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I am working on a Pitch Hold Auto-pilot for F-104, which has the following damping characteristics; Longitudinal Characteristics of F-104

which clearly shows that it has degenerated and unstable phugoid mode. well, I made a SAS which consists of 2 loops / Feedbacks. 1) The angle of attack feedback to increase the Cm alpha or static margin of the aircraft 2 Pitch rate feedback to dampen the short period mode. but it has eventually increased the natural frequency to almost 365 rad/s as shown below, SAS damping characteristics

but this frequency is not according to the handling qualities as it should be less than 3 rad /s. what feedbacks should I try so that all the states are stable as well as desired handling qualities can be achieved? states are velocity, angle of attack, pitch, pitch rate. Simulink Model of SAS: SAS Simulink Model

Root Locus of the outer Loop: Root Locus of the Pith rate loop

How can i decrease the frequency to the desired range keeping it stable and keeping in mind that it was unstable first. Also, with not much decrease in damping as it should be greater than 0.5.(short period mode) Phugoid mode >0.04 state space model

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  • $\begingroup$ Seems like your alpha feedback is too large? $\endgroup$ – JZYL Apr 14 '20 at 17:05
  • $\begingroup$ then what would be the best way to compensate it? $\endgroup$ – Adeel Ahsan Apr 15 '20 at 11:14
  • $\begingroup$ Can you provide the full state space matrices you're using? $\endgroup$ – JZYL Apr 15 '20 at 11:27
  • $\begingroup$ I have provided please check as i redited it. $\endgroup$ – Adeel Ahsan Apr 15 '20 at 17:41
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You seem to have too high an alpha feedback. For intuitive understanding, alpha feedback mainly increases the pitch stiffness, analogous to increasing the static margin; pitch rate feedback mainly increases the pitch damping. For your purpose, you would need to significantly reduce the alpha feedback.

For example, with your provided state space model,

$$\dot{x}=Ax+Bu$$

using a linear feedback,

$$u=Kx$$

where

$$K=\begin{bmatrix}0 & 0.5 & 0.25 & 0 \end{bmatrix}$$

satisfies your damping ratio requirements. SP frequency is 3.9 rad/s, SP damping is 0.53, Phugoid frequency is 0.023 rad/s, Phugoid damping is 0.14.

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  • $\begingroup$ yes it Worked but the frequency is little bit higher $\endgroup$ – Adeel Ahsan Apr 16 '20 at 8:06
  • $\begingroup$ @Adeel But it's much better than your 364rad/s. For high HQR, you typically want higher SP frequency. That being said, you can keep lowering the alpha feedback and increasing the pitch damping if you want to make further adjustment. $\endgroup$ – JZYL Apr 16 '20 at 11:09

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