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Tech Question Avid B Help Please

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Posted

Making an engine mount weld jig, and adjusting for thrust line, downthrust angle, etc.

 

I looked through the build manual that came with my B model, and in the Weight and Balance Section 1 it referred me to "Section 4.1" for leveling the airplane. But there was nothing in Section 4.1 about where you level the airplane from. The info may be missing, or I could just not have it in my manual.

 

What do I use for leveling the airplane in the pitch axis, with the horizontal tail removed from the airplane?

 

 

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Posted

level from the bottom of the door sill gets you really darn close.  I think the book actually says to use the 3rd rib out from the root, but when I did that on mine it was dead even with the level on the bottom of the door sill, and that is easier than holding a straight edge or a 4' level on the wing while playing with the jacks to level it.

 

:BC:

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Posted (edited)

some manuals - and I have both Avid Magnum and several Kitfox - say to use the door sill - while others say to use the cross tubes and longerons under the cabin - check them both out, and if they are the same, you cant go wrong. If not, then I would opt for the door sill.

EdMO

Edited by Ed In Missouri

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Posted

Thank you all... I have the fuselage apart... no tails mounted, no wings mounted. I'm just wanting to make my wood engine mount jig with the right amount of downthrust. Does anyone know if the upper and mid-section engine mount lugs (for the original Rotax mount) are vertical, or 90 degrees to level? I can put a vertical level on this easily enough.

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Posted

I would level it side to side across the door sills, and fore n aft using the door sill.  I could be remembering wrong, but I dont think they (engine mount holes) are lined up vertical.  you could pull a few measurements on it and check though.

 

:BC:

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Posted

The manual says use rib that has the middle flaperon hinge on the end of it. They show it on the left side here, I've always used the right side wing cause there is no adjustment in that wing. Here is that page from my MK IV manual. Jim Chuk

post-329-0-90834800-1418949967_thumb.jpg

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Posted

Dang, my memory was close.. I knew it was a third rib, just forgot which third one it was :lmao:  Thanks for setting me straight Mr. Chuk!

 

I hate being wrong but the wife says I am all the time.. I think she is mistaken :lol:

 

:BC:

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Posted

Guys... the wings are not mounted on the airplane. The ribs are on the other side of the garage :)

 

I tried measuring the lower door sill. I had to take an average between the two door sills. Because of the previous damage to the airplane, there is a slight difference between one door sill and the other. (When we un-twisted the fuselage we were only concerned with the wings. Fixing the fuselage so it was perfect would have required the original weld jig in Boise, if it still exists.)

 

Anyway, with the door sill (average) as close to level as I can get it with a bubble level instead of a digital protractor, I learned that the engine mount lugs on the fuselage are not vertical. So I can't use that for a "plumb line".

 

I did get enough of an idea from the door sills that I think I can get close to what I want as far as downthrust. All I want is two degrees, but it needs to be there because of torque, P-factor, etc.

 

I'll invoke the ancient spirits of free flight modelers throughout time, take a whiff of Nitrate dope and castor oil, and take my best shot at it.

 

The real bitch is going to be taking the fin offset out, because the Franklin turns the right way and the Rotax turned the wrong way. Lil John used VG's on one side of the fin and a big trim tab on the rudder... I'd like to do it more properly than that if I can, but I'm not re-building the entire aft fuselage.

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Posted

If its not perfect when you get done, you can take another whiff of the ole dope and add a washer or two under the engine mounts to get the thrust any place you want it.

 

The bottom longerons at the gear mounts should be running in the same plane as the door sill so you can level off that as well.

 

:BC:

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Posted

for "B" model S/N 232 weight and balance leveling instructions are as follows (from the manual 1986):

 

"Block under the main gear until the belly of the fuselage is level from the firewall back to the shock struts. On the tail wheel model, block under the tail wheel.

 

Level the airplane, fore and aft (see section 4-1)"

 

That's straight from the manual. however, the section 4-1 is for firewall installation and has no information about leveling. The manual was pretty crude back then.

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