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#1




Lift and Angle of Attack
"Dave Daniels" wrote in message
... [...] Firstly, can the lift acting on an aircraft go negative? Yes, though a nonsymmetrical wing (the most common type) will not have an exactly reversed lifttoangleofattack profile for negative angles of attack. That is, negative five degrees of AOA won't produce the same amount of lift downward as positive five degrees of AOA produces upward. Most wings are designed to optimize lift in the upward direction and so will get more lift for the same angle of attack in the positive direction, compared to the negative direction. [...] Secondly, does the angle at which an aircraft is banked affect the angle of attack? The only thing that affects angle of attack is the airplane's attitude relative to its path of flight. If I were writing a simulator, I would just use the velocity vector and the attitude (bank, roll, and pitch angle) to determine angle of attack. Whether roll angle affects the angle of attack depends on your definition of "affects" and how exactly you're calculating angle of attack. But generally speaking, if the flight path is not exactly aligned with the longitudinal axis of the airplane, the roll angle does affect the angle of attack, albeit in a small way. Aerodynamically there's another thing to consider, which is that the force called lift is directed perpendicular to relative wind. In anything other than straight and level flight, this means lift is not exactly opposite gravity. In order to calculate vertical acceleration, the simulator code will have to break the lift vector into multiple components, one of which will be the vertical component acting against gravity. Again, roll angle (among other things) needs to be accounted for to figure out the vertical component of lift. [...] Thirdly, induced drag: I have read that this is inversely proportional to the square of the aircraft's velocity, but I do not see how this follows from the equations I have seen. It appears to me that the greater the velocity, the greater the induced drag. I must be missing something fundamental here. Can anyone explain it? For a constant angle of attack, increased airspeed means increased induced drag. However, it also means increased lift. Induced drag is a byproduct of lift. In normal cruise flight, induced drag actually goes *down* as airspeed increases, because lift remains constant. The only way to keep lift constant is to reduce angle of attack as the airspeed increases, and reducing the angle of attack reduces drag faster than increasing airspeed increases it. Pete 
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#2




In article ,
ArtP wrote: Keep in mind when working with a program, the programmer frequently takes shortcuts to produce the desired result. While the AOA may not change the vertical component of lift will decrease as the bank increases. This particular simulator was written during the late 1980s and early 1990s. The target PC would have been something like a 20Mhz 386 with no floating point instructions then. I do not want to take anything away from the programmers for an interesting piece of code, but they were forced to take a lot of shortcuts. It sounds like you are confusing parasitic drag and induced drag. Parasitic drag (result of friction) increases as with speed. Induced drag (result of AOA) decreases with speed (the faster you go the smaller the AOA required to maintain altitude). That begins to make more sense. I think that my problem was that I was assuming that the induced drag decreased with velocity at any angle of attack or altitude. If the lift is constant, then higher speeds require a smaller lift coefficient and therefore the induced drag coefficient is smaller. Am I on the right track here? Dave Daniels 
#3




"Dave Daniels" wrote in message
... [...] If the lift is constant, then higher speeds require a smaller lift coefficient and therefore the induced drag coefficient is smaller. Am I on the right track here? Yes. It is, in fact, basically what all three respondents to your post (myself included) said. 
#4




Please go to http://airtrafficcontrol.noip.org:8080 or
http://privatepilot.noip.org:8080 . There had an answer for aerodynamics. (used for FAA written test) "Dave Daniels" wrote in message ... I have some questions about lift and angle of attack that I hope somebody can answer. First of all, I should say that I am not a pilot, nor am I an aeronautical engineer and my understanding of aerodynamics is superficial at best. Heck, even my maths is pretty ropey. (It reminds me a bit of someone being asked what skills they brought to a construction job and they replied "I can cut wood, but not in a straight line".) Anyway, I have been looking at the source code of a flight simulator and trying to work out what it does. I have a number of questions. Firstly, can the lift acting on an aircraft go negative? Instead of the force pushing the aircraft up, can it push the aircraft down if say, the angle of attack is negative? Secondly, does the angle at which an aircraft is banked affect the angle of attack? Trying to figure it out in my head, I would say 'no' but the program implies otherwise. What is the answer here? Thirdly, induced drag: I have read that this is inversely proportional to the square of the aircraft's velocity, but I do not see how this follows from the equations I have seen. It appears to me that the greater the velocity, the greater the induced drag. I must be missing something fundamental here. Can anyone explain it? Dave Daniels 
#5




Also try http://www.monmouth.com/%7Ejsd/fly/how/htm/how.html
Answers 1. yes 2. bank and AoA are independent 3. depends on whether you fix at level flight or constant angle of attack Joan == In message , "[email protected]" writes Please go to http://airtrafficcontrol.noip.org:8080 or http://privatepilot.noip.org:8080 . There had an answer for aerodynamics. (used for FAA written test) "Dave Daniels" wrote in message ... I have some questions about lift and angle of attack that I hope somebody can answer. First of all, I should say that I am not a pilot, nor am I an aeronautical engineer and my understanding of aerodynamics is superficial at best. Heck, even my maths is pretty ropey. (It reminds me a bit of someone being asked what skills they brought to a construction job and they replied "I can cut wood, but not in a straight line".) Anyway, I have been looking at the source code of a flight simulator and trying to work out what it does. I have a number of questions. Firstly, can the lift acting on an aircraft go negative? Instead of the force pushing the aircraft up, can it push the aircraft down if say, the angle of attack is negative? Secondly, does the angle at which an aircraft is banked affect the angle of attack? Trying to figure it out in my head, I would say 'no' but the program implies otherwise. What is the answer here? Thirdly, induced drag: I have read that this is inversely proportional to the square of the aircraft's velocity, but I do not see how this follows from the equations I have seen. It appears to me that the greater the velocity, the greater the induced drag. I must be missing something fundamental here. Can anyone explain it? Dave Daniels  Joan Walsh 
#6




In ,
Dave Daniels wrote: In article , ArtP wrote: snip It sounds like you are confusing parasitic drag and induced drag. Parasitic drag (result of friction) increases as with speed. Induced drag (result of AOA) decreases with speed (the faster you go the smaller the AOA required to maintain altitude). That begins to make more sense. I think that my problem was that I was assuming that the induced drag decreased with velocity at any angle of attack or altitude. If the lift is constant, then higher speeds require a smaller lift coefficient and therefore the induced drag coefficient is smaller. Am I on the right track here? Dave Daniels If you'd typed "If the lift is constant, then higher speeds require a smaller angle of attack and therefore induced drag {quantity of} is smaller", I'd have agreed with you immediately. I fail to see how coefficients (of anything) can become smaller, or larger. Sorry to be pedantic but, as far as I understand it, 'coefficients' are physical constants. In this case, they are characteristics of a particular airframe. The parasitic drag coefficient will go in proportion to a combination of frontal area and total surface area (note that the frontal area presented to the relative wind will change with variations in angle of attack, following a pitch control input). The quantity of parasitic drag increases with the cube of the airspeed, multiplied by this coefficient, mutiplied by the mass per unit time of air flowing over it. (in kg.m.s1 aka Watts, if I' m not mistaken) I was loose with the definition of airspeed there and since we're cubing it, we need to be more specific. Whilst TAS is fine for dead reckoning navigation calculations, I think IAS is a better measure of airflow over the wing because the stalling speed is the same, expressed in IAS, at all altitudes. (And when that stalling speed converges with the highest achievable forward speed in level flight, the plane reaches its 'service ceiling'). IAS falls short of TAS by an increasing margin, as altitude increases and I think the drag should be calculated on the basis of the lower, IAS value. As I see it, a given mass of air (density * volume), moving over a wing in unit time will produce a given quantity of lift, at a specified AoA, whether you prefer the Bernoulli principle, or the airdeflection theory (or both) for what generates it. Therefore, at altitude you *can* fly faster and further (in unit time) but you also *have to* in order to generate enough lift. (You have to fly through a larger volume of lessdense air for the mass to be the same, which can only be done by travelling further forward and, if done in unit time, this obviously means faster). As mentioned in previous replies, induced drag is the result of the production of lift which, by not being perpendicular to the wing (as viewed in crosssection) retards forward motion to some extent. At higher speeds, the required amount of lift can be achieved at a reduced angle of attack. This brings the induced drag vector closer to the vertical and the retardation component of it is thus minimized (it never totally goes away). A useful side effect of this is that, by presenting a reduced frontal area to the relative wind (nearly edgeon), the element of the planes parasitic drag, contributed by the wings, is reduced and higher speeds become more possible. The induced drag coefficient, however, remains a constant, for that particular aircraft. One slight complication (for the programmer) is that real wing profiles are not uniform across the span  the wingtips may present a slightly different AoA to a particular relative wind than the wing roots. It can be designed that way so that *part* of a wing will stall at a slightly higher speed than (and thus in advance of) the section where the control surfaces are. The turbulence of the stalled portion causes a vibration which the pilot will be able to feel, as a warning that a full stall is about to occur and give them time to take action. Conversely, when no stall is occuring, the fractionally higher AoA means the wing roots generate more lift than the wingtips, so the strongest part of the wing takes a larger share of the overall load and wing flexing is minimized. As far as PC simulators are concerned, modelling multimodal wings like that, or the fluid dynamics calculations of air over wings is still far too complex, so they have to treat a plane as a point mass, with moments of inertia (if that's not a contradiction in terms) and calculate the forces acting on it. Separate factors for induced drag, surface area drag etc go into the flight model but these may be empirically determined values which just happen to make the overall flight model performance feel 'right', across a range of conditions, rather than having any resemblance to corresponding factors for the real thing.  regards, Mark 
#7




"Mark Cherry" wrote in message
... Sorry to be pedantic but, as far as I understand it, 'coefficients' are physical constants. In this case, they are characteristics of a particular airframe. You're not being pedantic. You're just being wrong. A "coefficient" is simply a number in an equation. It may or may not be a physical constant, but in the case of lift, it is not. It varies with the angle of attack, as does the coefficient of drag. You can break the coefficient of drag down into its component parts, induced and parasitic (aka "form" or "friction") drag. But even the parasitic component changes slightly with angle of attack, simply because of the change in the crosssection of the airframe presented to the relative wind. As you noted, the parasitic component changes even more when flying in uncoordinated flight, such as a slip or skid. [...] Whilst TAS is fine for dead reckoning navigation calculations It's not just fine. It's necessary. [...] (And when that stalling speed converges with the highest achievable forward speed in level flight, the plane reaches its 'service ceiling'). Wrong again. An airplane's "service ceiling" is defined to be the altitude at which the climb rate is lower than 100 feet per minute. An airplane's *absolute* ceiling is the altitude at which the airspeeds corresponding to best rate of climb (Vy) and best angle of climb (Vx) converge, which is the altitude beyond which the airplane simply cannot climb. The airspeeds Vx and Vy, the same at absolute altitude, are well above stall speed. There IS another altitude that is of importance for airplanes that can climb especially high and which go particularly fast. That is, the point at which stall speed and the Mach buffet speed converge. At that altitude, going slower stalls the airplane and going faster results in Mach buffet. Perhaps that is what you were thinking of. [...] The induced drag coefficient, however, remains a constant, for that particular aircraft. As I mentioned above, no. The induced drag coefficient changes along with the lift coefficient, and depends not only on the airfoil, but also on the angle of attack. [...] The turbulence of the stalled portion causes a vibration which the pilot will be able to feel, as a warning that a full stall is about to occur and give them time to take action. One will experience turbulence *prior* to stall even in an airplane with a constant angleofincidence airfoil. The prestall buffet is caused by the airflow separation, which starts to happen before the actual stalling angle of attack, along with this turbulent airflow striking the horizontal stabilizer (the latter happens only in aircraft in which the horizontal stabilizer is in the path of the airflow coming off of the wing, of course). Pete 
#8




In ,
Peter Duniho wrote: snip Okay, thanks for those corrections. Looks like I'd started out on a false premise and, once started, the tendency is to followthrough to the finish. g Apologies for wasting anyone's time. I'm familiar with Cd figures quoted for cars  which present a more or less fixed cross sectional area to the airflow  and the editable drag parameters within the old flight dynamics editor s/w for Flight Shop created aircraft. Even if it's not so in reality, it looks as if the sim's flight model treats these factors as constants. shrug I really should read more but I enjoy simming far, far too much.  regards, Mark 
#9




In article , Mark Cherry
writes Sorry to be pedantic but, as far as I understand it, 'coefficients' are physical constants. In this case, they are characteristics of a particular airframe. True, they are, but the lift coefficient is a property at a given angle of attack so they are characteristics of an airframe but only for a given angle of attack. For small angles of attack then the lift coefficient depends almost directly on the angle of attack. The parasitic drag coefficient will go in proportion to a combination of frontal area and total surface area (note that the frontal area presented to the relative wind will change with variations in angle of attack, following a pitch control input). The quantity of parasitic drag increases with the cube of the airspeed, multiplied by this coefficient, mutiplied by the mass per unit time of air flowing over it. (in kg.m.s1 aka Watts, if I' m not mistaken) Generally speaking parasitic drag depends on the square of the speed. Power required depends on the cube of the speed. Drag is a force, measured in Newtons or pounds force. Drag is a force; watts are power. The classic drag equation is D = 0.5*(air density) * (reference area  usually wing area) *( (velocity)^2) * (drag coefficient) where velocity is TAS NOT IAS. You seem to be confusing Power required with the drag force. But I see Peter Duniho has already posted corrections!   David Francis EMail reply to  
#10




In ,
David wrote: In article , Mark Cherry writes Sorry to be pedantic but, as far as I understand it, 'coefficients' are physical constants. In this case, they are characteristics of a particular airframe. True, they are, but the lift coefficient is a property at a given angle of attack so they are characteristics of an airframe but only for a given angle of attack. For small angles of attack then the lift coefficient depends almost directly on the angle of attack. The parasitic drag coefficient will go in proportion to a combination of frontal area and total surface area (note that the frontal area presented to the relative wind will change with variations in angle of attack, following a pitch control input). The quantity of parasitic drag increases with the cube of the airspeed, multiplied by this coefficient, mutiplied by the mass per unit time of air flowing over it. (in kg.m.s1 aka Watts, if I' m not mistaken) Generally speaking parasitic drag depends on the square of the speed. Power required depends on the cube of the speed. Drag is a force, measured in Newtons or pounds force. Drag is a force; watts are power. The classic drag equation is D = 0.5*(air density) * (reference area  usually wing area) *( (velocity)^2) * (drag coefficient) where velocity is TAS NOT IAS. You seem to be confusing Power required with the drag force. But I see Peter Duniho has already posted corrections! Similar thanks go to you. Dunno how that 'cubed' nonsense got in there. Call it 'brain fade'... Now let's see how much of this I remember in 6 month's time. ;)  regards, Mark 
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