Dynamic Soaring: Explaining the Errors of an Innovative Theory

[RossFW]

Hi!

Slightly oddball request.

A nice, but mistaken guy has produced this website-http://www.dynamic-soaring-for-birds.co.uk/

and further written about his ideas here-http://www.bou.org.uk/dynamic-soaring/

He contends that Dynamic Soaring is possible utilising a constant horizontal wind.

This is, of course, a clear violation of C of E and the error is using the Earths surface as an absolute frame of reference.

It is beyond my limited physics skills to clearly explain this.

Could someone with a little more formal training write a brief synopsis of the errors in his logic and, with your pemission, I will direct the Gentleman here?

 

[rcgldr]

Without a wind gradient, then constant wind is the same as no wind to a bird flying in the air, since the air is the medium that the bird is flying in. The only diffference is the ground speed, but that doesn't matter to the bird unless the bird is flying in some type of ground effect where there is a wind gradient.

In the case of "classic" bird type dynamic soaring with wind shear, it's not clear how much addtional lift is provide by updrafts coming from waves in the water.

 

[RossFW]

Yes, agreed totally- but it needs to be put such that the gentleman understands.

He also subscribes to the "Downwind turn" myth, all related to a belief that momentum and KE have absolute values and are measured reference the Earth's surface.

As to Wind Gradient soaring, while I'm sure the extent to which birds utilise it is unknown, it has been used by Model Gliders to achieve quite extrordinary speeds.

 

[rcgldr]

He also subscribes to the "Downwind turn" myth, all related to a belief that momentum and KE have absolute values and are measured reference the Earth's surface.

What if I put a very large treadmill below birds circling in a no wind condition so that the Earth's "surface" moves at any speed I want it to? Assuming the treadmill doesn't affect the wind (no wind gradient), then does the speed of the treadmill have any effect on the circling birds?

As to Wind Gradient soaring, while I'm sure the extent to which birds utilise it is unknown, it has been used by model gliders to achieve quite extrordinary speeds.

The most recent speed record I'm aware of is 468 mph == 749 kph (Feb, 2011). Hard to see in this video until the camera guy takes over and reaches a speed of 405 mph.

http://www.youtube.com/watch?v=rfoxjNg-eg0&hd=1

 

[CWatters]

RossFW.. Have you looked at the issue of what happens when descending through wind shear on landing? For example as a glider descends through wind shear (eg reducing head wind) it should in theory accelerate and maintain a constant air speed. However it takes time and height to get a glider to accelerate. It appears quite possible to descend fast enough that the airspeed falls faster than the aircaft can accelerate. The danger is that the airspeed may fall to below stalling speed. To counter this effect in windy weather glider pilots typically add half the wind speed to their normal approach speed to provide additional safety margin.

 

[RossFW]

RCG,

The big treadmill is a brilliant illustration, thanks!CW,

I'm actually a professional Airline Pilot, so considering the effects of Windshear is kind of my stock-and-trade. Yes, if wind speed changes, either through descending through a vertical shear of flying horizontally through a rapid wind change (micro-bursts associated with Thunderstorms being one example) airspeed will vary.

What WON'T happen (but the Author of the web-site thinks it will) is that there will be any variation to airspeed if you change direction in a constant wind.

 

[CWatters]

I think it depends how you turn. Clearly in a nice slow co-ordinated turn I agree with you. But what happens if you just stand on a rudder pedal?. The aircraft will usually skid to some degree. The airspeed certainly changes during that sort of event and a I don't mean due to increased drag but also because the aircraft isn't moving in the direction it's pointing. It's very obvious in a helicopter where it's possible to have negative airspeed.

 

[RossFW]

I think it depends how you turn. Clearly in a nice slow co-ordinated turn I agree with you. But what happens if you just stand on a rudder pedal?. The aircraft will usually skid to some degree. The airspeed certainly changes during that sort of event and a I don't mean due to increased drag but also because the aircraft isn't moving in the direction it's pointing. It's very obvious in a helicopter where it's possible to have negative airspeed.

yes, but none of that will be influenced by the wind, from the POV of anyone on board the aircraft.