Fun Flight “Prevailing Wind” Methods

Flying Out Into The Wind

One of my “long distance” Fun Flight methods starts with a daybreak flight into the prevailing wind. I have found that by doing this I am able to sustain minimal headwind resistance. By maintaining a “nap of the earth” flight plan the headwind is of relatively little significance. Testing of this concept has proven that winds of 20 to 25 mph measured even as low as 500 feet AGL (Above Ground Level) are reduced to no more than 5 mph at Ultralight Flight Level 75 (that would be 75′ above the ground). Sometimes there is no measurable headwind at this low altitude AT ALL. This only works early in the morning. I have found that the headwind combines with rising thermals at roughly 10:30 am on, causing the favorable “near earth boundary layer” phenomenon to disappear. Thus these trips become “dawn patrols” in order to take advantage of these elementary physical principles.

Flying Back With The Wind

Then on the return flight, the rising velocity and building thermal activity allows me to “ride the wind” on my return trip.  It’s like saving desert until the end of the meal.

A Practical Aspect of it is that this method also helps to insure against the possibility of fuel starvation on the way home. You know, having made your outbound destination when fighting the light headwind, that the same (re-filled) fuel tank will get you homebound riding a faster tailwind.

Riding The Wind

Now you know why I call myself The Sky Surfer.

Crosswind Journeys

A permutation of this method is the Crosswind Journey. As I watch the wind direction trend during the week preceding the flight, I pick out a destination that will provide a course as close to 90 degrees off the wind. This way I can reduce the effect of the wind in either direction. This Fun Flight Method is more problematical than the

Aerodynamic Observation (The Butterfly Method)

Years ago I was given a “naturalistic observation” that provided some of the basis for my Long Distance Ultralight Flights. I was in my backyard with my dogs. The wind was about 20 miles per hour from the South with gusts to 30. It was windy enough that the birds were sitting it out. Out of the corner of my eye I saw a small, bright, yellow object making its way across the face of my fence. It bobbed up and down and side to side but maintained a constant movement forward into the wind. It was a butterfly! By all rights, it should not have been able to fly in that wind at all, much less make headway against it. At first I assumed it was only able to make forward motion while being partially shielded by the fence. The fence was North/South – in line with the wind – but I decided it was shielding the little creature somehow. Then, amazingly, the butterfly flew above the top of the fence and kept going though an area with many bushes and trees reaching altitudes of 2o to 30 feet until it disappeared behind the trees.

I noticed when it was clear of the fence its “zig-zagging” increased in amplitude but its forward motion continued unabated. I pondered this for some time. Since then I have witnessed other butterfly flight demonstrations of this sort and have drawn some conclusions.

Boundary Layers

In the study of aerodynamics or fluid mechanics is the concept of a boundary layer. In the context of aircraft, this describes the air layer that is “attached” or “viscous” to the object near it – generally a wing, propeller or fuselage. What it tends to do is to “grab on to” and “distort” the on-coming, non-viscous air mass. Most study of this effect regards high airspeed and has been conducted to enhance the ability of high performance aircraft to cut through the air with less drag. I can’t remember seeing anything about boundary layers at low air speeds. But, as I thought about it, it seemed obvious that the butterfly was making use of boundary layers at relatively low air speeds. In windy conditions there is a boundary layer around all types of objects. In the case of the butterfly, his objects of interest are trees, bushes, tall grass, and fences. Because the wind inside the boundary layer is “viscous” and therefore considerably slower than the greater moving air mass around it, the butterfly was able to make headway by flying through the slower moving boundary layers around the trees, bushes and fence. Thus, while he appears to be flying forward against a strong wind, he is actually making decent headway by flying through relatively slow boundary layers attached to these objects.

A Man Has To Know His (Machine’s) Limitations

Harry Callahan wasn’t talking airplanes when he uttered that line but the wisdom is well taken. A lot of pilots would rather think of themselves as a Golden Eagle when flying. That really misses the point for Ultralight Flying. Taking inspiration from the Boundary Layer discussion above, if you are an Ultralight Flyer, it is far more useful to think of yourself as a butterfly. Ultralights have everything in common with them: low wing loading; lots of drag; relatively low power. A few years back, I adjusted my mind to this. After that my Ultralight Flying experience made a quantum leap. It has become far more interesting and challenging. Now it is more like airborne judo or kung fu. Now I don’t take the hit straight on. I allow it to move past me.

Here’s how that plays out. I stay extremely low and when possible fly around, rather than over, hedgerows. It works well and has permitted me much farther range on the limited fuel I can carry. But, if you try this, don’t forget that Butterfly Boundary Layer Flying can only be done if you have good “stick & rudder” chops and maintain constant vigilance lest one of the objects adjacent to the boundary layer reach out and smite thee.