


Indonesia, East Java, Malang
Joined Oct 2012
22 Posts

Discussion
Flap Rate VS Weight Load
need help...Does anybody know how many flaps per second for certain weight load?please show me the source..




Joined Jun 2012
53 Posts

Which depends on the wingspan also...



Indonesia, East Java, Malang
Joined Oct 2012
22 Posts





this is a post from another member of the forum
This may help you design your ornithopter: The lift and drag produced by a flapping wing that rotates about an axis is strongly affected by flap frequency, flap angle (also called amplitude) and wingspan. Assuming the arc over which the wing oscillates about has a fixed distance, only the time it takes for the wing to pass through that act is what determines it's velocity. If the frequency increases, the time over which this distance is covered decreases and the velocity increases. As lift increases with the square of the airflow velocity, it can be said that lift the lift produced by a flapping wing also increases with the square of the flapping frequency. t = 1/f V = d/t = d*f L ~ V^2 = (d*f)^2 where: f = flap frequency d = arc distance t = time V = velocity L = lift The wingspan also plays a similar role. The arc distance traveled by the wingtip increases with both wingspan and the angle of the arc. For some given angle, if the wingspan is doubled, the arc distance must double in order to keep the same angle. The same applies if the angle is changed. Thus the arc distance is the product of the angle in radians and the wingspan. This means the velocity is directly affected by the wingspan and as such affects the lift. d = a*s v = d*f = a*s*f L ~ V^2 = (a*s*f)^2 where: a = amplitude s = wingspan The lift produced by a wing is proportional to the square of the airflow velocity times the net wing area. The area of a rectangular wing is the chord times the wingspan. If the planform is non rectangular, a constant can be multiplied to both values to obtain the area. Since lift is linearly proportional with wing area times the velocity squared it, this is what happens: A = s*c*K L ~ A*(a*s*f)^2 = s*c*K*s^2*(a*f)^2 = s^3*c*k*(a*f)^2 where: c = wing chord K = planform adjustment constant Thus it can be said lift is proportional to the square of flap frequency, amplitude and the cube of the wingspan. Since drag is also affected by these same variables, the effects are also true for it. If you decrease the wing span, the drag decreases, the wings can flap faster, the lift increases, but so does the drag again. In short if you increase or decrease the wing span in very small increments you can obtain the highest lift, for the lowest current from of your motor. That is assuming the chord and amplitude don't change. An easier way adjust the loading on your motor is to just change the wing chord. It affects drag linearly, so if you half it, you half the drag. I can't really say which is the most optimum solution, but if you want the most static thrust, use the highest amplitude possible for the lowest flap frequency possible. This is due to inertial resistance created by the oscillating mass of the wing changing linearly with amplitude, yet increasing with the square of the frequency. Static thrust only matters if you want to have a slow flying ornithopter or a ornithopter capable of hovering. The thrust that keeps an ornithopter flying forward is more depedant on wing angle of attack than any other variable. It's mostly affected by wing flexibility and forward velocity. If you want a nice forward flying ornithopter (like a bird), my recommendation would be something with a 60 degree flap angle (+35,  25, 5 degrees of dihedral), a 35 cm span for each wing (70 cm total wing span) with a 10 cm wing chord and a half eliptical planform. The relatively high aspect ratio would also give you a nice gliding ability. If this overloads the motor chop off 5cm from the wingtips. I Hope this long post helps you some i just prefer to build and modify till it works cheers brian 


Indonesia, East Java, Malang
Joined Oct 2012
22 Posts

Quote:
oh, what is K? is it like cl (coefficient lift) ? 




thank george aka plantoflap i think hes the one who posted cheers Brian



Joined Sep 2003
371 Posts

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George. http://www.rcgroups.com/forums/showt...ghlight=xxxmag 



Indonesia, East Java, Malang
Joined Oct 2012
22 Posts

Quote:





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This difference in area can be considered as a ratio. The area of the wing divided by the area of the rectangle gives this ratio. If you know this ratio, and you know the length and width of the rectangle, you can calculate the area of the wing. Curiously enough, the length of the rectangle is the wing's span, and the width of the rectangle is the wing's maximum chord, so this concept can be directly used in ornithopter design. Both of these distances are known especially when working with membrane wings, and since most ornithopters use these, it's even more applicable. Mathematically K can be described as: K = Aw/Ar = Aw/(S*C) where: Ar = rectangle area Aw = wing area S = span C= Maximum chord As I said before, if you know Ar, S or C, you know Aw: Aw = K*Ar = K*S*C 



Indonesia, East Java, Malang
Joined Oct 2012
22 Posts

Quote:




Joined Sep 2003
371 Posts

One easy way to find the wing area is to draw a wing to scale in a rectangle, cut away all the paper outside the rectangle then weigh the rectangle. Then cut away the part of the rectangle that is not 'wing' and weigh the 'wing' part. 'K' would be wing/rectangle. wing area can then be deduced too.
Finding the 'effective' wing area of a membrane wing, or its lift and drag, by computation alone is almost impossible. Its like trying to calculate how a flag in the wind behaves. As a wing's stiffness approaches that of a rigid wing computations make more sense. By that I mean add battens. Check how Mr. Kazu Kaku aranges the battens on his birds. Some work better than others. The best orni wing design 'tool' available to us is Gravity, and its free. The better you can make your membrane wing glide, while retaining enough tip flexibility to twist under powered flight, the better it will fly with power later. If you build some 'test' wings with a stick fuselage and airplane 'tail' you can work out where and how many battens you need. The wings will distort more as weight (clay) is added so take lots of 'in flight' pictures as you change things. George 



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