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Old Sep 24, 2012, 03:39 AM
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Well the good news is that it looked a lot more stable in pitch with the more forward C of G.
It does look like a combination of adverse yaw and lack of side area.

The adverse yaw (ie as you roll right, it yaws left) can be improved by a number of methods. Roberts suggestion of moveable leading edge controls will very likely help, and is an elegant solution, but obviously involves a fair amount of work. A quick work round would be to program in lots of aileron differential, such that the up going aileron moves considerably more than the down going one. Then add a couple of small 'cheat' fins on the underside of the wing tips. As you have a lot of sweep back, these will be more effective the further out you place them. Another trick is to toe them in by 20 degrees or so. That way, as the bird yaws, the drag increases more on the outboard wing than the inboard one, so tends to self correct the yaw better.
You can make the fins out of clear acetate sheet so they don't show, but whatever you use, keep the weight you are adding to the wing tips as low as possible.

On one of my birds, (the Red Kite) I use 100% differential ie the ailerons only move up, combined with drag plates that are fixed to the bottom of the aileron and project forward of the hinge line. This way, as the aileron deflects upwards, the drag plate drops into the airflow below the wing and gives yaw control.

I have found that several of my birds will behave ok in gliding mode, but any bad traits will show up under power, when you have to cope with the torque reaction from the motor and the spiral flow from the prop. - Your model seems to exhibit the same tendencies as it flew straight on the glide, but rolled / yawed left under power.

Hope that has given you some ideas. Don't give up, it is almost there!

Malcolm
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Old Sep 24, 2012, 07:04 AM
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You know, I just noticed something just before you crashed. First question. Was your V-tail set as elevator only, or was it failed in for rudder?

The reason that I ask is because before you crashed you V-tail was turning to the left. (Thanks to your slow motion)

Another is did you put any down right to the motor? Lee suggested that with his falcon kits; and as also many airplane manufacturers design it in their planes.

Also, could you wings be a little warped? What I mean is, when looking straight at the birds leading edges and trailing edges from the front, are they in opposition?
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Old Sep 24, 2012, 09:46 AM
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Great tips guys. I plan on checking for warp today. I too noticed that the right elevator seem to be up and the left had no delection. The plane was set up for elevator, aileron only and I cannot duplicate the anomaly on the ground. It may be an optical illusion. I had intended to add a verical rudder, but was having so much trouble balancing the plane I did not want any additional weight on the tail. The glide test seem to indicate it was not needed. Malcom, can you post some photos of the cheat fins and drag plates you speak of?
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Old Sep 24, 2012, 04:01 PM
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Managed to find a suitable slope today and had about a 10 mph breeze. This is the first breeze since I started test flying.
I removed the motor to save the shaft and added lead in its place.I cut a balsa vertical rudder and jammed it up you know where. I added differential to the ailerons and experimented with different flap settings. I found I got my longest flights with aileron trim just a fraction above nuetral. The wing tips have 2 degrees of washout built in.
I also took some measurements on the wings prior to flying. The left wing seems to have 1/2 centimeter more dihedral over the span. As I repaired the wing last night , I am not sure how they compared in previous flights. I have found the weak spot in the wings as hard impacts produce a tear starting at the trailing edge an running along the root. I have taken advantage of this today to repair the dihedral. \
At one point I was actually able to slope soar briefly until I encountered a tree. This prompty broke off the vertical rudder. Further flights after this produced no discernable difference in Yaw. The bird responds to both left and right aileron even at these slow speeds. The last flight the plane banked and turned left and when I picked up the plane the left rear elevator was jammed in the down position. I was a litte surprised as how much it effected the flight. I may go back to the V-tail aileron setup. Final trim reguired just a bit of up elevator. I plan on reinstalling the motor with 3 degrees of right thrust.
The poor bird is beginning to look like its been shot at and hit. I have been posting results in several threads and getting good advice from a number of people. Thank you all.
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Old Sep 24, 2012, 05:17 PM
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Quote:
Originally Posted by Joel K. Scholz View Post
Malcom, can you post some photos of the cheat fins and drag plates you speak of?
Here you go Joel.

Cheat fins as fitted to my Vulture - these are larger than you'll need, as they had to overcome the destabilising effect of a bulky camera system this bird had to carry. The angle of the fins increases drag, but does help to control yaw. Worth playing with both size and angle of fins to get best performance. - Start a bit big to be safe, then cut down as you get a feel for the bird.

Aileron drag plates as fitted to my Red Kite - just a bit of thin ply stuck to the bottom of the ailerons. Obviously you need 100% differential with this arrangement as the ailerons cannot deflect downwards
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Old Sep 24, 2012, 07:25 PM
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Thanks Malcolm. I learn something new every post.
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Old Sep 24, 2012, 07:33 PM
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Quote:
Originally Posted by highviz View Post
Here you go Joel.

Cheat fins as fitted to my Vulture - these are larger than you'll need, as they had to overcome the destabilising effect of a bulky camera system this bird had to carry. The angle of the fins increases drag, but does help to control yaw. Worth playing with both size and angle of fins to get best performance. - Start a bit big to be safe, then cut down as you get a feel for the bird.

Aileron drag plates as fitted to my Red Kite - just a bit of thin ply stuck to the bottom of the ailerons. Obviously you need 100% differential with this arrangement as the ailerons cannot deflect downwards
Uhh...Highviz, you KNOW you're gonna have to give us more pictures of that bird of yours!! AND the build ..if you can.
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Old Sep 24, 2012, 11:20 PM
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Yeah, Highviz, you using a bungee launch for your vulture?
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Old Sep 25, 2012, 04:24 AM
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Quote:
Originally Posted by G-LO View Post
Uhh...Highviz, you KNOW you're gonna have to give us more pictures of that bird of yours!! AND the build ..if you can.
Quote:
Originally Posted by bluefalconhollan View Post
Yeah, Highviz, you using a bungee launch for your vulture?
I don't want to hi-jack this thread and contractually can't say too much about this project, as it was carried out on behalf of a production company who own the rights.

But can give you this link that they released http://www.bbc.co.uk/programmes/p00nngm0

Bear in mind that there is a certain amount of artistic licence involved in this clip. It's actually a compilation from five days flying and filming on the mountain, but good landings don't make such entertaining TV!

Malcolm
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Old Sep 25, 2012, 04:34 AM
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So you're saying this company will produce this bird for possibly selling to the public, or not?
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Old Sep 25, 2012, 04:53 AM
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So you're saying this company will produce this bird for possibly selling to the public, or not?
No - It was a one off project specifically commissioned for the TV series, but understandably they don't want too many trade secrets given away, so have the say in what and when information is released. If I want any future work with them I have to respect that!

Of course that does not stop me continuing to build models in a similar style and publishing information about them if I want to. - But before you ask, no I am not intending to go into production

Malcolm
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Old Sep 25, 2012, 05:10 AM
Blue Falcon
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ahh..John Downer. Yes, he makes the best video's ever! They already produced excelent video's about the andes condor.

Earthflight (Winged Planet) - Condor's Birds Eye View of the Andes (Narrated by David Tennant) (1 min 3 sec)
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Old Sep 25, 2012, 11:04 AM
What could possibly go wrong?
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African Vulture

Here's a picture of the real thing. Is there anything here that might help us to design yaw stability into our birds?

Nick
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Old Sep 25, 2012, 12:25 PM
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Originally Posted by nickchud View Post
Here's a picture of the real thing. Is there anything here that might help us to design yaw stability into our birds?

Nick
You can control a bird model by tip feathers that makes the bird stable too. Also a gyro is a big plus. The best is a central vertical fin. Here I fly the bob hoey's turkey vulture, this model is even able to fly without vertical fins.

RC Turkey Vulture Thermalling (3 min 11 sec)


Jaap Kooy's albatros with a vertical central fin:

Albatross soaring along the North Sea coast (2) (1 min 30 sec)
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Old Sep 25, 2012, 12:55 PM
What could possibly go wrong?
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Gyros?

That's an interesting subject. Where do we start? One of these?
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