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DooMaw - build log

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Head in the clouds

Well-Known Member
Joined
Mar 11, 2012
Messages
1,981
Location
Gold Coast, East Coast of Australia
Here we go again ...

I ran into a mental block with AussieMozzie. Among other things I had one problem after another with perceived engine cooling issues. The external plumbing for the liquid cooled heads on the R912 creates a real headache for making plenums or ducting to get air to flow reliably around the fins on the barrels. Not so much of a problem on a tractor configuration but a real nightmare for a pusher installation.

I also came to dislike the control system, I'd underestimated the size of torque-tubes for the aileron and elevator actuation so both of them are too springy and might be prone to propagate control surface flutter - and it'd be very difficult to increase their size without a series of major modifications because they're so embedded within structural members or passing each other with minute clearances. And - while my plan of installing the joystick and associated linkages in the central structural tunnel seemed like a good idea at the time because it kept the controls free of the risk of foreign object interference, it also made them very difficult to inspect and service. They're all good lessons learned for next time but in my case have probably cured me of pusher aircraft for good.

Then I lost more friends over the last two years, they were highly experienced and in crashes that could have been survivable if they hadn't been in spam cans that collapsed on them - and the Moz is worse than most in that regard, being a rear-engine spam can. For some time SVSUSteve's constant preaching about crash-worthiness had been nagging away at me and I guess this was one of the last straws.

I've also have had so little spare time lately which has meant that the Moz build took much longer than intended and in the meantime my requirement of aircraft type has changed ...

Much as I hate not completing a project I have decided to move on and build something entirely different. For many years I have had a hankering to build a really rugged trailerable bush-plane with 'extreme' STOL capability. Something that could take me to some of the places I used to be able to get to back in the grand old days when I had helicopters. If it was to be very docile at low speed, with no tendency to bite by dropping a wing when stalled or at minumum controllable speed, then there'd probably also be a market for such a beast on the cattle stations.

My first insight into the real possibilities came about when I saw this video of the Just Aircraft Highlander SuperSTOL.

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I think what they've done in making the craft controllable and docile while operating well behind the power/drag curve, is remarkable - and the plane is clearly capable of getting into the confined spaces in remote areas that I have in mind. Consequently, whilst being my own original design, mine does have a fair few similarities to the Highlander. I hope they won't be too peeved - after all, imitation is the sincerest form of flattery.

As far as steel sizing and structural calcs are concerned I've had the enormously time-saving advantage of having Bill Whitney, one of Australia's most regarded aeronautical engineers, give me permission to use plansets of two of his similar-sized designs, so I have been able to lift a lot of his sizing and adopt some of his clever design concepts for the tubing structure. Bill isn't known for the lightest possible structures but they are very strong, several of them are FAR23 certificated, and designed for limited aerobatics. Consequently mine won't be as incredibly light as the SuperSTOL but it will be stronger, and it's certainly still a lightweight - the combined steel structures of fuselage, landing gear, fin, HS, elevators and rudder all come to just 64kg/141lbs.

This will have slats or slots, I've not decided whether to make them fixed or auto-deploying yet, or they could start out fixed and be changed later perhaps. Also large slotted flaps and long travel highly damped suspension with medium sized tundra tyres. I've incorporated as many as possible of the other smart design concepts as I have come across over the last 2-3yrs. It will have my version of Daffyd Llewellyn's (another of Oz's respected aero engineers) Airflow Kit which he developed for the Seabird Seeker, and which allows that aircraft to be flown fully unco-ordinated with full back-stick, or to be thrown into a hard turn at marginally low speed and stalled whilst doing so, and still not drop a wing. It's a very clever concept, the description of which can be found in the Do vortex generators really work. thread on the Australian Recflying forum.

I've kept all the flail clearances at least as large as were demonstrated to be necessary for the FAR23 certification of Whitney's Boomerang, as determined in actual impact testing. There is a large amount of stroke available under the seats without any hard structure to hit. The steel fuselage floor structure under the seats is double layered by having the gear mounting truss below the tubular fuselage floor structure, and the forward part of the lower gear structure forms an angle to the bottom edge of the firewall helping to prevent the firewall digging in and causing a sudden stop in event of a bad arrival. The engine mounting structure continues that angle forward, further improving the 'bounce off', rather than 'dig in' situation.

The overhead structure to the spar attach points has crossed bracing rather than a single brace preventing the spar attach points intruding into the cabin space in event of impact by one wing or the other, avoiding what has been a fatal flaw in some Cub crashes.

I'm using the BD4/Tailwind style of square windshield/firewall front end purely for simplicity in construction and jigging, and it certainly doesn't seem to disadvantage those particular designs. That shape also provides more foot and leg-room which has allowed me to reduce the overall fuselage width to just 36"/915mm and the extra shoulder room that will be needed as a result will be provided by bubble doors. That also enhances downward visibility which is an advantage for a plane designed for ground-related activities.

A further difference which sets this design a little apart from anything else that I am aware of, is the wing and tail folding mechanism. Early last year I came across a plane called "The Mistress" that was for sale in Oz. It was designed by a gifted German toolmaker and has a wing folding feature that makes the folded plane into a more compact package than any I have seen before. With the benefit of CAD I have been able to enhance his concept a little and devise a system which will allow the wings and tail to fold in a genuine 'less than 2 mins', as well as having complied with Autoreply's insistence about the need for the controls to attach and detach automatically to avoid any possibility of the control connections being neglected while assembling the plane prior to flight. This will also address the situation where non maintenance-approved pilots might be expected to have an engineer to sign off the plane after each fold/unfold.

The difference between this wing folding and others, apart from its almost 'press-button' simplicity to operate, is that the wings and HS end up folded flat against the side of the fuselage while still being supported by the struts (see pics of the Mistress below - notice the picture of it in the trailer, the wings are lying flat against the side of the fuselage). This means that the widest part of the folded package is the wheels and they can easily be kept within the maximum width for standard trailers on the road. Other folding systems where the struts are not removed generally result in the wings simply rotating back around the lower strut attach point and the rear spar - the Kitfox/Skyfox being good examples (see pics following) - but that method imposes a restriction on the wing chord if trailerable width is a consideration. With the wings folded flat against the fuselage there is no chord restriction, but admittedly there is a fair bit more hardware required to achieve the folding geometry.

Amidst the 'real work' I have to do on weekdays, when I finally get this finished I'm hoping it's a plane that'll allow me to do more than other types might, so I call it DooMaw.

Since folks can't comment on the build log I've started a discussion thread as well - DooMaw discussion thread

The images below show -


The CAD model of DooMaw
The Mistress - showing wings and HS folding against fuselage sides
Kitfox wings folded


snap_037.jpg snap_038.jpg snap_044.jpg

95-10 010.jpg flying 141.JPG 20140215_132225.jpg

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