"Best Glide Speed"

[tlewis95]

Ok, as a glider pilot, I get to debate this with my instructors all the time.

Whenever we do maneuvers ("steep spiral" is the main one) that require us to stay in the same area when doing a power off descent, I am always told to fly best glide (best L/D) speed.

This would be the speed to use if we were trying to glide somewhere in a straight line with no wind. Whenever I apply wind corrections to give us our "speed to fly" the CFIs seem to disagree.

Also, if we are staying in the same place, just spiraling down to altitude, minimum sink speed is the logical speed to use in my mind. Now I know that there isn't one published for any of the aircraft that I fly, but knocking 3-5kts off of best glide is probably going to get you min-sink speed.

So technically, the only time you would want to use "best glide" speed, is when you are trying to get to a landing site with no wind.

Does it make sense to anyone else?

Just a thought.

 

[bLizZuE]

Deviation from best glide on the faster or slower side will only induce more 'induced' drag or 'parasite' drag. So I can't see how decreasing the airspeed would give you less of a 'sink'.

 

[tlewis95]

Deviation from best glide on the faster or slower side will only induce more 'induced' drag or 'parasite' drag. So I can't see how decreasing the airspeed would give you less of a 'sink'.

It just gives you a lower sink rate. You aren't going to go as far as you would at Best Glide, but you will be in the air longer.

 

[tgrayson]

Does it make sense to anyone else?

Min sink speed makes sense when you're trying to maximize your time in the air, as you would with a glider. This speed is theoretically .76 of the Vldmax, but the idle thrust would screw that up a bit. But it's easy to figure out in an airplane...just minimize the VSI.

I also agree that min sink makes sense for the steep spiral, but this velocity will be well below stall speed in a 60 degree bank, hence not advisable. If you're spiraling down to altitude for some other reason, it may or may not make sense to use min sink; sometimes you're in a hurry and you want to use max sink.

Note to all: best glide speed does NOT give you the minimum sink rate; it gives you the best ratio of forward velocity to sink rate. You can achieve a smaller sink rate with a lower velocity, but your forward velocity is reduced even more, reducing your glide range.

 

[bLizZuE]

We actually teach steep spirals at best glide +10 to ensure no accidental accelerated stalls. I understand why you may want to use a minimum sink speed, however we use a steep spiral maneuver so as not to leave our reference point, key position, or emergency landing field. If you want to have more time to troubleshoot a problem, I think best glide would give you more time in the air.

Essentially, I don't understand why you would want to use min sink.

 

[tgrayson]

more time to troubleshoot a problem, I think best glide would give you more time in the air.

Min sink gives you max time in the air. The difference between min sink and best glide is similar to the difference between Vy and Vx. Min sink is the minimum altitude loss per unit time, whereas best glide is minimum altitude loss per unit distance.

 

[bLizZuE]

Hrmph.

 

[Douglas]

You are right tlewis, but for reasons brought up by Tgrayson, your instructors are right, even though they don't know why. I shouldn't say that, maybe they do know why.

 

[PeterPilot]

Does it make sense to anyone else?

Yes it does, but good luck explaining it to most airplane pilots.

What you are describing is the core skill of speed-to-fly that every successful cross-country glider pilot uses. It is explained in Chapter 5 of the FAA’s Glider Flying Handbook (FAA-H-8083-13). The principle applies in gliders, airplanes, and rotorcraft alike. The only difference is the shape of the performance curve.

Unfortunately, most people do not understand the principle and only have one “speed to fly” in mind, which is the max L/D in still air. As you have noted, this can create a problem when being “checked” by someone who only knows one “speed to fly”.

 

[MNFlyGuy]

The CFI I did my spin training with was also a glider CFI. After we finished my spin training we got into a discussion of polar curves and glider performance. I could not recreate what he showed me that day, but I still retatin my initial reaction of "wow, that is cool"

 

[tlewis95]

Yes it does, but good luck explaining it to most airplane pilots.

What you are describing is the core skill of speed-to-fly that every successful cross-country glider pilot uses. It is explained in Chapter 5 of the FAA’s Glider Flying Handbook (FAA-H-8083-13). The principle applies in gliders, airplanes, and rotorcraft alike. The only difference is the shape of the performance curve.

Unfortunately, most people do not understand the principle and only have one “speed to fly” in mind, which is the max L/D in still air. As you have noted, this can create a problem when being “checked” by someone who only knows one “speed to fly”.

Sounds about right.

Thanks.

 

[Realms09]

Deviation from best glide on the faster or slower side will only induce more 'induced' drag or 'parasite' drag. So I can't see how decreasing the airspeed would give you less of a 'sink'.

Even if your goal is max range, do you see how the winds could make your ideal speed slower or faster than max L/D speed? In an infinitely strong tailwind your maximum range will be obtained by flying at your minimum sink speed, not your maximum L/D speed.

 

[PeterPilot]

To see the other side of picture, put a fixed power setting on an airplane that allows it to climb (e.g. an AMEL struggling to climb on one engine). Then draw a line from the origin of the graph to the polar curve to find Vx. The top of the polar curve is Vy. This graphically demonstrates why Vx is always less than Vy.

Incidentally, if you are a glider pilot and really understand your art, try explaining to airplane pilots why the best glide speed changes with weight, but the glide ratio remains the same.

 

[MidlifeFlyer]

Yes it does, but good luck explaining it to most airplane pilots.

Most airplane pilots know that gliders can't possibly fly anyway - since they don't have power, either they can't climb or they can't control their airspeed (depending on which version of the gospel the airplane pilot buys into)
:argue:


Sorry, but with the "slow flight" thread in another part of the forum I cou;nd' resist. Anyone got a nice flamethrower smiley?

 

[tlewis95]

Most airplane pilots know that gliders can't possibly fly anyway - since they don't have power, either they can't climb or they can't control their airspeed (depending on which version of the gospel the airplane pilot buys into)
:argue:


Sorry, but with the "slow flight" thread in another part of the forum I cou;nd' resist. Anyone got a nice flamethrower smiley?

Doing static displays with gliders, you would be amazed at how many airline pilots didn't get what we were doing. Mostly when we tell them about a recent flight (maybe 250nm) all they can comment about is how windy it must have been that day, like it was the wind keeping the glider in the air, like it was some sort of kite.

 

[c172captain]

Doing static displays with gliders, you would be amazed at how many airline pilots didn't get what we were doing. Mostly when we tell them about a recent flight (maybe 250nm) all they can comment about is how windy it must have been that day, like it was the wind keeping the glider in the air, like it was some sort of kite.

I thought it was a kite

 

[///AMG]

Incidentally, if you are a glider pilot and really understand your art, try explaining to airplane pilots why the best glide speed changes with weight, but the glide ratio remains the same.

Not hard to understand, even for this "airplane pilot"

 

[fish314]

Even if your goal is max range, do you see how the winds could make your ideal speed slower or faster than max L/D speed? In an infinitely strong tailwind your maximum range will be obtained by flying at your minimum sink speed, not your maximum L/D speed.

Well, in an "infinitely" strong tailwind you'd have a hell of a time, what with all those relativistic effects! Just kidding, and you've got a great point.

When I explain this to guys, though, I usually use a headwind as an example.

Students will (hopefully) tell me that the best glide range occurs at the best glide speed (L/D max). So then I ask them to imagine an airplane for which best glide speed is 100 knots (true) flying in a 100 knot direct headwind. They understand that in that condition the airplane goes nowhere, but if we speed up we are at least making some forward progress. So our glide range has improved by flying OFF of the L/D Max speed.

 

[tlewis95]

So then I ask them to imagine an airplane for which best glide speed is 100 knots (true) flying in a 100 knot direct headwind. They understand that in that condition the airplane goes nowhere, but if we speed up we are at least making some forward progress. So our glide range has improved by flying OFF of the L/D Max speed.

:yeahthat:

Pretty good way to explain it without getting too technical I think.

I like it.

 

[Realms09]

Well, in an "infinitely" strong tailwind you'd have a hell of a time, what with all those relativistic effects!

Tangent:

Hey, turns out it's a real field! I listened to an astrophysics guy give a talk on relativistic fluid mechanics. These situations occur in gas nebulae out there in the cosmos. I don't remember much of it, but if you think about it there are some similarities to "normal" fluid mechanics. In aeronautics the relation of flow speed to the speed of sound tells how information can be transmitted in the fluid through pressure variations. In supersonic flow an object must be in another object's Mach cone to be affected by it. Now we zoom out to the universal scale. Here, the speed of light limits the transmission of information and an object must be in another object's "light cone" (in space-time) to be affected by it.

Also, if I remember correctly the Prandtl-Glauert rule for compressibility has a similar form to the Lorentz transformation in relativity.

Alright, enough nerd for one post.