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.
- 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.
- 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.
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?