Another A5 Icon crash 7/27/19

[FastEddieB]

FastEddie, you are correct but have an over-simplified understanding of the situation. You are ignoring the effects of momentum (energy) of the aircraft that must be overcome, especially if the turn is quick.

It’s momentum relative to what? The air mass or the ground?

Analogy...

You’re walking down the aisle of a train moving at a constant speed of 60 mph. You stumble and fall. Is it relevant to your injury likelihood whether you’re walking in the direction the train is moving or in the opposite direction?

In the case of a plane, the only thing that matters is its momentum relative to the air mass. In a 60 kt wind, a 100 kt plane could be moving over the ground at 40 kts into the wind or 160 kts flying downwind. In both situations the “lift” and stall likelihood are identical. The plane has no way of knowing what the ground is doing*. In your scenario, something would change momentum-wise if the plane started doing 360’s in that wind as the groundspeed went from 40 kts to 160 kts. It doesn’t. Under a hood you could not feel anything different from doing the same 360’s in still air.

Back to the ICON, as it made the right turn in an airmass moving left to right in the video, it would behave exactly as it would in still air. A wing does not know or care what the ground - or water - below it is doing.


*The only way to determine what the plane is doing relative to the ground is via navigation sources - or to look.

[Wm.Ince]

A 15 knot loss of airspeed (as in a, non-microburst, wind shear event)...

Of course. Stipulated.
But not relevant to a discussion about the effect of a downwind turn on lift and/or stall.

On the contrary, I think it may be very relevant. Review what happens to the ICON just before contact with the tree.
Forget about what happens over the ground reference (water), that is irrelevant. But the airmass the airplane is surround by, that's what is relevant, especially at low speed and high angle of attack.

[TimTaylor]

Momentum relative to the earth and the earth's gravitational field. If the aircraft had zero mass, you would be correct. However, it has mass and does not instantaneously change directions such that the relative wind is constant. It takes some time for the aircraft to "catch up" such that the relative wind is constant again. How long is proportional to the mass and power, I suppose.

[FastEddieB]

A 15 knot loss of airspeed (as in a, non-microburst, wind shear event)...

Of course. Stipulated.
But not relevant to a discussion about the effect of a downwind turn on lift and/or stall.

On the contrary, I think it may be very relevant. Review what happens to the ICON just before contact with the tree.

It could have been a gust from the rear. It has nothing to do with whether the plane was turning upwind or downwind at the time - the result would be the same.

[FastEddieB]

Momentum relative to the earth and the earth's gravitational field.

So doing the 360’s in a 60 kt wind can be felt by a pilot?

The earth’s gravitational field is “pulling” things straight down to the earth’s center. It is not somehow “pinning” things not in contact with the surface to a particular location to which things can be referenced inertia-wise.

Admittedly, I never took physics in school, so if I have something wrong I can accept that. But I still hold that we can set up an experiment on a day with a steady, strong wind: fly perpendicular to the wind, and do 90° turning stalls turning in either direction. I hold they will be identical, and such is my experience. I take it you hold the “downwind” turn will stall sooner. In that case I’m virtually sure you’re wrong.

[Wm.Ince]

. . . But I still hold that we can set up an experiment on a day with a steady, strong wind: fly perpendicular to the wind, and do 90° turning stalls turning in either direction. I hold they will be identical, and such is my experience. I take it you hold the “downwind” turn will stall sooner. In that case I’m virtually sure you’re wrong.

Using your logic . . . then why add "half the gust spread" to Vref.
Energy management.

[Wm.Ince]

Of course. Stipulated.
But not relevant to a discussion about the effect of a downwind turn on lift and/or stall.

On the contrary, I think it may be very relevant. Review what happens to the ICON just before contact with the tree.

It could have been a gust from the rear. It has nothing to do with whether the plane was turning upwind or downwind at the time - the result would be the same.

I think that's possible.
"the result would be the same" . . . but not immediately!
And an immediate gust from the rear would result in a dropoff of airspeed, thus getting very close to critical angle of attack. Remember, he was at a very low speed to begin with, on his climbout.

[TimTaylor]

I took lots of physics, but not as much as Paul. So, how does inertial navigation work? Again, Eddie, you are looking at part of the issue and are correct for the part you are looking at. However, you are ignoring 1/2 of the situation or maybe less than 1/2. It depends on the mass of the aircraft and maybe some other variables.

[FastEddieB]

You’re talking gusts. I’ve already stipulated that gusts and shear are real and have real effects.

What started this was an allegation that turning downwind can cause a loss of lift. That’s not true.

Yes, you approach at a faster airspeed when gusts are present. But it’s significant that you do not adjust approach airspeeds for headwinds or tailwinds. The way some are talking here, you’d expect to need to approach faster with a tailwind due to the “loss of lift”. You don’t, and there’s a reason for that.

[FastEddieB]

I took lots of physics, but not as much as Paul. So, how does inertial navigation work?

Easy to Google. And irrelevant to any discussion of aerodynamics.

[Wm.Ince]

Paul . . . where are you? You need to weigh in on this issue.

[drseti]

Paul . . . where are you? You need to weigh in on this issue.

Where do you think, Bill? Been flying all day, and I'm now sitting with my feet up and a cold beverage in hand. Will weigh in after I cool down a bit.

[Warmi]

Do you schedule your students , especially with very few hours, around midday ?
I wonder how do they take the usual thermals and bumps etc...

Now that I recall , due to my work and other commitments , pretty much all my lessons were either before 9am or after 5 pm - I never really experienced thermals until I started flying my own plane heh.

[drseti]

The plane has no way of knowing what the ground is doing

There you go again, Eddie, anthropomorphizing an inatimate object. You mustn't do that. They hate it!

[Wm.Ince]

Paul . . . where are you? You need to weigh in on this issue.

Where do you think, Bill? Been flying all day, and I'm now sitting with my feet up and a cold beverage in hand. Will weigh in after I cool down a bit.

Good!
Have one on us.