Which is faster?

[Big Beer Belly]

Ok, lets try the other question folks have a hard time getting.

Which decends faster, a heavy aircraft, or a lighter aircraft?

(Lets just say a 747 at max gross, or at a weight a couple of hundred thousand pounds less)

Vmo is Vmo ... weight plays no role in maximum speed (presuming we are not thrust limited.)

BBB

 

[GVFlyer]

Ok, lets try the other question folks have a hard time getting.

Which decends faster, a heavy aircraft, or a lighter aircraft?

(Lets just say a 747 at max gross, or at a weight a couple of hundred thousand pounds less)

I think what you are attempting to ask is, "What effect does weight have on the power off glide range of an airplane."

If you examine the forces acting on a gliding aircraft, you will see that the gliding ratio is numerically equal to the lift-drag ratio.

For example, if a transport in a glide has an L/D of 16, each mile of altitude will be traded for 16 miles of horizontal glide. Thus, the aircraft should be flown at L/Dmax to obtain greatest glide distance.

So to directly answer your question, since the maximum lift-drag ratio of any given aircraft is an intrinsic property of aerodynamic configuration, gross weight will not effect gliding performance.

If your B747 has an L/Dmax of 15, the whale can get 15 miles horizontal distance for each mile of altitude. This would be true at any gross weight if the jet is flown at the angle of attack for L/Dmax. The gross weight would, of course, affect the glide airspeed necessary to to obtain this angle of attack, but the glide ratio would be unaffected.

GV

 

[mar]

No question

Which decends faster, a heavy aircraft, or a lighter aircraft?

(Lets just say a 747 at max gross, or at a weight a couple of hundred thousand pounds less)

Absolutely no question here. An empty 747 will descend faster for the same IAS.

By the way, interesting discussion so far. I think a new debate (like Pitch vs. Power) has been born.

 

[SeaSpray]

By the way, interesting discussion so far. I think a new debate (like Pitch vs. Power) has been born.

No we haven't.

GVFlyer is correct - it's basic aerodynamics.


SS

 

[~~~^~~~]

Mar is correct.

The answer is that a heavier airplane takes longer (further, slower decent rate) to come down than a lighter aircraft. Works in the flare too, a heavier airplane floats down the runway further than a light aircraft.

Conservation of momentum. http://www.grc.nasa.gov/WWW/K-12/airplane/conmo.html

A typical RJ at 46K to 47K will decend at 1,700 to 1,800 feet per minute at idle while holding 250 KIAS. A "very light" 43K pound plane needs about 2,100 to 2,200 feet per minute simply because there is less mass pulling it towards Earth. The effect is more pronounced with decelleration. My 2,800lb Mazda stops a whole lot shorter than the 5,640lb F150 Lariat.

 

[GVFlyer]

Deleted. Duplicate post.

 

[GVFlyer]

F=ma

Mar is correct.

The answer is that a heavier airplane takes longer (further, slower decent rate) to come down than a lighter aircraft. Works in the flare too, a heavier airplane floats down the runway further than a light aircraft.

I'll buy that. It's a different question than the one I answered, yours is a straight F=MA application.

On an ISA day, in a 90,000 lb G550 at FL450 it will take 27.2 minutes and 157 nautical miles to descend to sea level at Mach 0.75 with PLA=0 . In a 50,000 lb jet, it will take only 19.8 minutes and 115 nautical miles to make the same descent under the same conditions and at the same speed.

GV

 

[mar]

What? No discussion?

No we haven't.

GVFlyer is correct - it's basic aerodynamics.


SS

Well. I didn't say who I agreed with, but there seems to be some disagreement.

 

[bubbers44]

At best L/D aoa both light and heavy aircraft would have the same glide ratio. Practically, a pilot descends at a higher than best L/D aoa.
That is why a heavy aircraft requires more distance to descend from a given altitude than a light one. The heavy aircraft descending at M80/320 knots is closer to best L/D aoa than the light one.

 

[bubbers44]

At best L/D aoa both light and heavy aircraft would have the same glide ratio. Practically, a pilot descends at a higher than best L/D aoa.
That is why a heavy aircraft requires more distance to descend from a given altitude than a light one. The heavy aircraft descending at M80/320 knots is c
loser to best L/D aoa than the light one.

I meant to say higher speed than best L/D aoa speed.

 

[Muddauber]

Well. I didn't say who I agreed with, but there seems to be some disagreement.

According to my Aerodynamics for Naval Aviators GV is precisely correct.

In the his glide example airspeed varied so that the both aircraft could maintain LD max.

In his fixed speed example (M0.75) the heavier aircraft is much closer to LD max than the lighter one for the same reason that gliders carry water ballast in competitions.

Muddy











`

 

[mar]

Confusion

Ok whatever.

The debate I was referring to was the original "Which is faster" question.

I think we got our wires crossed.

GVFlyer is indeed a gentleman and a scholar.
Now leave me alone.