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Return to runway after engine failure

pierre smith said:
The airplane DOES in fact, know about the wind! Many of our guys have stalled and busted their tails during an abrupt turn to downwind with a loaded airplane!
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Sorry Pierre, but the laws of physics don't support you. If you are indicating 120kts on climbout, and make a turn upwind or downwind while still indicating 120kts, the aircraft hasn't a clue whether you've turned into or away from wind.

What is different is that your groundspeed can change dramatically in a strong wind, and as a result the sight picture for the pilot changes dramatically. This is why people stall and spin on downwind turns close to the ground... They "think" they are going faster all of a sudden, and try to slow down by raising the nose... Thereby lowering their indicated airspeed and increasing their angle of attack until the two meet at the stall point.

At 7,500' feet and a 3 minute turn, you won't feel or see any difference in airspeed or stall speed turning either into or downwind.
Do the same thing near the ground and it's a whole 'nuther story!
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The aircraft doesn't know whether it is at 7500' or sea level. Aircraft fly on indicated airspeed and stall on angle of attack. Everywhere.
 
Snowflake said:
Sorry Pierre, but the laws of physics don't support you. If you are indicating 120kts on climbout, and make a turn upwind or downwind while still indicating 120kts, the aircraft hasn't a clue whether you've turned into or away from wind....
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The statement is essentially correct, Rob, but in the circumstance of a severe wind change or turning away from a severe headwind, to maintain a constant IAS may well be the last thing you observe before hitting the ground.
 
I think we were taking it as read that all bets are off in the case of sudden wind shear. But if you're airborne and flying down the runway centerline with a strong crosswind, and make a turn downwind, maintaining your indicated airspeed will not require you to descend. If you're in climbing attitude, and maintaining your climb speed, you will keep climbing at the same rate. This is what people aren't getting. If the indicated airspeed doesn't change, it's like the plane is flying in still air. You just start moving faster over the ground.

Unless you stand the plane on one wingtip and haul a 6g turn, you can't get a sudden enough change to the relative wind moving over the aircraft. But if you did do that sudden 6g turn, you'd lose indicated airspeed due to the increased g, so again it's not the same situation.
 
return

An option that I haven't seen mentioned that would be particularly important inearly flight testing: If there are no other safe options and your plan is to return to the runway at x altitude, if there is a significant crosswind, at least let the airplane drift downwind. Now instead of a more than 180 degree turn, at some point the turn becomes a true 180. One of the problems with saying never to the turnback is that we have no way of knowing how many have done a sucessful turnback.
Also consider the airplane: a Champ or Taylorcraft the turnback in most cases would be a non event, not much different than a low performance glider. A Bonanza A36, absolutely nuts to even try it below at least 1500 feet agl.
A friend with a non RV homebuilt made a sucessful turnback where there were NO other options. This obviously did not go into any report or database.
 
downwind

Regarding Pierres post, I at least partially agree. But the issue is not a pure downwind turn but rather an issue of wind gradient. This can involve a shear that is very turbulent but frequently there is little or no turbulence. The example is a large western airport in the mountains where I have made hundreds of landings. When the winds aloft are from the southwest I have seen 50 knots or more tailwind on short final. Usually this diminishes around 2-3 hundred feet agl. I have seen it several times where a very strong tailwind existed below 100 feet and the surface winds were either calm or a few knots headwind.
Now lets look at the opposite. A takeoff in a slight wind into a rapidly increasing headwind will increase performance. A takeoff into a rapidly increasing tailwind will decrease performance. The airplane sinply cannot instantly "adjust" to a 50 know plus change in headwind/tailwind and during the adjustment period there is a change in performance.
If the wind is steady(no gradient) and the aircraft speed is stable, there is no difference in upwind/downwind turn.
 
johnny stick said:
I reason this because momentum is in relation to a fixed reference point on the earth, not to the wind speed or wind vector. Sometimes experience is more real than physics.
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Again, you're forgetting the fact that the aircraft doesn't know where the earth is or how it's moving relative to the earth, once it's in the air. Momentum relative to the earth is meaningless.

There is no difference in the momentum of a J3 Cub cruising in still air at 60mph indicated, vs. a J3 cub "hovering" in a 60mph headwind and indicating 60mph.
 
Consider the following experiment:

You, the observer, are in a hot air balloon, drifting with the wind. In the basket you feel no wind at all. It's dark, so you cannot see the ground, so you don't even know you're drifting. Now you watch an airplane takeoff. The pilot makes turns on instruments, because he can't see anything either. The airplane turns left, turns right, into the wind, with the wind, any which way. All of these turns look exactly the same to you - the same way they look when there's no wind. That's because the airplane doesn't "know" there's a wind.

If there's any wind shear - changes in direction or speed - then that's different.

BTW, the laws of physics - PROPERLY APPLIED - have, so far in history, usually given the right answer. When they give the wrong answer, the "law" is deemed incorrect and people look for a new, more correct law to replace it. e.g., we know Newton's laws of motion are incorrect. But at aircraft speeds they have always given the right answer to the best of our ability to measure. They've been around for a long time so I wouldn't bet against them.
 
I think a good scenario to help understand this is to assume you are hovering say in a constant 50knot wind, stationary over the ground.

At this point if you could instentaneously reverse your direction (180 deg turn), at the first instant you would still have 0 ground speed but a negative air speed relative to the air. At that first instant you would essentially stall.
It would take a little bit of time before your airspeed indicated 50knots again.

So I believe the rate of turn does make a difference on windy days.
 
Oh yeah?

Snowflake said:
Sorry Pierre, but the laws of physics don't support you. .
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...that may be but experience does!

I was a CFI when I started my ag Career and the first time that happened to me, I said, "WTF just happened?" so I asked my boss, a returning WWII C-47 driver from the Berlin Airlift and military CFI who had taught basic in AT-6's and ATP rated, what happened, because I barely saved my bacon with full power, staggering back to "on the step."

He said, "Throw the textbooks out the window and listen to me! Everything changes near the ground. What you just experienced has killed more guys than I can remember, because it ain't textbook. You've just learned a valuable lesson about winds and turns near the ground. If you've gotta turn downwind, heavy, do it veeery gently! The airplane's gotta 'catch up' with the wind."

You go with the laws of physics and I'll keep listening to the voice of experience. It has allowed me to survive 41 years of ag work, heavily loaded and near the ground....two very dicey conditions.

Best,
 
Niko said:
At this point if you could instentaneously reverse your direction (180 deg turn), at the first instant you would still have 0 ground speed but a negative air speed relative to the air. At that first instant you would essentially stall.
It would take a little bit of time before your airspeed indicated 50knots again.
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This is correct, but an unrealistic situation. To see why, consider flying at 50mph in still air. If you suddenly made a 180 degree turn, you'd be moving backwards through the air, right? Well, no. You'd still have a positive airspeed, but it would be lower by an amount commensurate with the amount of "g" necessary to make that 180 degree turn. Hovering in a 50mph headwind in a J3 cub, it's entirely possible to make a 180 degree turn without the indicated airspeed changing. Or your altitude. Or your power setting.

You can't be lulled into the thought that because you have a tailwind, that you're somehow being "pushed" by the wind, because that's never the case. Aircraft fly relative to the air mass they are operating in, not relative to the ground.

Pierre, you make a good point about being near the ground (You ag-pilots are a crazy lot... :). Within 1-2 wingspans of the ground, you do get a boost in lift from the effect of the ground being there. Lifting out of that ground effect layer, and climbing into a wind gradient, could certainly cause problems. But this thread is about the turnback manoeuver, which generally wouldn't be attempted below a few hundred feet, and well away from any ground effects.
 
We need to keep Pierre's experience in context. He spends a lot of time flying very close to the ground where there will be a wind gradient with altitude. The is a lot of "friction" between the wind and the ground, so there may be a significant change in wind speed in the lower 50 to 100 ft. The turns between dusting passes start off as climbing turns.

If he is doing a dusting pass into wind, the wind will increase during his climbing turn to reverse direction. This may cause a greater loss of airspeed than he would see if the wind was calm.

But, while Pierre's experience is quite valid, it doesn't say anything about the effect of wind during turns with a constant wind.
 
Kevin Horton said:
But, while Pierre's experience is quite valid, it doesn't say anything about the effect of wind during turns with a constant wind.
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Right, Kevin. Isn't it interesting that, between the statics problems in the torque wrench thread and the fluid dynamics problems in this thread, we often see this disparity between the intuitions of engineering scientists and pilot practitioners. We should all be humbled by the apparent fact that experienced pilots are often right about the real world (although sometimes wrong about the reasons) while experienced engineers are often right about the reasons (but sometimes weak in applying our scientific models to all the complex vagaries of the real world). We really have much to teach one another if we'd all just listen more.

--
Stephen
 
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perspective

I think Stephen is on to something.

Like I said earlier, I have experienced Piere's downwind turn phenomenum. And I have researched the physics and think I understand it.

Regardless, I have found that when the experience does not fit the "physics" invariably the "physics" is not complete; more often than not there are other effects the theoritical guys forgot. Let's just say that when theory does not fit experience, then the theory is usually incomplete. No matter what you believe in theory about downwind turns, believe in Pierre's experience, before it (the lack of the complete theory) kills you.
 
Thank you!

fatherson said:
). We really have much to teach one another if we'd all just listen more.

--
Stephen
Click to expand...

..Couldn't have said it better:)...thank you.

A little more gas on the fire here: Believe it or not, if there is a 10 knot wind blowing near the ground, there may well be 20 or more knots, within the next 20 feet up! My downwind passes, in the field, will INDICATE a much higher airspeed than my upwind passes!

In discussions about this seemingly unbelievable phenomenon with other graying, very experienced ag pilots, we still scratch our collective heads:cool:

We kinda, sorta concluded that the upper third of our rather large airplanes (8,000 lb. gross) and with 50' wingspans, have the upper part of the fuselage in the faster air, while the lower part and the pitot tube is in the slower air???

....and my 496/groundspeed, agrees!

Go figure.

Best,
 
pierre smith said:
A little more gas on the fire here: Believe it or not, if there is a 10 knot wind blowing near the ground, there may well be 20 or more knots, within the next 20 feet up! My downwind passes, in the field, will INDICATE a much higher airspeed than my upwind passes!
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If you're flying in the same air mass, with a 10 knot wind blowing near the ground, your indicated airspeed (all other things being equal) will be the same in both directions. If it's not, then you've either bumped the power setting or you're climbing or descending on each pass without knowing it.

I suspect that the real world effect here is that you're either consciously or subconsciously adding power on the upwind pass and removing it on the downwind pass, because the sight picture out the front window looks different in each direction... You're rocketing downwind, but slogging along upwind. You'd want the same speed each way for uniform spraying, wouldn't you?

We kinda, sorta concluded that the upper third of our rather large airplanes (8,000 lb. gross) and with 50' wingspans, have the upper part of the fuselage in the faster air, while the lower part and the pitot tube is in the slower air???
Click to expand...
It's a good theory, but it still requires that the air mass be somehow different when you're travelling upwind vs. downwind. If you're at the same altitude in each direction, and the wind is the same at that altitude, the airplane again doesn't know the difference.
 
johnny stick said:
Regardless, I have found that when the experience does not fit the "physics" invariably the "physics" is not complete; more often than not there are other effects the theoritical guys forgot. Let's just say that when theory does not fit experience, then the theory is usually incomplete. No matter what you believe in theory about downwind turns, believe in Pierre's experience, before it (the lack of the complete theory) kills you.
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In my experience it's been about 50/50 between the physics not being complete and the real world explanation not actually referring to the physics being discussed.

Ag pilots making U-turns at the ends of spraying runs is not the same situation as making an upwind turn vs. downwind after takeoff (which is what this thread is about). Correct me if i'm wrong, Pierre, but I understand that the U-turns you're making are sort of like one half of a lazy-8, right? You're decelerating up to half way through the turn, then diving back down and accelerating back up to spraying speed, right? If nothing else, this fits what I think i've seen every time i've seen a spray plane working. I can totally see how one could get too close to their stall speed while doing this, there's a lot of changes going on very rapidly both to the aircraft's configuration and the wind gradient above ground.

An aircraft making a turn up- or down-wind on takeoff is usually at a stabilized climb rate, and a stabilized airspeed. It's just climbing out and turning to the next heading. In that situation, there is no difference between an up- or down-wind turn. If the indicated airspeed is constant, your performance is the same either way. (to be fair, there will be a slight difference due to the fact that your pitot is on one side of the airplane or the other).

By extension, if you suffer an engine failure after takeoff and want to do a turnback, once you've stabilized your glide speed, turning up- or down-wind should probably be chosen more based on how close you want to stay to the airport, and tempered with what ground conditions exist below (houses, fields, etc.). Not because of tall tales about downwind turns.
 
Last word

Rob I agree with you. I am still learning, but I believe I do not know enough to attempt a downwind turn. I would rather crash in an upright position near stall away from the airport, then face first in a stall recovery on the runway's edge. JMO. Thanks for expressing your opinion, everyone's count.
 
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johnny stick said:
The physics I learnt at UC Berkeley says a plane traveling down wind has more momentum than one traveling upwind at the same airspeed (relative to a fixed reference point, earth).
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Strictly speaking, Johnny, what you wrote above is correct, as momentum is mass * velocity, and velocity can be defined as the rate of change of the position of your aircraft versus a fixed point on earth. The problem with your reasoning is that in aviation we call your definition of velocity by the alternate name, ground speed. And ground speed has nothing to do with how the airplane flies through a moving mass of air. Since your terms are imprecise, it's not entirely clear to me what you are arguing, but I'm trying hard to follow you. Can you help me understand why you think momentum (relative to the earth) should have some influence on how your airplane flies?

Edit: and to be clear, when I say "ground speed has nothing to do with how the airplane flies through a moving mass of air", I mean at a high enough altitude, like say 500 ft, where the air mass is moving almost entirely independent of its proximity to earth, not at the kinds of altitudes where Pierre works, where various ground effects influence both the wing and the wind much more aggressively.

--
Stephen
 
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The motor and the mind on the turnback

I am talking about the body, mind interaction of the turn back upon engine failure. A little background on my call sign --Go to Col.Grossmans aritical
On Sheep, Wolves and Sheepdogs then you will know my mindset. I have over 30 yrs. in Law Enforcement and am Currently a Firearms Instructor in Handgun, Shotgun and Patrol Rifle, and have been thru the FBI sniper program plus 10 yrs on a special operations team as one of the team leaders. I built my RV 6 over a period of years and do pay attention to the wisdom of my flight instructor
Probably wondering what this has to do with the turnback. I can teach anyone how to pass the FBI handgun course, We run training to be as realistic as possible with active shooter courses and many other courses ment to raise the body and mind interactions to provide as much stress as possible as to give that officer a card in the rolodex in that persons mind how to react for that situation. Same as providing engine out procedures and other emergencies in the air environment.
But there is a big difference in practicing and the real emergencies.
During a real emergency the body go's into a fight or flight,( no pun intended)
mind, body reaction. During this time blood rushes from the extremities to the core. That means the dexterity in the hands and arms are like handling clubs instead of being able to thread a needle. Also you experience tunnel vision and you have exceptional strength your Heart rate will increase to over 200 bpm and your breathing rate will be off the charts. In an RV the combination of ham hands, and extra strength may be a BAD THING.
The reason I bring the COP Training thing up is in a REAL EMERGENCY like a GUNFIGHT trained police officers who were in a gunfight could only HIT THE GUY TRYING TO KILL HIM between 20 and 25 percent of the time from a distance INSIDE OF 10 feet, these are just hits not incapacitating hits. These same officers under the stress of training I know could hit the paper training target every time in record time 100 percent of the time from 10 feet. I know I just came back from a new acadamy class that have very little training and no street experience and from 5 yards they could perform at 100 percent in training
You can train for this turnback over and maybe knowing the engine is still running providing a ho hummer turnback and landing down wind hopefully with out pranging your bird. But in a REAL emergency I know you will not perform to your expectations unless you Train in Real Emergencies. It takes
at least 2000 attempts before your mind and body can react as one then it takes real encounters with a real emergency to begin to slow your breathing and heartrate down to be able to react to the problems at hand.
Please watch the low and slow videos with the bad outcomes that had power and listen to the people who on these posts are trying to save your families from bad decisions you make. Oh by the way my minimum for turning back is pattern altitude and I will fly the pattern otherwise it will be straight ahead and remember fly the airplane and BREATH IT UP is probably more important to bring calm to you and lower your heartrate.
Please if anyone wants to explore this more contact me
Ken Asbe SHEEPDOG
 
BTW, Johnny, it's only the physics I'm trying to clarify up above. I hope my tone comes across as sufficiently civil and contrite. I'm trying to reach a clear understanding of what's going on in the air.

But we already agree completely on this point:

johnny stick said:
I am still learning, but I believe I do not know enough to attempt a downwind turn. I would rather crash in an upright position near stall away from the airport, then face first in a stall recovery on the runway's edge.
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Me, too. Pierre already has many more continuous years in the air than I will ever hope to have. I will never have his experience with low speed, low altitude maneuvering. I won't be attempting a turnback if my engine ever fails below pattern altitude because I'd rather survive a glancing blow with the earth ahead of me than a head on blow with the earth below me.

--
Stephen
 
Okay, if you want a physics explanation, here it is:

One of the important principles of physics has to do with "inertial reference frames"; that is, a point of view from an observer who is not being accelerated.
The principle is that the laws of physics are the same in all inertial reference frames - there is no one (say, on the ground) reference that is any more correct than another (say, floating in a balloon). They all have to give the same observable answers, if you calculate properly. One of the tricks of the trade is to choose a reference frame where the calculation is easy. To be concrete, suppose there is a 20 kt wind out of the north. Since we are going to want our observer in the balloon to be unaccelerated, THE WIND MUST BE STEADY for this approach to work. If I choose my frame of reference to be the ground, the calculation is hard. I have to include the effect of the wind on the airplane. But if I choose my reference frame to be a guy in a balloon (who is moving south at 20 kts, as seen from the ground) the calculation is easy, because this guy observes no wind, so the airplane flies, as seen by him, exactly as in does on no-wind days. Of course this observer also thinks the ground below him is moving north at 20 kts, so the airplane's path over the ground is different than on no-wind days. If the pilot maintains 1.3 Vso as seen by this observer (and note, this is also the true airspeed that would be measured onboard the aircraft) and makes a 30 degree bank, the airplane will not stall. It does not matter if the guy on the ground observes the plane turning cross wind, downwind, upwind, whatever. The guy in the balloon - the "easy to calculate" frame of reference - sees all these turns as exactly the same.
Pilots get into trouble only when they see the ground! e.g., their turn radius, over the ground, doesn't look normal (due to the wind drift) so they increase the bank angle.
Finally, we all agree that below the tree line, or a hill, or a barn or a hangar, the wind can change very abruptly. If the observer in the balloon were down there, he'd be accelerated all over the place so he would not be a valid inertial frame of reference.
 
Agreed

Bob, I agree completely with your explaination concerning inertial frames of reference. But I have also have experienced Pierre's phenomenum, so I conclude the downwind turn is a complex physics problem. A shout out to my friends up there. My apoligies to Stephen, I guess he was saying the same thing, but I just did not hear.
 
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A thought experiment

Imagine that you are flying along at 100' above the runway, at 65 knots IAS. There is a 65 Knot headwind. So your groundspeed would be zero.

(I have seen this in a Cub, with an approx 35 kt wind. In fact I have actually had a negative groundspeed in a Cub. Backing up over the ground at a few knots while the IAS was around 30.) But I digress........

Now, imagine that SUDDENLY, in fact, instantaneously, the wind stops completely. (OK, in real life, probably wouldn't happen, but this is a thought experiment.)

Your aircraft would suddenly see zero airspeed. (Airplanes don't fly particularly well at zero IAS.) It would take a finite amount of time, (and altitude), to regain your 65 knot indicated airspeed.

That is due to the inertia of the airframe. The more massive (heavier) the airframe, the greater the inertia. The point is, the more rapid the change in apparent wind, the more likely you will see it on the airspeed indicator. So if the wind SUDDENLY changes, or if you SUDDENLY turn downwind, it will show up on the airspeed indicator, which indicates indicated airspeed. :p

That's why the Big Boeing's can "see" a "+20" or a "-20" on short final. There is often dramatic wind shear near the ground, and it takes TIME to change airspeed.

I'm sure a physicist could come up with a formula to predict this behavior, and I am fairly certain that formula would include not only mass, but also a time factor.

Boiling it all down, Pierre has it right, IMHO.

Disclaimers: One guy's opinion. YMMV. I could be wrong, it actually has happened before. $.02 doesn't go very far these days, in fact, I think we should eliminate the penny. A little bit of humor intended. I may have had a glass or so of Red Wine, which may or may not have had an effect on my application of logic, or the lack therof. You get the drift.

Size (weight, mass) matters.:rolleyes:
 
uneven airmass

No physicist or ag pilot here, I do have some down low airshow experience, but during an acro routine, my rule was high if slow and fast if low.

Anyway - it occurs to me that the air mass an airplane is in is not a homogenous mass of air all moving at the same speed, especially within 50' or even 100' of the ground. Even at cruise altitude this is true - anywhere you feel even light turbulence, the air you are in is all moving at different speeds, and most likely different directions. Close to the ground I doubt there is any mass of air as large as the wing span of an airplane that is all moving at a constant speed or even in the same direction. Updrafts, downdrafts, eddys, whirpools, all from ground interference, solar heat, etc. I bet if you could see all the currents it would be a real mess, and perhaps that is where classroom physics meets the real world.

Just my very uninformed guess.:)
 
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PCHunt said:
Imagine that you are flying along at 100' above the runway, at 65 knots IAS. There is a 65 Knot headwind. So your groundspeed would be zero.
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Imagine the same situation with *no* headwind, and your groundspeed and indicated airspeed equal at 65kts.

Now, imagine that SUDDENLY, in fact, instantaneously, the wind stops completely. (OK, in real life, probably wouldn't happen, but this is a thought experiment.)
Click to expand...
Understood. Imagine that suddenly, in my situation above, that you could suddenly stop your Cub in mid-air.

Your aircraft would suddenly see zero airspeed. (Airplanes don't fly particularly well at zero IAS.) It would take a finite amount of time, (and altitude), to regain your 65 knot indicated airspeed.
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Agreed. In either case, if you stop first, then you need to get started again. Basic physics! :)

That is due to the inertia of the airframe. The more massive (heavier) the airframe, the greater the inertia. The point is, the more rapid the change in apparent wind, the more likely you will see it on the airspeed indicator. So if the wind SUDDENLY changes, or if you SUDDENLY turn downwind, it will show up on the airspeed indicator, which indicates indicated airspeed. :p
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I have no argument with any of this. If you change the conditions, the conditions will change. Yes! But if you *don't* change the conditions, ie. if you maintain your indicated airspeed, the conditions do not change!

This whole debate reminds me of th downwind-faster-than-the-wind debate, which will also never end... :p
 
Sheepdog hit the nail on the head.

You can have a firm understanding of the phisics of the situation but when the chips are down, the brain may fail to function adquately.

Thats it is worth considering KISS and land straight ahead.
 
pierre smith said:
..Couldn't have said it better:)...thank you.

A little more gas on the fire here: Believe it or not, if there is a 10 knot wind blowing near the ground, there may well be 20 or more knots, within the next 20 feet up! My downwind passes, in the field, will INDICATE a much higher airspeed than my upwind passes!

In discussions about this seemingly unbelievable phenomenon with other graying, very experienced ag pilots, we still scratch our collective heads:cool:

We kinda, sorta concluded that the upper third of our rather large airplanes (8,000 lb. gross) and with 50' wingspans, have the upper part of the fuselage in the faster air, while the lower part and the pitot tube is in the slower air???

....and my 496/groundspeed, agrees!

Go figure.

Best,
Click to expand...


This is explainable, in that you may have actually been doing a bit of "dynamic soaring" in the wind gradient you describe. It's similar to how the wandering albatross exploits the energy that can be extracted from the wind gradient - the difference in wind speed caused by frictional slowing of the wind very near the surface, compared to the higher wind speed a bit higher up over the ocean. Your maneuvers in Ag flying, using your example, where your upwind pass is close to the ground in the slower air moving 10mph caused by the wind gradient, followed by your zoom climb while still going upwind into the faster moving 20mph air up higher, followed by your 180 deg. turn, and then a descent while traveling downwind back into slower moving 10mph air near the surface, are a close approximation to part of what the Albatross does to soar thousands of miles over the ocean without flapping at all. The bird gains energy when climbing
into the faster moving air while going upwind in the zoom climb, and again gains energy again in the dive when going downwind and entering the slower moving air near the ground. You also gained some energy in these maneuvers. Of course the energy that can be extracted from the 10 mph difference in the wind speed you described is not going to enable your heavily loaded Ag plane to "Soar", but you did actually gain some airspeed, which is because you gained some "total energy". This gain in total energy can be taken or traded off by us in different ways. When climbing into a wind gradient on take-off going upwind, most would take it as a better than normal climb rate. Of course it only lasts as long as while you're actually transitioning through the layer where the wind speed is changing. Once you reach the altitude layer where the wind speed is constant again, the effect is gone, although you still have the extra energy you
gained in the form of some extra altitude you wouldn't otherwise have at that instant. When flying into a the slower moving air while diving towards the ground going downwind, you tend to take the gain in energy as an increase in airspeed, which is what you noticed. This gain in airspeed in this case is not related to being in ground effect, it is related to the gain in energy from transitioning through the wind gradient from the faster air up high to the slower moving air down low as you descended through it at the beginning of your downwind pass.

Radio control glider pilots have become very
adept at doing this type of dynamic soaring. They make loops diving in an out of the slow moving air in the lee of a ridge going downwind, and zoom up into going upwind into the faster air on the upwind side of the ridge. With each loop, they gain more energy and have
gotten their gliders over 400mph in recent years doing this. Full scale glider
pilots mainly only dream of being able to soar like this, although a few have
claimed to do similar maneuvers to the albatross across a wind gradient found at higher altitudes, such as near temperature inversions, which can provide
wind gradients at altitudes that are safer in which to operate full scale gliders. Ingo Renner, former world soaring champion was one of the first to
write about a soaring flight using dynamic soaring.

The only problem with Ag flying is that on the other end of the field, where
you're going downwind near the surface, and do a zoom climb up into the faster air while going downwind, followed by a 180 deg turn and a dive back down to the surface going upwind, you actually are going to lose some energy in those maneuvers. The Albatross doesn't do that. After diving going downwind near the surface, he makes a 3g zooming 180 deg turn into the wind but then
immediately turns downwind again and dives right away back towards the surface going downwind, instead of diving straight into the
wind like you would have to do spraying crops. So you're gaining energy 1/2 the time, and losing energy the other 1/2 of the time in a wind gradient like you described. I think if Steve Smith our resident aerodynamicist was reading this thread, he'd probably agree with what I'm trying to say.

http://en.wikipedia.org/wiki/Dynamic_soaring

http://esoaring.com/albatros_presentation_esa.pdf

http://www.youtube.com/watch%3Fv%3DOix6sHKzOLU&sa=U&ei=LS9NT_L5OamfiALE-b3BDw&ved=0CBsQtwIwAw&usg=AFQjCNEz6FFQ_esbeN5PIWrnPIWWRbU7Sw
 
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Could be.

I think you're onto something, Alex.

I didn't change power but the airplane is usually so much faster, downwind, than upwind...by INDICATED airspeed, it's crazy! Obviously, groundspeed would be greater downwind but I've tried shallow turns without much climbing to avoid the high downwind speeds because it messes up our chemical calibration....we put out less chemical on the downwind pass because of the added airspeed, and more on the upwind passes.

None of this happens higher up...even at 300' ferrying but, man, does it manifest itself near the ground!

We usually try to spray in a crosswind, which helps drift our spray downwind and we don't have to breathe those fumes and the airplane stays cleaner as well. Field layout doesn't always make this feasible though.

Thanks,
 
here is how this goes.....

Joe RVator is flying slowly, and quietly up stream, at 100 mph into a 20 mph headwind, looking at the wildlife in River.... When he spots some wildlife participating in a "natural activity...."

He hauls back in a climbing turn... the airplane slows, and the headwind turns to a crosswind, while his airspeed has dropped to 80 his ground speed is nearly the same at 80 mph...

He is focused on the "activity" on the ground, but his perception of speed is still that he is plenty fast.

The nose starts to fall thru as the airplane picks up a tailwind, and it feels like it is accelerating... In reality it is starting to mush but the increasing tailwind provides a feeling of acceleration, and he keeps pulling back....

About the time he is headed back down stream with the full effect of the tailwind making his now 60 mph airspeed feel like his 80 mph groundspeed, which was exactly what it was when he was flying comfortably upstream, into the wind...

The wildlife has been distracted from their activity just in time to see the airplane's nose rise, wobble briefly, then snap over instantaneously and impact the ground, right across the river from them in a ball of dust and branches.....

This is not about the physics of a downwind turn, it is about the perception of a downwind turn, and whether it is an EFATO, or turning back to look at a pair of bare B--bs on the beach, the perception is real, and it is deadly.....

The perception is that as the airplane turns downwind, it accelerates, relative to the ground, and an pilot wanting to extract more performance from the airplane will sense that perceived acceleration and try to harvest that energy unconsciously....

It is as real as physics and not to be ignored.... But physics of the downwind turn remain, the airplane doesn't know.... As usual, it is the pilots where the trouble comes from.......

Tailwinds,
Doug Rozendaal
 
People are in two camps, they're either of the "Groundspeed = Airspeed" camp or the "Airplane only knows about airspeed so groundspeed always = airspeed plus the current head/tail wind component" camp. Both are wrong. The former camp are the ones who say "my gps says I'm doing 220 mph, therefore I have a 220 mph top speed" obviously they're idiots. The latter camp is basically repeating what they were taught in PPL groundschool, and are WAY less wrong that the first group, but still wrong.

Follow along:

In a uniform moving mass of perfectly still air, and airplane's airspeed is always equal to it's groundspeed. We all know that.

In a uniform mass of moving air (so, winds are constant with altitude and position over the ground, most of the time we fly in winds that are like this, we don't notice large gradients with small changes in altitude or position). In this mass of air an airplane with a constant airspeed and direction with have a ground speed of GS = TAS + TW (tailwind component, a headwind would be negative). Everybody would also (hopefully) agree with that.

Now, when you do physics equations, things like the kinetic energy of an airplane should be computed with respect to the ground, not the air. In other words, when you hit the ground in a crash, or use you brakes to slow you down after a landing, it's your ground speed that matters, not your airspeed. Your brakes must convert the energy associated with your ground speed into heat, your airspeed has nothing to do with it. This is why we land upwind, lower groundspeed means less work for the brakes (and slower groundloops :) ).

Anyway, to explain the missing link, lets imagine Pierre in his BIG, HEAVY airtractor, full of poison at 2000 AGL practicing spraying maneuvers at altitude for safety. He's in a large mass of air that is blowing right down the crop rows at a consistent 20kt. As he makes his UPwnwind pass at 100kt airspeed, his groundspeed is only 80 kt. When he reaches the end of the row (at a safe altitude) he puts his BIG HEAVY airplane in a 60 deg bank turn. What happens? As the airplane starts the turn, the headwind component starts to decrease, as he's directly crosswind, the wind changes from a headwind to a tailwind. When Pierre is facing perfectly upwind, his airspeed should be 100IAS and his groundspeed should now be 120IAS, right? Well...maybe. If Pierre left the power alone and made the turn, he'd have either lost altitude or airspeed due to the increased drag in a 2G 60deg turn. That would happen instill air, too though, so lets say Pierre knows how to compensate. Thing thing is, there's something else going on. In a quick enough upwind to downwind turn, the air doesn't have enough time to accelerate the airplane to what it's groundspeed SHOULD be. Remember, the kinetic energy state of the airplane is in reference to the ground. So before the turn, the airplane has an 80 kt groundspeed, a minute after the turn, it'll be 120kt. The difference in those speeds is 2X the windspeed, and that kinetic energy is supplied to the big airplane by the wind during and after the turn. For a short time after the turn, Pierre's groundspeed will be a little below what it's "supposed" to be. That means his airspeed will be a little low, too. I don't know how much. In an ultralight, the wind doens't have to do much work to get the airplane accelerated and the ultralight is draggy enough that the wind catches it pretty quickly. In a million pound C5, it will take longer for the wind to accelerate all that mass (especially given the lower drag coefficient).

People talk about the illusion of tight turns close to the ground in wind, and that' important, but there's also the effect that the wind has on the kinetic energy state of the airplane, that causes a short drop in airspeed as an airplane turns upwind to downwind. This may or may not be important in an RV, or even in a C5, assuming they aren't able to turn faster than the wind can accelerate the airplane, but it is an effect that's really there. Think about how long it'd take a 20kt wind to accelerate an 1800lb airplane to 20kt if the airplane was sitting on a frictionless frozen lake......that's what's going on here.
 
Thank you.

Very well said, Chuck. I'd just about "dusted my feet and left" as my Bible instructs, because what I've told here comes from 41 years of ag experience near the ground and I can't adequately, or reasonably, explain the phenomenon, but it's killed a bunch of ag pilots worldwide!

I'll make an open offer to anyone who can squeeze in beside me in my Air Tractor, for a firsthand experience on a windy day:)

DSC00223.jpg


Best,
 
I don't think so..

If I am following GTMULE's argument, he is saying that the airplane's kinetic energy is a function, in part, of groundspeed. I don't think so. My airplane does x knots TAS on y THP. It does this no matter what the wind is doing. When I measure that with my GPS I get three different ground speeds, then use the NTPS spreadsheet and compute my TAS. Meanwhile, my IAS has not changed, my engine's behavior has not changed, etc. It takes HP to get to a given state of kinetic energy. The energy he is talking about is actually potential energy.

Now let's assume that I have a 200 mph wind on the nose and I am flying at 1 ft AGL at 200.01 mph TAS. What's my kinetic energy? It depends on what I hit. A bird? A little shrub?

When I fire a rifle, say a .30-06, I momentarily create a projectile with a muzzle energy (kinetic energy) of about 3,000 foot-pounds if memory serves. If I fire it at a retreating target of sufficient speed, it will gently bump the target but if I fire it at a target that is stationary with respect to me, it will hit it pretty hard.

Not that it's relevant, but I just remembered a conversation with an air force pilot who was one of the first to shoot himself down (or at least damage the plane with his own bullets).
 
hevansrv7a said:
If I am following GTMULE's argument, he is saying that the airplane's kinetic energy is a function, in part, of groundspeed. I don't think so. My airplane does x knots TAS on y THP. It does this no matter what the wind is doing. When I measure that with my GPS I get three different ground speeds, then use the NTPS spreadsheet and compute my TAS. Meanwhile, my IAS has not changed, my engine's behavior has not changed, etc. It takes HP to get to a given state of kinetic energy. The energy he is talking about is actually potential energy.

Now let's assume that I have a 200 mph wind on the nose and I am flying at 1 ft AGL at 200.01 mph TAS. What's my kinetic energy? It depends on what I hit. A bird? A little shrub?

When I fire a rifle, say a .30-06, I momentarily create a projectile with a muzzle energy (kinetic energy) of about 3,000 foot-pounds if memory serves. If I fire it at a retreating target of sufficient speed, it will gently bump the target but if I fire it at a target that is stationary with respect to me, it will hit it pretty hard.

Not that it's relevant, but I just remembered a conversation with an air force pilot who was one of the first to shoot himself down (or at least damage the plane with his own bullets).
Click to expand...

The energy state of the airplane is releative to the ground. Here's a thought experiment....

What's the kinetic energy of your airplane when it's tied down on the ramp in a 20 kt headwind?

Make sense?
 
The main (some say only) reason why people get killed in downwind turns is that they are looking at the ground and not the instruments. It has nothing to do with kinetic energy of the wind, but everything to do with the perceived speed over the ground by the pilot and hence letting the plane get too slow.

Does a downwind turn at 10,000 ft AGL present any danger to the pilot that a turn in any other direction would? I sure don't hear about inadvertent stalls making turns high above the ground.

If you want to talk wind shear I'll listen. Otherwise, the speed of the wind the airplane is flying in is immaterial to the stall speed of the airplane.
 
Energy State

gtmule said:
The energy state of the airplane is releative to the ground. Here's a thought experiment....

What's the kinetic energy of your airplane when it's tied down on the ramp in a 20 kt headwind?

Make sense?
Click to expand...

Yes, that is my point. If an object carried by the wind strikes my airplane there will be energy evident. My energy state with respect to the ground is, of course zero. What if there is air but no ground? What if it is two object orbiting the earth at intersecting vectors? The correct answer to all of these thought experiments is that the energy state is dependent upon the relative motion of the two objects if/when they meet. However, to disregard the molecules of the air as objects ignores a physical truth.

If I make turns at 30k MSL or at 10 AGL, the ground is equally irrelevant until I "find" it. For all practical purposes, the airplane is stationary and the ground moves. Then the airplane is not accelerating upwind/downwind.
 
gtmule said the magic word. Acceleration. The airplane turning from upwund to downwind has to accelerate with respect to the inertial reference frame (more or less the ground). This does not happen instantly but seems too in small light airplanes. But the heavier the airplane and the faster you make the turn, the more pronounced the effect.

We now return to our physics free programming.

Now, where did I leave that torque wrench! :D

Whoops. forgot that a frame moving uniformly with respect to an inertial frame is itself an inertial frame. So that makes Newtons math work in an airmass based frame, which means airspeed stays constant, for a perfectly uniform moving airmass.
 
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I regress

GTMule, I was thinking like you until Bobturnner set me straight. Kinetic energy is not an absolute value unless it is described in terms of the speed of light. Where I earlier went wrong, and bobturner corrected me, was that kinetic energy is relative and can be traded for potential energy (zoom climb). i believe the downwind turn phenomum has to do with the pilot looking out the window, track along the ground, wind shear, etc and not kinetic energy change with respect to the ground. JMHO
 
As a bit of a aside to this argument one of the most missed simple questions by pilots is what will glide further. A aircraft operated at a very light weight or the same aircraft at maximum weight if all other variables are the same. I think most on this forum will get the answer right.

George
 
ref to what

Yes, kinetic energy ref to earth is near zero, but reference to the moving airmass is quite high. I can prove it, pull up and trade kinetic energy for potential energy can be done regardless of speed relative to ground. Hit the ground and you are safe, pull up and you will climb.
 
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The energy state of the airplane is releative to the ground.
Click to expand...

The airplane turning from upwund to downwind has to accelerate with respect to the inertial reference frame (more or less the ground).
Click to expand...

This student of physics remains unconvinced. :D

The inertial reference frame relative to a horizontal turn is the air mass. If the air is moving at a steady state velocity, it matters not one whit whether it is moving at 30mph (relative to the earth) or 1000mph (relative to the moon.)

I remain open to persuasion if someone can explain, in physics terms, why the earth's gravity affects the horizontal force vector required to turn the aircraft. :)
 
would Pierre's DGPS provide data to answer the turn in wind question?

Pierre, do you get groundspeed, position, time information a couple times a second? Got any data from when there was a cross wind?
 
Nope.

Negative, Wallace. I watch several different clues to determine wind...ponds are shiny and still, on the upwind side, cloud shadows moving as I ferry and of course, laundry on the clotheslines and the occasional flag and sometimes smoke.

The 496 shows groundspeed and also helps in determining winds at my altitude.
 
Ted Johns said:
This student of physics remains unconvinced. :D

The inertial reference frame relative to a horizontal turn is the air mass. If the air is moving at a steady state velocity, it matters not one whit whether it is moving at 30mph (relative to the earth) or 1000mph (relative to the moon.)

I remain open to persuasion if someone can explain, in physics terms, why the earth's gravity affects the horizontal force vector required to turn the aircraft. :)
Click to expand...

Quite right. After I woke up I realized my error. A frame moving along with the wind is perfectly correct to use, and the motion there for this case is just simple circular motion. constant speed wrt the airmass.

I think the problem with observing this experimentally is finding a smooth airmass and flying the airplane very precisely. Closest I've come is flying thermal turns in a glider relatively low in a strong wind. You cna also see it with a model by trimming up a nice steady turn then leaving the model to fly itself while it drifts downwind.

Another place to observe that the airplane doesnt care about the wind is the case of a sailplane flying in strong wave conditions. Its easy to find 100 kt winds aloft in wave. A nice smooth turn downwind is totally without airspeed variation, as it has to be unless you have shear.
 
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Go test it!!

On a windy day, climb to an altitude with 30 kts of wind or more.

Hopefully you have an AOA indicator....

Trim yourself into a level 60 deg AOB level turn.

Once you are stabilized and level in the turn

Observe AOA, A/S and GS as you translate in an out of the wind line. You'll have your answer.

There is no mystery that wing lift coefficient is a result of airfoil configuration and AOA. If you remain level throughout the turn and AOA remains the same then wind does not cause the stall (directly) during the downwind turn. If AOA increases then you have a case... I know the answer - Go find out yourself.
 
For those so inclined, here's a reasonable physics analysis of the issue:
http://www.av8n.com/how/htm/maneuver.html#sec-infamous-downwind-turn

Another is in John T. Lowery's book "Performance of Light Aircraft".

Worth noting is that John S. Denker (PhD Physics, CFII), and John T. Lowery (MS Aeronautical Science ERU, ATP, CFII, etc.) are in complete agreement regarding this issue.

Conclusion: If the air mass is steady state, there is no difference between 0mph and 1000mph wind relative to the ground. :D
 
I am very sorry I started all this, but, for those who are genuinely interested, here is how you have to calculate the "upwind to downwind turn" if you use the ground as your frame of reference:

You can put yarn on the sides of the plane and you will see the yarn always goes straight back if the pilot flies a coordinated turn. The wind will never hit the side of the airplane, the indicated speed will not change, the wind does not accelerate anything.

As seen from the ground, the plane holds a constant bank angle, but it does not make a constant radius turn (upwind turn radius is smaller, downwind it's larger).
As seen from the ground, the aircraft is flying in a crab (fuselage not aligned to the current path over the ground).
As seen from the ground, the wing is producing a force in the "inward" direction, due to the bank, which we normally call the centripital force, but due to the crab, this force has some component which is in the forward direction, too. This is the force which accelerates the aircraft in this frame of reference, starting at TAS-Wind and ending at TAS + Wind. (For upwind turns the crab and this force are reversed). Now, the upward force produced by the wing has to hold the aircraft up, so it is proportional to the aircraft's mass. So, then, is this forward component, so the acceleration forward is independent of mass - small or large airplane, it makes no difference. A heavy airplane's wing generates larger forces than a lighter plane's wing. All of this varies smoothly thru the turn. What a mess! A complicate vector problem. Much easier to calculate in the frame of reference which drifts with the wind, where we know the plane will fly a simple semi-circle. To translate to the ground reference frame, just add (using vector addition) the wind vector to this motion.
A few other things:
the actual numbers for things like "momentum" and "kinetic energy" change as you move from one frame of reference to another.
Calculation of ground speed is NOT ground speed = true airspeed plus tail wind component. You need to find the magnitude of true airspeed plus total windspeed, added like vectors.

Finally, the same airplane, light or heavy, will glide the same distance (no wind). But best glide speed is lower if you're lighter, higher if you're heavier.
 
Imagine going 100 mph into a 100 mph headwind. Your groundspeed is zero.

Now imagine simultaneously killing the engine and reducing the wind to zero. Would you rather be 500' AGL or 5000' AGL?
 
Having the engine quit and simultaneously flying into a 100 mph wind shear, I'll take choice three: I'd rather be at home. -:)

But what does that have to do with turning in a constant wind?
 
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