Which is faster?

[~~~^~~~]

Alex - If anyone mentioned the effect of True Air Speed, I missed it. As a helo driver, it is worth remembering. Roughly, you can figure TAS as Indicated Airspeed * ( 1.0 plus 2% of yor Flight Level)

So at FL180 a 300 knot cruise gives you 408 KTAS
At FL330 a 300 knot indicated cruise gives you 498 KTAS

and so on until you realize the Concorde was only indicating around 450 in cruise while scooting along over 1,000 miles per hour.

As a bored RJ pilot who is usually trying to make up lost time, I've found the published climb profile not only works well for fuel burn, it also gets you to altitude the most efficiently where you can enjoy the much higher cruise speed. The CRJ200 published profile is a 290 knot IAS climb until M 0.74. I have found there is variation. A heavier airplane climbs better faster and a lighter airplane can climb better at a slower IAS.

The mention of L/D Max comes into play here - the wing is most efficient at a certain angle of attack, which is roughly speed and load dependent. Also, other factors come into play. The cowling on almost all turbofan and turbojet engines acts as a compressor. The air actually slows and increases pressure before the fan scoops it through the bypass. It then expands and increases speed as it decreases pressure. The more air you can shove in the cowling, the more air you have to work with, the more fuel you can mix with it and the more thrust you can make. In addition the Supercritical wings on many turbine aircraft do a lousy job at making lift at slow speeds.

So flying on profile is about as fast as you can go in many airplanes. If the trip is short you might come out slightly better at a "barber pole" climb, but on most jets the specified climb profile will get you to altitude most efficiently where you can really haul.....

The airplane I fly is limited at 330 to 335 KIAS. At 10,000 feet that is 396. I like the flight much better at 370, which even at a 280KIAS cruise works out to 487. Saving money and going faster.

 

[MalteseX]

Just wondering, given a constant power setting, say at an N1 of MCT, is it faster to climb to your flight level asap or to fly as fast as possible during the climb while maintaining a 500 fpm climb?

Does anyone have any data to support either argument? It seems like pilots that are behind schedule like to decrease the rate of climb and increase the airspeed on the way up. My hunch is that it would be faster to climb on up to cruise where the engines are more efficient and your TAS is higher.

It's faster to increase the airspeed on the way up--but it will burn more gas... maybe much more gas depending on your type of aircraft. Climb profiles are built to give speed, but include a more beneficial fuel burn. If you want to factor in time ONLY, climb at max airspeed.

 

[~~~^~~~]

If you want to factor in time ONLY, climb at max airspeed.

I disagree unless you are talking about a trip in a C172. Aircraft that can exceed their max operating speed in level flight will almost always benefit by getting to their cruise altitude faster so that they can attain a higher true airspeed faster. Even in the turbocharged Beechcraft, Mooney's and Pipers I've flown in General Aviation, this holds true.

Delaying your climb in a jet will not only increase your fuel burn, it will increase your block time as well. From your profile I can not tell if you are flying anything with an FMS, but if you have a FMS, plug in your non standard climb and see what the results are. The FMS's are usually a little optimistic on climb performance on the RJ's, but even so you can see for yourself the effect on block time.

For us a 330 KT climb increases trip time by 5 to 12 or 15 minutes depending on altitude, temperatures, and trip length.

If the airplane has a climb profile - it is optimal.

 

[Big Beer Belly]

With that beer belly I think radiation exposure at high-level cruise is the least of your worries. (how do you get/keep a medical with that anyway?)

I'm a right SEXY b-a-s-t-a-r-d, ain't I?


BBB

 

[MalteseX]

For us a 330 KT climb increases trip time by 5 to 12 or 15 minutes depending on altitude, temperatures, and trip length.

If the airplane has a climb profile - it is optimal.

I can't speak for all aircraft, only the jets I have flown --- and I was being overly simplistic. But the climb profile in aircraft I have flown did not give the highest TAS--it gave the highest specific fuel range.

This may not be the case for other aircraft.

Climbing so you attain the highest TAS in the shortest amount of time is the way to get to a destination quicker (disregarding wind) If you take wind into account, then the highest GS. In the aircraft I have flown, the climb performance was great enough to overcome the effects of the lower TAS at lower altitudes--so climbing at "barber pole" was the way to go (still got 3000 fpm at low alts and 1500 to 2000 fpm in the 20's).

However, at 27500 or so, the "barber pole" IAS limit was transferred to the Mach limit. As you hold the mach limit and climb higher, the TAS decreases. So for normal turbofan transport aircraft, the fastest climb to approx FL 280 and then flying your Mach limit, will give the highest TAS attainable for the conditions that day. (ie. For a given Mach number, TAS decreases with increases in altitude)

If winds are to be taken into account, then a wind correction for your altitudes can be applied to determine what the effects are on you GS at various altitudes.

I "flight tested" this once flying from Europe back to the mid western US in 1990. One of our company aircraft (identical aircraft, and fuel load) departed 20 minutes ahead of us. We were on identical flight plan with the 20 minute separation. I changed the flight plan to level at 280 and stay there for the entire flight. The first aircraft departed 28 minutes ahead of us and flew the flight plan and recommended profiles. We flew at "barber pole" and 280 the entire way. We passed them at about somewhere in the mid atlantic ( I don't remember the longitude). Anyway, we arrived 18 minutes ahead of them (a total gain of 46 minutes)--and neither aircraft had any ATC delays, we had top ATC priority for those flights. We had both higher TAS at FL280 as well as more favorable winds, since we were headed westbound. But we burned nearly 40K lbs more fuel than they did (almost 1000 lbs per minute gain).

This may not be the same for your aircraft --as shown by your FMS; so I guess the answer to the original poster's question is that ---"it's probably aircraft dependent."

 

[Big Beer Belly]

Not to sully a dang fine thread (thanks for the fantastic post, GV), but I have myself an idear:

This was demonstrated to me in a King Air 200 by a 757 AA CA. He spoke for literally ten minutes straight, saying this is a perfect piece of information which, when used, separates the pilots from the aviators.

He kept calling it L/D Max, L/D Max... I understand the drag curve and performance, and where you can get the dreaded bimodal distribution (you get the same climb at two speeds), but there is no free lunch in physics. Those two humps will come together at some regime.

My question (BBB or GV or any other Carl Sagan Gene. E. Yuss) is this: What is the key to causing this? It happens on a 757, it happens on a Metro, it happens on my Dash 8, it happens on BE20s.

Induced drag ... at least on a swept wing, symmetric airfoil. The Blue Angels usually demonstrate rather extreme angle of attack slow flight ... albeit with excess thrust available to accelerate in level flight ... bottom line, the same thrust setting will produce 130kt flight (high AOA,, high induced drag) or high subsonic/supersonic flight (with corresponding low AOA, low induced drag) ... discounting bow wave/transonic drag spikes.

Been many moons since I taught any of this though. YMMV.

BBB

 

[~~~^~~~]

Climbing so you attain the highest TAS in the shortest amount of time is the way to get to a destination quicker

We were on identical flight plan with the 20 minute separation. I changed the flight plan to level at 280 and stay there for the entire flight. The first aircraft departed 28 minutes ahead of us and flew the flight plan and recommended profiles. We flew at "barber pole" and 280 the entire way.

This may not be the same for your aircraft --as shown by your FMS; so I guess the answer to the original poster's question is that ---"it's probably aircraft dependent."

We agree.

You are right, there is a "sweet spot" between FL290 and FL310 where a given Mach will translate into a higher TAS. The exact location of the sweet spot depends on temperature - sounds like you nailed it

 

[~~~^~~~]

If I was flying an aircraft that couldn't get out of the thirties, I'd climb at the fastest speed that would still give me a 500 fpm climb. Your manufacturer's cruise climb speeds are normally predicated on the most cost effective climb that will allow you to reach the maximum range capabilities of your aircraft, that is to say saving fuel, not time.

GV

~

I know you guys love him.... but he's wrong. Not to pick nits, but many First Officers have the wrong idea about how to get the shortest block time from the airplane and this comes up more frequently than it should.

The speed that gives you the best climb profile and gets you efficiently to the altitude where you can go fast turns out being the fastest. Doing 330 in the climb out of 10,000 feet (396TAS) is not quicker than climbing at 290 at FL280 (452TAS).

His GV and 550 perform a whole lot better than most commercial aircraft operating near max gross weight, so he probably does not notice the difference the way a pilot of a relatively low performance RJ does.

 

[Muddauber]

The speed that gives you the best climb profile and gets you efficiently to the altitude where you can go fast turns out being the fastest.

That's not what my FMC says...


Muddy

 

[~~~^~~~]

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)

 

[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.