Is airspeed constant or cyclical when flying model aircraft in windy conditions? I need help to settle an argument about aeronautics. Particularly model aircraft. 
It has been observed by some that when a model airplane flying with the wind turns back into the wind , some aircraft tend to pitch up and gain altitude indicating an increase in airspeed. Alternately, when flying into the wind, after turning around tend to lose altitude indicating a lower airspeed.
Most model pilots say that this is an “illusion” or “pilot error” and that given no change in throttle or thrust the airspeed remains constant. I disagree! It is my theory that in the first case forward momentum is carried through the turn causing a momentary increase in airspeed and when the model turns to fly with the wind it takes a while for the model, now with a slower ground speed to get up to optimum air speed. Thus airspeed is not constant but cyclical. 
I realize that there are many factors involved and that some of my detractors base their belief on their training in full scale aircraft. Model aircraft tend to be smaller, have lower glide ratios and are often based on high performance aircraft designs as opposed to your typical civilian aircraft.  Flying style is also quite different with faster scale speeds and sharper turns for the models. 
Am I right? Wrong?
How do Newton's laws of motion relate to this? 
 A: I don't think you're right, because wind is simply the motion of the air mass over the ground. Since any aircraft, even a paper airplane, is entirely supported by the air, it simply doesn't "know" what the wind is. To illustrate the point, if you are at a lattitude of about 40 degrees north or south, you are actually traveling toward the east at roughly 1,200 km/hr, and not only that, you are traveling at roughly 60,000 km/hr in orbit around the sun, but you are not aware of it. The model airplane has no way to be aware of the relative motion between the air and the ground.
Turning upwind or downwind does not change the airspeed, because when the plane is in the air, it only sees the air mass itself, not the fact that the air mass is moving with respect to the ground (which is what wind is). You would see the same behavior flying a race-track pattern even if there were no wind.
(However, if there are obstacles around, like trees or buildings, such that the wind speed is different from place to place, then the airplane might well notice a difference if it goes from one place to another.)
What you will see is something every pilot learns - that when you go into a turn you descend unless you compensate by pulling back on the elevator and/or applying power, because the lift vector is no longer vertical. You get lateral acceleration to make the turn by giving up some vertical acceleration to match gravity.
Coming out of a turn, the opposite happens - if you don't relax the back pressure or throttle you applied during the turn, you find yourself rising.
Maintaining constant altitude going into and coming out of turns is something you learn with practice.
(When I first did that, my instructor said "You just blew your checkride".)
A: I'm afraid the airspeed can change as the model turns downwind. You are forgetting inertia. Irrespective of its airspeed, a model flying into the wind will lose some of that airspeed as it turns down wind. Lets look at a turn of 180 degrees. If the turn is VERY gentle, there will be no noticeable difference, but if the turn is more rapid, the aircraft will have to maintain an airspeed (x) from (x -groundspeed) to (x + groundspeed) in just a few seconds. Inertia is not sensitive to airspeed, only to groundspeed. The heavier the aircraft (and the sharper the turn) the more intertia will affect it.
If you want an example, dynamic soaring (see albatrosses and petrels) is heavily dependent on inertia. That is why these birds are big and have a high wing loading. Except that, instead of losing/gaining airspeed through the windshear, they maintain airspeed and convert it into height (and vice versa). Little, lightweight birds cannot do this.
Finally, full-size pilots are taught that in the event of engine failure during take-off NEVER TURN BACK to the airfield. Fly straight ahead and pancake into a ploughed field if you must. If you turn back, you will lose airspeed (which is already struggling at take-off minimum) and you will probably spin. Look on YouTube for several examples...
