning have been told that a motor works harder on the ground then in the air. The motor, gas or electric will "unwind" once in flight. Power requirements (amps) will be reduced at cruise speed for the SAME throttle setting. Works that way in my Cessna too. Before takeoff, when I apply 1600 RPM for the mag check the throttle is at about 1/2. At 1600 RPM on the downwind leg for landing it's only at about 1/3. Same power, less fuel. The only differance is that the 80 MPH WIND is assisting the prop.
So I have a theory. I don't think that aerodynamics has anything to do with this unexplained current drop in bench vs in flight test. What if heat is the real difference? Metal conductivity increases with heat. In flight I would imagine a motor runs significantly cooler than on a bench.

Copper has a resistivity of about 1.68 e-8 ohms at 20 degrees celcius. Not very much. But if the temperature of the copper is 200 degrees c, like say in a bench test with no cooling the resistance jumps to around 2.86 e-8 ohms. Lets say that in flight the temp of the motor is somewhere in between say 100 c, at this temp copper has a resistance of 2.2e-8.

Now all of these resistance values are very small, but their relative difference is very significant. If voltage remains a constant and resistance increases, then we should see an increase in current. Since temps in flight are lower than at rest, this could explain why current draw is lower.
Last edited by Dkumagai; Feb 28, 2012 at 03:36 PM.
Feb 28, 2012, 03:44 PM
Bill
United States, NE, La Vista
Joined Jul 2007
261 Posts
Originally Posted by HiFlyer371
Thank you for that!
I've been a R/C modeler for almost 40 years and from the beginning have been told that a motor works harder on the ground then in the air. The motor, gas or electric will "unwind" once in flight. Power requirements (amps) will be reduced at cruise speed for the SAME throttle setting. Works that way in my Cessna too. Before takeoff, when I apply 1600 RPM for the mag check the throttle is at about 1/2. At 1600 RPM on the downwind leg for landing it's only at about 1/3. Same power, less fuel. The only differance is that the 80 MPH WIND is assisting the prop.

Quote:
 Originally Posted by Dkumagai So I have a theory. I don't think that aerodynamics has anything to do with this unexplained current drop in bench vs in flight test. What if heat is the real difference? Metal conductivity increases with heat. In flight I would imagine a motor runs significantly cooler than on a bench. Copper has a resistivity of about 1.68 e-8 ohms at 20 degrees celcius. Not very much. But if the temperature of the copper is 200 degrees c, like say in a bench test with no cooling the resistance jumps to around 2.86 e-8 ohms. Lets say that in flight the temp of the motor is somewhere in between say 100 c, at this temp copper has a resistance of 2.2e-8. Now all of these resistance values are very small, but their relative difference is very significant. If voltage remains a constant and resistance increases, then we should see an increase in current. Since temps in flight are higher than at rest, this could explain why current draw is lower.
Interesting and well thought out.
But it doesn't explain why the full size Cessna uses 1/2 throttle and 4 gallons per hour of fuel at 1600 RPM on the ground and 1/3 throttle and 3 gallons per hour in the air at the same RPM setting. This is repeatable. The next student in the airplane is required to do the same Mag Check (RPM test) and fly the same pattern at the same power settings. It has been a long established fact that a propeller moving through the air in more efficient than a static propeller.
 Feb 28, 2012, 03:46 PM Registered User United Kingdom, England, Ripon Joined Dec 2011 309 Posts I dont understand why people are making this so complicated, Im not the best when it comes to physics but to me its just a bit of common sense that a plane being held on a bench has to move static air upto the speed the props pitch is supposed to, whilst in flight the plane is already moving through the air so the load is less........simples!!!!!!!
 Feb 28, 2012, 03:47 PM Better then Sliced Bread! United States, CA, Arcata Joined Oct 2011 2,555 Posts I still think the airflow of movement itself will have an effect on the prop too. Take into account a column of air moving at 5 mph, your plane is traveling at 25mph into it, your airspeed is 30mph, there is an extra 5mph of air flowing into the prop, which actually should increase it's RPM speed and cause less power draw, going into the wind, let's say we are going 25mph with the wind of 5mph, our airspeed is now 20mph, however the motor still has airflow with it (as we are going 20mph airspeed), but 5mph less wind then before, so RPM should be a little lower then going with the wind. Latest blog entry: Team Skunk FPV Video: Across the Bay
Feb 28, 2012, 03:49 PM
Registered User
United Kingdom, England, Ripon
Joined Dec 2011
309 Posts
Quote:
 Originally Posted by NorCalMatCat I still think the airflow of movement itself will have an effect on the prop too. Take into account a column of air moving at 5 mph, your plane is traveling at 25mph into it, your airspeed is 30mph, there is an extra 5mph of air flowing into the prop, which actually should increase it's RPM speed and cause less power draw, going into the wind, let's say we are going 25mph with the wind of 5mph, our airspeed is now 20mph, however the motor still has airflow with it (as we are going 20mph airspeed), but 5mph less wind then before, so RPM should be a little lower then going against the wind.
Again why make it so complicated lol
 Feb 28, 2012, 03:50 PM Better then Sliced Bread! United States, CA, Arcata Joined Oct 2011 2,555 Posts It's not about being complicated, it's about learning the principles behind it Latest blog entry: Team Skunk FPV Video: Across the Bay