L/D Max Headwind / Tailwind..?

A

awacs94

Well-Known Member
So here's the situation:

Your in a single engine airplane, Engine fails: You pitch for L/D Max (Best Glide) Is it better to be at 15,000 feet with a headwind, or at 7,000 feet with a tailwind. Which will result in the greater distance if the landing site is at sea level?

Thanks,

-Brian
 
Is this a quiz or a real question? I'd have to say 15,000 feet with a headwind. It almost seems like a no brainer, but I could be wrong.
 
Looking for the most distance... Headwind / Tailwind = Would Be the same wether it be 10 or 20 or 30 kts ... etc..
 
I'm just guessing here, but...

I'd rather have the headwind. Increased perfomance. The tailwind makes the wing less efficient.

In the end, it's probably six in one, a half dozen in the other...
 
The answer is.......it depends!

The strength of the head/tailwind in relationship to the aircraft's speed will dictate which one is best.

Let's run some numbers as an example.

Best glide speed = 100 knots
Rate of descent = 500 fpm
Aircraft "A" is at 15,000 feet with a headwind,
Aircraft "B" is at 7,000 feet with a tailwind.

Example #1 head/tailwind = 10 knots
Example #2 head/tailwind = 60 knots

In both examples aircraft "A" will be in the air for 30 minutes (15,000 feet / 500 fpm = 30 minutes) and aircraft "B" will be in the air for 14 minutes (7,000 feet / 500 fpm = 14 minutes).

Example #1;
Aircraft "A" is traveling over the ground at 100 - 10 = 90 knots.
It travels (90 nautical miles / hour)(1 hour / 60 minutes)(30 minutes) = 45 nautical miles.

Aircraft "B" is traveling over the ground at 100 + 10 = 110 knots.
It travels (110 nautical miles / hour)(1 hour / 60 minutes)(30 minutes) = 25.67 nautical miles.

Example #1 it is better to be at 15,000 feet with a headwind.

Example #2;
Aircraft "A" is traveling over the ground at 100 - 60 = 40 knots.
It travels (40 nautical miles / hour)(1 hour / 60 minutes)(30 minutes) = 20 nautical miles.

Aircraft "B" is traveling over the ground at 100 + 60 = 160 knots.
It travels (160 nautical miles / hour)(1 hour / 60 minutes)(30 minutes) = 37.33 nautical miles.

Example #2 it is better to be at 7,000 feet with a tailwind.

So the answer is.........it depends!

:D
 
If you have a strong head or tail wind like the above example you probably will not actually want to pitch for best glide. The best glide speed will maximize the distance you can fly with no power, with no wind. If you have a strong head wind it is better to glide at a faster speed, so the head wind will have less of an effect on your performance. You will not stay in the air as long, but will cover more ground.

In the example with the strong tail wind, you can go for the minimum sink speed, which is less than best glide. Just experiment until you see the lowest sink on the VSI. By staying in the air longer, you can glide farther due to the tail wind.
 
goredbirds said:
I'm just guessing here, but...

I'd rather have the headwind. Increased perfomance. The tailwind makes the wing less efficient.
Click to expand...


You guessed wrong.

The wind has zero effect on the efficiency of the wing. Tailwind or headwind, it makes no difference, L/D max never changes. As long as you are flying through a steady wind the wing dosen't know or care.

What does change is your ground speed resulting in the differences that anonaman mentioned, however the wing dosen't know or care about what is happening to the ground track.



Disclaimer: L/D max DOES change depending on weight and configuration, but not based on wind.
 
ananoman said:
If you have a strong head or tail wind like the above example you probably will not actually want to pitch for best glide. The best glide speed will maximize the distance you can fly with no power, with no wind. If you have a strong head wind it is better to glide at a faster speed, so the head wind will have less of an effect on your performance. You will not stay in the air as long, but will cover more ground.

In the example with the strong tail wind, you can go for the minimum sink speed, which is less than best glide. Just experiment until you see the lowest sink on the VSI. By staying in the air longer, you can glide farther due to the tail wind.
Click to expand...

Now you're changing his scenario. He didn't ask what is the best thing to do, he asked which of two alternatives is better given the criteria he outlined.

Reminds me of the olde sign: "Warning! This way be threed creep!"

:D
 
"The wind has no effect on the efficiency of a wing"

So taking off and landing into the wind has been a stupid idea all along?
 
SteveC said:
Now you're changing his scenario. He didn't ask what is the best thing to do, he asked which of two alternatives is better given the criteria he outlined.

Reminds me of the olde sign: "Warning! This way be threed creep!"

:D
Click to expand...


That correct.. Which is the best out of the two situations...
 
goredbirds said:
So taking off and landing into the wind has been a stupid idea all along?
Click to expand...


The wing dosen't know or care. You rotate at a certian AIRspeed, not groundspeed.


Now OTOH, the wheels care greatly. As should you, since takeing of into the wind will create the shortest GROUND roll.
 
Lets try this example instead: Why is it such a big deal when an aircraft experiences a low level windshear from a headwind to a tailwind? A SIGNIFICANT reduction in performance is experienced...
 
awacs94 said:
Your in a single engine airplane, Engine fails: You pitch for L/D Max (Best Glide) Is it better to be at 15,000 feet with a headwind, or at 7,000 feet with a tailwind. Which will result in the greater distance if the landing site is at sea level?
Click to expand...

SteveC said it best with his example of how it depends. Let me go a little further by saying how best glide is computed...it's not L/D max. L/D max will give you the lowest sink rate, but not the best distance (best glide is determined for distance covered, not time aloft).

The total drag curve is directly proportional to descent rate for a given airspeed. The bottom of the curve will give you the longest TIME aloft, because sink rate is at its minimum.

Best glide in no wind is determined by drawing a tangential line from the origin of the graph (0 speed, 0 drag) to the underside of the curve. That will give you the best airspeed/sink rate ratio. In other words, you will go the furthest DISTANCE possible, but will be in the air for less time.

If you have a tailwind, the line should be started back further (negative x-axis), or if you have a headwind, begin the tangential line ahead further (positive x-axis). So if you have a 10 knot tailwind, draw the tangential line beginning at (-10 speed, 0 drag) and if you have a 10 knot headwind, start at (10 speed, 0 drag).

You should be able to see the tangential line touches the curve at a slower airspeed with a tailwind because it starts further back and therefore has a shallower slope on the graph. The opposite is true for headwinds.

As I said in my monsterously long post in the "pitch for a/s, power for alt", I wish I could draw this out. It makes more sense visually.
 
goredbirds said:
Lets try this example instead: Why is it such a big deal when an aircraft experiences a low level windshear from a headwind to a tailwind? A SIGNIFICANT reduction in performance is experienced...
Click to expand...

Yes, but only for a few seconds.

After a few seconds the airplane "Catches up" with the air mass it' is moving through then it is back to "normal"

Now if this happens at low altitude and/or low airspeed it can be deadly, but it is only a temporary situation brought on by the SUDDEN CHANGE of the wind.

In my answer I was refering to a steady tail or headwind.
 
jrh said:
SteveC said it best with his example of how it depends. Let me go a little further by saying how best glide is computed...it's not L/D max. L/D max will give you the lowest sink rate, but not the best distance (best glide is determined for distance covered, not time aloft).

The total drag curve is directly proportional to descent rate for a given airspeed. The bottom of the curve will give you the longest TIME aloft, because sink rate is at its minimum.

Best glide in no wind is determined by drawing a tangential line from the origin of the graph (0 speed, 0 drag) to the underside of the curve. That will give you the best airspeed/sink rate ratio. In other words, you will go the furthest DISTANCE possible, but will be in the air for less time.
Click to expand...

I think this is inaccurate info. While I'm not an aero engineer...this is contrary to anything I've ever studied. My understanding is that Best Glide (jet or prop) is at an angle of attack for L/Dmax. In fact, the glide ratio of the airplane is identical to the L/Dmax ratio. However, it is true that L/Dmax is best endurance for a jet...but max range for a prop. Max range in a jet will be at some higher airspeed...derived by some crazy formula.

Also, if you draw a line tangential to the drag curve...it is or is very close to L/Dmax.

Seagull?
 
goredbirds said:
"The wind has no effect on the efficiency of a wing"

So taking off and landing into the wind has been a stupid idea all along?
Click to expand...


The wind will alter the dynamic pressure acting upon the wing which affects lift....but the efficiency of the wing is unchanged.
 
goredbirds said:
"The wind has no effect on the efficiency of a wing"

So taking off and landing into the wind has been a stupid idea all along?
Click to expand...



Initially when taking off with a tailwind, the aircraft has to accelerate to the speed of the tailwind for the wing to be at an indicated 0. The aircraft must accelerate further to Vr.


*Looking at the members map, I think you live like a few blocks or houses down from me :D*
 
B767Driver said:
I think this is inaccurate info.
Click to expand...

Oops, you got me. You're right. I was confused about the definition of L/Dmax. L/Dmax is the point at which the tangential line from the origin touches the curve. L/Dmax equals best glide without power and best range with power.

L/Dmax is NOT the minimum drag point, as I earlier stated. However, that minimum drag point at the bottom of the drag curve, whatever it is called, will give you best endurance in a prop powered aircraft.

AOPA has a good article on gliders that explains all of this:
http://www.aopa.org/members/ftmag/article.cfm?article=649
 
goredbirds said:
Lets try this example instead: Why is it such a big deal when an aircraft experiences a low level windshear from a headwind to a tailwind? A SIGNIFICANT reduction in performance is experienced...
Click to expand...
winshear is a CHANGE in speed of the wind. if you are flying at 100 kts IAS, and experience a 50 knot change in relative wind from behind the wing, your IAS will drop to 50 knots, until the plane catches up with this new relative wind, or stalls and hits the ground on short final.

this is why you build a gust factor into your final approach speed. (1/2* the change in wind) + your approach speed. so if the wind is steady at 15 knots and gusting to 30, you add 7.5 knots to your approach speed. good in theory, anyway.
 
You must log in or register to reply here.
Back
Top