Best Glide

C

cre8flyer

New Member
I've always understood that to maximize glide range (simulated engine failure) you should use the published best glide speed. . . makes sense to me. . .

Recently a glider pilot told me that when there is a headwind on final, increasing your airspeed (above published Vg) will help you reach the runway, and that all glider pilots know and practice this.

Can anyone shed some advanced aerodynamics light on the apparent contradiction here?

Thanks!
 
cre8flyer said:
Can anyone shed some advanced aerodynamics light on the apparent contradiction here?
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The glider pilot is correct, but it's not really an aerodynamics issue. Within the body of air that the aircraft is moving, best glide will be whatever your POH says for your weight.

Let's consider an extreme example: If you're in a C172N, with a best glide of 65 knots, what sort of range would you have going into a 65 knot headwind? Zero! Obviously, you would get a better range, with respect to the ground, by increasing your airspeed over best glide. Rule of thumb glider pilots use is 1/2 the wind velocity.
 
Yes, interesting explanation. . .

What is the precise intention in calculating best glide airspeed?
1. Minimize sink rate (In this case your explanation would be correct)
2. Maximize horizontal range
3. Something else?
 
cre8flyer said:
Yes, interesting explanation. . .

What is the precise intention in calculating best glide airspeed?
1. Minimize sink rate (In this case your explanation would be correct)
2. Maximize horizontal range
3. Something else?
Click to expand...

Best glide is NOT minimum sink rate. It's the highest ratio of horizontal velocity to vertical velocity. You can achieve a lower sink rate by decreasing airspeed to below best glide, but your forward velocity suffers by a greater amount, leading to an overall reduction in range.

Real world, with wind, we want to maximize the ratio of ground speed to vertical speed and best glide is only a good approximation to that when winds are light.
 
tgrayson said:
The glider pilot is correct, but it's not really an aerodynamics issue. Within the body of air that the aircraft is moving, best glide will be whatever your POH says for your weight.

Let's consider an extreme example: If you're in a C172N, with a best glide of 65 knots, what sort of range would you have going into a 65 knot headwind? Zero! Obviously, you would get a better range, with respect to the ground, by increasing your airspeed over best glide. Rule of thumb glider pilots use is 1/2 the wind velocity.
Click to expand...

1/2 wind velocity increase to Vg? Should a 172 pilot consider the same factors?
 
cre8flyer said:
How do you know all this stuff anyway? How can I get that smart too?
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This sort of information generally is a by-product of reading about basic aerodynamic and performance theory. While the books may not *directly* answer your question, they give you the tools to find the answer. If you don't take an interest in the tools, then you won't find any answers in the books. In this way, knowledge is hidden from the unworthy. ;)

Gliding performance is often displayed in "hodographs" or "speed polar" diagrams, which basically graph a horizontal velocity with respect to a vertical velocity. A line drawn from the origin tangent to the graphed curve shows directly the angle of descent. You can take into account wind by shifting the line's origin left or right, which will show you the optimal glide speed and angle of descent. Many glider pilots are aware of these diagrams.
 
meritflyer said:
1/2 wind velocity increase to Vg? Should a 172 pilot consider the same factors?
Click to expand...

No, increase to Vbg. And, sure, any pilot should take this into account, assuming that his goal is to maximize his range. The flip side of the situation the OP brought up is having a tailwind. Here you should decrease your best glide speed by 1/2 the wind velocity, and you'll get a lot more range that flying against the headwind.
 
tgrayson said:
While the books may not *directly* answer your question, they give you the tools to find the answer. If you don't take an interest in the tools, then you won't find any answers in the books. In this way, knowledge is hidden from the unworthy. ;)
Click to expand...
I, too, was struck by the simplicity and directness of this quote, and worthy of repeating.

As an instructor, I spend way too much time trying to "handle" the tools for the student, in a hope that he/she will 'take an interest', and learn to "handle/use" the tools.

Your quote made this very clear to me.
 
tgrayson said:
This sort of information generally is a by-product of reading about basic aerodynamic and performance theory. While the books may not *directly* answer your question, they give you the tools to find the answer. If you don't take an interest in the tools, then you won't find any answers in the books. In this way, knowledge is hidden from the unworthy. ;)
Click to expand...

Yes, that is a brilliant paragraph. You could pull out the words "basic aerodynamic and performance theory" and insert similar descriptive words on just about any topic and it would be true as well.
 
seagull said:
Yes, that is a brilliant paragraph. You could pull out the words "basic aerodynamic and performance theory" and insert similar descriptive words on just about any topic and it would be true as well.
Click to expand...

I wish I had meant it to be brilliant, because then I'd be pleased. As it is, I'm blushing.
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