Vg

[innovator152]

Vg is the airspeed for best glide, or greatest distance traveled per loss of altitude (best range) right?
is there an airspeed that would give you the lowest rate of descent (best endurance)?
or are they one in the same?

i thought i remember reading or hearing of such a thing, any help would be appreciated.

 

[The Gardener]

Sure, minimum sink. It is generally a few knots slower than best glide but it isn't a published speed. In a true emergency you'd be hard pressed to justify changing speeds because the benefits would be largely negligible and time could be better spent on other tasks (IMO).

 

[inigo88]

Minimum Sink will keep you in the air longer but will shorten your gliding range.

Check out the first couple sentences in Polar Curves.

 

[ryan1234]

Vg is the airspeed for best glide, or greatest distance traveled per loss of altitude (best range) right?
is there an airspeed that would give you the lowest rate of descent (best endurance)?
or are they one in the same?

i thought i remember reading or hearing of such a thing, any help would be appreciated.

To be overly precise, V-best glide will change slightly with weight and wind - depending on the airframe. It probably wouldn't hurt to think of V-best glide as a function more of total ground distance covered... or a function of groundspeed.

For example, If a small aircraft had a published best glide of, say 45 knots, and you were fighting a headwind of 45 knots.... best glide speed would not be 45 knots. You would want a faster speed to make forward progress.... or in a pragmatic situation, if you weren't over water, you could just turn around - and then you would want to take advantage of the tailwind, and best glide would be closer to the min. sink airspeed.

As far as weight goes, it would be better to be heavy during a headwind and light during a tailwind to cover the most distance.

How is this in any way a real world situation? Winds aloft and TAS change with alititude for the most part - how can you make the best of your range if you were over water or rugged terrain?

 

[The Gardener]

To be overly precise, V-best glide will change slightly with weight and wind - depending on the airframe. It probably wouldn't hurt to think of V-best glide as a function more of total ground distance covered... or a function of groundspeed.

For example, If a small aircraft had a published best glide of, say 45 knots, and you were fighting a headwind of 45 knots.... best glide speed would not be 45 knots. You would want a faster speed to make forward progress.... or in a pragmatic situation, if you weren't over water, you could just turn around - and then you would want to take advantage of the tailwind, and best glide would be closer to the min. sink airspeed.

As far as weight goes, it would be better to be heavy during a headwind and light during a tailwind to cover the most distance.

How is this in any way a real world situation? Winds aloft and TAS change with alititude for the most part - how can you make the best of your range if you were over water or rugged terrain?

All true, however if a pilot counting on a tailwind or accelerating into a headwind to make a landing site they should probably have a good think about how and where they are planning flights.

 

[Fly_Unity]

All true, however if a pilot counting on a tailwind or accelerating into a headwind to make a landing site they should probably have a good think about how and where they are planning flights.

???

I never really worried about headwinds or tailwinds during training or instructing until I started flying gliders.

Now its more of a natural instinct I guess, but my power off 180's and emergencies have became quite a bit better if I may say so. I still have to do a yearly single engine check-ride and maximizing my glide sure saved my day a few times.

 

[The Gardener]

???

I never really worried about headwinds or tailwinds during training or instructing until I started flying gliders.

Now its more of a natural instinct I guess, but my power off 180's and emergencies have became quite a bit better if I may say so. I still have to do a yearly single engine check-ride and maximizing my glide sure saved my day a few times.

I was more imagining a pilot sitting at 5000'agl saying, if I can just pitch for minimum sink I'll be able to just barely make the shoreline! Agreed on what you say about landing 100%. Maybe I was off context.

 

[ryan1234]

All true, however if a pilot counting on a tailwind or accelerating into a headwind to make a landing site they should probably have a good think about how and where they are planning flights.

I always end up getting to this point - but the reason why I say it is because a situation happened to me a little over a year ago (and can happen to anyone) on a long ferry flight in a single engine Cessna from FL to Puerto Rico. After taking off from the Turks (which is pretty much all water until PR), at about 12,000 the fuel valve failed something like about 120+nm out of Puerto Rico...So how were the winds going to affect me? Based on what I had calculated for winds aloft, Punta Cana was the best alternate... plotted about 90nm out. The "good" fuel tank was all of the sudden indicating about 4 gal - how accurate are those gauges anyways? I was beyond radio coverage anywhere, approaching about 5pm (which was getting dark). Best economy - super lean (who cares about the debate on long term engine effects!).... how should I save what TAS I have? Min sink? The course to Punta Cana luckily had a modest tailwind up high - minimum sink at a reduce power setting seemed to be the best choice. Say I have 2.5-3gal left..... Did lots of Math! No dice trying to contact San Juan Center - finally a Jet Blue flight relayed my situation and I eventually was able to get in touch with Punta Cana to declare the emergency.

It was great to see the coast, but I wasn't as high as I'd like... made a dog leg base to final for 9 after they cleared the runway as the wing dropped the engine gave up the ghost pretty much... and by the taxi-way turn off the prop had stopped spinning.

So... do the details matter? Well maybe in most cases they don't... but it's certainly good to know just in case.

 

[The Gardener]

I always end up getting to this point - but the reason why I say it is because a situation happened to me a little over a year ago (and can happen to anyone) on a long ferry flight in a single engine Cessna from FL to Puerto Rico. After taking off from the Turks (which is pretty much all water until PR), at about 12,000 the fuel valve failed something like about 120+nm out of Puerto Rico...So how were the winds going to affect me? Based on what I had calculated for winds aloft, Punta Cana was the best alternate... plotted about 90nm out. The "good" fuel tank was all of the sudden indicating about 4 gal - how accurate are those gauges anyways? I was beyond radio coverage anywhere, approaching about 5pm (which was getting dark). Best economy - super lean (who cares about the debate on long term engine effects!).... how should I save what TAS I have? Min sink? The course to Punta Cana luckily had a modest tailwind up high - minimum sink at a reduce power setting seemed to be the best choice. Say I have 2.5-3gal left..... Did lots of Math! No dice trying to contact San Juan Center - finally a Jet Blue flight relayed my situation and I eventually was able to get in touch with Punta Cana to declare the emergency.

It was great to see the coast, but I wasn't as high as I'd like... made a dog leg base to final for 9 after they cleared the runway as the wing dropped the engine gave up the ghost pretty much... and by the taxi-way turn off the prop had stopped spinning.

So... do the details matter? Well maybe in most cases they don't... but it's certainly good to know just in case.

Great story and thanks for sharing. A pilot I know of took off with a fuel drain open and flooding fuel out onto the ground. After rotation the power was lost and he ended up stopping at the end of the over run with just a few feet to spare before the dirt. You are right that in some cases, just a few feet might make it or break it!