Gross weight vs Glidespeed

[KickSave]

I'm tuning up my aerodynamics knowledge for my checkride and I came across a question that I haven't been able to find a solid answer for in my text books.

How does gross weight affect the IAS you want to pitch for in a power off glide (best glide speed)? So far, each of the few POH's I have read only list one Vglide for that make/model. The only reference to this specifically that I can find is in the Handbook of Aeronautical Knowledge, page 1-24 on Forces in a Glide. It states that variations on gross weight do not affect glide angle provided that the optimim IAS for "each gross weight" is used. Then it says that the fully loaded airplane will sink faster, but at a greater forward speed (ground speed) and reach the ground faster, yet travel the same distance as the lighter airplane. This seems to say that as your plane is heavier, you would pitch for a higher Vglide IAS.

But later it states that the higher airspeed corresponding to the increased weight is provided automatically by the larger component of weight acting along the glide path. This seems to say that you pitch for the same VGlide IAS regardless of weight, and your groundspeed will vary based on weight, but your IAS will not.

So I'm confused. I've read all my other sources and none of them cover this in enough depth. Do you pitch for different Vglides based on your weight, higher IAS for higher gross weight? Or do you always have only one IAS for VGlide, and the ground speed variation based on weight is the only thing that changes?

Any insight would be appreciated

/Dave


PS, upon further review, there is a line that states the speed for L/Dmax will vary for different airplane weights, but it's doesn't specify if this is IAS, TAS or even ground speed, to which the glide angle really depends on. Still confused

 

[52vincent]

This link may help you out.

http://avstop.com/AC/1-23.html

 

[KickSave]

52, thanks, but that is the exact wording of the section I already quoted from the Pilots Handbook of Aeronautical Knowledge(AC 61-23C). Although it's the best reference I've found on the subject, it still left me with the questions I posed originally.

 

[midlifeflyer]

There are a bunch of speeds that are published at max gross weight and reduce at lower weights. Vs, Va, and Vg are among them. So it's a matter of flying at a lower IAS at a lower weight.

The universal formula is

V1=V2 * Sq Root (W1/W2)

Where:
V1=Target Airspeed
V2=Published airspeed for a given weight
W1=Weight for which airspeed is to be determined
W2=Gross weight for which airspeeds are published

It's not an enormous issue in light aircraft because the weight range isn't that great, and when we're talking about glide speed, the fastest speed isn't all that fast. But if you look at a POH for something big, you'll all of see these speeds listed at various weights. Because of the importance of Va, you'll often see the different listings even is small airplanes (like a 172).

We're dealing with IAS here. GS is a function of wind not weight. I think what the manual is trying to tell you is that assuming no winds, even though you will drop faster, you will also be moving forward faster, so the net glide change is negligible. Remember that best glide is least loss of altitude over distance, not least loss of altitude over time.

 

[KickSave]

Midlife, bingo, that was what I was looking for, thanks!

Basically all the POH's I own give one Vglide. As you mention, in larger airplanes, the POH will give different Vglides, just like the 172 POH gives different Va's.

The formula you posted fills in the rest.

Although a 172 POH only gives a single 65 kias as the Vglide(assuming they intended that speed to be at max gross wgt), using that formula you gave would produce 58 kias if you used a weight of 2000 lbs instead of max gross of 2450. Not a huge difference, but it could be the difference between hitting the numbers and landing in the dirt, I'd imagine. In the ArrowIII that I'm taking my ride in, the differences become more pronounced, about 11 kias.

Confusion lifted... thanks

BTW where did you find that formula, or do you have any recommended reading references that cover this stuff in more detail than AC61-23C ?


/Dave

 

[GeorgeTG]

I'm tuning up my aerodynamics knowledge for my checkride and I came across a question that I haven't been able to find a solid answer for in my text books.

How does gross weight affect the IAS you want to pitch for in a power off glide (best glide speed)? So far, each of the few POH's I have read only list one Vglide for that make/model. The only reference to this specifically that I can find is in the Handbook of Aeronautical Knowledge, page 1-24 on Forces in a Glide. It states that variations on gross weight do not affect glide angle provided that the optimim IAS for "each gross weight" is used. Then it says that the fully loaded airplane will sink faster, but at a greater forward speed (ground speed) and reach the ground faster, yet travel the same distance as the lighter airplane. This seems to say that as your plane is heavier, you would pitch for a higher Vglide IAS.

But later it states that the higher airspeed corresponding to the increased weight is provided automatically by the larger component of weight acting along the glide path. This seems to say that you pitch for the same VGlide IAS regardless of weight, and your groundspeed will vary based on weight, but your IAS will not.

So I'm confused. I've read all my other sources and none of them cover this in enough depth. Do you pitch for different Vglides based on your weight, higher IAS for higher gross weight? Or do you always have only one IAS for VGlide, and the ground speed variation based on weight is the only thing that changes?

Published best glide is typically for max gross so you will need to reduce the speed for lighter weights. Use the formula above for calculating the speed reduction.

Beware of the fact that L/D max is not the best glide speed.
Best glide is achieved at the tangent to the power required curve.

If you like graphs (more intuitive than math I think) try this:

Take your power required cure and mirror it vertically
(it looked like a valley, now it looks like a hill)

Change the label on the vertical axis to sink rate

L/D max is now Minimum Sink

Starting at the origin (zero airspeed, zero sink) draw a tangent to the curve. Where the tangent touches the curve you have best glide.

Heavier weights will shift the curve down and to the right. (at heavier weights you will stall at a faster speed and your minimum sink rate will be greater)

more discussion here (with math;-)
web.usna.navy.mil/~dfr/flying/weight1.pdf

Cheers
George

 

[midlifeflyer]

BTW where did you find that formula, or do you have any recommended reading references that cover this stuff in more detail than AC61-23C ?

I picked up the formula long ago in another forum, but it can be found in AC 23-8A, the Flight Test Guide for Certification.

GeorgeTG gave you part of the rest of the answer. If you can get a hold of Aerodynamics for Naval Aviators, that really is one of the best volumes on this and other aerodynamic subjects. Although it can get really technical, it =is= designed for pilots. I took a quick look and here is that book answering your question

An unbelievable feature of gliding performance is the effect of airplane gross weight. Since the maximum lift-drag ratio of a given airplane is an intrinsic property of the aerodynamic configuration, gross weight will not affect the gliding performance. If a typical jet trainer has an (L/D)max of 15, the aircraft can obtain a maximum of 15 miles horizontal distance for each mile of altitude. This would be true of this particular airplane at any gross weight if the airplane is flown at the angle of attack for (L/D)max. Of course, the gross weight would affect the glide airspeed necessary for this particular angle of attack but the glide ratio would be unaffected.

It's available for sale in various outlets. I seem to recall that there was a huge pdf file download available somewhere, but I don't have the link anymore.

 

[KickSave]

Much thanks to both George and Mark. I'll grab those references to keep the learning up

/Dave

 

[cookmg]

Another element to the best glide subject that I find interesting is the effect of winds.

In a headwind, you should increase your glide speed (IAS) to reach max glide. By how much? I would like to know. I've read 1/2 the headwind component, but that wasn't in 61-23C.

Interesting thought . . . if two airplanes at different weights are in a glide into a headwind, would the heavier plane go farther as it will spend less time aloft and therefore less time in the headwind.

 

[KickSave]

Interesting thought . . . if two airplanes at different weights are in a glide into a headwind, would the heavier plane go farther as it will spend less time aloft and therefore less time in the headwind.

According to 61-23C, assuming the same make/model and flown at the same airspeed, the heavier plane will travel the same distance across the ground, it will just get there faster (less time)

Weight effects endurance, wind effects range. The best bet is to be light and glide downwind(best range, longest endurance). The worst glide performance will come when you are heavy, heading into the wind (shortest range, shorter endurance).

 

[GeorgeTG]

The worst glide performance will come when you are heavy, heading into the wind (shortest range, shorter endurance.

Not quite
as cookmg pointed out:

heavier weight into headwind is better.
(same glide ratio in air, but less time exposed to headwind)

Thye worst case is lightweight into headwind. (same glide ratio in air, longer time exposed to headwind)

Case in point: high performance sailplanes carry water ballast in the wing for high speed runs on cross coutries.

Cheers
George

 

[KickSave]

Not quite
as cookmg pointed out:

heavier weight into headwind is better.
(same glide ratio in air, but less time exposed to headwind)

Thye worst case is lightweight into headwind. (same glide ratio in air, longer time exposed to headwind)

Case in point: high performance sailplanes carry water ballast in the wing for high speed runs on cross coutries.

Cheers
George

Ok now ya lost me. If we agree that headwinds reduce range(Same time in the air, but less ground covered), and higher weights reduce endurance (less time in the air, but the same ground covered), my feeling is that in an emergency glide, you want the most distance AND the longest time in the air - to reach the most suitable landing sight and to have more time to prepare.

So why then would it be advantageous to be heavier? Are you saying that if you are lighter, the longer time you spend in a headwind will decrease your range? My understanding is that the range and endurance profiles are, all other things being equal, not effected by the same factors.

I guess it doesn't matter too much in a typical light single, since we can't change weight too much once the engine quits, but we can effort to glide down wind.

Man I need to go to sleep now

 

[midlifeflyer]

Another element to the best glide subject that I find interesting is the effect of winds.

In a headwind, you should increase your glide speed (IAS) to reach max glide.

I think that's accurate, but lets watch out for the language we are using. I noticed KickSave's next message while I was typing this. The confusion may be the result in cross-using the terms.

The adjustment for wind is not creating a new "best glide" speed.

"Best glide" is the airspeed that will give you the least vertical loss over horizontal distance. It answers the question, "how far can I go in ideal conditions." It's an aerodynamic number that sort of exists in a vacuum and doesn't change based on winds.

"Maximum endurance glide" sometimes called "minimum sink" is the airspeed that will give you the least vertical loss over time. Also aerodynamic, it answers the question How long can I stay up?

The kick in the "best glide" definition is that it doesn't take wind into consideration. What you are referring to as "max glide" is neither. You are of course correct that winds will affect how far you can go. As an extreme example, if your best glide is 70 and your headwind is 70, you won't go any distance at all. And you're right that you'd have to increase your IAS to compensate for the headwind to get anywhere at all.

But the result isn't a new "best glide" speed in the strict sense. It's a speed that takes best glide and adjusts it for conditions. It the old trading altitude for airspeed in reverse. You're going to accept a higher rate of descent in order to go forward further. Let's call it "max adjusted glide"

KickSave, keep in mind that there is a difference between theory and practice. You said

my feeling is that in an emergency glide, you want the most distance AND the longest time in the air

Not necessarily. If your best landing spot is some distance away, you want the best distance. On the other hand, the field is within easy reach and your priority is troubleshooting the engine and securing the airplane for landing, you want the most time. Finally, let's say that you're at 3000' AGL and there's a suitable landing spot 1 mile and 45º ahead of you. Everything else is granite mountains with no options. You're in an ideal position, left base for a landing spot. But the problem is that you're too high. You don't really want more time and you definitely don't want more distance. You want to drop like a rock to get to an altitude where the rest of the approach and landing will be as "normal" as possible.

 

[kaufhaus]

Not quite
as cookmg pointed out:

heavier weight into headwind is better.
(same glide ratio in air, but less time exposed to headwind)

Thye worst case is lightweight into headwind. (same glide ratio in air, longer time exposed to headwind)

Case in point: high performance sailplanes carry water ballast in the wing for high speed runs on cross coutries.

Cheers
George

Man, that glider note got me digging back into the ole brain attic. I'll take a shot at elaborating a bit, though it's coming from old info, so set me straight if I wander from the true path.

Water ballast is used in racing when speed around the course is the goal and there is good lift. The result of the extra weight from the water is a reduction in L/D so distance covered per foot of altitude lost is less. BUT, the extra weight from the water also has the effect of significantly increasing the speed at which the plane makes best L/D. Therefore, if conditions are such that the pilot can expect to encounter strong enough lift to compensate for an L/D of 55:1 with water vs 60:1 without (don't quote me on these numbers, I'm pulling them out of my...er, thin air) while delivering that 55:1 glide ration at 75 KIAS instead of 60 KIAS without water, adding the extra weight is a good deal for a sailplane racer.

Of course, if the lift isn't as strong as expected, that extra 5 feet forward per foot down can make the difference between finishing and ending up in field somewhere miles away. Good thing it's dumbable.

 

[KickSave]

Not necessarily. If your best landing spot is some distance away, you want the best distance. On the other hand, the field is within easy reach and your priority is troubleshooting the engine and securing the airplane for landing, you want the most time. Finally, let's say that you're at 3000' AGL and there's a suitable landing spot 1 mile and 45º ahead of you. Everything else is granite mountains with no options. You're in an ideal position, left base for a landing spot. But the problem is that you're too high. You don't really want more time and you definitely don't want more distance. You want to drop like a rock to get to an altitude where the rest of the approach and landing will be as "normal" as possible.

Ok, I'll agree with that, with the one caveat that if you need to sink faster, you can always configure the plane to do so (flaps, gear, pitch) but if you need to extend a glide to make a safe spot, yer screwed somewhat. So while it requires planning to sink at the perfect rate to make a spot with little or no extra maneuvering, you're better off having extra time and extra altitude/distance, than not enough of either.

I know in the ArrowIII that I'm practicing in lately, if you want to sink like a rock, it will do that for you happily. So I'm just sorta used to preferring to be high, and bleeding off the extra, rather than being low and having little chance of actually making the spot.

I guess in real life, we don't always get to choose, eh?

/Dave

 

[midlifeflyer]

I know in the ArrowIII that I'm practicing in lately, if you want to sink like a rock, it will do that for you happily.

The Arrow is know for it's safe glide: glides like a safe!

 

[uwochris]

I don't know if this was mentionned above already, but there is one important thing to consider with L/D max.

When you are flying at the best glide speed, you are actually attempting to achieve the maximum ratio of lift to drag- this was mentionned above.

However, something to keep in mind is that L/D max is associated with a specific AOA, NOT an airspeed. Small trainers and Cessnas don't have angle of attack indicators, so the only resource we have is the ASI. This ratio is obtained from the coefficient of lift and drag curves, and finding the area where the ratio between the two is greatest.

While your best glide speed changes with weight, winds, etc, the specific AOA to use to achieve L/D max remains CONSTANT with altitude, weight, etc. This is important.

Now, let's consider what happens when weight increases; if weight were to increase, more lift would be required to maintain an equilibrium. There are 2 ways to generate lift - increase the AOA or increase IAS. Since L/D max is associated with a fixed AOA, you only have 1 option- increase IAS.

Therefore, when gross weight increases, use a slightly higher IAS for the best glide ratio.

Also, endurance is NOT affected by winds, but range is. With endurance, we fly at the minimum point on the power required curve- this provides minimum sink rate. Range is a function of distance... headwinds and tailwinds can therefore have an impact.

 

[midlifeflyer]

Now, let's consider what happens when weight increases; if weight were to increase, more lift would be required to maintain an equilibrium. There are 2 ways to generate lift - increase the AOA or increase IAS. Since L/D max is associated with a fixed AOA, you only have 1 option- increase IAS.

That is an =excellent= additon.

 

[Airpiraterob]

simple (?) math

mabye this was already said in a different way but i hope its something that can add to the discussion.

oh and definite credit to UWOCHRIS, absolutely correct info.

-best L/D does indeed remain constant. the speed at wich this is attained is not. best L/D speed increases with weight.
-best L/D is found at ONE angle of the wing to the airstream (Angle of attack) any higher or lower angle of attack increases the drag of the wing. usually this AOA is at a few degreese positive (3 perhaps)
-so whatever speed you are flying at, if the wing angle AOA is at 3 degreese positive in this case, youre flying at your best L/D
-racing gliders use water ballast. true. they do this to make the best L/D happen at a higher speed in relation to the first point above. how is following below:

look at this:
you only get a certian "coefficient of lift" at your best L/D (a Cl of say .2)
given the same AOA as a constant (+3 deg.) you need to go faster to generate more lift. again this is assuming that only ONE angle is used and that being the best L/D angle in this case. so...

a plane with a 200 square foot wing at .2 Cl (coefficient of lift) gives you 357#'s lift at 60mph, and 635#'s lift at 80mph. so this glider should have the same performance at 80mph as it does at 60 but weighs 278#'s more!

(wing area x rho/2 x velicity in FPS squared x Cl = pounds of lift)

the changing variable is the pounds of lift that has to equal the weight of the plane, as that goes up the only thing you can change in the equation is the velocity quotient.

AH HA! you were about to point something out to me but i caught it....the drag increase of travelling much faster is the penalty. your range at the best L/D will decrease as the drag sharply increases. with a 30% increase of speed you get a 78% increase of lift.....you also get a 78% incease of drag!! so your range at that higher speed is dimished...but not by much! (its all porportional to the square of the number...twice as fast...4 times the drag) so overall given no wind at all, the best L/D for maximum range glide is found at the lightest possible weight for the airplane. (get an enema in a sauna before you fly it this way for maximum effect)

just some notes:
the drag at 80mph is very minimal anyway and 78% on top of not much drag to start with is still not much anyway

i had alot of fun in the Schweitzer 1-26, the best glide and the minimum sink speeds were only 5mph apart as i remember. something like 59 and 54mph. i think....its been a while.

 

[KickSave]

Rob, excellent post.. you had me right up until this point:

>>> (get an enema in a sauna before you fly it this way for maximum effect)<<<


Can't I just leave the flight bag full of FAR AIM's at home instead of that?


/Dave