Very simple question here. I know what Laplace transforms are, but except for solving ODE's I haven't really used them in a real project or thought about it in a very pragmatic way.

I am currently reading "Small Unmanned Aircraft : Theory and Practice" from Randal W. Beard, Timothy W. McLain and am a bit lost on how exactly I should incorporate/use a specific equation.

My question is about this equation that relates the aileron deflection to the fixed wing UAV's roll angle: enter image description here


enter image description here

It seems like the aileron's deflection angle $\delta$ is a function of s, ie $\delta(s)$.

So my first question is: suppose you want to imply the aileron rotated 45degrees how does that translate to $\delta(s)$?

Secondly, I imagine I should let s vary from 0 to infinity, that will give me a graphical representation in the s domain, but that I think won't tell my how many degrees my roll angle is relative to some axis. Or am I missing something?


Another link to the book, with mirrors (libgen.io, libgen.pw, bookfi.net, bookzz.org) to its content depending on your location: http://libgen.io/book/index.php?md5=7314182B194BAB33173B521614B42663

I am trying to achieve latteral tracking and/or terminal control on a fixed wing UAV for autodidactical purposes.

  • 1
    $\begingroup$ I could not open the book you reference from your link and therefore cannot see the context of the equation. $\endgroup$
    – Koyovis
    Jan 1, 2018 at 17:12
  • $\begingroup$ @Koyovis added a link at the bottom of my post $\endgroup$ Jan 2, 2018 at 5:44

1 Answer 1


It looks like a transfer function for a feedback loop, which is a function of time and transformed into the frequency domain. Input is the aileron deflection $\delta(s)$, output is the roll angle $\phi(s)$. When deflecting the ailerons, the aircraft is accelerated around the X-axis. The roll movement creates a damping force proportional to span $b$, which quickly dampens out the roll acceleration into a steady roll rate.

So achieving a roll angle depends on the moment of inertia around the X-axis, and wing area geometrical data, plus the time that the aileron is deflected at a certain angle. In order to get to a specific bank angle, the aileron needs to be deflected, then returned to zero before the bank angle is achieved - damping and inertia will then continue to roll the aircraft until the target bank angle is achieved.

The transfer functions defined in the frequency domain are used for determining gain and phase shift, more info for instance here, section 6.3 onwards. Using a signal in the frequency domain gets to be a pain once the deflection is a real world time history and not a mathematical function like a sine wave or a pure step input.

enter image description hereImage source

Once the gains are determined with the Bode plots, the block diagram of the transfer function is pretty useful for determining bank angle as a function of the time history of aileron deflection: just read the actual bank angle, subtract from the desired bank angle, and set the aileron deflection angle in real time according to the difference signal.


Have managed to have a look into the book. I'm familiar with the equations, describing stability and control of aircraft. Your question on how to define an input signal in the frequency domain is a pertinent one: it would have to be defined mathematically in the frequency domain as well. I have only seen use of the frequency domain equations in off-line analysis, not in real-time use since the inputs are never purely mathematically defined. Equations such as 5.53 for the time domain. Then any input signal can be added in real time, multiplied with the gains found from the Laplace transform, integrated and output.

So my first question is: suppose you want to imply the aileron rotated 45degrees how does that translate to δ(s)?

It depends how fast you bring the ailerons to 45º, for how long you keep them there, and what happens next. If it is a step response, the function is $\frac{0.785}{S}$. The aircraft will respond with a steady roll rate (in aeroplane axes) and will never establish a constant bank angle.

  • $\begingroup$ You probably couldn't open the links due to your location. Hadn't thought about that :), I added a link to a webpage with multiple mirrors in order to have access to the book's content. $\endgroup$ Jan 2, 2018 at 5:43
  • $\begingroup$ If I'm not mistaken according to the book this is an open loop control system (block diagram: 5.2 page 69). You say TFs are used for determining gain and phase shift, that's indeed what I learned while being at uni. But how is this going to be useful here? How do you suggest me to proceed with this equation (or another one in this book) to know how much I need to deflect my ailerons to get a certain roll angle? $\endgroup$ Jan 2, 2018 at 5:52
  • $\begingroup$ How much and how long for, given the dynamics of the aircraft and of the aileron deflection. The new link shows the book behind a pay wall, sorry still cannot see the block diagrams and the context. I've never used the frequency domain equations directly in real-time code, only equations in the time domain. $\endgroup$
    – Koyovis
    Jan 2, 2018 at 6:15
  • $\begingroup$ I can try to share the book this way: dropbox.com/s/ueiuu7ulhn2h7ti/… I personnaly think it is a very comprehensive book. $\endgroup$ Jan 2, 2018 at 6:26

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