Development of a dream

Hi Guys, having introduced myself a couple of days ago (did you read that?) I have now opened an album for pictures of my single seat, mid wing pusher, with a difference. Before you leap there though, a bit of back ground info.

Here in Oz we have our (Recreational Flying | The most popular home of Recreational Pilots and Ultralight Flying Enthusiasts) forums and some time ago some of our members tried to design an aircraft by committee, so to speak. Well, the inevitable happened and everyone put forward their ideas, wants, wishes etc etc. but no basic guide lines or rules were established.

Consequently, nothing came of it. I was still interested in designing something that a rank amateur could build. Something that was a bit more modern than the old Thrusters and Drifters, but not as involved and time consuming as a full metal, or composite. Whilst us in the know would be ok with building in wood, this might seem like 'old' technology to 'newbies'. Therefore, Rag and Tube seemed to be the way to go but, could everyone weld. No. So a steel frame was out. I opted for Alloy tubes that could be bolted, or riveted together as a start. I had a dream for something different and thought my ideas might also be appealing to others as well.

I proposed the forum might like to be involved in the development of my design but very little came of it from the older (wiser?) members. It was voiced that litigation as to who was ultimately responsible should the craft be built and fail in flight was something to be avoided. The younger set, quite liked my idea so I kept working on it and am now on Mk 2.

I have been reading your posts for quite sometime and I like your approach to the subject. BTW, thank God you didn't sub-divide this particular forum. Without your knowledgeable input, there just might be at least one fellow dreamer heading for disaster.

OK, here we go. First view Picasa Web Albums - Doug - Ultralight De...

Now, some more info.

You may ask why position the prop where I have it. All I'm going to say is 'Why not?' It's only drawback is cost and the need for some of us to have to farm it out a bit. (subcontract)

The tail feathers will be removable as a one piece assembly.

The boom tube will be internally strengthened as per the wing main spars.

Power will most probably be from an 1800cc VW flat 4, set fairly well back to counter the forward position of the pilot. I want that to be as far forward as possible the give a better rearward, under wing view. Ever lost the field on late downwind? Prop will initially be driven via multiple V belts.

My original intent was to use a ply/foam sandwich for my ribs but was kind of guided away from that idea, so then modeled solid ply. Having now read the comparison tests here within your forums, I will revisit that plan.

I've simplified my flaps. The original Youngman type had their merits but involved too much work for a beginner. They would also come too near to the prop for my liking.

So, what do you think. Viable? safe? If you're knowledgeable, be brutal, I can take it

Just one thought among many:

I am not sure about what your goals are with your main spar design. I assume you have a cantilever wing, from the drawings. You have three tubes inside the main outer spar tube, presumably riveted to the outer tube? You show one on top and two on the bottom. If this works at all, it would be better to have two at the top and one on the bottom to better resist compression in the top and tension in the bottom of the spar. But perhaps you have a reason for doing it this way?

What is your reasoning behind this kind of spar?

Interesting. I recall a small, single-seat canard aircraft that had an engine and prop installation virtually identical to yours, back in the mid-eighties. It flew as far as Oshkosh one year (and appeared in Sport Aviation), and then vanished from the scene. Fatigue and/or weight in the prop hub seems like it would be the major stumbling block, or possibly exciting a resonance in the tail boom at certain throttle settings.

I'll echo PT's concerns about your spar design. Innovative, but inefficient. A conventional cap-web design will be lighter and much stiffer. At the very least, invert what you have, per his suggestion.

And since the three tubes don't seem to be connected outside of the big tube the only shear path is the ribs, which will most likely fail. Once outside of the main tube the three tubes should be connected with webs.

However if weight is your goal and you still want to stick with tubes, you can just do the round tube to the tips and using a hole saw, cut out a series of circular plugs along the span to reduce excess weight.

Check out this French site for a simple ply spar and some other really good tips ( use translator to view text )
www.quaetaers.club.fr/index.htm
And this excellent German website about building a PIK 26 ( its in good English and German )
www.pik-26.de/index.html
And this French site about building a 2 seat wooden aircraft
www.gazaile2.nmr7.free.fr/
A good Composite web site about building a KR@ look a like
www.krsuper2.com/
This good site about the Colombans Cri Cri metal aircraft
www.cricri.zoomshare.com/12.shtml
And the SD Minisport a wooden single seater
www.sdplanes.com/sd1specsa.htm

Lots of photos and good building ideas
Flyoz

My thoughts on the spar design.

As I was aiming this project at the younger, and less patient (include me), members of the aviation world, I thought that something that was quick to show progress was needed, and interest thereby maintained. How many incomplete builds are there out there due to lost interest and drive? Using tubes seemed to provide part of the answer. I realized that a single tube wouldn't be stiff enough so came up with the internal bracing. Weight was on my mind when I decided to terminate the main tube just prior to the stress-wires crossover point. I don't like the idea of putting holes through tubes (weakening?) and thought there was sufficient length stiffened. I haven't show any bonding methods yet. I've considered epoxy clues for the tubes to spacers but my latest idea is to drill a pilot hole through the tube and into the spacer, indent the outer hole, then insert a coarse threaded s/s self tapping screw through the lot. Once home and secure, smooth the head off to blend with the tube profile, making sure not to remove too much and losing the heads holding power. I tossed up as to use 3 or 4 inner tubes but came back to the weight problem again. I haven't done much research in this area yet. For modeling purposes, I have used a 2mm (0.078") wall thickness. The spar is 4" O.D. and the inner's 1". Thanks for your advice on inverting the inner assembly. This is just the sort of advice I need. (where's your clap smilie?)

Extending the outer tube to the tip end would make things a lot easier. Just modeling those 3 holes in the right place was a bugger of a job for me . Using just one rib template will make the job easier and faster.

I hadn't thought about the inner spar pulling out although it is anchored by the stress wires.....not good, and I thought that the 3/4" ply ribs would be strong enough to hold any shear loads put on them. They would be either epoxied or glassed to the tubes.

Orion, do I understand you correctly. You say cut holes in the out tubes to save weight? Doesn't that weaken it significantly?

Topaz, got any more info on the 80's plane? I'd like to view it if you can find any.

No one questioned my leading edge. The intention here is mainly to eliminate a scalloped wing, so past that picture. I included the spar to add strength and provide additional anchorage. The 2mm ply covering is not intended to provide strength or stiffening, purely to prevent scalloping of the film skin.

The aft spar like wise provides extra anchorage. It's position isn't set yet. It was there when I had Youngman Flaps in the design.

I forgot to mention up front that the wings are intended to be cantilevered and removerble for trailering or long term storage.

If you're interested, here is a link to the Oz forum thread re this design.
How To: Design and build an 'Ultralight' - Recreational Flying

FlyOz, thanks for the links. I once got all excited by the Pic-26 then saw it's stall speed. Pity though, nice little plane. As for the Cri-Cri, gee there'd be a load of them built here if we could have 2 engines.

Originally Posted by deskpilot

Orion, do I understand you correctly. You say cut holes in the out tubes to save weight? Doesn't that weaken it significantly?

Like with any structure, you have to actually analyze the situation and optimize it for the loads and design conditions. Tubes (in bending) are actually very inefficient for use as a spar since you have material where you really don't need it (on the sides). Furthermore, tubular spars will most likely fail through a process of membrane instability, which results in failure stresses that are dramatically lower than what simplistic analysis might suggest.

However, even so, the sides will tend to be much thicker than needed for the shear loading so boring holes through the fore and aft "faces" is perfectly OK. But to do it right you do have to analyze the case otherwise you introduce stresses you had no intention to.

Lots of things to consider
Lots of good info to read first on Homebuiltairplanes.com
My two cents worth .......
Make the spar simpler - check out the MAG 01 Plywood spar - attaches real easy and has completed the static testing so far or go for a metal spar like the Cri Cri or Banbi
Pilot saftey - what happens if it goes nose over - the full weight of the engine behind the pilot - what sort of roll over protection ?
Nosewheel retract not worth the effort at the speeds i think and adds complexity
Just some ideas not criticism thats what this site is about
Flyoz

The 4" diameter tube spar is almost certainly too small.
The Kolb uses a 5" tube spar, I think, and may have gone to 6" in later models.

A cantilever wing has extreme stress at the wing root so a tube would not have the desired gradual taper in thickness that most designs require for minimum weight.

I bought a 5"x .063 6063 irrigation pipe and tested it with my body weight. It bucked by membrane instability as orion described. I was not impressed, perhaps a thicker tube would work.

The BD-5 has a tube spar about 3" in diameter, but it is more than .25" thick. It has large holes near the wing tip. (and a very short span)
BB

Many builders have also incorporated internal reinforcing for the BD spars in order to give them improved margins and a bit more stiffness.

As far as general comments are concerned, the layout looks feasible and has the potential of being simple and attractive, if a bit of attention is paid to the details. As was mentioned above, the idea of mounting a prop onto the tail boom in this manner has been done at least once in the past but for one reason or another, the concept did not survive. One of the big questions here has to do with service: How do you replace the reduction drive belt(s)?

Tube spars are simple but coming up with an effective attachment mechanism for the ribs is often a problem. Usually the solution is either inadequate or expensive. So far I've never really seen an optimal configuration. You might have better luck with a built-up rectangular tube spar instead - it's a bit more work but in the end it can be made more optimally from both, the standpoint of weight and manufacturing efficiency.

I might also suggest that you omit the retractable nose gear. The drag reduction versus a well designed gear is pretty minimal and if the goal here is simplicity, then the retraction mechanism goes sort of counter to that.

But your prop will be the biggest problem - you will have to make sure that there is no deflection or flexing between the prop and the engine otherwise your belt life might be measured in minutes. I'm guessing that section will have to be a separate assembly of some sort or the boom will have to be heavily reinforced or made of steel in the section in order to attain the maximum rigidity in the assembly.

I think the prop hub will also end up rather massive since the hoop and twisting stresses set up by the blades will be significant.

I think the concept is attractive but as they say, the devil is in the details and some of the aspects of the configuration as you have it will make this a rather challenging design problem.

Originally Posted by orion

And since the three tubes don't seem to be connected outside of the big tube the only shear path is the ribs, which will most likely fail. Once outside of the main tube the three tubes should be connected with webs.

However if weight is your goal and you still want to stick with tubes, you can just do the round tube to the tips and using a hole saw, cut out a series of circular plugs along the span to reduce excess weight.

Besides the shear load, these separate tubes will also buckle in compression because there is nothing to stabilize them. The separate tubes would act like 3 fishing poles and would deflect sideways under load. Also as mentioned above you would also want the 2 tubes on the top for + G's. Assuming the concept could be made to work you might want 4 tubes if the one on the bottom will still buckle when it undergoes -G's and compression loads.

Running the main tube all the way out would be the way to go. It would be excellent for torsion loads but a bit inefficient for normal wing loadings. It would work though and would be a good shape to take all loads simultaneously.

Originally Posted by deskpilot

... Topaz, got any more info on the 80's plane? I'd like to view it if you can find any...

Well, I just ran a Google image search a couple of different ways, and turned up nothing. Doesn't surprise me terribly - this was a one-off aircraft that appeared at Oshkosh for one year only, then disappeared. If you're really interested, take a look at the "Oskosh Review" issues of Sport Aviation for the early- to mid-eighties. Your local library might have them, or you might be able to get copies somewhere else. There were three or four images (and a brief article) about the airplane in one of those four or five issues, but that's about as close as I can come to getting you an image.

The aircraft appeared to be built to an identical specification as you're using now: single seat, light-duty sport plane. It had a nicely-faired fuselage 'pod' (looking rather BD-5-esque as far back as the wing), a canard on the nose with the leading edge at the tip of the fuselage pod, a shoulder-mounted wing, and a single rectangular vertical tail mounted on a short (~4') tubular tail boom aft of the pod. The engine was mounted in the pod below the boom, and drove a propeller that rotated around the boom through a belt drive. The belt sheave was integral with the prop hub. My recollection is that it had fixed gear (tricycle), and that it had at least flown prior to Oshkosh that year, and may have flown in to the event that year.

It was a one-off, never kitted, and completely disappeared off the scene after that one appearance at Oshkosh. Whether that was because of some technical flaw or simply that the designer/builder just kept the thing to himself and flew it, I simply don't know.

At least one motorglider had a pusher prop around the tailboom. I don't recall the name.
A set of spare belts could be held in place on the side if replacement is difficult. Or it might be simpler to just install the extra belts and get a long life.

On the triangle space frame truss...
I have pondered this quite a bit. It might be better to place one large tube on top rather than two tubes as suggested. I have not checked, but sometimes one large tube is better than two small tubes of the same weight because the large tube is stiffer in all modes of flex. (one 2 inch tube would make a better wing strut than two 1" tubes side by side, for instance)
BB