CFI myths..Special VFR day/night et al

[bigD]

Good to see you back, Avbug! Hope things are well with you.

As a science man, I know the "fatal downwind turn" is a myth, however I'm not going to just wave my hand and discount any pilot that has seen otherwise in actual practice. So there must be a logical explanation for what some pilots are seeing. For example Avbug, are you SURE that the wind you're dealing with in the instances where you see the airspeed fluctuations is constant? No shear at all?

 

[avbug]

Sometimes yes, sometimes no....but no, I'm not sure. In fact, in some cases, most definitely it was due in part to a shearing effect. I would attribute that in many cases to climbing from ground effect into an increasing gradient: certainly a quick pull up coupled with a rapid turn can cause this. Also, the types of turns I'm talking about are turns that most people won't ever see or do in their flying (specifically the ag turn).

Every ag turn is started with a pullup for obstacles and then a turn downwind, followed by a turn back into the wind to reduce the turn: the object being to get turned around as fast as possible and to complete the turn one wingspan farther upwind for the next pass.

Many would be quick to lable what I'm saying as junk science (and they'd be right), but I'd be interested to see what they'd have to say after a morning of back-and-forth with a little wind. There really is no such thing as a down-wind turn. Except.

I'm not back yet...just stealing some unauthorized access on a terminal that doesn't belong to me.

As a sidenote, I have recently observed that there are 24 hours in a day rather than the 18 I thought were available. Being internet-free does wonderful things to one's schedulebook.

 

[bugchaser]

Guys, I have been watching this discussion and can't help but put my two cents in. What Avbug is saying is absolutely true. I have seen this thousands of times and there is a difference between a downwind and upwind turns. I have observed this many times when flying a field upwind/downwind. When pulling out of the field going into the wind, the airplane will climb like a rocket. Often when loaded light it is possible to pull up almost verticle right out of the field. One the other end of the field, you will bust your *** if you even think about this kind of pullup. What is the difference in these two pullups/turns. The only difference is in the wind direction. I have thought about this many many times and believe that some of the explanation lies in the fact that often there may be a wind gradient at low levels. What I mean is that the wind speed increases as the airplane moves from just above the ground up through 200-400 ft. Also many times the wind is blocked by trees, etc. Anyway, the effect is that at low levels, the downwind turn is much more difficult to make and the airplane does not perform nearly as well. As Avbug stated, these kinds of things are much more evident when you spend lots of time flying around heavily loaded and right on the edge of a stall. You get very sensitive to small changes in the way the airplane behaves. I know many of you may still disagree and give examples from the "book", but until you spend a year or three down in the dirt, don't discount this as a myth.

 

[100LL... Again!]

I think that wind gradient and surface friction would explain this.
Even a 10 knot difference would yield a noticeable result - both upwind and downwind. Unles you can independently verify that there is no 'layering' of wind speeds, the science must hold fast.

As the aircraft climbs and turns, there is a very slight shearing effect from the increase in tailwind component from turning downwind.

Therefore, the theoretical physics of this question hold true, yet it does not explain the observed phenomenon due to the lack of inclusion of this additional factor.

bugchaser-
I notice you referred to pulling up into a turn. This correlates with the slight change in wind component. I'm guessing that you rapidly transit a few hundred feet of altitude.

This explains why no one sees this effect during turns around a point or steep turns.

See there? Everyone was right after all.

 

[avbug]

Several hundred feet? Thems some mighty big trees, thar.

75' to 150' is closer. The idea is to get turned around and headed the other way in the absolute minimum time possible. Time spent in the turn is time spent not spraying; that means time wasted and profit lost.

A properly executed ag turn resembles an aerobatic maneuver more than something you might picture a student pilot doing.

 

[100LL... Again!]

Still enough to account for a few knots of wind shift, which is enough to be noticeable.

 

[bugchaser]

I think the idea of the wind gradient makes a lot of sense. 100LL, I think you better explained what I was trying to get across.

Avbug, personally I like to pull up high during turns. That is just my style of flying and it fits the airplane that I fly. It is very common to be up 200-300 ft during a turn. You are right about not wanting to waste time during the turn. Don't worry I'm not wasting any time out there. Are you doing any ag flying this season? Just curious if you had a seat this year.

 

[avbug]

No crop work (yet), but probably taking a leave to do SEAT work in the drom again.

 

[VNugget]

I don't see what the big mystery is, without even having to bother with wind gradient. If the downwind turn (assuming you were tracking straight in the crosswind) was such a myth, wouldn't we all be taking off downwind half the time depending on our mood?

 

[JimNtexas]

How about turning in the jetstream? If you turn a 360 over a solid overcast at night in smooth air, but with high winds present, will you notice the airspeed changing wrt to the wind?