Throttle back on downwind

Gosh! We haven't seen a discussion on the "Infamous Downwind Turn" in ages!

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;^)
--
Paul McIntosh
RC-Bearings.com
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Don't wake the sleeping giant (or maybe trolls, in this case?) :-)
Good flying, desmobob
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"desmobob" wrote:

Okee. I'll take a stab at it. :-)
Nah, goes back to discussion of throttling the turbines down and taking too long to spool up. One thing I noticed in the crash video was the ominous dark sky, increasing wind, that of an approaching storm.
Conditions are contrary to what would make for a successful flight day.
Seems he pro'ly made a common mistake that has gotten general aviation and commercial aviation pilots into problems, throttling back to match ground speed in a gusty tail wind condition, putting plane into a stalled condition in which it could not recover.
AKA, pilot error. Did the Ministry of Model Aviation Transportation Safety publish its findings? ;-)
--
HPT

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That's pretty much normal weather around there.
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Paul McIntosh
RC-Bearings.com
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"Paul McIntosh" wrote:

Windy, yes, stormy, save for another day.
--
HPT

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On Fri, 10 Mar 2006 14:12:14 -0500, "Icebound"

Makes no sense to throttle back on a downwind leg. Sure, the groundspeed will be faster downwind than upwind and you might have to turn a bit harder when turning back to upwind, but if the airplane can't handle those stresses, it's better off on the ground. It all amounts to relative wind across the wings, not groundspeed.
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The OTHER Kevin in San Diego <skiddz "AT" adelphia "DOT" net> wrote:
| Makes no sense to throttle back on a downwind leg. Sure, the | groundspeed will be faster downwind than upwind and you might have to | turn a bit harder when turning back to upwind
Why?
| but if the airplane can't handle those stresses, it's better off on | the ground. It all amounts to relative wind across the wings, not | groundspeed.
True.
However, while most of the differences between flying in wind and in calm air are are strictly perception related (i.e. your plane *looks* like it's going faster downwind) there are some real differences, mostly related to a the wind gradient found near the ground.
Right at the ground, there is no wind, thanks to the drag of the ground. And up at a large altitude, you'll have the full effect of the wind, but as you get lower, the wind speed will decrease.
This gradient causes at least two problems close to the ground --
1) when you're landing upwind, you lose airspeed as you descend. This is generally a good thing, as it helps `suck' the plane into the ground and prevent balooning, but it does mean you don't want to come down for a landing right at your stall speed.
But when landing downwind, you gain airspeed as you descend, which causes balooning, and the effect is very pronounced.
2) if you're doing a steep turn at low altitude, the lower wing will be in air with a lower wind speed than the upper wing, due to this gradient. The effect is especially high with gliders with long wings.
In any event, if you're flying upwind at a low altitude and enter a tight bank, the airspeed over the high wing will be higher than that over the lower wing, which generally means that it'll create more lift. This will tend to pull the plane into an even tighter bank and has probably caused the death of more than one full scale glider pilot (since they're low and don't have enough altitude to recover from a nearly 90 degree bank.)
Actually, I may have answered my own question about `why you'd have to turn tigher downwind' -- going downwind, the airflow over the lower wing would be higher than that over the higher wing, which would tend to pull your plane out of the bank, requiring more pressure on the stick. Of course, this would only be a signifigant factor close to the ground and especially with a plane with a large wingspan.
Of course, this is all about steady winds -- if you have gusts of wind, or lulls in the wind, they'll certainly affect your airspeed.
--
Doug McLaren, snipped-for-privacy@frenzy.com
If I were stuck on a desert island with only one person, one book, and
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On Sat, 11 Mar 2006 19:56:05 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote:

To maintain your ground track. We (I say we in including anyone else who flies "full scale") practice maneuvers all the time during initial pilot training to maintain ground track. Not a week ago I was doing high-recon in a helicopter and really had to turn hard from the downwind leg to keep from being blown away from the pinnacle I was about to land on. Turning from the upwind I had to finesse the helicopter around the turn to keep from being blown towards the pinnacle.
Same thing happens in a traffic pattern.

Not entirely true. Near the ground, friction reduces Coriolis Effect, it doesn't eliminate wind altogether. Sure, the surface may slow it down a bit, but it can also increase it. (Look up Katabatic Winds) You should try changing the oil on my truck in my driveway. There's ALWAYS a good breeze blowing as I live at the bottom of a valley. Unless I put up some sort of wind barrier, the oil goes everywhere but the drain pan.

Actually, you lose groundspeed. Airspeed is the speed the aircraft "feels" as it moves through the relative wind. If I have a 20 knot headwind right off the nose and I'm indicating 70 knots, my groundspeed will be about 50 knots. The airplane "knows" it's flying at 70 knots and doesn't care it's only covering 50 nautical miles in an hour.
This is exactly why runways are built to "face" the prevailing winds and why aircraft carriers turn into the wind and pour on the coals, to increase lift. As for landing, you DO want the aircraft to stall just as it touches down.

Nope. You actually lose airspeed and gain groundspeed. Again, 20 knot tailwind, indicating 70 knots my ground speed will be around 90 knots. Ask a pilot friend with a GPS to take you up and check the groundspeed on the GPS against the airpseed indicator. How many aircraft have impacted the ground short of the runway when they encounter a headwind to tailwind sheer??
I asked a student pilot a couple months ago, "Can you fly a helicotper backwards at 25 knots on a calm day?" He answered, "No." I then asked if he could hover a helicopter with a 25 knot tailwind. he said "Sure." I then asked him, "What's the difference?" He didn't understand the relative wind across the rotor disc and along the aircraft would be 25 knots from the rear in either case and the aircraft would only know it was flying "backwards" at 25 knots.

Has nothing to do with the wind. Once you're aloft, the aircraft moves along with the airmass and unless it's really gusty, the airplane doesn't care what the wind is doing. All it knows it that it's flying at some airspeed and attitude.
The death spiral you describe is a pitch problem, not a bank problem. Altimeter is winding down, plane is banked hard over and the pilot hauls back on the yoke in an attempt to stop the descent rate. This only tightens the turn even further..

True, but again, once aloft, the aircraft doesn't "feel" the wind. It only moves along with the airmass it's flying in.
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The OTHER Kevin in San Diego <skiddz "AT" adelphia "DOT" net> wrote:
| >Right at the ground, there is no wind, thanks to the drag of the | >ground. And up at a large altitude, you'll have the full effect of | >the wind, but as you get lower, the wind speed will decrease. | | Not entirely true. Near the ground, friction reduces Coriolis Effect, | it doesn't eliminate wind altogether. Sure, the surface may slow it | down a bit, but it can also increase it. (Look up Katabatic Winds)
I didn't say near the ground. I said *right at the ground*.
If you look at your ceiling fan, the blades are probably dirty. Common sense tells you that the air would be rushing past there pretty quickly and so dust couldn't accumulate there -- but there it is. The reason is that there's a small film of air that is barely moving (compared to the fan blade) and that catches dirt.
| You should try changing the oil on my truck in my driveway. There's | ALWAYS a good breeze blowing as I live at the bottom of a valley. | Unless I put up some sort of wind barrier, the oil goes everywhere but | the drain pan.
Yes, but that's a foot above the ground -- not a fraction of a milimeter. Of course, you don't fly your plane's wing down to a fraction of a milimeter from the ground, so it's a moot point, and not something I'm going to argue about.
| >This gradient causes at least two problems close to the ground -- | > | >1) when you're landing upwind, you lose airspeed as you descend. This | >is generally a good thing, as it helps `suck' the plane into the | >ground and prevent balooning, but it does mean you don't want to come | >down for a landing right at your stall speed. | | Actually, you lose groundspeed.
Yes, I'm aware of that. That's the obvious reason why you land upwind, but it's not the only one. I'm talking about something more than that, something that's less obvious.
Do you have Martin Simon's `Model Aircraft Dynamics' ? If so, read section 4.14 `The wind gradient' -- that's what I'm referring to.
| Airspeed is the speed the aircraft "feels" as it moves through the | relative wind. If I have a 20 knot headwind right off the nose and | I'm indicating 70 knots, my groundspeed will be about 50 knots.
Of course, and this is well known, but it's not the entire story.. However, as you lose altitiude, the wind speed generally slows (due to the gradient), which will generally reduce your airspeed, which will reduce lift unless you correct for it.
I've had this discussion before -- even in the same context! I even typed out a paragraph of Mr. Simons' book there -- you can read it there, if you wish.
http://groups.google.com/group/rec.models.rc.air/browse_thread/thread/9a13d875a0532d0c/5b52a8e6e8b44e53#5b52a8e6e8b44e53
... maybe that'll help explain what I'm talking about.
| I asked a student pilot a couple months ago, "Can you fly a helicotper | backwards at 25 knots on a calm day?" He answered, "No."
Why would he say that? I can fly my R/C helicopter backwards, with or without wind. And I'm not even very skilled at helicopter flight!
| >2) if you're doing a steep turn at low altitude, the lower wing will | >be in air with a lower wind speed than the upper wing, due to this | >gradient. The effect is especially high with gliders with long wings. | > | >In any event, if you're flying upwind at a low altitude and enter a | >tight bank, the airspeed over the high wing will be higher than that | >over the lower wing, which generally means that it'll create more | >lift. This will tend to pull the plane into an even tighter bank and | >has probably caused the death of more than one full scale glider pilot | >(since they're low and don't have enough altitude to recover from a | >nearly 90 degree bank.) | | Has nothing to do with the wind. Once you're aloft, the aircraft | moves along with the airmass and unless it's really gusty, the | airplane doesn't care what the wind is doing. All it knows it that | it's flying at some airspeed and attitude.
You're ignoring the wind gradient. Figure 4.15 in Martin Simons' book covers exactly this case. Here's the caption --
Figure 4.15 Turning into the wind near the ground is dangerous. The lower wing enters the slow moving airstream and loses lift. The higher wing enters the faster aistream and gains lift. The result is a strong tendancy to increase the angle of the bank.
| The death spiral you describe is a pitch problem, not a bank problem.
I wasn't talking about a spiral at all, though what I'm talking about does sometimes result in death, especially to glider pilots making sharp turns near the ground.
I also don't think this is what happened to the B-52 model -- it looked like it had plenty of altitude, at least at first, which would make the wind gradient very small. But of course, I've only seen the video like everybody else, so I don't have any special insights about what really happened.
| >Of course, this is all about steady winds -- if you have gusts of | >wind, or lulls in the wind, they'll certainly affect your airspeed. | | True, but again, once aloft, the aircraft doesn't "feel" the wind. It | only moves along with the airmass it's flying in.
Of course, but your airplane certainly does `feel' *changes* in the wind, be them due to changes in the wind direction, gusts or a wind gradient. (And wind sheer is just an extreme form of wind gradient, though I was talking specifically about a gradient caused by getting closer to the ground.)
--
Doug McLaren, snipped-for-privacy@frenzy.com Save the whales! Collect the whole set!

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The difference in gradient isn't enough to be noticed. We don't notice it even in full-scale airplane flying.
Dan
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| >2) if you're doing a steep turn at low altitude, the lower wing will | >be in air with a lower wind speed than the upper wing, due to this | >gradient. The effect is especially high with gliders with long wings | | The difference in gradient isn't enough to be noticed. We | don't notice it even in full-scale airplane flying.
If you say so. Others disagree. (Though to be fair, I should have been more specific -- it's a steep *upwind* turn at low altitude.)
Granted, it'll be most noticable in a light, slow plane with a long wing span -- which describes most gliders -- and only if doing a relatively steep bank close to the ground, which is rarely a good idea. So it's not surprising that you've never noticed that particular effect of the wind gradient.
Of course, that was only one effect of the wind gradient. There are others that are more noticable in normal flying.
There's several pretty good pages out there that talk about wind gradients and how they affect flying. Some of the better ones I saw include :
http://en.wikipedia.org/wiki/Wind_gradient http://myweb.tiscali.co.uk/miskin/gliding/gliding/x_wind_gradient.htm http://home.comcast.net/~d-m-hague/ppg/ppg_and_wind.pdf http://www.glidingwa.com.au/Certificate.htm
Note that the last link given talks about the wind gradient causing a bank to become stronger --
13. What is the danger in banking too steeply near the ground in a strong wind? ... The top wing is in an air mass of different speed to the bottom wing (wind gradient). At low level, turning into a strong wind causes the glider to over bank, vice-versa if turning downwind. The effect may be beyond the pilot's ability to prevent it occurring.
... so it would seem that at least *somebody* has noticed it, in full scale flying.
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Doug McLaren, snipped-for-privacy@frenzy.com
Strangers have the best candy.
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On Mon, 13 Mar 2006 16:29:33 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote:

I asked a glider rated pilot here at work (I work with a bunch of pilots - everything from gliders to helicopters) and he said in larger sailplanes, you do notice the gradient difference between the high and low wing, and even when thermalling, but he's never noticed in an airplane.
I've never noticed it flying a helicopter or an airplane but in the helo, we produce our own differential lift since our "wings" spin... :)
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The OTHER Kevin in San Diego <skiddz "AT" adelphia "DOT" net> wrote:
| >| >2) if you're doing a steep turn at low altitude, the lower wing will | >| >be in air with a lower wind speed than the upper wing, due to this | >| >gradient. The effect is especially high with gliders with long wings | >| | >| The difference in gradient isn't enough to be noticed. We | >| don't notice it even in full-scale airplane flying. ... | I asked a glider rated pilot here at work (I work with a bunch of | pilots - everything from gliders to helicopters) and he said in larger | sailplanes, you do notice the gradient difference between the high and | low wing, and even when thermalling,
During thermalling there's another effect involved, unrelated to any wind gradient. If you're doing tight circles, like you do when thermalling, the lower/inner wing is moving slower through the air than the outer/upper wing, which causes it to create less lift, which will tend to pull it down more and tighten the bank.
The effect happens on any plane in a turn, but it's the strongest in a plane with a long wing span doing tight turns -- basically, gliders in a thermal.
http://www.skysailing.com/pages/theory.htm mentions this effect in the `overbanking tendency' section. http://www.av8n.com/how/htm/roll.html talks about it as well, in section 9.4, and figure 9.13 gives a good diagram of it.
Of course, in a thermal there's another effect that will counteract this to some degree -- the air in the center of the thermal tends to be rising faster than the air in the edge of the thermal, and if you've centered the thermal perfectly, your lower/inner wing will be in the air that's rising faster than your upper/outer wing. I'm not sure which effect is generally stronger, but from what I'm reading I'm guessing it's generally the overbanking tendency.
| but he's never noticed in an airplane.
A glider isn't an airplane? :) (Of course they are -- they just tend to be lighter, fly slower and have longer wings for a given weight -- all things that tend to make these effects seem more pronounced.)
| I've never noticed it flying a helicopter or an airplane but in the | helo, we produce our own differential lift since our "wings" spin...
Helicopters ought to be affected similarly, but there's so many other effects going on that this relatively minor effect would probably not ever be noticed.
--
Doug McLaren, snipped-for-privacy@frenzy.com
My favorite kind of wild animal is on a plate. --Homer Simpson
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On Mon, 13 Mar 2006 18:51:46 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote:

Yeah, I got the nth degree of an explanation halfway through my morning coffee.. He could have the decency to let me finish my 1st cup tho. hehehe

According to the FAA, a glider is it's own category. Perhaps I should have said "powered airplane". I think a modern sailplane is faster than his Citabria..

Actually, it is noticable as you pass through ETL on takeoff and again as you slow to land (along with transverse flow effect) but it kinda becomes an unconscious effort to apply the control inputs to compensate for it. Dissymmetry of lift is always there, but various rotor systems compensate for this by blade bending or flapping.
There's a very noticable effect when you get moving really fast and get into retreating blade stall.. Nose pitches up and the ship rolls to the stalled blade side (Left in the case of the helos I fly) but it's easily recoverable by applying aft cyclic to slow down and then rolling level again.
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On Tue, 14 Mar 2006 02:36:03 GMT, Joe Ellis

I'm amazed helicopters just don't fly apart in the 1st place. There's so much going on with those blades as they continuously change pitch, lead/lag, flap...
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On Tue, 14 Mar 2006 13:03:45 -0800, The OTHER Kevin in San Diego <skiddz "AT"

Time to trot out the moldie oldies:
"If the wings are traveling faster than the fuselage, it's probably a helicopter - and therefore, unsafe."
"If something hasn't broken on your helicopter, it's about to."
"Helicopters are a mass of rotating parts going somewhere to crash."
"It takes a college degree to fly, and crash a helo. It takes a high school deploma to fix'um and make'um fly again."
"A grunt is the true reason for the existence of the helicopter. Every helicopter flying in Vietnam had one real purpose: To help the grunt. It is unfortunate that many helicopters never had the opportunity to fulfill their one true mission in life, simply because someone forgot this fact."
"It is a fact that helicopter tail rotors are instinctively drawn toward trees, stumps, rocks, etc. While it may be possible to ward off this natural event some of the time, it cannot, despite the best efforts of the crew, always be prevented. It's just what they do."
"The terms "Protective Armor" and "Helicopter" are mutually exclusive."
"Loud sudden noises in a helicopter WILL get your undivided attention. The BSR (Bang Stare Red) Theory states that the louder the sudden bang in the helicopter, the quicker your eyes will be drawn to the gauges. The longer you stare at the gauges, the less time it takes them to move from green to red." "Helicopters are essentially, heaps of shrapnel travelling in a group, waiting to shower the countryside with howling doom."
"The helicopter is really just a machine for making bolts unscrew themselves." ============================================================================ "The thing is, helicopters are different from planes. An airplane by its nature wants to fly, and if not interfered with too strongly by unusual events or by a deliberately incompetent pilot, it will fly. A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other, and if there is any disturbance in this delicate balance the helicopter stops flying; immediately and disastrously. There is no such thing as a gliding helicopter.
This is why being a helicopter pilot is so different from being an airplane pilot, and why in generality, airplane pilots are open, clear-eyed, buoyant extroverts and helicopter pilots are brooding introspective anticipators of trouble. They know if something bad has not happened it is about to." =============================================================================
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Excellent... some there I hadn't seen. Enjoyed 'em all!
Good flying, desmobob
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On Tue, 14 Mar 2006 16:42:38 -0500, "Martin X. Moleski, SJ"

[snip]
Nice!
My favorite:
A helicopter is 10,000 parts spinning rapidly around an oil leak.
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On Tue, 14 Mar 2006 15:18:11 -0800, The OTHER Kevin in San Diego <skiddz "AT"

Heh heh. Of the making of helicopter jokes, there is no end!
Q: What makes helicopters fly?
A: There are various theories:
    1. Money.          2. They don't really fly; they beat the air into submission.          3. They're so ugly, the ground repels them.
                    Marty
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