Descend rate vs weight
Joined: Jan 2006
Posts: 255
Likes: 0
From: on the Blue Planet
heeeee...
What about the weight vs lift relationship? Higher weight requires more lift
Now slightly deeper...
What is, in the given case, the difference between the lighter and the heavier craft? Yes dude, weight.
To maintain a given airspeed (IAS), you will therefore need more lift for the heavier craft (climbing, level, descending).
More lift? Yes, increase your AOA. And produce more drag. Result is a lower ROC during the ascent, lower airspeed (or MN or FC) in the cruise, and a higher ROD pointing the nose to the Planet.
(much brain storming in this heat
)
What about the weight vs lift relationship? Higher weight requires more lift
Now slightly deeper...
What is, in the given case, the difference between the lighter and the heavier craft? Yes dude, weight.
To maintain a given airspeed (IAS), you will therefore need more lift for the heavier craft (climbing, level, descending).
More lift? Yes, increase your AOA. And produce more drag. Result is a lower ROC during the ascent, lower airspeed (or MN or FC) in the cruise, and a higher ROD pointing the nose to the Planet.
(much brain storming in this heat
)
Joined: Jul 2006
Posts: 204
Likes: 0
From: 'tween posts
2 a/c identical type similar atmospheric conditions but with diff weights...
lets assume both plan a descend at idle thrust and say 250 kts.....
they both are like gliders at this point(discount the residual thrust at idle pwr).the heavier one say achieves a rod of 2500fpm. now the lighter has to increase its rod to maintain 250kts. instead if it tries to maintain 2500fpm its speed would wash off to something lower.
WHY: because a component of the wt vector cotributes towards the speed............
hope that helps
lets assume both plan a descend at idle thrust and say 250 kts.....
they both are like gliders at this point(discount the residual thrust at idle pwr).the heavier one say achieves a rod of 2500fpm. now the lighter has to increase its rod to maintain 250kts. instead if it tries to maintain 2500fpm its speed would wash off to something lower.
WHY: because a component of the wt vector cotributes towards the speed............
hope that helps
Joined: Sep 2002
Posts: 2,188
Likes: 6
From: La Belle Province
gearpins has it the right way around.
at a fixed speed and fixed power setting, a lighter aircraft will have a greater rate of descent than a heavier aircraft, due, as he notes, to the weight component along the flightpath contributing to "thrust".
at a fixed speed and fixed power setting, a lighter aircraft will have a greater rate of descent than a heavier aircraft, due, as he notes, to the weight component along the flightpath contributing to "thrust".
Joined: Dec 2005
Posts: 700
Likes: 14
From: EU
Olendirk,
I agree with gearpin and Scientist versions but in other words my answer is:
Because an aircraft is restricted to maximum speed during descent, the heavier aircraft has to maintain a lower rate of descent than a lighter one, otherwise it would overspeed. Remember, heavier aircraft have a greater momentum and this weight driven momentum will produce a greater speed in in a vertical dive. Therefore, a heavier aircraft has to start its descent earlier than a lighter aircraft because it has to maintain a shallower descent.
In other words, a lighter aircraft can descent later and quicker than a heavier aircraft because it can maintain a greater vertical descent profile without overspeeding.
Hope that answers yours question.
TFCM
I agree with gearpin and Scientist versions but in other words my answer is:
Because an aircraft is restricted to maximum speed during descent, the heavier aircraft has to maintain a lower rate of descent than a lighter one, otherwise it would overspeed. Remember, heavier aircraft have a greater momentum and this weight driven momentum will produce a greater speed in in a vertical dive. Therefore, a heavier aircraft has to start its descent earlier than a lighter aircraft because it has to maintain a shallower descent.
In other words, a lighter aircraft can descent later and quicker than a heavier aircraft because it can maintain a greater vertical descent profile without overspeeding.
Hope that answers yours question.
TFCM
Joined: Sep 2001
Posts: 68
Likes: 0
From: Dubai, UAE
Another way to look at the same problem
Best glide speed, (max distance for min hieght, not to be confused with min ROD, which will normally be min clean) is a function of wieght, that is why gliders carry water ballast. They can go faster for the same hieght lost against distance.
Now, given the aircraft descend at the same speed ( assuming it is above best L/D) then the heavy aircraft is in fact closer to its (higher) best glide speed, so is more efficient.
If you change the descent speed, to say best L/D the lighter A/C will always win.
In the triple 7, ( esp the 300 or er) a heavy A/C can be a bi***h to get down if you get slowed down, 250 is close to best L/D and you need boards to keep any sort of profile.
Best glide speed, (max distance for min hieght, not to be confused with min ROD, which will normally be min clean) is a function of wieght, that is why gliders carry water ballast. They can go faster for the same hieght lost against distance.
Now, given the aircraft descend at the same speed ( assuming it is above best L/D) then the heavy aircraft is in fact closer to its (higher) best glide speed, so is more efficient.
If you change the descent speed, to say best L/D the lighter A/C will always win.
In the triple 7, ( esp the 300 or er) a heavy A/C can be a bi***h to get down if you get slowed down, 250 is close to best L/D and you need boards to keep any sort of profile.
