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.

 

[Timebuilder]

There's only one small problem - no physicist in the world agrees with what you just wrote.

There may be several reasons that no one agrees, but they don't worry me. In order for a calculation to be relevant, you have to have an equation that models the relationship that is the basis of the hypothesis. I suspect that this is the reason for disagreement.

All we are talking about is taking a pilot tube that is pointing in one direction where the forces are additive and turning it to a direction where the forces are subtractive. It's that simple. What makes it interesting is the fact that the pitot tube gives us a representation of the flow of relative wind over the wing. The question for the model is this: how quickly is the orientation being changed? Under normal conditions, the change is so painfully slow that the forces involved have plenty of time to "keep up" with the evolving situation, as the airplane is affected over time by the fact that its relationship to the wind is changing. The key is that at a point found by the extreme maneuvering of ag planes on a windy day, these changes happen much more quickly than than any "timed turn" experiment would reveal.

Perhaps additional variables are coordination and pitch attitude, since we are apparently not discussing an effect in a "normal" turn. In this case, we have to ask "how abnormal" a turn do we have to look at?

It's an interesting question. This would certainly explain problems that might arise during the modelling process. Does the turn have to happen so fast that it is a skidding turn?

 

[VNugget]

Why would a continuous steep turn not demonstrate the desired effect, if the airspeed is low enough and the load factor is high enough to reach come close to a stall? Why would a 90 degree show the effect but not a 360, or a 720, or a 1080, ad infinitum? My guess is because it is a less controlled experiment, and more likely to introduce factors such as trying to make the upwind and downwind turns with the same-shaped ground track, or in the same amount of time after a crabbed crosswind leg, etc., but then again, who am I to pass judgement?

 

[Checks]

Fact: For a given pitch and powersetting the airplane will maintain the same airspeed and rate of climb regardless of turning upwind or downwind.
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I agree but it takes time for the aircraft to achieve steady state conditions.

Ask any pilot who flies slow aircraft, close to the ground, whether the downwind turn is an illusion they will agree that is isnt.

 

[Sctt@NJA]

Ask any pilot who flies slow aircraft, close to the ground, whether the downwind turn is an illusion they will agree that is isnt. [/B][/QUOTE]


I don't have to ask one, I AM one. I have several hundered hours of 7ac aeronca champ time. Low, slow and way underpowered with 65 horse power. Much of that flying time is under conditions where I was flying lower than nearby terrain or obstacles. With this airplane you tend to fly between hills rather than over them.

And you don't have the power to get yourself out of trouble either. Its very possible to find yourself in a position where you are in a "bowl" with rising terrain on all sides and very limited climb performance to help you cope. Very important to use all advantages of terrain and wind to your advantage. Flying into the wind for better climb angle is critical. Locating a thermal helps too.

I have tested the downwind turn (with a bit of altitude) for loss of airspeed, and there was no loss. No gain turning into the wind either. These are my observations, so for folks who have seen otherwise I can't explain.

The science (which says you wont lose airspeed in a downwind turn- i mean other than induced drag) meshes with my personal experience in airplanes.

I just wanted to point out that I am not just speaking from a text book. And also, I wouldn't have tested it myself if I never had doubts!

 

[Checks]

I went out and did 20 degree bank turns with the AutoPilot on and power set yesterday.

I used 90kts and the airspeed would rise to 93kts as I went into the wind and then would decrease as I was turning downwind.

Maybe I did something wrong but I saw the airspeed change Yesterday.


I will say that all you naysayers have me wondering if I am missing something. The next day the wind is really howling I will do it again.

 

[100LL... Again!]

The real issue -

An airplane is not a laboratory, and it is not possible to isolate single factors. This is why flight test aircraft are often fitted with enough computers to run the space shuttle.

The physics say no. Some of the pilots say yes.

If the effect is there, it is for some other reason.