Advantage of higher cruising speed (headwinds)

[skeptic36]

I saw this on another forum.

 

To put it in context he is talking about how much closer to a tail wind it has to be before it becomes an advantage for a slower aircraft compared to a faster one

 

Firstly is he correct. Secondly am i the only one who didn't know this?

 

Assuming a random 30 knt wind aloft, then for a aircraft cruising at 60 knt TAS you will face a headwind 60% of your flying life - in other words: not until the wind is 15 degrees past perpendicular in your favor, will you see an increase in GS.

 

 

 

For a 100knt TAS aircraft, you will only battle headwinds 55% of your flying life and needing only 10 degrees past the 90 mark before you can call it is break even on the GS. Actually a little less difference than I thought, but this is an extra "tax" imposed on the slower aircraft aside from more obvious enroute figures.

 

 

Regards Bill

 

 

[Dafydd Llewellyn]

Neil McDonald, who used to ferry Transavia Airtruks all over the World - and who demonstrated one at the Paris Air Show - once remarked to me that if he ever actually experienced a tail wind, he was afraid he'd catch a chill . . .

 

 

[Guest Nobody]

I think your friend is right. To demonstrate I have drawn the diagram below. Look first at the left half.

 

The black circle is 60 units diamater. This represents the distance that the aircraft would fly in 1 hour at 60 knots.

 

The red circle is the same diamater just moved up by 30 units. This represents the possible points that an aircraft can make it to if there is a 30 knot wind if it started at the yellow point at the centre of the black circle. If it flies so that its ground track is directly 90 degrees to the wind then it will have covered 52 Nm after 1 hour. which is 87% of its speed(represented by the purple line). The angle dimension of 151 degrees represents all of the ground tracks that the distance covered is greater than the speed. If the wind comes from a random direction then the aircraft will be favoured 42% of the time and hindered 58% of the time.

 

The right side of the graph is the same diagram with the circles expanded to represent 100knots. If the ground track is 90 degrees to the wind then the plane covers 95 knots or 95% of its airspeed. In a randon direction wind the aircraft will benefit 45% of the time. (162.7/360)

 

 

So faster aircraft are less affected by winds and benefit from tailwind more frequently than a slower aircraft.

 

 

[Guest Andys@coffs]

its true but what isn't said (but is implied) is that the law of diminishing returns applies to the angle of favourable winds) .... the faster you go the closer you approach the 50% mark (180degrees) but you wont ever get there.......

 

What does need to be pointed out is that 30kts of headwind on a 60kt machine means that your segment times double for a true headwind. That significantly affects fuel usage and "what if" planning. On a 120kt machine the same headwind impacts only 1/3 extra time. Fuel usage in such circumstances isn't likely to create diversion requirements, those are more likely to be other weather driven circumstances......

 

I have a 55kt trike and a 120kt Jabiru...I have landed the trike and waited for headwinds to abate in the afternoon rather than face possible fuel exhaustion, I've never needed to do that in the Jabiru.

 

Andy

 

 

[robinsm]

horses for courses, slow and steady does me but then I am not normally in a hurry to get anywhere and can sit and wait for lower head winds. If they are to strong, I just don't go there. Simple.

 

 

[Guest Andys@coffs]

In rereading what I wrote it does sound as though the missing bottom line was "and that's why faster is better"

 

That's not what I meant. 55kts in an open cockpit in summer is simply magnificent, 40kts just tootling around in temps that are 5-10 degrees C cooler than on the ground watching the ants below in the heat is even better....120Kts open cockpit.....I suspect you have neck muscles that put body builders to shame.......in other words it might be someone's idea of fun....but not mine

 

Sorry if it came across otherwise

 

Andy

 

 

[Bruce Tuncks]

Here's how I did it: Remember the airspeed is 60kts , direction unknown, and the groundspeed is to be 60 knots , assume to the North. The wind is 30 knots, direction unknown.

 

Draw a 60mm circle (for 60 knots) and through the middle a vertical line for North.

 

Where the line crosses the circle at the top, swing a 30mm arc for the wind vector. ( Compass prick on the crossing point where the N line crosses the circle )

 

Draw a heavy line with an arrow from where the arc crosses the 60mm circle to the top of the circle where the prick was. Label this Wind Vector. Arrowhead towards the N line.

 

Draw a heavy line with an arrow from the center of the big circle to the tail of the wind vector. This is the Airspeed Vector.

 

Draw a heavy line with 2 arrowheads from the center of the circle to the head of the wind vector. This line goes N and is the resultant vector, or course made good . This line is 60mm long of course.

 

You will see that the wind vector is 254 degrees, so you need 16 degrees of tail component to not be slowed.

 

Now repeat with the big circle 100mm for 100 knots

 

You will see that the wind vector is now at 261.5 degrees, so you now only need 8.5 degrees of tailwind component.

 

So any wind between 254 degrees and 261.5 will act as a slight headwind for the 60kt plane and a slight tailwind for the 100kt plane.

 

This was interesting, I had never done it before either.

 

Bruce

 

 

[farri]

All good in theory but anyone flying a 60kt aircraft in 30 kt wind is looking for trouble.

 

Frank.

 

 

[facthunter]

They don't want to be looking for much, because they won't see much at 30 knots G/S. I remember heading for Newcastle Beach from Rutherford in a Tiger Moth with a good breeze blowing, one day and gave it away after an hour or so saw me at Beresford and I went down to 500'. to try and do better. The Tiger has an A/S of about 78 Knots... .Nev

 

 

[Yenn]

It is interesting to watch a GPS while in a slow plane and high wind. I did a trip in a Drifter, cruise speed 50K. My GPS was giving me time to run to destination and at one stage it was getting longer. On an 80 NM trip I was 30 miles from home before I could at last be sure I would not have to turn around and go back. Under about 1000' cloudbase and NE wind, when I arrived a friend told me to look at the carbies, and they were glistening with ice droplets all over the outside. no sign of carbie ice.

 

 

[turboplanner]

Depends what you want - quicker arrival or lower fuel cost.

 

The engine has to make enough power to overcome air resistance at cruising speed along with some other things like accessories and mechanical resistance.

 

Frontal area is a major part of the equation, offset to a small degree by streamline coefficient.

 

Power required to overcome air resistance goes up exponentially, so you have to burn a lot more fuel to cruise at the faster speed, given two basically similar aircraft.

 

So the percentage disadvantage of the slow aircraft is offset by lower cost. (I know a four place touring aircraft kills a Savannah for cost per pax per Nm, but that's another equation.)

 

 

[Guest Nobody]

Here's how I did it: Remember the airspeed is 60kts , direction unknown, and the groundspeed is to be 60 knots , assume to the North. The wind is 30 knots, direction unknown.Draw a 60mm circle (for 60 knots) and through the middle a vertical line for North.

Where the line crosses the circle at the top, swing a 30mm arc for the wind vector. ( Compass prick on the crossing point where the N line crosses the circle )

 

Draw a heavy line with an arrow from where the arc crosses the 60mm circle to the top of the circle where the prick was. Label this Wind Vector. Arrowhead towards the N line.

 

Draw a heavy line with an arrow from the center of the big circle to the tail of the wind vector. This is the Airspeed Vector.

 

Draw a heavy line with 2 arrowheads from the center of the circle to the head of the wind vector. This line goes N and is the resultant vector, or course made good . This line is 60mm long of course.

 

You will see that the wind vector is 254 degrees, so you need 16 degrees of tail component to not be slowed.

 

Now repeat with the big circle 100mm for 100 knots

 

You will see that the wind vector is now at 261.5 degrees, so you now only need 8.5 degrees of tailwind component.

 

So any wind between 254 degrees and 261.5 will act as a slight headwind for the 60kt plane and a slight tailwind for the 100kt plane.

 

This was interesting, I had never done it before either.

 

Bruce

Bruce,

If I understand correctly here is the the diagram for your explanation.

 

 

 

[Bruce Tuncks]

Yep that's exactly right. And you got it more accurate than I did with a cheap compass and protractor.

 

I'm sure impressed how you can draw those beaut diagrams on this forum. How do you do that?

 

regards,

 

Bruce

 

 

[PA.]

Autocad and save as a jpg.

 

 

[Guest Nobody]

Yep that's exactly right. And you got it more accurate than I did with a cheap compass and protractor.I'm sure impressed how you can draw those beaut diagrams on this forum. How do you do that?

regards,

 

Bruce

Rhino 5.0 is very simple but can handle surfaces in 3d as well. It's a shame it costs quite a few hours flying.....