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Sep 14, 2019, 05:45 PM
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Here is UIUC's wind tunnel Ct vs J for the APCE 8x8 prop at constant rpms. It does not show a bump as in your chart. What is the source for your graph?
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Sep 14, 2019, 06:07 PM
Sokol
JureZ's Avatar
see file attached here , Illini at work.


not my work , I just added the chart in the last spreadsheet in the book.
Sep 14, 2019, 11:56 PM
Registered User
Ok, I'm at fault here. I generated that Excel spread sheet and take responsibility for its defects. The reason for the shape of that curve lies in the way that it was extracted and represented in math form. It is represented by a polynomial of the form T = A + B*J + C*J^2. It is not a perfect fit, rounding out the actual shape for simplicity's sake. The result is that the curve bulges slightly where it should have been flat or slightly negative.

I will see if I can correct it. There may be other high pitch/diameter ratio entries that show similar bulges.

I have reviewed all of the 70+ APCE props in the data base and none show a bulge like that. Some do show odd behavior, such as discontinuities probably caused by measurement errors.

Thanks for the heads up.
Bill
Sep 15, 2019, 12:10 AM
Sokol
JureZ's Avatar
Quote:
Originally Posted by aeromodel03

Thanks for the heads up.
Bill

cool !
Sep 16, 2019, 01:34 AM
Registered User
Thread OP
Thanks for the learning lessons here!
Does this logic also apply to EDF-s?
Or because the propeller is spinning there so fast, we basically just need to consider the thrust?
Sep 16, 2019, 02:30 AM
Sokol
JureZ's Avatar
applies to EDF's also. They demonstrate very clearly how thrust reaches a maximum value after gaining airspeed.

Some EDF based models are impossible to hand launch.
Sep 16, 2019, 07:27 AM
Registered User
Thread OP
Quote:
Originally Posted by aeromodel03
The attached graph provides an estimate of the thrust as a function of air speed for a generic prop 15x6.5 operating at 8000 rpm. The thrust decreases nearly linearly from zero airspeed to max pitch speed.
Does this graph show true values only if propeller is facing the airflow( like normal pusher motor)?
If the pusher motor is with angle, the values would be different right?
Sep 16, 2019, 02:34 PM
Registered User
Quote:
Originally Posted by hans123
Does this graph show true values only if propeller is facing the airflow( like normal pusher motor)?
If the pusher motor is with angle, the values would be different right?
I'm not aware of any tests with airspeed on off-angle operation. The effect is likely to be small for small angles as the bulk of the net air flow direction over a blade is determined by the spinning blade air displacement, not by the frontal airflow.
Sep 17, 2019, 05:11 AM
Registered User
Thread OP
Quote:
Originally Posted by aeromodel03
I'm not aware of any tests with airspeed on off-angle operation. The effect is likely to be small for small angles as the bulk of the net air flow direction over a blade is determined by the spinning blade air displacement, not by the frontal airflow.
Yeah sounds reasonable. If we take quadcopter for example then it's propeller angle is near to zero and according to vector calculations airspeed doesn't play role in thrust.
Sep 20, 2019, 01:04 AM
Registered User
Thread OP
How does this logic apply to two motor setup?
If @ 20m/s, overall drag is 15N=1,5kg. 14x11 propeller has thrust @ 5500RPM, 20m/s just over 1,6kg. If I now use 2 mototrs instead of 1, Should I aim for 800g thrust for each motor to get the same result? So if we sum up thrust values we get 800+800= 1600g thrust total?
Sep 20, 2019, 12:08 PM
Registered User
Quote:
Originally Posted by hans123
How does this logic apply to two motor setup?
If @ 20m/s, overall drag is 15N=1,5kg. 14x11 propeller has thrust @ 5500RPM, 20m/s just over 1,6kg. If I now use 2 mototrs instead of 1, Should I aim for 800g thrust for each motor to get the same result? So if we sum up thrust values we get 800+800= 1600g thrust total?
Yes, the total thrust is the sum of the individual thrusts. The power required is the sum of the individual motor powers.


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