Induced Drag
07-28-2004 | 10:35 PM
#26
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From: Stuttgart, GERMANY
RE: Induced Drag
I think that defining "induced drag" simply as the "drag due to lift" misses some important subtleties. You can have induced drag with zero lift. Conversely, you can have drag due to lift caused by mechanisms that have nothing to do with induced drag.
First case:
Suppose you had a finite-span wing generating lift at say 5 degrees angle of attack (AOA). I think most would agree that such a wing would experience an "induced drag" force. Now suppose you were to take an identical wing, turn it upside down and attach it to the original wing by a set of long struts such that it was at -5 degrees AOA (kind of a mirror image biplane). If the wings were set far enough apart, both wings would generate roughly the same lift and drag as the original wing. The net lift would be zero (because the wings are lifting opposite each other), but the net drag would not. This is an example of induced drag with zero lift.
This is a silly example, but consider a wing with twist. There is some angle of attack where a twisted wing will generate zero lift. At the zero lift condition, some parts of the wing are pushing up and others down. There is induced drag associated with the local pushing even though the net lift is zero. The induced drag of a twisted wing can actually get smaller as it starts to generate lift. This makes it hard to identify induced drag as the drag due to lift.
Second case:
If you look at any wing section in "Theory of Wing Sections", the section drag coefficient (Cd) depends on the section lift coefficient (Cl). In other words every wing section experiences drag due to lift. However, I think most would agree that a 2D wing section experiences zero induced drag. This 2D "drag due to lift" is what I would call "profile drag", or drag due to viscous losses. Although some may bookkeep this as induced drag, the underlying mechanisms are entirely different.
First case:
Suppose you had a finite-span wing generating lift at say 5 degrees angle of attack (AOA). I think most would agree that such a wing would experience an "induced drag" force. Now suppose you were to take an identical wing, turn it upside down and attach it to the original wing by a set of long struts such that it was at -5 degrees AOA (kind of a mirror image biplane). If the wings were set far enough apart, both wings would generate roughly the same lift and drag as the original wing. The net lift would be zero (because the wings are lifting opposite each other), but the net drag would not. This is an example of induced drag with zero lift.
This is a silly example, but consider a wing with twist. There is some angle of attack where a twisted wing will generate zero lift. At the zero lift condition, some parts of the wing are pushing up and others down. There is induced drag associated with the local pushing even though the net lift is zero. The induced drag of a twisted wing can actually get smaller as it starts to generate lift. This makes it hard to identify induced drag as the drag due to lift.
Second case:
If you look at any wing section in "Theory of Wing Sections", the section drag coefficient (Cd) depends on the section lift coefficient (Cl). In other words every wing section experiences drag due to lift. However, I think most would agree that a 2D wing section experiences zero induced drag. This 2D "drag due to lift" is what I would call "profile drag", or drag due to viscous losses. Although some may bookkeep this as induced drag, the underlying mechanisms are entirely different.
07-29-2004 | 09:48 AM
#27
RE: Induced Drag
fair enough
I tend to look at the whole airframe and try to imagine what parts are working for me and what is working against me
If it detracts rather than adds - -I call it drag
If it helps rather than detracts -I call it lift .
So far -I have not found ANY parts which don't fall into one of these camps.
To me , many parts of the airframe change rolls from one to the other .
I cut and try to get best compromise.
I tend to look at the whole airframe and try to imagine what parts are working for me and what is working against me
If it detracts rather than adds - -I call it drag
If it helps rather than detracts -I call it lift .
So far -I have not found ANY parts which don't fall into one of these camps.
To me , many parts of the airframe change rolls from one to the other .
I cut and try to get best compromise.
07-29-2004 | 09:55 AM
#28
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From: Bloomington, MN,
RE: Induced Drag
I've been quietly following this thread and your criticisms to Dick's points regarding induced drag, lift etc.
I can no longer stay quiet.
You are heavily involved in a 'jot and tittle' clarification through some well meant (perhaps) criticism of Dick's plainly stated 'aerodynamic principles/conclusions'---{the I've done it and this is what happens school}-- while you appear to be placing yourself in a 'this doesn't pass muster' and incidentally, I'm far above it all elitist position. Your personal profile would seem to preclude your having any real authority to adopt this rather vaulted position. Perhaps there is much you have left out of your profile, but at this point, those are the facts.
I know of a number of Dick's very successful designs in Pattern, TOC, IMAC etc. Perhaps you should enlighten us all by posting some of your successful designs so that all may see and evaluate your background, which you feel entitles you to 'correct and/or pass judgment', with some inherent disdain incidentally, on the comments made by someone who is well known and quite successful in the very field in which you wish to occupy a self appointed position of 'learned critic'.
From where I sit, and I've had a few thousand hours of flight experience in this hobby we've got a proven designer/experimenter (Dick) politely responding to condescending comments from someone who has close to no credentials (you) . My 'jot and tittle' comment stands.
Since I am no match for Dick in the 'what makes them fly' category, I have nothing further to offer. My 45 years plus in building, operating and personal designs/modifications thoroughly aligns me with Dick's comments. You Sir are simply splitting hairs!
I can no longer stay quiet.
You are heavily involved in a 'jot and tittle' clarification through some well meant (perhaps) criticism of Dick's plainly stated 'aerodynamic principles/conclusions'---{the I've done it and this is what happens school}-- while you appear to be placing yourself in a 'this doesn't pass muster' and incidentally, I'm far above it all elitist position. Your personal profile would seem to preclude your having any real authority to adopt this rather vaulted position. Perhaps there is much you have left out of your profile, but at this point, those are the facts.
I know of a number of Dick's very successful designs in Pattern, TOC, IMAC etc. Perhaps you should enlighten us all by posting some of your successful designs so that all may see and evaluate your background, which you feel entitles you to 'correct and/or pass judgment', with some inherent disdain incidentally, on the comments made by someone who is well known and quite successful in the very field in which you wish to occupy a self appointed position of 'learned critic'.
From where I sit, and I've had a few thousand hours of flight experience in this hobby we've got a proven designer/experimenter (Dick) politely responding to condescending comments from someone who has close to no credentials (you) . My 'jot and tittle' comment stands.
Since I am no match for Dick in the 'what makes them fly' category, I have nothing further to offer. My 45 years plus in building, operating and personal designs/modifications thoroughly aligns me with Dick's comments. You Sir are simply splitting hairs!
I don't know what 'jot and tittle' means, but I can be a bit of a butthead in my posts. Mea culpa. Having said that, I'm not trying to adopt any particular position, and I haven't felt any need to supply a comprehensive personal profile. I don't have any successful designs to my credit, and don't make any such claims. I know a little bit about aerodynamics, and make posts on some topics on which I am comfortable. These are, after all, discussion forums. Apart from my bouts of unconstructive bad attitude, I am sometimes concerned about people who ask legitimate questions getting misleading or inaccurate information, and this motivates me to post. I'm in a twelve step program for the butthead problem, and your incisive comments will no doubt help me out.
Work at it a little harder, and I bet you can stay quiet for a lot longer.
banktoturn
07-29-2004 | 12:20 PM
#29
RE: Induced Drag
The models we enjoy ( ENJOY), allow one lots of latitude .
Split rudders for drag (rearward lift?) etc..
My use of corny/misleading (to some apparantly) terms is deliberate.
What fun is it unless you get to think about it a bit?
My original Rules of Aerodynamics are tongue in cheek
Some people see rules as gospel
I don't.
I poke fun at lots of stuff -
I try to not poke fun at things I don't understand -at least basically.
My Rules :
1.If wing loading is too high -the CG does not matter.
meaning -if it has a wing loading too high - don't bother -it is just a crash looking for an opportunity to happen
2. If wing loading is light enough - cg still does not matter .
meaning- you can simply drag it in any direction desired.
Somewhere between these two extremes -your dream model will be found.
What kills me is the full scale stuff which ignore rule number one.---
Don't ask--
Split rudders for drag (rearward lift?) etc..
My use of corny/misleading (to some apparantly) terms is deliberate.
What fun is it unless you get to think about it a bit?
My original Rules of Aerodynamics are tongue in cheek
Some people see rules as gospel
I don't.
I poke fun at lots of stuff -
I try to not poke fun at things I don't understand -at least basically.
My Rules :
1.If wing loading is too high -the CG does not matter.
meaning -if it has a wing loading too high - don't bother -it is just a crash looking for an opportunity to happen
2. If wing loading is light enough - cg still does not matter .
meaning- you can simply drag it in any direction desired.
Somewhere between these two extremes -your dream model will be found.
What kills me is the full scale stuff which ignore rule number one.---
Don't ask--
07-29-2004 | 09:59 PM
#30
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From: Stuttgart, GERMANY
RE: Induced Drag
The Drag on an object is traditionally defined as the force acting in the direction of the free stream (or opposite the object's the velocity vector if you insist on an earth-fixed reference frame). By this definition, a split rudder would generate primarily Drag and not Lift. This definition makes no distinction as to the utility of the force, only its direction.
Lift is traditionally defined as the force perpendicular to the free stream (or velocity vector). This definition doesn't quite go far enough as there are an infinite number of directions that are perpendicular to the free stream. In order to choose only one direction, the Lift on an object can be further defined as the force that acts perpendicular to the free stream, AND in the symmetry plane of the object. Since most aircraft have gross left-right symmetry (except Rutan designs), this definition is usually unambiguous (the lift force is "up" for an aircraft in upright, wings-level, constant-altitude flight). The force perpendicular to both the lift and drag is traditionally called Side Force. With these definitions, a wing would normally generate Lift and a vertical tail would generate Side Force.
The above definitions are at least somewhat arbitrary, and you could argue that there can be no incorrect definition for Lift or Drag. While that may be true, certain definitions have proven to be more useful than others, and those are the ones accepted by the "community". Personally, I think there is value to honoring conventional definitions. Clear definitions provide a framework for discussion that minimizes the potential for misunderstanding. You need to look no further than this thread to see cases where loose terminology has led to misunderstanding and has stirred up needless emotional debate. If this position makes me elitist, so be it. I don't think you need to be an expert to express yourself with precision, just disciplined.
If your goal is a series of heated and largely meaningless exchanges, go on with your imprecise self (I will be the first to admit that the entertainment value of certain meaningless exchanges should not be overlooked).
Lift is traditionally defined as the force perpendicular to the free stream (or velocity vector). This definition doesn't quite go far enough as there are an infinite number of directions that are perpendicular to the free stream. In order to choose only one direction, the Lift on an object can be further defined as the force that acts perpendicular to the free stream, AND in the symmetry plane of the object. Since most aircraft have gross left-right symmetry (except Rutan designs), this definition is usually unambiguous (the lift force is "up" for an aircraft in upright, wings-level, constant-altitude flight). The force perpendicular to both the lift and drag is traditionally called Side Force. With these definitions, a wing would normally generate Lift and a vertical tail would generate Side Force.
The above definitions are at least somewhat arbitrary, and you could argue that there can be no incorrect definition for Lift or Drag. While that may be true, certain definitions have proven to be more useful than others, and those are the ones accepted by the "community". Personally, I think there is value to honoring conventional definitions. Clear definitions provide a framework for discussion that minimizes the potential for misunderstanding. You need to look no further than this thread to see cases where loose terminology has led to misunderstanding and has stirred up needless emotional debate. If this position makes me elitist, so be it. I don't think you need to be an expert to express yourself with precision, just disciplined.
If your goal is a series of heated and largely meaningless exchanges, go on with your imprecise self (I will be the first to admit that the entertainment value of certain meaningless exchanges should not be overlooked).
07-29-2004 | 10:45 PM
#31
RE: Induced Drag
Never , was I looking to make someone angry .
My imprecise ramblings are intended to simply present another viewpoint
Albeit to some , a cockeyed one --
To pe perfectly blunt tho -- If I had persued model aerobatic design based on critical airfoils and closely limited CG positions, etc., I would have spent way too much time with very little improvement .
I instead , looked at power and wingloading as the only meaningful ingredients (as do all the new aerobats).
I did not realize how little TESTED, PROVEN info there was for these types of models.
As many advocates of 3D flying have found -- you can get a powered aircraft to do many things never considered possible
All with airframes and setups once dismissed as unworkable.
My imprecise ramblings are intended to simply present another viewpoint
Albeit to some , a cockeyed one --
To pe perfectly blunt tho -- If I had persued model aerobatic design based on critical airfoils and closely limited CG positions, etc., I would have spent way too much time with very little improvement .
I instead , looked at power and wingloading as the only meaningful ingredients (as do all the new aerobats).
I did not realize how little TESTED, PROVEN info there was for these types of models.
As many advocates of 3D flying have found -- you can get a powered aircraft to do many things never considered possible
All with airframes and setups once dismissed as unworkable.
