Guessing Down thrust...
 

On my 80" BUSA Citabria Pro, the plans call for 1 Dgeree of Engine downthrust. But, many who have built it say it flies much better with 2-3 deg of downthrust.

I have installed an Enya 155R 4c, and had 0 degrees to start with. It of course flew as if it needs downthrust, so I have added the following....

Under the top mount bolts, I installed two 1/16" thick washers between the mount and firewall, and it definitely has downthrust now. But, I am wondering exactly how much? I appears to be a good 2-3 degrees, but I have now way of really measuring exactly, so I was wondering what you all thought from your experiences of adding washers to attain an angle. How much does it generally end up with when doing this?

Patriot

RE: Guessing Down thrust...
 

On my 80" BUSA Citabria Pro, the plans call for 1 Dgeree of Engine downthrust. But, many who have built it say it flies much better with 2-3 deg of downthrust.

I have installed an Enya 155R 4c, and had 0 degrees to start with. It of course flew as if it needs downthrust, so I have added the following....

Under the top mount bolts, I installed two 1/16" thick washers between the mount and firewall, and it definitely has downthrust now.

>Make sure the washers are not going to compress into the firewall. I prefer
>to use them under the mounting lugs of the engine.

But, I am wondering exactly how much? I appears to be a good 2-3 degrees, but I have now way of really measuring exactly, so I was wondering what you all thought from your experiences of adding washers to attain an angle.

>The GP Laser Incedence meter will also measure down/side thrust.

How much does it generally end up with when doing this?

>I am more concerned with how the plane flies, than with how
>many degrees of down/side thrust I have. I use the amounts indicated
>on the plans as a STARTING POINT and go from there. I have had no
>problems using washers for shims.

RE: Guessing Down thrust...
 

Each washer is about 1/2 degree.

RE: Guessing Down thrust...
 

Ok, so with one big washer between the engine mount and the firewall, I should have a good 1/2-1 degree of downthrust. I am certain it is probably at least 1 degree, because you can really see the angle with the naked eye. The backplate of the spinner has dropped about 1/8" from centerline, so I'll give that a try. Unless you all have a better idea? Thanks.

Patriot

P.S. BTW, I named my cat "Campy", named after bicycle components..."Campagnolo".

RE: Guessing Down thrust...
 

You could buy, or borrow, an incidence meter and use it to determine exactly the amount of down thrust you have. It will tell you in 1/2 deg (1/4 deg if you interpolate) increments the amount of thrust variences you produce.

RE: Guessing Down thrust...
 

Why guess when it is so easy to measure? Just use the tangent function from your old high school trig class. Even most of the low priced calculators now have this function available. You know the distance between the lower bolt and the upper bolt holes for one side of the triangle and the thickness of the shim (washers) for the other leg.

RE: Guessing Down thrust...
 

I thought your question was more an experimental one. When your plane is trimmed straight and level at half throttle, go to full throttle. If the plane pitches up very quickly, you need more downthrust, if it pitches down, you have too much downthrust. When the plane remains fairly level until the speed picks up enough to make it climb, then you've got it.