CFI myths..Special VFR day/night et al

[SPBRIAN]

I enjoy science to make sense of things, and seeing a performance increase because of the wind doesn't make sense. But I will admit that there are things that science will not explain, and I have limited ammounts of flight time to have seen thise phenomenon.

Bugchaser I think that the reason that you notice the difference is quiet possibly the fact that you are trying to set up runs across the ground, and paying attention to the ground when turning with the wind of course the plane will not "perform" as well because of the increased groundspeed, so you will appear to be moving much faster while climbing at the same rate?

And on the phenomenon side, at our local airport is a fairly well known aerobatic pilot with many many many hours of low level airshow flying. When they are practicing he will swear up and down that when doing hard pulls to vertical he will get a few hundred more feet when he does the pull while going downwind. I should go bug him for an explanation... Time to put on the investigator hat...

-Brian

Glad to see ya AVBUG, hope the season is going well and everybody is staying safe.

 

[CLCAP]

Mountain wave:

Rising air - airplane tries to maintain altitude - speed increeses - Falling air - airplane tries to maintain altitude - speed decreases.

 

[VNugget]

I have heard the statement many times "Wind direction and/or speed has no effect on IAS". True. But, the rate of direction change or speed change does affect IAS. 'splain this one to me: If changing wind direction or velocity (at an extreme rate) has no effect on airspeed, then why have airplanes crashed when encountering microburst type windshear close to the ground. In a wind shear, you encounter a change of plus 30 or so down to minus 30 or so (just numbers) in a few seconds. Been there, done that....you WILL see an immediate IAS change, and it usually ain't good!

That's because airplanes have inertia, just like any other object in the physical world. If it's moving a certain groundspeed, it wants to stay with that groundspeed (Newton's 1st law.) So of course if we're flying a headwind and a gust blows, the airspeed will increase.

"Wind direction and/or speed has no effect on IAS" refers to steady states. That's why you can fly backwards in perfectly controlled flight on a really windy day.

 

[Timebuilder]

"Wind direction and/or speed has no effect on IAS" refers to steady states. That's why you can fly backwards in perfectly controlled flight on a really windy day.

I remember the first time I achieved a hover in a 172 on a windy day. I thought "Wow! Just like a wind tunnel!" Then the wind increased, and I started flying backwards. This was the best illustration ever that IAS was independent of either wind speed or goundspeed. A sharp turn of 180 degrees would have brought my TAS to almost zero, and I would have had an immediate loss of lift.

While the downwind turn is not a myth, it is hardly ever a factor that most pilots will need to worry about.

 

[A Squared]

A sharp turn of 180 degrees would have brought my TAS to almost zero, and I would have had an immediate loss of lift.

No, this is absolutely incorrect. If you weren't looking at the ground, you would have felt no difference nor seen any in your airspeed indicator. Granted, if you banked steeply enough, you *could* cause the plane to stall, but this would happen at the exact angle of bank at which the airplane would stall in still air. Let's say that you were flying at a TAS of 70 knots whan you were doing your hover. If you turned down wind you would accelerate to 140 knots. It seem appealing to beleive that the air just can't accelerate the airplane 140 knots so it will just drop out of the sky...nothing could be further from the truth. THe airplane is perfectly capable of accelerating to a ground speed of 140 iknots. What happens when you are doing the same 180 degree turn in still air? You are accelaerating from a groundspeed of 70 knots in one direction to 70 knots in the opposite direction, a velocity change of 140 knots, which is exactly the same velocity change that you get in a 70 knot wind.

140 knots is 140 knots, neither the air not the airplane care what the ground is doing underneath them.

 

[Timebuilder]

If I were wrong, there would be no such thing as wind shear, as the new wind direction would have no trouble accelerating a aircraft to the new airspeed.

Just as a change in wind can cause a plane to literally drop from the sky, a ligh maneuverable plane can have a radical change in the relative wind under the circumstances I descrtibed.

And it's the relative wind that keeps the plane flying.

 

[Timebuilder]

It would undergo the exact same change in relative wind as a plane turning in still air.

Not so.

In still air, a turning airplane has a nearly constant relative wind.

In my example, the the airplane is a "rest" for the purposes of Newton. The only reason it is flying is the relative wind. A sudden turn to the opposite heading will begin to to accelerate this resting body by means of a tail wind. Until the airplane is sufficiently acclimated to this new relative wind, it will have insufficient relative wind to support flight.

Airplanes are not capable of going from zero to 140 without the use of a steam catapult.

 

[VNugget]

Originally posted by Timebuilder

If I were wrong, there would be no such thing as wind shear, as the new wind direction would have no trouble accelerating a aircraft to the new airspeed.

Wind shear is a change in relative wind, so it has nothing to do with this topic, as we're talking about a steady wind. "Accelerate" is a very bad word to use, because it usually refers to a change in velocity due to a net force in a direction. It's best left to refer to groundspeed. Just to keep things clear, I will point out that I understand what you're saying, because a plane has inertia, and therefore wants to stay at a certain groundspeed, and therefore when a gust of headwind blows, the airspeed will increase, just as you said. (Of course, if it's not just a gust but the wind stays with its new speed/direction, the greater drag will cause the plane to gradually decrease its airspeed to its orginal value... and decelerate to a lower groundspeed.)

Anyway, back to the 70kt hover... we are dealing with a steady wind. There is no reason (if there is, give us one) that the plane would behave any different in the 180 degree turn with a groundspeed of 0 or 70. Let me emphasize that we are ignoring the ground, and NOT trying to keep any particular ground track... we are just turning in the air.

As A^2 said, the net work done on the airplane is still the same, regardless of whether we went from 70 to -70 or 0 to -140; and centripetal acceleration will be the same with any given bank angle. The turn will take the same amount of time to complete. Everything will be the same.

And it's the relative wind that keeps the plane flying.

Completely true.

a change in wind can cause a plane to literally drop from the sky,

Completely true.

a ligh maneuverable plane can have a radical change in the relative wind under the circumstances I descrtibed

It would undergo the exact same change in relative wind as a plane turning in still air.



When thinking about all of this, keep in mind that Newtonian physics has nothing to do with motion, but acceleration. Any pre-existing motion is just the baseline from which further changes in motion (by defintion, acceleration) takes place. Imagine that you want to fly a R/C plane inside a cargo plane at FL350. Would it behave any differently? Of course not; any forces (and the subsequent accelerations) on it would change the baseline velocity, but to the R/C plane, it feels no different. Come to think of it, you don't have to do this thought experiment at all, because it's already happening: we are on the surface of a planet that is spinning at god-knows-what-angular velocity around itself, and at god-knows-what velocity around the sun, which is spinning at god-knows-what velocity around the core of our galaxy, ad infinitum. Does it make any difference to us? No. In fact, we didn't have the slightest clue for the first several thousand years of our civilization.

You are flying a real plane in a mass of air, which just happens not to have walls, unlike the cargo plane.

 

[VNugget]

Whoops, trivial correction... angular velocity does matter, but it's so low in this case that it's insignificant... most of the time. Note that the Earth is wider at the equator.

 

[SkyWestCRJPilot]

Here's one to think about. You're flying on a 180 degree heading. The winds aloft according to your GPS or FMS say from 270 degrees at 10 knots. You notice at your 1 O'clock position a hot air balloon about 5 miles away. Are you on a collision course? (Here's a hint, it doesn't matter how fast your airplane is flying.)

Here's another: Say your flying a freight plane full of 500 ducks. Total weight of the ducks (no cages just birds walking around) is 2500 lbs. Your airplane's takeoff weight is 100,000 lbs. Once you're in the air all of the ducks start flapping their wings and are flying around the cabin. Does your airplane now weigh 97,500 lbs. or does it still weigh 100,000 lbs?