Do aircraft at cruise bob and/or skip? 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 &quot;bob&quot; 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 &quot;bobs&quot; 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 &quot;swoop&quot;. Each &quot;swoop&quot; starts with a stall where it'll &quot;flip over&quot; and dive down and gaining speed as it does so, the gain in speed results in the nose &quot;picking up&quot; resulting in a &quot;swoop&quot; back into a near vertical &quot;climb&quot;, 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 &quot;bobbing&quot; 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 &quot;bobbed&quot; back up to their pre-decent altitude?)
In the case of &quot;skipping&quot; 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?
Note:
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.
Note:
If it's relevant motorized foam is shaped like a flying wing; in case aircraft with tails behave differently.
Note:
I've certainly observed that the wing tends to &quot;pitch up&quot; in proportion to its speed above a certain speed; this is most noticeable in the cycloidal &quot;swooping&quot;. 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


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)
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
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