Large Model Association Forum

Hi

The latest project is well under way so I thought I'd try and post some pictures.
Please excuse the quality of the photos as I've only my iphone to work with.

It's a Don smith 335 plan blown up 20%. I've got the retracts from century jets. It will have two z62s with reduction drives.

The crux of the project is the 140cm rear prop shaft , will it work? Thats the question I'm looking forward to finding out!

Fantastic Karl . But the question is will it fit in the van lol will watch this one keep up the good work
Regards
Aaron

Update on the 335, airframe is now complete so the balsa bashing commences!
Below are a couple of pictures of the assembled 335.
The motivation to get it finished is to see whether the 140cm rear prop shaft works.
The prop shaft fits directly onto the prop driver without any flexible joint. To ensure correct alignment the engine former/engine/ shaft assembly was bolted together then lowered into the fuselage vertically . This was then epoxied to the fuselage former , allowing it to settle in its natural position where it's aligned perfectly with the fuselage. Hopefully it should work.

Karl

Other than at the rear, have you got any bearings/supports on the rear shaft Carl?

The one I'm working on has 3 over it's length & it's a chunk shorter than you're 1.4m. I have one bearing & aluminium housing just ahead of the anti-vibration engine coupling at the front, one at the rear + a 3rd in the middle of the shaft.

With no central support, I can see the shaft oscillating if it starts resonating putting a huge stress on your coupling. The percussive forces generated by the rear engine will also put huge stresses on a rigid coupling.

Just my 2p's worth.

Phil

Hi

Update on the project - I completed the fuselage and so was able to test the rear engine/ prop shaft , not wanting to put any more work into the project until I've tested the shaft.

I tried starting by hand and after several pops had to use my dynatron on 24v to turn it over.
It started! There were two problems, firstly the cooling fan on the rear engine is an airscrew and needs to be a pusher prop so engine not being cooled. Second problem was that the rear spinner backplate securing holes had been stretched by the torque of the starter causing the rear spinner to knock.
Below is a link to a short video showing the engine running. Excuse my stupid phone screwing up the orientation .

http://youtu.be/g5Z8Fiz-MT8

Phil - re your last post, I totally understand what you're saying, I'm not an engineering expert just a bloke in a shed with an idea whose only way to test the idea is to build it.

Karl

Both engines running!

Update on the this ridiculous idea that I've had to build.


After fitting a carb kit and overhauling the carb I've had the front engine running( a new piston and pot on a old engine) and so was able to get both engines running together.

The picture shows them both running, I have a short video that I'll post on you tube .

There appears no lack of power as she was able to push the brick chicks forward !

Karl

Nice one good luck with the maiden.

Below is the link to 36 second video on you tube showing the engines running.
This is the first time that I've had them running together and they need a little work to get them in sync. The power is clearly there as the model is able to push the house brick chocks forward.

http://youtu.be/LuWeC42IrP0

I started the 335 at the beginning of October, four months ago. I want to run the engines some more before taking it for its final inspection , however the crap weather isn't conducive to outside activities.
Therefore the next project is being unboxed and the wing spars being spliced - a Don Smith Tupolev tu95 bear, at 202" I thought I'd go for an easier project than the last one. I've purchased the kit from belair which is breaking from my normal routine of scratch building the model.


Karl

What diameter & pitch props are you running front & rear Carl?

Phil

Hi

The props are 28 x 12.

My concern is not the longevity of the propshaft but the longevity of the model due to its weight. With the two engines, propshaft , bearing and the undercarriage that has been stolen out of boeings spares bin the thing weighs more that dark matter.
The patch of alloy at the side of the rear engine is where the rear exhaust header pipe charred the wood.
The rear engine is cooled by a air intake pipe from the front supercharger and from the cooling fan at the front of the rear engine. How are you cooling your 335 rear engine?

Karl

Hi Carl

From all the research I have done, and people I have spoken to who have built & flown large 335's, it appears you have to run more pitch on the rear prop than on the front. There is a lot to it, but the simple explanation seems to be that because both engines are 'in line'......the air hitting the rear prop has already been accelerated/turbulated by the front prop, so if you have identical pitch on the rear to that of the front prop, it'll have little effect and create drag rather than thrust, hence if it were me, I'd be looking at something like a 26" x 14" on the rear.

I cannot confirm exactly what was done on the full-size, but I'm pretty sure I'm right in saying the 2 props were not the same (can check tomorrow when back at work). They varied in diameter which would suggest different pitches were used front to rear.........but considering both engines were the same, it points more towards 2 different props than 2 identical.

Re:- Cooling.......I have air entering the fuselage via both supercharger scoops, plus the large scoop under the rear fuselage. My exit area consists of the 2 large flaps under the stab (approx. 1" x 3"), a smaller one on the underside of the fuz ahead of the lower fin (approx. 2" x 1/2") plus through the rear fuz through slots in the rear former carrying the rear bearing housing. All of these are rear facing so forward movement & airflow passing back along the surface of the fuz will basically scavenge hot air out from inside the fuz. I have no internal cooling fan, but the main internal bulkhead the rear 45 is mounted to has a hole in it approx. 3" square directly in front of the cooling fins........this will be ducted so any air entering the fuz ahead of this bulkhead has to pass through said hole & through the cooling fins when flowing through to the rear fuz and the exit openings.

This is a proven kit and the system works. I have a customer with this kit over in Texas where it's VERY hot and he's had no overheating issues.......he has carried outs temp test and it shows the rear engine runs cooler (approx. 20 deg IIRC) with the front engine running, compared to it stopped, so the rearward airflow created by the front engine forcing air into the inlet openings & through the fuz I definitely having an effect.

Re:- weight.......what's she come in at?......and what's the wing loading.

Phil

karl,
Well done i like your bloke in a shed approach and totally different to Phils fancy German high tech build ..
Cheers Euan

Hi

The 335 is ready to test fly after sorting out the various problems that arise -
Cooling fan repeatedly trying to escape out of the fuselage( coupler modified) , rear exhaust leaking, faulty servos causing me to exchange wiring/ reciever.

After copious engine testing and with no sign of percussive forces( a good name for a rock band)
I decided to take it to a nearby field for some taxi testing.

With parental pride I watched its 35 kg bulk zig zag down the field. Both engines in sync it appeared like a moronic robotic anteater, going no faster than walking pace the noseleg gave way followed by the sound of the prop breaking .
The cause of the leg failure is poor workmanship on the bespoke century jet retracts , where the leg cam is merely tack welded. The mains legs broke during engine testing which I've already had repaired ( photo attached),
I have taken more photos of the cause of the fault but I'm having difficultly posting them.
I'm a little annoyed to say the least as my shed built prop shaft is working well yet I'm let down by professionally built equipment that cost a lot if money.

At least the rear engine kept running.

!!!!! MAIDEN FLIGHT!!!!!

Yes it took to the air and below is the link to its first flight which is posted on YouTube .


http://youtu.be/Ey-Cwi1Jd9E



It's only a short video as it was only a short flight, however the rear engine/ propshaft performed faultlessly, unlike the front engine which cut out after the front engine bolts came loose shutting the throttle(forgot to loctite bolts)
Yes , the canopy catch failed and the retracts weren't pumped up enough but it flew superbly managing a roll on its first circuit.
It's a victory for the man in a shed engineering.

Well done Karl, looks really good.