I have the 3rd edition of Thomas Kamps' 'Model Jet Engines' and am looking at building an engine to the plans in the book. I'm not a flyer; I would just like to be able to build a working engine, making as much as possible myself.
The KJ 66 is a (very) similar design, but I'm drawn to the Kamps' design because it should work (to some extent, at least) with a home made turbine wheel.
I know there have been previous discussions about the older design which used a hollow shaft. The design in the 3rd edition uses a solid shaft:
Is anyone aware of any problems with this design? - I don't see to many build threads for it anywhere.
I've also joined the GTBA and am asking there too - apologies if anyone sees it twice!
I may be able to get hold of some Inconel 617 for my turbine wheel (I know that Inconel 718 used to be the material of choice). Inconel 617 seems to have good hot strength, excellent creep resistance & is scale resistant but I'm not sure how workable it is in comparison to 718.
Anyone used 617 or know if it will stand being twisted into vanes?
however, they don't like to be twisted too far or stress fractures will appear at the blade roots...been there done that.
Thanks, I was going by the instructions in the Kamps book which says to twist the blades by 30-35° before grinding to shape - is that likely to cause a problem? (I can believe it will take a bit of force!).
You have to heat the blade to orange to bend it to 30-35 degrees. I've never made a turbine wheel from Inconel, just stainless. Even with heating, it takes a moderate amount of force to bend it with the claw tool.
If it's not going to power an R/C airplane, have you considered slightly enlarging the Kamps design? Reason being, the rotor becomes slightly more efficient the larger it's made.
have you considered slightly enlarging the Kamps design? Reason being, the rotor becomes slightly more efficient the larger it's made.
Thanks for the reply - I've already got the compressor wheel, so am intending to build to the book (however I'm just contemplating whether I could reduce the diffuser diameter by 1mm so that it will fit inside a camping gas canister!)
I have read that there are problems hot-forming inconel unless you get it to over (something like) 1000°C as moderate heat can make it more prone to cracking. I think this was a 600 series alloy (maybe linked to an incident with a thrown blade). I appreciate that all the alloys have different properties, and there are a bewildering number of them. Not sure that I can get that temperature without oxy gear (I reckon 1100°C is about the limit for an air blowlamp in ideal conditions - probably won't be able to get >900°C without insulation).
I have a 316 disc that I could use - I don't know whether just to go ahead with that, or (given the work involved) it would be worth starting with something better.
One of the things that I've picked up is the KJ66 should have the bearings preloaded towards the front of the engine. I assume that this will also apply to the very similar Kamps design?
The design in the book is for a spring preloading the rear bearing towards the rear. This shows the relationship of the rear end parts as dimensioned in the book (with the turbine wheel as a blank disc):
A spring force of 15N is recommended and Kamps states: "this value should not be exceeded by a significant amount" - I think that this is somewhat less than what is usually recommended for the KJ66.
I have seen various suggestions to reverse the preload direction of simply replicating the preload arrangements around the front bearing. I am uneasy about this from an engineering point of view, as one then loses the accurate location of the compressor wheel.
As an alternative, would it be possible to preload the rear bearing from the rear using a wave spring washer and a narrow threaded ring to set / adjust the preload?
This would mean lengthening the shaft tunnel by ~3 to 4mm if the bearing position remains unchanged, but there seems to be room to do this without fouling any of the other parts:
I haven't come across this arrangement before, and wonder if it is likely to introduce problems that I'm not aware of.
Thanks - I've seen wave washers used in the same place as the spring, but it was the idea of extending the tunnel slightly to fit a wave washer / threaded ring to the *rear* of the rear bearing that I haven't seen before. Can you see any reason why not to do it that way?
Thank you! - The arrangement in that first video is almost exactly what I'm considering doing (except I'm thinking of a threaded ring to retain the bearing rather than the circlip / snap-ring) - very helpful, as I hadn't seen that arrangement used before.
The arrangement in the second video is similar to what I've seen suggested as a modification for the Kamps. The disadvantage is that any expansion difference between the shaft and tunnel affects the compressor wheel clearance. Probably not a problem for an engineered production engine where these things can be taken into account (and if the shaft tunnel and shaft are both steel?), but I don't think is ideal for a one-off build in my shed with steel shaft and aluminium tunnel.
If you now imagine the rotor is inserted into the shaft tunnel and screwed to the front ball bearing and spring preload and compressor wheel, there is no longer any great movement in the axial direction, so the compressor wheel cannot rub against the diffuser.
In this picture you can see the inserted spring not pretensioned.