HALE VTOL Flight Dynamics

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Several people have been asking about the ability of a wing to generate lift when upside down. The answer is yes, an upside down wing will create lift if the angle of attack is high enough. Now being in a rotor configuration further complicates things, but the same principles of physics apply. There is a distributed lift along the wing based on velocity (v) and angle of attack (alpha).


3689513393?profile=originalWhen the sum forces of each wing are added, a single vector is produced for each wing. That vector will determine the stability of the wing (the moment force) and the total lift force generated. It is no different for a flying wing than a spinning rotor, the rotor just has a different load distribution due the the non-linear, radially distributed velocity along the span. These moment diagrams should hold true independent of factor such as wing twist and airfoil discontinuity. At any speed of rotation there will be a combination of left/right alpha which will result in a stable rotation in hover. This also holds true that there is a combination of alphas which will provide a stable rotation at any amount of lift for climb while at any power level. This relationship will most likely not be linear however, which means that as the controller adds power, the ratios of left and right alpha may have to change. this means that the vehicle may not be able to be "trimmed out" to stable but rather adjusted for stability depending on power level.This all sums up to "i may need a computer control system to incorporate a trim coefficient based on power input and collective control, if not a complete gyro stabilization system with a feedback loop. Either way, it can be done, and so help me, it will be. 

I have taken a couple measures to help enlarge the stability bucket to try to make a stable VTOL ascent/decent possible with manual controls. I have added a generous dihedral of 12 degrees to help compensate for dis-symmetry in the wing lifts, and I have put the cg in front of the control surfaces for each wing.  The wing is structurally very sound, made mostly from carbon fiber, so aero-elacticity should be minimized. To ensure that the design is in the ballpark, I will make a test stand to allow it to rotate at low speeds (below flight rotational speed) in order to inspect for any stability issues before full flight testing. 

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This complexity begs the question, "why not just use a symmetric airfoil???" Well, the answer is that fine tuning the aircraft for improved performance is what takes it from proof of concept, to a viable tool. I have been getting emails about an Air Hogs toy called a Switchblade. It is pretty much the same concept, but without any of the elegance or performance. The old saying, you can make a washing machine fly if you put a big enough motor on it comes to mind. Air hogs is known for making some really cool, really fun, really unique, and really low cost toys. Although the Switchblade meets all those standards, it doesn't fly very well, or for very long. It would not make the cut for a military or even commercial UAV system, and from the ratings I've seen online, it didn't sell very well as a toy either. 

At the end of the day, creative people will always try to push the limit. Some people will be neigh-sayers and rant about how an idea will never work, and other will be inspired, and take the technology to the next level. I'd rather be the later than the former. Besides, people aren't remembered for being dismissive and correct, they are remembered for breaking convention and proving the "dismissers" wrong. 

Thank you all for your comments and advice. Keep 'em coming!

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Comments

  • this sounds like a good idea with lots of development ahead!

    well, first of all, there are lots of things that you cannot predict. i would suggest you to start with a small foamy, with simetrical wing airfoil. it'll probably fly, but i'm sure it will also show you some problems. after the first sucessful flights, go ahead and try an assimetrical design. maybe you'll need 3 or 4 prototype versions, but if you have the will to do it, i'm sure you'll make it!

    just don't try to start with the full complex version. even if everything is calculated and simulated, i'm quite sure a little bug in the design will cause a complete system failure, and several hours of hard work will go straight to the trash. start small, and i'm sure you'll have a unique aircraft at least.

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