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My model aircraft seems to do phugoid oscillations and I'm having trouble landing. It is around 2 kilos, very high mounted wing, so bad landing takes out the underbelly everytime.

I'm wondering what's causing the phugoid, CG placement? The extremely high wing causing pitch oscillations due to inertia? Insufficient tail?

How can I solve this? I cannot change the wing configuration, but I can change CG along the fuselage length / modify tail.

Edit: More detail - design: Side View

Front view

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  • $\begingroup$ When does the oscillation happen? In level flight without control input? $\endgroup$
    – GdD
    Feb 1, 2018 at 13:28
  • $\begingroup$ @GdD yes in level flight. with inputs, it's larger $\endgroup$
    – NRB
    Feb 1, 2018 at 13:29
  • $\begingroup$ Before you do any design changes I'd check your control setup, make sure you aren't getting flutter or play in your control surfaces. $\endgroup$
    – GdD
    Feb 1, 2018 at 13:30
  • $\begingroup$ @GdD yeah I've checked them. They're stiff and stable. $\endgroup$
    – NRB
    Feb 1, 2018 at 13:32
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    $\begingroup$ it's an interesting question but very broad, possibly too broad for this site. Also, there's not enough detail, making any answer speculation, which is also discouraged. Pictures, designs and any other details would help. $\endgroup$
    – GdD
    Feb 1, 2018 at 13:35

3 Answers 3

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Move the center of gravity forward

What you experience looks almost like a phygoid motion (well, most of it is a phygoid), but involves stalling at the low-speed part of the cycle, so it is not the classical Eigenmode.

Rather, your aircraft's trim point is beyond its stall angle of attack. It will, therefore, stall when not actively steered towards a lower angle of attack and then, when lift on the wing diminishes and its center moves backwards while lift on the tail is still linear, the aircraft will pitch down and pick up speed again.

Maybe it will be already enough to change the setting on your elevator to a few degrees more trailing-edge-down deflection. What certainly will help is to move the center of gravity forward because this will increase static stability and require more trailing-edge-up elevator deflection to maintain the same trim point.

From your sketch I would also guess that your tail volume* is on the low end. Move the tail further back by lengthening the fuselage: This will increase stability and, especially, damping, so the pitch motion after stall becomes less violent.

* This is the area of the horizontal tail surface, multiplied with it's lever arm (distance between center of gravity and the tail's quarter chord point).

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  • $\begingroup$ So it all starts with the trim point way beyond stall, huh? So if I move the CG forward (which is the easiest modification for me) It'll increase tail length as well, increasing tail volume. I should see if it's sufficient. $\endgroup$
    – NRB
    Feb 2, 2018 at 0:59
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    $\begingroup$ @NRB: Yes, start with the center of gravity. But you will only need a small shift, so an extra lengthening of the tailboom is still advisable. Also, you might want to look at the trim point of the elevator: Give it a little more trailing-edge down deflection and see how that works. $\endgroup$
    – Peter Kämpf
    Feb 2, 2018 at 11:02
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The phugoid is an interchange between kinetic and potential energy. Anything that dissipates energy in the process will add damping - more parasitic drag from a speed brake for instance when talking about the landing. It's a bit of a dilemma, anything that is done to clean up the aerodynamic shape of the aircraft will decrease phugoid damping.

Control problems related to the phugoid are caused by the natural frequency being too high, this can be lowered by increasing horizontal tail size: a larger tail for a lower frequency, and more time to anticipate on the motion.

Helicopters have the phugoid behaviour exactly like fixed wing aircraft have, and the military have stability requirements for phugoid behaviour: from MIL-H-8501A

$$\begin{array}{|c|c|c|} Period & Visual Flight & Instrument Flight\\ \hline \text{< 5 sec} & \text{½ amplitude in 2 cycles} & \text{½ amplitude in 1 cycle} \\ \hline \text{5 - 10 sec} & \text{at least lightly damped} & \text{½ amplitude in 2 cycles} \\ \hline \text{10 - 20 sec} & \text{not double in 10 sec} & \text{at least lightly damped} \\ \hline \text{> 20 sec} & \text{no requirements} & \text{not double in 10 sec} \\ \hline \end{array}$$

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  • $\begingroup$ Near stall speed, the period is around 5 to 6 seconds. If horizontal tail can affect frequency, what will affect the damping (except parasitic drag)? Because the touchdowns are very hard (lots of residual amplitude). $\endgroup$
    – NRB
    Feb 1, 2018 at 15:06
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    $\begingroup$ The damping is only affected by $C_L/C_D$. A period of 5-6 seconds is too short for comfort - install a draggy large horizontal tail could be an option. $\endgroup$
    – Koyovis
    Feb 1, 2018 at 15:15
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There can be two factors causing the landing mishaps mentioned:

  1. Phugoid: the natural phenomenon of any aircraft.
  2. Pilot Induced Oscillations: the pilot-in-the-loop oscillation of the aircraft while in control of a pilot.

To test the phugoid, trim the aircraft to a calm condition and do not touch the controls for several seconds.

For improving phugoid (long period) dynamics, especially if it is already causing troubles, a pitch-rate-gyro can assist in augmenting the stability. A sample commercial product here.

The PIO is a far more important problem, and it's a combination of the pilot's behavior (his/her control dynamics) coupled with the airplane's dynamics. During final approach and landing, pilots tend to be more stressed, and if the phugoid is not a lucky one, the touchdown may happen at not the best point.

The gyro would also help with the PIO problem, if any.

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  • $\begingroup$ It oscillates without intervention for nearly a minute before it's damped completely. I could add gyro assist, but that'll be if I cannot mechanically solve the issue. $\endgroup$
    – NRB
    Feb 3, 2018 at 7:37
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    $\begingroup$ Ok, I assume it’s not a CG problem of controllability. still, you could try a forward CG configuration to make your touchdown easier. The phugoid is triggered by (sudden) disturbances (external or control commands), so a forward CG might improve a bit. Else, the gyro would be the way to go. $\endgroup$
    – Gürkan Çetin
    Feb 3, 2018 at 8:19

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