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Author Topic: NZ Jet Trainer,  (Read 775 times)
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Sandgroper
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« on: March 06, 2015, 10:10:25 PM »

Hi all,
I have always had a thing about pulse jets so a while ago I bought a HK pulse jet,not wanting to get dragged around faster than I can manage I decided a larger slower model would be the go,the first pulse jet I saw in the mid seventies was in a large twin finned model with a thick built up wing to keep the speed down and although practical it was very ugly as well so another approach was needed.
I came across the NZ Jet Trainer designed by Doug Kennedy in the 1963-64 Aeromodeller Annual and it looked right to me with its elliptical shaped 33 1/2"span 3/8" thick hard balsa wing.
 I am at the stage of making and fitting the bell crank before shaping the wing section,I have inlaid alloy tubes into the inboard wing but I notice many jet speed models have external controls,am I better off with the controls built in nice and tidy or just hang them out in the breeze with adjustable tip guides?the drawing is a little sketchy about the controls,the bellcrank appears to be under the wing but the leadouts and the pushrod appear to be internal.
The model will be carbon covered and run on a methanol nitro mix.

Philip
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noclassmac1972
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« Reply #1 on: March 07, 2015, 02:59:16 AM »

Hi Phillip it is much easier to put the controls on the outside of the wing as you are not going for speed. Also be aware if you are making the fuel tank as per plan. you probably will only get 5 laps with methanol based fuels. My T34 kit for the hobbykingjet has a 200cc tank and get 12 -14 laps

Robert Bolton ( Kiwi Jet Team )
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Sandgroper
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« Reply #2 on: March 07, 2015, 03:52:02 AM »

Hi Robert,

thank you for your advice the lead out tubes can go on the outside as a little tip weight, I will inlay a 4"x3/8" sq hardwood spar and mount the crank on top the inside wing.
 If I make up a shim tank to fill up the nose it will be around 110cc,I could make up a carbon fiber tank/nose and get more laps.
 I also see some jets use a mono wheel, it looks like a better idea than a dolly to me.

Phil
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noclassmac1972
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« Reply #3 on: March 07, 2015, 04:18:41 AM »

Hi Phillip if you want to stick with this design it would be best to run the unleaded fuel instead of the methanol fuels. will not be user friendly though. 110cc will be far to small for methanol fuel. Also please note the fuel tank is at the front so when the fuel is being used the models cg moves further back making it much more sensitve. I big headache for the one flying it.. Monowheel plus wing skids are a good idea. But you can also just skid it off with no wheel as theres no prop in the way  Wink

Robert
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Sandgroper
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« Reply #4 on: March 07, 2015, 04:25:05 AM »

Hi Robert,
I ordered three spare jet assemblies and two sets of valves,all of the jet assemblies were aligned correctly and I drilled one set out for methanol.
The petal supplied with the unit had a light scale on one side like traditionally heat treated spring steel plate and ground on the other, it had small pit marks in the valve sealing area that showed up under a magnifying glass after I gave it a nice polish with Vitasol, the replacement petal supplied with the jet was of a harder material ground on both sides, black on one side blue on the other and harder than the first petal as it took a bit to polish off the blue,the other replacement petals were ground and blue on both sides and the polish does not touch them,three petals all different material?

Philip
« Last Edit: March 07, 2015, 04:42:39 AM by Sandgroper » Logged
Sandgroper
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« Reply #5 on: March 07, 2015, 04:36:40 AM »

I always wondered why jets did not take off on a skid,I will take your advice on the fuel I have plenty of jets to play with, I always had a thing for Avgas since my days playing with a high comp Datsun 1200 engine in a Mallock U2 Mk11 Clubman sportscar ,is Avgas better than unleaded it always made my hopped up road car go well.

phil
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noclassmac1972
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« Reply #6 on: March 07, 2015, 04:38:11 AM »

As long as they are spring steel they should be ok. THe main thing is to get them to seal on the head..  Dick Harts article in the aeromodelers was spot on to follow

Robert
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Sandgroper
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« Reply #7 on: March 07, 2015, 04:50:18 AM »

Yes I read that,a sealing area near the threads had a nick in it so I gave it a rub on a sheet of 1200 wet and dry on glass,It was then I discovered the face is slightly concave but only after I took off a little anodizing ,it still seals up ok.
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