CFI myths..Special VFR day/night et al

[bigD]

Oh, and I'll also eat a dozen cake donuts. Although those probably won't taste any better than my DC's!

 

[VNugget]

The airplane, while in a fluid medium, simply does not move with the wind faster than it can turn.

...
Such an airplane moves with the wind, whereas this may not be the case for an airplane which is turned more rapidly than it accelerates with the wind.


Twice now you've mentioned this acceleration thing, without explaning it. Can you please explain it? I'm sure you're not talking about the net acceleration of the aircraft through a hypothetical 180 upwind-to-downwind turn, as it would be the same regardless of the wind strength and I don't have to show you why.

do you think you could demonstrate this downwind turn airspeed loss and upwind turn airspeed gain using a more pedestrian airplane

I'm sure just setting up a steep turn with a constant bank and altitude would eliminate all other factors and do the trick, no? If Avbug is right, the airspeed would oscillate with a period of 360 degrees, and if you steepen up the bank enough, you would get a stall buffet on one side of the circle.

An example would be an airplane on a stick. Point it into the wind, it "feels" or indicates the airspeed of the wind. Turn it downwind, and it experiences just the opposite. This isn't a perfect example, of course, as the airplane isn't stationary in the turn. It is moving in a fluid airmass. However, if you can turn that airplane downwind faster than it moves with the air mass (and it can be done), the result is a perceptable and often signficant airspeed loss.


What are you comparing with the term "faster?" Turn rate is measured in degrees per second, while wind is obviously distance per time. Can you please clarify?

 

[100LL... Again!]

Avbug-

An airplane IS a part of the atmosphere in that it is part of that frame of reference. I am not stupid enough to think that it is ACTUALLY homogenous with the atmosphere.

Your example of a model airplane on a stick is not valid, because once again, you are now referencing yourself to the ground, which becomes your inertial frame, and yes, the airplane will feel wind.

You cannot say 'forget the ground, forget groundspeed" and then talk about wind.

Wind is defined as airflow relative to the ground, right?

If you cannot reference the ground then there IS no wind.

I am absolutely certain there is some effect you are observing, and I have no doubt the airspeed variances are observable, but I don't accept the argument that one can always seperate illusion from fact, no disrespect intended. That's why illusions are powerful.

A steady-state wind that does have layers of varying speed CANNOT produce this effect.

Do you see why the 'airplance on a stick' analogy is not usable?

By the way, what is the proof that the airplane can "turn that airplane downwind faster than it moves with the air mass" ?

The airplane cannot at all accomplish this. The change of direction is limited by the aerodynamic force the wings can produce during the turn, which does not very. The idea that this can be done violates the law of consevation of energy.


Suppose you are jogging on a very wide treadmill with plenty of room to run around. Are you saying that you could 'turn downwind' on the treadmill at a faster rate than you could turn in still air? Your ability to accelerate 'downwind' is no greater than your ability to accelerate with the treadmill not moving.

When experience runs contrary to physics, it means that there are additional factors not being measured.

 

[avbug]

I'm done screwing around with this thread. Stupid inexperienced me in the cockpit has just been deceived all this time. Dang.

Perhaps I'll start reefing it downwind and ignore the loss. After all, theoretically it can't hurt me.

Not.

 

[100LL... Again!]

I never said you were inexperienced, stupid, or that the effect you obviously observed was not real.

Just having what I thought was a clarifying discussion.

 

[varicam]

This argument is pretty easy to settle: Fly a jet or turboprop to an altitude that has decent winds, maybe 80-100 kts just to get the full effect. Fly a series of 360 degree steep turns at a constant altitude, leaving the power setting alone after finding the one that will hold the desired airspeed. Note whether or not the airspeed varies when turning downwind and up wind from crosswind.

Been there, done that. Anyone who has taken a flight test has demonstrated steep turns with wind present. Have you ever found it necessary to add power to maintain airspeed on the downwind portion of the turn?

 

[avbug]

Okay, I was done...but no. That has been discussed, and doesn't have a bearing on the "downwind turn." Load that airplane heavily and reef it hard downwind. Not a constant turn, but one hard 90 degree turn downwind. Rather than drifting with the wind (as it is wont to do), the turn may be effected such that an airspeed loss occurs which would not otherwise occur in still air. Likewise, the same turn into the wind will effect an airspeed increase.

Somewhat a self-induced shear, rather like flying into a microburst (but different... )

 

[Timebuilder]

By the way, what is the proof that the airplane can "turn that airplane downwind faster than it moves with the air mass" ?

The proof is the loss of airspeed.

You don't need to have ground as a part of the picture, or frame if you like, in order to have a wind. All you need is the airplane, and you have relative wind. Wind has very little mass or inertia, and the airplane has a great deal of both, by comparison.

Now add to the relative wind a movement of the airmass compared to a fixed point in space. It doesn't have to be the ground, just a fixed point. Now, heading into that wind gives you an increase of relative wind, and indicated airspeed, since we measure airspeed from a probe that points forward. If you change directions slowly, the wind can continue to effect the aircraft through the turn as the airplane decellerates toward the fixed point, and accelerates away from the fixed point, describing a hyperbolic function. The mass of moving air has sufficient time to act upon the heavy mass of the aircraft, overcoming its inertia to return to a state of equilibrium. If the direction of the aircraft is changed too quickly, then there will be a lag time before that stablized state is regained as the air acts upon the aircraft, overcoming the inertia from one vector and begining to add energy to the new vector.

The only reason this effect exists at all is because an airplane is designed to fly in one direction at a time, and turns are a compromise to that design. If planes flew in any direction at any moment, measuring airspeed in whatever direction the plane was moving, there would be no difference in airspeed, and no measurable loss.

 

[bigD]

The proof is the loss of airspeed.

You don't need to have ground as a part of the picture, or frame if you like, in order to have a wind. All you need is the airplane, and you have relative wind. Wind has very little mass or inertia, and the airplane has a great deal of both, by comparison.

Now add to the relative wind a movement of the airmass compared to a fixed point in space. It doesn't have to be the ground, just a fixed point. Now, heading into that wind gives you an increase of relative wind, and indicated airspeed, since we measure airspeed from a probe that points forward. If you change directions slowly, the wind can continue to effect the aircraft through the turn as the airplane decellerates toward the fixed point, and accelerates away from the fixed point, describing a hyperbolic function. The mass of moving air has sufficient time to act upon the heavy mass of the aircraft, overcoming its inertia to return to a state of equilibrium. If the direction of the aircraft is changed too quickly, then there will be a lag time before that stablized state is regained as the air acts upon the aircraft, overcoming the inertia from one vector and begining to add energy to the new vector.

Timebuilder - with some of the words you're throwing around, you're beginning to sound like a physicist! There's only one small problem - no physicist in the world agrees with what you just wrote. Do some vector math and it'll become pretty obvious. Don't just think about it - sit down and do some calculations. You'll see!

Like I've said before, I don't doubt what Avbug has been seeing, but one thing is for sure, it's not for the reasons listed above.

 

[Vector4fun]

I had thought this thread would have died by now. The ONLY "downwind turn" effect on an aircraft in normal flight is a possible wind gradient as it gains altitude. This is a real phenomenom an might have a noticeable effect on a light aircraft climbing off the runway.

ANY AIRCRAFT is going to suffer an airspeed loss in a tight turn, even a freaking F-16 at constant throttle. Jesus, this isn't rocket science or even high school physics. I have personally witnessed bleeding pilots climbing out of the smoking wreckage claiming "downwind turn". BS. It was caused cranking an underpowered, high-drag airframe around in an estimated 40 deg bank at 100' altitude and not near enough speed. That's NOT downwind turn effect, that's INDUCED DRAG.

Try this test. Put yourself under a hood, (with safety pilot), climb up to 3000 or 4000 AGL in a 40 mph breeze but smooth air, and then do level, timed turns without reference to a compass, but only to the turn coordinator. I defy anyone to discern a difference in airspeed loss based on direction of turn. The only differrence noted will be a greater loss of speed with a higher bank/ load factor.