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Author
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Topic: Need a lifeline here...
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Sv
Pilot
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posted 06-16- 03:16 PM
   
I need a life-line here... I am stumped. I'll go with audience vote:What directions are the lift and drag forces from the wing applied? My first guess was lift is perpindicular to the wing itself and drag parallel to the wing. But this can not be the case, no? If you had 90 degrees AOA then lift would be acting as pure drag and drag would be acting as pure lift! Second guess was that lift is applied relative to the airstream. Next guess... the aircraft's physical direction? Maybe I am just confusing myself as I often do. But if I have 45 degree AOA, then is 1/2 my lift actually holding the aircraft backward? Or is the "virtual" wing still really parallel with the aircraft's flight path, just with more lift and drag due to the high AOA. This is my gut feeling... forces should be applied peropindulicar and parallel to the aircraft's flight path. What does the audience say?  btw, I have the dbug code to see the vectors, but I need to reinstall it. Also It is hard to tell slight angles with it as well as it is hard to sustain a very high AOA  ------------------
-Sv =FC=
WWI in SDOE!
IP: Logged |
Tailslide
Pilot
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posted 06-16- 03:54 PM
With the new debug version you can dump all this info out to a file an graph it in a spreadsheet, very informative. From my observations in SDOE, lift is perpindicular to the wing. At 90 degress AoA the airfoils generate no lift, why would this be drag? At 90 degrees they do generate a lot of drag but I don't know how this could be called lift if the force is opposite the direction the wing is travelling, which it appears to from watching the drag lines in debug mode. TS [This message has been edited by Tailslide (edited 06-16-2000).] IP: Logged |
jedi
Pilot
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posted 06-16- 04:02 PM
Lift is applied perpendicular to the chord line of the wing. Drag is applied opposite to the thrust vector (although both of those statements simply establish a frame of reference). You could also say that lift acts perpendicular to the surface of the earth, and drag acts opposite to the flight path of the aircraft, but in any case you're really talking about components of the lift and drag vectors and how they compare to weight and thrust.And if ever there was a question that threatened to spin off out of control into technospeak, this is it ------------------
--jedi-- IP: Logged |
Sv
Pilot
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posted 06-16- 08:24 PM
How about this: Take an aircraft that is flying low with 20 degree AOA - the aircraft is holding altitude. Shouldn't all the lift that this wing produces be lifing the plane? If the lift vector is indeed perp. to the wing, then part of this lift is now acting as drag... is this correct? The drag acting opposite the direction of the wing makes sense to me.It seems to me that at any AOA the resultant force from the lift eqation should be forcing the aircraft (relative to itself) upward - that is why it is called lift and not called drag, right? -Sv IP: Logged |
Sv
Pilot
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posted 06-16- 10:10 PM
Hmmm... I guess that "upward" relative to the aircraft is perp. to the wing... I never stop confusing myself -Sv IP: Logged |
Jaguar
Pilot
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posted 06-17- 02:28 PM
I'm going to take a stab at this............. I HEARD THAT! The forces are relative to the wing, since that is what is creating the forces. If your at a +45 AOA, then the drag/lift vectors change +45 deg too. The only thing, is that now you have 2 vectors relative to the ground instead of one. One vector lifting the plane up, one pulling the plane back the way it came from...... but the thrust from the prop will kep the plane going foward (+45 deg AOA, and foward relative to the ground) because it overcomes the "backward lift". Also rember that lift is a function of speed, the faster you go, the greater the lift. So you could hold an AOA of 20 while maintaining alt only at a certain airspeed. The lift vector (relative to the ground) here equils the weight vector (relative to the ground), and the thrust vector (relative to the ground), is greater than the drag vector (relative to the ground), even though the aircraft is at 20 deg..........................................................right? ------------------
Cheers!
Jaguar
The FS Hangar
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jedi
Pilot
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posted 06-17- 05:34 PM
The wing doesn't know which way it's oriented. It simply produces lift based on its shape. That force is always (with perhaps very small exceptions) perpendicular to the chord line. If the airplane is at the top of a loop, the lift force is actually DOWN towards the earth, unless you give the wing a negative angle of attack (i.e. push forward on the stick), which would be the case if you flew inverted trying to maintain level flight.Now, the component of the lift force that opposes gravity is always perpendicular to the earth's surface. If a plane is in a 45-degree left bank, the lift force generated is angled 45 degrees left. Part of that force is opposing gravity, and part is turning the airplane. If you want to make a level turn, you'll have to increase the angle of attack, say, 5 degrees (depending on speed). That will angle the lift force BACK 5 degrees. Now part of the lift force opposes gravity, part is turning you, and part is contributing additional DRAG. In general, any increase in lift will increase drag, whether that increase is due to an airspeed increase or a greater angle of attack. When you bank an airplane, the wing on the outside of the turn is generating more lift than the inside wing. This causes more drag on the outside wing, which yaws the airplane away from the turn (called adverse yaw). Thats why you have to use the rudder to make a coordinated turn. This is most noticeable on planes with long wings and "fat" airfoil sections. Gettin a headache yet?
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--jedi-- IP: Logged |
Sv
Pilot
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posted 06-17- 07:03 PM
All this would still make sense if lift force were applied perp to airflow and not the wing chord. The only directional dif would be the AOA which is almost always relatively low.It has allot to do IMG (in my gues) with how the coefs are measured in the wind tunnel. If the wing is in the tunel at 45 AOA, and the lift coef is determined by a vertical force sensort, then lift MUST be applied relative to the airflow, NOT the wing chord. If the wing is just a machine that turns airflow into lift, then it makes more practical sense to apply this lift perp to the airflow, not the wing. What is the wing chord really? It does not really exist! A wing produces lift at 0 AOA, this implies that that real[i] chord of this wing has AOA! Does positive lift always [i]need to have a rearward vector? -Sv IP: Logged |
Sv
Pilot
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posted 06-17- 08:35 PM
Let's look at a graph of lift and drag over AOA: 
AOA here is -180 to 180 left to right. Green is lift, red is drag Note that 90 degree AOA produces 0 lift. This works for both working theories (perp to chord and perp to airflow) when taling about application in the simulator. The "final answer" lies in how the lift coef is found in the wind tunnel. If it is a measure of force perp. to the chord of the wing, wouldn't one get the highest coef at 90 degrees? My theory is that the lift coef is determined my a measurment of force perp to the airflow in the tunnel... a simple up and down vector. This would give a lift coef graph much like you see above. If this is indeed the case, I also theororize that the lift force in the sim MUST be applied perp. to the airflow to produce the desired results. Well? -Sv IP: Logged |
Tailslide
Pilot
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posted 06-18- 01:31 AM
SV, just load up the debug version and look at the pretty blue lift lines at 90 degrees to the wing as you fly around
[This message has been edited by Tailslide (edited 06-18-2000).] IP: Logged |
Sv
Pilot
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posted 06-18- 09:58 AM
Have done TS, but it is hard to see the small diference between perp. to chord and perp. to airflow.Also, even if it is perp. to chord... why? I just want to understand... -Sv IP: Logged |
juzz
Pilot
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posted 06-18- 10:27 AM
Are we talking 2- or 3-dimensional flow here? Either way, lift should be applied normal to the relative direction of the flowfield. [This message has been edited by juzz (edited 06-18-2000).] IP: Logged |
Sv
Pilot
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posted 06-19- 08:51 AM
You rebel juzz! So that backs up my rationale, doesn't it? I will look for some details on the web...-Sv IP: Logged |
Kraftwerk
Pilot
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posted 06-19- 10:49 AM
AoA means angle relative to the flow/stream of air over the wings. If you yank back on the stick your little alpha indicator on the HUD will instantly jump to a higher value then as the wing bites into the flow of sorrounding air it will go back to zero. Even if you are at 45 deg angle to the ground. In that instant you have an alpha the drag component works parallel to the airflow, and the lift normal to it. Same goes if you're going ballistic and yank back. Then drag works normalto the ground (parallel to airflow) and lift parallel to the ground and normal to the flow. Purely 2-D of course. Then there's downwash and induced angles of attack and thereby induced drag due to lift but don't get me started on that.[This message has been edited by Kraftwerk (edited 06-19-2000).] IP: Logged |
closterman
Pilot
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posted 06-19- 12:30 PM
Bonjour!Just a little precision about this:
----When you bank an airplane, the wing on the outside of the turn is generating more lift than the inside wing. This causes more drag on the outside wing, which yaws the airplane away from the turn (called adverse yaw). Thats why you have to use the rudder to make a coordinated turn. This is most noticeable on planes with long wings and "fat" airfoil sections.----- This is 1/2 true, the adverse yaw was the result of more drag make by the outside aileron and not by the wing. The aileron make more drag in down position or the aileron make less drag in up position (I can't remember..)
Just try with the Lanc, put full aileron (with no rudder) and you can see the adverse yaw. But when you put the aileron to neutral the adverse yaw disapear. Some device was made to beat the adverse yaw like "aileron frise" (i dont know in english, sorry!!) like in the Spitfire or put the pivot point behind the leading edge of the aileron.
Closterman
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Tailslide
Pilot
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posted 06-19- 12:45 PM
"My theory is that the lift coef is determined my a measurment of force perp to the airflow in the tunnel... a simple up and down vector. " This is what it really all hinges on. We are trying to model the forces that were recorded in the wind tunnel. However these forces were measured should be how they are exerted in the sim. TS IP: Logged |
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