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.

Don't believe everything you here and read.

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.

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

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.

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.

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.

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.