hi_alpha on....

[mgh]

There was a post on AVSIM about the .cfg file multiplier entries for hi_alpha_on_roll, hi_alpha_on_yaw, and hi_alpha_on_pitch. (The poster found the last in an add-on .cfg file even though it's not documented in the SDKs).

Does anyone know what actual effects they have - which coefficients they apply to, and how is the multiplier bendied in as alpha increases?

 

[Roy Holmes]

SDK, Aircraft configuration files, [flight tuning] has notes on high angle of attack parameters.
air file sections 451,456,460,464 are worth a look. For example the first one is called AIR_ALPHA_ON_CL_BETA in the Prepar3D SDK

Aircraft_sim_tech_Zyskowski.pdf includes a paragraph on the subject.



Roy

 

[mgh]

Thanks.

The ALPHA_ON entries in the .air file are tables of multipliers against alpha.

I assume that the hi_alpha multpliers from the .cfg file aren't also applied to these but don't know which of the other derivati9ves they sdo apply to.

I checked with Zyskowski's paper, bit it's not clear how his modifiers, K, are derived from the hi_alpha values in the .cfg file. I'd guess it probabably varies linearly from 1 at alpha = 0 the to the .cfg file value at some, unknown, value for hi_alpha?

 

[Roy Holmes]

I beleive they alter the relevent coefficient as a function of AOA. For instance 460 varies the value of yaw moment coefficient (Cn) due to sideslip angle due to AOA.

The cfg scalar would increase or decrease that effect. I think it is an attempt to reduce stability at the stall

However I have spent little time looking at the subject

Zyskowski's paper was a slight opening of the kimono, not the full Monte!

Roy

 

[mgh]

I don't know if the .cfg mutiplier applies only to those coefficents that have an ALPHA_ON entry in the air or to all of the relevant ones.

Also, I've no idea what FS regards as "high alpha" or how the multiplier is applied to different types of aircaft. "High alpha" is likely to be very different for, say, a C172 and Concorde?

 

[Roy Holmes]

Since the high AOA tuning scalars apply to roll and yaw stability I'm inclined to think they affect 451 (AIR_ALPHA_ON_CL_BETA ) 456 (AIR_ALPHA_ON_CLP), 460 (AIR_ALPHA_ON_CN_BETA) and 464 (AIR_ALPHA_ON_CNR). These four have AOA input range from -180 to +180 degrees, the other Alpha tables have AOA input generally in the range of -15 to +25.

I compared the 172 and 737 entries for 451. The 172 for which CL max occurs at 15 degrees, has scalar values in 451 of 1.0 at zero AOA, 2 at 15 and 3 above 20 AOA. These would resist roll due to sideslip at high AOA.

On the other hand, the 737 with CL max at 16 and a swept wing, has 451 scalars of 1 at zero AOA, decreasing to 0.8 at 10 and 0.2 from 18 and above. These would make it increasingly likely to have roll due to sideslip at high AOA, like Dutch Roll which is a characteristic of swept winged aircraft.

That is my best guess at the moment

Roy