This is a series of letters between enthusiasts wanting more information on the use of "tip feather" type control systems for both models and full sized aircraft. Each of the letters has been answered by Bob Hoey, and we appreciate him taking the time to help with his comments and observations. Click here for more of Bob's work.
February 1, 2008
Hello Bob. Love your bird models.I have your plan for the Turkey Vulture and have yet to build it. Might try to have it ready for Soar Utah this September.
.......I have some questions about designing birds:
<>I assume the structure for all your birds is basically the same with the exception of the wing-tip controls. On the seagull and pelican, did you place the wing servos next to the body as you did on the vulture? I would think the complex dihedral/annhedral/swept spar would make it difficult to run a carbon rod from the fuselage to the wing tip. How did you do it? Was the servo placed at the outside wing beak?
Did you use the same airfoil for all your bird models? I am considering designing a albatross with a conventional structure (wood). I think that the basic structure of the vulture might work. I would redesign the wing to be a two piece affair for transport. Also, I would like to keep the wing-tip controls like your birds and not use ailerons. Are the twisting wing tips on your seagull effective?
Thanks for reading this and thanks for your time. <>
Jeff Meskey <Jeff.Meskey@SSAMarine.com>
You sound like an "experimenter" like me. An Albatross should be a good, but challenging, project. I can probably give you some tips that will help out. First I'll answer your questions directly.
Yes, the basic structure is the same for all, except for the wing spars, which I'll describe later.
The Seagull has servos at the wrist joint, and carbon torque rods from there to the tip aileron (like the vulture). It works OK, but goes against the desire to keep the wing inertia as low as possible (to keep the roll oscillations down).
The Pelican has the servos at the center section of each wing with a standard, flexible push-pull tube that operates a bellcrack near the tip. That is what I would suggest for your Albatross. There is a little more slop, but it works just fine.
Until recently I have used the same airfoil for all the birds - undercamber with reflex at the trailing edge. It works well for low speed thermal soaring but the undercamber provides a lot of drag at higher speed. I recently tried an airfoil that has the same upper surface as the Turkey Vulture model, but the bottom surface is flat between the leading edge and the forward edge of the trailing edge piece. This removes the undercamber, but retains the reflex at the trailing edge which is necessary for pitch trim. It flies just fine and seems to have better penetration at higher speed. Should work OK on the Albatross. (which, of course, is a pretty high wing loading, fast bird).
I agree with a 2-piece wing for transport, and also to allow you to change the dihedral by bending the connecting plate or tubes.
The seagull tip ailerons work just fine and will allow you to select a trim bias position that will eliminate adverse yaw that you would get from ailerons.
Now some suggestions;
(1) You should probably build in 3 or 4 degrees of washout in the outer wing panels outboard of the wrist joint. (I have 4 degrees on the Seagull)
The "sweet spot" for the seagull model is with the tip aileron at a -7 degrees incidence with respect to the wing center section. ( best coordinated turns)
(2) The wing spars for the Seagull and Pelican were built up as follows; The main spars (full depth) were cut from a sheet of 3/16 (or whatever, balsa) to the shape of the wing dihedral (front view). A 1/16" by 1/4" cap strip was glued to the top and bottom of the spar. A strip of .007" by 1/4" carbon ribbon was glued to the 1/16" cap strips. When the structure is completed, the wing is sheeted, top and bottom, with 1/16th sheet balsa forward of the main spar to the leading edge. This traps the carbon ribbon between the cap strip and the sheeting which produces a very strong, light weight "D" spar wing and helps retain the complex dihedral shape over the span of the wing.
A little bit complex, and very difficult to build without warps, but no one said it would be easy!!
All birds are pretty short-coupled in pitch- especially the albatross. And it doesn't have a very large tail. The reflex in the wing arifoil will be pretty important to allow you to trim it.
I have found that starting with the little round disks (mounted near the wing trailing edge, angled in about 20 degrees) helps to provide some level of directional stability during the early flights. The final tweeking (eventually eliminating the disks) should be done by adjusting the total wing dihedral. It only takes a couple of degrees to change from an unstable airplane to a marginally stable, but nice flying model.
I have attached some photos to help with the above comments. I have photos of the construction phase of the Seagull and Pelican model, but they have never been published. (Click on this link then go to the bottom of the Seagull and Pelican pages to view the images.)
If I can help in any way, let me know, and by all means, let me know how your Albatross flies!!
August 2, 2004
I live in South Africa, fly radio control gliders, love watching soaring birds and am fascinated by soaring flight. I happened across your bird models on the TWITT page and really enjoyed your very realistic looking bird models and must say that you did a good job on the building side too, with those compound curves. What grabbed my attention was the use of the tip feathers to do roll control. I made a 1/4 model and then later a full scale 10m glider with a team and entered the Red Bull Gives You Wings Competition of 2000 and 2002, using winggrid as tips for reduced induced drag and trying to optimize for slow flight and short span. On this glider i used the front winggrid for roll control almost like a spoiler, creating drag and reduced lift on the side i wanted to turn too,
figure 3 and 4 shows pictures of
and the actual glider). I would like to have more information on your
feather control system if any is available.
using positive pitching airfoils
rearward cg and fly by wire pitch stabilization, I was wondering if you
have considered trying this type of application on your models.
especially Figure 11, I realized that if the cg position could moved using the trim position as reference – resulting in no trim deflection (cg is adjusted as the center of lift moved from slow flight to fast flight), drag could be further reduced. The other method to do this of course is to adjust the wing planform in flight - move the aerodynamic center (moving the lift center relative to the cg), like the Exulans
This of course is not a problem
for a bird
in flight, and I think that birds therefore do not need reflexed
- resulting in less drag.
August 16, 2004
Andy at TWITT forwarded your
It sounds like we have a lot in common, or at least are both pursuing
same interests. One of my personal failings is that I'd rather
it myself" than do web research and find out what others have done. A
of years ago I DID spend some time looking at the "winggrid" concept. I
came away with SOME understanding, but an awful lot of puzzlement.
Stay in touch,
July 20, 2004Hello Everybody:
My latest brain spawn is in the Birdwing configuration. It comes from the idea to get a BLUM (Basic Ultralight Motorized) in a garage without a lot of work. No need to fold the wings, no need to dismount and carry large parts (like Mike Sandlin did with his GOAT glider. He quickly changed his opinion). Sources of inspiration are:
1) PelicanI use a easy to make contant chord central wing and I add smaller wingtip feathers to increase the span ... and hopefully the effiencienty of the square wing. To get the wing in the garage you need to flap the rear part of the wing upwards. It looks like a control surface, but it isn't. It is just a part of the wing that can be folded upwards to reduce storage size. This folding can be done by a easy piano hinge (I guess). No heavy or difficult hinge. If that part is folded upwards, the length of the airplane is reduce to 2,2 m!. OK, you need to remove the tail as well. But I saw that the Flying Flea tail is so easy to dismount and mount I want to use the same system. Again, not a great deal of work to create and so do at the airstrip to get it in flying condition.
........The feathers ... well... I am still hoping to find easy mount and dismount systems in other sports. That I still need to work out (in the basic concept idea). But, you will get easy to handle small parts. The elevons are hinges at a small section of the rear spar of the wing. So ... the first part still has the last part of the wing, but there is another part connected to that part and that is the rest of the elevon. Why this system? Well, I hoped to avoid complicated hinges this way. OK, it will not be a piano hinge, becauseit has to be dismountable. But ... It willstill be a easy hinge. The Flying Flea tail hinge still as inspiration.
........If all is dismounted you have a total dimension of 5 m by 2,2 m. Good to fit on a trailer and be pushed in a garagebox. I guess that the main design idea in this concept is: low wingloading (as the Vulture, Sea Eagle, Condor), higher efficienty of a rather square wing by those wingtip feathers, easy to make hinges and not a hard work in mount/dismounting the parts.
........I use the "cockpit" layout of the Superfloater. Anything easier ... send it to me. You will have noticed that the prop is rather large for this skid and wheel configuration. I hope to get the prop more efficient by giving it a wider diameter and to prevent the prop from touching the ground at take off. You need to run while two bars (those green things at the bottom of the wing) are tilted downwards and support the engine and prevent the prop from touching the ground. Once in the air, you place your feet on the footrests on the skid and you retract the green bars. Landing? I suggest to use a folding prop and just land on the skid.
........I am still doubting what kind of airfoils I should use. Autostable and have a tail with not so much negative AoA (angle of attack) or use a more lifting flat bottom airfoil and have the pitch moment corrected by a "classic" V-tail. Euh ... I think more about the autostable airfoil. Reason: I hope to make elevons in that split V-tail.
Now my questions:
1) I noticed in the TWITT that Bob Hoey uses -27° to 0° AoA for his set of birdfeathertips of his Pelican model. OK, those will give not much lift. Can they be counted in as wingsurface? I do mean ... I need about 12m2 (+/- 129 sq ft). Without those tips I have 1.7m x 5m (=8.5 m2) of surface. Can I make the rest in those feathers?........All comments are welcome. I will be off for some weeks soon and I hope to use your advise while drawing in my sketchbook during the holiday.
Koen Van de Kerckhove
August 14, 2004
Here are some off-the-top-of-the-head thoughts regarding your BULM Proposal.
(1) The wing aspect rato looks to be about 3.7, which is low, but probably OK. The good news for a low-aspect-ratio flying wing is that the chord is fairly long which results in a longer moment arm for pitch stability and controllability. The bad news is that the angle of attack range is quite large and you will need a LOT of power to take off and climb. You will have to use a reflexed airfoil, and it appears from your drawings that you have done that.Good luck!!