headwinds & tailwinds

I think you're on to something there.
I've not thought about it before or heard anyone mention it, but it is an interesting observation.

Take the two cases of a quartering headwind vs. a quartering tailwind (45 degrees off from your course line).

In the quartering headwind case, the headwind component is INCREASED because you have to turn the nose slightly into the quartering headwind to stay on your course line.

And, in the case of a quartering tailwind, the tailwind component is DECREASED because you have to turn your nose slightly to the side the tailwind is coming from in order to stay on your course line.

Interesting stuff to think about. Or, am I missing something?
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Rob,
That theory has been used by bicycle racers for years. If it isn't about a 40 degree cone behind you, it is hurting you.

You're probably spot on applying it to flying.

 

Yes, more than 180 degrees of the circle is an effective headwind. And I believe the stronger the wind speed, the more of the circle becomes a headwind due to the larger crab angle.

Another interesting thought. If you are making a roundtrip flight, it's quicker in no wind conditions than with a wind. Long ago I didn't believe that one. It seemed to me that the loss of groundspeed would be balanced by the gain in groundspeed the other direction. But you spend more time at a slower speed, and less time at a faster speed.

As an example: If you fly at 100 kts and fly a 100nm out and back, it would take 2 hrs.

But if you have a 50 kt wind, the oneway trip with the headwind is already
2 hrs, and you still haven't flown back yet.

If you are trying to save fuel, best to fly slower in a tailwind (to spend more time in it) and faster in a headwind (to spend less time in it).

 

I once had a 747 flight from JFK to LHR that arrived over Heathrow a full hour early! The flight had been a bit bumpy the whole way, sort of like riding in a car on a cobblestone road. The pilot attributed that to a heavy tailwind - I believe he said 180 knots - that actually put our ground speed at close to Mach 1.

A lot of good our early arrival did us - they weren't ready for us at Heathrow, so we entered a holding pattern and circled near London for some 40 minutes, wiping out most of our "early" arrival.

 

Yup, well know phenomenon and the old pilot saying is true... most winds are headwinds.

And you are correct that actually about 270 degrees worth of winds are headwinds, and only if there is a sufficient component to give you an increase in ground speed does it pay off.

That's life....

And no, the plane doesn't have any more drag crabbing, it's still flying in the "relative wind", which is to say there isn't any skidding going on, it's just that the relative wind makes you fly in a different direction than you really want to go to get there...

If you google "pressure wave" flying you will see the USAF in the 40's developed techniques to minimize fuel burn by flying long distance flights along "pressure waves", to take advantage of the relative winds aloft and maximize range. It doesn't sound logical, you would think that going the shortest distance would result in the shortest flight time, but when you aren't going 500kts (which they weren't at the time) and you are flying at higher altitudes where the winds can get really strong, the theory works.

 

Generally that is true. I use Flightsoft, which if you give it your plane parameter will suggest the most effect altitudes.

Art

 

Rob- KIAS or KCAS is speed through the airmass as measured by the airspeed indicator. Varies with altitude and an example is a U-2 that has a dual range airspeed indicator where at altitude (FL 650 or so) the difference between max q and stall is around 7 knots and KIAS is less than 200 knots because the air is so thin. KTAS is corrected for air density and represents the the actual velocity through the airmass, without regard for winds. So that U-2 might have 190 KIAS and 520 KTAS. Ground speed, KGS is KTAS corrected for winds, and represents actual velocity over the ground track.

 

One thing I see written a lot that's totally false....."be careful turning downwind as you're more likely to stall". I see it in RC magazines, typically in their flight reviews. I see it occasionally in Flying magazine. Unless I'm totally missing something, turning downwind, upwind, crosswind doesn't have any affect on flying.

 

A "downwind" leg is a term used to describe your position in relation to the runway you will be landing on.

Pilots have been known to get too slow in the pattern and turn a downwind leg and stall.

 

Yeah, I realize that. But the misunderstanding I see written is that turning downwind, or better stated, turning into a tailwind, causes a loss of IAS. And that simply isn't true. Wind has no effect on IAS, whether turning into it or away from it.

 

I guess it is if you're really slow. Maybe rc planes push the envelope a little more.

 

Actually I don't think it is. No matter what speed you're at. You are part of the airmass your are flying in. The airplane doesn't know if it's in a headwind, tailwind, crosswind. So doing a 360 circle in a constant 10 Kt wind, or 100 kt wind shouldn't make a difference.

Anyways, it's sorta off subject, but just something I thought I'd bring up.

 

United Airlines, I believe, did some analysis and found out that they are affected by a headwind component something like 70% of the time.

 

You can think of it this way. Lets say you are flying into a headwind, if the airplane could instantly do a 180 it would now have a tailwind the the result would really be a windshear. Now of course an airplane can't instantly change heading but a small airplane can have a fairly quick turn rate. Combine that with a wind speed of let's say 25 knots and an indicated airspeed of 65 knots the wind speed is a large percentage of the aircraft speed. A quick 90 turn could result in a loss of the headwind component.

 

I see what you're thinking, but I believe it's 100% incorrect.

The airplane is part of the airmass it's flying in. The airmass is moving at a constant, say 100 kts. It doesn't matter if you make an immediate 180, say an instant 180. You're airspeed won't change.

A feather drifting in a 100 kt wind doesn't feel any wind. An airplane flying in a 100 kt wind is the same.

We spin at over 1000 mph on the earth. When we make a 180, we don't get whip lash as we are now spinning backward. I know this sounds obvious, but I believe it's the same theory.

I know this all sounds wrong, but I believe it's 100% true.

Now, as winds typically increase as you climb, if you're climbing into a CHANGING windspeed, you will experience airspeed changes. This could happen when turning downwind as you could be climbing. But, if the wind is constant, there shouldn't be 1 kt of change.

I'm open to anyone wishing to try and change my mind. I'm open to all opinions. I used to believe turning downwind would cause an IAS loss. But I'd put money on the fact it doesn't.

OK, lets here from everyone...........

 

Lets say an airplane is flying at 100 knots going into a 100 knot head wind, in this case the ground speed is 0. If an airplane was flying the opposite direction its ground speed would be 200 (100 knots from the airplane flying thru the air and 100 knots from the tailwind). Now look at the first airplane. When it makes a 180 its ground speed has to go from 0 to 200. 100 of that is because of the tailwind the other 100 has to come from the airplanes mass changing direction. I'm sure I could explain my thoughts better in person than in writing. In this case the rate (degrees per second) has a major impact. It the airplane could do a 180 in one second it would essentially have a negative airspeed of 100. A more standard turn of 90 seconds for a 180 would allow the mass (or maybe the vector of the mass) to change and have less effect on the indicated airspeed.

 

No...

Forget ground speed... it has nothing to do with this discussion. Ignore it.


His airspeed stays at 100k.

 

The key to this discussion is the caveat that it is a CONSTANT wind speed when the plane turns downwind.
But often it is not constant, often the wind has occasional gusts within it.
And sometimes, the speed of the gusts can be significant to the usual wind speed that day.

So if the wind is 10 knots but gusting to 35 at times, there is potentially a 25 knot area of "wind shear" or "sudden wind speed differential" as one of those gusts goes by.

The lighter the airplane, the more it might be affected by the wind gust.
So, in the case of a small r/c airplane, even a 10 knot wind gust could be enough to have it drop quickly.
In a small GA plane, maybe a 25 knot relative wind gust could be an issue (if it happens to be perfectly timed with the plane in a banked turn going toward the downwind leg).

During my training, my instructor and I did stall training in a Cessna 172 on a day that had 35 kt gusts. When you raise the nose on a power-on stall and get close to stall speed, you can really feel the gusts as they come by and hit those wings.
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IF the wind is constant, turning downwind shouldn't have a factor, no matter the rate of turn. GS is a relation of IAS (or TAS...doesn't matter) to the earth. So it's irrelavent as far as IAS goes.

I don't know much about helicopters, so this may not make any sense at all. But lets try this:

1: Wind from 270 at 100 kts.
2: Helicopter facing into the wind, heading 270. IAS 0 Hover (while watching from the ground he would be going backwards 100 kts)
3: Helicopter pilot quickly steps on rudder pedals (anti-torque pedals...whatever they call them) to do a 180 and is facing 090 now
4: Helicopter is now heading 090. IAS 0 Hover (while watching from the ground he would be going forward at 100 kts)

No problem, right? If the helicopter pilot never looked outside, he would never know the wind was even blowing.


The same should hold true for the airplane.

 

Lou is correct. As long as you maintain the same IAS when you make a turn, makes no difference what the winds are for maintaining controlled flight. It will make a difference in your ground track, but the airplane will fly just fine, just not where you want it to go without a correction. Much like a fly doing 10 KIAS inside a railroad car doing 60 mph. Turning does not affect the fly's airspeed.

The problem on making turns in a crosswind comes from trying to maintain a ground track, and that definitely is affected by the wind. Try and adjust IAS to maintain ground track and you can definitely have a problem, especially with a tailwind component on crosswind or final turn.