Glide
Thread Starter
Joined: Mar 2002
Posts: 16
Likes: 0
From: Down Under
Glide
As a chopper pilot I am lacking in some fixed wing knowledge. The other day I heard that fixed wing glide furthur the heavier they are (eg full of pax/freight, as opposed to just the pilot). If this is correct can somebody please explain why.
Joined: Aug 1998
Aviation Qualifications: ATPL
Posts: 6,623
Likes: 847
From: Ex-pat Aussie in the UK
The gliding distance is unaffected by weight if you maintain the angle of attack for the best lift/drag ration. A heavy aircraft will require a higher airspeed to maintain the best lift/drag ratio than a light aircraft.
Competition gliders carry water ballast, not to fly further, but to fly faster - and they can dump that weight at low level in order to take advantage of the higher speed, for a zoom climb ability.
Pilots that use constant speed schedules (generally much higher than best lift/drag speed) for descent will notice an increase in the range for descent when heavy, simply because they are flying closer to the best lift/drag angle of attack when they are heavy compared to when they are light.
Competition gliders carry water ballast, not to fly further, but to fly faster - and they can dump that weight at low level in order to take advantage of the higher speed, for a zoom climb ability.
Pilots that use constant speed schedules (generally much higher than best lift/drag speed) for descent will notice an increase in the range for descent when heavy, simply because they are flying closer to the best lift/drag angle of attack when they are heavy compared to when they are light.
Joined: Nov 2001
Posts: 214
Likes: 0
From: KUL
bcp,
i believe this topic had been discussed before. if i remember off-hand from what was discussed, the gliding distance for the same airframe will be the SAME regardless of it's weight. however, the best gliding speed will not be the same; the heavier one being faster. apparently that's why glider racing pilots add ballast to their glider to increase their speed!
if however, if a CONSTANT speed is to be maintained, then the heavier aircraft will have a greater gliding distance. for most airline ops, we descend at a constant speed (at least we do), that's the reason we get a very steep descent path if the aircraft is light.
hope this helps...
SR
i believe this topic had been discussed before. if i remember off-hand from what was discussed, the gliding distance for the same airframe will be the SAME regardless of it's weight. however, the best gliding speed will not be the same; the heavier one being faster. apparently that's why glider racing pilots add ballast to their glider to increase their speed!
if however, if a CONSTANT speed is to be maintained, then the heavier aircraft will have a greater gliding distance. for most airline ops, we descend at a constant speed (at least we do), that's the reason we get a very steep descent path if the aircraft is light.
hope this helps...
SR
Joined: Dec 2001
Posts: 69
Likes: 0
From: Wisconsin
A glider has a glide ratio of 20/1, this can be expressed asa glide of 20 feet forward for every 1 foot down. The glide ratio can all so be expressed as L/D or 20 pounds of lift and 1 pound of drag. This ratio is fixed in the design of the aircraft but can change with the configuration of the aircraft (flaps, slats, etc).
The best L/D is found at the bottom of the drag curve and corrasponds to a given speed at a given weight. If you fly faster or slower at the same weight, your L/D and glide performance decreases.
How does weight affect the L/D ratio? Take two identical gliders with an L/D ratio of 10/1, in identical weather (no lift or sink, no winds) and forget about pilot techniques, the only differance is the gliders weight.
Glider 1 is at a max gross weight of 2000 lbs and its best glide speed (best L/D) is 70 knots. At this speed lift being created is 2000 lbs and the drag produced is 200 lbs (a 10/1 ratio).
Glider 2 is at a weight of 1500 lbs. If this glider flys at 70 knots it is producing 200 lbs of drag and it's L/D ratio is less than 10/1. How does glider 2 get back to the 10/1 ratio? Fly slower. Glider 2 needs to be at a speed to give it 1500 lbs of lift and 150 lbs of drag (a 10/1 ratio).
If both gliders are racing each other, both gliders would cover the same distance and lose the same amount of altitude but Glider 1 will do so at a faster speed. In other words cross the finish line first.
This is an over simplified version, but it should help.
The best L/D is found at the bottom of the drag curve and corrasponds to a given speed at a given weight. If you fly faster or slower at the same weight, your L/D and glide performance decreases.
How does weight affect the L/D ratio? Take two identical gliders with an L/D ratio of 10/1, in identical weather (no lift or sink, no winds) and forget about pilot techniques, the only differance is the gliders weight.
Glider 1 is at a max gross weight of 2000 lbs and its best glide speed (best L/D) is 70 knots. At this speed lift being created is 2000 lbs and the drag produced is 200 lbs (a 10/1 ratio).
Glider 2 is at a weight of 1500 lbs. If this glider flys at 70 knots it is producing 200 lbs of drag and it's L/D ratio is less than 10/1. How does glider 2 get back to the 10/1 ratio? Fly slower. Glider 2 needs to be at a speed to give it 1500 lbs of lift and 150 lbs of drag (a 10/1 ratio).
If both gliders are racing each other, both gliders would cover the same distance and lose the same amount of altitude but Glider 1 will do so at a faster speed. In other words cross the finish line first.
This is an over simplified version, but it should help.
Joined: Aug 2000
Posts: 3,648
Likes: 2
From: UK
At the same angle of attack (which can be chosen for the optimum L/D) the heavier aircraft will glide slightly further. The speed for that AOA is higher, which gives a higher Reynolds number, and thus a lower skin friction drag coefficient. It's a small effect though, and is probably not noticeable under most normal conditions.
Joined: Dec 2001
Posts: 941
Likes: 0
From: what U.S. calls ´old Europe´
Bookworm is right, higher weight results in higher speed for maximum L/D and therefor in higher reynolds number. For high performance gliders this means about 0.5% better L/D a t maximum weight.
On the other hand we talk about large transports, and there is another number getting important : the mach number. Especially at high altitudes the speed for best L/D is at a mach number above .5 and this results in increasing sonic drag with increasing mach number. Best L/D corresponds with quite large CL so highest local mach number on the wing´s upper surface might be twice the nominal mach number, so supersonic flow might exist at best L/D AOA and speed, if altitude is high.
But all these influences are marginal in comparison with the errors hidden in the ´rule of thumb´ values for optimum speed which is used by the pilot. So even a very good pilot will waste a few percent in glide ratio because he lacks the neccesary informations.
On the other hand we talk about large transports, and there is another number getting important : the mach number. Especially at high altitudes the speed for best L/D is at a mach number above .5 and this results in increasing sonic drag with increasing mach number. Best L/D corresponds with quite large CL so highest local mach number on the wing´s upper surface might be twice the nominal mach number, so supersonic flow might exist at best L/D AOA and speed, if altitude is high.
But all these influences are marginal in comparison with the errors hidden in the ´rule of thumb´ values for optimum speed which is used by the pilot. So even a very good pilot will waste a few percent in glide ratio because he lacks the neccesary informations.
Joined: Aug 2001
Posts: 117
Likes: 0
From: Europe
If you factor kinetic energy (mass) into the pure aerodynamic equation (all posts so far), a heavier airplane - and we’re talking up to 100 tons here - WILL gilde further, because at the end (or just before) of the “aerodynamic” glide, its greater inertia will cause a much slower speed decay.
