CFI myths..Special VFR day/night et al

[Sctt@NJA]

Both sides are right on this.

1. The airplane doesn't care if it is in still air or moving (as long as its a steady-state wind)

2. Downwind turns ARE dangerous (but only when close to the ground)

Concerning point 1: The inertia type arguments don't pan out. Try doing a 180 in 100kt plus winds and you will see- the airplane doesn't care.

Concerning point 2: Its simply a matter of climb gradient. The RATE of climb will be the same with a headwind or tailwind. The ANGLE of climb relative to the ground will change. (clearly a poor ANGLE of climb is what is hazardous when you need to clear obstacles)

So low-level downwind turns are more dangerous than upwind turns due to the increased groundspeed which causes a decreased climb angle (remember angle of climb is the aircrafts ability to climb over distance relative to the ground) and thus reduced performance in regards to clearing obstacles.

Note: When I say "low-level" I mean low enough that nearby obstacles are a concern. 1000 feet over prairy land doesn't count. 10 feet with a tree line ahead does.

I would speculate that the other phenomina observed by some pilots (eg. incleased RATE of climb when turning upwind) are an illusion based on groundspeed and angle of climb.

The same maybe for those who say you lose airspeed... if you were doing a low level climbing turn to an increasing tailwind component the decreasing climb angle (due to increasing groundspeed) would cause the pilot to compensate and get their climb angle back by raising the nose, thus losing airspeed. The proximity of the ground is providing a reference of climb angle that doesn't exist higher up.

 

[bigD]

Does anyone else have a decent explanation for this? I am very interested to see someone else's analysis.

SkyWest's explanation about the downforce is right on. Just remember that air is a fluid in many of the same ways that water is. If you had a fishbowl full of water, and started adding fish to it, you'd kind of expect the weight of the bowl to increase as you add the fish, right? The weight doesn't just suddenly disappear because the fish are swimming around in the water. It's the same kind of thing when dealing with airplanes, birds, or whatever flying around in a closed vessel filled with air.

 

[100LL... Again!]

How 'bout we get together next time its blowing about 30-40 knots. I'll take off and do a rapid downwind turn while maintaining airspeed. If the downwind turn theorem is true, I would have to desend radically to maintain airspeed.

$100 bet. (I'll feel bad taking your money).

 

[bigD]

Wait, a $100 bet that you'll have to radically decend to maintain airspeed?

You're on!

 

[Sctt@NJA]

You wont have to descend to maintain airspeed. But if you need to clear obstacles you would be better off turning upwind.

See? The hazardous downwind turn "myth" is true. Its just the reason for it thats misunderstood.

Fact: For a given pitch and powersetting the airplane will maintain the same airspeed and rate of climb regardless of turning upwind or downwind.

Fact: That same airplane while turning downwind will have a greater groundspeed and lesser angle of climb, so its climb perfomance relative to the ground will be worse.

The people saying the downwind turn is a "myth" are correct in the sense that rate of climb and airspeed are not affected by a steady state wind. They are wrong however in thinking that a low-level downwind turn is not dangerous. Its all about ANGLE of climb. (How high you can climb for a given distance over the ground)

 

[100LL... Again!]

Scott-

Check out my post again, I was in agreement with what you said.

I was saying that IF the downwind turn myth was true, then I would have two choices; attempt to hold altitude and lose arspeed, or maintain airspeed and lose altitude.

I agree with your point about the climb gradient. If you check the previous page, I mentioned it back there.

What I think happens insome instances is that during the downwind turn the pilot subconsciously notices the increased groundspeed, pitches up to 'compensate' for what he perceives as excess speed, and thusly loses speed. In this instance there is a speed loss, but it is self inflicted - not the effect of a changing wind component.

Life is full of optical illusions, and some are very convincing.

For example, I actually thought I would get paid actual money to fly airplanes, but it was an illusion for years.

 

[Sctt@NJA]

Yes, I do believe we are on the same page

 

[avbug]

It goes way beyond an optical illousion. That's part and parcel for the problems that many pilots encounter during a downwind turn, but far too many times even this summer I've encountered the "mythical" downwind turn at work.

Ignoring the ground, a hard turn downwind very often will see a marked airspeed decrease especially when heavy. A hard turn into the wind will see the opposite.

I fully comprehend the physics and aerodynamics and reason. I also know what happens in reality, and the truth is that the fabled downwind turn IS reality.

Most ag pilots don't give this a second thought. We know.

Take an airplane already loaded to the gills, headed on a direct crosswind. Fly it close to minimum airspeed, so there's no margin. Haul it hard downwind, and see what happens. Haul it hard upwind and see what happens. Do that day in and day out, thirty times an hour for eight hours a day, and you'll find religion when it comes to doing downwind turns.

Pilots do get caught by the "illousion." But for those who live in the environment, doing nothing but critical turns in wind all day and see the effects, do you really think we're that stupid? Uh-uh. We know.

I fully appreciate the science. I do. But then I've taken learned engineers into the shop to show them how their unbreakable design snaps off...and hotshot pilots and aerodynamicists into places whence their only reply was first to throw up, and then say, "oh."

Are downwind turns a reality, and the attendant airspeed with them not a myth? Yes, they can be, and are. It is, as they say, where the rubber meets the road.

 

[bigD]

Avbug - I've said this before in an earlier conversation we've had about this same subject - from a physics standpoint I find what you say hard to believe, but I also respect your experience and I'm certainly not calling you a liar, so there must be some explanation that fits your observations into existing physical laws.

Problem is, the owner of the flight school I rent from would likely frown upon me operating the Duchess in the kinds of conditions that you describe. So I can't verify what you say for myself. And the physicist in me NEEDS this verification.

So can I ride along with you? I know it's dangerous, but I'll sign a waiver absolving anyone from responsibility if I get killed!

 

[avbug]

Hop in, but bring a sick sack.

And a helmet. Right at the moment I have just one seat, and you'll be riding in the hopper. No windows and you can't see the instruments, but I'll tell you what happened.

In my day we didn't have windows. And we were grateful. (now I have to close my eyes, instead).

 

[bigD]

I've got my helmet and I'm ready to go! If I'm about to puke, you can just drop me out of the airplane!

And I'll be grateful!

 

[VNugget]

Take an airplane already loaded to the gills, headed on a direct crosswind. Fly it close to minimum airspeed, so there's no margin. Haul it hard downwind, and see what happens. Haul it hard upwind and see what happens. Do that day in and day out, thirty times an hour for eight hours a day, and you'll find religion when it comes to doing downwind turns.

In that direct crosswind, are you correcting for it and tracking straight down the ground, or letting the wind take you where it may and ignoring the ground? I obviously don't know jack about ag flying, but I bet it was the former.


Once again, confusion reigns supreme (for example, bigD's second-to-last post) because people are not setting their terms straight in the discussion and end up arguing about two different things.


Of course if you're trying to make turns from a crosswind with the same (well, mirrored) ground track, you're gonna need to bank higher and load the plane up more (and, of course, stall, if you're inside the difference margin) in the downwind turn. There's not the slighteset mystery; you all teach your students this in ground reference maneuvers, no?


The people who say the downwind turn is a mystery (such as bigD in this case) are talking about when you're ignoring the ground, which, as you realise, is a completely different situation. There is no argument, you have not contradicted each other!

 

[bigD]

VNugget - but remember that Avbug stated that, in his experience, a turn to upwind will result in an increase in airspeed. In your scenario (where you're trying to follow a specific ground track), the airspeed will decrease either way - it's just that turning downwind will result in a greater loss of airspeed.

 

[VNugget]

Which scenario?

 

[avbug]

No contradiction, but you obviously haven't read my previous posts, as each of your comments have already been addressed.

Forget outside references, or ground track. Fly 90 degrees to the wind, and turn hard. Some of the previous commentary on inertia and other factors does play here, as you can certainly see the "mythical" effects of the downwind turn.

As I've previously stated, on my shelf, I have the stick grip a pilot was holding when he impacted the ground in a pecan grove, after executing a downwind turn and being unable to recover.

Yes, the illousion makes a difference, but one must presuppose that some of us are able to see past that. Perhaps another 20 years more...but perhaps not. Perhaps we see things rather clearly now...hard not to, when you see it all the time.

Again, ignoring ground track, keeping the eyes strictly inside the cockpit and ignorning ground references, one can indeed see the difference in airspeed when turning downwind. The airplane, while in a fluid medium, simply does not move with the wind faster than it can turn. This is especially the case with a hard steep turn...a turn (for the purists) that would NOT result in a stall or appreciable loss in still air.

Someone had previously asked if this were the case in the jet stream; if one complete's a 360 degree turn, why is there not a point at which an airspeed loss, and an airspeed gain is noted. In such a case, the aircraft does not experience any change in indicated airspeed. Groundspeed, of course, changes, but this is not relevant. Make that a hard turn, especially when loaded heavily, when the airplane may be made to turn faster than it accelerates over the ground (ignoring all outside visual cues), and the airplane may experience an airspeed loss or gain.

Now I appreciate the illousion factor, and the gradient factors that have already been discussed...I discussed them myself in previous posts. But these are germain only to the concept of perceived loss...not actual airspeed loss (which does occur). Believe me, I had several occasions this summer in 30-50 knot breezes, turning downwind and into rising terrain while descending toward it for a drop, that the illousion was all but overpowering. And downright intimidating. But it's neither here nor there.

Of a greater interest to me at the time was the attendant airspeed loss. I could mitigate that by making a slower turn, but that resulted in unacceptable drift and difficulty obtaining the proper run-in line around a smoke column. The only choice was a harder turn, and to account for the loss by descending to make up for it.

All the same, it's a real phenomenon.

 

[VNugget]

Someone had previously asked if this were the case in the jet stream; if one complete's a 360 degree turn, why is there not a point at which an airspeed loss, and an airspeed gain is noted. In such a case, the aircraft does not experience any change in indicated airspeed.

I think I'm losing track here... is that your statement, or that of the person who asked the question? Because it's a direct contradiction of your claim that the airspeed would change.

Make that a hard turn, especially when loaded heavily, when the airplane may be made to turn faster than it accelerates over the ground (ignoring all outside visual cues), and the airplane may experience an airspeed loss or gain.

Are you still talking about the same scenario? If so, why would the result be any different, except in degree?

The airplane, while in a fluid medium, simply does not move with the wind faster than it can turn.
Can you please explain this in a little more detail?


These aren't arguments, per se, but I'm rather just trying to clearly unerstand your post.

 

[100LL... Again!]

The problem is that an aircraft is not enough of a controlled environment to be able to scientifically isolate the real causes of this effect.

Whatever the effect is, it must be attributable to something other than wind component.

The laws of physics do not allow for the effect due to the circumstances stated.

Simply put, an aircraft is part of the atmosphere in which it is flying and reference to the ground is not relevant in terms of aerodynamics.

An additional factor to consider is the effect of wind on turn radius. Keeping the turn the same size would require a steeper bank with a tailwind, as we all know. Even slight bank angle changes can generate a variance in IAS.

 

[avbug]

An aircraft is NOT part of the atmosphere, but a foriegn body which affects and is affected by the atmosphere.

Forget the ground. Forget ground reference, and turn radius. Throw them away. Take them out the picture, and throw them away.

Load that airplane heavily, to the gills. Controlled conditions apply. Fly it on a heading 90 degrees from the wind direction; in other words a direct crosswind. Make a hard turn downwind over a specific waypoint. Without looking outside, see what happens. Now go back to the same waypoint and do it into the wind. Now reverse headings, and do it the same way, experiencing the same loss or increase on opposite sides (right instead of left, due to heading change).

An ag pilot sees this thirty times or more in an hour. Discounting the illousions (GOTTA believe that someone who does this thousands of times a year isn't always so stupid that he or she can't separate the illousion from the fact...), you poll 100 pilots doing this and see what they have experienced. Get in the airplane and try it yourself.

The aircraft is in a fluid medium but not a part of it. This is why the airplane can move and do things that say, a hot air balloon cannot. An airplane can be turned downwind rapidly enough that it can experience a performance loss and airspeed loss, easily felt and discernable in the cockpit, without ever needing to look outside.

This is different than a standard rate turn performed in the jet stream or some other significant wind event. 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.

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.

The biggest loss I have seen doing this was in unusual circumstances in a Learjet 24D during a hard turn. (leave aside limitations discussions for now). I experienced a 60 knot loss (only part of which was due to the turn). Despite the magnitude of that loss, it was still hardly significant compared to a lesser loss in say, a Pawnee close to the ground and heavily loaded, and already on the edge of a stall in a normal turn.

 

[VNugget]

An ag pilot sees this thirty times or more in an hour.

Don't ag pilots correct for crosswind? If not, then I had a totally skewed conception. If so, well, we just introduced reference to the ground, which we were supposed forget last paragraph.

I have time to address the rest of your post later.

 

[bigD]

Avbug - you're going to get a lot of responses regarding your last post, but it'll all be a rehash of what's been said before. You certainly don't need to hear it again from me.

The bottom line is that posts trying to discredit your assertions using physics (hell, someone even pulled out the term "inertial frame") or whatever else aren't going to do a d*mn thing to convince you otherwise because the best proof is what you see with your own eyes in the cockpit. So all I'm trying to do is figure out why you're seeing what you're seeing.

Now, I obviously can't ride with you when you're firefighting, or ag flying, or flying a Lear 24D. But let me ask you this - do you think you could demonstrate this downwind turn airspeed loss and upwind turn airspeed gain using a more pedestrian airplane such as a Duchess without flying outside its operating limitations? If so, I'd happily fly to wherever you are, wait around for some good sized winds aloft, and hand the plane over to you to show me what you've seen.

Turn and bank as hard and fast as you want (please keep the wings attached), but if you can show me an increase in airspeed during a fast upwind turn, I'll eat my headset right then and there.