Descent distance

[gimmesumvalium]

Have forgotten the aerodynamics regarding descent.

Can anyone refresh me as to the aerodynamics (lift/drag/weight etc.) which convincingly explain why a heavy aircraft glides further than a lighter aircraft.

[Islander2]

That may be difficult, because it doesn't!

The glide distance is independent of weight as long as the aeroplane is flown at the speed for (L/D)max. The higher the weight, the faster is the best glide speed and the greater is the RoD, BUT the glide range remains constant.

[gimmesumviagra]

With Jet transports, a light aircraft falls out of the sky - with the 747 we used an adjustment (based on weight) to determine our top of descent.

My problem now is to justify it aerodynamically to my students!

[yoshiman]

Just something to add, hope this doesnt sound dumb, im just learning, but how come the best glide speed increases with weight? As the weight increases does L/D max increase too? Can someone please explain this.:ugh:

[Islander2]

With Jet transports, a light aircraft falls out of the sky - with the 747 we used an adjustment (based on weight) to determine our top of descent.Only because, in those circumstances, it's not desirable to reduce the speed by the amount needed to maintain glide range.

To justify it aerodynamically, you need to start with a graph of L/D versus Lift Coefficient - any good text book will provide. This shows L/D rising as CL increases until it reaches a peak, then falling for further increases in CL.

CL is proportional to W/(V^2), so (L/D)max occurs at a given W/(V^2).

For reduced weight, the glide range will either a) reduce if the speed was high enough to put you to the left of (L/D) max (which would be the case for a cruise descent), or b) increase if the aeroplane was flying slow enough to be on the right of (L/D)max at the reduced weight.

For GA single-engine flying, the most important case is achieving maximum range following engine failure. Here, you want to glide at the speed that gives (L/D)max ... and the glide range (nil wind) will then always be the same, regardless of weight. So to achieve the W/(V^2) that gives (L/D)max, if W is reduced then V needs to be reduced (although only by 0.5-0.6x the weight reduction %, because V is squared).

As an example, in my Bonanza a glide at (L/D)max gives a nil wind glide range of 1.7nm per 1000ft. The glide speed to achieve that, however, is 15kts lower when the aeroplane is at its lightest compared with when it's at MAUW - circa 95kts versus 110kts.

[B2N2]

I had a hard time getting my head around exactly that when I was still flying gliders.
With competition gliders you have the option of carrying water in ballast tanks.
Purpose being to increase the "best distance" glide speed while maintaining the best glide ratio.
This article explains it well:
http://home.comcast.net/~verhulst/GBSC/student/ballast.html

Here is a pic of a glider dumping water during a finish pass:

http://upload.wikimedia.org/wikipedia/en/9/9b/LS40075.jpg

[formulaben]

Just something to add, hope this doesnt sound dumb, im just learning, but how come the best glide speed increases with weight? As the weight increases does L/D max increase too? Can someone please explain this.:ugh:

I'll just keep it simple: many other other V speeds go up with weight; Vx, Vy, Vs, Va, Vref, etc. Vbg (best glide) is no different. Hope that helps.

[machonepointone]

The speed for best lift/drag ratio will depend on the AUW, therefore the heavier the aircraft the higher the speed. In still air conditions it makes no fifference to the range. However, when gliding into a headwind there is an advantage when gliding at a higher speed because the percentage reduction in groundspeed is less. This translates into better range.

[Neptunus Rex]

To keep it simple, let's ignore any headwind or tailwind effect. Consider a 747, or any other large jet airliner. The typical descent profile is a fixed speed from Mach number into IAS, then 250 kts below 10,000 feet. A heavy aircraft (eg freighter) landing at MLW will be closer to best L/D Ratio throughout the profile than a lighter aircraft, and so be gliding more efficiently.There is also more momentum to dissipate in deceleration. Both factors add up to more air miles in the descent for the heavier aircraft.
Neppy
:cool:

[wug]

I always thought that total energy was a factor. At a given height, a heavy aircraft has more potential energy than a light one. In order to overcome drag during descent, potential energy is converted to kinetic.
Therefore a lighter aircraft must descend more steeply to maintain a given airspeed, thus the shorter descent distance required.
wug :8

[Unhinged]

Glide Ratio (horizontal distance covered for height lost in nil wind) equals the Lift:Drag ratio.

Lift:Drag ratio depends on Angle of Attack (amongst other things), and has an optimum value at the most efficient AoA (usually around 4 deg nominal)

As weight increases the rate of descent will increase, and horizontal speed must also be increased in order to maintain the same Angle of Attack.

In nil wind conditions, with a constant AoA, glide distance doesn't change as weight changes. As the weight increases, you will glide to the same place but you will arrive there sooner.

[OpenCirrus619]

Unhinged :D

Best simple explanation, which also has no inaccuracies, I have heard for ages.

OC619