# How is d(Cl)/d(alpha)@stall and Cl increment to stall defined in Xrotor?

I hope there are some people around here with some experience in Xrotor than me. (Xrotor is:"XROTOR is an interactive program for the design and analysis of ducted and free-tip propellers and windmills." Found: here documentation is at the end of the site) For the Blade Section Properties, I need following:

========================================================================
1) Zero-lift alpha (deg):   0.00       7) Minimum Cd           : 0.0070
2) d(Cl)/d(alpha)       :  6.280       8) Cl at minimum Cd     : 0.150
3) d(Cl)/d(alpha)@stall :  0.100       9) d(Cd)/d(Cl**2)       : 0.0040
4) Maximum Cl           :  2.00       10) Reference Re number  : 2000000.
5) Minimum Cl           : -1.50       11) Re scaling exponent  : -0.2000
6) Cl increment to stall:  0.200      12) Cm                   : -0.100
13) Mcrit                :  0.620
========================================================================


Now does anyone know how 3) d(Cl)/d(alpha)@stall and 6) Cl increment to stall is defined?

If I had to guess 3) is just the slop after Maximum Cl. But 6) which is also mentioned in the doc as "delta CL for the stall transition region" gives me some headache.

Any help is appreciated.

Cheers

• I'm voting to close this question as off-topic because it is about using a software program, rather than about aviation, as defined in the Help Center. – Ralph J Jan 8 at 15:54
• @RalphJ This is indeed about software, but it's a software on the design of airplane rotor blades. Why are these types of questions not allowed? Where else would such questions be answered? – JZYL Jan 8 at 16:03
• Indeed, and it's one of the basic, most popular software applications used in aerospace engineering. I would also consider a question about Xfoil acceptable. – Daniel Jan 8 at 19:36
• @RalphJ I am new here and sorry if I posted this in the wrong forum but considering that this a software for propeller design, I thought I was in the right spot with aviation. As well as the tag propeller exists in this forum. On the other hand, if you are voting to close where should I post it in your opinion? – Polyp Jan 9 at 7:09
• @LucEvertzen could you help me with that? – Polyp Jan 10 at 14:50

I interpreted the values as follows:

1. The angle of attack at which the specified propeller airfoil generates zero lift
2. The change in lift coefficient per change in $$\alpha$$ in radians
3. The same as 2, except that it is this value around the stall angle (it is mostly a safety factor for numerical calculations I think, small but non-zero)
4. Maximum $$C_l$$, which is achieved at the theoretical stall angle
5. Minimum $$C_l$$, which occurs on the backwards sweep of the propeller
6. The $$C_l$$ increment from the last linear part of the $$C_l$$ polar to the stall point
7. The $$C_d$$ at $$\alpha=0$$
8. The $$C_l$$ at $$\alpha=0$$
9. Quadratic drag dependence on $$C_l$$, slope of the $$C_d,C_l^2$$ curve
10. The Reynolds number at which these values are obtained
11. Re scaling exponent as explained in the XRotor documentation
12. Moment coefficient $$C_m$$
13. The critical Mach number.
• Thank you very much for your reply! Now I have to get all this out of the poplars. One thing to 3) how did you define around the stall angle? I mean if you have maximum C_l then the gradient of the polar is zero. How far before or after maximum C_l do you get 3? – Polyp Jan 11 at 15:26
• I simply used the stall angle or just a super small increment beforehand. I can't tell you with a 100 percent certainty but as far as I can remember the results weren't that sensitive to values such as 3). If you want to be sure just play with the values a bit. Just increment these values by a small amount and see by how much your answer changes. If it changes a lot than we might need to take another look at it. Otherwise it doesn't matter as much. – Luc Evertzen Jan 12 at 17:30
• Ok that sounds good for me. Thank you again! – Polyp Jan 13 at 6:56