All entries for Friday 06 March 2009
March 06, 2009
Today I began doing some preliminary tests on my paper airplanes (or gliders). I just want to 1) try a few small things like folding the wing tip up/down, changing the wing area, to see what effect they have on flight time. I was actually quite surprised at the result especially when some of the modifications I made produced unexpected results (like when I expected not to fly, it did, when I expect it to fly, it crashes etc). Some of the more consistent finding I observed were
1) The way I throw launch the plane has a significant impact on the flight time/ distance
Initially I did not expect this to make much difference because I thought after the launch phase, the plane will just glide according to its aerodynamic properties conferred by its design. But I noticed sometimes on the same plane I get different results. When I tried to make my throw consistent, I noticed a relatively horizontal throw (very little angled throw) will produce much better flight distance than a more vertical throw (aimed higher say around 30 degrees from horizontal).
This finding made me reconsider whether to use throwing angle as my 'noise' factor in my Taguchi experiment. Initially I thought from a customer's view , one may not pay careful attention to the way he/she throws the paper aeroplane, so a 'robust' plane could be something that could be thrown at any angle and still produce good flight results. But since any throwing angle of above 20 degree or so may potentially reduce flight performance significantly, there may not be much point changing the angle of flight throw (i dont know...) .
Then I read about several different launch method. One is to throw the plane vertical as high as possible and let it decend through gliding. The second is to launch it from a high place. The second method is better for me because I don't think I could get my throw consistently high every time. But both methods require a large space such as the basketball court in the sport centre. This availability of space could be a potential constraint for my experiement. This means I may end up throwing it horizontally allowed by my height in a large classroom, or in a long corridor (such as the corridor in Claycroft).
2) The flight result of two identical planes are surprisingly similar
This is a result I wanted but I didn't expect the flight result of two identifical planes to be so similar to each other. Initially I thought the variation in my throw could have significant impact on the result, but to my surprise, when I perform the same throw on two planes with identical design (with a little practice to get consistent throw), they both landed at the exact same location! This is good news because it suggests with a careful attention to my throw, I could actually eliminate or reduce the influence of my throw on the flight result. This means the changes in flight result will likely be attributed to the design changes.
3) Somtimes it hard to determine whether improved flight is due to deliberate change in design or unknown factor
This is something I observed when I tried to throw a randomly folded plane (still maintain the basic design but has its wing tip folded down for no particular reason), I noticed the plane actually 'glide' quite nicely (slow descent) but in an upside down manner. But when I fold another plane properly with a much larger wing surface, it didn't glide but rather flies like a dart (very fast, no gliding) and descend very quickly. This suggest to me that for the first plane it wasnt the wing that was producing lift but when it turned upside down, the whole plane upside down was producing lift (confusing).
That's for now, I will think more about what control factors I will use for the Taguchi experiment to influence flight performance. I also need to think about the location where I perform the experiment. Initially I thought about doing this outside, but now I think the wind will have too much influence which could make my experiment invalid. So indoor is probably better option for now. But as mentioned before, there are also multiple options for indoor location.
Principles of flight
This is a simplisitic summary of how I understand paper airplane aerodynamics
How does plane fly?
Plane flys because when it flys lift is created on its wings.
How is the lift created?
This lift is created because when the air pressure is smaller above the wing and greater underneath the wing.
Why is airpressure different above and under the wing?
This happens because when wings travel through air, the air travels faster above the wing and slower underneath the wing.
Why air travels faster above wing?
The ways airfoil is designed (curvature on the top) means the distance to travel from the front end to the rear end is greater on the top surface than on the bottom surface (more distance to travel at the top, less distance underneath). When to bottom air reaches the rear end of the airfoil, the top end is still 3/4 of the way (see fig 2a), this creates a vacuum of air (vortex) at the rear end that pulls the top air to make it go faster (like how a vacuum cleaner pulls air) to meet the bottom air at the rear. (Note as of 16:40 06/03 a mistake in the description about the 'distance difference as a cause of lift generation' is recongised and will be corrected in a future entry)
How is air pressure difference related to lift?
Consider this; a motionless object is simply in a state where all forces applied to is equal in all directions. Pressure difference creates unequal balance in force. As the pressure above the wing lessens, the downward 'push' by gravity and weight of atomphsphere is overcomed by the upward 'push' of a denser under-wing air (like a hydrogen balloon, air inside ballon is 'lighter' (less pressure) than the 'heavier' air outside the ballon (higher pressure), therefore the plane rises up.