Originally Posted by Flyfalcons

Yes, what I am talking about is crabbing. Flying around with the rudder sticking out because of a nonexistant "weathervaning" effect while in flight is not crabbing.

You are not crabbing, you are slipping the airplane by using rudder this way.

I'm with Ryan on this one, consider that you are in a hot air balloon, a flag always hangs limp from a balloon AKA no wind is felt inside even if your traveling "with" a 25 MPH wind. OK so fly a circle around yourself (being "in" the balloon)... there is no upwind or downwind effects from that "viewpoint", you'll need not bank more on the downwind side nor less on the upwind side to perform a perfect circle "around the balloon". Simply consider that your aircraft is riding an escalator that's moving downwind and you are not! Hope that helped, for slope soaring it's very exaggerated and a eureka moment when you finally get it.

Originally Posted by reyn3545

When you are talking about flying in a coordinated heading, you are referring to flying a heading different than your destination, understanding that the wind will push you to that ultimate destination... (I assume).

In our circle flying r/c world, I think most people view "crabbing" as using the rudder to compensate for the weathervane effect, keeping the flight path parallel to the runway or keeping the path vertical on an upline. This probably isn't a correct use of the term for full scale flight.

That would be slipping, not crabbing. It only matters in reference to the trac across the ground.

I haven't read the whole thread somebody may have answered the question about the sensation of the model weathervaning... but I just wanted to confirm that you DO need to have a greater crab angle to track straight in an upline compared to horizontal flight, because:

In a crosswind, tracking straight & level at airspeed X, you're crabbing @ x1 degrees

In a crosswind, tracking straight in a vertical upline at airspeed Y, you're crabbing @ y1 degrees.

Angle Y is greater than angle X angle due to the decreased airspeed Y compared to airspeed X, because at a slower airspeed you need a greater crab angle to track straight. I'm sure someone has drawn a geometric proof of this.

An added factor may be due to the tendency of the wind to blow somewhat faster at higher altitudes due to orographic drag at lower elevations, thus necessitating an even greater crab angle with altitude. This is of course dependent on local conditions and is the same reason wind turbine operators get more for their money by building higher towers.

Originally Posted by kwilson

I was reading a few posts regarding the tragic accident over on the full scale forum, and the effects of the wind. I had some questions, but thought it would be better to discuss them here. Let me start by saying I only have about 20 hours in a Cherokee and 1 hour in an Extra 300, so I am by no means an expert on full scale flight. I do have several years experience flying r/c though, and have alway heard from experts that the airplane does not know the wind is blowing. This is assuming a steady wind, not gusts.
My question is this. Why does the plane "weather vane" into the wind? For instance, if you pull to a vertical upline in a cross wind, the nose will invariably start to point in the direction the wind is coming from. In fact, I usually need to hold opposite rudder in order to maintain a straight line.

If you fly your IMAC lines at the same speed I dont think vertical would be any different. Most planes now can maintain the same speed vertically and horizontally and usually do. Ive never noticed this affect in full scale of any kind. Maybe we compensate without knowing it. Some models do it worse than others. I suspect it is the shape of the plane and a ratio of tail size and its moment arm. I bet a longer tail length gets pushed more.

Cool, and of course constant speed is best - but my planes definitely don't go as fast vertically as horizontally....

Originally Posted by flyinryans

If you fly your IMAC lines at the same speed I dont think vertical would be any different. Most planes now can maintain the same speed vertically and horizontally and usually do. Ive never noticed this affect in full scale of any kind. Maybe we compensate without knowing it. Some models do it worse than others. I suspect it is the shape of the plane and a ratio of tail size and its moment arm. I bet a longer tail length gets pushed more.

This brings up an interesting phenomenon I noticed a few years back. A friend was practicing up lines and noticed that the aircraft tended to yaw progressively into the wind the further up the line it traveled. At first I thought he must be applying rudder unconsciously so I watched his fingers as he flew the uplines. He actually took his thumb off the left stick after establishing the upline and the aircraft continued to yaw into the wind. This yawing motion tended to keep the aircraft on it's vertical line. At first I thought this must be due to the thrust line of the engine or some other prop related effect. The thing is the wind was directly across the flight line and the model exhibited the same tedancies at both ends of the flight line with the wind off to both the right and left sides respectively. What was causing this? The only thing I could come up with was that the aircraft was climbing through progressively increasing winds as it climbed. We all know that an aircraft in flight does not feel the effects of steady state winds. We may have to crab to hold a navigational course such as maintaining the flight line but the aircraft is traveling straight through the moving air mass. Short term disturbances, turbulence or windshear if you will, can in fact affect the aircraft's attitude and therefore it's flight path. In this case the rapidly increasing wind velocity with altitude was effectively hitting the Vertical stabilizer and rudder causing the weathervaning tendency.

We observe what I assume to be a similar effect during a crosswind takeoff. The longitudinal axis of the aircraft is aligned with the centerline of the runway while the wheels are in contact with the ground creating the friction necessary to maintain runway lineup. As the aircraft leaves the ground it is essentially skidding at the moment of liftoff. The aircraft's natural stability tends to correct this hence the yaw on initial liftoff.

I've had points taken off doing Hammerheads at regionals in a 20 knot gusting 30 direct crosswind for having a "too aggressive" crab on my uplines that got progressively more acute the slower my plane went near the top of the maneuver. Despite this my uplines were straight as an arrow! I was using visual references on the ground and I did not waiver. But, I got knocked down anyway because of their flawed perception. Still took 5th out of 40 pilots, but I could have been third. Oh well, nobody is perfect.