View Full Version : A light aircraft will descend faster, so will a heavy one..

k.swiss

3rd Apr 2023, 16:01

Please, like I am a five-year-old child can someone explain this.

A heavy aircraft will descend faster because of the momentum it has.

A light aircraft will descend faster because it can reach a higher speed, less limited by VMO/MMO on the descent.

So which one is the truth, do you need to plan more track miles to descend a heavier aircraft or a lighter aircraft?

Additionally in terms of slowing down, which do you need to plan more track miles for? A lighter aircraft will take time to slow down due to its inertia. But a heavier aircraft will also take time to slow down because of its momentum, sometimes you need to drop the gear for example.

Would appreciate any underlying logic. Thank you.

MechEngr

3rd Apr 2023, 16:15

Is this the same aircraft at two different load states?

k.swiss

3rd Apr 2023, 16:18

Yes for discussion sake A320/737 one empty one full lets say. Still air.

Vessbot

3rd Apr 2023, 16:26

Please, like I am a five-year-old child can someone explain this.

A heavy aircraft will descend faster because of the momentum it has.

A light aircraft will descend faster because it can reach a higher speed, less limited by VMO/MMO on the descent.

So which one is the truth, do you need to plan more track miles to descend a heavier aircraft or a lighter aircraft?

Momentum has nothing to do with it, and they're both just as limited by Vmo/Mmo, unless I'm missing something from what you meant to say.

However, a heavier airplane (at the same speed as a lighter one) is closer to its (L/D)max speed, and therefore will glide at a shallower angle.

(Remember, as a baseline, that they have the same (L/D)max, yielding the same glide angle, but the heavier one occurs at a faster speed, and higher descent rate.)

MechEngr

3rd Apr 2023, 16:45

"A heavy aircraft will descend faster" is incorrect - they are capable of not descending at all until the fuel runs out and the engines are running. Momentum has nothing to with that. A plane heavy with fuel will do this for a longer time.

If the engines stop, then the best glide (for distance) speed will be higher for the heavier state because the lift requirement will be higher making the drag higher and therefore the greater rate required for turning altitude into kinetic energy to be bled off via drag. AFAIK the distance will be the same.

Vessbot

3rd Apr 2023, 16:53

Additionally in terms of slowing down, which do you need to plan more track miles for? A lighter aircraft will take time to slow down due to its inertia. But a heavier aircraft will also take time to slow down because of its momentum, sometimes you need to drop the gear for example.

I didn't see this before, I think you edited it in.

Inertia and momentum functionally mean the same thing here, and the heavier plane has more of it; and will take more time and distance to slow down because of it.

Central Scrutinizer

3rd Apr 2023, 16:57

What you are looking at is the descent angle (gamma) which is a function of the lift to drag ratio L/D.

As a first approximation, the aircraft with the higher L/D will descend at a shallower angle and will need more track miles to descend.

Vessbot

3rd Apr 2023, 17:01

What you are looking at is the descent angle (gamma) which is a function of the lift to drag ratio L/D.

As a first approximation, the aircraft with the higher L/D will descend at a shallower angle and will need more track miles to descend.

OK, and which aircraft has a higher L/D?

Central Scrutinizer

3rd Apr 2023, 17:21

OK, and which aircraft has a higher L/D?

Depends on the boundary conditions of the problem.

If the speed is free, max L/D is a function of geometry alone and not mass. The glide angle will be the same but the speed will vary.

Vessbot

3rd Apr 2023, 17:32

Depends on the boundary conditions of the problem.

Let's say, each aircraft is doing its own best trying to descend as steeply as possible?

AerocatS2A

3rd Apr 2023, 17:51

A light aircraft has a lower best glide speed so it has more margin to Mmo/Vmo. This allows it to achieve a steeper descent at Mmo/Vmo.

Central Scrutinizer

3rd Apr 2023, 17:54

Let's say, each aircraft is doing its own best trying to descend as steeply as possible?

Well then the limitation becomes the Vmo. Assuming Vmo is a fixed number independent of mass, both aircraft descend at the same IAS, the heavier one should have a higher L/D ratio (more lift for the same amount of drag) and therefore a shallower descent angle (more track miles).

Alex Whittingham

3rd Apr 2023, 21:08

This sounds like something out of the ATPL theory which is based in turn on the pre-war work on gliding theory. You balance the static forces in the descent and derive a formula that says sin gamma = (drag-thrust)/weight. You observe thrust is minimal so say that sin gamma = drag/weight, and then observe that, assuming weight is relatively constant, the smallest value of sin gamma occurs where the drag is least and make the first statement that the most shallow glide angle occurs if you fly at VMD for the weight.

Now hop back to the previous formula that says sin gamma = drag/weight and substitute CD/CL for drag/weight, this leads to the second statement that the glide angle is the lift drag ratio upside down, and this is independent of weight provided you fly at the correct VMD for that weight. Now we have statement #3 which is that, provided you fly at the correct VMD for the weight, the glide angle (flight path angle) will be the same for both a heavy aircraft or a light aircraft and therefore, in answer to the OP's question, the planned track miles will be the same.

Now rate of descent, and for this you need a dynamic diagram with a right angle triangle with descent angle gamma, TAS on the hypotenuse and rate of descent in the vertical. sin gamma = O/H = rate of descent / TAS, therefore rate of descent = Sin gamma x TAS. The heavier aircraft will have the same sin gamma in a VMD glide but IAS will be higher, so TAS will be higher, so rate of descent will be higher. Now we have statement #4 which develops from #3, which is that, provided you fly at the correct VMD for the weight, the glide angle (flight path angle) will be the same for both a heavy aircraft or a light aircraft but the heavier aircraft will have a higher TAS and therefore a higher rate of descent.

AerocatS2A

3rd Apr 2023, 23:37

That's all well and good, but we're not flying at VMD we're trying to fly a steep descent which means at Vmo / Mmo. At Vmo / Mmo a heavy aircraft is closer to its VMD and will have a better glide than the light aircraft. It's quite obvious in practice when flying older jets without VNAV.

Well then the limitation becomes the Vmo. Assuming Vmo is a fixed number independent of mass, both aircraft descend at the same IAS, the heavier one should have a higher L/D ratio (more lift for the same amount of drag) and therefore a shallower descent angle (more track miles).

Vmo is independent of weight. Buffet boundaries however are not, so may affect manoeuvre margins.

Have a look at this chart, and then perhaps consider your statement. You are around 90% there, but that means a 100% review.

Consider first level flight at a constant AOA; total lift variables for a comparison are Lift =CL.V^2, that's the set of variables. everything else is lipgloss in a comparative analysis. So, as Lift must equal weight for level flight, the heavier aircraft must either have a higher CL for a constant speed, or an increased speed for a constant CL. CL is related to the a-slope of CL v AOA. In general the relationship of almost all systems will be around 0.09 CL for every degree of AOA. There are a few things that will affect that, such as aspect ratio, and very slightly VGs can increase that slope... trailing edge mods may also increase that slope.

So, thinking laterally from that case, consider when the aircraft is in a straight glide.. the chart will get you to your journey.

May the forces be with you

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1006x666/screen_shot_2023_04_04_at_7_59_09_pm_6793822826792baa6963f41 68a4f14b485f9278f.png

mustafagander

4th Apr 2023, 11:54

Airlines usually have published descent profiles which are more or less independent of weight.

Within these parameters a heavy aircraft takes more track miles for descent coz it has more energy to dissipate.

On B744, in my experience, it's not a big deal coz of winds and ATC vectoring. Use the Perf manual figures and you won't be too far wrong then patch it up.

Capn Bloggs

4th Apr 2023, 13:55

There are two opposing scenarios here.

The theoretical scenario is where aircraft descend at the AOA for best L/D. In this case, a heavy aeroplane will descend on exactly the same path (AOA), but at a greater speed, than a light aircraft. Both will hit the ground at the same point, the only difference being that the heavier aeroplane will hit faster/quicker.

This is eloquently explained by none other than A C Kermode in The Mechanics of Flight, presented to me by my Dad in April 1975 (use my diagram below for the "Fig 6.1" discussion):

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1000x1272/effect_of_weight_on_glide_kermode_9a2cce9ea1fb42acbe4237350c 18ed125d3210b7.jpg

The Practical scenario is completely different. That is, we descend at (approximately) the same speed for all descents. We do not descend at Vmin+5 (or Green Dot?). We descend at say Mach 0.8 into 280KIAS. This completely changes the dynamics of the descent. Because the thrust is zero, the only way the aeroplane can counter the drag at the "fixed" speed is to use the weight, by diving. Now, if the aeroplane is heavier, the effective thrust from the weight will be more, and so it doesn't need to descend as steeply as a light aeroplane.

And that's all there is to it.

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/816x794/descending_faec5c1a1e869c06b2fb5a11e9811b0e67de3c81.jpg

Scarebus discussion on page 159 here:

https://www.skybrary.aero/bookshelf/getting-grips-aircraft-performance-february-2002

It's "Getting a grip on" or "Coming to grips with"! But then Airbii aren't English are they! :}

Vessbot

4th Apr 2023, 18:05

So, thinking laterally from that case, consider when the aircraft is in a straight glide.. the chart will get you to your journey.

May the forces be with you

Can you please say your conclusion, with respect to the question of the thread?

FullWings

5th Apr 2023, 01:38

So which one is the truth, do you need to plan more track miles to descend a heavier aircraft or a lighter aircraft?

If you descend at the same speed in both, then you will need more track miles in the heavier aircraft. Simple explanation: best L/D speed increases with wing loading, so your glide angle will be better in the heavier aircraft == more miles.

Additionally in terms of slowing down, which do you need to plan more track miles for? A lighter aircraft will take time to slow down due to its inertia. But a heavier aircraft will also take time to slow down because of its momentum, sometimes you need to drop the gear for example.

Again, a heavier aircraft needs more track miles. Simple explanation: as you are operating in a better L/D regime as you slow down, the deceleration will be less == more miles.

Capn Bloggs

5th Apr 2023, 02:08

Additionally in terms of slowing down

Again, a heavier aircraft needs more track miles.

Not necessarily. If your VNAV allows for the weight and places you higher if you are lighter, you won't notice any appreciable difference in slowdown-distance.

Certainly though, from the same point in the sky, say 5000ft at 20nm at 250KIAS, a heavier aircraft will take longer to slow down than a lighter one for the reason I showed above: the longer/larger weight vector contributes more to the effective thrust, meaning that either the heavier will glide further or, if forced onto the same flightpath eg 5000ft/20nm to 3000/10nm, it won't slow down as much.

Central Scrutinizer

7th Apr 2023, 12:20

Have a look at this chart, and then perhaps consider your statement.

I've had a look at the chart but don't get where you're coming from.

Could you please elaborate?

Vessbot

7th Apr 2023, 22:33

I've had a look at the chart but don't get where you're coming from.

Could you please elaborate?

I went back through your post with my quibbling hat on, and couldn't find anything wrong; so I'm curious too what the other 10% is.

k.swiss

8th Apr 2023, 00:21

Gents I understood nothing. :confused:

Either I dust off the books and get to grips with such concepts as drag-curves and LR ratios once again, or someone truly could explain like I am five. :O

Vessbot

8th Apr 2023, 01:58

Gents I understood nothing. :confused:

Either I dust off the books and get to grips with such concepts as drag-curves and LR ratios once again, or someone truly could explain like I am five. :O

Starting from the very basics. Straight and level, unaccelerated flight. Lift opposes Weight, Thrust opposes Drag. Everything is 90 or 180 degrees to everything else.

Then, think of a climb or descent. Weight no longer opposes Lift, but is slightly off (by the amount of the climb or descent angle). Due to this off-ness of angle, Weight becomes partially aligned with either Drag (in a climb) or Thrust (in a descent.) In the picture a few posts up, this is represented by the green forward-pointing vector in the bottom triangle. This is called the Forward (if descending, or Rearward if climbing) Component of Weight. Put more simply, going up hill, weight is pulling you back, and if going downhill, weight is pulling you forward. This is the same mechanic as what happens in a car going uphill or downhill. (Literally the same, it’s not an analogy or any other type of mental trick). I bet you already understood this in a common-sense gut level, if not by drawing triangles. This is why you tend to slow down uphill and speed up downhill.

Now, consider two planes descending at the same angle. If everything about them is the same, then their Weight is the same, and their Forward Component of Weight is the same, and their Thrust and Drag are the same, and their speed is the same. Everything. Now, imagine one is heavier. If we keep everything else the same (descent angle, Thrust, Drag) then the one with greater Weight will also have a greater Forward Component of Weight. Therefore it will try to go faster.

But since we want to keep to the same speed as the lighter one (it’s ATC-assigned speed, or it’s Vmo/Mmo, or whatever) and we can’t reduce drag since we’re already at idle, then the only option for the heavier plane is to reduce the descent angle. Once that’s reduced, then the Forward Component of Weight is reduced (and matches the lighter plane again) not because Weight is reduced, but because the descent angle is.

TLDR: Higher weight pulls the aircraft forward harder, and a reduced slope counteracts this effect.

TLDR in math: (gamma, γ, is descent angle) Longitudinal forces in a descent are T + W sin γ forward, balanced by D backward. If speed is constrained to be the same, then D is the same. T is zero (idle) therefore W sin γ must be kept constant. For that, higher W requires lower sin γ, which means lower γ.

AerocatS2A

8th Apr 2023, 04:30

Gents I understood nothing. :confused:

Either I dust off the books and get to grips with such concepts as drag-curves and LR ratios once again, or someone truly could explain like I am five. :O

Do you accept that the heavy aircraft and light aircraft both have the same glide ratio but on the heavier aircraft it occurs at a higher speed (min drag speed is higher on the heavy aircraft)? And that flying faster than this speed will increase the descent angle?

Imagine the heavy aircraft is so heavy that the best glide speed is the same as Vmo. In this case it can’t descend steeper than best glide. Now make it a bit lighter, give it a min drag speed of Vmo - 20 knots. Now it can go a little bit faster than best glide and therefore a bit steeper. The lighter it is, the more margin there is between best glide and Vmo, the steeper it will descend at Vmo.

So the lighter aircraft can descend more steeply. The lighter aircraft will also slow down more quickly due to having less momentum.

Now, imagine one is heavier. If we keep everything else the same (descent angle, Thrust, Drag) then the one with greater Weight will also have a greater Forward Component of Weight. Therefore it will try to go faster.Remember a chap writing that an emergency descent in an empty 747 gave a truly impressive view of the landscape, somewhat akin to a Stuka bomb run.

Central Scrutinizer

8th Apr 2023, 10:24

I went back through your post with my quibbling hat on, and couldn't find anything wrong; so I'm curious too what the other 10% is.

I guess it has to do with the level of simplification. My statement is correct as a "first approximation" with simplifying assumptions such as small angles etc. One can always go one step deeper in aerodynamics but a first order of magnitude is good enough here.

k.swiss

10th Apr 2023, 13:05

Starting from the very basics. Straight and level, unaccelerated flight. Lift opposes Weight, Thrust opposes Drag. Everything is 90 or 180 degrees to everything else.

Then, think of a climb or descent. Weight no longer opposes Lift, but is slightly off (by the amount of the climb or descent angle). Due to this off-ness of angle, Weight becomes partially aligned with either Drag (in a climb) or Thrust (in a descent.) In the picture a few posts up, this is represented by the green forward-pointing vector in the bottom triangle. This is called the Forward (if descending, or Rearward if climbing) Component of Weight. Put more simply, going up hill, weight is pulling you back, and if going downhill, weight is pulling you forward. This is the same mechanic as what happens in a car going uphill or downhill. (Literally the same, it’s not an analogy or any other type of mental trick). I bet you already understood this in a common-sense gut level, if not by drawing triangles. This is why you tend to slow down uphill and speed up downhill.

Now, consider two planes descending at the same angle. If everything about them is the same, then their Weight is the same, and their Forward Component of Weight is the same, and their Thrust and Drag are the same, and their speed is the same. Everything. Now, imagine one is heavier. If we keep everything else the same (descent angle, Thrust, Drag) then the one with greater Weight will also have a greater Forward Component of Weight. Therefore it will try to go faster.

But since we want to keep to the same speed as the lighter one (it’s ATC-assigned speed, or it’s Vmo/Mmo, or whatever) and we can’t reduce drag since we’re already at idle, then the only option for the heavier plane is to reduce the descent angle. Once that’s reduced, then the Forward Component of Weight is reduced (and matches the lighter plane again) not because Weight is reduced, but because the descent angle is.

TLDR: Higher weight pulls the aircraft forward harder, and a reduced slope counteracts this effect.

TLDR in math: (gamma, γ, is descent angle) Longitudinal forces in a descent are T + W sin γ forward, balanced by D backward. If speed is constrained to be the same, then D is the same. T is zero (idle) therefore W sin γ must be kept constant. For that, higher W requires lower sin γ, which means lower γ.

Thank you! Thank you! Thank you!

Appreciate everyone's replies here, read them all. Thanks guys for explaining it on a simple level, makes sense now.

FlightDetent

12th Apr 2023, 05:56

Do you accept that the heavy aircraft and light aircraft both have the same glide ratio but on the heavier aircraft it occurs at a higher speed (min drag speed is higher on the heavy aircraft)? And that flying faster than this speed will increase the descent angle?

Imagine the heavy aircraft is so heavy that the best glide speed is the same as Vmo. In this case it can’t descend steeper than best glide. Now make it a bit lighter, give it a min drag speed of Vmo - 20 knots. Now it can go a little bit faster than best glide and therefore a bit steeper. The lighter it is, the more margin there is between best glide and Vmo, the steeper it will descend at Vmo.

So the lighter aircraft can descend more steeply. The lighter aircraft will also slow down more quickly due to having less momentum.Excellent didactics! Will try to imitate ...:ok:

Locked door

13th Apr 2023, 02:27

two identical aircraft, one heavy, one light start a glide at the same place at their own best glide speed. They will reach the ground at exactly the same spot, the heavy one will get there first.

I remember being surprised by that during ATPL theory.

Capn Bloggs

13th Apr 2023, 02:51

Theory, yes, but totally different in the real world where we have to descend at the same speed, heavy or light. That's the whole point of the topic.

RichardJones

20th Apr 2023, 19:40

There are two opposing scenarios here.

The theoretical scenario is where aircraft descend at the AOA for best L/D. In this case, a heavy aeroplane will descend on exactly the same path (AOA), but at a greater speed, than a light aircraft. Both will hit the ground at the same point, the only difference being that the heavier aeroplane will hit faster/quicker.

This is eloquently explained by none other than A C Kermode in The Mechanics of Flight, presented to me by my Dad in April 1975 (use my diagram below for the "Fig 6.1" discussion):

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1000x1272/effect_of_weight_on_glide_kermode_9a2cce9ea1fb42acbe4237350c 18ed125d3210b7.jpg

The Practical scenario is completely different. That is, we descend at (approximately) the same speed for all descents. We do not descend at Vmin+5 (or Green Dot?). We descend at say Mach 0.8 into 280KIAS. This completely changes the dynamics of the descent. Because the thrust is zero, the only way the aeroplane can counter the drag at the "fixed" speed is to use the weight, by diving. Now, if the aeroplane is heavier, the effective thrust from the weight will be more, and so it doesn't need to descend as steeply as a light aeroplane.

And that's all there is to it.

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/816x794/descending_faec5c1a1e869c06b2fb5a11e9811b0e67de3c81.jpg

Scarebus discussion on page 159 here:

https://www.skybrary.aero/bookshelf/getting-grips-aircraft-performance-february-2002

It's "Getting a grip on" or "Coming to grips with"! But then Airbii aren't English are they! :}

Those vectors, explain why high performance sail planes take on water before departure. Mainly, I believe to get more speed for the optimum L/D ratio. Reduces the time flying between thermals, for example.

A heavy a/c at max landing weight, more distance is required. Takes longer to slow the aircraft down.

Hope that makes sense.

Weight does not effect the glide range within certain parameters. It does effect the TAS. Heavy = Fast.

Double Back

21st Apr 2023, 06:38

For You as a Swiss guy You will understand next, I will try to keep it as simple as possible.

Driving uphill in a car requires energy. Driving the same car, but now with 4 people on board, requires much more energy. The more people in the car, the more energy needed. Going down, the heavy car will need using the brake much more often than the same, but lighter car.

Weight is "the engine" during descent, to keep the car rolling, the more weight, to more "power" You have. You cannot switch off the "weight engine".

So once two cars are on top of a mountain, the light one has much LESS (potential) energy stored than the same car, with 4 people in it.

But You cannot change the % of slope on a mountain, airplanes CAN change their descent angle.

Example: two identical airplanes, both at 10km of cruising altitude, one at max allowable weight, and the other one, as empty as possible, will have a enormous difference in energy state.

For a normal descent, both will fly at the same SPEED. But the heavy one needs to select a LESS STEEP descent angle, otherwise the excess energy will drive the plane into crazy dangerous speeds. OR it needs to pull the speed brakes (like the car), but that is highly uneconomic.

In the 747 I flew for 22 Years (all types), descent planning was done initially by looking at a simple table: The more WEIGHT You had, the MORE distance You needed to start the descent before destination, in order not to overshoot the destination.

By dusted memory, at a very light weight you needed about 110 Nm before destination, but when You were heavy, it would be 130 NM or so.

Remember, airliners use idle power during descents, so that is not a factor. Here again, weight is the engine that maintains flying speed.

Ik kept out wind influence, difference in gliding performance at different weights, I think those are NOT the factors, TS is looking for.

Modern airliners do the calculation of where would be the optimal TOD (Top Of Descent), that flight, electronically, but it also uses actual weight (together with things like forecasted winds) as the MAIN factors to determine that point.

Richard

Capn Bloggs

21st Apr 2023, 09:21

Weight does not effect the glide range within certain parameters. It does effect the TAS. Heavy = Fast.

Incorrect. That is what the thread is all about. Doubleback's explanation is spot-on.

Weight has a direct effect on the "gliding" aka idle-thrust descent range because we are descending at the same speed. The higher the weight, the more descent distance a jet needs at the ATC speed.

RichardJones

21st Apr 2023, 11:49

Incorrect. That is what the thread is all about. Doubleback's explanation is spot-on.

Weight has a direct effect on the "gliding" aka idle-thrust descent range because we are descending at the same speed. The higher the weight, the more descent distance a jet needs at the ATC speed.

I stand corrected Captain, thankyou. I was an aviator, never much of an academic and less able to explain myself at times.

blind pew

21st Apr 2023, 12:08

A lot of theories but trust me as I got 16,700 fpm out of an empty F100 going into Rome before I chickened out and was still a good 30 knots below VNE whilst looking at the terrain below us..wasn’t popular with the crew…

JABBARA

21st Apr 2023, 20:36

“If you can’t explain it to a six-year-old, then you don’t understand it yourself” Albert Einstein

Following figure is extracted from AIRBUS Getting the Grips with Aircraft Performance P 163 (attached note boxes are mine)

Commented as:1. For different weights, if respective Green Dot (GD) speed is maintained, Descent Angle (DA, in another word Glide Path Angle, GPA,) and Descent Range remains same; however with increasing weight, Rate of Descent (RD) increases or vice versa

2. For all different weights, if a fixed (same) speed value, within Standard Descent Speed Range (eg 280K), is maintained, as weight increases, DA (=GPA) and RD decreases, consequently Descent Range increases

3. A counter-intuitive point: Why DA (=GPA) decreases with increasing weight?

ANSWER: During glide, the drag along flight path is balanced with the component of weight along flight path, in the opposite direction with drag. Therefore, as weight increases, less GPA (=DA) becomes enough to counter drag; result is less RD

Full stop

NOTE I have a thread titled "AMC3 CAT.OP.MPA.182 Fuel/energy scheme — aerodrome selection policy — aeroplanes" I need some comments and replies please

Thanks

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/932x330/jpeg_df12adf8ff98f2e7f7df483523b227da5a4728de.png

Vessbot

22nd Apr 2023, 00:38

Weight does not effect the glide range within certain parameters. It does effect the TAS. Heavy = Fast.

Incorrect. That is what the thread is all about. Doubleback's explanation is spot-on.

Weight has a direct effect on the "gliding" aka idle-thrust descent range because we are descending at the same speed. The higher the weight, the more descent distance a jet needs at the ATC speed.

You are both right, under different conditions.

If speed is unconstrained, then best glide angle is the same at all weights. But at a higher weight, that best glide angle is achieved at a higher speed.

If speed is constrained to a certain number, then glide angle (not best, just glide angle) is shallower at a higher weight.

Pugilistic Animus

24th Apr 2023, 07:09

The heavier plane at glide speed will reach the same position as a lighter plane, only faster...if a made an error don't crucify me :}

JABBARA

24th Apr 2023, 11:38

Pugilistic Animus

The heavier plane at glide speed will reach the same position as a lighter plane, only faster...if a made an error don't crucify me https://www.pprune.org/images/smilies/badteeth.gif

Correct if you fly with (L/D)max speed (=GD) of each weight.

Otherwise, if you fly faster but same speed for any weight, the heavier has more range.

BraceBrace

24th Apr 2023, 12:11

Just glared over all responses and one thing always amazes me in these discussions... nobody talks about angle of attack.

Best glide is a result of a wing characteristic, aka Cl/Cd curve, polar curve,... The wing will create best glide at a certain fixed angle of attack. As long as you don't change the wing profile, this angle of attack is a fixed value. To me, one of the most beautiful words in aviation is "finesse"...

Then you look at the aircraft and I guess it's easy to understand heavier weights require higher speeds for to fly this optimum angle of attack.

I've never been a military pilot, but always envied aircraft with AoA indications. Climbing and descending would be so much easier... and safer.

Double Back

24th Apr 2023, 12:16

Pugilistic, no, it doesn't.

Imagine a bobsled downhill, they use a "heavy" crew of 3-4 to include more energy, so more speed. That is what THEY are looking for, the quicker they are down, the higher their ranking. BUT sleds (or cars driving downhill) CANNOT change their "descent path".

Airplanes (airliners here) can select their own (descent) path angle, but speed is many times limited to MMO (Max Mach number), or later, IAS. In general, You will make a kind of "cruise descent" with about the same speeds as You were cruising level, before the descent.

The heavy one HAS to select a flatter angle in order not to overspeed. That flatter angle will stretch his glide 20-30% !!!! (guesstimate).

The light one will reach terra firma at way less distance form the Top Of Descent (TOD) than the heavy one, which follows a flatter angle and travels further.

Agree, many people have problems that a "heavy" thing, in a descent where most of the time distance is important, will GLIDE further than a light one. I taught many students to fly, it was one thing they had problems with to comprehend, it looks completely unnatural. (AS was the influence of wind on the plane's IAS, if interested, google with the "downwind syndrome"). No, I never crucified one :).

Ask ANY glider pilot that flies contests (distance) why he/she takes up to a few HUNDRED kilos with him/her in water ballast.....???? Isn't that strange, because many people think gliders should be as light as possible? Forget it, the heavy one gets way more distance if compared with a light one from the same altitude. Modern performance gliders have complex navigation/performance computers on board that calculate available energy versus distance over ground, so the pilot can see continuously how far he can get from that position. Like when to start making the "final glide" towards the finish line. Many variables are included like weight, height and wind as being the main drivers.

Double Back

24th Apr 2023, 12:29

BB, true, but for the sake of simplicity, I left that out. Any POH/AOM will show You the best glide speed increases with weight.

JABBARA

24th Apr 2023, 12:37

The wing will create best glide at a certain fixed angle of attack. As long as you don't change the wing profile, this angle of attack is a fixed value.

That is correct at lower mach number where compressibility effect is low (approx Mach <0.4 )

At high Mach number,, mentioned AoA also changes, therefore for same weight as fly higher altitude, (Cl /Cd)max (=(L/D) max for certain weight) Green dot speed increases

Note for an Airliner wing profile (Cl /Cd)max AoA is aproximately 4 degree at low mach number

Pugilistic Animus

[QUOTE] The heavier plane at glide speed will reach the same position as a lighter plane, only faster...if a made an error don't crucify me https://www.pprune.org/images/smilies/badteeth.gif[ /QUOTE]

Correct if you fly with (L/D)max speed (=GD) for any weight.

Otherwise, if you fly faster but same speed for any weight, the heavier has more range.

So... I want more range so I am going to add a few tons to my Cessna 140 or Piper cub, and that will give me intercontinental range? Excellent.

For a descent with idle thrust (residual) the greatest range will occur when the aircraft as a system is operated at the tangent to the L v D curve which is LD MAX. For a heavier aircraft, the speed will be greater, and sink as well as velocity will be greater, range itself doesn't increase.

Lets put this to bed, again,

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1114x932/screen_shot_2023_04_24_at_9_07_56_pm_a40954984ac16fc271b3eb3 0fa90f2d5f03ec9ea.png

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/968x138/screen_shot_2023_04_24_at_9_08_58_pm_5f0741bf4a3c6eaa7a32aaa 9ef55213e95e233ea.png

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1298x824/screen_shot_2023_04_24_at_9_13_25_pm_9cb09c2b7569a08d12861eb 62ee0f476a8b84d98.png

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1308x856/screen_shot_2023_04_24_at_9_14_25_pm_dc76e71c62b94d855c549fc 5c58bb07f64320de1.png

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1330x1166/screen_shot_2023_04_24_at_9_15_21_pm_aef83edc419d5179201efdd 80e08cfa074deeca2.png

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1290x270/screen_shot_2023_04_24_at_9_16_24_pm_a8da8b3b754179ff0180f57 dfa467251c73a44e4.png

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1302x1094/screen_shot_2023_04_24_at_9_17_11_pm_e51148eb24dff111a50be00 a98d53b83dfabc3a4.png

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1302x360/screen_shot_2023_04_24_at_9_17_49_pm_c8673cc5f9d9f33afeb9ec3 a4b649ff991d83e69.png

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1274x1024/screen_shot_2023_04_24_at_9_18_42_pm_a2fcf97db5628a88cd167bd 3396ad98ebca93efb.png

It is a little protracted but is simpler using the FTM-108 Ch 8 reference than many others.

Gliders will have greater speed when heavier, they don't go further just because of speed for heavier weights, but that changes when wind effects arise, where having a higher IAS for best L/D may be helpful, in headwinds etc.

Pugilistic Animus

25th Apr 2023, 00:17

Oh math! Unfortunately, I m almost sick and tired of math because I teach a thermodynamics 1 And 2 for two semesters, and Part one during summer session but nevertheless I'm gonna take some time after May to indulge in those free body diagrams provided by Mr. FDR

JABBARA, very nice of you the way you commented. Much appreciated

:)

JABBARA

25th Apr 2023, 11:12

Thank you

Pugilistic Animus (https://www.pprune.org/members/160551-pugilistic-animus)

Vessbot

25th Apr 2023, 18:35

The distinction between the free speed condition (under which glide range is always the same, but achieved at a higher speed for the heavier plane) vs the assigned speed condition (under which the heavier plane, at the same speed, has a longer glide range) has been made how many times by now? Yet people continue making posts under one condition, then get told they're wrong by someone, thinking under the other condition, both providing reasoning (under the respective condition) that has already been posted half a dozen times each.

So, once again...

The heavier plane at glide speed will reach the same position as a lighter plane, only faster...if a made an error don't crucify me :}

Pugilistic, no, it doesn't.

...

Airplanes (airliners here) can select their own (descent) path angle, but speed is many times limited to MMO (Max Mach number), or later, IAS. In general, You will make a kind of "cruise descent" with about the same speeds as You were cruising level, before the descent.

The heavy one HAS to select a flatter angle in order not to overspeed.

You are both right, under different conditions.

If speed is unconstrained, then best glide angle is the same at all weights. But at a higher weight, that best glide angle is achieved at a higher speed.

If speed is constrained to a certain number, then glide angle (not best, just glide angle) is shallower at a higher weight.

AerocatS2A

26th Apr 2023, 02:29

Don’t all PPRuNe threads start going in circles after page one?

Double Back

26th Apr 2023, 10:47

True, I was looking at how a heavier weight is handled in practice. I thought that was more likely answering the initial TS question.

Early in my career, still 2 stripes as cruise reliever on the DC10, I asked the capt if we could do an "optimum" descent, speed around 250 KTS (close to minimum clean speed), would love to see that in stead of the usual 300+ KTS. It took for ages, so we cancelled it halfway, never to repeat that again. I never investigated what would be the effect on range, possible a lot better.

Capn Bloggs

26th Apr 2023, 11:39

It took for ages, so we cancelled it halfway, never to repeat that again.

We call you types leadfoots, hoons, speed demons, etc etc! :E