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Matt6461
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How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 7:57 am

Over the years here, I've seen and participated in many threads that get derailed and/or waste time because the participants disagree over very basic concepts like what causes drag, what determines fuel efficiency, how do wings bend, what does an empennage do and how etc.

I've been saying for years that we should have an "A.net aero/engineering basics" thread pinned on the front page of Civil and TechOps (as New Rules and "A.net acquired by..." were), with all posts subject to editorial review by a committee of credible experts.

Clearly the Mods/Devs haven't taken my suggestion, and probably with good reason (who'd be accountable for having the final word? Can we expect folks to write, basically, a lesson plan for free?).

So I'm opening this thread as hopefully the next best thing. I'd like for members to post fundamentals questions here, and I hope some of our resident experts will be willing to enlighten us.

I am no expert, just a curious amateur, so I don't intend my statements to be authoritative on even the basics.
Nonetheless, I'll have the courage to look stupid and get the ball rolling by proposing some answers to basic issues that I've seen in the past - with citations to credible sources.


A few common issues (there are more...):
  • 1. What is the relationship between L/D and weight?

    My answer: In cruise, basically none*. An airliner flies at an optimal cruise Lift Coefficient (Cl) regardless of weight.** This means an airliner climbs as its weight changes, maintaining constant Cl** and constant cruise L/D. This dynamic doesn't apply to takeoff and, to lesser extent, climb: you can choose a cruise FL but you can't dictate runway altitude.

    *L/D changes slightly at different cruise flight levels (FL) due to Reynold's number effects and other minor issues. But this is a rounding error when using one significant digit.
    **Due to 1,000ft FL intervals, there's some deviation from optimal Cl between step-climbs. But mean Cl tracks optimal Cl.
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  • 2. What determines parasitic drag (Dp)?

    My answer: Dp is largely a matter of wetted area, which causes skin friction, which dominates Dp.

    Reference:
    This means 75% of our Parasitic drag is made up of air friction drag against the aircraft’s wetted surface.
    There are other drag factors, but these are the main ones[]. The important ones are Air friction drag and Induced drag. These represent 85% of total drag of an aircraft.
    This is why aircraft designers try to minimize the total surface of the aircraft at the same time as they try to make the wingspan as wide as possible.


    https://leehamnews.com/2018/03/09/bjorn ... more-26557

    Common A.net myth: There is such a thing as "frontal area drag" and it is REALLY important.
    Truth: There is no such thing as frontal area drag for airliners. Frontal area impacts drag via the fineness ratio (Fuselage LOA / diameter). A higher fineness ratio means a higher Coefficient of Parasitic Drag (Cdp). The difference between the finest (e.g. 757-300 or DC-8-73) and the stubbiest fuselages (e.g. A310 or 787-8) rarely exceeds 5% Cdp ratio, and is around 2% for 10 versus 12 fineness (approximately A350-900 versus A350-1000).

    Frontal area also has some effect on area-ruling for transonic airliners (all contemporary jet airliners). That is usually handled with fairings to avoid excessive local velocities and nearly all modern airliners avoid wave drag except on the wing crests.

    None of these considerations mean frontal area drag is some independent drag component. Frontal area has some relatively small knock-on effects on the main drag components, but shouldn't considered in isolation.
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  • 3. Induced drag (Di): common myths, oft-ignored factors, and basics.

    Common myth: Induced drag is determined by aspect ratio

    At the airplane-level, Di is inversely proportional to the square of span. Ref: https://en.wikipedia.org/wiki/Lift-induced_drag (contains further references to authorities). Higher AR is good because it enables greater span (Di reduction) while adding less wing wetted area (i.e. less Dp) and while adding less wing weight (up to a point- extremely high AR wings will be heavier than a lower-AR wing of equal span).

    Relationship between FL and Di: Di increases with altitude. Many miss this effect. More precisely, Di increases linearly with the inverse of air pressure (=air density in the Stratosphere/Stratopause). Ref: same Wikipedia article (for now, I'm being lazy)

    Note that while Di is proportional Lift^2 (=weight^2 during cruise), the inverse linear relationship between Di and air pressure means that, due to step-climb as we lose weight, Di is functionally linear with weight during cruise. Dp is also functionally linear with weight, as Dp is linear with air pressure. Seeing these two relationships explains how L/D is functionally weight-independent during cruise: its two main components are linear with weight, which equals lift during cruise.

    Common myth: Winglets reduce Di only by reducing tip vortices

    Even industry publications make this mistake.

    Truth: Winglets work by changing the lift distribution of the entire wing, thereby altering the flow and energy of the trailing vortex - the strength of which largely determines Di magnitude. Ref: http://www.fzt.haw-hamburg.de/pers/Scho ... -09-10.pdf

    Also our friend Bjorn (aka Ferpe) at Leeham:

    Induced drag is not a tip phenomenon.
    It’s important to understand, induced drag is coming from the global spanwise change of direction of the air around a wing, Figure 4.

    Image

    The global spanwise circulation around the wingtips is the source of induced drag, not the very visible wingtip vortices. The air behind a wing creating lift is forced down into a giant vortex sheet (Figure 5) which continuous down behind the aircraft.

Ok so that's a start. Hopefully others will ask fundamentals questions and hopefully our aerodynamic luminaries will provide us with definitive answers.

Please reply only if you can claim credible expertise in aero/engineering or have cites to credible and on-point sources, if you have points of clarification, or if you have fundamentals questions that, hopefully, one of our experts will answer. If the latter, please check any apprehensions of embarrassment and ask your question no matter how basic. Then we can all learn something from replies.

I'll start with a basic question: For airliners with 50/50 ratios of Di/Dp, why would you ever climb above the isothermic stratopause (usually ~FL370)? You'll see no SFC benefit from lower temperature, likely a decrease to L/D. You'll spend energy on climb, not all of which is recovered in descent. Is it a wave drag issue? But lower Cl should mean lower wave drag. What am I missing?
 
flipdewaf
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 11:41 am

Matt6461 wrote:
I'll start with a basic question: For airliners with 50/50 ratios of Di/Dp, why would you ever climb above the isothermic stratopause (usually ~FL370)? You'll see no SFC benefit from lower temperature, likely a decrease to L/D. You'll spend energy on climb, not all of which is recovered in descent. Is it a wave drag issue? But lower Cl should mean lower wave drag. What am I missing?
If you are not at the coffin corner and can effectively ignore the effects of that and the transonic drag rise is not becoming an issue yet then look to the breguet range equation and see that there is a significant part of that that is the velocity. As you increase your altitude the density decreases your equivalent airspeed reduces or more to the point the equivalent airspeed stays the same (because the pilot says so) and your actual airspeed (the one that counts for range) goes up.

So in essence, the SFC might not improve but if the increase in true airspeed is greater than the increase in transonic drag then up you go!

Yes under ISA conditions the top of the troposphere is about 11km as you say (somewhere between 36 and 37kft) but in reality that standard can go all over the place it can easily be under FL300 at the poles and over FL500 at the equator. Th standard is just a convenient guide but isn't all that standard.

Fred
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FlyHossD
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 3:02 pm

Matt6461 - I like your idea as I'm a curious amateur, too. 6 of the last 7 books I've read have been engineering related texts.

You wrote, "Can we expect folks to write, basically, a lesson plan for free?" No, I don't think we can expect that. However, we could post relevant links here.

Matt - what is your background?
My statements do not represent my former employer or my current employer and are my opinions only.
 
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Matt6461
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 7:33 pm

FlyHossD wrote:
Matt - what is your background?


Legal. Absolutely no industry experience. Just a geek.

Well done on your research. I've signed up for a basic engineering principles class at local extension campus. Autodidact can only take me so far; I want face time with an expert who can fit the pieces together. Haven't been to school in a decade or so; will be nice to be back.
 
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Matt6461
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 7:45 pm

FlipDewaf wrote:
in reality that standard can go all over the place


Yeah that makes total sense. Thanks.

flipdewaf wrote:
As you increase your altitude the density decreases your equivalent airspeed reduces or more to the point the equivalent airspeed stays the same (because the pilot says so) and your actual airspeed (the one that counts for range) goes up.


I can't grok this answer. By my lights, cruise TAS is limited to critical mach regardless of IAS - TAS subject to mach variation with T. In fact, I never see any use for TAS aside from pilots. If there are Breguet models that use IAS, it seems simpler to break out its pressure components and use TAS.
Last edited by Matt6461 on Tue Mar 13, 2018 8:00 pm, edited 1 time in total.
 
LH707330
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 7:50 pm

You're forgetting one very important one: forget what they told you about thrust, weight, lift, and drag, money is what keeps planes in the air!

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flipdewaf
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Re: How airplanes fly: fundamentals and common mistakes

Tue Mar 13, 2018 9:23 pm

Matt6461 wrote:
FlipDewaf wrote:
in reality that standard can go all over the place


Yeah that makes total sense. Thanks.

flipdewaf wrote:
As you increase your altitude the density decreases your equivalent airspeed reduces or more to the point the equivalent airspeed stays the same (because the pilot says so) and your actual airspeed (the one that counts for range) goes up.


I can't grok this answer.
firstly, thanks for the new word, I had to look it up. It I shall endeavour to use it in conversation .
Matt6461 wrote:
By my lights, cruise TAS is limited to critical mach regardless of IAS - TAS subject to mach variation with T.
there isn't a hard do not cross Mach number, my understanding of the critical Mach number is just the point where the effects of compressibility start to occur and there are some areas of >M1 airflow over the aircraft, normally the wing. As you go higher compressibility drag will go up with increased speed and if speed goes up quicker than drag then all things being equal you will have better fuel burn. Conversely if you are lighter and can go at a similar TAS and and similar compressibility drag level and keep an optimal Cl by reducing IAS.
Matt6461 wrote:
In fact, I never see any use for TAS aside from pilots.
True airspeed is harder to measure but is truth so is incredibly useful for calculating range and as fuel burn at a fundamental level is specific range it's also critical for that.
Matt6461 wrote:
If there are Breguet models that use IAS, it seems simpler to break out its pressure components and use TAS.
TAS is critical for Breguet, I can't see a benefit to using a version with IAS.

Fred







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Starlionblue
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Re: How airplanes fly: fundamentals and common mistakes

Wed Mar 14, 2018 12:35 am

LH707330 wrote:
You're forgetting one very important one: forget what they told you about thrust, weight, lift, and drag, money is what keeps planes in the air!

Image


Alternative explanation.

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"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WIederling
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Re: How airplanes fly: fundamentals and common mistakes

Wed Mar 14, 2018 3:50 pm

flipdewaf wrote:
grok
firstly, thanks for the new word, I had to look it up.


Read the book ( "Stranger in a Strange Land", R.A.Heinlein ) it is a rich read.
And I think it is a very "current" read in today's world of alternate truth, strange politics and deranged religiots.
Murphy is an optimist
 
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Matt6461
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Re: How airplanes fly: fundamentals and common mistakes

Fri Mar 16, 2018 6:51 pm

flipdewaf wrote:
there isn't a hard do not cross Mach number, my understanding of the critical Mach number is just the point where the effects of compressibility start to occur and there are some areas of >M1 airflow over the aircraft, normally the wing. As you go higher compressibility drag will go up with increased speed and if speed goes up quicker than drag then all things being equal you will have better fuel burn. Conversely if you are lighter and can go at a similar TAS and and similar compressibility drag level and keep an optimal Cl by reducing IAS.


Yeah no hard Mdd (drag divergence mach); it's just a convention for the point where you see substantially more wave drag delta than speed delta. Boeing uses some ratio of (Drag delta) / (speed delta) as its Mdd definition but I can't recall the precise figure. The cost index for choosing cruise mach also reflects this tradeoff (cuz this is a fundamentals thread, I'm just stating what might be obvious for many).
 
flipdewaf
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Re: How airplanes fly: fundamentals and common mistakes

Tue Apr 10, 2018 1:13 pm

Best place to pose this question as I cannot seem to make my model work and I'm getting stumped with my half hour per day that I spend on this over my lunch break (if I have one).

My model now predicts weight based off the stamford weight breakdown method but I have found to get reasonable numbers for real airline (what seem like anyway) OWE then you have to assume a full max exit capacity cabin but the numbers seem reasonable. I have got a full (nearly) aero package that uses data for the dimensions of the aircraft to determine a drag polar at any given speed at any given height based on the standard atmosphere. There are some issues with the model, in particular the drag seems too low at takeoff but I have not yet added any additional drag due to high lift devices to the model but the piece that is really bugging me is that the whilst I am able to get a good feel for where the Drag divergence will occur which seems to work quite well (Mdd + Cl/10 +t/c = 0.95) I cannot find a good way to implement this in to the model where and how the drag rise should occur from this. I would expect to run a few ms^-1 faster than Mcrit which is where Mdd is but I am not able to put a figure on what that additional drag is or is this something that only a computer simulation is going to help with.

Fred
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