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Old Feb 13, 2013, 10:28 AM
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Angle of Attack

My question is simple; What determines angle of attack for a wing? Been flyin off and on since late 80's and I fell in love with the Sophisticated Lady. I have recently got my hands on a new kit. IM also startin to play with foam wings and airframes. If I change the wing, How do I set the angle of attack for a given wing? Is it relative to centerline?


Thank You
Slider n Copa
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Old Feb 13, 2013, 10:35 AM
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AOA doesn't exist until you have airflow, and then it is the difference between the direction of the airflow and the wing. I think you are thinking of incidence, which can be measured a couple different ways but i usually do it relative to the tail plane. Should be relative to the thrust line but that's harder to visualize.
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Old Feb 13, 2013, 10:46 AM
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Sounds interesting. Listening in.
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Old Feb 13, 2013, 11:22 AM
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"Imagine" you were holding your right hand directly out the passenger car window into the airstream while traveling at 30mph with your hand held flat relative to the ground - this would of course be on the passenger side with no obstructions like mail boxes, tree's etc.

With your hand held parallel to the road, you feel the air drag of the relative wind trying to push your hand toward the back of the car. As you rotate your hand slightly (say +/- 5-10 degrees), you will feel a lifting force (or downward force) depending on the angle. The front of your hand goes up relative to the back and it will drive your hand and arm upward. When the front of your hand goes down relative to the back, it drives your hand down. Small angles on your hand will cause a pretty large change in force with not much change in felt drag (the force trying to push your hand/arm to the back of the car).

This relative angle between the angle of your hand and the airstream is the Angle of Attack. When your hand is "lined" up with the wind, you don't get lift and you are arguably at "0" angle of attack.

In the real world, you have an airfoil (due to the need to have a spar) that has camber or curvature which makes it non-symmetrical relative to the line drawn between the front and trailing edges. This also can cause lift in addition to the Angle of Attack.

This is one reason that an airfoil like a Clark Y has lift at 0 angle of attack and also why a symmetrical airfoil has no lift at a 0 angle of attack.

Scott
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Old Feb 13, 2013, 11:29 AM
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Nice explanation Scott! I always forget about the car window thing.
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Old Feb 13, 2013, 11:43 AM
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Incidence angle

From the original question it is apparent that you are really concerned about the relative angle of the chord line of the wing as compared to the chord line of the horizontal stabilizer. Normally in aircraft design this factor is used in conjunction with the length measured between a point at 25% of the wing mean chord and a point at 25% of the horizontal stab mean chord to establish stability and handling characteristics. If you are working with existing conventional designs as mentioned in the original post, a certain experience dictated rule of thumb can be used as a close approximation. Most, but not all, airfoils used on such designs will require a positive angle of incidence, ie wing nose up with respect to the stab, and the initial configuration is in the ball park of an easy solution. Set the stab on your bench with the chord line parallel to the bench top, and then set the wing on the bench. The relative angle thus formed is usually very close, ( if anything a tiny bit too large). Then visualize the plane in gliding flight, which will require the wing to have positive angle in relation to the actual glide path of a few degrees. Mount the wing such that the fuse is in its minimum drag orientation as it flies down the flight path, the wing is a few degrees positive, and the stab is aligned as previously determined on the bench. You will now need to realize that the wing/fuselage angle is speed dependant, as is the wing/stab angle, ie only perfect for one airspeed. In the case of the planes mentioned this speed was selected as slow cruise. The planes, as designed, were not optimized for high speed. Out of the box the planes have too much relative angle between the wing and stab (makes the plane very forgiving to fly, ie slow and positively stable) and the fuse/ wing angle is optimized for best performance at that slow and stable speed. Flattening things out will move the optimum performance point to a higher airspeed, vice-versa increasing the angles will move the optimum to a slower airspeed. Bear in mind also, that the structure of these aircraft were also designed for the design flight regime and often are not capable of withstanding the loads at significantly higher airspeeds, particularly the twisting forces on the wings caused by pitching moment (a whole 'nother can of worms). Isn't aero theory grand?
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Old Feb 13, 2013, 02:24 PM
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The way I read the question, is what's the angle of attack with a different wing?.
So it seems the fuse/tail is to be used with a different wing and different airfoil (possibly).
So would a flat bottom foil have a different "angle" than a semi-symmetrical? I'd imagine that if the fuse was unaltered, and flattest section of wing was against the fuse, that the semi airfoil would have a bit more up angle since the leading edge would be higher.
Of course, there's many many other factors involved with this situation then just incidence.
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Old Feb 13, 2013, 02:55 PM
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Quote:
Originally Posted by Slider n Copa View Post
My question is simple; What determines angle of attack for a wing? Been flyin off and on since late 80's and I fell in love with the Sophisticated Lady. I have recently got my hands on a new kit. IM also startin to play with foam wings and airframes. If I change the wing, How do I set the angle of attack for a given wing? Is it relative to centerline?


Thank You
Slider n Copa
I believe you have some terminology errors in your question.

AoA is the angle of the main wing in relationship to the Relative Wind.
All references are to a centerline or Datum line of the fuselage. Some designers have the fuselage fly at a slightly nose down angle. The main wings angle is the chord line in reference to this datum line. A rough rule of thumb is angle the wing leading edge up approximately one-half to one degree for each degree of camber of the airfoil. The horizontal tail is there to counteract the negative pitching moment of the main wing. If there is any minor up/down elevator trim for an articulated tail, ignore it.

If you get the angle wrong all that happens is the fuselage will fly at a different angle which may increase fuselage drag.

Hope this helps
Curtis
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Old Feb 13, 2013, 08:41 PM
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Upstate NY
Joined May 2005
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Quote:
Originally Posted by Slider n Copa View Post
My question is simple; What determines angle of attack for a wing? Been flyin off and on since late 80's and I fell in love with the Sophisticated Lady. I have recently got my hands on a new kit. IM also startin to play with foam wings and airframes. If I change the wing, How do I set the angle of attack for a given wing? Is it relative to centerline?


Thank You
Slider n Copa
If you desire the plane to fly with the fuselage relatively level to the ground at slow/normal speeds (like landing approach), you want a relatively "larger" amount of wing incidence which I will define as an wing leading edge up angle relative to the fuselage center line (in real aircraft it can be several things but likely it is the line of minimum drag for the fuse). Typically this value for our models is somewhere around 3 to 4 (look at your other planes) degrees - give or take.

a) This angle depends on the wing loading of the plane (oz/sq ft) for a given trim speed. The higher the wing loading, the more angle you need on the wing to keep the fuse level for a given trim speed of the plane. This is due to the wing having to generate more lift to carry that weight for the same speed and the wing has to "fly" at a higher angle of attack to generate the needed lift.

b) The angle depends on the trim speed of the plane - a hotliner will NOT want a large amount of incidence because its normal speed is quite high. A lightly loaded wing will also not need much wing incidence for the same reason as cited above - the wing doesn't have to work as hard. A "heavy" glider will need a higher angle of incidence in order to fly for a given speed when compared to a "lighter" glider.

c) The angle depends on the airfoil as well. A symmetrical airfoil will need more than say a highly cambered airfoil like for example the ubiquitous Clark Y.

Note that I have given you no exact formula here, a lot of this is guesswork for models and is why you can thank the original designers for working out for most planes.

If you have ever seen a Hobie Hawk, they typically have a lot of incidence and look kind of funny with the tail up in the air at their normal speeds (relative to what you think they should have).

Here is a good read...
http://www.charlesriverrc.org/downlo...sis/da_web.pdf

Best Wishes,
Scott
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Old Feb 13, 2013, 11:16 PM
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Joined Feb 2013
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Thank you

I appreciate the input, Makes sence to me
Quote:
Originally Posted by Gil Gauger View Post
From the original question it is apparent that you are really concerned about the relative angle of the chord line of the wing as compared to the chord line of the horizontal stabilizer. Normally in aircraft design this factor is used in conjunction with the length measured between a point at 25% of the wing mean chord and a point at 25% of the horizontal stab mean chord to establish stability and handling characteristics. If you are working with existing conventional designs as mentioned in the original post, a certain experience dictated rule of thumb can be used as a close approximation. Most, but not all, airfoils used on such designs will require a positive angle of incidence, ie wing nose up with respect to the stab, and the initial configuration is in the ball park of an easy solution. Set the stab on your bench with the chord line parallel to the bench top, and then set the wing on the bench. The relative angle thus formed is usually very close, ( if anything a tiny bit too large). Then visualize the plane in gliding flight, which will require the wing to have positive angle in relation to the actual glide path of a few degrees. Mount the wing such that the fuse is in its minimum drag orientation as it flies down the flight path, the wing is a few degrees positive, and the stab is aligned as previously determined on the bench. You will now need to realize that the wing/fuselage angle is speed dependant, as is the wing/stab angle, ie only perfect for one airspeed. In the case of the planes mentioned this speed was selected as slow cruise. The planes, as designed, were not optimized for high speed. Out of the box the planes have too much relative angle between the wing and stab (makes the plane very forgiving to fly, ie slow and positively stable) and the fuse/ wing angle is optimized for best performance at that slow and stable speed. Flattening things out will move the optimum performance point to a higher airspeed, vice-versa increasing the angles will move the optimum to a slower airspeed. Bear in mind also, that the structure of these aircraft were also designed for the design flight regime and often are not capable of withstanding the loads at significantly higher airspeeds, particularly the twisting forces on the wings caused by pitching moment (a whole 'nother can of worms). Isn't aero theory grand?
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Old Feb 13, 2013, 11:25 PM
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May Be I asked the wrong question, Seems like I should have asked how to determin the nutral angle for a given airfoil.
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Old Feb 13, 2013, 11:32 PM
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Thank You

Thank You, Haven't messed with this stuff in awhile.
Quote:
Originally Posted by jasmine2501 View Post
AOA doesn't exist until you have airflow, and then it is the difference between the direction of the airflow and the wing. I think you are thinking of incidence, which can be measured a couple different ways but i usually do it relative to the tail plane. Should be relative to the thrust line but that's harder to visualize.
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Old Feb 13, 2013, 11:43 PM
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Airfoil changes

Thats the idea, I wanna try different airfoils on the fuse, What else do I need to consider.
Quote:
Originally Posted by cityevader View Post
The way I read the question, is what's the angle of attack with a different wing?.
So it seems the fuse/tail is to be used with a different wing and different airfoil (possibly).
So would a flat bottom foil have a different "angle" than a semi-symmetrical? I'd imagine that if the fuse was unaltered, and flattest section of wing was against the fuse, that the semi airfoil would have a bit more up angle since the leading edge would be higher.
Of course, there's many many other factors involved with this situation then just incidence.
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Old Feb 14, 2013, 12:47 AM
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Thanks to All

Just wanted to Thank All that gave input, Lots of gd info there Thanks again
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Old Feb 14, 2013, 01:35 AM
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United States, CA, San Jose
Joined Mar 2012
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Quote:
Originally Posted by Slider n Copa View Post
Thats the idea, I wanna try different airfoils on the fuse, What else do I need to consider.
There's a whole lot math that makes me glassy-eyed when people talk about it, but different foils will have a different dynamic interaction with a fixed fuse/tail. All the moment arms and coefficeints and what-not. Might be hard to predict and require trial and error regardless of where it's set.
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