# Is the maximum lift-drag ratio found at minimum drag?

user2168
• Is the maximum lift-drag ratio found at minimum drag? user2168

Figure 10-5 of the FAA's Pilot's Handbook of Aeronautical Knowledge shows:

I didn't think $L/D_{MAX}$ coincided with $D_{MIN}$. Is this Figure accurate?

• Well, for all L/D curves and D curves, the assumption is that the weight of the aircraft is constant and that there is no acceleration. Therefore the lift equals the weight (neglecting the small vertical component of thrust). So lift is a constant in these curves.

The rest is simple mathematics; the maximum of $\frac{1}{f(x)}$ occurs at the minimum of $f(x)$ (when $f(x) > 0$), so the maximum of $\frac{L}{D}$ coincides with the minimum of $D$.

• Well, your lift equals weight, or the airplane drops out of the sky or climbs into orbit. Therefore, lift is constant. Then the point with minimum drag must be the one where L/D reaches it's maximum.

• It doesn't have to, and almost certainly doesn't coincide. There is not data here on how lift varies with speed. A NASA chart I found shows it rising quadratically with airspeed. Similarly, the drag is probably a quadratic minimum, so $D \propto a+b(v-v_m)^2$, where $v_m$ is the speed of minimum drag. Then $\frac LD =\frac {C_Lv^2}{a+b(v-v_m)^2}$. The fact that the numerator is rising will move the max $\frac LD$ point above $v_m$, but not by much.

Tags
Related questions and answers
• See Wikipedia:Drag polar and Wikipedia:Polar curve (aviation) for example. These curves are not on a polar coordinate system. Why are they called polars?

• Figure 10-5 of the FAA's Pilot's Handbook of Aeronautical Knowledge shows: I didn't think $L/D_{MAX}$ coincided with $D_{MIN}$. Is this Figure accurate?

• Antennas sticking out of an aircraft obviously increase profile drag, but the folks who design antennas have done a lot to improve aerodynamics: High performance aircraft can have antennas mounted... (e.g. blade-style transponder antennas are available which produce about 80% less drag than older spike-and-ball style antennas), measured at 250 MPH. Since the average light GA aircraft has a VNE below 250 MPH and isn't generally covered in antennas, how significant is the antenna drag on a typical light GA airframe? For example, would stripping the ~6 pounds of antenna drag from your average

• I know, flaps are mostly used during take-off or landing to generate both lift and drag simultaneously. I am wondering if the pilot uses flaps to generate more lift so that they can climb in less distance (due to drag of flap) and possible reasons for doing it. I have an idea that the flight control system may prohibit flap deployment above a certain altitude or at higher speeds due to flap structural limitations.

• reduce - or even completely remove - the induced drag around the tips of the fan blades? Would it be possible to have a similar setup for a standard prop, it doesn't even have to be stationary, it could be a ring connecting the tips of the prop, spinning with it. Like an infinite winglet. :) It has the added safety benefit that it'll be visible when the prop is spinning. And I figure if the ring is strong enough to maintain its circumference, the load on the prop should be marginal since it's spinning around its own center of mass. Is the induced drag on the prop not large enough to warrant any

• On SIGWX charts, it shows pairs of symbols with, say, */** or **/**. I know what the symbols mean on either side, but why are there two, and what does the slash indicate? Would love good resources that explain more, too. Example chart here, from the FAA sample questions (caution: 37 MB download), Figure 20, over Southern California. I’m also interested in knowing what a dot with R underneath means.

• If you fly low, air is dense so you can get more thrust from your engines, but you get more drag. On the other hand if you fly higher you have less drag but the output of engine decreases as well. So what's the optimum altitude to fly at, and how does one determine it?

• The G1000 has a feature where it can put up a winds aloft arrow/speed in the PFD. I've been sort of wondering how exactly this is determined. My guess is that the aircraft (or the G1000) knows your airspeed via the pitot/static system and heading via the fluxgate/mag compass. Then, it uses your GPS position to figure out where you should be every X seconds given the airspeed and heading. Any difference must be attributable to winds aloft. Does this make sense?

• I've read a lot of NTSB crash reports regarding small, GA aircraft (just trying to figure out what went wrong and what to avoid.) There seem to be a lot of reports that talk about "low altitude high speed stalls" happening on approach. What is a high speed stall, and how is it created? What is the best way to avoid one? As they seem to cause a lot of GA accidents...

• Most wings suffer from induced drag due to a pressure difference above and below the wing causing air to sneak around the tip, forming a vortex. There are various methods to minimize these effects, such as winglets. However, looking at the Synergy aircraft as an example, box wings have no wing tips. Disregarding any other parts of the aircraft, are the wings actually free from induced drag? Or are they still causing induced drag, just in a way I'm not able to think of with my limited fluid dynamics experience? I've read somewhere that a traditional bi-plane design is less efficient due

Data information