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A light aircraft will descend faster, so will a heavy one..

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A light aircraft will descend faster, so will a heavy one..

Old 24th Apr 2023, 12:11
  #41 (permalink)  
 
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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.
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Old 24th Apr 2023, 12:16
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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.

Last edited by Double Back; 24th Apr 2023 at 12:19. Reason: addition, correction
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Old 24th Apr 2023, 12:29
  #43 (permalink)  
 
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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.
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Old 24th Apr 2023, 12:37
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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

Last edited by JABBARA; 24th Apr 2023 at 12:48.
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Old 24th Apr 2023, 14:23
  #45 (permalink)  
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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 [ /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,
































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.







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Old 25th Apr 2023, 00:17
  #46 (permalink)  
 
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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

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Old 25th Apr 2023, 11:12
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Thank you
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Last edited by JABBARA; 25th Apr 2023 at 11:26.
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Old 25th Apr 2023, 18:35
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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...


Originally Posted by 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
Originally Posted by Double Back
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.
Originally Posted by Vessbot
​​​​​​
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.
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Old 26th Apr 2023, 02:29
  #49 (permalink)  
 
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Donít all PPRuNe threads start going in circles after page one?
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Old 26th Apr 2023, 10:47
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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.
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Old 26th Apr 2023, 11:39
  #51 (permalink)  
 
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Originally Posted by Doubleback
It took for ages, so we cancelled it halfway, never to repeat that again.
We call you types leadfoots, hoons, speed demons, etc etc! ​​​​​​​
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