Downwind Turns

[Wannabe Cubdriver]

Ladies and Gentlemen,

The number of posts I have sifted through with regard to downwind turns in the ultralight crash topic has me confused a little.

My question (I know it has probably already been answered a dozen times already, but I am seeking clarification) is:

What are the hazards in executing a downwind turn on a windy day?

My background: low time spam can pilot, mostly Cherokee time. I have enough experience to know I am as much a hazard to pers on the ground as I am in the air and am always wanting to learn more. My flight training was in a fairly windy part of the country, my flight exam was 29 gusting 35kts (straight down the pipe). Winds exceeding 15kts are routine. What danger was I in flying countless (seemingly) circuits in these conditions? My instructor (very new) told me there was no issue as long as I was aware of "illusions created by drift" (an entire set of lessons).

I guess I just want to know if I am lucky or not.

Thanks for your time.

Cheers!

[S2D]

Turns are only an illusion. actually the earth is rotating in such a way as to make it appear that you are turning Carry ON!!!!

[mvivion]

Good grief, Man!! Read the thread....don't start ANOTHER one fer cryin out loud!

MTV

[Wannabe Cubdriver]

I did read the thread, like I said I did. No one wants to agree unless they pull out a slide rule. Sorry if my concern for keeping my a$$ out of the trees due to a training deficiency disturbs you, I just thought maybe my 21 yr old 3 digit total time instructor might have missed something. I don't care about the effect of wind over the prop in the turn or what if I was in a rotary, yadayadayada, I want a simple yes/no answer. Am I OK flying a properly loaded, standard category a/c in these conditions or am I oblivious to a hazard that's gonna make me ride one in. I apologize if my questions offend you, I'll stop asking. Maybe someday when I consider my opinions and experience on flying worthy of contribution, I'll do my best to help out, however, my experience is as such that I would rather ask the questions and soak up what I can.

Maybe I'll hang up the headset and save my hard earned money, then I'll have more cash for hunting.

Thanks.

[aalexander]

I did read the thread, like I said I did. No one wants to agree unless they pull out a slide rule. Sorry if my concern for keeping my a$$ out of the trees due to a training deficiency disturbs you, I just thought maybe my 21 yr old 3 digit total time instructor might have missed something. I don't care about the effect of wind over the prop in the turn or what if I was in a rotary, yadayadayada, I want a simple yes/no answer. Am I OK flying a properly loaded, standard category a/c in these conditions or am I oblivious to a hazard that's gonna make me ride one in. I apologize if my questions offend you, I'll stop asking. Maybe someday when I consider my opinions and experience on flying worthy of contribution, I'll do my best to help out, however, my experience is as such that I would rather ask the questions and soak up what I can.

Maybe I'll hang up the headset and save my hard earned money, then I'll have more cash for hunting.

Thanks.

The point that you seem to be missing is not that you shouldn't be interested, but that if you weren't satisfied with what you read on the thread, what on earth made you think you would get something different by attempting to drag the discussion to another thread???

[Lippy]

I guess I just want to know if I am lucky or not.

...one word answer? No.

Lippy

[Wannabe Cubdriver]

The point that you seem to be missing is not that you shouldn't be interested, but that if you weren't satisfied with what you read on the thread, what on earth made you think you would get something different by attempting to drag the discussion to another thread???

Exactly that, I wasn't satisfied with what I read in the other thread, which was titled "Ultralight Crash", so I figure someone with a definite answer might just read this topic title and respond, rather than overlook an older 10 page topic with a title that may not be of any interest to them.

My intent was not to initiate another 10 pages of opinions, but rather to try and appeal to someone who soundly knows this subject and can actually answer yes or no with regards to the kind of flying I do, without contradicting follow up posts. I didn't think it was too much to ask.

[aalexander]

My intent was not to initiate another 10 pages of opinions, but rather to try and appeal to someone who soundly knows this subject and can actually answer yes or no with regards to the kind of flying I do, without contradicting follow up posts. I didn't think it was too much to ask.

And you've read all 10 pages (13 if you count the breakaway thread that someone else started on the same topic) and you *seriously* believe that you're going to get some simple one word answer? Do you honestly think that is a realistic expectation?

I'd be interested in hearing you rationale for believing that days and days and pages and pages of intense disagreement will suddenly vaporize merely because you started a new thread.

I'm listening. This ought to be good.

[aalexander]

Since you're in search a simple one word answer to a question which precipitates disagreement on many levels, I'll help you out;

A number of people strongly believe that the your answer is no. If you don't like that answer, a number of other people strongly believe that your answer is yes.

Pick which ever one word answer suits you best.

Or you could attempt to understand the discussion and make up your own mind about the situation, but you've already said you don't want to do that.

[Wannabe Cubdriver]

Since you're in search a simple one word answer to a question which precipitates disagreement on many levels, I'll help you out;

A number of people strongly believe that the your answer is no. If you don't like that answer, a number of other people strongly believe that your answer is yes.

Pick which ever one word answer suits you best.

Or you could attempt to understand the discussion and make up your own mind about the situation, but you've already said you don't want to do that.

So, I am to believe that this is a chicken - egg deal, nobody knows, and only best guesses count. I have attempted to understand the discussion, that's why I asked the question again, for clarity, there is far too much disagreement. I am not looking for opinions, I am looking for facts. I guess I cant have the facts given to me by someone who knows without my needing a degree in aerodynamics.

I'd be interested in hearing you rationale for believing that days and days and pages and pages of intense disagreement will suddenly vaporize merely because you started a new thread.

Like I tried to say, I am looking for a fact, not an opinion, I thought there was a difference. I guess my easy answer is that if the answer is not KNOWN, don't post.

...one word answer? No.

Thanks Lippy, at least I got an answer and not an attack.

I sure like the treatment the newly experienced guys get here. Forget I even asked.

[Lippy]

1. 2. 3. 4. 5. 6.
Six Word Answer: No No No

[Clyde and Susan]

Those of us that know are not believed by those that don't understand. The fact is that once you are free of the ground the wind makes no difference. Just watch out for the illusion.

I understand your frustration but please don't give up on us. You just touched a sensitive spot. ...Clyde Davis

[Rob]

I guess I just want to know if I am lucky or not.

...one word answer? No.

Lippy

Please do not take this as an attack, as it is not.... but... if you have to ask then the only proper response is YES

Those of us that believe, will generally do OK, as we are aware of what can happen. Those that do not believe, still fly with airspeed as a buffer, so they are generally equally covered... The "fence sitters" flying safe "most of the time" are the ones likely to fall out of the sky when they just happen to push that last little bit.

Lots more than one word... but probably the real answer to your question

Take care, Rob

[Dave Calkins]

Wannabee, wow. Rob probably gave the answer you wanted. Guys, I agree we've been through this, but Yikes!, a bit rough on the guy.

A question for Rob:

Those of us that believe, will generally do OK, as we are aware of what can happen. Those that do not believe, still fly with airspeed as a buffer, so they are generally equally covered...

Rob, I'm not quite sure which camp I am in if it's about believing or not. However, I think you would put me in the "non-believers", except that I am feeling this stuff and not using airspeed as a buffer, but I have an apt and sufficient explanation, that you guys happen to not buy.

I think that my description of the scenario is sufficient, but I do not think that having a "good" (I think it's good) description removes me from the crowd who fly by my seat.

Why did you say it like that? No offense taken, by the way.

[Wannabe Cubdriver]

I'd like to thank Lippy, Rob, Clyde and Dave for their input.

I guess I will continue to fly as I have been, not pushing the envelope. I just thought that when at circuit level my envelope may have been substantially decreased in these conditions.

I do not believe that there is no science to prove this as fact, touchy subject as it might be, magic is not the reason an airplane flies (or stops flying). Like I said, 10 pages of disagreement with ideas to support both camps does not prove either. I am not here to stir the pot, simply to try and keep my carcass from needing saving 'cuz I was just hanging there, fat, dumb and happy.

Thank you for the positive input, and as for the rest, thank you for your remarks. There are also two camps with regards to pilot stereotypes, and both of these have became evident here.

[Clyde and Susan]

The absolute best explanation is the one posted by AAlexander Repeated below. ...Clyde Davis

"aalexander



Joined: 10 Jun 2005
Posts: 534
Location: Anchorage
Posted: Sat Aug 30, 2008 6:54 pm Post subject:

--------------------------------------------------------------------------------

Coming late to the party here, I've been in Tampa in simulator training I'm trading in the DC-6 for the Herc. Anyway, against my better judgment I'll weigh in here.

First off, I don't take the position that there isn't "bad air" particularly down low. No doubt that with convection, wind gradients, obstacle effects on the wind, and a myriad of other things, some days are better for flying on the ragged edge than others.

That said, it appears from the reading I've done (and I admit that haven't read every post on all 10 or so pages of the 2 threads) that there are folks who believe that at say, 10,000 ft, in a steady, uniform, consistent wind, with no shear, or vertical air motion, you can turn an airplane "downwind" and stall the airplane in a turn which wouldn't stall the airplane if that same turn was performed in still, calm air.

The fallacy comes because it *seems* like the inertia of the airplane will resist the acceleration required to maintain airspeed in the downwind turn, and that the airplane requires more acceleration to maintain airspeed in the downwind turn. This fallacy is seductive, because on the face of it is *seems* so *right* and I can certainly see the appeal of that line of thinking. The trouble is, it is just a fallacy, the aircraft turning "downwind" accelerates at the identical rate as the airplane turning at the same turn rate in still air. Let’s take all the "seems like", "common sense tells you" and other fuzzy thinking out of the equation and analyze what is actually happening, using numbers. It’s not hard to do.


Acceleration is defined as change in velocity per unit of time. So what is the required acceleration?

Take an airplane flying first directly north at 50 knots, then turning at standard rate, constant altitude and airspeed turn 180 degrees to directly south..

We’ll consider only north/south winds, so we need only consider north/south acceleration. East west acceleration becomes irrelevant.

So, what is the acceleration when this is done in still air? Immediately before the turn, the velocity is 50 knots north, and immediately after the turn the velocity is 50 knots south, so the velocity change is 100 knots (168.8 ft/sec). The change takes place in exactly one minute, so the average north-south acceleration during the turn is 100 knots/minute , or 2.81 ft/sec./sec


Ok, now what happens when we make the same turn in a wind? Lets say we have a 25 knot wind out of the north. Now, I’ll consider the groundspeed here, because that is what causes the erroneous perception. In reality, it’s only the airspeed that matters, but the results won’t change if you consider the groundspeed correctly.

What is the groundspeed before the turn? 25 knots north
After the turn? 75 knots south
Net change in goundspeed? 100 knots (The velocity difference between 25 knots one way and 75 knots the opposite direction is 100 knots or 168.8 ft/sec)

Time to turn 180 degrees in a standard rate turn? 1 minute.

Average acceleration ? 100 knots per minutes or 2.81 ft/sec/sec. The average acceleration through the turn, after you add a wind, is identical, right out to however many decimal places you want to carry it out to.

OK, how about a really big wind, surely if we use enough wind the airplane will have to accelerate faster to keep flying speed right? Well let’s take a look. Let’s say we had a 200 knot wind out of the north.

Groundspeed before the turn 150 knots south (50 knots in a 200 knot head wind).
Groundspeed after the turn 250 knots south. (50 knots plus a 200 knot tail wind)
Change in groundspeed 100 knots (168.8 ft/sec)
Time to turn 180 degrees in a standard rate turn? Still 1 minute.

Average acceleration needed to maintain 50 kt. airspeed? Still 100 knots in one minute, or 2.81 ft/sec/sec.

I know what some of you are thinking. You’re saying, well a standard rate turn isn’t a much of a turn, it’s such a gradual turn that the airplane has enough time to accelerate downwind, you need a faster turn to get the "wind blowing backward over your wing causing you to fall out of the sky" effect.

OK, let’s use a faster turn, see how that changes things. Lets use a 45 degree bank. Lets say that our airplane can bank 45 degrees at 50 knots without stalling, and that it has sufficient power to circle indefinitely at 45 degrees bank and 50 knots in a level turn without loosing airspeed. And lets further say that 50 kt is hte slowest this airplane can fly in a 45 degree bank, 49.5 knots and it falls out of the sky. 45 degrees bank at 50 knots will give a turn rate of approximately 22 degrees per second (from the chart on pg. 179 of Aerodynamics for Naval Aviators) That means that the 180 degree turn will take about 8.2 seconds.

So what is the acceleration in still air?

50 knots north to 50 knots south is still a change of 100 knots. The 180 degree turn now takes 8.2 seconds, so the average acceleration needed is 100 knots/8.2 seconds, or 20.6 ft/sec/sec.

Now, let’s see what the acceleration is in the dreaded downwind turn. Let’s skip right to a really big wind and not piddle around with insignificant 25 knot winds. Let’s use the same 200 knot north wind we used before with a standard rate turn.

Groundspeed before the turn? 150 knots south
Groundspeed after the turn? Must be 250 knots south, 249.4 knots across the ground (49.5 kt. airspeed) and the airplane stalls
Total velocity change required. 100 knots (168.8 ft/sec)
Time for velocity change 8.2 seconds
Average acceleration needed to maintain 50 kt airspeed? 168.8/8.2 = 20.6 ft/sec/sec

Huh, turns out even in a really steep bank, really low airspeed, high turn rate and ridiculously huge wind, the acceleration needed to maintain airspeed in the turn with a 200 kt. wind is identical to the same turn in still air.

And that is where the rubber meets the road. All the downwind turn theories depend on "inertia causing the airplane to not accelerate fast enough to maintain airspeed" with the fundamental flawed belief that the acceleration *must* be greater if you turn downwind or you lose airspeed.

However, when you actually analyze what the required acceleration is, by taking the velocity before, the velocity after, and the time of the turn, (change in velocity divided by time is the only valid way you can consider acceleration, because that is the definition of acceleration) we find that the wind makes absolutely no difference in the acceleration required through the turn in order to come out the other side with 50 knots of airspeed.


Now some of you are probably thinking, yes but that is the *average* acceleration, the acceleration varies during the turn. Well, yes, the acceleration in hte north/south direction does vary, but just like the *average* acceleration remains identical out the nth decimal place, regardless of the wind, so does the acceleration at any particular point during the turn. Think about it, the acceleration during the turn changes significantly, but the *average* acceleration remains the same? Not likely. For those of you who are unswayed by the unlikeness of that, we can take a more analytical look at the situation.

Yes, it is true that the acceleration during the turn changes, more specifically, the acceleration in the North/South direction (which is what we’re interested in) changes, the total acceleration during a constant turn remains the same. The north south component of the acceleration will be greatest at the 90 degree point of the turn and the least at the 0 degree and 180 degree points in the turn. So lets look at the point where the acceleration is the greatest, that’s where we’ll fall out of the sky right?

First off, what is the lateral (across the ground) acceleration? Well, it we stick with the 45 degree bank, the lateral acceleration is conveniently, 1 g , or 32.2 ft/sec/sec. I see some of you saying, no, a 45 degree turn is more than 1 g, well yes and no. It gives a "load factor factor" of 1.414 but that’s 1 g of gravity vertically and 1 g of horizontal acceleration, add Pythagorean theorem and that gives you 1.414 g aligned with your butt in the seat. So, 1 g lateral acceleration for a 45 degree banked turn. Now in a zero wind situation, what is our groundspeed (or airspeed, no difference if there’s no wind) at the exact 90 degree point in the turn? Zero, right? That’s the precise point when our groundspeed (and airspeed) is reversing from just a little in the north direction to just a little in the south direction. And right at the reversal, north/south groundspeed (and airspeed) is zero. So let’s examine that one second bracketing the precise 90 degree point. One half second after the reversal, the north/south groundspeed is 16.1 ft sec (9.5 knots) to the south, Acceleration is 32.2 ft/sec/sec, and we’ve accelerated at that rate for 1 half second from zero north/south groundspeed, (32.2 * 0.5 = 16.1 ft/sec). Similarly, at one half second before the 90 degree point the north/south groundspeed (and airspeed) is 9.5 knots in the north direction. So in that second the groundspeed and airspeed changes from 16.1 ft/sec (9.5 kt.) north to 16.1 ft/sec (9.5 kt) south for a total velocity change of 32.2 ft/sec in one second which is an acceleration of 32.2 ft/sec/sec or one g.

Well what if we throw a wind in there? Again, we’ll use the 200 knot wind because that ought to magnify any small effects. So, in a 200 kt wind out of the north, what is the airplane’s north/south groundspeed at the exact 90 degree point? As we saw in the still wind case the north south airspeed goes to zero, so the groundspeed must be 200 kt. (337.6 ft/sec) to the south. A half second before the 90 degree point, groundspeed is 190.5 kts. (321.5 ft/sec) south (200 knots wind to the south plus 9.5 knots airspeed to the north = 190.5 knots to the south) one half second after the 90 degree point the groundspeed is 209.5 kt (353.6 ft/sec) to the south (200 knots wind to the south plus 9.5 knots airspeed to the south = 209.5). So, over that one second centered on the 90 degree point in the turn the north/south groundspeed goes from 190.5 knots south to 209.5 knots south, a change of 19 knots (32 ft/second) in one second when is 32 ft/second/second, which is the same acceleration we had in the no wind condition.


I could go on, and do the same analysis for the 45 and 135 degree points for the turn, or any other point, but the math will start to get complicated, and the answer will always be the same. A constant, unaccelerated wind will not change the acceleration of the plane in the turn, one iota, nor will it change the acceleration required to maintain airspeed as you turn down wind.

Last edited by aalexander on Sun Aug 31, 2008 5:28 pm; edited 1 time in total "

[Dave Calkins]

As I said in the other thread: What does a DC-6 pilot know about low, slow, heavy, and underpowered?

Wannabee, don't give up on those guys that were hard on you.

[akbhm]

Wannabe,

Most discussion critiques given so far to your question ignored your note that winds were high, with serious gusts present. I suspect you are asking really about wind shear or rotors, which everyone knows we cannot see, but we sure can feel the effects when we get into one.

Any type of turbulence should make you cautious.

Every time I can feel the shear effects in the seat of my pants, I exercise extreme caution. Like the time landing on Iliamna Lake at Pedro Bay, when at full speed of about 120 knots, an attempt to turn downwind resulted in about an 800 foot loss in altitude, with near total control loss, in spite of full throttle response. Lucky for me I started at 1200 feet, with sufficient altitude to recover.

Or the time in my cub going to Kenai to land. I saw a C130 on a long final to land (but did not know he had been doing touch and goes in an extended pattern). So I proceeded merrily in on the base leg, only to suddenly find myself in the wake turbulence he had generated several miles from the airport. Again, luck was on my side, with total control loss and several seconds of unusual attitudes, when I exited the rotor without damage.

Up here in Alaska we have plenty of mountains and lots of huge lows coming in from the gulf, so we routinely see the effects of severe wind shear and rotors when flying close to the mountains. Afternoon winds always seem to be generated during the summers as well, which make for some merry rides and updrafts. That is why most mountain pilots fly in the early morning only, forgoing the turbulence, wind gusts, and shear that develop later in the day.

My advice is to exercise real caution when flying in unstable weather conditions, especially when in the vicinity of terrain elevation changes.

Fly early in the day and also remember to avoid all the towering Q's!

Good luck and have a ball flying! I hope you excel and are able to stimulate many others to get into aviation.

We need all the new aviators we can find nowadays, since so many of us are now gray hairs.

[gulfstream]

Your instructor was/is exactly correct.

Regards,

Bill

[JimC]

.......So, I am to believe that this is a chicken - egg deal, nobody knows, and only best guesses count.

You're wrong for three reasons. Most folks who deal with basic computational aerodynamics know quite well what's happening and aren't guessing. Further thoughts.....

1) best guesses will bust your butt when they aren't in accordance with physics.
2) When doing a climbing downwind turn at a low enough altitude to be within the HAG related windshear, your airspeed will deteriorate as you turn downwind (until you get high enough to clear the shear).
3 When doing a constant HAG downwind turn, your airspeed will not deteriorate -- BUT
4) The pyschological impression of increasing speed as you turn downwind (due to increasing groundspeed) may result in your making control inputs that will reduce your airspeed -- which will bust your butt.

In a nutshell, for both perceptional and physical reasons it might be safer for you to avoid low level downwind turns when a shear is present, and it might be best for perceptional reasons to avoid them even when a shear is not present.
JimC

[12 Geezer]

Jim, what does "HAG" mean?

[GeorgeMandes]

I am another person late to this party. Based on best practice for flying a helicopter in the mountains, as taught by Canadian Helicopters, and adapted to my fixed wing mountain flying, I am very cautious with downwind turns at low altitude with strong winds. The reason has nothing to do with a possible increase in stall speed downwind, and everything to do with where you are going to park the machine when the engine quits.

I will happily fly along in the helo at 100 agl (in the right terrain) with a strong wind on my nose, because if the engine quits, I am landing straight ahead. With that hypothetical 20 knot wind on my tail, I may want to be at least 500 agl, because I need enough altitude to turn back into the wind for an engine out landing. The same goes for a fixed wing. As a result, I factor in wind velocity and adjust the altitude that I will turn out of the wind up as the velocity increases.

George

[S2D]

Jim, what does "HAG" mean?

Height Above Ground ?????

[StewartB]

I am another person late to this party. Based on best practice for flying a helicopter in the mountains, as taught by Canadian Helicopters, and adapted to my fixed wing mountain flying, I am very cautious with downwind turns at low altitude with strong winds. The reason has nothing to do with a possible increase in stall speed downwind, and everything to do with where you are going to park the machine when the engine quits.

I will happily fly along in the helo at 100 agl (in the right terrain) with a strong wind on my nose, because if the engine quits, I am landing straight ahead. With that hypothetical 20 knot wind on my tail, I may want to be at least 500 agl, because I need enough altitude to turn back into the wind for an engine out landing. The same goes for a fixed wing. As a result, I factor in wind velocity and adjust the altitude that I will turn out of the wind up as the velocity increases.

George

This could be a whole new thread. Tom Wardleigh, a very well known and well respected instructor, used to drill me on 180*, power out, "return to runway" turns. It was great training. In the 180 at climb-out power and speed he'd pull the power to idle and we'd see how little altitude I could lose during a simulated emergency 180* turn. I can say without hesitation that on a relatively calm day if I experienced an engine out at a few hundred feet I'll make the turn. On a windy day, not a chance. In that discussion the "perception" issue is a valid one since flight is temporary. Time and space are very limited and the ground very real. I want my return to earth to be on my terms. Disclaimer: If you want to experiment with this maneuver, get a good instructor to go with you.

Stewart

[GeorgeMandes]

Unless there is a high rise or forest fire ahead of you, with a long runway behind you, I don't see the attractiveness of the 180 turn, especially with any significant wind. It takes one emergency, the engine failure, and adds a second emergency -- executing the low altitude, low airspeed, high AOA turn to land at a higher ground speed.

Simcom used to devote a fair amount of time to doing low level 180 degree turns back to the airport in their initial and recurrent training for the PC12. I was puzzled why they didn't just suggest people land straight ahead in a relatively slow landing aircraft like the Pilatus. Subsequently, a former Pilatus demo pilot and a PC12 owner parted out a PC12 in Montana killing both, practicing such a 180 degree turn.
http://www.ntsb.gov/ntsb/GenPDF.asp?...06FA126&rpt=fa

Rather than be spring loaded to turn back to the airport, my default is to land straight ahead into the wind -- especially in aircraft like helicopters, Cubs/Huskies and Skywagons.

[StewartB]

My decision would be based primarily on what's in front of me.

Stewart

[JimC]

HAG = height above ground

JimC

[bob turner]

This turned out to be an excellent thread. I am going to save Alexander's analysis.
I don't know what a "few hundred feet" are, but six hundred is very comfortable in a J3 with a total power failure, in almost any wind you can land downwind in. With today's traffic, I often get the airport to myself, and practice these a lot. At 400', things get exciting fast, and at "a few hundred feet" I will aim the 180 at what looks like the least hurtful surface, given the wind, and go to full flaps when I can make it. No way will I turn 180 degrees at "a few hundred feet" even in the J-3. I have to admit, in the last week, I have only done six such turn arounds, so I am not the world's expert. And I am doing them on an infinite airport with no trees or hills . . .
The one to watch is a catastrophic engine failure below 100' and at Vy. Ouch! Even worse with flaps extended.
Opinion.

[Broknbone]

I've been away from the site for a handful of days and I see that the original thread has fallen off. Just as well. I wanted to start fresh anyway. Since my last post, much to my wife's chagrin, I've have pondering, debating, and researching this issue pretty much continuously, taking only occasional breaks for silly things like work and sleep.

Aalexander, you are correct (don't gloat yet). I believe the rest of us are correct as well and that the two sides of the debate have been trying to compare "apples and oranges".

The key to understanding the debate is in understanding the concept of "inertial frame of reference". Aalexander, 12geezer, mvivion, Galileo, etc. all have a good grasp of physics within an "inertial frame of reference" and they are correct.

The rest of us have been trying to describe what happens in an "accelerating" or "non-inertial inertial frame of reference". I think the two sides have been trying to argue "apples against oranges".

I am suggesting that a plane sitting on the ground is stabilized in a different "inertial frame of reference" than the plane stabilized in a moving mass of air. The plane on the ground must transition through an "accelerating frame of reference" in order to stabilize in the "inertial frame of reference" of the moving mass of air. I think I can prove that it is during this "transition" that problems with flight can occur.

Before I go further, I am going to pause and refer everyone, who might still be interested in this debate, to the following URL: http://id.mind.net/~zona/mstm/physic...tialFrame.html

This site talks about the concepts of inertial & non-inertial frames of reference in layman's terms (without math) and uses a juggler on a bus instead of fish in bowl on a ship as an example.

I have made tentative plans to meet with the local physics professor on Saturday at the local java shop. I'll be ordering a double espresso. Then again, if the weather is good, I'll probably go flying. Screw physics.

[12 Geezer]

"I am suggesting that a plane sitting on the ground is stabilized in a different "inertial frame of reference" than the plane stabilized in a moving mass of air.

Pete, That is correct as far as it goes, although just because the frames of reference are different does not necessarily imply that stability in one precludes stability in the other. In fact, it's just the opposite, if both are inertial frames. In that case, stability in one inertial frame automatically means stability in any other frame that is simultaneously inertial (non-accelerated).

The plane on the ground must transition through an "accelerating frame of reference" in order to stabilize in the "inertial frame of reference" of the moving mass of air.

This is not correct. The airplane setting on the ground is stable in both frames of reference, because neither frame is accelerating. The airplane moving at a constant velocity with respect to either frame is also moving with constant velocity with respect to the other frame (not the same velocity - it's just that both are constant). It then follows that the airplane accelerating with respect to either frame experiences an identical acceleration in the other frame. This is the essence of the Galilean transformation: that the equations of motion for an event are exactly the same with respect to any inertial frame.

An accelerating frame occurs in gusting conditions. Here the body of air is accelerating (changing velocity), thus the frame of reference 'attached' to the body of air is also accelerating. Accelerating frames invoke inertial forces, sometimes called pseudo forces, to account for the changing motion of the frame.

You deserve congratulations for digging into this! Feel free to take what I've written for critique by your prof friend. Where your conversation with him (her?) may get muddied is in the case of turbulence. In turbulence the frame of the air is unpredictably accelerated with respect to the ground - so pseudo forces become part of the analysis.

[StewartB]

I learned that a sound scientific hypothesis should be able to be validated by experiment and the results should be repeatable. 12 Geezer, the last time I posed this question you didn't answer. I'd like to provide you another opportunity.

Let's say you take-off in a crosswind with the necessity of making a hard climbing turn and you know you'll begin the turn in ground effect and complete the turn out of ground effect. Would you prefer to turn upwind? Downwind? Or like many guys here have theorized, is there absolutely no difference? After all, this is the meat of the topic, right? Steady rate turns in steady state winds at 10,000'? Who cares!

Let's define the crosswind at something sporty but not crazy, like 20 mph. Straight ahead is not an option. Which way will you turn?

I had the opportunity to take off of an obstructed strip yesterday in about a 20mph crosswind. I turned into the wind even though the obstructions were equal in front and on both sides. I was not at max performance. I was at about 1.1 Vx over the obstacles and during the turn. I didn't even think about this topic until after I'd departed. I didn't need to. My actions were instinctive.

I'm looking forward to your answer.

Stewart

[Broknbone]

Gordon,

I wanted to test my idea with the professor before I posted more, but I'm too intrigued by the whole debate to sit still. I agree with everything you said for the most part accept, I'm not sure I am getting my idea across to you. I'll try to give you an idea of where I am going with this. I am going to steal some verbiage from the website I referenced.

"An inertial frame of reference has a constant velocity. That is, it is moving at a constant speed in a straight line, or it is standing still. Understand that when something is standing still, it has a constant velocity. Its velocity is constantly zero meters per second."

Situation 1: The plane sitting on the ground (or turning circles in a no wind situation) is in an inertial frame of reference with a constant velocity of zero. No net movement in any direction. Agree?

Situation 2: The plane turning circles in a 20 knot moving mass of air is in a in inertial frame of reference with a constant velocity of 20 knots. Agree?

I'm suggesting that the difference between situation 1 & 2 is 20 knots of kinetic energy. In order for plane #1 to join Plane #2, it must past through an accelerating frame of reference, gain 20 knots of kinetic energy, and stabilize in a new inertial frame of reference with a constant velocity of 20 knots.

I'll leave it at for the moment.

[Dave Calkins]

In response to StewartB:

Being one of the "many guys who have theorized here", I must interject and correct that I never said there was "absolutely no difference", as SB has construed my statement.

[StewartB]

Dave,

Don't be so sensitive. My comment wasn't directed at you. It was a summation of the comments by many. I defined my interests in this subject. I'm interested in the context of these small, light planes that we operate. My question is valid and you're the only one who answered it. Into the wind is what you said, right? I don't understand the hesitation of those who are adamant that there is no danger in a downwind turn. Like I said before in the other thread, its a simple question.

Stewart

[behindpropellers]

Stewart-

I would go into the wind also. If the motor quits at 100' and you are into the wind you just reduced your crash/land speed by 20MPH which would equal about 50% in a cub.

Tim

[StewartB]

Assume the motor won't quit. That condition has nothing to do with the problem we're discussing. Not that you're incorrect, because you aren't. Let's just focus the scope to a good running airplane in a max performance turn.

Stewart

[AKClimber]

If I may interject a bit.
I agree that there seems to be a bit of an apples and oranges comparison going on here.
Some of us are trying to approach it like a scientific inquiry and examine each variable at a time, whereas others throw all the variables in the equation all at once resulting in a different answer.
So in an attempt to establish an equal frame of reference, we need to start with the simplest scenario and then add variables one at a time which can then be debated.
1 - Can we all agree that once we are established in a flight in a mass of air, there is no aerodynamic difference between downwind and upwind turns if done at the same rate without reference to ground?
Ex. A boat on a step in a fast flowing river if kept just on step will stay on step no matter which direction it's turned, provided the boats water speed stays the same.
2 - Can we all agree that when flying close to the ground, the visual cues from the ground reference can be mistaken for air mass reference resulting in steeper turns when turning downwind and shallower ones upwind. This is especially true when trying to avoid obstructions.
Ex. One will have a tendency to drive a boat at a faster water speed going up river than downriver - mostly due to needing more time to avoid obstacles. This will result in sometimes coming off of step when going downriver, but hardly ever when going up river.

Once we can agree on the effect of the above variables, let us throw in a more contentious one:
3 - When taking off, the aircraft has to stabilize in the moving air mass and upwind turns are safer than downwind turns - this is similar to what the ag pilots describe when making climbing downwind/upwind turns.
Ex. To continue with the boat example, as having 2 dimensions might be easier to visualize - when a boat that is on step enters a fast flowing river from a slough, there will be some time/distance before the boat is stabilized in that current. That is if the moment you enter that fast flowing river and turn upstream, you will stay on step (initial increase in water speed), whereas you will fall off of step when turning downstream (initial increase in water speed).
In other words, if you throw a stick into a river, it will not instantaneously flow at the speed of the current, but will need to accelerate at some rate.

I think these are the most important variables that are necessary to establish as the foundation of this argument before the downwind ag-turn can be fully examined.

[mvivion]

I recommend that those posting on this thread read the posts on the Ultralight Crash thread to avoid redundancy.

Thanks,

MTV

[ag-pilot]

Stewart-

I would go into the wind also. If the motor quits at 100' and you are into the wind you just reduced your crash/land speed by 20MPH which would equal about 50% in a cub.

Tim

Tim actually you reduced your crash speed by 40 mph by turning into the 20 mph wind verses turning down wind.
Dave

[S2D]

http://www.youtube.com/watch?v=NzlG28B-R8Y