![]() |
If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#51
|
|||
|
|||
![]()
"Greg Esres" wrote in message
... relatively small fraction of the total weight of the airplane in the first place, less than 10% in at least some cases, perhaps most cases) Lift in a 10 degree climb should be reduced about 1.5%. Yes. So? Not relevant to the statement you quoted (which was about thrust). I'm not sure your theory holds up very well. "His" theory is mentioned in a number of aerodynamics books. Fantastic. It would have been nice of you to provide the name of one popular (i.e. easy to find) one, so that I can read up on it. [...] If you had enough unused AOA left to generate a load factor, you could change the flight path then return the AOA to its original value. The aircraft may be able to stay on a steeper flight path due to the reduced parasite drag and reduced effective weight. Don't forget that thrust will increase slightly with a lower airspeed. I admit, I didn't consider scenarios where one is taking advantage of transient changes in drag and lift. Still, there's not much "unused AOA" in the regime of flight we're talking about, nor did David suggest that might be required (his implication, to my reading, was that his suggestion applied generally, not with very specific pilot techniques and situational characteristics). To have a vertical component high enough to support the airplane will require a horizontal component so high that the airplane won't slow. Not really clear on what you mean by that. Yeah, I was posting pretty late. That wasn't clear at all. My point is simply that I don't see how you can increase thrust enough to support the airplane significantly, while still managing to slow the airplane down to theoretically lower-drag steady state. Perhaps the zoom maneuver you described is the answer to that. All of the above is very vague. What I hear you say is "I don't want to believe you." ;-) Yes, I admit that readily. But the reason I don't want to believe is that the proposal bears no resemblance to the behavior of any airplane I've flown, not while I've been flying it anyway. I agreed up front that my response is as much hand waving as anything else. But then so is David's. I'd be more than happy to see someone step in with some real math that shows the answer one way or the other. I don't happen to be patient enough with the math. There's a reason that, when I was working on my math degree, I focused on theory and stayed away from numbers. ![]() There are an infinite number of steady states; every time I move the elevator, I create a new steady state. It seems to me that in this context, my qualification of "new steady state" (and David's for that matter) should have been clear. That is, he's proposing that at the same speed, there are multiple steady states that produce different amounts of drag. Lift is always generated perpendicular to the wing's chord. No, for subsonic flight, it's perpendicular to the *local* relative wind, the relative wind that is modified by wingtip vortices. Mea culpa. Still, in a climb (or descent), lift is not being applied entirely to counteracting weight. If lift were perpendicular to the chordline, you would have induced drag in a wind tunnel, and you don't. I understand my error regarding chord versus relative wind. Still, I'm boggled by the lack of induced drag in a wind tunnel. If the wing's not creating lift (0 AOA), I can see how there wouldn't be induced drag. But this would happen in the real world too. If the wing is creating lift, shouldn't there be a measurable force parallel to the relative wind? Even in a wind tunnel? You can measure lift in a wind tunnel. Why not induced drag? Pete |
#52
|
|||
|
|||
![]()
Yes. So? Not relevant to the statement you quoted (which was about
thrust). The issue under discussion was how much less lift was needed when thrust supported some of the weight of the aircraft. The reduction in necessary lift could accommodate a lower airspeed at the same AOA, or a lower AOA at the same airspeed. But, as you said, the reduction in lift is not a whole lot. It would have been nice of you to provide the name of one popular (i.e. easy to find) one, so that I can read up on it. It's not always easy to find a reference to something I've read; I often lose hours doing so. Anyway, here's one: "Introduction to Flight", by John D. Anderson. p. 290. Quote: "As seen in this example, for steady climbing flight, L (hence Cl) is smaller, and thus induced drag is smaller. Consequently, total drag for climbing flight is smaller than for level flight at the same velocity." Still, there's not much "unused AOA" in the regime of flight we're talking about, nor did David suggest that might be required (his implication, to my reading, was that his suggestion applied ?? True But the reason I don't want to believe is that the proposal bears no resemblance to the behavior of any airplane I've flown, not while I've been flying it anyway. If the effect exists, I agree that it would probably be small and might well be lost in the wash. I'd be more than happy to see someone step in with some real math that shows the answer one way or the other. Anderson shows some numbers. I hate trying to depict the math in this forum, because it looks so ugly. I don't happen to be patient enough with the math. There's a reason that, when I was working on my math degree, I only delve into math when the concepts are not clear. Putting some numbers to the theory makes things real sometimes. That is, he's proposing that at the same speed, there are multiple steady states that produce different amounts of drag. There are precedents. A banked aircraft at a given airspeed will have a larger AOA than a non-banked one, and thus incur larger amounts of induced drag. I envision that a climbing airplane is essentially a lighter one, since thrust will support a small amount of weight. Still, I'm boggled by the lack of induced drag in a wind tunnel. If the wing's not creating lift (0 AOA), I can see how there wouldn't be induced drag. But this would happen in the real world too. If the wing is creating lift, shouldn't there be a measurable force parallel to the relative wind? Even in a wind tunnel? I should qualify that. The great body of wing sections that NACA tested in the early part of the last century were placed flush against the walls; no wingtips. It is the wingtip vortices which cause the local relative wind to be different from the "real" relative wind. Absent that, the total aerodynamic force is perpendicular to the incoming freestream. They did this intentially, since the actual induced drag on a real wing will depend on its aspect ratio. Better to make their data "pure" and allow builders to adjust it to fit their specific planforms. There is still drag, of course, but just not induced drag. |
#53
|
|||
|
|||
![]()
Greg Esres wrote:
relatively small fraction of the total weight of the airplane in the first place, less than 10% in at least some cases, perhaps most cases) Lift in a 10 degree climb should be reduced about 1.5%. How did you arrive at 1.5%? Lift is always generated perpendicular to the wing's chord. No, for subsonic flight, it's perpendicular to the *local* relative wind, the relative wind that is modified by wingtip vortices. If lift were perpendicular to the chordline, you would have induced drag in a wind tunnel, and you don't. Come on Greg, you're telling me that wings in wind tunnels have no induced drag? That's ridiculous. How about if the wind tunnel was 1000 miles long by 10 miles high - would the wings in that wind tunnel have induced drag? (I seem to remember this same argument a few months ago) Hilton |
#54
|
|||
|
|||
![]()
Hilton wrote:
Greg Esres wrote: No, for subsonic flight, it's perpendicular to the *local* relative wind, the relative wind that is modified by wingtip vortices. If lift were perpendicular to the chordline, you would have induced drag in a wind tunnel, and you don't. Come on Greg, you're telling me that wings in wind tunnels have no induced drag? That's ridiculous. How about if the wind tunnel was 1000 miles long by 10 miles high - would the wings in that wind tunnel have induced drag? (I seem to remember this same argument a few months ago) Just to follow-up my own post, here is a line from the NASA web site: "During the winter, with the aid of their wind tunnel, they began to understand the role of high induced drag on their aircraft's poor performance." http://www.grc.nasa.gov/WWW/Wright/airplane/drag1.html Sorry Greg, "no induced drag in a wind tunnel" is simply not true. Hilton |
#55
|
|||
|
|||
![]()
How did you arrive at 1.5%?
L = Wcos(theta) Come on Greg, you're telling me that wings in wind tunnels have no induced drag? That's ridiculous. How about if the wind tunnel was 1000 miles long by 10 miles high - would the wings in that wind tunnel have induced drag? (I seem to remember this same argument a few months ago) Argument? No, Todd Pattist and I attempted to educate you on this subject, but apparently failed. The size of the wind tunnel is irrelevant. What matters is that the wing tips are flush against the walls of the wind tunnel. This produces 2-D airflow, rather than 3-D. In 2-D flow, there are no wing tip vortices and thus will have no induced drag. (If you would read something other than Aerodynamics for Naval Aviators, you'd understand this.) |
#56
|
|||
|
|||
![]()
Sorry Greg, "no induced drag in a wind tunnel" is simply not true.
Once again, you don't understand. If you put a 3-D wing in a wind tunnel, you will get induced drag. The article you posted even documents the effect: ----------snip-------------- This drag occurs because the flow near the wing tips is distorted spanwise as a result of the pressure difference from the top to the bottom of the wing. Swirling vortices are formed at the wing tips, and there is an energy associated with these vortices. ----------snip-------------- See that? Flow "near the wing tips is distorted"....vortices are formed at the wing tips 2-D flow is achieved when the wing tips are flush against the sides of the wind tunnel. No wing tips = no pressure leaking around the wing tips = no wingtip vortices = no induced drag. |
#57
|
|||
|
|||
![]()
"Greg Esres" wrote in message
... "As seen in this example, for steady climbing flight, L (hence Cl) is smaller, and thus induced drag is smaller. Consequently, total drag for climbing flight is smaller than for level flight at the same velocity." I'm not questioning whether thrust contributes to lift, and thus reduces the total lift requirement. It is patently obvious to me that a force directed at least partially downward contributes to lift. That quote says nothing more than that. What I am questioning is whether for a given performance scenario there are multiple drag scenarios. That is, he's proposing that at the same speed, there are multiple steady states that produce different amounts of drag. There are precedents. A banked aircraft at a given airspeed will have a larger AOA than a non-banked one, and thus incur larger amounts of induced drag. It's clear that I continue to fail to state my objection properly. Let me try again... David's post implies that for a given performance scenario (straight and level flight, for example) you can nudge the airplane into a "new steady state" where drag is lower. Your examples of climbing and turning don't address that issue; they are entirely different performance scenarios (that is, the airplane is doing something different) than the scenario to which drag is being compared. According to David's original post (if I read it correctly), there are multiple drag scenarios for a given path of flight. Each time you come up with an example, it starts out by assuming a new path of flight compared to the "base case". I envision that a climbing airplane is essentially a lighter one, since thrust will support a small amount of weight. Seems reasonable to me. But what if you don't want to climb? And in particular, if we're talking about comparing one airplane in straight and level flight to another in straight and level flight, introducing a climb to the discussion doesn't help much. Pete |
#58
|
|||
|
|||
![]()
Greg Esres wrote:
How did you arrive at 1.5%? L = Wcos(theta) *If* you assume level and climb airspeeds are the same. [zap: induced drag disagreement] (If you would read something other than Aerodynamics for Naval Aviators, you'd understand this.) Yeah, I also hate it when people use decades of research, hours of wind tunnel testing, and accepted aerodynamic principals in these newsgroups. ![]() Hilton |
#59
|
|||
|
|||
![]()
According to David's original post (if I read it correctly), there
are multiple drag scenarios for a given path of flight. I didn't pick up on that, but if so, I agree with you. That scenario seems unlikely. |
#60
|
|||
|
|||
![]()
On Fri, 31 Dec 2004 at 02:06:08 in message
, Peter Duniho wrote: Peter, I am sorry that this reply is a bit delayed as we have had visitors over the last three days. I have gone back to basic equations and looked again at them! I now have to confess that you are, at least in part correct. I offer my apologies. "David CL Francis" wrote in message ... [...] So under some conditions if you just raise the nose a little you can find a new steady state where speed is slightly reduced but with the same thrust you can climb at the same AoA.. Your theory sounds wonderful, but I doubt it holds water in practice. My statement above was wrong as written. It is true in the sense that you can find a new steady state at the same AoA and a slightly reduced speed but only by applying more thrust. You apply more thrust and if the AoA stays the same the velocity will initially increase and the flight path will curve upward arriving at a new steady state where there is a climb, but at a slightly reduced speed. Theta is the angle of climb in degrees. If the L/D is fixed at 10 then here is a little table. I hope it comes out not too screwed up by different fonts and line lengths: weight Lift/ Theta Drag Thrust Lift Lift Velocity lb. drag deg lb. lb. lb. % % 10000 10 0 1000 1000 10000 100 100.00 10000 10 1 1000 1174 9998 100 99.99 10000 10 2 999 1348 9994 100 99.97 10000 10 5 996 1868 9962 100 99.81 10000 10 15 966 3554 9659 97 98.28 10000 10 30 866 5866 8660 87 93.06 10000 10 45 707 7778 7071 71 84.09 10000 10 60 500 9160 5000 50 70.71 10000 10 90 0 10000 0 0 0.00 In practice aircraft cannot be controlled at zero velocity and more thrust would be needed at the higher angles of climb to maintain a controllable airspeed. Note that lift is reduced to zero in a vertical climb as it must be. I freely admit that these equations give only a simplified demonstration. There are many refinements to be added but the basics are generally accepted I believe. For example at the same power setting thrust is not independent of velocity, the thrust is not always exactly opposite to drag and control deflections also have an effect on drag and total lift. Weight is the gravitational force on the aircraft and points to the centre of the earth. Thrust is defined, in this simplification as a force along the flight path and drag is a force in the opposite direction. Lift (you seem to have got this wrong further down in your post) is defined, in the usual way, as acting at right angles to the flight path. Drag acts along the flight path in the opposite general direction to thrust. It has two components. One is the standard parasitic drag and the other is induced drag which can be looked at as the drag component due to the tilting back of the force vector to allow for the deflection of the air stream by the generation of the lift. By making a few assumptions about the size of the airflow that is actually deflected this can be shown to be proportional to the square of the lift coefficient. Thrust does not reduce the required lift by much, especially not in the light planes we tend to fly. Steady-state pitch angles in climbs tend to be modest, meaning a tiny fraction of the thrust vector is the downward component. Just 17% of the total thrust, for a 10 degree pitch angle. Given how little thrust a light plane has in the first place (only a relatively small fraction of the total weight of the airplane in the first place, less than 10% in at least some cases, perhaps most cases), even taking 20% (or even 34%) of that and applying it to lift just isn't going to help that much. The thrust must be equal to the drag or the aircraft will not fly at all! If the thrust is smaller than 10% of the weight then that implies that the lift/drag ratio is better than 10 for it to even fly at best lift/drag ratio. A motor glider requires little thrust to fly but may not climb very well. Note that the pitch angle, depending on how it is defined, is not normally the same as the angle of climb Even assuming an airplane with a thrust-to-weight ratio of 1.0 (a rare occurrance, but they do exist...some F-16s, for example), I'm not sure your theory holds up very well. You might think that you could simply increase thrust as you slow the airplane in order to allow a smaller AOA to suffice to provide the remaining necessary lift. But there's a problem with that idea. See the table above. I think you are using what you call 'my theory' in a strange way in the above. If an aircraft is capable of climbing vertically and steadily then thrust must always be greater than weight so that a velocity can be maintained sufficient to maintain control Under those circumstances the situation requires that lift is zero. If the AOA is kept small, then the increased thrust will prevent the airplane from decellerating. To have a vertical component high enough to support the airplane will require a horizontal component so high that the airplane won't slow. If the AOA is allowed to increase, then what the increased thrust is actually allowing is for the wing to stall without the airplane being pulled downward by gravity (the wing WILL stall at the appropriate AOA, regardless of airspeed). It's not demonstrating anything about some "new steady state". I do not understand the above and in particular your remark that the 'aircraft will not slow'. If AoA is not changed then increased thrust will increase speed and increased speed will increase lift. That, unless checked, will lead to the flight path curving upward and a climb beginning until a new state is reached. The gravity component then also plays its part in speed reduction. The above thought experiment should also illustrate another problem with your theory: it ignores the change in the portion of lift actually contributing to counteracting gravity that occurs due to pitch changes. See below for more commentary on that. Again I am not sure what you mean there. If you mean it ignores the effect of a climbing flight path then it does not. I have no special theory - just an attempt to explain basics in the simplest reasonable way using the simplest possible static equations. AFAIK there are an infinite number of different possible steady states in the flight of an aircraft. Of course, to me the biggest problem intuitively with your theory is that I am sure that aerodynamics involves only continuous functions. Given that, if you assume more than one steady state, you are claiming that there are multiple local minima/maxima between which are apparently "lower efficiency" areas. And of course, if there's more than one, I see no reason to believe that there are only two. This would then imply that the flight envelope has numerous of these local minima/maxima points. I certainly agree that aerodynamics is a series of continuous functions with changes of those functions at certain values. I am not sure about the rest of your paragraph above. An aircraft can fly at any speed and angle within its capabilities. Change any of the basic 4 forces and a new situation that can be maintained may or may not be possible. Given that in more than 100 years of study, this concept has never shown up as a noted element of the relationship between speed, drag, and lift, I'm inclined to believe that it's just not true (just as the idea of "cruising on the step" is not true). What concept are you talking about? I am just using simple balance of forces equations - apart from when I get it wrong of course! :-( I admit that I have not brought out the equations and proved my point irrefutably. Someone like Julian Scarfe or Todd Pattist would probably do a better job discussing this, since they seem to be more "math oriented" (that is, they don't mind crunching some equations now and then ![]() feel reasonably confident that there's no secondary "new steady state" one can achieve by increasing AOA and taking advantage of thrust. I would welcome Todd's participation, he has corrected me a few times but we have often agreed. The extra work comes ONLY from a net surplus of power. I agree with that. However that net surplus can come from either more engine power or reduced drag. Because even in a modest climb the engine thrust vector plus the lift vector combine to match the weight. I assume by "the engine thrust vector" you really mean "the vertical component of the engine thrust vector". Assuming that, I agree with your statement, but I don't find it informative. The engine thrust vector has a non-zero vertical component even during level cruise flight, and yes it does contribute to counteracting gravity, reducing the lift required. Fair enough and I have made a part of that point above somewhere. It is a part of the simplification involved. Put another way if you are flying below the maximum lift drag ratio and you increase the AoA to the optimum whilst keeping the same power the aircraft should climb. This is self evident if you are flying level at high speed and at a climbing power setting. Bring the nose up increasing the AoA and your aircraft will definitely climb. Agreed? I never said anything to the contrary. If you are at an angle of attack lower than the best L/D AOA (and thus at an airspeed higher than the L/Dmax airspeed), increasing pitch angle without a change in power will result in a climb, yes. I am glad we agree on that! But that has nothing to do with what happens at an airpseed near stall, which occurs below the L/Dmax airspeed, and well above the best L/D AOA. Agreed. You cannot decrease drag by increasing AoA near the stall. This is part of where we were at cross purposes. I was not thinking specifically about near the stall. This seems like a good point at which to mention something else you've left out and which I alluded to earlier... Lift is always generated perpendicular to the wing's chord. Some people like to call just the vertical component of this force "lift", but the amount of force acting through the wing is the force perpendicular to the chord. That is wrong. Lift is always defined as perpendicular to the free stream velocity of the aircraft. In the same way lift is not defined relative to gravity either except that in steady level flight it just happens to be exactly opposite and balancing gravity. However you label things, you cannot avoid the fact that when you change the angle of the lift vector, the portion of the force created by the wing used to counteract gravity is also changed. In particular, in the low-airspeed, power-on example, even as thrust is helping support the airplane, you are using your lift less efficiently, which means that the wing needs to generate more total lift just to provide the necessary vertical component. Insofar as I understand it, I disagree still with that statement. If thrust is providing a vector in the lift direction then in steady flight less lift is required as my table does show. This is similar to the required increase in lift while in a turn, but due to redirecting the lift vector in a different way. No, it is different to that because the increased force required in a turn is needed to produce an acceleration which is felt as 'g'. That does not apply in steady level, climbing or gliding flight when just the normal 1g is felt. Since the lift vector points slightly aft in level flight, even at high airspeeds when the angle of attack is low, it's easier to see how this negates at least some of whatever contribution thrust might make as the angle of attack is increased. However, in gliding flight, the vector is pointed forward, helping counteract the contribution drag makes to lift. The lift vector points at right angles to the line of flight no matter what. Lift and drag coefficients are measured like that unless things have changed a lot since my long ago student experiments in a wind tunnel.. The only time you consider a backward shift of the 'lift' vector is in calculating induced drag using a simplified method. By including a separate term for induced drag proportional to Cl^2 that effect is allowed for. Lift and drag coefficients for an airfoil are determined for infinite aspect ratio. This is getting too long to deal with like this! It has got to the point where we would need to discuss this face to face to achieve much more and come to a common understanding of terms and simple equations. I wrote "at least some" up there, but it should be apparent from the disparate magnitudes of the lift and thrust vectors that you get a much more significant change in lift than you do in thrust. Not at small angles of climb as my correction and table indicates. Small angles of climb mean a small change of the lift vector component in the truly vertical direction vector. The bottom line he even though thrust does contribute at least a little to counteracting gravity, it does not do so in a way significant enough to change the fact that, as you slow the airplane from any airspeed at or below L/Dmax airspeed, you experience increased drag. That is true and I do agree with that and apologise for the confusion. Only if the L/D increases with AoA do you get a benefit of reduced drag. Although the L/D of an aircraft may be 10 or more at maximum. at full speed in level flight it will be much lower, perhaps as low as 2 or less. At the zero lift AoA the Lift/drag ratio is also zero but you cannot then maintain level flight!!!. I hope that helps to clear some of the misunderstandings between us. My only excuse is my great age! I repeat my apology for my error. I always enjoy your posts even if I don't quite agree! -- David CL Francis |
Thread Tools | |
Display Modes | |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
PIREP--CO Experts low level carbon monoxide detector | Jay Honeck | Piloting | 10 | December 3rd 04 11:21 AM |
What's minimum safe O2 level? | PaulH | Piloting | 29 | November 9th 04 07:35 PM |
Altimeter setting != Sea Level Pressure - Why? | JT Wright | Piloting | 5 | April 5th 04 01:04 AM |
The Internet public meeting on National Air Tour Standards begins Feb. 23 at 9 a.m. | Larry Dighera | Piloting | 0 | February 22nd 04 03:58 PM |
flight level in Flight simulator | Robert | Piloting | 3 | August 20th 03 07:37 PM |