Large Model Association Forum

I'm pleased to say that this project is now registered (Over 20Kg category), and, is finally underway. No quarter-scale documentation has been found so far, so, I expect that everything will have to be adapted from other scales. This may mean that I'll have to teach myself CAD ! (Normally, I'm more at home on the drawing board). One advantage of CAD is the ease of re-sizing things plus you can give a "DXF" file to the laser cutting boys once you have worked out all the wood sizes etc.

The spec' for the model is very ambitious and includes working variable pitch contra-props, double wing-fold, sliding canopy and actuating deck hook as per full-size. To date, I have been working on the design of the contra gearbox with some initial ideas on how I'll integrate this with the variable pitch mechanism.

On the workbench now is a 20% mock-up of the two co-axial props together with 'Mk 1' of the planetary gearset. This will be used to drive the front prop, reverse the rear prop and accept the input from the turbine's reduction gear. So far, the props have been run up to 2400 rpm without problems. Blade pitch is fixed at present as the rear prop has been adapted from a right-hand tractor but still gives a 'healthy draught' when reversed against the front prop. The planet gears have suffered a few broken teeth. They are only in wood at present so no surprises there ! However, the exercise has taught me the correct number of teeth, pitch, profile and gear ratios. It is helpful to sort all this out now before going into metal...

I'm really enjoying the project so far. There will be many other engineering challenges along the way but, I hope that working out the driveline and prop control issues, up-front, will pay dividends in the long run. My wife is the photography expert, so, as soon as I have the bugs out of the contra gearbox, I'll post some pic's of it in operation.

Hi Dave,
Have just taken delivery of all the available parts from Dave Wigley for his 1:5 Wyvern. I was thinking this was pretty ambitious with a home manufactured undercarriage and contra-prop assembly (just a free-wheeler front prop) but am now suitably humbled.

I look forward to your future posts with great interest and wish you luck with this demanding project.

(yet another) Dave.

Hello David. Nice to see another Wyvern engineer-pilot in the making ! I'm just convalescing from a hospital op' and last week read Harald Penrose's autobiography - chief test pilot for Westlands (he did a lot of the original test /development work on the Eagle, Clyde and Python variants).

I've also corresponded with Dave W in the States as part of my research (I now owe him some photo's of the contra-box...) however, today, I had to make two new planet gears due to some over-enthusiastic 'bedding-in' of the Mk 1 set. You have chosen well - his design is very well thought out and proven, and, yours will probably be flying miles ahead of mine.

At the present time, I'm getting quotes for plan enlargements, CAD files and metal gearsets etc. However, I have a sneaky feeling I will end up drawing it and doing a lot of the machining myself.

Happy to share info' / news with you. Are you going the turbojet route, or, the more 'sensible' IC /Electric one ? I now have an inspector assigned for the over 20Kg category and he has been in touch once with some good advice.
Dave C

Hello Dave,
Sorry so long with the reply. I will be going the 'sensible' route following Dave's design very closely and will be using a BME110 engine as most of the other alternatives are too wide to fit the cowl. Not started it as yet as it was a spur of the moment decision with the investment in plan, parts and engine and will have to finish two or three other projects first. Yes I will have to register soon. There is a possibility ( but only that) that the weight could come out under the limit, but as you intimate advice is always welcome.
With the warmer weather my modest machine shop is now habitable so I plan to make a start on the metalwork in the near future. Your gearbox sounds very interesting ( and complicated) and will be quite an achievement, what turbojet are you aiming to use ?
Hope the convalescing worked and you are now fighting fit.

David Q.

Hello David

Making a good recovery thanks, but, the workshop is still a bit too cold for serious build sessions. Still shaking the bugs out of the contra-box design but am now close to ordering the metal gearset from a specialist supplier.

So far, I've only looked at the Wren 44 and Jetcat TP's. One's a bit low on power and the other's quite expensive - have AMT and a few others to research as well.

Another thread that may interest you is one of mine under "Technical Help Required" - I'm trying to settle on a wing section with washout that will also work with the wing-fold and flap mechanisms. Could you have a look at Dave W's plan and let me know which root and tip sections he's used ?

For the undercarriage, Dave has given a series of images of the undercarriage under construction together with descriptions of his machining op's. I think the link is on my "Project Feasibility" (Tech Help again) thread...let me know if you can't find it and I'll dig it out for you.

Hope to post some pics of the gearbox internals soon...

Dave

Hello David,
DaveW only shows the ribs themselves with no type name. However the latest post in the Tech Help looks like you may be sorted, full size anyway.
DaveW has supplied me with machining drawings and is very approachable for advice so I should be OK.

Dave.

The Mk 1 contra-gearbox is nearing the end of it's test life. The rear prop has been repaired twice and I have now consulted with a specialist gear manufacturer for some quotes and extra assistance with tooth profiles, gear ratios etc.

Lessons learnt so far :-
1. Wooden mock-ups are great for getting ideas 'into-the-flesh' but they have their limitations !
2. Getting the shaft centres and tooth PCD's correct (to within 0.5mm) is vital for smooth running.
3. To get the correct relative speeds for the front and rear props will need a compound planet set not a single-stage one as per the test unit.
4. Working with small-scale props has introduced too many packaging problems and difficulties with manufacture of the close tolerance gearsets.
5. A plus point ! - the gear manufacturer's catalogue has pointed the way for the variable pitch change system at the hub end.

The Mk 2 gearbox will be to scale and should contain proper "involute" teeth profiles plus a two-stage planet set. I still feel the need to prove everything in wood first before ordering /making parts in metal...the hunt is now on for a reasonably priced lathe, dividing head, milling facility etc. as the cost of bespoke gears is quite prohibitive

Pics to follow as soon as I can persuade the wife to get her camera out.
Dave

This is a cut-away of the full-size pitch-change system - needless to say the 1/4 scale version will be somewhat simplified !

Hello Dave,have a look at the link click on TC Engineering he's a fellow modeller and a mate he might be able to help you. http://midlandslargemodelflyinggroup.com/

Cheers,Dave

Thanks for the engineering contact David. As soon as I've finalised the design I'll make some detail working drawings and get in touch...
Dave C

A quick update on progress : The Mk 1 gearbox contained a fatal flaw in the tooth design. This has now been identified, corrected and a quote received for a new design. The fault was actually a geometrical one (by me, not the supplier) and it involved the teeth spacing of the internal ring that reverses the rear prop. Justification for a wooden mock-up ! Due to packaging issues with the little planet gears that mesh inside the ring gear, 1/5 scale is now definitely out. This is a pity as there is already a well proven plan available at 1:5 and none (as far as I'm aware) at 1:4.

Demand for paintings and music gigs has kept me out of the workshop this 'summer', but, now that I'm officially an "OAP", I should be able to make some progress with the Mk 2 gearbox over the winter. As I'm going to start from scratch with the 1:4 plan, I've decided to go for the S2 Wyvern (Rolls Royce Clyde engine) rather than the larger S4 that most people model. This has the advantage of having the same fuselage size /shape as the one in the FAA museum in Yeovilton. So, theoretically anyway, research should be that little bit easier...

Dave C