L / D Max with Headwind / Tailwind...

[avbug]

Putting the wind on your tail will always give you more choices of where to put it down.

I agree with most of your statement, except that.

A tailwind from one thousand feet is not at all the same as a headwind from fifteen. A small airplane with a sixty knot glide speed and a five hunded foot per minute rate of descent in that condition (assuming no other changes with altitude) provides a 50 knot airpane traveling forward into the wind for 30 minutes, or a 70 knot airplane traveling with the wind for two minutes.

In either case, the airplane at fifteen thousand has a decided advantage, particularly if he or she elects to fly with the wind rather than against it.

As others have correctly noted, however, achieving a safe touchdown is the goal, as opposed to merely traveling as far as one can possibly go. If one is flying that single engine airplane properly, one has kept a suitable forced landing site underneath for the duration of the flight, and knows where it is long before the power has failed.

 

[dictatortot]

Reading through the posts, it sounds like everyone has the right idea. I'll try to add my two cents with some examples of situations where you'd use L/D max vs. best glide, plus that "speed to fly" thing.

Most pilots have a good grasp on the idea of best glide speed. After an engine failure, I think of it as a compromise between trying to minimize your rate of descent while actually making some progress over the ground on your way to your forced landing site.

What if the engine quits right over a nice big field and you're just going to spiral down and land there? In this situation your progress across the ground doesn't matter anymore and L/D max (also called min-sink) would give you your maximum time aloft to run checklists and prepare for the forced landing.

Now what if the engine quits over the mountains with only one airport within gliding distance? You slow to best glide speed and point the nose towards the field but then you realize you've got to buck a 30 knot headwind to get there. In this situation, you're more interested in your progress over the ground, and in order to make some headway into the wind, you've got to increase your speed a bit faster than best glide in order to make the field.

Speed to fly ties these two situations together. If your engine quits and you've got a tailwind on the way to the forced landing site, slow up to min-sink speed (this was only three knots below best glide speed in the gliders I used to fly, but I imagine it's quite a bit slower than best glide speed in powered aircraft.) If you've got a headwind on the way to your forced landing site, add half of the headwind to your best glide speed to help make progress across the ground.

 

[A Squared]

Reading through the posts, it sounds like everyone has the right idea. I'll try to add my two cents with some examples of situations where you'd use L/D max vs. best glide, plus that "speed to fly" thing.

...............
In this situation your progress across the ground doesn't matter anymore and L/D max (also called min-sink) would give you your maximum time aloft to run checklists and prepare for the forced landing.

L/D max is best glide, not minimum sink.

 

[dictatortot]

L/D max is best glide, not minimum sink.

You're correct; I guess I rushed a bit at the end there and got mixed up.

Has anyone plotted some L/D curves in anything powered? Is there any other way to obtain min sink speed other than plotting the curve and drawing the tangent line?

 

[VNugget]

I was actually planning on doing that in a 172 sometime in the near future, for a similar thread on ***************. There's disagreement whether going faster than L/D max helps or not in a headwind.

 

[Huggyu2]

As the tailwind increases, and your "best glide distance" speed decreases, is it valid to say that when you get to min sink speed, it will stop there, even if the tailwind continues to increase?

 

[A Squared]

You're correct; I guess I rushed a bit at the end there and got mixed up.

Has anyone plotted some L/D curves in anything powered? Is there any other way to obtain min sink speed other than plotting the curve and drawing the tangent line?

You had me wondering there. You obviously grasp the concepts ... and then you say min sink happens at L/D max.......Hmmmm.

I was actually planning on doing that in a 172 sometime in the near future, for a similar thread on ***************. There's disagreement whether going faster than L/D max helps or not in a headwind.

Not sure where this discussion is taking place, but that's not exactly a debate. That's just someone who just doesn't grasp the principle. The soaring community has known this and proved it (and utilized it over and over) as long as there have been gliders. It's a simple and fairly obvious principle. "debating" it is like debating the sum of 2 and 2. You could take the position that it's 5, but you're just wrong.