L / D Max with Headwind / Tailwind...

[awacs941]

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? The winds will be equal wether they are a head wind of 10 or tailwind of 10.

Thanks,

-Brian

 

[stagger]

L/Dmax

who cares how far you can glide, find a piece of terra firma,, put yourself in high key (remember afm51-37) and land before the speed runs out... along with aircraft control.And next time check your fuel guages before you fly..ha ha

 

[r1830]

Your answer would depend on many variables.

For example
You lose an engine at 10000 feet
The aircraft glides at 60 mph (best glide speed)
It descends at 1000 feet per minute.
You are heading 090 degrees.
The wind is constant from sea level to 10000 feet at 15 mph out of the west (270 degrees)

With no wind
It will take you 10 minutes to lose 10000 feet at 1000 feet per minute.
The aircraft is moving forward a 1 mile per minute (60 miles per hour).
In ten minutes your aircraft will travel 10 miles in the air.

15 mph headwind
Add the 15 mph head wind; the aircrafts groundspeed will be 45 mph (60-15).
It will still take 10 minutes to descend. But you will only travel a distance of 7.5 miles over the ground.

15 mph tailwind
Make it a 15 mph tailwind and the groundspeed will be 75 mph (60 +15).
During the 10 minute descent you will travel a distance of 12.5 miles.

So for any given altitude you will travel a further distance over the ground by gliding with the wind as opposed to against it.

 

[172driver]

Ummm...that wasn't the given scenario, 1830.

In the given scenario you would be better off at 15,000 ft. with the 10 kt HW. Using your numbers for descent rate and glide speed, you would have 15 min at 50 kts as opposed to 7 min at 70 kts, or 12.5 NM vs 8.2 NM.

However, we also have to consider the difference in TAS, and thus GS, between the altitudes at which the engine supposedly failed. There is going to be a 40 kt difference for the first 7 minutes, then decreasing by 5 kts/min, once the higher airplane reaches 8000 ft, and the lower one hits the ground. Of course, along with this goes a higher descent rate to get the same IAS. Either way, the higher airplane will glide much further.

 

[Gutenberg]

Is a best glide of 75 knots still your "best" glide in a 75 knot headwind? nupe.

 

[r1830]

Ummm...that wasn't the given scenario, 1830.

My apologies, I was just trying to make a general statement regarding gliding distance from a constant altitude with the variables being into a headwind or a tail wind.
______________________________

This is what I stated in the beginning of my previous post.

Your answer would depend on many variables.

For example
You lose an engine at 10000 feet
The aircraft glides at 60 mph (best glide speed)
It descends at 1000 feet per minute.
You are heading 090 degrees.
The wind is constant from sea level to 10000 feet at 15 mph out of the west (270 degrees)

_______________________________________

I will go back to the original scenario and make some observations based on the original information given. I will have to add some other variables to complete the calculations.

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? The winds will be equal wether they are a head wind of 10 or tailwind of 10.

Thanks,

-Brian

For this example (15,000ft with a 10kt headwind)
1. The aircraft has a best glide speed (L/D Max) of 10 knots.
2. The aircraft descends at 500 feet per minute at best glide speed.
3. The aircraft is at 15,000 feet
4. The aircraft is flying into a 10 knot headwind.

To lose 15,000 feet at a rate of 500fpm would take 30 minutes (15,000/500=30)
The aircraft has a ground speed of 0 knots (10kt headwind negates 10kt glide speed).
During the 30 minute descent the aircraft travels 0 miles.
___________________________________________

For this example (7,000 ft with a 10kt tailwind):
1. The aircraft has a best glide speed (L/D Max) of 10 knots.
2. The aircraft descends at 500 feet per minute at best glide speed.
3. The aircraft is at 7,000 feet
4. The aircraft is flying with a 10 knot tailwind.

To lose 7,000 feet at a rate of 500fpm would take 14 minutes (7,000/500=14)
The aircraft has a ground speed of 20 knots (10kt tailwind plus the 10kt glide speed).

During the 14 minute descent the aircraft travels 4.6 miles
(14min/60 min per hour = .23 hours) (.23 hours X 20 Knots per hour = 4.6 nautical miles)

_______________________________________

Comparing the two, we come to the conclusion it is better to glide from 7,500 feet with a 10kt tailwind than it is to glide from 15,000ft with a 10kt headwind if the aircraft has a 10kt glidespeed resulting in a 500 fpm descent.

The original question needs more info to answer correctly.
We need to know:
*Best glide speed
*Descent Rate at best Glide Speed
__________________

If you are in an aircraft at 15,000 feet and flying into a headwind when you lose your engine.
If you pitch for L/D Max and turn around so the wind is now a tailwind you will go further. In fact you will go just over twice as far than if you started at 7,000 feet with a tailwind.

 

[PaulThomas]

Is a best glide of 75 knots still your "best" glide in a 75 knot headwind? nupe.

Best glide speed has nothing to do with the wind. The wind will only affect the distance you travel.

I undertand your argument that you would remain stationary and would be better off going down faster to have a higher airspeed in order to move forward, but if you do that, you're not using a best glide speed anymore, even if it gives you the result for the best glide in that direction.

 

[PaulThomas]

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? The winds will be equal wether they are a head wind of 10 or tailwind of 10.

Thanks,

-Brian

It's imposible to answer the question without further information as it depends on the best glide speed of the aircraft. If the aircraft best glide speed is 400knots, 10 knots of headwind or tailwind don't matter. Now if your best glide speed is 8 knots and you have a 10 knot heading... well you're going to be moving backward.

 

[MauleSkinner]

It's imposible to answer the question without further information as it depends on the best glide speed of the aircraft. If the aircraft best glide speed is 400knots, 10 knots of headwind or tailwind don't matter. Now if your best glide speed is 8 knots and you have a 10 knot heading... well you're going to be moving backward.

Actually, you can figure it out...if your best glide speed + 10 knots of tailwind is more than 15/7 of your best glide speed - 10 knots of headwind, you're better off with the tailwind. Now, since your best glide speed is PROBABLY more than 20 knots (especially considering TAS at altitude), I'd take the altitude and the headwind, given the choice.

Fly safe!

David

 

[Lead Sled]

May I suggest that you guys go spend a little time in a glider. While you're at it, ask the CFI-G about the differences between Best L/D Speed, Minimum Sink Speed and Speed to Fly. The concepts apply directly to powered aircraft as well.

'Sled