Why increase speed in a headwind?

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Martymccasland

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M.McCasland
Many places I come across advise to lean/reduce power if you have a tail-wind but just go ROP and hammer if you have a head-wind? The latest occurring this morning in an aviation article on AvWeb. My CFI, other very good pilot friends say the same. But I just don't see the light.

What is the thinking behind this?

If I run LOP, my TAS drops from 168-170 to 157-159, my fuel burn drops 3-5gph, my range is improved (not to mention the engine is very nice and cool -- which is a safety and long-term maintenance money saving item to me).

Why would I give that up in a headwind? Unless the trip is short, the headwind is going to reduce my range anyway. Running WOT and ROP is going to reduce it even more -- likely requiring a fuel stop on a longer flight. Running 65-70% LOP is just going to put you a few minutes behind the fire-breathing plane going flat-out up ahead -- and is very likely going to allow you to keep flying when the faster plane stops for fuel. Even if the fast plane was able to go 100 knots faster by running WOT/ROP, the slower plane is going to beat them unless the next leg is extremely long.

Example: on a 480nm trip right now with a 21 knot headwind component, running LOP makes the trip 3:43 mins at 44.6 gallons. Running wide-open ROP makes it 3:27 at 56 gallons. To me the fact that there is a headwind or tailwind is irrelevant. If anything, a huge headwind makes the case for easing up, burning less, forgoing a stop. And we're only talking 15 minutes over nearly 500nm.

What am I missing?
 
The effect is not in proportion as you might expect. The slower you go, the more important the headwind is. Let's use an extreme example to illustrate.

Suppose your airplane is really slow and flies 105 knots flat out and 95 knots with range maximized. You come across a 100 knot headwind. You'll get there really slowly flat out, and not at all leaned and slow.

But I think people are chasing the next digit here, and it won't make that much difference.
 
It is the time.

You spend more time in a headwind than a tailwind, so increasing speed reduces the amount of time fighting the headwind.

That is why winds don't affect fast airplanes as much as slow airplanes. A 50 kt headwind doesn't impact your flight time as much when flying at 500 kts as it does when flying at 100 kts.
 
It's an algebraic equation that requires the headwind to be a certain percentage of your airspeed before it is beneficial to increase the speed and burn more fuel. In your example of 21kts headwind, you haven't reached that point, yet. Keep bumping the headwind up until you see where the advantage comes into play.
 
Conversely, the more time you spend with a tailwind, the more it benefits you. That's why reducing power and aggressively leaning is the most cost effective flying when you have a 35 to 40 kt tailwind (or more). When you true out at 130 to 135 like I do, it's far too much fun to see a 170kt ground speed to consider slowing down though.

So, I guess I'm with MAKG. I don't worry about it. For the most part I fly at 22"/2300rpm leaned until rough, enrichened until smooth, regardless of the conditions or altitude.
 
Always fun to look down in a strong headwind and see cars on the highway passing you.
 
drotto said:
Always fun to look down in a strong headwind and see cars on the highway passing you.
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that happened to me in a Cherokee 180 leaving Hilton Head. Ground speed of 67-70!! :rolleyes2:

I knew right then that I needed something faster.
 
Last time I flew into a good headwind, I was flying over I-35 doing 42kts groundspeed. The irony was hysterical. Got a pilot's cert, got a plane, going 1/2 the speed of the cars below and paying 2X the price for gas.

That said the trip back GS was 147
 
The relationship between distance and time is what makes this problem difficult for me to comprehend. It helped me to think about an automobile trip first.

Suppose I want to average 60 mph, but for 15 miles I am forced to drive at 45 mph. To make up for it, I decide to drive 75 mph for 15 miles. Will than give me an average of 60 mph. NO, because ave speed = total distance divided by total time. Driving 15 miles at 45 mph, requires 20 minutes. Driving 15 miles at 75 mph requires 12 minutes. The total is 32 minutes. So the total distance of 30 miles took 32 minutes. My average speed is not 60 mph; to meet that average I'd have to do it in 30 minutes, not 32 minutes.

Think about a round trip flight with a constant wind. The time you lose speed due to a head wind is GREATER than the time you are in the air with a tail wind. So it's better to increase your airspeed in a headwind and reduce airspeed in a tailwind.

But when does it really matter. That's the tougher one to call.

By the way, once in a head wind I not only saw cars passing me, but a DOG chasing a car passing me.
 
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I do love that in theory you could be hovering over the ground.
 
Given enough wing area and enough wind you can go backwards.
 
EdFred said:
It's an algebraic equation that requires the headwind to be a certain percentage of your airspeed before it is beneficial to increase the speed and burn more fuel. In your example of 21kts headwind, you haven't reached that point, yet. Keep bumping the headwind up until you see where the advantage comes into play.
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I plugged a few numbers in a spreadsheet for my plane (A36) and see what you are talking about.

A few interesting things popped out.

The time savings is maximized at short trip distances and huge headwinds. e.g. The maximum time savings I saw considering trip ranges from 200-800nm in headwind components from 10-95 knots was 39.7 minute advantage at 300nm and a 90 knot component.

As distance got longer, the time advantage drops due to the WOT plane reaching range limits at ever decreasing headwind component values, thus decreasing the maximum time advantage figures. i.e. at 350nm, the most one could save is 31.9 minutes. At 400nm, 26.7 minutes. At 500nm, 21.7min.

As the distance got shorter, the time advantage also dropped as you are not in the air long enough to rack-up the time advantage.

Then range starts to work the other way at longer trips. The WOT plane is obviously out of fuel quicker as headwinds increase -- leaving the LOP plane anywhere from 72 nm extra range (on medium range flights with huge winds) to 175nm extra range on longer range flights with even a more common 10 knots of headwind.

So the moral of the story, at least for my plane, is hammer so long as the trip is relatively short and wind is hurricane level and above (>70 knots) to get any sort of reasonable payback in terms of fuel per hour additional cost. From more normal headwind components (40 knots and less), the time advantage decreases while the cost to save that time increases substantially to shave off a few minutes. Even at that, one can't expect to save more than 40 minutes maximum in any of the headwind speeds I considered (10-95 knots) over the entire range envelope of the plane. And to shave off those extra minutes, the cost in doing so can be as little as $78 per hour of time saved (short trips, howling wind) to $250 per hour of time saved (longer trips and more common headwinds of 10-20 knots).

For longer trips, max range comes into play and allows the leaned plane to beat the WOT plane, completely erasing any time advantage.

I plugged in different speeds to simulate something slower, like a 172 or Cherokee -- and the values changed but held the same trend. Yet, due to the lower speed, there was a point where there was a maximized / no added cost to hammer and time savings were positive: 300nm with a 70 knot headwind component -- you're money ahead to hammer. At other speeds, the cost to go faster drops to negligible cost for hammering ($11.50 extra for 350nm at 60 knots headwind saving 154 minutes, etc.).

Maybe that the moral of the story is if your plane's TAS is 150+, you'd have to get up 100+ knot headwind components (flight levels?) for the 1st 1/3 or so of your plane's range to have any huge time impact -- and the impact only decreases from there, regardless if the trip is shorter or longer. On the other hand, if your plane cruises closer to 100 knots, more common headwind components of 30-40 knots can create situations where you actually come out ahead to hammer. In all planes, the advantages maximize in the 1st 1/3 or so of the range curve (i.e. if you plane can go 800nm, expect maximum time savings around 250-350nm.)

For really fast planes, burn a little more Jet-A, get above it all, and laugh at the guys burning 100LL...
 
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drotto said:
I do love that in theory you could be hovering over the ground.
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Not just in theory - I've done it in a Cub. At about 45k.

The "Stick and Rudder" test is to then do 360º turns under the hood.

Your groundspeed will zoom from zero to 90k, then back to zero, then zoom to 90k, and so on.

And, without looking at the ground (or a GPS), you absolutely, positively cannot tell when you're in the "headwind" or "tailwind". Really.

I'm working on a long thread about "Stick and Rudder" moments - will try to have it presentable in a day or two.


Back on point, imagine your best glide speed is 70k and you're at 1,000' 1/8 mile from a runway with a failed engine. Problem is, you have a 70k headwind.

It should be clear that to make any progress towards the runway, you'll have to glide faster than your best glide. Once you get a handle on that, the need for more speed in a headwind and less in a tailwind may become clear.
 

If you ever get a student that is a sharp engineer, have them create a spreadsheet with all the variables included (fuel burn/hour at various power settings/altitudes/wind speeds/and the like) and see for yourself what the tradeoff is in both time and $$ (and they are never the same).

Jim
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drotto said:
Always fun to look down in a strong headwind and see cars on the highway passing you.
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My airplane cruises at 60 mph. Any headwind is strong by the above definition.
 
timwinters said:
So, I guess I'm with MAKG. I don't worry about it. For the most part I fly at 22"/2300rpm leaned until rough, enrichened until smooth, regardless of the conditions or altitude.
Click to expand...

You'd have a hard time pulling that off at FL190. :D
 
Ted DuPuis said:
You'd have a hard time pulling that off at FL190. :D
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True! No oxygen on board in my case. I'd love to see how high she'll go someday though.

BTW...to the OP...

A similar mental exercise applies to best glide also. To achieve true best glide, you increase your speed into a headwind, reduce it when you have a tailwind.
 
timwinters said:
True! No oxygen on board in my case. I'd love to see how high she'll go someday though.
Click to expand...

I was referring to the 22" part. When I was up there in the 310 a couple months back it was closer to 14". But who knows, it may have changed. Global warming and all. ;)
 
Martymccasland said:
Many places I come across advise to lean/reduce power if you have a tail-wind but just go ROP and hammer if you have a head-wind? The latest occurring this morning in an aviation article on AvWeb. My CFI, other very good pilot friends say the same. But I just don't see the light.

What is the thinking behind this?

If I run LOP, my TAS drops from 168-170 to 157-159, my fuel burn drops 3-5gph, my range is improved (not to mention the engine is very nice and cool -- which is a safety and long-term maintenance money saving item to me).

Why would I give that up in a headwind? Unless the trip is short, the headwind is going to reduce my range anyway. Running WOT and ROP is going to reduce it even more -- likely requiring a fuel stop on a longer flight. Running 65-70% LOP is just going to put you a few minutes behind the fire-breathing plane going flat-out up ahead -- and is very likely going to allow you to keep flying when the faster plane stops for fuel. Even if the fast plane was able to go 100 knots faster by running WOT/ROP, the slower plane is going to beat them unless the next leg is extremely long.

Example: on a 480nm trip right now with a 21 knot headwind component, running LOP makes the trip 3:43 mins at 44.6 gallons. Running wide-open ROP makes it 3:27 at 56 gallons. To me the fact that there is a headwind or tailwind is irrelevant. If anything, a huge headwind makes the case for easing up, burning less, forgoing a stop. And we're only talking 15 minutes over nearly 500nm.

What am I missing?
Click to expand...

Nothing IMO, you are correct as I see it. If we are talking about fuel reserves then slowing down always helps. Increasing your NMPG is all that matters.

If we are talking about time and no fuel stop variable, then OK I guess someone can always get somewhere a couple of minutes faster by hammering it. The juice is rarely worth the squeeze IMO.

Perhaps your CFI is used to flying wet rentals?
 
Ted DuPuis said:
I was referring to the 22" part. When I was up there in the 310 a couple months back it was closer to 14". But who knows, it may have changed. Global warming and all. ;)
Click to expand...

Oh I knew what you were getting at. Just saying I'd like to try it sometime.

Actually, at about 9,000' & above I can no longer pull 22". So, I'll typically increase the RPMs.
 
FastEddieB said:
Back on point, imagine your best glide speed is 70k and you're at 1,000' 1/8 mile from a runway with a failed engine. Problem is, you have a 70k headwind.

It should be clear that to make any progress towards the runway, you'll have to glide faster than your best glide. Once you get a handle on that, the need for more speed in a headwind and less in a tailwind may become clear.
Click to expand...

Why try to make the runway when you can land "in place". No ground roll, small flare (travel backwards?!?) Don't need to pull the chute if you've got a cirrus.
 
Here's another oddity: If you're in a crosswind situation, the wind has to be at least a few degrees behind a direct crosswind before it acts as a tailwind. So, if you're headed due east at 100 knots, and there's a crosswind blowing from 356 degrees at 30 knots, it will still act as a headwind, because you have to head into the crosswind in order to stay on course. Kind of a bummer when you think about it, the majority of winds are working against us.
 
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