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fat avid 912 engine mount

13 posts in this topic

Posted

The mount is under way.  making the cups that all tubes to mount to the firewall is 16 pieces welded together to make the cups.  i think i know why kitfox and rotax charge what they do for these.  next one i will have the discs cut on the water jet and save some shop time by getting a whole bunch done at once.  The firewall jig is based off the pattern that Randy Tyler made of his firewall.  This will locate the mount flanges and basic tube lay out.  I still have to fabricate the stand offs to hold the cups for welding the tubes.  Our plan is to move the 912 forwards about 5 inches to keep the cg the same as it was with the subaru engine.  Its a fair bit of work to get the first one perfected but future mounts will be easy to weld.  I will use hardened all thread to locate the cups so it will be easy in the future to make the prop flange fall where ever the builder desires to make the cg so engine swaps wont require a bunch of battery moving etc.

:BC:

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Posted

Any offset to engine?  EDMO

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Posted (edited)

Any offset to engine?  EDMO

straight ahead and 2 deg down.  Pics will follow later, its not letting me upload from my phone.

Edited by akflyer
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Posted

Any offset to engine?  EDMO

straight ahead and 2 deg down.  Pics will follow later, its not letting me upload from my phone.

The 2 degrees down - Was that just to line up prop flange with cowl, or other reasons?   EDMO

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Posted (edited)

Ed,

When we got to measuring my Subaru mount we discovered that Airdale made it at a 6.5 degree down angle.  The only thing I can figure is that they had their jig set up wrong or the welder screwed up.  Anyway, it explains why Jack's plane and my plane fly so different when everything else is pretty identical.  The reason we are going back to (1.5 degree down Leni) is to get it more standard, better takeoff performance and less drag in cruise.  (Jack's is 1.6 degrees down).  Many of the cub guys are going to in line mounts now.

I swore I would never go back to a Rotax because I hate the way they treat their customers with their pricing but I wanted more horsepower and less weight and could not figure out how to get it with my Subaru without adding even more weight of a supercharger and probably would cost more in the end with custom heads, etc; so I bit the bullet and bought a used 912 ULS and just ordered a high HP Zipper kit from Hal last weekend. 

I really like the Yamaha idea but it is still a bit in the development stage for me to make it a quick swap project that I wanted.

Hope to have the new extended wings with 3/4" washout, much stiffer spars/inserts, lengthened lift struts, extended flaperons and engine swap done by end of summer next year (although that may be a bit ambitious).  I sure appreciate the Magnum wing build info you sent, these wings will be identical to the magnum wings dimensionally but with the flaperon hangar spacing of the heavy hauler wings. 

Still plan to use the great Subie motor again though, want to extend the new MKIV fuselage I have and put it together with the Subie and current wings.  You can't beat the ease of operation and reliability of the Subie!

Having Leni building the mount for me is awesome; Many thanks Leni!

Randy

 

Edited by SuberAvid
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Posted

I hope your Rotax project works well for you, Randy, and doesn't make a liar of me. 

We (as 912 operators) are finding more and more aftermarket and non-traditional sources for parts for the 912 engines all the time. We are also coming up with new and better tuning tricks to make them run better.  Please don't be shy to ask if it gets funny with you. 

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Posted

 

 

 I sure appreciate the Magnum wing build info you sent, these wings will be identical to the magnum wings dimensionally but with the flaperon hangar spacing of the heavy hauler wings. 

 

 

 

 

where can a guy get a hold of this info?

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Posted (edited)

 

 

 I sure appreciate the Magnum wing build info you sent, these wings will be identical to the magnum wings dimensionally but with the flaperon hangar spacing of the heavy hauler wings. 

 

 

 

 

where can a guy get a hold of this info?

TJay,  I don't know if this is in Files and Forms, and I sold my Magnum book.  I think I helped Randy decide to get the Just inserts, like you have.  Moving struts outward more than Avid might be a benefit too.  Magnum wings are comparable to Kitfox, except 4 feet longer span and 083 spars.  Larger flaperons should help him too, but not as much as my flaps and ailerons, IMO.   HH rib spacing would be good.  That's about all I can remember.   I don't know if he is going to use Kitfox 4 ribs like Leni plans to do on the Coyote.   Maybe Randy can tell you more?   EDMO

Edited by EDMO

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Posted

Randy, thats interesting to hear your soob mount was angle down 6 degrees , I had the same problem with mind ,same guy must of built them. It caused alot of head scatching trying to get it to fly level,good luck with the new projects

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Posted

Hi Randy,

My fat Avid with 912 weighs under 700 empty if it were not for the larger tires I have on it, so it is pretty much as light as it can easily be made. In other words not a lot of extra weight in any location that would mess with basic weight and balance.

I do not know where the subaru mount placed the engine, but I can tell you that the "stock" (whatever that means :-) Avid 912 mount places the prop in the right place (front to back) to allow the motor to fit a stock mkiv airframe and cowl without modifying the cowl.

With that mount on my Fat Avid the plane sits about an inch in back of the front CG limit when empty. So, it's much better with regard to CG than putting a 912 on a stock MKIV that is not stretched because the empty CG will fall in front of the forward limit.

Anyway the reason I am posting is that if I had put my engine 5 inches forward of where it is located on my plane the CG would be super too far forward. As it is, I have temporarily placed 10lb of lead the in the Fat Avid tail until I can move the battery back because it flys so much nicer than it does an inch back of the forward limit..... and I can still load it up with a lot of shit and not go aft.

If you haven't completed the mount yet I would highly recommend placing the engine in the same place as Avid placed it with their various 912 mounts. I've seen three and they are all different, but they did all place the prop in the same position in very close to the same place as the 582 mount place the prop for that engine.

Chris

 

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Posted

Leni - I think Chris is correct, 5 in. forward sounds like way too much. Lowell Fitt built a 912 ring mount for his KF4-1200 where he added 2.2" forward and later claimed it was probably 1" too much. He was trying to compensate for added tail weight calculated to be just under 7lbs. at an 11' arm after installing speed rib Intercoastals and trim etc. There's obviously more length and weight in the A+ empennage than KF4-1200 but surely not >15lbs.?

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Posted (edited)

Leni - I think Chris is correct, 5 in. forward sounds like way too much. Lowell Fitt built a 912 ring mount for his KF4-1200 where he added 2.2" forward and later claimed it was probably 1" too much. He was trying to compensate for added tail weight calculated to be just under 7lbs. at an 11' arm after installing speed rib Intercoastals and trim etc. There's obviously more length and weight in the A+ empennage than KF4-1200 but surely not >15lbs.?

Randy hauls huge Moose and Caribou parts and camping gear out of the Bush - Maybe that's why he needs forward engine weight?  Evidently he is trying to keep the same CG that the Subaru engine had because he has been flying it that way and it works for him.   EDMO

Edited by EDMO

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Posted (edited)

Thank you all for the info, it will likely save Me from positioning the engine in the wrong place (and Leni from a lot of work and rework).  I was trying to keep the CG pretty close to where it is currently with the Subie motor since as Ed says, I tend to load my plane pretty heavy at times and it has a baggage area that extends back 6' behind the truss that the bungees wrap around.  I also keep my tool box at the very back of the baggage area right now but moved the battery to the cabin side of the firewall when I went to the EarthX battery.  My Subie motor is about 195 lbs dry without exhaust and I think the 912 is around 130 lbs dry, but is quite a bit longer motor and the redrive sticks out a lot farther, so my position estimation may be off.  Leni was going to take some more measurements of the motor and CG length of it before we made any final decisions but this is great help!.

Chris, can you give me a length from your firewall to the redrive prop flange?  My plane is still going to be quite a bit heavier than yours, likely in the 770 lb range but that will still give me the best known starting position.

Larry, you can bet I will be hitting you up for help as I try to learn the care and operation of the 912.  I also had Hal send me the parts to convert to the 1" manifold crossover tube.  He says between that and the lighter pistons of the Zipper kit the engine will run smother and idle down to 1000 rpm without chattering everything.  I am used to pulling my Subie down to 600 rpm when landing and find it a bit difficult to land short while trying to keep the engine at 1400 rpm like Rotax says.  Hopefully this will help in that regard.

Willis, look forward to hearing about how your Yamaha conversion performs as well.

Edited by SuberAvid

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