Flying Wing for Search and Rescue

Hey Guys,

I've been working on designing a flying wing UAV since 2 months now. Human search and rescue using an autonomous drone is the prime objective of this project.

First Prototype.

One of the major aim of our project is to show how a visual human detection system that uses images from a normal camera can be implemented in software. The system would fit into the larger context of disaster management and more specifically how it can benefit search and rescue operations. 

So the first prototype was made but it couldn't fly all that well. The tips stalled while rolling(probably because of no twist and less sweep) and led to a disastrous crash. As soon as our first prototype failed we got down to designing the second version.

I need some advice with the design of the second air-frame. 

So I'm pretty satisfied with the numbers Xflr is churning up for the second prototype. But I'm unhappy with the kind of performance I'm getting. 

Also I'm a bit skeptical about the amount of twist I've to give to prevent my wing tips from stalling. I'm using the Pankin twist formula and it gives approximately -9 degrees of twist. I'm not sure if this is usual for flying wings or not. I was told anything between 4-15 is normal. 

So my doubts about the design are : 

1) Is the twist alright or should i reduce it a bit ? 

2) What can I do to improve my max Cl/Cd ? 

3) Any other design faults you guys see in the design ? 

Also some updates on the Image processing part. 

HOG detector was successfully tested on a sample camera feed.


Thank you for your time and consideration.

Cheers,

Karan

Views: 1903

Comment by Martin on October 25, 2014 at 7:14am

You should try to keep the twist as small as possible. There will be a huge performance penalty with 9 deg of twist when flying at any speed other than the design airspeed.

Tip stall can be combated with a good tip airfoil (and a little bit of twist). The tip airfoil should be thinner and have less camber than the root airfoil.

Performance can be gained by increasing the wing aspect ratio. You should decrease the root chord and increase the wing span. I would reduce the root chord to about 25 cm, the tip chord to 20 cm and then increase the span to 2 meters.

Another thing to optimize is the lift distribution.

Also, I would recommend using the MH45 airfoil at the root and a thinned and a thinned and decambered version of the root airfoil at the tip.

Comment by Karan Chawla on October 25, 2014 at 7:27am

I'm currently using E334 at the root and MH 49 from where the plank section ends , including the tips. I can probably reduce the thickness of MH 49 at the tips ? 

Wing span is a constraint. So, I can't increase that. I'll try reducing the root chord and post the results. 

From the research papers I've read, Bell shaped lift distribution is considered the most efficient for the tailless designs and to achieve that I've given 1/3rd of the twist inboard, kept it constant over the remaining 2/3 section of the wing and given the remaining at the tip. 

Are yous saying I should try to achieve elliptical for this design as well ?

Thanks.

Comment by Martin on October 25, 2014 at 7:44am

If span is a constraint then the elliptic lift distribution is the most efficient. There is no one ultimate lift distribution. It depends on the constraints and the geometry of the wing - planar/non-planar.

In regards to tip stall you should also look at the CL distribution. The CL at the tips should be lower than at the root.

Comment by Nikola Rabchevsky on October 25, 2014 at 7:58am

I think the image processing aspect is cool but I doubt it will work for SAR activities.  Assuming that A) you're flying aroudn 400 feet AGL and B) the subject is down, the camera likely won't have enough resolution to do image processing.  I have the same reservations about FLIR for this type of work.

Comment by james johnson on October 25, 2014 at 8:23am
Is there a particular reason behind using a wing in your design? A polyhedral high wing design is far more forgiving and efficient. The natural self-leveling characteristics would require less power for a high endurance craft to maintain stable fight.
Comment by Martin on October 25, 2014 at 8:38am

Sweep already has dihedral effect.

Comment by Karan Chawla on October 25, 2014 at 9:04am

@Martin I'll try to achieve elliptical and post some updates by tomorrow. what does "CL at the tips should be lower than at the root" ?  Isn't that always the case ? I think having near zero or negative lift at the tips would greatly reduce induced drag. 

@Nikola It's just something I'm trying to achieve. I have no plans of making it commercially available.

@James Wings because they're highly portable and they are sleek which reduces the chances of any major damage to the wing during a hard landing. 

Comment by Martin on October 25, 2014 at 9:42am

That is not always the case. Having higher CL at the tip is the cause of tip stall. The tip airfoil reaches its maximum CL sooner than the root airfoil and hence tip stall.

If you have negative lift at the tips then your lift distribution wouldn't be elliptical anymore. If you would like to have less drag and same lift you would have to increase the wingspan.

For a given amount of lift and span the elliptical lift distribution is the optimum.

http://aerocrafty.blogspot.com/2013/06/bell-shaped-lift-distributio...


Moderator
Comment by Gary Mortimer on October 25, 2014 at 9:52am

You can refine designs in simulation for hours, its the tweeks that make the platform work in the field that add value. Its all about compromise. The most efficient foils and thin sections might be tricky to make work structurally in a platform destined to land in grass and rough fields all day long. One good clout and a dent and all the aerodynamic work is borked. Also end users taping cameras or other sensors on and operating with the CG well out of whack. But I digress. Always love seeing proper builds here.

Comment by Karan Chawla on October 25, 2014 at 10:04am

@Martin Thanks for the blog post. So basically with span constrained I should try to achieve elliptical lift distribution. 

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