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DooMaw - building a STOL


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I've had a few people show interest in the plane I'm currently building and they asked me to post a discussion about the build.

 

I've called it the DooMaw, on the basis that it's intended to be quite versatile and allow me to 'Do More' than just use it for transport from airport to airport. In fact it's intended to be mainly used on unimproved surfaces around the bush and the coast – a huntin', fishin', explorin' kind of thing.

 

The design was influenced by the obvious capabilities of the Just Aircraft Highlander SuperSTOL, but with a few additions.

 

I was very impressed with the reported docile handling of the Seabird Seeker which resulted from the 'Airflow kit' developed by Dafydd Llewellyn – there's a discussion about it on the do vortex generators really work thread, so I plan to incorporate some of that cleverness too.

 

The shortage of affordable hangarage is an increasing problem too, so DooMaw will have quick-folding wings. The best wing-fold I've ever come across was on a small parasol aircraft that forum member M61A1 had for sale about a year ago. It was called the 'The Mistress' and at little more than a pull of a button at the wingtip, the wings fold aft and rotate about the rear spar and strut, resulting in the wings folding flat against the fuselage sides, like a bird's wings, while still being supported by the struts – a truly ingenious arrangement, so I have incorporated that into the design as well.

 

The design parameters then -

 

  • STOL performance with heavily damped, sturdy long-travel landing gear and tundra tyres for 'smack-down' landing capability.
     
     
     
     
  • Staged stall control via the principles of the Llewellyn Airflow Kit for dependable low flight speed handling characteristics.
     
     
     
     
  • Less than 2 minute wing-folding (the horizontal stabiliser folds up against the fin/rudder also) with auto-connect/disconnect control linkages. This will make it easier to get shared hangarage because DooMaw can fit into a hangar next to another fully rigged plane. Alternatively half a dozen of them could fit into a hangar that would normally only fit one rigged plane, or it can be kept in a trailer and towed to and from the flying field.
     
     
     

 

 

Below is the video of the SuperSTOL performing the 'smack-down landing', for those who may not have seen it.

 

 

Below are some pictures showing 'The Mistress' and its ingenious folding wings – note that the wings are on the plane in the trailer, folded flat against the fuselage sides.

 

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I was fortunate to have a very experienced aeronautical engineer assist me with the member sizing for the structure and then I 3D modelled it in CAD so that I could produce CNC cutting templates later. Below are some images of the CAD model of the DooMaw.

 

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I started the build about six months ago, worked a month or so on it and then was delayed for four months by life's necessities and only recently got back to it. In the next few days I'll post excerpts from the build log to bring this thread up to date with the construction so far.

 

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Hi Alan, good to see you are back building again. Is the fuselage going to be built with 4130? I guess this will be discussed in your next posts.

 

Cheers Mike

 

 

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Thanks for showing your interest fellas, it makes it worth the time taken to put fingers to keyboard - and if the whole exercise perhaps encourages someone to build their own plane I would be really pleased.

 

I almost forgot to mention another of the primary design considerations - crashworthiness. Like a lot of others, I'm absolutely appalled at the fatality rate in our sport in recent years and consequently have given crashworthiness a lot of thought.

 

In my mind there are two main conclusions. A higher proportion of crashes result in fatalities these days, when compared with the proportion of yesteryear because the planes fly (and crash) much faster these days, and in general the modern airframes provide less occupant protection than the earlier ones did.

 

In the 1980s we used to mainly fly in groups, we kept our planes in trailers and took them to various airfields and paddocks, assembled them and flew all weekend, camping in tents or the trailers overnight. We had lots of crashes, so I don't think the number of crashes per number of flights has increased these days, it's probably reduced. We regularly had to help someone retrieve a damaged plane and load it piece-meal into their trailer. The difference appears to be that those crashes happened at slow speed in strong tubular steel or aluminium airframes so we rarely had an injury, let alone a fatality. These days the crashes happen at high speed in airframes that collapse structurally and offer little or no occupant protection, so they frequently involve fatalities.

 

With the above in mind, and in any case wanting a tough structure for a rugged bushplane, I am building DooMaw with a welded chromoly tubular steel cage fuselage since they are demonstrably stronger in an impact than other types.

 

Given that a bushplane does tend to get regular use close to the edges of the envelope and is therefore more likely to be involved in a mishap, I have also incorporated as many as possible of the other crashworthiness features I have come across, some of them were pointed out in discussions we had a while ago on this site.

 

The intention is to give the occupants as good a chance of survival, and preferably without injury, as if they were involved in a car crash at similar speeds. After all, we don't expect to be injured or killed in a car these days, except in a very major crash, so DooMaw has been designed from the beginning with crashworthiness as a primary consideration.

 

Bill Whitney kindly provided me with some very helpful documentation which came from the FAR23 certification of the Whitney Boomerang. I've kept all the flail clearances at least as large as were then demonstrated to be necessary, as determined in actual impact with crash test dummies.

 

There is also a large amount of stroke (empty space) available under the seats without any hard structure for the occupants' spines to hit, that space will be filled with memory foam to provide additional shock protection and progressive deceleration.

 

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 up 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 and skip along', rather than 'dig in' situation.

 

The overhead structure between the spar attachment points has crossed bracing rather than a single diagonal brace, helping to prevent the rear spar attach point on one side intruding into the cabin space in event of impact by the wings, going between trees for example, avoiding what has been a fatal flaw in some Cub crashes.

 

And the very nature of the landing gear provides excellent progressive deceleration, having long-travel pneumatic oleo struts and low pressure, large diameter tundra tyres.

 

Moving away from crashworthiness and onto the general design aspects - 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 the performance or handling of 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 which enhance downward visibility, an advantage for a plane designed for ground-related activities.

 

Onto the actual build – last year we moved home to a nice old elevated Queenslander half-way between the Gold Coast and Brisbane, with a bit more space than I had previously, and there's plenty of room for storage under the house. There's also parking for two cars under there and my wife's car just fits but unfortunately mine is too high and so has to live in the garage/shed in the garden which I'd hoped to use exclusively for building the plane.

 

Consequently I needed to be able to pull the car out to work on the plane and that meant building a mobile project bench which would be perfectly flat each time it was in position for working on the build. So the bench became the next design challenge and the first part of the build.

 

The bench was straightforward but I found I couldn't work in the shop to build it after midday. It was a very hot summer and the sun would stream in the open front door and rapidly took the internal temperature over 40C, so I had to build a roll-up awning first. That made all the difference and it stops any rain blowing in too.

 

That was in January this year, the pictures below should tell the story, they show -

 

New home

 

The shed awning

 

CAD model of the bench

 

Bench under construction using self-levelling laser and chains with turnbuckles to keep it all straight

 

Addition of the wheels and lever-lift for moving it about

 

Adjustable feet to take account of the uneven floor

 

By placing it on the same marks each time the bench is flat and level to within 1mm over the whole surface

 

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Next I added the bench tops and cut them to shape for the fuselage whether on it's side or upright - the wider end has four outriggers added to the steel structure to support the timber overhang.

 

The CRMO tubing all arrived in one tightly wrapped package so I had to cut it open to check the contents against the delivery note. To prevent any rust getting hold of it before I cut it all up I resealed all the different sizes in individual plastic sleeves and stored them under the house.

 

Then it was back to CAD work and produce some printed drawings for the forward fuselage section and main landing gear legs so that I could lay them out on a grid on the bench and install wooden blocking to hold all the members in position for tack welding.

 

More pics show -

 

Adding the bench-tops

 

The CRMO tubing in plastic sleeves

 

Printed drawings laid out for positioning the blocking

 

The blocking added and paper stripped away.

 

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Next was to print and cut out the paper wrap templates for the gear legs. I started with the gear legs as they are thicker walled tubing and would be more forgiving while I got up to speed with TIG welding again - it had been a while.

 

I opened up the appropriate plastic tubes that I had sealed the tubing into and then cut pieces of steel tubing to length for all the parts for the gear legs, then put the rest back and taped up their covers. Have to be very rust conscious here ...

 

I held each piece of tubing central in a vice and wrapped each end of it with its wrap template, held in place with a rubber band, and gave it a squirt of auto paint to mark out the notches. It was a very quick process and easy to judge the indexing alignment of each end by eye just by keeping the split in the template at the top (Pics 1-3 below). Four of the tubes were too short to be able to hold them in their centre and still have enough room to wrap the template on each end without moving the tube so I made an indexing line along the tube by scraping a sharp blade along it while holding it in the vice. I used an old electric planer blade which was easy to hold horizontal while running it along the tubing so it made a line that was true and didn't spiral around the tube.

 

Once the notches were marked and the paint dry I cut the bulk away with a thin cutting disc in a 4" grinder and shaped up the curves to the paint line with a 1/4" thick disc dressed to a rounded edge with a diamond wheel dresser. As long as I didn't try and grind too aggressively the paint stood up to the heat well and provided a good reference for accurate notching (Pic 4).

 

I laid the parts in the jig and they fitted perfectly which was very satisfying (Pics 5-8). I did have one small problem though. I had a software glitch that prevented me notching a tube end for more than one other interfering tube so where a longeron/chord, and two web members all came together the wrap template only showed the notch for the longeron and not the other web member, so I had to cut them by eye. Three turned out fine but one had a gap of about 1/16" so I had to remake that one (Pics 9 & 10). Later I sorted out the software glitch so it won't be a problem again.

 

The second last pic shows the completed members for the port gear leg in the jig and starboard leg stacked above.

 

I visited the welding supplies place nearby and re-established my Argon supply for the welding, tracked down the filler rods I need (ER70S-2 1.6mm dia), and I ordered online what looked like a nifty little device that mounts a small angle grinder, turning it into a mini drop-saw to make it quicker and easier to lop my tubing lengths.

 

The drop-saw attachment arrived a few days later and predictably it was among the worst of Chinese crap but was a starting point for making something useful ... (Pic 12)

 

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In the 1980s we used to mainly fly in groups, we kept our planes in trailers and took them to various airfields and paddocks, assembled them and flew all weekend, camping in tents or the trailers overnight. We had lots of crashes, so I don't think the number of crashes per number of flights has increased these days, it's probably reduced. We regularly had to help someone retrieve a damaged plane and load it piece-meal into their trailer. The difference appears to be that those crashes happened at slow speed in strong tubular steel or aluminium airframes so we rarely had an injury, let alone a fatality. These days the crashes happen at high speed in airframes that collapse structurally and offer little or no occupant protection, so they frequently involve fatalities.

Nicely said.

 

 

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On and off during February I rebuilt the little grinder attachment by re-working the hinge arrangement to much closer tolerances and equipped it with a new dedicated 5" grinder, mounted it all on blocks of wood with rails to feed the material in square and an exit rail with attached measure and stops for cutting repetitive lengths. All a bit fiddly to do but the end result is well worth it, making it a breeze to accurately cut the material to length before notching.

 

I installed the welder under the bench so that it would be always with the work and also out of the way. That was a mistake which I later changed, but is how it was for a while.

 

Before diving into welding the landing gear legs I ran a few practice lines and was a bit disappointed how much I'd forgotten so it took a fair few practice pieces before I was happy enough to start work on the real thing.

 

The next thing was to exactly position the main members in the landing gear jig (post#7, second last pic) and to do that I needed to have the bosses installed. The bosses being the pieces at the top of each of the main gear members which the bolts will go through to attach the legs to the fuselage. Naturally those bosses need to be in exact alignment with each other to allow the legs to hinge up and down without binding. I've had trouble with keeping them aligned in similar situations before, if they're not very firmly held in place they tend to get pulled around during the welding. Consequently this time I machined up the bosses and made a spacer bar to go between them temporarily - the ends of which I bored and tapped so that I could bolt the bosses to them until the welding is completed. It worked well and they were still well aligned after the legs were fully welded out.

 

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Tacking up the gear legs in the blocking jig, then label (in case I want to build another at some stage) and remove the blocking from the bench and weld out the first leg.

 

Using simple stands to avoid too much out-of-position welding, and aluminium foil to make argon dams to keep the weld well immersed in inert gas.

 

Once I got that far I had an enforced break of four months. Too much real work getting in the way and then holidays overseas, some of that spent in South Africa visiting and then travelling with another forum member and his wife, a fascinating couple. Before he retired he flew 747s for SAA and then went on to flying support missions for various militaries around southern Africa, he had some hair-raising tales to tell ...

 

His grandson is now an aeronautical engineer employed on the development of the AHRLAC (Advanced High Performance Reconnaisance Light Aircraft) and we were able to have a very privileged personal tour of the build/test facility. Unfortunately I wasn't allowed to take any photos because they had it stripped down for inspection after the first round of flight tests but I was able to see and learn some very clever construction methods they have developed.

 

For those who don't know of the AHRLAC it's a multi-role aircraft being developed in Pretoria by a private syndicate. It has a modular pod system that can convert it in minutes from its main role as a high performance reconnaissance aircraft to a personal transport or a fighter or a ground-attack configuration, or medivac and many etceteras. It has extremely rugged landing gear and can operate from a ploughed field with ease, a stock pic below, and here is a link to

 

https://www.youtube.com/watch?v=gqyrftQUKkg, a Youtube search will produce some interesting vids of the early flight testing.

 

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L.ove the bearing jig, and the wrap templates. A bit of CAD time obviously saving hours of fettling there.

 

The portable workbench is a good solution to limited workshop space. Mine was to leave the car outside to take its chances :) Are you happy that the (19mm?) plywood top will stay completely flat as is? From your pix there doesn't appear to be any supports under the outer edges. My concern would be that it may warp in your sunny Qld weather.

 

I welded my workbench from 50 x 100 box, then put a 19mm chipboard top on, followed by a second 19mm melamine board. That gave me a nice white plasticized surface to draw on, glue on, screw to, etc. It is 14'6" long by 4' wide (sorry about the mixed units - I'm building an American plane), and is flat & level to as near as dammit. It's also very solid! Pic of framework here:

 

http://i1303.photobucket.com/albums/ag145/beragooBruce/016Medium_zps1db12389.jpg

 

I am looking forward to seeing your STOL progress - more power to your Tig hand.

 

Bruce

 

 

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Thanks Soleair. I couldn't bring myself to leave the car outside, I've only just changed it over for a newer one after I left the last one outside for three months and it turned to a bucket of rust. The Gold Coast is terrible for rust-as-you-watch.

 

You're quite right about the (12mm) plywood top warping. I thought it would be OK as I had earlier added outriggers to the wider end of the bench and the narrow end only overhangs the frame by 150mm each side, I thought that would be OK and I wanted a clear edge for clamping. However when I got back to the project mid last month I found all the edges wavy by as much as 20mm! Previously the benchtop had been flat to within one millimetre. Before I put the front fuselage sides into their jig I added timber rails at the end of the outriggers under the edges of the wide end and that pulled it all back level again, I'll do the same at the narrow end when I need it flat for joining the two fuselage sides with the cross-members.

 

Yours is a nice solid looking bench - and great plane build pics following that, Minimax is looking good!

 

When I got back on the project in mid June (last month) I had decided to get a much smaller TIG torch. I'd put it off before because I'd been chasing one here for some while and could source various parts of it but not the whole kit so I just ordered the lot from China and when it arrived was good timing as I had some spare time again to get back to the welding, so I used it to weld out the other gear leg which I had tacked in February. It was a vast improvement and the gas lens and flexible head made welding a joy.

 

I also discovered 2% Lanthanated tungsten electrodes which I found I could run right down to 1mm diameter and they still handle the HF start nicely and coped well enough with the heat when you have to give it a high burst for tricky spots deep in the throat of a cluster. I was really pleased to change away from thoriated tungsten as I've recently lost a friend to cancer of the esophagus and associated parts and it's likely it was a result of long term grinding of the tips of thoriated tungsten. He was a many-year welder for the RAAF. Thoriated tungsten is sufficiently radioactive to carry warnings about the risks of their use and particularly the dust from grinding them.

 

I followed that with making the bosses that carry the axles, got a friend with a larger lathe than mine to broach the keyway into them and mill a matching key-way into the axle bolts (the key stops the axle rotating under braking loads) and set up a simple jig for installing the axle bosses at the right angles and welded them in.

 

Then I needed to make the cleats to weld to the sides of the axle boss which will pick up the bottom of the oleo strut so I modelled them in CAD, created a drawing of them full-size, printed the drawing and cut it out to use as a template. Held the paper templates to some sheet CRMO with magnets and a squirt of spray-paint produced shapes to cut out. Soon I'll need to get on and model all the rest of the cleats for the project and produce the CNC files so I can get them laser cut, it's pretty tedious doing them with an angle grinder and floor drill -

 

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This looks really interesting and up my alley. I am looking to build another aircraft that is STOL and the tail dragger config is also what I was looking at. What MTOW weight did you design it for and what do you hope the empty weight will be ?. The word is that RAA will be up to 750kg by the end of this year and if you designed it to this weight or could be increased it will allow a lot more design features as well as weight for fuel and people

 

Mark

 

 

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Alan, known as Head in the clouds, taugh me how to fly . I have had other instructors but he taught me the important stuff. My instruction that finished off my flying was by John McBride many years later. I have had the best instructors when learning. I will be for ever grateful.

 

Cheers

 

 

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.... What MTOW weight did you design it for and what do you hope the empty weight will be ?. The word is that RAA will be up to 750kg by the end of this year and if you designed it to this weight or could be increased it will allow a lot more design features as well as weight for fuel and peopleMark

Hi Mark. The old weight debate ... it's a tough one for any designer, there are many considerations. A 750kg MTOW is an increase of 150kg or 25% and of course that means that many people start thinking they can load an extra 150kg on the plane whereas actually it's less than half that. Considering that the 600kg MTOW plane has a useful payload in the region of 250kg the plane:payload ratio is 7:5 so a 750kg MTOW plane would have a payload of 5/12x750=312kg which in a practical sense is only a 62kg increase in useful load. Given that it's a larger plane and now carrying more gear you can bet your last dollar folks will want an increase in fuel capacity since your miles/lt will likely reduce a bit so if we allow an extra 12kg for fuel your pax and baggage increase will only go up by 50kg.

 

So what happened to the other 100kg? More structure of course, to carry the extra weight because the real extra weight is the full 150kg ... the old law of diminishing returns kicking in. OK, some will point out that as planes get larger they have an increase in payload percentage rather than a decrease - perfectly true of course but that is gained by changes in the technology used in the structure, Boeings aren't built the same way our sportplanes are, for example. At the other end of the scale consider the performance of RC models vs our sportplanes.

 

Does that mean I wouldn't design for the 750kg? At this stage yes it does mean that because of the massive increase of cost that a 750kg plane will have when compared with a 600kg plane. Only a tiny increase surely ... or only a 25% increase in cost I hear some more realistic folks say ... or is it? Anyone want to take a guess why our current 600kg planes cost more than twice as much as the original 450kg planes cost? An early Lightwing, allowing for inflation only cost around $50K in today's money because at that weight it could perform well with a Rotax 582, they're still available brand new at 40% the price of a 912 ... that's part of why we're now paying well over $100k median price for a basic LSA.

 

But wait ... will the 750kg plane fly happily enough with a 912? Well it would but folks being folks they'll see it as a bit sluggish compared to a 600kg plane so they'll want more power - the new 915iS will do nicely thanks, so add yet another $12k for the engine price increase (hell the powerplant now costs nearly as much as the entire new 450kg Lightwing). And don't forget we just lost another 15kg payload for the bigger/heavier engine ... uh oh we're down to a total payload increase of only 35kg for this 750kg plane that now costs 50% more than the 600kg plane - bloody thieving designers and manufacturers!

 

OK, a bit of a sardonic take on it perhaps, but not too far from the truth regrettably.

 

The thing is that going to 750kg is very far from just a matter of approving the weight increase. DooMaw would lift the extra weight with ease and would require little more than a slight beef-up of the main spar to retain its plus/minus 6g capability, and it would still do it within the 45kt stall requirement (at 600kg it'll stall at around 32kt) but it would no longer be a STOL. Instead it would be using its STOL features (lift enhancers, if you will) to be able to operate within the regulations but that's not what the aircraft is about. You'd end up with a plane with a small performance envelope since it would be flying overloaded, it'd take off much faster than intended and would have low cruise speed.

 

The best answer would be to design a different plane for the increased weight category. If one was determined to take this one to the 750kg category and still make it perform well it would need more wing area and that could only be achieved with an increase in span (if chord was increased it would need a longer fuselage, then taller gear to retain the angle of incidence on the ground). A span increase would have to be a greater increase than it might at first appear because if the parallel chord wing was just lengthened it would hit the ground when they're folded, so the tips would have to taper in the fashion of Cessna 172 wings. Not impossible but that span/bending moment increase would mean much heavier spars and struts. The fuselage wouldn't need any significant changes, it's already very solidly built to cope with the heavy landings and rough terrain.

 

So - is all lost? No, it isn't! We all know that weight is the enemy where aviation is concerned and especially so where STOL is the goal. I've shown above that a 150kg weight increase doesn't give you anything like a 150kg payload increase, it's more like 35kg given the extra fuel and heavier engine. Weight advantages aren't gained at all easily. So - we have to look at other ways to gain that extra 35kg. If we can do it in the 600kg category we can save a heap in costs compared with a 750kg plane, and that has to be a plus. Simply put, we need a stronger and lighter airframe than what you're flying now. I don't know for sure whether DooMaw has that but I'd wager it probably is at least 50kg empty weight lighter than a 701. I can't say for sure because I haven't completed the wing design yet nor have I estimated their weight. I do know that the fuselage, HS, fin, rudder and landing gear are all CRMO steel and the total weight of all of them is only 64kg plus welding filler and cleats, say 66kg, and the similar parts of the structure of sheet-metal planes I have been involved with have been over 100kg, so at this stage we're very comfortably ahead.

 

 

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Interesting observations. Your point on the escalation of structure weight to accommodate a larger engine could perhaps be addressed by using a 2-stroke engine.

 

I realize there is a deep seated mistrust - even contempt - for these engines from some, but I wonder how much of this is based on first hand experience with a modern 2 stroke, as opposed to 3rd or 4th hand anecdotal assertions.

 

Using a lighter engine means lower structure weight and therefore increased useful load. True this is offset to some extent by the higher fuel burn, but even here modern 2 strokes are considerably improved on say, Rotax 503 levels. Properly looked after & operated, 2 strokes can be as reliable as 4 strokes (perhaps even more reliable than some [pause to don flak jacket]). This is unsurprising considering a single cylinder 2 stroke has just 3 moving parts. And as you point out, they are considerably cheaper.

 

They don't sound as nice, though . . .

 

Bruce

 

 

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There's lots of other factors. There's really only one two stroke around A Rotax 582 and it's only got a 300 hr rated life. We know they can do a fair bit more but that discussion has been had before, and as time goes on there are less being used, so you become something of an oddity..

 

Other factors include: Noise. They don't sound like an aeroplane. They must be geared, They smell bad. They use a lot of fuel. They have heavy and often protruding into the airstream mufflers mounted on rubber that breaks. The 582 is liquid cooled. extra weight servicing and drag (generally but not always).

 

I'm not trying to bag them but just bring in a few considerations.

 

Power for weight they win, but a wankel type rotary could tip them off their perch, especially if you are chasing real horsepower. Nev

 

 

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Hi Alan

 

I understand exactly what you are saying. I agree also but it does all come back to your empty weight. My Sav is 320 kg empty and now is 600kg so can take 280kg in pax and fuel. I am currently 109kg (was 132) and the mrs is currently **** better not say but she has lost 20kg so we are now 209kg (do the math so I wont get in trouble) also in about 6 months both of us should be around 170kg so that leave us full 90kg for fuel so around 126 litres capacity for MTOW in a pretty good STOL aircraft not the best but up there but still pretty good.

 

The reason for the 750kg was I was looking at a Vision which is 750kg MTOW and is "supposed" to be every bit in specs as good if not better than the Savannah. Its empty is proported to be the 340 to 355kg empty it has much more room in the cabin and is slightly longer and a bigger wing. So I understand it would be heavier but if you can keep your new one at say 280kg MTOW then you are on a huge winner. A 701 is supposed to be around 270 to 280 so if you got it 50kg lighter that would be incredible and would be such a good aircraft.

 

Please keep us all well informed as I am sure you will have a line up for plans or kits if you are going to do that. I know I will probably be in that line up

 

Mark

 

 

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.... I am currently 109kg (was 132) and the mrs is currently **** better not say but she has lost 20kg so we are now 209kg (do the math so I wont get in trouble) also in about 6 months both of us should be around 170kg so .....

Congratulations to you both Mark, that's truly fantastic! Bloody hard to do but easier than getting weight out of an airframe. Until recently I've always been very fortunate in not being overweight but my job has been more sedentary recently and consequently I progressively lost sight of my feet ... I lamented that I had put on 10kg in six months and I just couldn't get rid of it again, even with plenty of exercise, and I didn't feel good either. My brother suggested I carry a backpack around for a full day with 10kg in it, just to give me some incentive. It worked, I've lost 9kg of it during the last 9 months.

 

All power to you for getting the rest off 107_score_010.gif.2fa64cd6c3a0f3d769ce8a3c21d3ff90.gif

 

but if you can keep your new one at say 280kg MTOW then you are on a huge winner. A 701 is supposed to be around 270 to 280 so if you got it 50kg lighter that would be incredible and would be such a good aircraft.

I didn't know what the intended EW of a 701 is but the one I weighed was 325kg, so we're in the same ballpark. I'll be quite happy if mine comes out around the 270kg mark but it may be a bit more or less. As I said I haven't any numbers for the wings yet and in any case I'm not building it to a target weight but to as light as possible without any compromises on ruggedness.

 

Please keep us all well informed as I am sure you will have a line up for plans or kits if you are going to do that. I know I will probably be in that line up

Thanks for the encouragement. I'm avoiding having any specific future intentions for it at this stage. It certainly wouldn't be a plans-only thing but a partial kit and plans for the rest of it is a possibility. I'm not likely to want to spend my whole time building the kits so it'd depend on finding people who might be interested in joining a business model along the lines that Jabiru developed. Theirs is a clever concept, where lots of people produce the parts in their own workshops and sell them to Jabiru who complete the assembly. At least, I understand that's how they do it.

 

It means they limit their capital outlay and make their suppliers responsible for QA and production efficiency, and no problems with employees, wages, insurances, workplaces ...

 

 

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I've built three steel fuselage aircraft before and always had the same problem, rust coming through the frame paintwork after a while. Admittedly it has been because of poor preparation for the painting and using the wrong paint, on one occasion the dope applied to the fabric ate into the paint ... and in other areas the paint hadn't adhered well and could be scraped off easily.

 

Part of the reason for the poor preparation is that it's a real PITA abrading the airframe once it's all welded together, getting into the all the nooks and crannies is a real problem. Additionally cleaning all the mill scale off the areas to be welded is quite difficult, so this time I decided to take a different approach and get the scale cleaned off all of the steel tubes before they were notched for welding, and then get a coat of primer on it to prevent it rusting. I could easily enough clean the paint off the areas that will be welded when I was ready for that, re-prime them after welding and eventually apply a two part epoxy topcoat.

 

At the suggestion of a friend on another forum I put the tubes in the lathe, spun them fast and wrapped sandpaper around them. It ripped the scale of beautifully. Then I went to finer and finer sandpaper (to 180 grit) and finished them off on a wire-wheel in the bench grinder. Painted them with the suggested Zinc Phosphate primer and was ready for the next stage - so I thought. I didn't get back to them for a week but even having cured for that long I found I could easily scrape the primer off with my fingernail. I use the same primer in aspects of my work and it adheres tremendously well to mild steel so I reckoned it was most likely the chromium in the chromoly that was making it so hard to get paint to stick well.

 

I arranged a visit with the paint company's chemist to see if he could shed some light on what was going wrong. It turned out the preparation was 'too good' meaning that the fine git and wire-wheel had left the surface too smooth and therefore without sufficient keying and also he suggested that a self-etching primer would be better. Etching primer is usually only used for non-ferrous metals like aluminium, zinc, brass, copper and stainless steel, and it's used on stainless because of the chromium so it's a reasonable assumption that some of the problems I have had have been due to the chromium in the chromoly.

 

So - the tubes went back in the lathe to sand all the initial primer off and were then masked to avoid having to clean off the areas to be welded, and then painted again with etch primer. Much better adhesion this time.

 

Pictures show the descaling of the tubing, painting, easily removed first primer, PVC tube 'bath' for washing the tubes in thinner prior to painting and the second round of primer -

 

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What about sandblasting the completed frames or complete airframe then getting it powder coated. We get lots of stuff we make here that sits outside on wharves and water treatment plants and we get them all power coated and never have any issues with any corrosion on the cabinets. I may add a little extra weight but would seal off air to it and it shouldnt rust.

 

 

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Powdercoating is great but not suited to building a one-off over a period of time as I am with this one. If it was a production run it would be a different matter. There are two problems - first that sandblasting of thin walled tubing can be a showstopper if you don't do it yourself using low air pressure and working slowly. Garnet or even glass bead blasting can be very aggressive so if you send it away to be done you might end up with a destroyed airframe. Home garnet or bead blasting isn't safe (or legal in most States) unless you have a proper booth and respiration equipment, and who wants silicosis anyway? Soda blasting isn't aggressive enough to provide sufficient keying to the surface.

 

Then the second part of the problem is the time it takes to complete a one-off build if you have to fit it around 'real work'. This project has already taken 9 months since the steel arrived here and by the time I weld the last cleat on and could get it powdercoated it would be a bucket of rust here on the Gold Coast. With sweat off your hands in the summer, the constant salt-laden south easterly trade winds and the damp winters you simply have to manage the rust protection as you progress through the build. Hence my process of painting the majority of the steel as I go and then the welded areas as they get done. I can always clean off some primer to add a cleat or whatever and re-prime that area afterwards.

 

In addition to that I have every part covered with plastic sheets and tarps whenever they're not being worked on and especially at night. There's more than just the corrosion prevention consideration, keeping the areas that are to be welded bright and clean makes a huge difference to the quality of the welds too, so there is a lot of incentive to be particular about this.

 

We're getting close to catching up to the present time with these log excerpts now - the next stage was to manually abrade and paint the landing gear legs. I hadn't de-scaled the tubing for them in the lathe so that was a job that cost a fair bit of skin off the fingers, back-ache and some muttered obscenities.

 

It seems a lot longer ago but just two weeks ago I finished wrap templating and notching the ends of all the tubing for both of the forward fuselage sides. Once they were notched they were too tight a fit so I then had to adjust them slightly to fit them up in the wood-block jig which I had set up in January. That's when I discovered the unsupported edges of the ply had warped a fair bit and had to add some timber bracing underneath which pulled it back straight again.

 

By that stage I had spent 135 hours on the project (apart from 2 years of spare time designing it) but included in that is the time to make the bench and little dropsaw and setting up the welder.

 

Pics of the painted gear legs, keyed axle bolts and the first side of the forward fuselage in the jig waiting to be tacked -

 

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As I mentioned earlier, after I used it a bit I didn't like the arrangement of having the welder under the project bench so a couple of weeks ago I retrieved an old wheeled cabinet from my storage, refurbished and painted it and turned it into a dedicated welding trolley. Now I can move it around the shop easily and have good access for changing the welder settings, a rack of sealed tubes for the different filler rods, the cupboards below contain all the torch accessories and from a surplus shaded pole motor I also made a dedicated small wheel grinder for dressing the tungsten electrodes.

 

During last weekend 17-19th July I tacked the first forward fuselage side together, removed it from the jig and on Saturday fitted up all the parts for the second side, tacked that one, labelled and removed all the jig blocks and tacked the other side of both frames.

 

On the Sunday I completed the welding of one of the sides. The welds were quite pleasing, and still improving -

 

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Total hours to that stage were 149hrs.

 

 

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