Negetive Power Coefficient(CP) in Propeller

[Inseo Choi]

Hi, I found that the strange results from performance of the propeller rotating 'reverse'
Im using openvsp 3.30.0 ver.
The fisrt result is the propeller rotating cw



and the second result is the propeller rotating ccw

Next, these are the results from the above propellers.

ct is same each others.

But the problem is shown at cp

The only difference setting is the rotation direction 'Rev'

But the results of the cp is very different between CCW and CW.

It seems like the cp of CW is just negative value of the CCW

Is it right ? or Is it real negative value ?

Thank you.

The propeller is the NACA 5868-9

Here is the .vsp file.

Best Regards

Inseo.

[Inseo Choi]

2023년 3월 14일 화요일 오전 10시 25분 34초 UTC+9에 Inseo Choi님이 작성:

[Rob McDonald]

You can safely take the absolute value of these.

I thought I fixed it at one point, but who knows...

OpenVSP feeds in reverse rotations to VSPAERO with negative RPM.  In CT, RPM shows up as an even power -- in CP, it shows up as an odd power.  Consequently, the sign of CP flips.

Rob

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[Inseo Choi]

Hi Rob.

CT = Thrust / (rho n^2 D^4)
CP = Power / (rho n^3 D^5)

So, the value of the CP when the reversed RPM is must be negative value.

Thank you.

Inseo.

2023년 3월 14일 화요일 오후 1시 46분 11초 UTC+9에 Rob McDonald님이 작성: