Date: Mon, 3 Apr 1995 11:02:35 -1000 From: sasaki@netopd.harvard.edu (Marty Sasaki) Message-Id: <3lpnpb$8dh@netope.harvard.edu> Organization: Harvard OIT Network Services Subject: plate lift and more aerodynamics (Re: Materials was Re: Beginner Kite Maki) In article , perry.farmer@thefarmbbs.com writes: |>I agree, in a turbine, this kind of lift (if you want to call it lift) would |>be considered reactionary. |> |> \ |> \ |> ------------------->\ |> \ \ |> \ \ |> \ |> -> |> |>In this example there would be almost no Benoulli effect due to the upper |>part of the wind being almost totally stalled. Sorry, I'm right and you (and Dave Lord) are wrong. I've seen wind tunnel photographs (with smoke lines). They show that above a certain point the air actually flows over the top and joins up behind the wing. The path over the top is longer. With a flat plat, the point where the the flow is the same is at 90 degrees. At this point the wing is totally stalled. Let's suppose that all of the air that hits the surface below the top is diverted downward. Since the airflow is restricted (or the travel length is longer, take your pick) the air speed is higher, so the pressure will be lower. Since the back side has lower velocity air in this case the force will be downward. Only if the momentum is large enough to overcome the air pressure source will the net force be upward. Air has low density, and in this case low velocity. The momentum is tiny, not enough to lift kite off of the ground. With planes at high altitude, the static pressure is very small, so the Bernoulli effect is also small. At high velocities, the momentum is greater, so it can overcome the Bernoulli effect, plate lift dominates. One of the problems with applying airplane aerodynamics with kites is that most of the data stops when the angle of attack is greater than the stall angle. Above the stall angle, lift drops dramatically, and is uninteresting for airplanes. Airplane designers are interested in avoiding stalls and in recovering from stalls. Kites spend a lot of their time operating with the angle of attack above the stall angle. If you continue to look at lift past the stall angle, you will see that it decreases and approaches zero as the angle of attack reaches 90 degrees. I've heard it said many times that a single line kite flying at it's highest angle is flying is very nearly stalled. Usually, this is false. A single line kite at it's highest angle has all of the forces acting on it in equilibrium. Another problem is that an airplane operates within a small range of wing angle of attacks. A kite operates over a very large range of angles of attack. !> Think of it this way. Wind would strike the kite |>and would spill off the bottom edge. Movement would be reactionary. Once the |>kite picks up speed such that the reactionary movement is producing enough |>thrust (most likely a better term for this) the Bernoulli effect would start |>to come into the picture. No, the increase in a kites speed decreases it's angle of attack, which results in an increase in lift. Remember, lift is a vector, and it has direction as well as magnitude. Part of the lift translates into pulling against the lines, and part of the lift translates into a forward force which keeps the kite moving in the direction the nose is pointed. This is as good a place as any to describe how a stall happens, and it doesn't have anything to do with "dumping the air" from a sail. Let's say you are doing a ground pass and you do a good push/push turn directly down wind, the kite turns up into what we kite flyers call a stall. What happens here is that the kite is turned up in a way that kills it's forward speed. Suddenly, the angle of attack goes from a small angle, to a large one. There isn't enough lift being produced to push the kite upward, so it stays in one place. !> Only directly overhead, would the kite be into |>what would come close to being called "total Benoulli lift". It is at this |>point that with my Prism Eclipse, I can actually let go of the handles for a |>few seconds. Directly overhead, the lift is generally lower than elsewhere in the wind window. The kite has little forward velocity, so the apparent wind and the actual wind are almost identical. The angle of attack is lower. -- Marty Sasaki Harvard University Sasaki Kite Fabrications sasaki@noc.harvard.edu Network Services Division 90 Melrose Street 617-496-4320 10 Ware Street Arlington, MA 02174 Cambridge, MA 02138-4002 phone/fax: 617-646-1925 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Mon, 3 Apr 1995 19:03:16 -1000 From: lord@eskimo.com (David Lord) Message-Id: Organization: Eskimo North (206) For-Ever Subject: Re: plate lift and more aerodynamics (Re: Materials was Re: Beginner Kite Maki) In article <3lpnpb$8dh@netope.harvard.edu> sasaki@netopd.harvard.edu (Marty Sasaki) writes: >From: sasaki@netopd.harvard.edu (Marty Sasaki) >Subject: plate lift and more aerodynamics (Re: Materials was Re: Beginner Kite >Maki) >Date: 3 Apr 1995 21:02:35 GMT >In article , perry.farmer@thefarmbbs.com writes: >|>I agree, in a turbine, this kind of lift (if you want to call it lift) would >|>be considered reactionary. >|> >|> \ >|> \ >|> ------------------->\ >|> \ \ >|> \ \ >|> \ >|> -> >|> >|>In this example there would be almost no Benoulli effect due to the upper >|>part of the wind being almost totally stalled. >Sorry, I'm right and you (and Dave Lord) are wrong. >I've seen wind tunnel photographs (with smoke lines). They show that >above a certain point the air actually flows over the top and joins up >behind the wing. The path over the top is longer. With a flat plat, >the point where the the flow is the same is at 90 degrees. At this >point the wing is totally stalled. Marty I have also seen the same smoke lines but I have a totally different interpretation. What I see is a very large separation bubble that covers almost the total back side. This separation bubble consists of very low velocity , nearly stagnate air that cannot supply any lift because its pressure is very near static pressure. >Let's suppose that all of the air that hits the surface below the top >is diverted downward. Since the airflow is restricted (or the travel >length is longer, take your pick) the air speed is higher, so the >pressure will be lower. Since the back side has lower velocity air in >this case the force will be downward. Only if the momentum is large >enough to overcome the air pressure source will the net force be >upward. I agree totally and it is the momentum of the air that causes the reactionary movement of the kite, otherwise the kite would never launch. >Air has low density, and in this case low velocity. The momentum is >tiny, not enough to lift kite off of the ground. The kite is light and the reactionary force is enough. >With planes at high altitude, the static pressure is very small, so >the Bernoulli effect is also small. At high velocities, the momentum >is greater, so it can overcome the Bernoulli effect, plate lift >dominates. >One of the problems with applying airplane aerodynamics with kites is >that most of the data stops when the angle of attack is greater than >the stall angle. Above the stall angle, lift drops dramatically, and >is uninteresting for airplanes. Airplane designers are interested in >avoiding stalls and in recovering from stalls. Kites spend a lot of >their time operating with the angle of attack above the stall angle. >If you continue to look at lift past the stall angle, you will see >that it decreases and approaches zero as the angle of attack reaches >90 degrees. I also agree with this. But it is not linear with angle of attack. It very quickly reaches a small value and doesn't change much on the way to zero. >I've heard it said many times that a single line kite flying at it's >highest angle is flying is very nearly stalled. Usually, this is >false. A single line kite at it's highest angle has all of the forces >acting on it in equilibrium. Yes. Our only disagreement is on what the forces are. It is my opinion that most of the force is reactionary. >Another problem is that an airplane operates within a small range of >wing angle of attacks. A kite operates over a very large range of >angles of attack. >!> Think of it this way. Wind would strike the kite >|>and would spill off the bottom edge. Movement would be reactionary. Once the >|>kite picks up speed such that the reactionary movement is producing enough >|>thrust (most likely a better term for this) the Bernoulli effect would start >|>to come into the picture. >No, the increase in a kites speed decreases it's angle of attack, >which results in an increase in lift. Remember, lift is a vector, and >it has direction as well as magnitude. Part of the lift translates >into pulling against the lines, and part of the lift translates into a >forward force which keeps the kite moving in the direction the nose is >pointed. Aren't the above two paragraphs saying the same thing. I interpret them as identical in meaning? >This is as good a place as any to describe how a stall happens, and it >doesn't have anything to do with "dumping the air" from a sail. >Let's say you are doing a ground pass and you do a good push/push turn >directly down wind, the kite turns up into what we kite flyers call a >stall. >What happens here is that the kite is turned up in a way that kills >it's forward speed. Suddenly, the angle of attack goes from a small >angle, to a large one. There isn't enough lift being produced to push >the kite upward, so it stays in one place. No. The Bernoulli lift goes to nearly zero and reactionary force causes the kite to stay in one place. The bridle must be adjusted properly for the wind speed, or the flyer must walk either backward or forward to compensate. >!> Only directly overhead, would the kite be into >|>what would come close to being called "total Benoulli lift". It is at this >|>point that with my Prism Eclipse, I can actually let go of the handles for a >|>few seconds. I totally agree. >Directly overhead, the lift is generally lower than elsewhere in the >wind window. The kite has little forward velocity, so the apparent >wind and the actual wind are almost identical. The angle of attack is >lower. If the L/D is any good the kite will assume quite a high flight angle and a low angle of attack. The lift force need only be enough to support the kite in a more or less steady state, no acceleration required. I admire your positive attitude about being right but I am even more positive that I am correct. Dave Lord = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =