Recently I started flying a model aircraft, admittedly a kindness for a motor stuck onto a piece of foam, and have noticed that it either bobs, for lack of a better term, like something afloat on water might when depressed or that it skips, for lack of a better term, like a stone thrown across the surface of a pond might.

In the first case the motorized foam is at cruise at constant speed at some height when I force it into a dive it seems to "bob" back up to it's original height when I stop giving it the input to dive. I suspect I'm observing a little Newtonian mechanics here trading potential energy for kinetic energy during the dive and that this gets converted back as it "bobs" up again.

In the second case when the motorized foam is at cruise at constant speed at some height it tends to undulate. This is exacerbated if I increase the angle of attack(?) such that in the extreme, where it stalls, the motorized foam goes into a cycloidal "swoop". Each "swoop" starts with a stall where it'll "flip over" and dive down and gaining speed as it does so, the gain in speed results in the nose "picking up" resulting in a "swoop" back into a near vertical "climb", at roughly the same height as when it first stalled it'll stall again and repeat the process.

Do larger scale aircraft, that is anything with at least a human on board, behave in this manner as well? In the case of "bobbing" do pilots use this in any way to their advantage e.g. does it help them maintain altitude or is it helpful in some way during dire situations? (My imagination can't help but wonder if there was ever a pilot who forgot to throttle down on decent, intending to land, who found they simply "bobbed" back up to their pre-decent altitude?)

In the case of "skipping" I can't imagine passengers being very agreeable to such a motion, especially not the stalling swoop variant, but I'm fairly certain I've felt a plane undulating slightly on a flight before. Do pilots have to constantly fight such undulations or do they reduces/eliminate this through proper trim?


I'm not talking about wing in ground effect, I observe this when the motorized foam is at roughly tree top height. The veldt I fly in comprises of knee high grass so I doubt I'll observe this until I fly over a lawn or something.


If it's relevant motorized foam is shaped like a flying wing; in case aircraft with tails behave differently.


I've certainly observed that the wing tends to "pitch up" in proportion to its speed above a certain speed; this is most noticeable in the cycloidal "swooping". Initially I thought this was a CG thing or that the motor was off center and generated a moment force but I see this is already discussed. It's more likely my trim is off.




Update :

I phugoid it out, I believe, stuck a coin under the battery (rotating it made it poke out like an Adam’s apple) and I trimmed the trim to something slim. Did the glide test and she stays level as a lake wafting gracefully from the skies like Dumbo with his trunk out, tail tucked and his C.G. Is under his chin ! Thanks all for the helpful hints and the epoxy related non-confessions. (When the coin is worth more then the model I’ll fork out for an upgrade. Till then I’ll wait for the back of this cold front)

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    $\begingroup$ +1 for "a model aircraft, admittedly a kindness for a motor stuck onto a piece of foam" and "the motorized foam"! Nice! :) $\endgroup$
    – FreeMan
    Jul 8, 2020 at 18:06
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    $\begingroup$ "in case aircraft with tails behave differently." They do. We put the tails there for a reason. :) In particular, they're quite useful for stability. $\endgroup$
    – reirab
    Jul 8, 2020 at 18:45
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    $\begingroup$ "...rivers of epoxy..." - when I was learning to fly R/C airplanes many years ago the standard beginners aircraft at our club was a foam model of a Piper Cub because it was A) easy for a beginner to put together, and B) could be repaired on the flight line by the appropriate application of five-minute epoxy. So a beginner could take his plane up (usually on a buddy cord with a more experienced pilot), fly for a while, crash his plane, gather up the pieces, smoke a cigarette/calm down, fix his plane, and be ready for another flight within 15 minutes or so. Don't ask me how I know... :-) $\endgroup$ Jul 9, 2020 at 12:57
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    $\begingroup$ The air mechanics "epoxy tears" are super handy, not that you'd know of course, given that the odd passer/family friend by who unwittingly shows even the slightest interest is given the controls and a crash course. Teenagers seem to hit the dirt uttering "... but I play video games !". The septuagenarians try maintain an air of decorum and a facade of nonchalance towards "toys" as they insist on another turn and laugh manically when they collide with the trees. Women, curiously, feign a timidness on the first throw then calmly and determinedly get airborne before any "man" can. $\endgroup$
    – Carel
    Jul 9, 2020 at 13:41
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    $\begingroup$ @FreeMan the duct tape "trim tab" is more to stop the white duct tape "wing tip guards" stay in place, similarly the duct tape "winglet guards" hold the duct tape "skids" in place during brick pavement and dirt road landings and power line "touch 'n go's". Thanks though :D I'll keep that in mind while getting thrifty with the pennies.The sticky faces of most such modifications are now all coated in a thin veneer of and mother natures' "dandruff" and in need of replacement. $\endgroup$
    – Carel
    Jul 9, 2020 at 13:59

3 Answers 3


It sounds very much what you're experiencing is a Phugoid cycle

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To put simply, your aircraft is not trimmed correctly. As it descends slightly it gains speed, which increases lift over the wing making it climb. This has the effect of it losing some speed, reducing the lift over the wing causing a descent, increasing the speed - and on it goes.

Full scale aircraft from light to airliner can and do experience the same thing. One notable case which comes to mind is the accident sequence of United Airlines 232 which lost all hydraulic pressure and thus had no control surfaces operational. They were controlling pitch (and roll) somewhat using engine power only, and thus did not have exact control of pitch and experienced significant Phugoid cycles.

Side note: You're unlikely to be experiencing ground effect at treetop height unless you have very tiny trees or very large wings.

When an aircraft flies at a ground level approximately at or below the half length of the aircraft's wingspan there occurs, depending on airfoil and aircraft design, an often noticeable ground effect source

  • $\begingroup$ Does this apply to both the situations? $\endgroup$ Jul 8, 2020 at 12:05
  • $\begingroup$ @Jameic that is exactly what I am observing, thank you. I'm certainly to high for Wing in Ground effect I've read that only occurs in roughly 2.3 chord lengths. My longest chord length is perhaps 15 cm the grass in the field is knee height so there is no change of observing that. When I'm more adapt at controlling the thing I'll try a smoother terrain and test WIG out. Is there a difference betwee dead stick oscillation and phugoid cycles or are they the same thing ? $\endgroup$
    – Carel
    Jul 8, 2020 at 12:29
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    $\begingroup$ As a model airplane pilot I can confirm that this behavior sounds very much like an out of trim model. It sounds to me like it's a little too nose heavy with a lot of up trim. This isn't necessarily a problem if you are new and appreciate the corrective nature of a plane setup this way, but trying to move the center of gravity a bit toward the tail could help tame these tendencies. $\endgroup$ Jul 8, 2020 at 18:01
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    $\begingroup$ "You're unlikely to be experiencing ground effect at treetop height unless you have very tiny trees or very large wings." file under Classic :) $\endgroup$
    – Fattie
    Jul 9, 2020 at 12:20
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    $\begingroup$ It's worth noting that due to us living in a three dimensional universe, the physical laws do not scale with linear dimension and so the flight dynamics of a drone or model aircraft will still be wildly different than a real aircraft. In a model plane you might wobble through 2-3 phugoid cycles in a second or two. In a full sized airliner this will happen on the scale of minutes - it's not nearly as dramatic. $\endgroup$
    – J...
    Jul 10, 2020 at 11:15

If there ever was a pilot who forgot to throttle down on descent

Full scale pilots trim their elevator for speed, and use their throttle to climb or descend.

"Bobbing or skipping" behavior in cruise is a sign of instability, not surprising for a tailless piece of foam. If this plane has a published CG location, thrust angle and control surface throws, start there.

If you have a hill nearby, power off glides will help determine if your CG is OK.

If you are nose heavy, the plane will dive to a higher than desired glide speed and require a lot of up trim, but should not constantly pitch up and stall. Moving weight forward generally improves directional stability.

If you are tail heavy the plane will be more difficult to control and tend to pitch up sharply with speed, sort of what you are seeing.

So work with weight first, in small increments. You want a slight tendency to pitch up with increased speed in glide so it can self control its speed. After you get it gliding stably at a speed comfortably above stall...

Make sure your control throws are not too much or too little, then ...

For powered flight, you want your thrust angle slightly down to help counter act the pitch up tendency at higher speeds. Just a few degrees, again adjusting at small increments. Models tend to be overpowered, this may take a few tries. The plane should track level or climb a bit when throttle is applied, without excessive trim required (some is normal).

Planes with tails generally are more stable in pitch, but even these will "bob" if they are not set up properly.

  • $\begingroup$ At FL-Trees, no power, and a "flat" angle of attack it glides quite stably without any input, picking the nose up makes it "waddle" as it descends "resting" on the flaps rather then the wing. Under power I've seen it has two "stable" modes, the first is a bit of nose up and enough power not to stall, if I can get it "flat" under the same power it starts "ascending" (Which is weirdly pleasing). The undulation occurs with more power. The motor seem just "big enough" taking around three circuits to reach FL-Trees. Thanks for the guidelines though I'll try them out on Friday/Saturday :D. $\endgroup$
    – Carel
    Jul 8, 2020 at 23:32
  • $\begingroup$ You have a rear motor mount. Pushing up in the back makes the nose go down. But before changing that I would move the CG up a bit. Check your elevons. If they are pitching the nose down (with aft CG) it will fly, not as easily. As you speed up the elevons force the nose down as well, which may be starting your "phugoid" oscillation. Move the weight up and DO THE GLIDE TEST. You want the weight forward and the elevons slightly up so it is more stable. Then try powered flight. It might be better. Good luck! $\endgroup$ Jul 9, 2020 at 0:55
  • $\begingroup$ Thanks for all of the help and pointers @Robert DiGiovanni, I included a full image of the model now as well. $\endgroup$
    – Carel
    Jul 9, 2020 at 12:37
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    $\begingroup$ @Carel "FL-Trees". What the heck do trees in Florida have to do with anything?? Coffee starts to kick in Oh... wait... that's what you mean! You get a definite +1 from me for creative writing. Well done!! $\endgroup$
    – FreeMan
    Jul 9, 2020 at 12:49
  • $\begingroup$ @Carmel thanks for the picture. Your elevons seem a bit up, nose weight and a little less up might be better. Establish your glide speed, then try power (a little downthrust may help). A "skippy" weight back plane will be a nightmare to land, anyways. Maybe get another one, and totally Frankenstein the one you have now, to find what's best. It will be worth the effort. $\endgroup$ Jul 9, 2020 at 14:34

In my experience with model airplanes the bobbing and dipping motion occurs when the center of lift is forward of the center of mass and represents a series of mini-stalls and recoveries that persist and repeat.

Since simple model planes generally do not have elevator trim, the solution is to move the wing attach point aft in small increments until the plane does not "do the dip". Back in the day, all my free-flight rubber-powered kit planes were set up to make this pitch trim adjustment easy.

If you go too far and move the center of lift too far behind the center of mass, then the plane immediately plunges into a nosedive.

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    $\begingroup$ Or add pennies to the nose... $\endgroup$ Jul 8, 2020 at 20:08
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    $\begingroup$ yes yes, like taping nickels to the end of a skipping tone arm! $\endgroup$ Jul 8, 2020 at 22:04
  • $\begingroup$ Moving batteries around a little could be a better use of mass. Even flipping a battery end for end could be enough change. $\endgroup$
    – Criggie
    Jul 8, 2020 at 22:24
  • $\begingroup$ The model is of a flying wing so there isn't really a wing attachment point but I'll rotate the battery and get thrifty with the duct tape and pennies. It seems stable when doing a speed of "No thank you gravity" but anything above "Are we there yet ?" results in the skipping; initially I assumed this was a mutually empathetic exuberance for being airborne. Also I'll have to wait until the cold front abates this evening saw me in two trees and a fence. $\endgroup$
    – Carel
    Jul 8, 2020 at 23:03

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