Gliding Distance

azpilot

azpilot

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azpilot
In another thread, the topic of glide distance came up. James comments that two identical airplanes can glide the same distance, in the same conditions, even if one of the airplanes is heavier than the other. I did some reading on this topic, but it still baffles me. This is how the conversation went:

James331 said:
brcase said:
The one question a remember from my commercial checkride, quite some time ago...

If you have two identical airplanes at the same altitude and location, one at minimum flying weight and the other at Maximum gross weight, which one will glider farther?

Brian​
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​
Spoiler alert, they both glide the same distance, the fat one just goes faster.

As for CPL, my initial was in a seaplane, but it was a easy ride, just a fancy PPL ride with tighter landing tolerances.
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So, I am not going to deny that this is true, but I wonder if someone can help me understand this more clearly. I can understand the best glide speed going up with weight, but I can't understand how the glide distance will remain constant. If the weight goes up, and the glide speed goes up, there will by definition be more wind resistance, which will chew up energy more quickly. I am just not getting this.
 
azpilot said:
If the weight goes up, and the glide speed goes up, there will by definition be more wind resistance, which will chew up energy more quickly. I am just not getting this.
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You are correct that there will be more speed and more "wind resistance" (drag). But the issue is not drag alone, but optimal lift to drag ratio (which is the glide angle). The same glider at a heavier weight will fly faster and generate more lift as well as more drag, both by the exact same factor, which will then cancel out when calculating the lift to drag ratio. Therefore you'll end up with a constant optimal lift to drag ratio, i.e. same glide angle.

Edit: Here is the theory, for the curious.
 
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RotorDude said:
You are correct that there will be more speed and more "wind resistance" (drag). But the issue is not drag alone, but optimal lift to drag ratio (which is the glide angle). The same glider at a heavier weight will fly faster and generate more lift as well as more drag, both by the exact same factor, which will then cancel out when calculating the lift to drag ratio. Therefore you'll end up with a constant optimal lift to drag ratio, i.e. same glide angle.

Edit: Here is the theory, for the curious.
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This really helps. The element I was missing was that lift increases as the speed increases. So the lift increases by the same factor that the drag increases. This makes sense now.
 
Another part of the equation is angle of attack. Essentially every airframe has an Ideal angle of attack that correlates to the best gliding distance.
So if the best L/D angle of attack is 2 degrees (selected arbitrarily for the example) both airplanes will need to fly at the same angle of attack to get the same gliding distance. But the heaver airplane will need to fly faster to maintain the 2 degree AOA than the lighter airplane will.

Brian

PS.
While I am not much of a fan of AOA indicators as a safety device, after all most planes we fly already have an AOA warning. AOA indicators do work very well as a performance indicator, as in the above example one can easily fly the best glide speed no matter what the weight using and AOA indicator.
 
The shape of the airframe doesn't change, which is what is setting the glide distance. Glide *speed* is a function of weight to lift. So if two airplanes are the same size and shape and one weighs twice as much:

1) the lighter airplane will come in at a relatively slow, gentle speed on a glide based on its shape;
2) the heavier airplane will come in on the same glide slope, but screaming down at a much higher speed.

Does that help at all?
 
Okay I don't mean to jump to conclusions (but permit me to)... I think the OP doesn't want theory...he wants it to make sense... And for that I will try to make it make sense.

The initial human thought is... More weight means it takes more energy/work to maintain the objects height. Therefore the lighter object "should" glide farther. This is the initial human thought.

What we are forgetting and why the glide distance is the same is that it took more energy/work to get the heavier plane up to the same altitude as the lighter plane. The key to understanding is this...

The heavier object at the same height above terrain has more potential energy than the lighter object at the same height above terrain. Because potential energy equals Mass x Gravity x height above terrain.

The heavier plane had more "energy in the bank" when the engine quit, but it must spend said energy at a higher rate to glide so that at the time heavier plane has only kinetic energy remaining (hits the ground) both will spend all their saved "energy in the bank" at the same distance.
 
Which is why racing sailplanes have water ballast tanks. When strong thermals are available you fly heavy, climbing slower but gliding faster at the same angle that an unballasted ship flies. When weak thermals are expected you dump the ballast so that you can climb better while giving up some straight line speed.

Or, as shown in my avatar, you dump the water in a high speed pass down the runway leaving a contrail as you pull up into a Chandelle and land victorious!
 
It's a simple matter of weight ratios. A 5 ounce bird cannot carry a 1 pound coconut.
 
A glider's thrust comes from gravity. The more the glider weighs the greater the thrust.
 
brcase said:
Another part of the equation is angle of attack. Essentially every airframe has an Ideal angle of attack that correlates to the best gliding distance.
So if the best L/D angle of attack is 2 degrees (selected arbitrarily for the example) both airplanes will need to fly at the same angle of attack to get the same gliding distance. But the heaver airplane will need to fly faster to maintain the 2 degree AOA than the lighter airplane will.
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^^ This is the Good Stuff(TM) right here.

Now everyone go apply the knowledge in this thread to how much air has to be displaced to keep a Big Honkin Airplane(TM) aloft, and why they have incredibly powerful wingtip vortices... :)
 
FastEddieB said:
European or African?
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Well then of course, African swallows are non-migratory. ;)

It's funny how their conversation sound like many of the threads I read here. :D

 
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denverpilot said:
^^ This is the Good Stuff(TM) right here.

Now everyone go apply the knowledge in this thread to how much air has to be displaced to keep a Big Honkin Airplane(TM) aloft, and why they have incredibly powerful wingtip vortices... :)
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They both . . . float?

Because, like, lift is proportional to speed?
 
As others have pointed out, the drag polar principle is what comes into play here. This is a concept that applies to anything with wings. Power pilots just aren't introduced to it because your average CFI out there hasn't flown a glider.
 
This misconception raises its head whenever someone mentions that they fly a tighter pattern when heavy, because they "won't glide as far" if something happens.

Assuming best glide in most POH's is given for max gross, if you use that same speed at lighter weights, it's when you're lighter that you "won't glide as far". Unless you adjust your glide speed downward for that lighter weight, not something that most pilots are taught to do, in my experience.
 
FastEddieB said:
This misconception raises its head whenever someone mentions that they fly a tighter pattern when heavy, because they "won't glide as far" if something happens.

Assuming best glide in most POH's is given for max gross, if you use that same speed at lighter weights, it's when you're lighter that you "won't glide as far". Unless you adjust your glide speed downward for that lighter weight, not something that most pilots are taught to do, in my experience.
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Nor are they taught to adjust it for wind effects (increase/decrease for head/tailwind).
The good thing is that if you are stretching a long final glide into the wind, being lighter than gross will reduce your best glide speed, while the headwind will increase it, so they tend cancel each other out somewhat.
 
FastEddieB said:
This misconception raises its head whenever someone mentions that they fly a tighter pattern when heavy, because they "won't glide as far" if something happens.

Assuming best glide in most POH's is given for max gross, if you use that same speed at lighter weights, it's when you're lighter that you "won't glide as far". Unless you adjust your glide speed downward for that lighter weight, not something that most pilots are taught to do, in my experience.
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Which is why an AOA indicator is nice.
 
I'm getting it that when weight changes, airspeed must change to maintain the best glide angle available . But I'm having trouble getting my head wrapped around that the glide angle will be shallower and I will travel farther, or the same distance over the ground when I'm heavier. Wouldn't that make the whole Newton and the Apple thing a lie?

I've just started on getting a Glider add on rating. Here's a question on the Pre-solo test I've been given to do.

A sailplane pilot should do which of the following when flying his final approach into a 20 mph headwind and seems to be under shooting.

a) Raise nose to slow just above stall speed and decrease the sink rate.

b) Use spoilers

c). Lower nose to increase penetration

d) Stretch the glide by flying at minimum sink speed

Everything I've read so far tells me that "minimum sink speed" is the same thing as "minimum rate of descent"' so I'm throwing out a) and d) because angle is what I need, not rate. I'm tempted to go with b), but that implies I have some spoiler out and my "use" would be to bring them back in. c) I don't get. Is there some thing called "penetration" that is the answer? All I can see is penetrating something short of the runway.
 
Not glider rated, but see if this thought experiment helps...

Your glider has a best glide speed of 30k. You're 1/8 mile from the runway, flying at 30k into a 30k headwind.

What action would be most likely to get you to the threshold? I'm thinking "c".

"Penetration" is new to me in this context, but I'm sure a glider rated pilot can clarify it.
 
Cooter143 said:
Which is why an AOA indicator is nice.
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No, AoA won't cut it. As I noted above, the concept of best glide distance is that it depends on several variables, one of which is the wind. AoA at best will give you your still air L/D speed, but not the correct one for the current conditions.
 
luvflyin said:
I'm getting it that when weight changes, airspeed must change to maintain the best glide angle available . But I'm having trouble getting my head wrapped around that the glide angle will be shallower and I will travel farther, or the same distance over the ground when I'm heavier. Wouldn't that make the whole Newton and the Apple thing a lie?

I've just started on getting a Glider add on rating. Here's a question on the Pre-solo test I've been given to do.

A sailplane pilot should do which of the following when flying his final approach into a 20 mph headwind and seems to be under shooting.

a) Raise nose to slow just above stall speed and decrease the sink rate.

b) Use spoilers

c). Lower nose to increase penetration

d) Stretch the glide by flying at minimum sink speed

Everything I've read so far tells me that "minimum sink speed" is the same thing as "minimum rate of descent"' so I'm throwing out a) and d) because angle is what I need, not rate. I'm tempted to go with b), but that implies I have some spoiler out and my "use" would be to bring them back in. c) I don't get. Is there some thing called "penetration" that is the answer? All I can see is penetrating something short of the runway.
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That question is doubly-ill-defined.
First, since we don't know if the runway is makeable, even assuming perfect flying, the best course of action might be to find a suitable nearby farmer's field and "land out".
But even if we assume that the runway is within gliding range, the way the question is phrased we might be landing short because we are flying either above or below the optimal speed.
If we do know that we are going faster than the correct speed (which should allow for wind as well as weight as I noted above), then obviously we need to slow down.
If we know we are going too slow, we need to speed up, but we really don't know, the way it's phrased.
What we do know, in all cases, is that if we seem to be landing short, adding spoilers is not the answer. Removing some might be, if we already have some spoilers in.

If you want to know more, read about "polars" here.
 
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If you know the best L/D speed for your aircraft and weight, that speed equates to the AoA that will get you the farthest for the height you can glide. If you have a headwind, that distance will be less than if you have no wind or a tailwind. Flying a different airspeed will not get you farther. Glider pilots speed up in sink so they get out of the sink faster so they can find some lift and they have a formula to determine an optimum speed to fly in sink based on the existing lift they expect. When you're in the pattern to land and at best L/D and it looks like you're going to be short because of a headwind, you've painted yourself into a corner and better find a soft spot to land short. Flying faster or slower than the best L/D speed ain't gonna stretch the glide.
 
Witmo said:
If you know the best L/D speed for your aircraft and weight, that speed equates to the AoA that will get you the farthest for the height you can glide. If you have a headwind, that distance will be less than if you have no wind or a tailwind. Flying a different airspeed will not get you farther. Glider pilots speed up in sink so they get out of the sink faster so they can find some lift and they have a formula to determine an optimum speed to fly in sink based on the existing lift they expect. When you're in the pattern to land and at best L/D and it looks like you're going to be short because of a headwind, you've painted yourself into a corner and better find a soft spot to land short. Flying faster or slower than the best L/D speed ain't gonna stretch the glide.
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Wrong.
To convince yourself, just assume you have both a headwind and best still-air L/D speed of 40 kts.
If you fly best still-air L/D, you'll fly 0 miles before landing. If you add, say, 10 kts, you'll fly a mile or more, depending on initial altitude.
If the headwind is less than your max still-air L/D, then the correct maximum distance speed is higher than the still air L/D by roughly half the headwind component.
I suggest you read up a bit on this topic before replying.
 
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RotorDude said:
No, AoA won't cut it. As I noted above, the concept of best glide distance is that it depends on several variables, one of which is the wind. AoA at best will give you your still air L/D speed, but not the correct one for the current conditions.
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The point I quoted and was referring to was referring to weight's effect on speed. AoA automatically compensates for weight reduction.
 
FastEddieB said:
As RotorDude points out, it most definitely will.
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I stand corrected now that I think about it. The rule of thumb I was taught was to increase the L/D speed by half the wind and all of any gust factor.
 
Cooter143 said:
The point I quoted and was referring to was referring to weight's effect on speed. AoA automatically compensates for weight reduction.
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But only gives you best power off range in still air, which is rare. Normally you have some wind component, in which case your AoA is only a reference point, but not "speed to fly". Therefore, in a power off glide, you are better off just knowing your approximate max L/D speed at your current weight, and adding/subtracting the wind component factor (and sink/lift to be pedantic). The AoA in this case is not really helpful.
AoA would be helpful when you lose your engine (or tow rope in a glider) right after takeoff and want to stay right above the stall for minimum sink while executing a 180 or 270 back. Once you are out of the turn and on a long final glide, you are better off using airspeed with wind and weight correction, as per above.
 
RotorDude said:
But only gives you best power off range in still air, which is rare. Normally you have some wind component, in which case your AoA is only a reference point, but not "speed to fly". Therefore, in a power off glide, you are better off just knowing your approximate max L/D speed at your current weight, and adding/subtracting the wind component factor (and sink/lift to be pedantic). The AoA in this case is not really helpful.
AoA would be helpful when you lose your engine (or tow rope in a glider) right after takeoff and want to stay right above the stall for minimum sink while executing a 180 or 270 back. Once you are out of the turn and on a long final glide, you are better off using airspeed with wind and weight correction, as per above.
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You seem eager to disagree. AOA corrects for weight, how is that not helpful?
 
Cooter143 said:
You seem eager to disagree. AOA corrects for weight, how is that not helpful?
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Not at all eager to disagree, just eager to set the record straight, since many newbies are reading this.
AoA corrects for weight, which is good, and it also corrects for bank angle, which is even better. But when you are trying to stretch your glide, it could be a distraction. The weight correction compared to your typical flying configuration would likely be much smaller than the likely wind correction. Also, when stretching your engine-less glide, you are unlikely to make too many turns, so the bank correction should not be an issue.
Bottom line: when losing an engine and making tight turns at minimum sink, AoA is very helpful, whereas on a long final glide you are better off using your known approximate L/D speed plus a wind correction.
BTW, it would be nice to have a GPS-calculated range, based on current ground speed and rate of descent. That would actually take into account all parameters, if you fly to maximize that number.
I am not sure if any (power) GA installations have that feature, however.
 
RotorDude said:
But when you are trying to stretch your glide, it could be a distraction.
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I don't really see how, especially if you are used to using it. It seems more likely that trying to do the mental math of finding and correcting for GW would be more of a distraction.

weight correction compared to your typical flying configuration would likely be much smaller than the likely wind correction.
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Depends on what you fly.

My point was pretty simple, AoA gives more information not less and it is nice to have. And as a practical matter, any adjustment needed for winds after establishing Max Range AoA is usually just a couple of clicks of trim.
 
FastEddieB said:
You're no fun.

We came here for an argument;)!
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I'm starting to think that's why a lot of people come here! It does make for good entertainment though, it can get kind of boring if everyone agrees.
 
Cooter143 said:
I don't really see how, especially if you are used to using it. It seems more likely that trying to do the mental math of finding and correcting for GW would be more of a distraction.


Depends on what you fly.

My point was pretty simple, AoA gives more information not less and it is nice to have. And as a practical matter, any adjustment needed for winds after establishing Max Range AoA is usually just a couple of clicks of trim.
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I am not against AoA, esp. for slow speeds and tight turns where it shines. But for glide stretching it might create a distraction under stress. Maybe not to you, but to an average pilot, IMO.

Edit: I'd like to add that perhaps I am too focused on the typical case of a GA pilot who flies a single light plane. If you fly a heavier/larger plane, or perhaps several different types, and/or the load varies a lot, then the AoA would be a good way to get the reference L/D speed, to which you can add the wind correction.
 
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Seriously, the idea to get across is that when facing a headwind, you want to minimize the time in that headwind, so you go a bit faster. With a tailwind, you want to maximize the time in the tailwind, so you go a bit slower. Exactly how much faster or slower involves math, which is not my forté. But don't let the perfect be the enemy of the good - a bit faster or slower as appropriate can only help.

Applies to cruise as well as gliding, which is both obvious if you think about it, and has real-world implications when flight planning.
 
Ok. I'm at the Gliderport. c) is the answer. The point is increase airspeed to increase energy so ya got some to deal with things. It isn't about trying to "stretch" the the glide by shallowing the glide angle which obviously won't work. It may get you down in ground effect which might get you to the runway. If there are "things" on short final you might have some energy to "jump" them.
 
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