Glide Ratio Question

[blueloo]

Can anyone help me with the following:

If 2 jets are identical in every respect but different weights, why does the heavier jet glide further?


If both glide (or for all intents descend at idle power) at 300kts which will go further?

If both glide at their best respective min drag which will go further?



I always thought that the heavier aircraft would have to convert its height into speed, and because it is heavier it would need a steeper descent to maintain the speed, and therefore distance covered would be less......

[Chimbu chuckles]

From memory, in still air, they both glide the same distance...the heavier one gets there quicker...that's if they are both gliding at their respective CLmax.

If you're talking both doing 300kts the heavier will travel further....probably because it's closer to it's CLmax than the lighter one....certainly in the real world you tend to need to start your descent earlier when heavy than when light...because we descend to a fairly fixed speed schedule....M.8/290/250kts or similar...rather than at CLmax.

[Capt Fathom]

These figures are rough, but should help with your query.

At higher weights, the best Lift/Drag Ratio speed is around 300 kts.
At lower weights, the best Lift/Drag Ratio speed is around 260 kts.

Descent is generally made at a fixed airspeed of around 300 kts , which is great when you are heavy, but not so great when you're light. (Around 300kts on descent is standard to help Air Traffic Control with sequencing of aircraft).
So at light weights and 300 kts (300kts is the speed for heavier weights), there is a large increase in drag over a 260 kt descent.

So if you descend at 300kts at light weight, the increase in drag brings you down in a shorter distance. If you descend at 340kts, shorter distance again.

Every Weight has a equivalent 'best Lift/Drag Ratio' speed. If you descend at a speed higher than that, you will descend over a shorter distance due to an increase in drag.

Bloody Hell...am I close?

[blueloo]

Thanks I see what you guys are getting at, but for some reason I still have this thinking from physics (which i was /am terrible at) - about kinetic energy and conversion, about having to convert energy forms, in the sense that the altitude is the energy, and we are converting that height into speed, surely the heavier jet needs to use more of the height to maintain an equivalent speed........

I am not sure where i got this thinking from, and I am pretty sure I am wrong..... hmmm

[Chimbu chuckles]

The heavier jet needs to lower it's nose a little more to angle the total lift vector forward enough to attain and maintain it's respective best glide speed...CLmax. Remember how weight acts straight down towards the center of the earth and lift acts normal to the chord of the wing....more or less?

Any slower than best glide speed and induced drag increases, any faster and form drag increases....just the right speed gives max lift for min drag.

The heavier jet glides at a faster speed than the lighter one...but they go the same distance...assuming still air and both at their respective CLmax.

That's not how we descend so pilots get the impression that heavier jets glide further period.

Someone may ask why do gliders bother with ballast then...if it makes no difference to distance...ballast, in a glider, is all about going faster while maintaining a reasonable glide ratio to get to the next updraft quicker...not about going further in a straight line, nil wind/no thermals, drag race.

[BackdoorBandit]

An aircraft must convert all its Potential Energy, due to height, into Heat Energy, which is formed by Drag.

The heavier the aircraft, the higher the P.E. and the greater the amount of energy needed to be converted to heat.
The drag at say 260kts is prety much a constant (you will have slightly increased induced drag, but airframe drag will remain about the same). Hence our conversion from P.E. to heat is pretty much constant regardless of weight.

So in order to convert the higher P.E. of the heavier aircraft, it will have to stay at 260kts for longer or descend at a higher airspeed.

If you have a fixed descent speed, then you must stay at 260kts for longer, and you must start you descent earlier.

If you remain at say 260kts for longer, then your distance for descent must also increase.

It is basically all about the conversion of Potential Energy into heat, and the means by which it is done.
Flaps will increase drag and hence convert energy faster, resulting in reduced glide distance.
Spoilers will do the same.

[blueloo]

Thanks all for the replies, i think i can make sense of it now.
cheers
loo

[Milt]

blueloo

The glide angle is a result of the ratio between lift and drag.

Mathematically the angle is the Tangent of L/D.

So best glide angle should be close to the lifting surfaces' maximum coefficient of lift.

Normal way to increase glide angle is to increase drag.
Spoilers, speed brakes, flaps, windmilling props, landing gear and higher speed all add drag to get you down at steeper angles.

Higher weights alter the required lift to carry the increased weight so you have to fly faster to reach the maximum coefficient of lift. The higher speed also raises the drag. If lift and drag then have changed by the same ratio the glide angle remains the same. But rarely will lift and drag vary by the same proportions. For practical purposes they will be close.

Conclusion. At higher weights you have to fly faster to achieve the smallest angle of glide.

Hope this doesn't confuse you more !

[jimshutt]

to maintain the same L/D ratio with an increase in weight, the A/A must remain at the min drag angle and therefore the speed must be increased. The energy required is provided by the increase in the ` forward component of the weight ` and it works ask any glider pilot. John.

[ftrplt]

similar question (http://www.pprune.org/forums/showthread.php?threadid=129617&highlight=glide+distance)