Birdwing BULM

my own BULM proposals
This time I am only sketching BULMs (Basic UltraLight Motorized). And now I tried to find a way to get a BULM parked inside your garage without the need for complex hinges, impossible folding systems and so. I wanted just a way to get your BULM inside and outside your garage and ready to fly with not much work. And ... it all had to be simple to built.  

Using nature as inspiration can lead to designs like you will see later on this page.

Basic idea:


Use a straight wing without sweep or dihedral. The maximal wing dimensions would normally become: 5m (16.40 ft) by 2,2m (6.56 ft). And that might not be enough as wing area and ... a straight wing does not have a good efficiency. Adding some extra surface seems the only solution. But ... how?

Adding large wingtips is not what I want. Mounting large parts on a "delicate" wing is not a one-man-job. I thought about mounting "feathers" like the Birdwing has. It might do a double task:

  • Adding surface
  • increasing the efficiency of the wingtip to normal standard.

Now you still have two possible versions: Pusher or tractor prop. I tried to make a proposal for both.

Proposal 1:

I add a copy of a mail I sent to the Fly5k Yahoo group.

A in-flight drawing of this proposal.Note the retracted prop-protectors.

"Hallo everybody,
My latest brain spawn is in the Birdwing configuration. Sources of inspiration are:

  • Pelican (Go see my Pelican page)
  • Superfloater (ultralight glider)
  • Mosquito harness for hanggliders

Simply stock the central part in your garage and fit the feathers and V-tail at the airstrip. I guess it can lead to a rather easy and light construction. I think about using wood to be able to make it myself. The ribs are all the same. So, rather easy to make. I hope to increase the efficienty of the wing by the useof those birdfeathers at the tips.
I use the "cockpit" lay-out 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 it 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.Just after take-off. The legs are still hanging. the prop-protectors not retracted yet. I am stil 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 inthat split V-tail."

Proposal 2:

The second proposal has a tractor prop and a enclosed cockpit. I hoped it to be a bit winter-proof. Here I use some other thinking.
I add a mail I made about this proposal and the reply I got from Bob Hoey, who is working on birdlike models for years.

"This creation is my last brain spawn. It comes from the idea to get a ultralight 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 hisGOAT glider. He quickly changed his opinion).
I use a easy to make constant chord central wing and I add smaller wingtip feathers to increase the span ... and hopefully the efficiency 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 theairstrip to get it in flying condition. The feathers ... well... I am still hoping to find easy mount and dismount systems in others 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, because it has to be dismountable. But ... It will still 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 garage box. I guess that the main design idea in this concept is: low wingloading (as the Vulture, Sea Eagle, Condor), higher efficiency of a rather square wing by those wingtip feathers, easyto make hinges and not a hard work in mount/dismounting the parts."

I added these drawings.

A side view of the proposal. Note: a square is 20 x 20 cm.

I hope to show here that the outside view still is OK. You see the landinggear which is inspired on the Henri Mignet's Flying Flea.The sides are very straight to be able to built it in wood. The pillars between the windows canbe reinforced as they do with the Skypup. There they use foam between two wooden parts to make the cockpit pillar. Go see some construction pictures at . OK, I added a round edge between the pillars. I hope they add strength, but they sure add ... looks and a more natural shape.

A topview. Here you see that the top of the cockpit is transparent. The rudder has a part in front of the rudder hinge as well. You see also that the feathers are not the same length. Go see the Birdwing page to see why.

A detail of the last feather. You see that a small part of the rear feather still hangs on the main wing. The mentioned hinge will be placed there. The part outside the main wing has the airfoil of the aileron.

Reply of Bob Hoey (Go see some of his models in the TWITT-site, here is a direct link):

  1. The wing aspect ratio 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.
  2. Don't expect anything dramatic by way of increased efficiency from the bird-feather wing tips. I don't think they are aerodynamically any more efficient than a normal tip. The trailing-edge-feather elevons that you show will produce a lot of adverse yaw. Since you show a rudder, you can link the two and probably create a reasonable turn capability, but I suspect you may have trouble if you also intend to use those surfaces for pitch control (tip stall, etc.). You might consider using the forward 2 or 3 feathers on aspan-wise axis for roll control (as I have been doing on my bird models), and a separate, inboard trailing edge surface for pitch control.  The advantage of using the forward feathers is that the neutral or zero setting for both feathers can be adjusted by rigging to eliminate any adverse yaw.[ed.: I didn't mention in my letter that I only hoped to get close to the efficiency of a rounded wingtip.]
  3. It looks like your placement of the pilot is a little too far aft. I suspect that your cg will be behind the best starting location of about the1/4 chord. Even if the airplane can be balanced by the engine and fuel,your cg will vary greatly with the weight of the pilot. Try to put his belly button directly under the 1/4 chord.
  4. Regarding the rear tip feather never being stalled, I suspect that is not true. The flow over the rear feathers depends on what is happening on the forward feathers. There is no magic formula for determining the relative angle of incidence of the individual feathers as you progress aft. What is right for one condition may be very wrong for another condition. (of course the birds can vary these individual angles at will!). What I am saying is that a stalled condition on any of the forward feathers (either from toomuch positive, OR NEGATIVE angle of attack) will mess up the flow over the top of that rear feather.
  5. The overall configuration has some merit, and would be worth building an R/C model to establish the best control system, airfoil, cg  location, etc. You could also incorporate some of your hinging ideas to see how they work.

Good luck!!
Bob Hoey

Well, these are the proposals this far. I hope they can inspire somebody. It might be that this proposal will become my second project to be built. But before that I will need to do a LOT of RC models. And ... I need to restart that hobby from zero. It is 15 years ago that I built and flew a RC trainer. Man, what a challenge it will be.