Flighty
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Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 3:28 pm

Self explanatory. Is this an incentive to cruise at high altitude when possible?

I find that in cars at high altitude are quieter and probably more efficient at cruise at altitude, so...
 
thegman
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 4:09 pm

As you increase altitude your true airspeed increases as a factor of air density and temperature, etc. Wind direction either increases or decreases your ground speed.

http://www.experimentalaircraft.info...ning/aircraft-navigation-speed.php

^This explains it much better than I can.
 
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 5:57 pm

Specifically as asked then yes wind resistance (drag) is lower at high altitudes. The reason is the air is thinner than at low altitude.

An analolgy may be that sea level the air is like treacle and at high altitude the air is like water. Obviously it is easier to push something through water than it is through treacle.

There are other factors that do come into play. There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air.
 
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 5:58 pm

Quoting Flighty (Thread starter):
Self explanatory. Is this an incentive to cruise at high altitude when possible?

I think you're asking if air resistance (profile drag) decreases with altitude.

Short answer is, for a constant true airspeed, yes it does, and yes that's a big reason to cruise at higher altitudes.
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 7:33 pm

Depends on your point of view. as stated above for a given true air speed you have less drag the higher you go.
BUT
Induced drag is calculated similarly to lift. It is basically the same formula but with a drag coefficient instead of the lift coefficient. This means that The higher you go the less lift you produce with a constant true airspeed. Thats why you go pretty fast up there to produce the lift you need.
The lift you need is dependant on your weight. When you want to climb you need a little bit more, when you want to descent you need a little less. But for a given weight the lift you need stays more or less the same throuhout the flight. So when the lift stays the same, so does the drag.

This means that in reality you have almost the same induced drag no matter what altitude you are at (with a given indicated air speed). But as your true air speed is way greater at high altitudes than it would be near the ground, you spend less time in the air thus saving fuel.
 
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 10:59 pm

Quoting StTim (Reply 2):
There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air.

In practice you just get less power.
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 11:48 pm

Quoting StTim (Reply 2):

There are other factors that do come into play. There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air.

The difference in gasses percentage wise is insignificant at cruising altitude. Less power is simply less air being drawn in, the same reason you have less power at lower throttle.

[Edited 2016-01-13 15:49:15]
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RE: Is Wind Resistance Lower At High Altitude?

Wed Jan 13, 2016 11:54 pm

Quoting nomadd22 (Reply 6):
Quoting StTim (Reply 2):

There are other factors that do come into play. There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air.

That's wrong. The difference in gasses percentage wise is insignificant at cruising altitude. Less power is simply less air being drawn in, the same reason you have less power at lower throttle.

Less air and thus less fuel to keep the fuel to air ratio constant.
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RE: Is Wind Resistance Lower At High Altitude?

Thu Jan 14, 2016 1:36 am

Quoting Flighty (Thread starter):
I find that in cars at high altitude are quieter and probably more efficient at cruise at altitude, so...

I find them lacking in the power that they typically have at sea level myself   And I grew up in a place where the average elevation was about 4,000' AMSL. Whenever I drive a car home, it becomes a real pig at high altitudes   Unless it has something in it to keep the air pressure up, like a turbocharger or a supercharger. Even with one of those, hot days at high altitude takes its toll on engine performance  
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RE: Is Wind Resistance Lower At High Altitude?

Thu Jan 14, 2016 11:36 pm

Quoting KELPkid (Reply 8):
I find them lacking in the power that they typically have at sea level myself

That's one of the reasons I'm switching to a Tesla. About 700 HP at altitude from a standing start.
 
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RE: Is Wind Resistance Lower At High Altitude?

Thu Jan 14, 2016 11:49 pm

Thanks for replies. I was just excited to notice that it wasn't just imagination that there was less wind noise at say 8000ft. Of course there is; the wind resistance is lower! Should help efficiency too. I have one turbo car that experienced much more lag at high altitude, but it may still achieve peak power eventually.

Hearing about induced drag was very interesting. Clearly at high altitude there is less lift. It seems there is no way to easily climb to say FL650 and enjoy tremendous fuel economy. Otherwise Airbus and Boeing would make their longha aircraft do that..
 
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 1:09 am

Quoting Flighty (Reply 10):
it wasn't just imagination that there was less wind noise at say 8000ft. Of course there is; the wind resistance is lower!

"Wind noise" is a subjective measure. It wouldn't necessarily decrease with altitude. An F-18 doing 0.98 Mach number at 8000 ft. will produce more "wind noise" than a Cessna 172 doing 90 kts IAS at 500 ft.
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 1:57 am

Quoting Flighty (Reply 10):
Hearing about induced drag was very interesting. Clearly at high altitude there is less lift. It seems there is no way to easily climb to say FL650 and enjoy tremendous fuel economy. Otherwise Airbus and Boeing would make their longha aircraft do that..

You'd need a very large wing, which leads to lots of parasite drag before you get to your cruising altitude.

Maximum operating altitude has other limitations though, in particular the requirement to be able to execute an emergency descent within a specified time.
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 8:53 am

Quoting Flighty (Reply 10):
Hearing about induced drag was very interesting. Clearly at high altitude there is less lift. It seems there is no way to easily climb to say FL650 and enjoy tremendous fuel economy. Otherwise Airbus and Boeing would make their longha aircraft do that..

There are other effects preventing cruising at these altitudes. While your true Airspeed is increasing with the altitude, you come closer and closer to your maximum Mach-Speed. That means although the air gets thinner, it will still flow around your aircraft at nearly the speed of sound. This limiting effect starts already at around FL290. Going higher, air temperature is decreasing, thus speed of sound is decreasing. Therefore your maximum possible speed will decrease until it is equal to your minimum speed (with some margins). This will define your maximum altitude. With decreasing weight your minimum speed will get lower, allowing further climb with a lighter aircraft.
This negative effect of the Machnumber at high altitudes is even stronger than the positive effect of thinner air in regards to your maximum speed. So you would be able for your fastest cruise at altitudes around FL290, but at the cost of a very high fuel consumption. Therefore we prefer to climb higher and fly slightly slower but much more economic.

[Edited 2016-01-15 00:56:56]
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 11:58 am

A good explanation of what is known as coffin corner.
 
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 3:40 pm

Quoting glen (Reply 13):
So you would be able for your fastest cruise at altitudes around FL290, but at the cost of a very high fuel consumption. Therefore we prefer to climb higher and fly slightly slower but much more economic.

Great thanks, I guess yes it occurred to me after posting this thread that FL650 or so would likely require the aircraft to go supersonic (otherwise no speed/efficiency benefit). I think that's the key issue I was missing. Otherwise, why not have an SST that faces very little resistance at stratosphere altitudes and goes mach 3. I never realized that that was the key hope of SST projects maybe (if I am not wrong): to get out of this coffin corner and reap the speed benefits in thin air. But obviously, it wasn't easy.
 
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 15, 2016 6:46 pm

Quoting Flighty (Reply 15):
Otherwise, why not have an SST that faces very little resistance at stratosphere altitudes and goes mach 3.

The "Blackbird" family (A-12, YF-12, SR-71) routinely cruised for hours at 3.2 Mach number above FL700. However, they were in afterburner the whole time and consumed enormous quantities of fuel.
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RE: Is Wind Resistance Lower At High Altitude?

Sat Jan 16, 2016 10:21 am

The higher you go the less drag that is true. However, you have to offset that by a lack of thrust coming out of the engines. You gain up until a certain altitude, then you start losing efficiency above a certain altitude. In the turbo-props i've flown this crossover point was generally in the lower 20's (SF340, BE-90, DHC-8), for the classic 737's and the MD-80's i've flown, this happened in the lower 30's where the 737NG's that I fly now have this cross-over in the mid-30's for the 800/900's and in the upper 30's for the 700...all estimates at average-ish operating weights of course. If you are super light these altitudes are higher up to the max ceiling, if you are heavier the optimum altitudes can be substantially lower.
 
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 2:45 am

Drag = Cd*1/2*rho*M^2

rho=air density
M=mach speed squared.

Lift=CL*1/2*rho*M^2

Both Cd (coefficient of drag) and CL (coefficient of lift) are a function of Mach #.

Lift must be great enough in commercial flight to climb 100ft/min or more. Any less and no certified flying higher.

In general, flying higher is less drag, but there is an optimum Mach # and thus the step climbing as aircraft weight reduces (more surplus lift allowing climbing to the next altitude). Step climbing as flights are, in each direction, 2000 ft incriments.

Newer aircraft have CFRP wings which allow more wing area and more wingspan that increases the optimum cruise Mach number. This is one reason newer aircraft have a higher optimum cruise altitude.

Quoting horstroad (Reply 4):
This means that in reality you have almost the same induced drag no matter what altitude you are at (with a given indicated air speed). But as your true air speed is way greater at high altitudes than it would be near the ground, you spend less time in the air thus saving fuel.

Except engines prefer a certain loading for optimum cruise. Thus there is an optimum cruise altitude for each weight. As the aircraft becomes lighter, the engines are less loaded and are less efficient (relatively, they're still burning less fuel). So some of step climbing is to gain back the engine loading. Put bluntly, engines idle poorly. They work best highly loaded. But go to fast, and shock waves produce much more drag (there is a small optimum window of flight speed). Climb too high and the engines have parts hitting shock wave drag (little added net thrust for a lot more fuel burn).

New engines have modified blade profiles which help, but there is a range of Mach #'s the engines are optimized for.

For example, the Geared turbofans currently being delivered start having issues above Mach 0.84. That would be a poor choice for a business jets that fly Mach 0.85 to 0.93. Hence why the PW800 have much smaller diameter fans for the same low compressor and high spool. Everything is optimized together (the days of an airframe being competitive with mis-matched optimum Mach #s are in the past).

The higher the cruise Mach #, the higher the optimum cruise altitude. Hence why Gulfstreams fly higher than commercial jets.

Quoting PassedV1 (Reply 17):
The higher you go the less drag that is true. However, you have to offset that by a lack of thrust coming out of the engines.

It is thrust and wing loading. The 737-800 has about 12% higher wing loading than the CS300. One would expect the C-series to be able to step climb earlier and thus achieve a higher cruise altitude.

But your summary is accurate. The MD-80s were optimized for lower altitude.


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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 8:21 am

Quoting lightsaber (Reply 18):
Drag = Cd*1/2*rho*M^2

Lightsaber thanks for this summary. Just a few questions of clarification:

-the quoted formula is for Cd0, right? Induced drag varies inversely with air density IIRC.

-is there a slight difference in the quoted formula depending on local speed of sound? I always thought Cd0 related to Vtrue^2. True airspeed would give different Mach numbers depending on local speed of sound. I know it's a small difference but just trying to see if I'm understanding correctly. The intuition in my head is that kinetic energy of the freestream/airplane interactions varies with Vtrue, while compreesibility/wave properties vary with M.
 
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 12:07 pm

Guys,

This thread demonstrates the difficulty of trying to simplify some technical ideas for the general public to understand.
The aerodynamics have been dumbed down so much that , as a matter of fact, we are in false / erroneous territory.
Does that help the OP or any reader of the thread ?
Don't think so.
So back to the subject :

"Is wind resistance lower at high altitude ?"
Basically, any object in an airstream will only be influenced by the number of air molecules it encounters ; that number is - broadly - a function of air pressure.
We could say that a speed indicator reflects the number of molecules that affect the sensor in the airstream. That indicated airspeed has to be corrected for density / pressure... in order to compute true airspeed - TAS.
SO, the answer to the question is :; "Yes, the air resistance is lower at high altitude ; the amount is proportional to the square root of density.

Quoting horstroad (Reply 4):
BUT
Induced drag is calculated similarly to lift. It is basically the same formula but with a drag coefficient instead of the lift coefficient.

I disagree. At that stage we only compute a phenomenon called *DRAG* without any attribution to any influence.

Quoting Flighty (Reply 10):
it wasn't just imagination that there was less wind noise at say 8000ft.

The main reason is that below 10 000 ft, an airliner is restricted to a maximum IAS of 250 kt.

Quoting glen (Reply 13):
There are other effects

I basically disagree with your post. The coffin's corner is the by-product of the lift equation taken to its limits : Clmax * M^2

Quoting lightsaber (Reply 18):
Drag = Cd*1/2*rho*M^2

Is that an ( over) simplification of the drag f(M) equation ? because I generally use the classic
Drag = Thrust = .7 Ps S M^2 Cd. (coeffs through metric units)
where the constants S - for wing area - and Ps - static pressure - are reestablished.

Quoting lightsaber (Reply 18):

Both Cd (coefficient of drag) and CL (coefficient of lift) are a function of Mach #.

Agreed with the proviso that the main influence is the angle of attack. The Mach number influence is harder for us to compute, especially when for all intents and purposes, we fly at a CI which brings an AoA at a very narrow band of M... until very late in the flight when weights become very low.
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 3:21 pm

Quoting lightsaber (Reply 18):
higher the cruise Mach #, the higher the optimum cruise altitude.

This doesn't seem to be correct.

Quick check for the A330 T700 engines at 200t cruise weight:

0.80M opt altitude 36 500ft.

0.82M opt altitude 36 600ft

0.84M opt altitude 35 300ft
 
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 3:51 pm

Quoting Chaostheory (Reply 21):

This doesn't seem to be correct.

Quick check for the A330 T700 engines at 200t cruise weight:

0.80M opt altitude 36 500ft.

0.82M opt altitude 36 600ft

0.84M opt altitude 35 300ft

You're just posting specs for one particular aircraft and engine. That has little to do with the general rule.
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 3:54 pm

Quoting nomadd22 (Reply 22):
You're just posting specs for one particular aircraft and engine. That has little to do with the general rule.

I checked for the falcon 900 and g3 too.

Same applies. I'll post them later tonight if i can.
 
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RE: Is Wind Resistance Lower At High Altitude?

Sun Jan 17, 2016 7:20 pm

Quoting Pihero (Reply 20):
This thread demonstrates the difficulty of trying to simplify some technical ideas for the general public to understand.

Indeed. Lightsaber made a great effort to dumb down the issues at hand in plain English for the benefit of this thread. Mastery of the material is necessary to do this. Anyone can quote the textbook, but it's not easy to explain why, for example A380 do not cruise higher to encounter less drag. But the thread was beyond my hopes to learn something! I am so grateful for everyone's time!

Quoting Pihero (Reply 20):
The main reason is that below 10 000 ft, an airliner is restricted to a maximum IAS of 250 kt.

Oh apologies; I was comparing automobile wind noise at 8,000ft vs sea level. It seems quieter at 8,000 ft. The aerodynamic "wall" in fact occurs at a quite higher speed (maybe 85 mph instead of 70 mph). I hadn't thought of this ever before, but wanted to learn why airplanes do or don't capitalize on this. And they do; likely this is why they ever climb above say 30,000 ft. But airplanes need lift as well, forming a coffin corner; land vehicles do not have this limitation (except Earth's surface does not have highways tall enough).  
 
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RE: Is Wind Resistance Lower At High Altitude?

Mon Jan 18, 2016 4:25 am

Quoting Matt6461 (Reply 19):
-is there a slight difference in the quoted formula depending on local speed of sound? I always thought Cd0 related to Vtrue^2. True airspeed would give different Mach numbers depending on local speed of sound. I know it's a small difference but just trying to see if I'm understanding correctly. The intuition in my head is that kinetic energy of the freestream/airplane interactions varies with Vtrue, while compreesibility/wave properties vary with M.

Yes the value is local mach #, but it is always oversimplified as one value that is a function of aircraft global mach # and angle of attack.



Quoting Pihero (Reply 20):
Is that an ( over) simplification of the drag f(M) equation ? because I generally use the classic

It is over simplified, I didn't put in area. But the constants are different (two different wing areas). But no, do not recalculate Ps, use the density. It is the 'classic' way American aerodynamics is taught. We end up with different Cd curves, but the same answer.

Quoting Pihero (Reply 20):
The Mach number influence is harder for us to compute,

CFD is used to compute the Mach # influence and verified by flight test. In general, able to be accurate within 1/2%. It has become too good. Management is cutting from the flight test budgets the time to verify assuming 'it will just be right.' Now this is at the big airframers. The little guys often miss... Cest la vie.

Quoting Chaostheory (Reply 21):
This doesn't seem to be correct.

Higher the design cruise mach #, the higher the altitude. What is happening is engine effects. The base aerodynamics should be the faster you fly, the higher the altitude. It is a general rule.

The trend is right from Mach 0.80 to 0.82. Mach 0.84 is odd... What is the fuel burn difference by altitude? Is a thermal limit? For lower altitudes have more cooling (denser air). I'm talking fuel burn. Other system parameters effect overall optimum and that gets complicated quickly.

We're also neglecting angle of attack in our discussion. So for the A330, I wouldn't be surprised if it flew with less angle of attack at the lower altitude shifting the Cd lower that overcame the air density penalty. What are your numbers for the Falcon 7X, G650, or other aircraft with more modern airfoils? The aircraft you listed are all older designs with interesting 'shock wave traps' were tricks might have been found to optimize range. There is a reason aerodynamics engineers like clean wings, it simplifies such discussions.   

Yes, everything is oversimplified. But it gets very complicated in airframe optimization. My favorite part of aircraft design is the final design of the airframe/engine mating. Where the two are optimized together as a system. Could then engine do this... well if the airframe did that... A pair of teams working together will produce an airframe where the system is better, but not the engine nor the airframe that was expected at the start of the study. It is intense... I worked with Airbus where we put out no fewer than 30 concept engines, each a tweak. The goal was the same, the best plane for the money. But it was "if we take this out of the engine and spend the money on the airframe" what do you get? Or... If we spend more on the engine but less on the wing, what is the result.

But it does depend on what you optimize:
fuel burn
range
maintenance
How much design (MTOW and thrust) growth is built in?
Short field
What is the optimum range?

The more that is 'hung on the wing,' the more complicated the discussion becomes. For shock waves change the optimum angle of attack and that changes the CL/Cd ratio, but only a little. We're getting beyond a classroom setting and getting into industry optimization.

But in general, the faster you fly, the higher the optimum altitude. Engines with poor cooling... will have to fly lower. That is one reason the Silvercrest went back for redesign. It wasn't getting enough cooling at high speed cruise at altitude to meet range at speed promises. Well... and that case issue... But both had to be corrected.

Quoting glen (Reply 13):
This negative effect of the Machnumber at high altitudes is even stronger than the positive effect of thinner air in regards to your maximum speed. So you would be able for your fastest cruise at altitudes around FL290, but at the cost of a very high fuel consumption. Therefore we prefer to climb higher and fly slightly slower but much more economic.

Yes, you can fly faster at lower altitudes, but at the cost of fuel. I was optimizing range. Since all modern aircraft fly 36,000+ in the stratosphere, there is no longer a temperature effect with altitude.

You couldn't fly faster at FL290 in most airspace, it is too crowded with aircraft flying Mach 0.74 to 0.78. Modern business jets will instead fly up higher at Mach 0.9+. There is a fuel burn penalty above Mach 0.85, but there are a growing number of business jets that fly there. Bombardier has been surprised at how the majority of flights the GE6000 are flown at Mach 0.87+. Gulfstream expected the G650 to be flown at Mach 0.87 to 0.88 most of the time and instead it has been Mach 0.9+.

We need to be careful about talking about older established types. There is a reason certain business jets are doing better than others and the common theme is range at speed. Not speed on its own, not range on its own, but how fast certain long range missions can be performed. New buyers of business jets tend to be the most intense users, so they have the more stringent demands. That drives the market. Anyone sensible, is buying a used business jet.

Commercial jets are benefiting from CFRP wings and that is changing the flight profiles nicely. Longer wingspans and more wing area for the same or reduced weight. I fully expect CFRP wings to take over all development, even smaller business jets. The new Lithium Aluminum will keep the fuselage beer can for a long time. Why do I bring this up? Weight determines cruise altitude to a high degree and the newer airframes have much lower wing loading which nicely changes the flight optimization. The new wing profiles are also far better for fast cruise.

One easy example to compare is the G280 vs. G200. While the tail aerodynamics were improved greatly, the big change was a wing profiled with much more advanced cross sections. Cross sections that were better than the G550 (as much as Gulfstream tries to imply they kept to those cross sections... nope... they are improved). The newer cross sections have much better characteristics for shock waves at speed, so we'll see performance more in line with the theoretical numbers.

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RE: Is Wind Resistance Lower At High Altitude?

Mon Jan 18, 2016 7:27 pm

Quoting lightsaber (Reply 18):
Lift must be great enough in commercial flight to climb 100ft/min or more. Any less and no certified flying higher.

In general, flying higher is less drag, but there is an optimum Mach # and thus the step climbing as aircraft weight reduces (more surplus lift allowing climbing to the next altitude). Step climbing as flights are, in each direction, 2000 ft incriments.

Isn't it more surplus thrust allowing climbing? ROC is generally proportional to excess thrust.

Quoting Pihero (Reply 20):
Quoting horstroad (Reply 4):
BUT
Induced drag is calculated similarly to lift. It is basically the same formula but with a drag coefficient instead of the lift coefficient.

I disagree. At that stage we only compute a phenomenon called *DRAG* without any attribution to any influence.

   Induced drag refers specifically to lift-induced drag, which is just one component of total drag. When talking about "wind resistance", you're more likely talking about profile drag, or friction.
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RE: Is Wind Resistance Lower At High Altitude?

Tue Jan 19, 2016 2:44 am

Quoting vikkyvik (Reply 26):
When talking about "wind resistance", you're more likely talking about profile drag, or friction.

Eh... all of the above!

I guess we have 'concluded' that total drag per distance actually reaches a minimum at a certain altitude..? It sounded like induced drag actually can rise if cruising too high, maybe because AoA has risen? Leading to a potential stall? Presumably, minimum total drag per distance equates directly to best fuel efficiency.
 
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RE: Is Wind Resistance Lower At High Altitude?

Tue Jan 19, 2016 4:53 am

This is a fascinating thread. Many thanks to all of the contributors and for the OP for asking the question.

Going back to the beginning of the jet age, the 737, DC9, and 737 all used effectively the same engine, PW JT8D.

737 (Jurassic) cruised at a slow Mach .74 ish. DC9 similiar.

727 cruised extremely fast, high 0.8X and up to 0.9 Mach.

Both used effectively the same engine as I understand. Where any major changes necessary to accommodate the huge speed deltas? Was the 727 extremely inefficient from a engine perspective? We're the dc9/737 not optimized? We're neither frames well optimized for the engine?

Obviously wing design drove the speed differences but I'm curious how the (seemingly) same engine hurt / affected either design.
 
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RE: Is Wind Resistance Lower At High Altitude?

Thu Jan 28, 2016 7:52 pm

Quoting lightsaber (Reply 18):
Except engines prefer a certain loading for optimum cruise.

Could you please explain the "loading?" Is that the force on the fan blades compressing the air mass? Like a propeller? If so, why not have "variable" pitch blades?
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 29, 2016 1:30 am

Quoting bhill (Reply 29):
Quoting lightsaber (Reply 18):
Except engines prefer a certain loading for optimum cruise.

Could you please explain the "loading?" Is that the force on the fan blades compressing the air mass? Like a propeller? If so, why not have "variable" pitch blades?

It has to do with RPM. Jet engines are most efficient in a certain RPM range, typically around 75-85% N1.

Variable pitch fan blades are a neat idea but I imagine the design and construction issues are rather large, especially with todays large wavy blades. However there is variable geometry in jet engines, in the form of variable stator vanes.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mandala499
Posts: 6589
Joined: Wed Aug 29, 2001 8:47 pm

RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 29, 2016 7:06 am

Quoting Flighty (Thread starter):
Is this an incentive to cruise at high altitude when possible?

Not when there are strong headwinds at that "ideal altitude"...

Best speed is around FL270 - 280... but poor fuel economy. Above that, mach limit begins to cap how fast you can go...

So it's all a trade off...
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flipdewaf
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RE: Is Wind Resistance Lower At High Altitude?

Fri Jan 29, 2016 10:04 am

Remembering back to my degree I think the best interpretation was that the drag was pretty much exactly the same at all altitudes (neglecting mach effects) you just happen to be going faster at higher altitudes. You get up higher and the speed of sound gets in the way.

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Pihero
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RE: Is Wind Resistance Lower At High Altitude?

Sat Jan 30, 2016 2:25 pm

Quoting flipdewaf (Reply 32):
Remembering back to my degree

Not very long ago, hence should be more accurate...

Quoting flipdewaf (Reply 32):
the drag was pretty much exactly the same at all altitudes

...for the same indicated airspeed (IAS)...

Quoting flipdewaf (Reply 32):
you just happen to be going faster ( i.e at a higher true airspeed, TAS ) at higher altitudes.

  

Quoting flipdewaf (Reply 32):
You get up higher and the speed of sound gets in the way.

...and your drag will start increasing more and more... for a constant IAS but at an increasing Mach number
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bhill
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RE: Is Wind Resistance Lower At High Altitude?

Wed Feb 03, 2016 4:53 pm

Quoting Starlionblue (Reply 30):
It has to do with RPM. Jet engines are most efficient in a certain RPM range, typically around 75-85% N1.

So as the density of the gas decreases with altitude, does the RPM have to increase to get the same N values/loading? Seems counter intuitive.
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rwessel
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RE: Is Wind Resistance Lower At High Altitude?

Thu Feb 04, 2016 5:21 am

Quoting bhill (Reply 34):
So as the density of the gas decreases with altitude, does the RPM have to increase to get the same N values/loading? Seems counter intuitive.

No, the Nx values are based strictly on RPM. But the amount of airflow (on a mass basis, which is all that really counts) through the engine is (roughly*) proportional to the local air density, so engines produce less power (and burn less fuel) at high altitude. But as power goes down, so does the load from the fan and compressors (since they're also dealing with less airflow), so things still balance. There is usually a drastic difference between maximum, sea-level, take-off power and the maximum amount of power available at cruise**, the typical cruise power is usually 20-40% down from that.


*Ram recovery and some flows do vary with *true* airspeed, but for subsonic engines, the effect is fairly minor.

**For example, you'd expect your GE-90-115 (115klbs at sea level) to produce only about 28klbs at 35,000ft. A 750,000lb 777 probably has an L/D of around 20:1 in cruise configuration, so it would need about 37,500lbs of thrust, or about 67% of the maximum available thrust at 35kft. And that's also why a grossed out 777 can't maintain 35kft on one engine (single engine thrust would reach 37,500lbs at about 28,000ft).
 
masi1157
Posts: 402
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RE: Is Wind Resistance Lower At High Altitude?

Thu Feb 04, 2016 1:16 pm

Quoting hivue (Reply 11):
"Wind noise" is a subjective measure. It wouldn't necessarily decrease with altitude. An F-18 doing 0.98 Mach number at 8000 ft. will produce more "wind noise" than a Cessna 172 doing 90 kts IAS at 500 ft.

"Wind noise" is not at all subjective, it can be measured and predicted quite accurately. On the same A/C flying at constant Mach, it does in fact decrease with altitude.


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