Throttle back on downwind

| >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.

| 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.)

Kevin didn't say "in extremis;" I did. Definition 2. (Did you view the video? It might have made more sense if you had.)

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

;^)

Don't wake the sleeping giant (or maybe trolls, in this case?) :-)

Good flying, desmobob

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? ;-)

Learning R/C from flying single channel airplanes, I learned that once the engine cut, I had to keep the glide speed up. You learn to judge airspeed and sink rate.

Also I learned that a small 1 lb. single channel plane didn't rekit itself like a 4 lb. plane, so I was more tempted to do gutsier things with it. Also they were easier to repair and get back to flying.

I don't know if I would want to take on a project with 8 turbine engines in it. Although such a project is extremely ambitious, more often than not, ALL MODEL AIRPLANES CRASH.

It's like taking a brand new expensive motorcycle and getting in trashed on a high speed lowside, IYKWIM.

The falling wing has a higher angle of attack than the other wing, so it's stalling sooner. It's the same effect on AOA as in a descending turn: the inside wing has a higher AOA. Applying opposite rudder doesn't bring the wing up by increasing its speed; that speed difference is too small. It brings it up by reducing the AOA on it as a slip is produced, and the reduced AOA reduces the stall severity anmd allows it to fly again.

Dan

The difference in gradient isn't enough to be noticed. We don't notice it even in full-scale airplane flying.

Dan

| >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 :

1. 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.

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... :)

| >| >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.

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.

Ask a full-scale V22 Osprey test pilot about it... one of the ones that's survived.

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.

That's pretty much normal weather around there.

Windy, yes, stormy, save for another day.

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...

On Tue, 14 Mar 2006 13:03:45 -0800, The OTHER Kevin in San Diego wrote in :

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." ==============================================================================

Excellent... some there I hadn't seen. Enjoyed 'em all!

Good flying, desmobob