Flying With Zero Ground Speed

[Konrad]

Well, the seasons are changing here in California. Along with the massive temperature drop of 10 degrees or so, there is an increase in wind.  Being as the wind is often very laminar coming off the ocean it offers a unique opportunity. That is if the wind speed is higher than the models stall speed one can fly in very small airspace (box). By the end of winter most of us can fly (hover) doing rolls and loops inside airspace box of 2 to 3 meter.

This style of flying does bring to light that the wind has a very noticeable speed gradient as a function of altitude. Once you are about 3 meters above the ground the wind speed is pretty constant. But as you get below 2 meters the wind velocity drops in a rather predictable profile.

At about 2 meters as the model gets closer to the ground the model will have to start moving forward to a void stalling. That is, what air speed is lost from the slower moving wind needs to be made up with ground speed.

You might have seen this when guys are landing in high winds. The plane looks in control dropping real nice then at about 1 meter above the ground a wing suddenly drops. Many guys claim that the issue is turbulence near the ground when in fact the plane just lost airspeed and tip stalled.

When I practice this “hover style” of flying I like to use a model that has a means to add washout to the wings, be it flaps of upward going ailerons (spoilerons). This adds protection against tip stalling should I mess up my energy management.

All the best,
Konrad

[Jack Plane]

Konrad, it is also feasible that there is a slight shift in wind-direction as much as wind-speed due to the friction-effect of the ground, which could catch the unwary pilot unaware as it combines with the lower wind-speed to allow one wing to stall?

In sailing in the northern hemisphere( the directions are opposite in the southern one), as an onshore wind reaches the land the wind-direction backs (moves anti-clockwise), and as it leaves the shore on the other side - imagine a peninsular - it veers (moves clockwise).  Similarly, the wind-direction at the head of a boat's sail will be slightly veered compared to its direction at the foot, i.e. the surface of the water has an effect on direction as well speed.  This friction-effect should be just the same on land.

A typical RC pilot wouldn't normally be aware of these subtle changes as they come in to land, but flying in a small box you might be more sensitive to it?

[Konrad]

Can't say as I've noticed and Coriolis effect in my flying. I suspect that any anomalies (hills, trees) near by would have a greater effect. But then I only fly with about 3 meter altitude variability. 

I can't say as I, or any of my captains, ever took this effect into account when rigging the sails on my sailboats, 16 meter masts. I will say that the vultures and hawks around here core thermals in random direction.

[Kurt]

Konrad,
a very interesting contribution, the zero ground speed flying. I have observed often the dicussion of the 'ground turbulence", where there is in reality a stall.
I myself have already damaged some planes by this phenomenum, but by the time it became clear that it was'nt turbulence.
I was not active in this forum for some time, because I changed my eye lenses from bio to plastic, which took about half a year of adaptation (focus on the left eye 2m , on the right eye infinite, poor brains!)

Greetings from Rio

Kurt
www.kurt-rio.com

[Konrad]

I too have problems training my brain. I'm try to adapt to progressive lens. I'd hate to have try to learn to use one eye for up close and the other eye for distance work. How has this effected your stereo vision (depth perception)?

All the best,
Konrad

[lincoln]

I think both wind gradient AND turbulence are significant when flying close to the ground.

The wind speed often continues to increase as the altitude goes up, though the gradient is less. At my home field, people have observed that the wind at 500 or 1,000 feet is often much stronger than at, say, 200 or 300 feet.

If you're near the coast, of course, you don't really need a motor at all for zero ground speed. Slope soaring is sufficient.