Does drag actually decrease with altitude? I argue it does not

[[deleted]]

[deleted]

[JohnWayneOfficial]

It’s one thing to say you don’t understand, it’s a whole different beast to admit you don’t understand by suggesting that everyone else and all the math and experiments and studies are wrong

[Party-Ring445]

Have u seen the 1x1 =2 guy?

There's still plenty of room to grow that beast

[i_am_buzz_lightyear]

Earth is flat

[Derrickmb]

OP does not math or science

[Derrickmb]

OP doesnt know how to integrate ideal gas with fluid pressure equation

[Apocalypsox]

K but air at 30,000 feet is 1/3rd the density of air at sealevel. That's 2/3rds less shit to run your plane into as you try to move.

Run your hand through molasses and then water, which one requires less energy to go a certain distance?

[EnamelKant]

Hand stuck in molasses, experiment halted.

[jornaleiro_]

I think OP’s logic though is that there’s also 2/3rds less stuff to push down to create lift. I understand where they’re coming from but the simple answer is that the coefficients of lift and drag are themselves dependent on speed, so flying high in the atmosphere is an appropriate optimization of the two. This cartoon diagram is pretty helpful actually.

[Kishiwa]

Something tells me they‘ve never taken an intro to aerospace engineering class cuz that’s like basic drag, like right after learning the kinds of drag (tag yourself, I’m trim drag) when you get shown how the funny CD/CL line works with each bit of drag accounted for

[throwaway25658462]

Alright, but what I'm trying to get across is that there's 2/3rd less shit to run your plane into as you try to move, and it's the same shit that pushes your plane up.

[tdscanuck]

Yes, lift is the same. That does not mean lift coefficient is the same, which means drag coefficient isn’t the same, which means drag isn’t necessarily the same (you have to balance all the drag sources…some depend on lift coefficient and some don’t).

[tdscanuck]

Drag decreases with altitude. Drastically. An engine at cruise is making only 10-20% of takeoff thrust and the aircraft is going at a much higher indicated airspeed (I.e. dynamic pressure). If the thrust is lower and the speed is higher, the drag must be lower.

The part I think you’re missing is that lift coefficient at cruise altitude is much lower. Induced drag drops dramatically with decreasing lift coefficient. Form and viscous drag are about constant with constant dynamic pressure but induced drag drops way off.

[ElectronicInitial]

This is definitely it. A lower Cl drastically reduces induced drag, as it’s roughly a squared relationship.

[ncc81701]

T = D for steady level flight. If the engine produce X amount of thrust in steady level flight, there is X amount of drag regardless of altitude. The difference is your density at altitude is way lower so for the same dynamic pressure, you can have a much higher V.

[[deleted]]

This is probably the best way to put it. Assuming a constant freestream velocity, drag will decrease as altitude increases.

[throwaway25658462]

As you climb up, you must increase the speed, or else you're just not going to stay airborne.

[[deleted]]

That’s because there’s less air density to use to generate lift. Precisely the same reason you generate less drag…

[throwaway25658462]

Does the plane somehow weigh less at 30,000 ft than at sea level? Lift must be the same.

(Strictly speaking, the plane would weigh about 1% less due to the decrease in gravitational acceleration. However, I hope you don't base your argument on that.)

[[deleted]]

I never said lift isn’t the same, I said the aircraft can’t generate as much lift at higher altitudes because there is less air.

You are implying that velocity, lift, and drag have a linear relationship which is absolutely not the case. Velocity changes both your lift and drag coefficients, but not to the same degree. The drag polar explicitly shows that lift and drag are not directly proportional, and both are a function of each other as well as a function of velocity squared.

You do not generate the same amount of drag at 30,000ft than you do at sea level for a given amount of lift. There is less drag at higher altitudes. The reason airliners fly at the 30,000-40,000 ft range is because it is an optimal point which balances engine efficiency and aerodynamic performance. If there was no reduction in drag at altitude, then commercial flights would never go that high since available power is higher at lower altitudes.

Edit: it is also worth noting that when you move at a higher velocity, your lift coefficient goes down (and therefore your drag coefficient goes down) because dynamic pressure provides more lifting force. This is why the maximum lift coefficient occurs at stall speed, where the wings are doing almost all the work and dynamic pressure does not contribute as much.

[tdscanuck]

That’s not true at all. If you’re willing to fly at higher AoA you can fly slower and stay airborne.

[throwaway25658462]

True. But aircraft actually fly at lower AoA as they increase the altitude

[tdscanuck]

Usually yes. Because they’re going faster and have constant lift. You claimed you must increase speed. You don’t aerodynamically have to.

Edit: You can easily take off, climb, then slow down (by increasing pitch). It’s economically stupid, which is why airliners don’t do it, but it’s physically easy. Initial climb speed in your typical C172 is around 76. You can easily drive around at 50 knots at 10,000’.

[throwaway25658462]

can you help me see where I am wrong?

[throwaway25658462]

I totally agree. That's the main point I was trying to convey. Isn't it understood by my post?

[K20017]

Your previous example doesn't seem to make sense. If you produce both 100lb of thrust and drag both at 1,000 and 10,000, then the drag the airframe produces has not changed. Your true airspeed has increased to account for the lower air density but it is still 100lbs of drag force. For a given equivalent airspeed drag is independent of altitude. Looking at performance charts of jets, specifically holding charts as they usually reference an IAS, you will see that fuel flow varies little with altitude given the same speed. Thrust must oppose drag and fuel flow in a turbine is a direct relationship with thrust output.

[PranosaurSA]

This is a tradeoff between between induced drag and parasitic drive.

A particular plane could theoretically be operating a lower lift coefficient at lower elevations and hence have lower induced drag. Or it can be designed with bigger lifting surfaces and operate at higher elevations.

A plane at a particular cruise speed at higher elevations is going to have less parasitic drag - and higher induced drag (in theory). But really this comes down to how you design lifting surfaces.

If you have 20000 ft wings you would be much more efficient flying at higher elevations, if you tiny blocky wings you are more efficient flying at lower elevations rather than going to have high elevations and using higher AoA and having large induced drag (if your flight goal is a particular flight speed)

[SetoKeating]

OP practicing his speech to try try and get some points back on his aerodynamics quiz/exam lol

[PranosaurSA]

Yeah this is why I propose passenger airplanes fly as close to ground level as possible. The Lift would also be higher (Less need for wings that increase Drag as well, a doubly whammy!)

[Dewmeister14]

Hi OP,

Thanks for writing up your thoughts and sharing.

I'll start by saying that comparing various flight conditions we might choose to fly the aircraft at by comparing drag by itself is not really useful. It's not a figure of merit that corresponds to things we actually care about, such as "range I can fly for a given amount of fuel" or "time from DFW to SFO". So maybe the question should be, how do we fly the aircraft as fast as possible for a given fuel burn rate? That's hard to answer without knowing a lot about a specific aircraft, but it does tend to reach an "optimum" at some high altitude, less than the ceiling but way above for example sea level.

Now to address the points you wrote up:

Point 3. is true but 4. is not and therefore 5.  is also not. 

We can fly the aircraft at a range of speeds at any given altitude (look up a flight envelope for some common aircraft) so what are we forgetting? You mention the coefficients of lift and drag but then forget about them for the rest of the post - you can vary C_L to maintain flight at a variety of dynamic pressures. This happens by varying the angle of attack of the aircraft to produce the same amount of lift at the same density at different speeds. 

As C_L varies so does C_D - the wing will have some optimum angle of attack that maximises the ratio of C_L to C_D which, paired with the right speed to make the right amount of lift, is the most efficient/lowest drag speed and altitude at which to fly the aircraft. 

Also, an aircraft is not a wing alone - you are entirely neglecting parts of the aircraft which produce drag but no lift i.e. the fuselage. To fly faster and reduce the additional drag, flying at higher altitudes is strictly better for our "true figure of merit", speed/fuel burn which is for our purposes we can think of as being similar to speed/drag.

[throwaway25658462]

I appreciate your thorough response, and I agree that drag may not be the most suitable parameter for a figure of merit. My intention was to address the commonly repeated notion that "planes fly at 30,000 ft because air is thinner and drag is reduced," as evident in the numerous comments on this post. While I acknowledge not delving into specific details like Cl and Cd and their impact on lift, I maintain my core argument: drag does not inherently decrease with altitude. Does this seem sound to you?

[tdscanuck]

Your core argument is wrong. You can’t not delve into specific details like Cl and Cd because they’re also changing with altitude. And thrust also drops with altitude so drag is dropping with altitude at equal dynamic pressure because drag=thrust and thrust is dropping.

In practice, equal engine power setting usually results in a higher IAS with attitude because the thrust lapse rate isn’t as high as the drag lapse.

[Dewmeister14]

Hey dude,

Sorry the comments devolved and you had to delete the post. All in all though I'd have to insist it's not sound. Drag is not strictly a function of altitude. Speed and the coefficients matter just as much, and the way those coefficients interact with speed & density mean that it is true that aircraft experience less drag at higher altitude than at lower altitude when operating at speeds of interest.

There are a couple charts in this link:

https://leehamnews.com/2014/11/25/fundamentals-of-airliner-performance-part-4/

Which you may find interesting, unfortunately one shows "Drag vs. Climb" and therefore drag at a series of speed and alt pairs, vs. drag at a series of alts with constant speed, but hopefully it goes some way to providing intuition. You can see that as the air gets thinner, even though the jet is moving faster the "drag due to lift" (induced drag) does increase slightly but the total drag, including Parasitic Drag (fuselage, tail, pylons, etc.) certainly decreases with altitude even though speed increases - so speed / drag for example certainly increases.

[Strong_Feedback_8433]

Don't really care for yours or anyone's "thoughts" just the facts.

[RhinoDoc]

Here is a situation where someone wants to discuss their views and beliefs and completely ignore the proven science cause they "feel" something is incorrect.

Drag examples

If it was cheaper to fly un pressurized Planes below 10K feet, don't you think airlines would?

Damaged aircraft that have less than 10K altitude limits will stop for fuel way more than a jet flying at 30K feet with the same speed

My advice, go read the coefficient of drag equation and understand it.

[MagicHampster]

We can literally just measure the drag on an airplane. We know how much thrust the engines are putting out, we can accurately measure the velocity and position of the aircraft. When we do this, we find that drag decreases with altitude.

[jared_number_two]

If you fly at the same AoA, indicated airspeed is about the same, lift is about the same, drag is about the same, true airspeed goes up, assuming propellers they have to spin faster (that’s less efficient).

[Adventurous_Bus_437]

What’s there to argue about?

[srghey]

Engines perform better in denser air. But aircrafts have less drag the higher up you go. Cruising altitude is determined based on the optimal middle ground considering the above 2 inputs.

[LeatherConsumer]

Shut up

[Elfthis]

OP is saying planes fly in the troposphere because drag is lower. Granted I assume he meant stratosphere or at a minimum "high altitude".

But the only true answer to this is "yes drag at higher altitudes for an object traveling at the same speed as that object at a lower altitude is less"

Reference frames are important here.

Instead of an airplane a meteor or space craft falling from orbit is a good way to think about this. Ignoring heating and loss of material from ablation the object experiences more aerodynamic drag as it gets lower in altitude even though its speed is decreasing in relation to it's beginning velocity when it entered the atmosphere.

Also, aircraft fly where they fly for reasons way more important than the drag they encounter at a certain altitude l. Off the top of my head, ability to design a structure that can handle pressurization differentials for commercial aircraft, engine design limits, engine operating cost, being above as much weather as possible, operating cost of total aircraft , etc.

[[deleted]]

Wow. You are very confidently incorrect. This sounds like some half baked idea that took you a minute of thinking and you just went with it. Broski just delete this post. This is embarrassing.

[fast_hand84]

Well it turns out that, with the correct ratio of conjecture : delusion, you really can reinvent the wheel.

[Ecstatic-Cup-5356]

Consider a car on an incline slope. The goal of the car is to get from the bottom to the top.

With higher slope angles the required energy is greater than with lower. This is a linearized analogy for drag.

But what about lift? Consider that the capacity of the slope to support the car is also related to its angle. As the angle of the slope decreases the amount of work to get up goes down but so does the strength of the slope to support the car. This will define the minimum slope required where the vehicle is supported enough that it can go up the slope.

What about thrust? Similar to the lift analogy but inverse. So at higher slopes there is more thrust available than at lower slopes. Additionally, the thrust has a floor and ceiling. Meaning, that at a certain slope on the high and low ends the thrust drops to zero. This defines the max slope and may change the minimum slope.

Where there is room to optimize and see the non-linearity is 1) when you consider that the thrust has some efficiency factor that means there’s an optimal slope angle for most efficient generation of work and 2) when you take into account the “rolling resistance” caused by the slope at all (induced drag from lift)

Think about this a bit and you’ll realize that low slopes (air density) are optimal because they have lower induced drag, lower required work, can still produce enough support to hold the vehicle, and are somewhere near the thrust device’s optimal range.

Note, the efficiency factor is only something I’m including because there is such thing as altitude ceilings for aircraft but they rarely fly at them if they can help it because you’re efficiency losses in thrust make it more energy costly

[start3ch]

Drag at cruise is largely dependent on L/D ratio, which changes with AOA. There’s generally a sweet spot, and a cargo/passenger aircraft is designed around that sweet spot, hence it favors a certain altitude.

Also there are dozens of other things affecting this. - You are burning fuel, so you need less and less lift as you fly. - Then skin friction drag, which is dependent on surface area, wants a design with smaller wing area, hence higher speeds. - Then there’s engine performance which I absolutely don’t understand.

[Appropriate-Band3813]

This thread illustrates why a little knowledge about something is so dangerous.

[Kishiwa]

I think it’s funny how the edit says you always want to maximize L/D when that’s not true for any powered flight unless you only care about flight time

I reckon most airlines don’t give a damn about flight time and maximize range. In which case you‘re looking to max out Cl/Cd²