Al,

Gee, I thought that was clear as mud, but then again it always sounds perfectly clear to the guy who is writing it! Let me see if I can muck it up a bit more. Seriously, let me take one more shot at this.

Groundspeed is airspeed plus or minus the wind. Airspeed is the actual speed of the relative wind over the airplane. 40kts airspeed with a 20kt headwind equals a 20kt groundspeed. 40kts airspeed with a 20kt tailwind equals a 60kt groundspeed. In both cases, your airspeed is still 40kts.

It's all very easy to understand as long as you are dealing with a steady state wind. When the wind gets gusty or rapidly changes direction, the dynamics change. In that case, indicated airspeed can fluctuate greatly with no change in power.

The first issue deals with aircraft momentum or lack thereof. It you have 40kts of airspeed in 20kts of steady state headwind and the headwind suddenly stops, what happens? Is your airspeed still 40kts? No! Believe it or not it's 20kts! This confusing little issue has been the cause of a number of windshear crashes before the phenomenon was understood. Let's say an airliner is on final at 130kts and flies into a downburst. These typically hit the ground and fan out, so if you fly into one you get hit with an instantaneous headwind, followed by a horrific downdraft, and then a huge tailwind. So let's say an airliner is initially hit with an instantaneous headwind of 50kts. The pilot will see an instantaneous increase of 50kts on his airspeed indicator. He pulls back the power to maintain approach speed. Next, he hits the core of the downburst then rapidly enters a 50kt tailwind, which results in an instantaneous decrease in indicated airspeed of 50kts. So there's the airliner, thrust pulled way back to maintain 130kts, while his groundspeed is actually 80 knots and he's getting pushed into the ground by a downdraft. Next, he gets hit with an instantaneous decrease in indicated airspeed of 50kts. In that scenario there is no chance of recovery. A crash is inevitable. That's why airliners use wind additives on their approach speed, so the airplane will have enough momentum to deal with the airspeed fluctuations. Nowadays, of course, thunderstorms and downbursts are avoided, and it is a typical policy to abort the approach if airspeed fluctuates by more than plus or minus 15kts below 1,000 feet.


The second problem deals specifically with model airplanes. How do we judge airspeed? By eye. By observing how fast the airplane is moving. But relative to what? The ground! We fly by groundspeed because that is all we have to work with. So we fly by "perceived" airspeed. Here is were the problem lies, and as Erik has said, it has claimed many a model airplane! Let's say our model stalls at 30kts and we are flying into a 20kt headwind. A normal approach speed would be stall speed times 1.3, or about 39kts. We fly our model at what LOOKS LIKE 39kts to us. Now lets say we suddenly lose that 20kt headwind, our airspeed will instantaneously drop to 19kts! So based on the concept of "perceived airspeed" we kind of have to make a little adjustment in our heads. If we adjust our throttle setting to maintain a constants perceived airspeed in gusty situations, we are actually adjusting our actual airspeed without knowing it! This problem manifests itself in the worse way under three situations; the turn to downwind, the turn to final, and final approach under gusty wind conditions.

Granted, this is all a bit much for an RC flyer to keep in his head. The bottom line is to remember we fly model airplanes based on perceived airspeed, not actual indicated airspeed. The actual airspeed of the airplane may be faster or slower than it appears. Be cautious going from headwinds to tailwinds. Keep in mind the airplane cannot make instantaneous changes in actual airspeed to account for sudden fluctuations in wind.