Power Kite Forum

Camber vs. AoA

harddrive8 - 27-2-2009 at 11:55 AM

I've been told several times that the difference between an Apex and Montana, at least in terms of depower, is the Apex changes the AoA, while the Montana changes camber as well. I've really just accepted this as change of camber = good quality, but my curiosity has risen about the difference here lately. Can someone help me understand the difference between AoA and camber in terms of depower???

acampbell - 27-2-2009 at 12:53 PM

I'll try and others can amplify...
It's a matter of improving the efficiency of the airfoil for any given AoA. When you do that, you improve the range of de-power, because the wing can operate at a broader range of AoA without stalling.

With your Apex, the airfoil shape stays roughly the same at various settings of the AoA. Very simple from the standpoint of rigging and mechanical complexity, but at the cost of efficiency, especially at high AoA, where the foil will stall ata certain point because the airflow cannot "turn the sharp corner" over the top of the wing without turbulating and tumbling, and that's what kills the kite.

Now sit just behind the wing of an airliner when it is landing and watch the flaps move outward and down as the plane slows for landing. As the plane slows, the angle of attack increases, but the wing does not stall, in part because the camber has changed and the airflow is offered a smoother more graceful path over the wing, as opposed to hooking a sharp turn around a big (relatively) flat slab. The change in camber makes the wing more efficient at lower speeds and higher AoA, producing more lift under those conditions than it could have otherwise.

Your Montana and other similar foils do not just tilt the slab of the wing. The compound action of the pulley mixer system, along with the pull on the rear lines direct to the trailing edge bridle, produce an independent and progressive action on each of the A, B, C and D (brake) bridles, enabling a more gradual curve of the airfoil as the air rushes over the top. Thus, like the airliner, the wing can function at a higher angle of attack before it stalls.

So being able to change the camber gives you more de-power since the foil can operate at a broader range of AoA (and spped).

mgatc - 27-2-2009 at 01:35 PM

When Angus puts on his kite professor hat and begins tutoring, there is little amplification or embellishment left to do. He has a true and enviable grasp of the science behind our play toys.:thumbup:

harddrive8 - 27-2-2009 at 02:08 PM

I think that cleared it up a bit for me. Let me recap and make sure I understand though...

When just adjusting the AoA (Apex), the kite just changes the angle at which it sits in the window. Little or no shape change occurs.

When changing the camber(Montana), the kite actually changes it's shape a little on the z axis between the A, B, C, and D points. Example being going from a straight line like an "l" to being like a "C". I know that may be over dramatic in the shape change, but just trying to get the idea of how the kite changes when integrates camber. Probably would be closer to an "f" without the horizontal line?

acampbell - 27-2-2009 at 02:55 PM

Quote:
Yeah, that's it. One thing..

Quote:
Originally posted by harddrive8
When just adjusting the AoA (Apex), the kite just changes the angle at which it sits in the window. Little or no shape change occurs.


to split hairs a bit, I would change the phrase "at which it sits in the window" to "at which it meets the on-coming air". Other things being equal, where a kite sits in the window will by itself change the angle of attack of a kite. That is the only way we can control the power of a fixed bridle kite.

When changing the camber(Montana), the kite actually changes it's shape a little on the z axis between the A, B, C, and D points. Example being going from a straight line like an "l" to being like a "C". I know that may be over dramatic in the shape change, but just trying to get the idea of how the kite changes when integrates camber. Probably would be closer to an "f" without the horizontal line?


If I had to choose my keyboard character, it would be the closing parenthesis... ")" ;)

harddrive8 - 27-2-2009 at 03:21 PM

Yeah ")" works too :smilegrin:

I understand now how the camber and AoA differ. Thanks for the help and explaination.

flyboy15 - 2-3-2009 at 10:41 PM

Someone either already has their private ticket or should probably get one :P . Campbell, you got aerodynamics for sure. I still know people who have flown kites for years and only know that the kite pulls more when u pull the bar, and stalls (just using the word) when they pull too much. Hes got it right, take if from a pilot, Apex owner, and Montana borrower.

acampbell - 3-3-2009 at 05:59 AM

Quote:
Originally posted by flyboy15
Someone either already has their private ticket or should probably get one :P . Campbell, you got aerodynamics for sure. I still know people who have flown kites for years and only know that the kite pulls more when u pull the bar, and stalls (just using the word) when they pull too much. Hes got it right, take if from a pilot, Apex owner, and Montana borrower.


Heh heh, thanks. Soloed gliders at 14 and soloed in a Super Cub at 16- before I had my driver's license. Then got my A&P. The wife is happy that I have kites 'cuase she knows they are cheaper than airplanes or sailboats.