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.
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
Now you still have two possible versions: Pusher or tractor
prop. I tried to make a proposal for both.
I add a copy of a mail I sent to the Fly5k Yahoo group.
My latest brain spawn is in the Birdwing configuration. Sources of
- 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. 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."
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
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.
Reply of Bob Hoey (Go see some of his models in the TWITT-site,
here is a direct link):
- 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
- 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.]
- 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
- 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.
- 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.
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.